Critical Review

Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX pubs.acs.org/est

Degradation of Anthraquinone Dyes from Effluents: A Review

Focusing on Enzymatic Dye Degradation with Industrial Potential
Eleni Routoula and Siddharth V. Patwardhan*
Department of Chemical and Biological Engineering, University of Sheffield Mappin Street, Sheffield, United Kingdom, S1 3JD
*
S Supporting Information

ABSTRACT: Up to 84 000 tons of dye can be lost in water,

and 90 million tons of water are attributed annually to dye
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production and their application, mainly in the textile and

leather industry, making the dyestuff industry responsible for
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up to 20% of the industrial water pollution. The majority of

dyes industrially used today are aromatic compounds with
complex, reinforced structures, with anthraquinone dyes being
the second largest produced in terms of volume. Despite the
progress on decolorization and degradation of azo dyes, very
little attention has been given to anthraquinone dyes.
Anthraquinone dyes pose a serious environmental problem
as their reinforced structure makes them difficult to degrade
naturally. Existing methods of decolorization might be
effective but are neither efficient nor practical due to extended time, space, and cost requirements. Attention should be
given to the emerging routes for dye decolorization via the enzymatic action of oxidoreductases, which have already a strong
presence in various other bioremediation applications. This review will discusses the presence of anthraquinone dyes in the
effluents and ways for their remediation from dyehouse effluents, focusing on enzymatic processes.

1. INTRODUCTION: THE PROBLEM regulating and monitoring the dyeing industry in Europe and
A very important factor of our life is water, as it is not only vital the United States;8,9 however, these are not clearly defined and
for our physical existence but it is also necessary for numerous not comparable across countries in regards to the color
activities in domestic and industrial fields, varying from intensity of the discharged effluents.9 These issues make the
cleaning and agriculture to cooking and product formation.1 monitoring of colored effluents released in the environment
Unsustainable exploitation and uncontrollable contamination quite a challenge. The problem of the dye contaminated water
are currently the “hot issues” regarding water management. is especially evident in Asia, which contributes about 50% of
The limited water resources need to be adequately distributed textile exports and more than 50% of world’s consumption of
and carefully used to fulfill the constantly rising agricultural dyes. However, many of the countries involved lack sufficient
and industrial demand due to population growth.1 The main legislation about environmental protection relevant to textile
strategies to address water scarcity are prevention, demand industries.10 Having said that, there have been efforts for color
management, and revalorization of water.2 Following that restrictions to be included in legislation.7
perspective, industrial wastewater should be recycled and Although currently the relevant legislation might be vague
reused. and not properly applied,11−16 it is clear that not only the
The dye sector and the sectors relevant to dye applications volume of discharged effluents needs to be minimized, but the
(textile, tannery, paper) are recognized among the most
quality of industrial effluents discharged in the environment
polluting industries, based on both the volume and the
needs to be fully monitored as well.
composition of effluents.3,4 Effluents released in the water
The dyeing and textile industry is responsible for dye
bodies create aesthetic and environmental issues5,6 with a high
societal unacceptance. Furthermore, they can cause pipe discharge in the effluents, as well as for a plethora of other
corrosion, blockages, and bioaccumulation,7 and result in the hazardous and potentially hazardous substances. Such
production of hazardous sludge.7 The presence of dyes in substances, mostly surfactants and persistent organic pollu-
effluents makes their reuse difficult, as the presence of color tants, are used to accentuate dye stability/fastness or color
and other substancesaffects consecutive dyeing cycles.7
Awareness of environmental protection has increased and Received: June 24, 2019
minimization of water usage and wastewater production is Revised: December 12, 2019
required, in addition to the limitation on the amount of Accepted: December 17, 2019
pollutants released to the environment. There are legislations

