Journal of Environmental Management 294 (2021) 112926

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Journal of Environmental Management

journal homepage: www.elsevier.com/locate/jenvman

Review

Recent advances in technologies for removal and recovery of selenium from

(waste)water: A systematic review
Izba Ali a, *, Vaibhav Shrivastava b
a
InOpSys - Mobiele Waterzuivering voor Chemie en Farma, Zandvoortstraat 12a, 2800, Mechelen, Belgium
b
Faculty of Bioscience Engineering, Coupure Links 653, 9000, Gent, Belgium

A R T I C L E I N F O A B S T R A C T

Keywords: Selenium (Se) is distributed into different environmental compartments by natural and anthropogenic activities,
Selenite and generally discharged in the form of selenate [SeO2− 2−
4 ] and selenite [SeO3 ], which are both toxic. Physical-
Selenate
chemical and biological treatment processes have been reported to exhibit good treatment efficiencies for Se
Wastewater treatment
from aqueous streams, only a few demonstrated to achieve effluent concentrations <5 μg/L. Moreover, there are
Advanced reduction process
Legislation only a few numbers of studies that describe the progress in technological developments over the last decade.
Mining wastewater Therefore, to unify the state of knowledge, identify ongoing research trends, and determine the challenges
associated with available technologies, this systematic review critically analyses the published research on Se
treatment. Specific topics covered in this review include (1) Se chemistry, toxicity, sources and legislation, (2)
types of Se treatment technologies, (3) development in Se treatment approaches, (4) Se recovery and circular
economy and (5) future prospects. The current research has been found to majorly focused on Se removal via
adsorption techniques. However, the key challenges facing Se treatment technologies are related to the presence
of competing ions in the solution and the persistence of selenate compared to selenite during their reduction.

1. Introduction monitoring in recent years. Mine drainage and volatilization of

Se-containing gases from fossil fuel comprise the largest sources of Se in
Selenium (Se) is an essential element, playing a vital role in the the environment, from where Se can further migrate to aquatic envi­
metabolism of organisms. In humans, it can act as a powerful antioxi­ ronments (Santos et al., 2015). Although Se exists in low concentrations
dant and can even reduce the risk for some cancers. Along with this, it usually ranging from a few μg/L to mg/L, it can still have a severe impact
also boosts up the immune system and has shown to be involved in on ecosystems if discharged without proper treatment as it tends to
successful defence mechanisms against viruses such as HIV (Stone et al., bioaccumulate in the aquatic environment (Mehdi et al., 2013). Oxides
2010). Remarkably, probably owing to its antiviral properties, also of Se are stable throughout the entire pH scale, making Se available in
significantly higher cure rates in COVID-19 were observed among the the form of selenate [SeO2− 2−
4 ] and selenite [SeO3 ] in aquatic ecosystems
population of Enshi city, located in a Se-rich area in China, compared to (Janz et al., 2013). Stringent standards are therefore set to limit the
people living in Wuhan city. This was related to the much higher dietary concentration of Se in wastewater as well as drinking water. The
selenium intake of people from Enshi (Zhang et al., 2020a, 2020b, maximum concentration set by the World Health Organization (WHO,
2020c). Furthermore, in another study, the Se-containing compound 2011) for Se in water fit for consumption is 50 μg/L and the EU uses an
ebselen was reported to have the best antiviral properties against even more precautious value of 10 μg/L. The US Environmental Pro­
COVID-19 in high throughput in silico screening involving over 10.000 tection Agency (EPA) has placed a regulatory limit of 3.1 and 1.5 μg Se/L
compounds and in subsequent cell-based trials (Jin et al., 2020). Despite for water lotic and water lentic ecosystems respectively (USEPA, 2016).
being an important micronutrient, Se has also been identified for its As a consequence, over the past decade, there has been a huge increase
toxic effects that also occur within a small concentration range. Sele­ in research related to the treatment of Se containing water, for example
nium pollution has become a great concern for both industries and the from mine drainage, flue gas desulfurization (FGD) exhaust water and
environment owing to the advances in water quality and pollution agricultural water, as these sectors are currently confronted with

* Corresponding author.
E-mail addresses: izba.ali@inopsys.eu (I. Ali), vaibhav.shrivastava@ugent.be (V. Shrivastava).

https://doi.org/10.1016/j.jenvman.2021.112926
Received 27 February 2021; Received in revised form 6 May 2021; Accepted 8 May 2021
Available online 9 June 2021
0301-4797/© 2021 Elsevier Ltd. All rights reserved.
I. Ali and V. Shrivastava Journal of Environmental Management 294 (2021) 112926

challenges to meet the new standards (Lenz and Lens, 2009; Staicu et al., concentrations. These are water-soluble, bioavailable and toxic (Sim­
2017; Tan, 2018). mons and Wallschläger, 2005). Selenate [Se (VI)] is relatively more
Selenium is present in (waste)water streams in varying compositions, stable in comparison with selenite [Se (IV)], as it has a symmetric tet­
concentrations and forms (speciation) which further confounds the rahedron structure. On the other hand, asymmetric structure and
development of technology that applies to a wide range of industrial chemical polarity make Se (IV) comparatively more reactive. Se (VI) is
sectors. In water Se generally occurs in the form of its oxyanions, sele­ very challenging to remove from water, because (i) despite having a very
nate [SeO2− 2−
4 ] and selenite [SeO3 ], in very small concentrations. Most of
high reduction potential, Se (VI) ions are considered kinetically
the technologies focus on the removal of Se (IV) and no universal non-reactive rendering very slow kinetics for its reduction to Se (IV) or
technology yet exists to remove Se (VI) below 5 μg/L levels collectively lower oxidation states, (ii) it is similar to sulfate from a chemical point of
or selectively from waters of any industrial sector (Murphy, 1988). Also, view which is a co-contaminant in most cases and acts as a competing
Se-containing wastewater originating from mine drainage or flue-gas anion. Selenium-laden water generally contains other co-contaminants
desulphurization (FGD) typically has a pH value around 4–5, which [e.g., nitrate (NO−3 ), sulfate (SO−4 2) etc.] at around 100 to 1000 times
hinders the Se-reducing bacterial activity and is to the detriment of higher concentrations compared to Se (VI) (Lens and Lens, 2009; Nish­
biobased technologies (Sánchez-Andrea et al., 2014; Soda et al., 2011; imura et al., 2007).
Winkel et al., 2012). Selenium oxyanions have minimal exposure to cations in natural
Over the last few decades, various biological, physio-chemical, or aquatic settings (e.g., Ca2+, Mg2+) and thus are only eliminated by
combined treatment techniques have been applied for Se removal. Un­ precipitation in a small fraction from the solution (Seby, 1999). They
fortunately, research studies introducing new treatment options often do also have increased solubility and mobility with increasing pH as
not sufficiently provide data on the applicability in the case of real opposed to metals (Chapman et al., 2010). On the other hand, elemen­
(waste)water or at full scale. Moreover, review studies providing in­ tary selenium Se0 is solid and fairly non-toxic, although the detrimental
formation regarding the ongoing trends in the development of Se effects found for nano-selenium are recently concerned among several
removal/recovery technologies are seriously lacking. Moreover, major researchers (Zhang et al., 2005; Schlekat et al., 2000). Elemental sele­
arguments such as implementation, removal efficiency, legislative nium (Se0 ) is prone to re-oxidization to SeO2− 2−
3 and SeO4 in aquatic
compliance, selenate/selenite specific removal remain unaddressed in habitats (Zhang et al., 2004). Along with this, it is also known to be
recent studies (see Table 1 in supplementary material). Therefore, to organically toxic to various species of fish such as filter feeders (Luoma
better assess the current trends, a systematic review has been performed et al., 1992; Schlekat et al., 2000; Li et al., 2008).
to describe the development of Se treatment technologies from the years
2010–2020. This study will help to investigate the following objectives:
1.2. Sources of selenium
• Country-based legislative requirements on selenium discharge and
ecological risks possessed by un-monitored discharge. The abundance of Selenium in its natural form is quite rare, con­
• A systematic review of the treatment technologies (physical, chem­ sisting of about 90 parts per billion in the Earth’s crust. Its concentra­
ical and biological) developed in the recent past (2010–2020) co- tions in the environment change from one geographic location to
comparing Se removal efficiencies. another. In natural environments, it is often found in combination with
• Investigate the current trends and future prospects and provide a the ores of other heavy metals (such as copper, mercury, lead, or silver)
detailed summary of the potential of newly developed technologies (Brasted, 2019). However, Selenium is mostly associated with phos­
to treat Se laden (waste)water. phate and fossil fuel (e.g., oil, coal, tar shale) but also found in selenif­
erous soils (Chapman et al., 2010). Coal is also one of the major sources
of natural selenium. Chinese stone coal, for example, was reported to
contain up to 6500 mg kg− 1 Se (Plant et al., 2003).
1.1. Selenium chemistry and toxicity Natural processes such as terrestrial weathering of rocks and soils,
wildfires and volatilization from soils, vegetation and water bodies are
Selenium (Se), a chemical oxygen-based element (Periodic responsible for the distribution of Se in various environmental com­
Table Group 16 [VIa]), is closely associated with sulfur and tellurium in partments (NAMC, 2010). Furthermore, the volcanic emissions (pro­
its chemical and physical properties. The Swedish chemist Jacob Ber­ duction and deposition of fly ash) also contribute as one of the major
zelius, who was employed in a chemical plant producing sulfuric acid sources of Se in air and terrestrial ecosystems (Haygarth et al., 1994).
and nitric acid, found selenium in 1817 in Gripsholm, a city in Sweden During the cretaceous period, the volcanic gasses and dust that accu­
(Eisler, 1985). In the 1950s, there was a renewed interest in the bio­ mulated in marine sediments enriched these sediments with Se signifi­
logical functions of selenium, when it was found to have toxic effects. cantly (to 100 mg kg− 1) (Kabata-Pendias, 2000). Soils that formed on
Increased accumulation led to cardiac muscular dystrophy or acute these Cretaceous parent rocks are, therefore, naturally rich in selenium
hepatic necrosis among the biotic species (Kieliszek, 2019). (i.e., seleniferous). Many of the geographic locations such as the Western
The toxicity due to selenium has gained focus following the occur­ and the Central US and Central Valley in California contain soils
rence in North Carolina (United States) in the mid-1970s of Se waste developed from such Cretaceous rocks. Furthermore, mineral fractions
emanating from a coal-fired power plant which resulted in massive in sediments, shales, phosphatic rocks and organic deposits are the
death of local fish populations in Lake Belews (19 out of 20 species were primary natural sources of selenium in terrestrial systems (Fernandez-­
eliminated) (Lemly, 2002). Later in the 1980s, at San Joaquin Valley Martinez and Charlet, 2009).
(California) resulted in the disappearance of the migratory bird pop­ While the contribution of natural processes is important, human
ulations, due to discharge of selenium-laden agricultural drain water in activities have intensified and changed the natural cycle of selenium.
Kesterson Reservoir (Ohlendorf, 1989). The cases of selenium contamination are majorly found in soil, plants
Today, researchers are particularly interested in sorting out the and surface water ecosystems (Etteieb et al., 2020). Selenium is released
narrow gap between the essential and toxic limits of selenium (Levander into surface waters through the extraction and processing of rocks and
and Burk, 2006). Selenium is present in (waste)water streams in varying minerals for instance copper mining (Brasted, 2019). Similarly, the en­
compositions, concentrations and forms (speciation) which further ergy generation in coal-fired power plants also releases Se via leaching
confounds the development of technology that applies to a wide range of into rivers and surface waters, further contaminating soil and plants
industrial sectors. In water Se generally occurs in the form of its oxy­ ecosystems. These activities could result in concentrations of >435 mg
anions, selenate (SeO2− 2−
4 ) and selenite (SeO3 ), in very small Se/kg in soils (Etteieb et al., 2020; Favorito et al., 2017). Depending

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I. Ali and V. Shrivastava Journal of Environmental Management 294 (2021) 112926

upon the organic matter, soil pH and climatic conditions, the bioavail­ effects of chronic long-term exposure to selenium, the EPA has set the
able Se is further up taken by plants, resulting in the contamination at maximum selenium levels of drinking water at 0.05 parts per million
the food chain (via grazing of cattle and other omnivores activities) (50 parts per billion) (USEPA, 2020). On the other side, the maximum
(Etteieb et al., 2020). selenium level in irrigation waters. Recommended by the United Nations
Nearly half of the world’s selenium demand comes from the metal­ Food and Agriculture Organization is 20 μg/L. Furthermore, the Cana­
lurgical industry (Fig. 1), followed by glass (20%) and PPCPs (10%) dian Ministry of Health has indicated a maximum acceptable concen­
production. Coal combustion for power generates volatile selenium tration (MAC) of 0.05 mg/L of Se (50 μg/L) in potable water (Health
species and selenium-enriched waste (fly ash, bottom ash, scrubber ash). Canada, 2014). Furthermore, British Columbia has developed a Saline
Coal is currently one of the most significant energy sources, accounting Water Quality Guideline which limits the Se concentration to 2 μg/L
for 40% of the global primary energy share. This trend is expected to (Nagpal and Howell, 2001). The maximum concentrations set by the
grow in the upcoming future (due to population explosion) and will World Health Organization for Se in water fit for consumption is 50 μg/L
further increase the release of selenium in the atmosphere (IEA, 2014). and the EU uses an even more precautious value of 10 μg/L (WHO,
Due to the combustion of coal, the Se is enriched into the ash and the 2011). The Mongolian maximum permissible concentration limit for
ratio of initial selenium in coal to selenium in the ash can be 1200 times drinking water was recommended to be 10 μg/L (Golubkina et al.,
higher than the parent feed coal (Lemly, 2004). In 2014 alone in the US, 2018). Furthermore, in Flanders, limits for companies discharging Se
approximately 130 million tons of coal ash was produced (USEPA, through their wastewater are also becoming increasingly stringent as the
2014). In December 2008, the Tennessee Valley Authority coalfired basic surface water quality standard in Flanders is currently 2 μg/L for
power plant discharged 1.5 million tons of ash into the environment due Se. For the freshwater ecosystems, USEPA, Australian and New Zealand
to dike failure, contributing to heavy Se toxicity in the adjoined Environment and Conservation Council and French Ministry have set the
ecosystem (TVA, 2009). Another study done on the accumulation of limit of 5 μg/L (USEPA, 2016; MDDEP, 2009; ANZECC, 2000). The
selenium in archived soil and herbal specimens at Rothamsted Experi­ target guideline for total selenium content was set at 1 μg/L in surface
mental Station (UK) concluded that an increase in Se concentration over waters (Canadian Council of Ministers of the Environment, 2007). In the
the last 100 years (0.15% per annum) is directly proportional to the coal Netherlands, the limits for Se in freshwater systems were set at 0.09 μg/L
combustion, reaching the highest level between 1940s and 1970s, when (short term) and 5.4 μg/L (long term) (Warmer and Van Dokkum, 2002).
the combustion of coal was at its peak for fulfilling the global energy For soils, the Canadian Council of Ministers of environment has rec­
demands (Haygarth et al., 1994). ommended 1 μg Se/g for agricultural, residential or parklands and 2.9 μg
The oil industry also contributes to the production of selenium and Se/g for commercial and industrial lands (CCME, 2007).
environmental pollution. Selenium enrichment in crude oils occurs in Selenium has been identified as a priority pollutant in the existing US
the same manner as coal, subject to high temperature and pressure over National Recommended Water Quality Criteria for freshwater ecosys­
long geological periods in fossil organic carbon-rich basins. The pro­ tems with a threshold value of 3.1 μg/L (USEPA, 2016). However, in the
cessing of crude oil and its by-products possesses a higher environmental context of the EU, selenium is not listed in the European Commission’s
risk of Se release in comparison to the processing of coal (Presser and Dangerous Substances Directive or the Environmental Quality Standards
Luoma, 2006). Another environmental concern is the disposal of ash at Directive (Environmental Quality Standard Directive, 2008/105/EC).
landfill sites that produce hazardous landfill leachate which further
creates danger of contamination to groundwater (Lemly, 2004). 2. Methodology
Additionally, essential sources of selenium are phosphate mining and
processing, through the leaching of surface waste rock dumps (Hamil­ The systematic review conducted in this study has been performed
ton, 2004). Selenium associated with the sulfide ores’ mineral matrix following some of the guidelines suggested by the Preferred Reporting
(copper, zinc, lead, gold, nickel) is easily released and emitted into the Items for Systematic Reviews and Meta-Analyses (PRISMA) statement.
atmosphere during the smelting process. Volatile selenium can then be As there is no such study presented previously on the particular subject,
accumulated in terrestrial or marine habitats and can adhere to the there was no registered protocol for the present study.
aerosol. The plumes will travel long distances, often over a distance of
100–200 km from the source of the emission depending on the local 2.1. Data and literature sources
topography and atmosphere conditions (Hodson et al., 1984).
The selection of studies for the systematic review has been summed
1.3. Concerned legislation at different countries up based on relevance from several electronic databases such as Web of
Science, Journal Storage (JSTOR), ScienceDirect and Google Scholar.
Selenium is exclusively regulated in drinking water, but allowable The strategy for further research was based on the usage of headings, e.
limits between different regulatory agencies vary by a factor of 5–10. For g., water, selenium, etc. And the corresponding key words. To obtain
instance, to protect customers served by public water systems from the more topic-oriented results, further filters were included in the meth­
odology. These filters included search terms such as (1) Selenium
removal, (2) (waste)water and (3) treatment. These search terms were
used in the “Advance Search” field of all the databases, therefore the
articles which contained these terms in their abstract, title or text were
filtered out. Subsequently, some additional records were also searched
manually and added to the dataset. This led to the inclusion of a few
more relevant studies.

2.2. Study selection

The selection of the articles for further assessment firstly proceeded

via elimination of the duplicates. Following that, the remaining articles
were entered in Kopernio, a web application to help in the blinded
screening. The first assessment of the selected articles was based on the
Fig. 1. Distribution of world selenium demand (by application) eligibility of titles and abstracts alone. This further led to the categori­
(McGroup, 2020). zation of articles as “included” or “excluded”. The following criteria

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I. Ali and V. Shrivastava Journal of Environmental Management 294 (2021) 112926

were chosen to find out the appropriate studies for a systematic review. iii Year of publication: any study done before 2010 was not
included.
i The study had to refer to Se species i.e., Se (IV), Se (VI) or Total Se. iv Language of study: papers available only in Japanese and Chinese
ii The study must report the influent and effluent concentrations of Se languages were excluded.
in water or a combination of either one of the above two values and v Type of publication: review studies and book chapters were not
the removal efficiency. considered.
iii The publication year of study should be between 2010 and 2020. vi Studies involving only removal efficiency were eliminated.
iv If the language of study is English, Dutch, French, German or Spanish vii Studies with irrelevant abstract or with no full text available
it should be included. through any source were discarded.