© XXXX American Chemical Society A DOI: 10.1021/acs.est.9b03737

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Environmental Science & Technology Critical Review

intensity, to assist the process of dyeing and to give specific by “anthraquinones” (15%), and indigoids in respect of the
characteristics to textiles, among others.3,17 present chromophore group.22
It is difficult to quantify the amount of dyes lost during While for azo dyes, relevant data is easily available, it is
production or during application on textiles, as the available difficult to find current or accurate data for the annual
figures from the literature are based on estimations, or are production of anthraquinone (AQ) dyes. Nevertheless, data
representative of very specific types of dyes or applications. found from previous years can be used to roughly estimate a
Nevertheless, it is important to discuss those data to production volume. For the U.S., within a period of about 15
understand the importance of the problem. Dye production years (1986−2002), the annual production of anthraquinone
may vary between 10 0008 and 770 00018 tons per year, and (a precursor for dyes and other chemicals) had a staggering
the losses are estimated around 2% during production and 5000% increase (from 500 to 25 000 tons).23 Given the
increase in production volume of dyes, it is safe to assume that
around 10% during application,7 with wastewaters being
the production of AQ dyes increased as well; a rough
discarded directly into the environment in developing
estimation of about 100 000 tons of AQ dyes per year can
countries.19 Based on the data from 2013, the annual be made.
production of textiles was around 30 million tons, increasing The specific chemistry of the anthraquinone group is based
every year.18 Each ton of textile requires around 30 tons of on the anthracene and consists of three fused benzene rings
water for the dyeing process,10 while each ton of dye (basic anthracene structure) with two carbonyl groups on the
production requires an average of 200 tons of water.5,20 That central ring (highlighted in Figure 1). This structure is
means a total of 80 million tons and 90 million tons of water, naturally colorless, but substitution of the aromatic rings gives
respectively, is attributed to dye production and textile dyeing color and controls its intensity.24 Color gets deeper with
process per year. Taking into account the amount of increased basicity of the substituent. For example, for an
contaminated water (2% and 10%, respectively, during their aniline-based substituent (NHC6H5), the maximum adsorption
production and application), a staggering sum of about 11 wavelength can rise from 327 nm to 508 nm.24
million tons of water is polluted per year, making the dyestuff The difference from azo dyes is that, in the anthraquinone
industry responsible for about 20% of the total industrial water structure, the carbonyl group acts as an electron acceptor, thus
pollution.21 It is thus evident that water pollution from dyes is requiring an electron donor to react and break their structure.5
an existing and growing problem that demands attention. This combined with resonance effects among the anthracene
The majority of dyes industrially used today are aromatic structure leads to higher difficulty in AQ dyes degradation
compounds with complex, reinforced structures, leading to compared to azo dyes19,25 and makes the choice of an
difficult degradation.18 Of the industrially important dye appropriate degradation/decolorization method challenging.26
categories (Figure 1), the most common “azo” dyes make up The majority of the industrially important AQ dyes are derived
from anthraquinonesulfonic acids, using sulfonation or
of almost 60% of the synthetic dyes used industrially, followed
nitation,27 and research has shown that presence of sulfone
groups in dye structure can reduce their degradability.28 Due
to their highly stable structure, AQ dyes are known for their
great fastness, stability, and brightness.24