Scientific articles that did not meet these inclusion criteria, were
excluded from this study. The exclusion criteria used in this study were 2.3. Data extraction
as follows.
For the extraction of data from the shortlisted studies, a Microsoft
i Elemental state of selenium: Se2− is not considered. excel based electronic data sheet file is used for compiling the results.
ii Studies reporting soil treatment for Se removal were excluded. The data extracted is as follows – (1) Form of Selenium – selenate,
selenite, total selenium or selenocyanate, (2) Technology implemented,

Fig. 2. Data screening and selection steps for systematic review.

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I. Ali and V. Shrivastava Journal of Environmental Management 294 (2021) 112926

(3) Scale of implementation: lab scale, pilot scale and full scale, (4) Type 3.1.1. Filtration
of water – synthetic or real, (5) Sample size of test, (6) Initial Concen­ This technique was again found to be implemented to treat Se
tration of Se, (7) Final Concentration of Se, (8) Removal Efficiency, (9) (selenite and selenate) by using membranes such as nano-filtration (NF)
Duration of Study and (10) Reference – authors’ names and DOI of and reverse osmosis (RO).
study. Nanofiltration. Various types of composite nanofiltration mem­
branes have been applied for both selenite and selenate removal. He
2.4. Overview of the data collection et al. (2016) developed a new thin-film nanocomposite NF membrane
comprising polyhedral oligomeric silsesquioxane (POSS) in the selective
In total, we selected 69 studies published between 2010 and 2020. layer. For an initial 1000 ppm concentration of both selenite and sele­
Before any screening was performed, the raw database consisted of 672 nate, it removed 93.9% and 96.5% of SeO2− 2−
3 and SeO4 respectively.
data points. Out of these, 183 studies were identified as duplicate Later, He et al. (2018a, 2018b, 2018c) further developed a thin-film
sources among the various databases. Further screening (explained in composite (TFC) membrane comprising of a water-soluble zwitterionic
Fig. 2) led to a final database consisting of records of 69 studies co-polymer (P [MPC-co-AEMA]), which improved the RE to 98.2% and
reporting the removal of Se in the form of selenate, selenite, total Se and 99.1% for SeO2− 3 and SeO2− 4 respectively. On the other hand,
selenocyanate. A full list of the studies and the details of data extracted Na-functionalized carbon quantum dot NF demonstrated 98% RE (He
from these studies is given in Appendix 1 (in supplementary material). et al., 2018a, 2018b, 2018c).
From among the selected literature, sorting and filtering of the data A different kind of filtration methodology, developed by employing a
were done in 3 phases. ceramic filter (made of clay soil and rice bran), was used to reclaim
storm water (Si = 22.63 μg/L). It was able to successfully remove total Se
• In the initial phase 1, data (n = 69) was sorted according to the scale below 0.8 μg/L (Shafiquzzaman et al., 2020). However, the possibility of
of a particular study. It was observed that about 88% of the studies Se entrapment and removal attached to the suspended solids, present in
were performed on the lab/batch scale and only 4 were done for high concentration (721 mg SS/L) in stormwater, which in turn resulted
each, pilot- and full-scale, respectively. in high RE. The RE may decrease in case of real (waste)water matrices
• The phase 2 literature survey categorized the data based on the type containing Se mostly in dissolved form. Another possibility could be the
of Se species treated or reported in the study. The studies reporting presence of specific functional groups in the clay/rice barn (filter media)
selenate removal were 36, only 2 were carried out to determine which is helping in the removal of Se oxyanions from stormwater.
selenocyanate removal, while the remaining discussed both selenite Reverse osmosis. Reverse osmosis (RO) was reported to achieve
and selenate removal. About 17 studies reported the treatment effi­ greater than 98% (Sf = 0.4 mg/L) removal in 24 h from mining waste­
ciency of a particular technology in terms of total Se removed/ water (Si = 0.002 mg/L) at Richmond, British Columbia, CA (Man et al.,
treated. 2016).
• In the third phase, securitization was done based on the type of From the results stated above, it can be inferred that the NF was
technology used for the treatment purpose. A comprehensive search majorly applied for synthetic waters only, however, all NF related
in this phase divided all the removal technologies into 3 major studies demonstrated its potential to remove both selenate and selenite
treatment processes – physical, chemical and biological (Table 2). above 94%. RO and NF result in almost the same range of removal ef­
ficiency, but RO achieved it in significantly lower time interval (<24 h).
Micropore ceramic filtration is a new technique and offers to provide
2.5. Review limitations
good RE at low costs, but as stated earlier, its workability needs to be
assessed for complex water matrices such as industrial effluents. More­
The review contains the collection of data from multiple sources such
over, this technique requires a longer time (>48 h).
as high and low impact journal articles, non-commercial trial registry
Even though RO and NF attain a high RE throughout studies, its
records, grey literature, company-owned trial registry records, confer­
research focus for the removal of Se from wastewaters has significantly
ence abstracts, etc. Due to the diversification in the collection of data,
decreased in the last 10 years. This could be due to the high operational
the quality and authenticity of data cannot be verified.
and maintenance cost of the membranes and additional treatment
From the selected dataset, it was assessed that most of the technol­
requirement of the reject stream, forcing industries to focus on the other
ogies yet await the implementation on full-scale or real wastewater.
alternatives for efficient removal. However, potential intervention in
Moreover, this review also contains some (pilot-/full-scale) technologies
replacing the membranes materials with materials made from natural
currently implemented at various industries for the removal of pollut­
organic fibers can help in solving this discontinuity, but the operational
ants (especially Se) from wastewater. Therefore, it is worth highlighting
and maintenance cost may not noticeably different from other mem­
that the efficiency of the technologies reported in the studies could be
brane operations. Furthermore, enhancing the reusability of the mem­
ambiguous to a certain degree.
branes can help in decreasing the overall maintenance costs for NF.

2.6. Data analysis

3.2. Chemical treatment
A detailed analysis of the final dataset identified the trends in the
development of removal technologies and improvements in their Chemical treatment was subcategorized into three main processes
removal performance. A comprehensive overview is presented as namely – adsorption, oxidation/reduction and precipitation. A thorough
follows. review of the selected dataset identified a significant inclination of
ongoing research towards the application of adsorption technology. Out
3. Results and discussion of 69, 25 studies were related only to adsorption techniques.

3.1. Physical treatment 3.2.1. Adsorption

A wide range of adsorbents has been tested over the past 10 years to
The research done in the physical treatment technologies only per­ determine their effectiveness to remove selenite and/or selenate. How­
sisted for the filtration techniques and no further research was continued ever, all the selected studies reported results from laboratory scale
on evaporation ponds (EP) after 2010. This could be attributed to the experimentation utilizing synthetic or real wastewater. Therefore,
inefficiency of EP to treat Se below regulatory standards (NSMP, 2007). instead of explaining each adsorbent separately, all the adsorbent based

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I. Ali and V. Shrivastava Journal of Environmental Management 294 (2021) 112926

lab studies have been categorized into two subheadings: (i) treatment of char carbon showed 95% RE for selenite and only 58% for selenate while
synthetic wastewaters and (ii) treatment of real wastewaters. Moreover, treating a solution containing 1 mg/L of both species (Jegadeesan et al.,
as the ion exchange is also a type of adsorption process, but differs in 2015). Interestingly, a study done by Yamani et al. (2014) showed very
performance mechanism, it has been discussed separately from other promising results (RE >95%) at the start when chitosan-copper beads
adsorbents. were used to treat Se (Si = 2 ppm) in a 40 mL solution. However, when
the phosphate (competitive ion) was added to the solution the RE
I Treatment of synthetic wastewaters. drastically dropped to 30% even after continuing the adsorption for 7
days (Yamani et al., 2016). This trend did not change any further even in
Most of the studies were performed on lab-synthesized solutions with the more recent studies. For instance, Thakkar and Mitra (2017) pub­
various concentrations of Se oxyanions. lished only 40% RE in their study while treating 50 mL of 5 mg/L
Selenite removal. In the case of selenite, overall, the RE ranged selenate solution with bimetallic diatom composite. Kalaitzidou et al.
from 81% to greater than 99%. Awual et al. (2015) showed that the (2019) also tried to initially remove both selenite and selenate by
ligand functionalized organic-inorganic based composite adsorbents iron-oxy hydroxides (FeOOHs). The results soon revealed that this
were able to achieve 95–100% selenite removal, but due to the lack of technique was ineffective for selenate. The study was continued for
information provided regarding initial concentration, the results cannot selenite (Si=500 μg/L) removal only and showed >90% RE (Sf < 25
be compared to real scenarios. In another study conducted by Bakather μg/L) in 4 h.
et al. (2017), iron oxide impregnated carbon nanotubes were used as an In another study done by Opiso et al. (2016), the immobilization of
adsorbent. This study showed that complete (~100%) selenite removal Se was tried to be achieved by using a combined principle of adsorption
was achieved for an initial concentration of 10 ppm, but the final con­ with co-precipitation. For achieving this, hydrotalcite and
centration was not mentioned in the study. There is a possibility that serpentine-like minerals were used in the study. However, the removal
remaining concentration was below the detection limit of their was not very effective for the higher concentrations of selenium,
instrument. showing only 60% removal (for Si = 100 μg/L) in 7 days.
In 2019, Jacobson and Fan reported removing selenite (Si = 150 μg/ Total Se removal. The study carried out by Vilardi et al. (2018)
L) to less than 5 μg/L, within 30 h, by using natural goethite as an employed carbon nanotubes with zero-valent iron and reported only the
adsorbent, whereas Bandara et al. (2019) were able to reduce the total Se removal. The RE obtained was around 80% while initial and
adsorption time to less than 24 h by using Graphene Oxide Nano­ final total Se concentrations were 10 mg/L and <2 mg/L, respectively.
composite Hydrogel Beads adsorbent. However, the removal efficiency As the separate treatability of Se species was not provided, this study
(81–85%) was significantly reduced too. cannot be compared with the other technologies due to the lack of
In 2019, Zeolite based adsorbent emerged as an efficient and faster homogeneity.
option, reducing up to 97% of selenite from the influent (Si = 10 mg/L),
within 10 h time (Zhang et al., 2019). However, the greatest RE (>99%) II Treatment of real wastewaters
was recorded by Magnetic iron oxide nanoparticles adsorbents. It was
documented that these adsorbents removed 10 ppm (Si) of selenite to Only a few studies uncovered the behaviour of adsorbents for real
less than 0.77 μg/L in 24 h. However, it is worth mentioning that 20 mg water matrices.
of adsorbent dosage was used for a sample volume of only 5 mL (Evans Domestic wastewater. A 100 mL sample of domestic wastewater,
et al., 2019). containing 197 μg/L of total Se, was treated with eggshell and orange
Thus, from an overall observation, it can be interpreted that the peels-based adsorbents. Adsorbents developed by orange peels per­
adsorption efficiency has improved over time while the retention time formed better (RE = 74.8%) than those obtained from eggshells (RE =
remains more or less the same (around 24 h). 47.3%) (Mafu et al., 2014).
Selenate removal. The four (selected) studies, published only on the RO concentrate. Lin et al. (2014) carried out adsorption experi­
topic of selenate removal, were found to exhibit different removal effi­ ments on RO concentrate obtained from Kay Bailey Hutchison Desali­
ciencies. Whereas around 80% RE was reported by magnetic nation Plant in El Paso, Texas, that treats brackish groundwater.
nanoparticle-graphene oxide composites (MGO) (Fu et al., 2014), 98% Originally the sample contained 11–12 μg/L of Se, but to determine the
by gracilaria modified biochar (Johansson et al., 2015), 95–98% by ZnO selectivity of adsorbent (drinking water treatment solids), the sample
nanocomposites (Gurunathan et al., 2019) and >95% by Copper-coated was spiked with selenate to get 0.2 mg/L solutions. The treatment was
activated carbon (Cu-AC) (Zhao et al., 2020a, 2020b). The high RE (e.g., not very successful and only attained RE less than 40%.
95% and 98%) reported above does not guarantee that these adsorbents Uranium production bleed water (PBW). Schilz et al. (2015) re­
would be effective in case of real wastewater. This was further proven ported the use of cupric oxide nanoparticles for the treatment of PBW.
when Johansson et al. (2015) used the same adsorbent (gracilaria Only 30% RE was recorded while treating 50 mL of sample (Si = 1.8
modified biochar) to treat mining wastewater. The RE dramatically mg/L, Sf = 1.3 mg/L).
dropped to 3%. Although Gurunathan et al. (2019) had not mentioned Drinking water. A recently published study on drinking water
final selenate values in the treated effluent, the other two studies re­ discovered the potential of mesoporous activated alumina to treat
ported huge variations in selenate removal with fluctuation in pH, for selenite and selenate (total Se initial conc. = 200 μg/L) and effectively
instance in case of Cu-AC, when pH increased from 4 to 6, the RE was removed not only selenite (80%) but also selenate (72%) (Meher et al.,
dropped from above 95% to less than 10% (Zhao et al., 2020a, 2020b). 2020). However, the effect of competing anions is not present in the case
Moreover, high ionic strength (100 mM NaCl) was also recognized as a of drinking water matrix which could explain this high RE. Hence, from
noticeable inhibitor to selenate removal (RE = 30%) (Zhao et al., 2020a, the above data analysis, it can easily be implied that the adsorption
2020b). technique, although studied extensively over the years, yet remain
Selenite and selenate combined removal. Many studies encom­ inefficient in its application for the treatment of Se in real waters.
passed the treatment of both selenite and selenate together. A study In case of selenate, the adsorbents were not found very effective. For
conducted by Roberts et al. (2015) reported exceptionally good RE the technologies which were examined for the adsorption of both sele­
(95–99%) for selenite as well as selenate. In this study, Fe-treated Gra­ nite and selenate, it was observed that selenite was much more effec­
cilaria edulis (biosorbent) was employed to treat a synthetic solution tively removed compared to selenate. The selenate removal efficiency
containing 500 μg/L of both Se oxyanions. However, in general, the laid around 30–80% compared to selenite (RE = ~95%).
selenate removal was noticed to be more challenging compared to In recent years, the usage of adsorption techniques for Se removal
selenite. For example, in a study conducted in 2015, fly-ash extracted from wastewater has gained significant attention. This technological

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I. Ali and V. Shrivastava Journal of Environmental Management 294 (2021) 112926

boom has resulted from the inexpensive and relatively simple To remediate this problem, a pre-treatment with barium chloride was
manufacturing process of adsorbents at the lab as well as the pilot scale. introduced which raised the treatment efficiency to 34% and 84% for
Another reason for their popularity is the possibility to produce them in selenite and selenate, respectively. In another study, Li et al. (2017)
an environmentally friendly way (eggshells, orange peel, fly ash, etc.) analyzed Se removal in the presence of sulfate by using granular layered
which further contributes to the circular economy through the appli­ double hydroxide (LDH) materials. It was noticed that the RE decreased
cation of resource recovery. Furthermore, the inexpensive regeneration from 74% to 33% when a ground water sample, containing high sulfate,
of the nanoscale adsorbents and their effective RE after regeneration was treated with LDH compared to a synthetic solution containing the
further contributes to their popularity among researchers. same initial selenate concentration of 100 ppm. To improve the treat­
On the overall analysis of the studies, it can be recognized that the ment efficiency sulfate reduction was, once again, carried out by barium
use of adsorption techniques at pilot or full scale is not yet popularly precipitation which enhanced the RE up to 65%.
implemented for Se removal as (1) the technology is relatively new and Even though the RE for Se is quite high (in both synthetic and real
more materials are still being synthesized to obtain better RE, (2) the wastewaters) for all cases discussed throughout this review for the ion
removal of selenium in comparison to the amount of adsorbent used is exchange process, a compulsory requirement of pretreatment (with
relatively low, (3) the lab tests done on real wastewaters have shown BaSO4 ) is observed. Additionally, the alternative use of CaSO4 (in
very poor RE, suggesting potential inefficiency at full-scale. Addition­ replacement of BaSO4 ) for precipitation of sulfate is not satisfactory, still
ally, while dealing with toxic compounds like Se, the adsorption does leaving appreciable amounts of sulfate in solution competing with se­
not eliminate the pollutants but simply transfers the pollution load from lenium (Tait et al., 2009). To handle this problem, precipitation via
the aqueous to the adsorbent phase. For the remediation of the afore­ biologically mediated reduction or using ettringite,
mentioned problem, the researchers should divert their focus to alter­ Ca6 Al2 (SO4 )3 (OH)12 .26H2 O (addition of alum, gypsum and lime) could
native pathways for Se recovery from wastewater. be researched in future experiments (Dou et al., 2017). Moreover, during
The studies further showed limited RE in case of selenate (on the regeneration of the ion-exchange chamber, a heavily polluted
average < 50%) due to its relatively stable atomic configuration in concentrate is generated that needs to be treated and disposed effec­
comparison to selenite. To increase RE, the process could be conjugated tively without creating any environmental concerns.
with other technologies such as precipitation, coagulation and biolog­
ical processes. Furthermore, the selenate RE could also be enhanced by 3.2.2. Oxidation/reduction
first reducing it to selenite (e.g., via selenium reducing bacteria or The oxidation/reduction (redox) treatment technique involves the
advanced reduction processes) followed by a subsequent removal using application of various types of approaches such as zero-valent iron
adsorption. reduction, electro-chemical reduction and photocatalysis.
The maximum adsorption capacity of various adsorbents across the
studies are mentioned in Table 3. The adsorbent capacity depends upon I Electrocoagulation
several factors such as pH and temperature, etc. It was noted that
maximum studies have shown the high REs at acidic pH (pH = 3–6). This There are only a limited number of studies describing Se treatment
could be due to the presence of functional groups (e.g., –OH and through the electrocoagulation technique. A study by Kazeem et al.
–COOH) which get protonated in acidic conditions. This leads to elec­ (2019) evaluated selenocyanate removal with the electrocoagulation
trostatic attractions with the negatively charged Se species. However, in process. The effect of applied current and initial concentration was
the study done by Fu et al. (2014), it was encountered that the RE monitored. This process involved two steps: (i) selenocyanate was first
drastically reduces for the pH < 2. This is because of changes in surface oxidized to selenite and selenate and (ii) Se oxyanions were then
properties at very low pH, due to which MGO loses its magnetic prop­ adsorbed onto iron hydroxide or aluminium hydroxide produced from
erties leading to a decrease in RE. Furthermore, all the studies showed Fe or Al ions released at the anode. A high RE (98%) was obtained by
insignificant RE for pH values greater than 11. increasing the applied current and decreasing the initial concentration.
In another study, Hansen et al. (2019) looked at the treatment of
III Adsorption via ion exchange selenite containing wastewater from a petroleum refinery (Si = 0.3
mg/L) by using an electrocoagulation process in a batch airlift reactor.
Ion exchange is a promising technique to treat Se-laden wastewaters, The sacrificial electrodes were made of iron which produced ferrous ions
however, the RE is severely affected by the presence of competing anions for the coagulation process. This technology produced satisfactory re­
such as sulfate. To overcome this problem, a pre-treatment with barium sults (RE = 90%) and the concentration in the effluent was reduced to
chloride to precipitate sulfate was proposed which has another concern 0.03 mg/L within 6 h.
of being an expensive alternative. According to Staicu et al. (2016), the The use of electrocoagulation showed significantly high RE for all the
use of FerrIX A33E as a resin for the ion exchange process resulted in cases found. Even in the case of petroleum wastewater, the efficient
96–100% removal of both Se oxyanions (Si = 1240 μg/L). However, in removal of 90% indicates its ability to process multiple contaminants
the presence of high sulfate concentrations, while treating FGD waste­ (SO2− 2−
3 = 0.04 mg/L and PO4 = 2.5 mg/L) in a single pass, not creating
water, the RE was diminished to 25% for selenite and 33% for selenate. any type of competition in-between different ions present in wastewater.
The selenium reaction intermediates have not been described in any
Table 2 of the studies, which does not allow to establish the underlying mech­
Treatment processes diversification in the selected dataset. anisms involved in the selenium removal. To remediate this, future
studies can focus on the mechanistic and kinetic modelling of the sele­
Treatment type Treatment technology Number of studies
nium removal process to get better insights into the process. This will
Physical Membrane filtration 5
further contribute to analyzing those process variables which can
Chemical Adsorption 25
Oxidation/reduction 12
contribute to a higher RE.
Precipitation 1
Biological Microbial reduction 3 II Zero valent iron
Bioreactors 13
Algal treatment 3
Over the last decade, ZVI nanoparticles have also been studied as an
Constructed wetlands/phytoremediation 5
Combined treatment processes 2 attractive option to treat Se-rich waters (He et al., 2018a, 2018b,
2018c). Tang et al. (2014) discovered a synergistic effect on selenate
Total 69

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I. Ali and V. Shrivastava Journal of Environmental Management 294 (2021) 112926

Table 3
Adsorption capacities of various materials for selenium oxyanions.
Adsorbent Water matrix Selenite Selenate Total Se Reference

Copper-coated activated carbon Synthetic – 4.23 mg/g – Zhao et al. (2020a, 2020b)
Chitosan beads Synthetic 5.5 mg/g 2 mg/g – Yamani, Lounsbury and Zimmerman,
2013
Magnetic iron oxide nanoparticles adsorbents Synthetic 47.8 mg/g – – Evans et al. (2019)
Zeolites Synthetic 4.16 mg/g – – Zhang et al. (2019)
Iron-Oxy hydroxides Synthetic 4.3 mg/g 10 μg/g – Kalaitzidou et al. (2019)
Graphene oxide nanocomposite hydrogel beads Synthetic 1.62 mg/g – 30 g/L Bandara et al. (2019)
Mesoporous activated alumina Drinking water 9.02 μg/g 5.38 μg/g – Meher et al. (2020)
ZnO nanocomposite adsorbents Synthetic – 160.5 mg/g – Gurunathan et al. (2019)
Natural goethite Synthetic 7.74 mg/g – – Jacobson and Fan (2019)
Carbon nanotubes with zero-valent iron Synthetic – – 2.5 mg/g Vilardi et al. (2018)
Iron oxide impregnated carbon nanotubes Synthetic 111 mg/g – – Bakather et al. (2017)
Bimetallic diatom composite Synthetic 227 mg/g – – Thakkar and Mitra (2017)
L-cysteine modified cellulose Synthetic 105 mg/g – – Shen et al. (2016)
Fly-ash extracted char carbon Synthetic 0.68 mg/g 0.44 mg/g – Jegadeesan et al. (2015)
Ligand functionalized organic-inorganic based composite Synthetic 90 mg/g – – Awual et al. (2015)
adsorbent
Gracilaria modified biochar Mining wastewater – 3.8 mg/g – Johansson et al. (2015)
Magnetic nanoparticle-graphene oxide composites Synthetic 27 mg/g 13 mg/g – Fu et al. (2014)
Fe- treated Gracilaria edulis Synthetic – 2.6–2.72 mg/ – Roberts et al. (2015)
g
Egg shell and orange peels-based adsorbents Domestic – – 160 μg/g Mafu et al. (2014)
wastewater

- not reported.

removal efficiency when ZVI treatment was coupled with the addition of system containing an iron anode and a copper plate cathode was
dissolved ferrous ions. The RE was significantly enhanced from around investigated by Baek et al. (2013), to remove selenate from synthetic
4%–74% with only 0.1 mM Fe2+ addition, after 24 h contact time. The wastewater. The selenate removal was attributed to ferrous hydroxide,
removal rate was further increased with the addition of more ferrous released into the solution from the iron anode. The removal efficiency
ions and nearly 100% RE was achieved within 7 h by augmenting the varied between 45.1% and 97.4%, depending upon applied current and
solution with 1 mM Fe2+ . initial Se concentration. The amount of ferrous hydroxide produced, and
Taking into account the very high selenite removal rate and treat­ reduction rate of selenate was directly proportional to the applied cur­
ment efficiency (>99% RE in 3 min) of nanoscale zero-valent iron (nZVI rent, whereas selenate removal was inversely proportional to its initial
= 5 g/L), Ling et al. (2015) tried to uncover the reduction mechanism. concentration. Even though the residual selenium (0.79 mg/L) in the
The results demonstrated that selenite was transformed to selenide and effluent was higher than the discharge limit, the electrochemical system
elemental selenium, which subsequently were separated from the solu­ was found effective for selenate removal (Baek et al., 2013). At higher
tion due to encapsulation into nanoparticles. levels of selenate, nitrogen purging was required (to inhibit partial
Furthermore, in the study by Liu et al. (2016), the removal rate of ZVI oxidation of ferrous) to ensure efficient RE. This further makes the
was reported to be improved significantly (>95%) by producing stabi­ process expensive for high selenium-laden wastewaters. Moreover, all
lized ZVI using compounds such as sodium carboxymethyl cellulose or the studies involved in this review for electrochemical reduction have
starch. However, total organic carbon was identified as a considerable not considered the efficiency of the process for selenite removal. So,
inhibitor to the removal process. It was confirmed that the selenate was future research could also potentially focus upon the combined removal
first reduced to selenite and elemental Se, which were then removed (selenite and selenate along with competing anions) from various
along with the nanoparticles. wastewater matrices.
The discussed studies for ZVI have shown significant removal of Se
from synthetic wastewaters. Moreover, it is expected to show the same IV Photocatalysis
pattern for groundwaters (using permeable reactive barriers for treat­
ment) as the studies have already shown its high RE for other pollutants In recent years, photocatalysis has gained wide attention in waste­
such as Arsenic (Panda et al., 2020). However, owing to the presence of water treatment due to its energy efficiency and strong redox ability.
dissolved oxygen in groundwater, the ZVI nanoparticles could form a Furthermore, it is also beneficial for the recovery of heavy metals from
layer of iron oxide around them, further inhibiting the reactivity of ZVI contaminated media (Vohra and Labaran, 2020). In the last decade, this
for the removal of Se. technology has been widely used in the advanced oxidation process for
The future trends in ZVI are currently focusing on enhancing its the degradation of organic compounds, selective oxidation, disinfection
surface properties to avoid passivation for gaining better RE. However, of microorganisms and many more. However, only limited research has
to date, the research on this aspect is very limited and these trends are been done to show its efficacy for selenite and selenate removal from
needed to be further investigated for analyzing the removal of Se in near marine environments.
future. TiO2 photocatalysis. In the study performed by Vohra and Labaran
(2020) the removal of selenocyanate by the use of TiO2 photocatalysis,
III Electrochemical reduction with EDTA as a hole (exciton)/H+ scavenger, was investigated. A high
RE of 87% was obtained within 5 h for an initial selenocyanate con­
In recent years, electrochemical systems have been investigated to centration of 10 ppm. However, for a higher value of selenocyanate (Si
overcome the limitations associated with ZVI. An electrochemical sys­ = 20 or 30 ppm), the study has shown a sharp delineation to 20–30%
tem involves oxidation of iron anodes which produce ferrous and ferric REs. This is possibly due to increased competition for both limited TiO2
ions, further leading to the formation of their respective hydroxides. The adsorption sites and photogenerated holes at higher selenocyanate
formation of iron hydroxides prevents rusting of anode due to the for­ concentration causing its lower removal.
mation of oxygen and water in a mixed cell. A batch electrochemical Advanced reduction process. The advanced reduction process

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I. Ali and V. Shrivastava Journal of Environmental Management 294 (2021) 112926

(ARP) has recently been developed as a novel and powerful technique 2015) and I1− at 195 nm and 225 nm (Truesdale, 2007). The production
for the treatment of oxidized contaminants in (waste)waters. The ARP of radicals from these reagents can also occur at the wavelength any­
process was found to be effective in the chemical reduction of various where around their respective absorption peaks obviously with a
oxidized (in)organic contaminants in water, such as nitrate (Botlagu­ compromise in the efficiency. Although different reagents exhibit
duru et al., 2015), perchlorate (Botlaguduru et al., 2015; Vellanki and different absorption peaks (AP), the studies carried out so far have not
Batchelor, 2013), 1,2-dichloroethane (Liu et al., 2014; Yoon et al., 2014) implied the same irradiation to the respective reagents (Wang et al.,
and various micropollutants (Schoutteten, 2017). ARP is similar to the 2018; Duan et al., 2018). However, in real cases, an advantage of using a
already more established advanced oxidation processes (AOP) in the UV light for activation is that UV sources are economical and are often
way that both involve the strategy to combine an activation method and installed already on wastewater treatment sites for the disinfection of
a reagent, but in ARPs reductive radicals are generated versus oxidative water as a part of tertiary treatment. There is also a lack of information
radicals in AOPs. Several activation methods, for instance, E-beam, on the relationship between quantum yield of ARPs, the reactivity of
γ-radiation, ultrasound waves, or ultraviolet (UV) light, can be used to reagents and removal percentage of Se. Thus, an in-depth identification
produce highly reactive reductive radicals. The reductive reagents that and quantification of reduction products should be conducted, and the
can be used in this process are a variety of ions such as SO2− 2−
3 , S 2 O4 , different mechanisms involved should be studied. Furthermore, as Se
2+
Fe , HS , Cl etc. Which can be selected based on their ability to
− − (IV) and Se (VI) can exist in the water at the same time, it is not only
produce reducing radicals and their effectiveness. enough to study the effect of ARP on each of them separately, but also
The ARP (using UV-ferrous) studied by Duan et al. (2017), reported the effect of ARPs on Se oxyanions present together should be deter­
to have obtained 19% and 61% reduction efficiency from selenite and mined. Accordingly, there is a need to optimize and demonstrate the
selenate respectively, having an initial concentration of 5 μmol/L of feasibility of ARP for Se removal from wastewaters at pilot/full scale.
each. This study introduced a new Se treatment approach. Although the
RE was not very remarkable, it was observable that this technique can 3.2.3. Precipitation
potentially overcome selenate treatment-related problems. In another The approach of researchers to discover novel strategies to remove
study, Duan et al. (2018) utilized dithionite as a reagent along with UV. Se from wastewater has led to the development of new techniques. Zou
The results showed >46% RE for selenite (initial – 5 μmol/L) and >52% et al. (2020) proposed a very innovative approach for the in-situ treat­
for selenate (initial – 5 μmol/L). ment of Se-loaded FGD slurry obtained from a coal-fired power plant.
The above studies indicate that the selection of a reagent in this Instead of treating the Se in wastewater, it was suggested to transform
process is very important because upon excitation these ions tend to selenite into solid-phase ferric selenite, by dosing ferric ion into the FGD
eject an electron into the solution [hydrated electron (e−eq )] which is a slurry, subsequently trapping it on to gypsum. The process showed very
powerful reductant (SRP = − 2.87 V), freely available to undergo re­ little effectiveness towards selenate, proving that high selenate con­
actions with different molecules in the solution. The faster reaction of centrations will give rise to poor performance of this method. However,
e−eq with Se (VI) (1.1 × 109 M− 1 s− 1) and selenite (1.2 × 107 M− 1 s− 1) in the case of selenite, 46% elimination was yielded.
6 − 1 − 1 In the discussed study, for achieving the high Se removal from
(Anbar and Hart, 1968) than with SO2− 4 (<1 × 10 M s ) (Buxton
wastewater, the dosage of ferric ion was much higher than the selenium
et al., 1988) indicates that ARP is a promising technique to selectively
concentration. This can further induce the problem of excess ferric in
reduce selenate and/or selenite in the presence of common
wastewater. To remediate this, the precipitation of ferric ion as ferric
co-contaminants, such as sulphates. This faster reaction of selenate
hydroxide can be done via addition of hydrated lime. Moreover, the
compared to sulfate was also confirmed by Wang et al. (2018), where a
produced gypsum, if used as a resource for another purpose, can induce
notable selenate reduction (from 10 mg/L to <0.1 mg/L; >99%) was
a problem of selenium leachability into the environment over time.
observed, under batch ARP (UV/sulfite) experiments, in the presence of
However, the leachable ratio was found to be 1.4–9.5%, further making
high sulfate concentrations (1000 mg/L) in a short timeframe (<1 h).
it applicable to be used as raw material in the construction industry for
The process remained effective even in the presence of chloride, sulfate
the manufacturing of wallboard and cement additives (Zou et al., 2020).
and phosphate too.
Another study by Hu et al. (2015) showed enormous removal effi­
Accordingly, ARP seems to have potential to transform selenate and/
ciencies for selenite (RE >98%) and selenate (RE >96%) with the help of
or selenite selectively to more reduced species (e.g., selenite or Se0 ), that
coagulation (using Fe- and Al-based coagulants) followed by precipita­
are easier to remove through adsorption and/or precipitation processes,
tion. Within 90 min, the concentration of both, selenite and selenate,
in presence of higher concentrations of matrix compounds that are
was reduced from 250 μg/L to below 5 μg/L. However, the RE was
typically competing with those Se species for removal in current treat­
severely affected by the presence of other oxyanions. For instance, the
ment technologies. Moreover, in comparison to biological treatment,
RE decreased to <15% in the presence of 1 mM PO3− 4 in the wastewater.
ARP technology has an expeditiously high rate of reaction, resulting in a
required hydraulic retention time (HRT) of typically 40–60 min versus
at least 4–6 h for bioreactors removing Se (Hill, 2010), whereas it is also
3.3. Biological treatment
still effective under conditions that are toxic to micro-organisms.
However, the application of this technology in real Se-containing
While removal of selenium can potentially be accomplished by
water matrices, such as surface waters, drinking water and industrial
physical and chemical methods, these methods are not always effective
wastewaters, has not been explored. No research studies have been
and can be quite expensive. One of the areas of important studies has
conducted to determine the application of this process using realistic
been the use of biological treatments which is divided into four cate­
concentrations of Se and competing co-contaminants being present in
gories – microbial reduction, bioreactors, algal treatment and con­
actual (waste)water. Moreover, the effects of irradiation alone, before
structed wetlands (Paul and Saha, 2019).
the addition of reagent and after its consumption, on the Se species has
not been demonstrated for (waste)waters with complex composition
3.3.1. Microbial reduction
(containing sulfates, nitrates and heavy metals). More scientific insights
A substantial number of ongoing studies focus on the identification
into the process mechanism and kinetics as influenced by various con­
of new bacterial species with high Se tolerance and reduction capacity.
stituents of the matrix are needed.
Although aerobic reduction of selenate has not been ubiquitously
Moreover, the optimal UV wavelength at which the reagents excite
observed, many bacterial groups have shown great potential in reducing
varies. For instance, SO2−3 exhibits an absorption peak at around 210 nm selenite even under aerobic conditions, e.g., Pseudomonas moraviensis
(Fischer and Warneck, 1996), S2 O2− 4 at 315 nm (Botlaguduru et al., has been reported to reduce selenite to Se0 in aerobic environments at a

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I. Ali and V. Shrivastava Journal of Environmental Management 294 (2021) 112926

high rate (0.27 mM h− 1) (Staicu et al., 2015). The strain was ineffective 12.7 mg/L.
towards selenate but reduced 10 mM of selenite in 2 days (RE = >95%)
and exhibited to withstand high selenite concentrations (up to 120 mM). III Electro biochemical reactor
In case of real wastewater such as mine wastewater, the bacterial Se
reduction can be inhibited by the presence of elevated levels of nitrates. The EBR technology is based on the theory that microbes mediate the
To overcome this issue, Subedi et al. (2017) enriched a bacterial con­ removal through electron transfer between metal and inorganic pollut­
sortium (Pseudomonas-, Lysinibacillus- and Thauera-related species) from ants (redox processes). In the electro-biochemical reactor produced by
a mine impacted natural marsh sediment having the capability to reduce INOTEC Inc, the microbes are directly supplied with the electrons
selenate and nitrate all together. These consortiums can potentially treat needed by employing a low voltage potential (1–3 V) and utilizing
wastewaters containing both pollutants at the same time. graphite electrodes (Opara et al., 2014). The decrease of Se concentra­
Although many microorganisms were reported to remediate Se- tions (Si = 35–531 μg/L) from mining wastewater below the release
contaminated water, only a few described the kinetics involved in the targets (5–10 μg/L) was recorded from a laboratory bench and on-site
process. In a study by Nguyen et al. (2019), selenium reducing bacteria pilot-scale tests, whereas the efficacy was observed to increase further
from activated sludge were isolated and the reduction kinetics were with increasing HRT (from 6 to 18 h) (Opara et al., 2014).
investigated. It was observed that bacteria performed better in simple
organic substrates (such as acetate and lactate) compared to more IV Fluidized bed reactor
complex organic sources (e.g., propionate, butyrate and glucose).
A wastewater stream from coal mining containing 22–500 μg/L of Se
3.3.2. Bioreactors was treated to concentrations below 10 μg/L. The treated wastewater
Experimental studies of bioreactors utilize different microbial com­ was further transferred through a separation system where the liquid
munities to treat Se-contaminated (waste)waters. and solid fractions were separated (Envirogen, 2011). This technology
was patented in 2011 by Envirogen Technologies. The application of
I Passive biological treatment such technology to reduce Se produces insoluble elemental selenium
nanoparticles integrated into the biosolids which can easily be separated
Various large-scale studies simulating principles of bioreactors have by centrifugation. However, the separated biosolids would require to be
been conducted to treat Se-rich effluents, however, the long-term RE thickened and dewatered before disposal. Moreover, if an excessive
could be challenging. A simple pilot-scale bioreactor system was amount of nitrates is co-existing with Se in the solution and the avail­
developed at Grande Cache, Coal Inc., AB, Canada, to treat non-acidic ability of carbon source (COD) is limited in the solution, an external
coal mine effluent (Si = 85 μg/L) (Luek et al., 2014). The reactor was supply of carbon substrate will be required. Besides, the media also need
inoculated with bacteria obtained from the same end-pit late sediment to be replaced periodically.
where effluent was discharged and successfully achieved >95% RE in
48 h. The system was found consistent in achieving the same RE without V Hydrogen-based membrane biofilm reactor
being affected by the temperature variations over the year. The study
suggested the possibility to create a cost-effective on-site treatment The H2-based membrane biofilm reactor (MBR) delivers bubble-less
system. H2 gas, through membranes, to a biofilm of microorganisms. The mi­
Therefore, based on their previous study results, Luek et al. (2017) crobial growth on membranes is supported by oxidizing the H2 and
conducted another study on full-scale over the whole end-pit lake reducing soluble electron acceptors present in the solution such as NO−3
ecosystem near Grande Cache, AB, Canada. Nutrients (N and P) were and SeO2−4 .The product is trapped or released from the membrane after
dosed into the lake to enhance primary production, which further gave reaction (Yuan et al., 2018). In the context of Se oxyanions removal from
rise to favourable conditions for Se- and sulfur-reducing bacterial wastewater, a study done by Van Ginkel et al. (2011) reported having
growth. The Se levels dropped from 6.5 μg/L to below 1 μg/L in shown a maximum 362 mg m− 2 d− 1 selenate removal flux for actual FGD
approximately 2 years. wastewater containing 10 mg/L (RE >95%). In contrast to this, Lai et al.
The lab-scale studies which are generally carried out under (2014) observed that selenate removal efficiency was only about
extremely controlled conditions cannot be representative of real con­ 40–60% in the reactor and was severely affected when nitrates were
ditions. Moreover, if the stimulation of these conditions will be practiced introduced. Thus, the use of a pre-treatment step for the removal of
under real conditions, it will be very difficult and expensive to manage excess nitrate was required to avoid competition during the Se treatment
these parameters on-site. Realistically, the treatment should be done in step. On the other hand, in a recently published study by Zhou et al.
situ. Therefore, the aforementioned technique can be implemented at (2018), selenate removal in the presence of nitrates and sulfates was
other industrial locations, but one of the main constraints to this tech­ investigated. The MBR system resulted in 95–100% selenate (Si = 2
nique is that the Se-laden effluents are generally acidic. mg/L) removal accompanied with ~100% nitrate removal (initial – 10
mg/L) in the presence of 50 mg/L of sulfate. However, the sulfate con­
II Sequencing batch reactor centration used in this experiment is relatively very low compared to
real (industrial) wastewaters. An important concern associated with this
The presence of competing anions has been the biggest challenge technology is the use of hydrogen gas which is highly inflammable and
during any Se-reduction process; therefore, many research studies have relatively explosive (Lai et al., 2014).
been published on this subject. A study by Kim et al. (2020) shows that
the inhibition effect of nitrate and perchlorate can be diminished VI ABMet® System
completely (~100% RE in 5 h) in an SBR by enriching the selenate
reducing bacteria under appropriate temperature (30◦ C) and sufficient The application of ABMet® has been found to sustain over the last
acclimation period (>40 days). two decades due to its ability to produce an effluent with less than 5 μg
In another latest study, aerobic granular sludge was employed for the Se/L. The reactor is loaded with granular activated carbon (GAC) and
first time in an SBR to determine the efficiency of selenite reduction selected strains of microorganisms specialized in removing Se. A pilot
simultaneously along ammonium removal. The SBR setup worked under study (19–38 L/min) performed by using ABMet® to reduce selenium
phases of anaerobic (8 h) and aerobic (15 h) conditions and without levels from refinery wastewater demonstrated selenium reduction from
compromising ammonium treatment efficiency, it yielded almost com­ 368 μg/L to 2.3–4.7 μg/L after 3 weeks (Nurdogan et al., 2012). This
plete removal of selenite (Sf = 0.02–0.25 mg/L) while treating up to

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I. Ali and V. Shrivastava Journal of Environmental Management 294 (2021) 112926

technology requires long retention times (6–24 h) before producing the and selenate (70–74%). The enhanced RE was attributed to the combi­
effluent with desired concentrations. Moreover, to avoid the problem of nation of two processes (i) biological reduction of selenate to selenite
plugging, the reactor must be periodically backwashed. The bacterial and (ii) adsorption of selenite by AA.
culture could seriously be affected by variations in their environment The biological treatment technologies have gained ample attention
(such as the occurrence of shock loads). The effluent also requires for many years mainly due to low cost as well as the ability of microbes
post-treatment (aeration to increase DO level) before final discharge. to adapt to different conditions and capability to work even in very small
Se concentrations. Anaerobic biological treatment is generally applied
VII Up-flow anaerobic sludge blanket reactor to treat Se-laden waters but the biogenic selenium (Se0 ) remains are
nanoparticles that can wash out and can re-oxidize to oxyanions in
Anaerobic granular sludge obtained from an up-flow sludge blanket aquatic systems. A post-treatment can be applied to remove (Se0 ) from
reactor (Industriewater Eerbeek B.V., Eerbeek, The Netherlands) was the effluent but this leads to the production of biological sludge which
batch tested by Zeng et al. (2019) to evaluate the removal of selenate further needs to be properly handled and disposed.
along with cadmium. Within 1.5 days of batch incubation, the selenate
and total Se levels were reduced by 98% and 70% respectively. Although X Fungal bioreactor
this technology is independent of attached growth principle, still the
high HRT and SRT requirements result in larger footprints. Moreover, in As the currently available technologies for Se removal have varying
case of poor solid-liquid separation in the reactor, the granules may costs and efficiencies, Sabuda et al. (2020) has investigated a novel so­
wash out. lution by using two Se-reducing fungi (Ascomycota) for removing both
Se (IV) as well as Se (VI). However, as with the other technologies, fungi
VIII Combined process of zero-valent iron and selenate reducing too were more efficient for Selenite removal (78%, Si = 2000 μg/L) than
bacterium for selenate (28%, Si = 25 μg/L).

In most of the studies discussed above, a standalone treatment was 3.3.3. Algal treatment
ineffective for the removal of selenium from wastewaters. Moreover, Algal treatment can be carried out in two ways; (i) adsorption on to
even if the studies showed some promising results in standalone treat­ dead algal biomass (ii) Se uptake by living algal biomass. The use of
ment, the after-effects of those technologies (residual products) were algal treatment for the biosorption of selenium has come forward as a
mostly left undiscussed by the researchers. To compensate for this, some promising clean-up biotechnology after establishing some successful
of the recent research studies explore the combination of standard results for heavy metals (such as zinc, lead, cobalt, cadmium, nickel) in
technologies for better removal of Se and address the issue of end previous studies. This could be further confirmed by the study done by
products which also usually create some environmental concerns. In this Filote et al. (2017), where the green algae (Ulva rigida and Cladophora
section, all the studies will be discussing the removal of selenate from sericea) obtained from the Romanian Black Sea coast was used to remove
wastewaters. Se by adsorption. The results showed REs ranging between 80 and 95%
The use of a combination of Zero-Valent Iron (ZVI) and Se (VI)- within 5–7 h, for an initial Se concentration of 25 mg/L. Furthermore,
Reducing Bacterium (SRB) is implemented into two studies. In the first the adsorption capacity of the algae for selenite and selenate was
study conducted by Okibe et al. (2015), the use of Thaurea selenatis for measured to be 0.5 mg/g and 0.2 mg/g respectively.
the reduction of selenate (Si = 8–11 mg/L) in the presence of SO2− 4 (200 A study performed by Gan et al. (2019), has used Chlorella vulgaris for
mM) and Cl− (300 mM) has been employed. The use of T. selenatis under treatment (by biological uptake) of selenate and selenite. The high
anaerobic conditions was severely inhibited and thus no microbial removal efficiency of 89% was observed for the initial Se concentration
growth was observed. However, while using it for micro-aerobic con­ of 1580 μg/L. Furthermore, the concentration of selenate and selenite
ditions, a Se removal of 55% was observed. The efficiency was improved dropped down to 20 and 10 ppb respectively, in the effluent. Interest­
to 98% by combining T. selenatis and ZVI. In a study by Liu et al. (2018), ingly, the problem of disposal of C. vulgaris (after accumulation of Se)
the process parameters have been monitored for various bacterial spe­ was also discussed. For decomposition of Se laden algal biomass, a
cies (Clostridium, Sphaerochaeta, Synergistes and Desulfosporosinus) with combined process of centrifugation followed by introduction of acti­
the combination of ZVI for the removal of Se from mining industry vated sludge was carried out that resulted in the removal of 41–47% Se
wastewater. Under the abiotic control, the selenite concentration has from the algal biomass. Another similar study was performed by Liu
shown a drastic reduction to <0.5 μg/L from an initial value of 1.5–5 et al. (2019) on the use of Chlorella vulgaris for Se removal (by biological
mg/L. Furthermore, the results were also significant for selenate uptake) under varying conditions of initial Se concentration, algal
reducing its concentration in the effluent to 5 μg/L. density, temperature and pH. This study has also shown approximately
In case of using ZVI + SRB, the results have shown a significant 90% removal for the Se ranging between 1000 and 3000 μg/L while the
decrease in Se even in the presence of competing anions. However, the tolerance against Se toxicity was found at 6000 μg Se/L.
major concern is to monitor the inhibition of SRB at varying kinetic and These studies suggest that high RE can be attained using algal
thermodynamic parameters. Furthermore, the usage of SRB requires treatment in Se laden wastewater. However, the risk of toxicity (in
enrichment of bacteria (which requires 1–4 weeks), making it a more aquatic environments) brought by the direct discharge of algal biomass
tedious process. Furthermore, studies will be needed to determine what is significantly high due to the lower decomposition rates of Se-laden
forms of selenium are produced in the process and under what condi­ algal biomass Gan et al. (2019). Moreover, the process is relatively
tions elemental Se is the dominant form. Additionally, the problem of re- slow and takes more than 48 h to exhibit an effective RE. Even though
solubilization of Se was also encountered in the study, further adding to the algal biomass can be easily available, the possible requirement of the
the inefficiency of the process. pre-treatment (such as protonation, hexadecyltrimethylammonium
(HDTMA) and ammonium), to enhance adsorption properties, increases
IX Combined process of activated alumina and bioreactor the complexity of the process.

Owing to the troublesome presence of selenate ions as it shows 3.3.4. Constructed wetlands
limited reactivity towards any type of treatment in comparison to sele­ The use of Constructed wetlands for selenium removal has shown a
nite, the biological treatment could offer the solution by reducing sele­ trend of varying results since the last decade. During data pooling, 4
nate to selenite. In a continuous flow activated alumina (AA) packed studies were selected based on applicability of selenium removal via
bioreactor, Ji et al. (2020) reported to significantly reduce both selenite

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CW. Out of these 4, 3 studies particularly deal with the removal of total cost. Therefore, many attempts have been made to recover selenium
selenium while only one study presents separate REs for selenite and from wastewater which will not only offset the treatment cost but will
selenate. A pilot-scale study was conducted by Chakraborti et al. (2015) also open a corridor for circular economy by making this pricy scarce
on a bulrush (Schoenoplectus californicus) based CW located in Oxnard, element available for reuse. One such way is to reduce selenate into
California. Even though this study was not particularly focusing on Se elemental selenium nanoparticles, which may further be recovered as a
removal, the concentration of reclaimed concentrate obtained from a RO valuable resource (Nancharaiah and Lens, 2015). It could be done in an
unit drastically reduced by 61% (from a value of 0.002 mg/m2/day to HRT controlled membrane biofilm reactor (Zhang et al., 2020a, 2020b,
0.001 mg/m2/day) for an average flow of 1.2 L/min. 2020c), or by employing various bioremediation techniques during FGD
Zhu and Bañuelos (2017) performed a mesocosm study for a varying wastewaters (Cordoba and Staicu, 2018; Gingerich et al., 2018). For
Se concentration ranging from 0.25 mg/L to 5 mg/L. The plant used instance, Hageman et al. (2017) show that the composition of Se
(Poplus trichocarpa) was already identified to be salt and boron tolerant. nanoparticles can be controlled by temperature and pH in a fed-batch
The study showed a significant decrease of 50–70% selenium for an bioreactor. Another possibility is to bioaccumulate Se in
initial flow of 30 mL/s. Another similar mesocosm study performed by algal-bacterial biofilms while treating Se-rich wastewaters in biofilm
Zhou et al. (2019) used floating-leaved plant (Nymphoides sp.), snails batch reactors so that it could potentially be applied as a Se-rich bio­
(Physella sp.), paradise fish (Macropodus sp.), watermilfoils (Myr­ fertilizer (Han et al., 2020). Moreover, Kagami et al. (2013) have also
iophyllum sp.) and waterweeds (Elodea sp.) for the setup of CW to treat an introduced a technique for recovery of (recyclable) Se from wastewaters
initial selenite concentration of 600 μg/L. The results were observed preceded with volatilization of Se (IV and VI) by microbes (such as
over 21 days, but still no higher than 40.4% RE was obtained. Pseudomonas stutzeri NT-I).
The most detailed pilot study of the use of CW for selenium removal
was conducted recently by Zhao et al. (2020a, 2020b), in which 4 3.5. Overall comparison of various technologies
different experimental arrangements were used for observing the effect
of various parameters on Se removal. The effects of organic amendments Fig. 3 shows a comparison of the trends in removal efficiencies for
and alternating wet/dry cycles were closely monitored over 8–20 days. various treatment technologies. The best results with the lowest stan­
The results exhibited a removal efficiency <94% within 8 days for dard deviation were obtained for filtration and oxidation-reduction
moderate and low organic carbon content and 98% (RE) within 20 days processes. Moreover, the combination of ZVI and SRB for treatment
for the 2-days wet/dry cycle. To clone the effect of on-field CW, the also showed a mean value of 95–100% RE across the studies. It is worth
cattail treatment system was added with a layer of litter, which reduced mentioning that the use of ZVI, employed in both cases (oxidation-
waterborne Se much more rapidly, achieving a 77% removal of Se reduction and combined treatment), resulted in high RE. Furthermore,
within 4 days. the use of bioreactors has also shown high efficiency (80–97%),
The major concern related to the use of CW (especially for Se considering the fact that the data originated from 12 documented
removal) is its potential risk to the adjoining ecosystem. If left under studies.
uncontrolled supervision, the wildlife can feed on the CW plants, hence The highest standard deviation was shown for the studies using
creating risks of biomagnification and biotransformation across the food adsorption (55–97%) and photocatalysis (35–85%). However, the re­
web. Moreover, other uncontrollable constraints such as temperature, sults of adsorption were significantly distributed over the data of 25
flow and climate in the full-scale conditions also adversely affect the studies (maximum for any process) for a wide range of adsorbents,
working of CW. Additionally, the use of CW for achieving an overall which could be a possible explanation for this large standard deviation.
removal of contaminants (instead of particularly focusing on Se) has Alongside removal efficiency, the economic analysis also plays a vital
shown more efficient results across the discussed studies. role in selecting a technology that could be installed and operated to
remove/recover Se, especially in the areas of poor or emerging econo­
3.3.5. Phytoremediation mies. For instance, the study performed by Dessì et al. (2016) considered
Phytoremediation is often considered an attractive option when it the cost of removing Se via mesophilic/thermophilic UASB reactor to
comes to the treatment of soils/wastewaters cost-effectively and sus­ vary between 0.51 and 1.06 €/kg Se. This is significantly high consid­
tainably. Many recent studies describe the uptake mechanism and ering the cost for the installation of bioreactor. In another study done by
metabolism of hyperaccumulator plants for Se-removal (Hasanuzzaman Zha et al. (2020), the treatment of selenium sludge (recovered from
et al., 2020). Ohlbaum et al. (2018) have shown that the plants like wastewater) by pyrometallurgical oxidation-reduction costs to be
Lemna minor and Egeria densa can be used to remove Se up to 76% (Si = $1730/ton Se (~1.4 €/kg Se). Even though the cost is higher in com­
74 μg/L) from seleniferous soil leachate. However, phytoremediation is parison to bioreactors, the recovered selenium generated a revenue of
generally a long-term treatment project (harvesting annually for 10–15 0.6 million € for the company (Zha et al., 2020). However, the cost
years), for which giant reed plants are found capable to withstand required to recover the Se sludge at initial level was not considered in
changing climatic conditions along with absorbing Se from agro­ the study. Other factors such as available capital, geographical location,
ecosystems (El-Ramady et al., 2015). In comparison to phytor­ legal restrictions, generation of commercial byproducts also influence
emediation, Garousi et al. (2016) has illustrated the competence of the selection of the best available technology.
rhizofiltration systems using sunflower (Helianthus annuus L.). The re­
sults show that sunflower exhibits high tolerance towards Se-toxicity 4. Conclusions
during hydroponic culture cleanup, hence making it opt able for treat­
ment purposes. These Se-enriched plants (used during phytor­ A systematic review of different physical-chemical and biological
emediation) can further be utilized as a fertilizer/fortified food treatment technologies which have been developed over the last decade,
(Schiavon and Pilon-Smits, 2017) or for the production of renewable for the removal of Se oxyanions (selenite and selenate) and selenocya­
fuel/petrochemicals by pyrolysis (Miranda et al., 2014) hence closing a nate, was conducted. Albeit all the three stated Se species have been
loop in circular economy aspect. identified as toxic, Se oxyanions have gained more attention, mainly
because of their persistence and bioaccumulation effects in aquatic en­
3.4. Selenium recovery and circular economy vironments. It is inferred from this study that much current research is
mainly focused on chemical adsorption and biological reduction-related
To comply with the increasingly stringent discharge regulations, technologies. Various adsorbents have been developed and tested over
various combinations of physicochemical and biological treatment the years, and although their efficiency has been improved over the
technologies are investigated. However, this raises the overall treatment time, but they are still found less applicable since most of them require

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I. Ali and V. Shrivastava Journal of Environmental Management 294 (2021) 112926

Fig. 3. Box and whiskers comparing the range of selenium removal for various technologies.

relatively acidic environment and generate sludge at the end. While Author contribution
many studies have been carried out to determine the removal efficiency
of Se from aqueous media, only a few have demonstrated the perfor­ Both authors contributed equally to the research and writing of this
mance at pilot or full scale. Moreover, many technologies have reported article.
obtaining remarkable removal efficiencies in laboratory-scale setups,
but most of these technologies render poor performance when fed with Formatting of funding sources
real (waste)waters. Technologies such as reverse osmosis, redox treat­
ment by ZVI and combinations of different treatment approaches show This research did not receive any specific grant from funding
the best results among all (>96%) with a faster removal being obtained agencies in the public, commercial, or not-for-profit sectors.
by ZVI (<24 h). Generally, most of the technologies were not effective
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