2. AVAILABLE METHODS FOR DECOLORIZATION

2.1. Industrially Available Methods. The most known
and extensively applied methods in the industry are adsorption,
coagulation, membrane filtration, as well as various oxidative
processes.10,29 Regarding biological methods, aerobic and
anaerobic processes are currently widely applied for general
water treatment, offering distinct advantages compared to
physicochemical methods (e.g., products of added value,
environmentally friendlier), but also facing challenges regard-
ing their efficiency (e.g., sensitivity, long contact time).29
There are many examples of papers reviewing the current and
future industrial methods for general dye decoloriza-
tion.3,8,30−34 Every method has advantages and disadvantages
related to the following criteria: efficiency under various
conditions, practicality, requirements of pre- and post-treat-
ment, and environmental impact; ultimately relating to the
cost. Given this complexity, a single method can rarely satisfy
these demands simultaneously;5 hence, typically, a combina-
tion of available and under-development methods is preferred,
thus maximizing their strengths and minimizing their
disadvantages.17,29
Despite the significant amount of research about the
decolorization and degradation methods applied for azo
Figure 1. Representation of the two most important chromophore dyes,6,35−39 not much research is available on AQ dyes, with
groups, with examples shown being Acid Black 1 (azo dye) and only two reviews available, both discussing AQ dye
Reactive Blue 4 (anthraquinone dye). decolorization, mainly by biological methods,40,41 and
B DOI: 10.1021/acs.est.9b03737
Environ. Sci. Technol. XXXX, XXX, XXX−XXX
 Table 1. Exceptional Performance of Decolorization of Anthraquinone Dyes by Various Physical and Chemical Methodsa
dye name/initial dye % max
concentration decolorizationb throughput
decolorization method (mg/L) /time scale (h) (mg/L/h)c comments ref
adsorption (activated Reactive Brilliant about 81%/0.5 121.5 pseudo second order kinetic model used, higher capacity than commercial carbon products 46
carbon from T. dealbata) Blue X-BR/150
adsorption (on silica) Reactive Blue 19/800 99%/4 198 amount of silane affects decolorization (the more the better) and elution of dye 55
adsorption (on clay) Acid Blue 25/100 about 100%/1 100 full material characterization and adsorption kinetics analysis, chemisorption dominates, lower pH facilitates adsorption 98
AOP (wet air, wet Reactive Blue 4/100 100%, 100%, 99%/ 100 very high degrees (>75%) of mineralization, wet peroxide oxidation worked best, examination of degradation pathway 93
peroxide, photocatalytic, 1 and 100%/0.75
Fenton)
AOP (photodegradation) Reactive Blue 19/30 over 95%/0.5 126 degradation based on ZnO and TiO2 nanoparticles assisted by photocatalysis, multifactorial design analysis and optimization, ZnO performs 25
+ nanoparticles −70 better (both cost of energy + dye degradation effect), TOC analysis shows low residual toxicity, activity of ZnO regulated only by pH
AOP (photodegradation + Reactive Blue 19/800 about 75%/3 200 examination of various factors affecting dye degradation such as dye, catalyst and peroxide concentration, identification of intermediate 99
TiO2) products through UPLC-MS, fragments show reduced cytotoxicity
Environmental Science & Technology

AOP (ozonation) Reactive Blue 19/200 almost 100%/0.3 200 ozone feed rate and presence of electrolytes affect decolorization, identification of oxidation products through IC 100
AOP (Fenton reaction) Reactive Blue 4/100 100% /0.5 200 coupled process enhances degradation due to enhancement of Fenton process reaction, high concentration of dye is prohibitive, metal 67
coupled with pyrite ash removal is necessary
ozonation and UV Reactive Blue 19/111 100%/0.1 1665 ozonation is better for decolorization, combination with UV radiation is better for mineralization, proposed degradation pathway, toxicity 68
radiation studies
AOP (Fenton/photo- Reactive Blue 81%-98%-42%/ 0.3 1050−2450 decolorization examined in pure dye (higher) and simulated effluent (lower), AOPs are more effective than UV radiation, dye structure 70
Fenton reaction), UV 19/2500 affects efficiency of each process, optimization study for each process
radiation
electrochemical Reactive Blue 95% /0.1 19 000 use of Fe (better action) and Al (can have reversed effect depending on pH) as coagulants, higher voltage increases removal percentage, 73
coagulation 19/2000 overall quite fast method, no mention of proposed sludge treatment

C
a
Please refer to SI Table S1 for biological methods. bValues shown are for the optimized methods as presented by researchers and refer to removal of color unless stated otherwise. cArbitrary value
calculated to show the maximum removal capacity of any given method within an hour, based on the best results presented in each reference. In cases where the time scale of the decolorization is within a
few minutes, the assumption of decolorization ability over continuous use for 1 h is made.
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DOI: 10.1021/acs.est.9b03737
Environmental Science & Technology Critical Review

comparably fewer research papers compared to those available flocculation is being replaced by newer methods or used in
on azo dyes1. What is worth mentioning, is that there is a combination with other methods, in order to reduce the effect
review paper focusing on the degradation of a specific AQ dye, of some major drawbacks such as potentially toxic sludge
reactive Blue 19, covering various methods and research production and the need for further treatment of the effluent.61
examples dated up to 2011.42 The principle of coagulation and flocculation methods is the
2.2. Physical, Chemical, and Biological Methods opposite charge between the soluble pollutant (e.g., dye) and
Applied for AQ Dye Removal. The most common physical the (usually) aluminum, iron, or, most recently, polymeric
methods for treatment of dye house effluents are adsorption, coagulant, that makes the pollutant insoluble.61 The factors of
and filtration (using membranes and reverse osmosis). As there importance during coagulation are the type and dose of
are numerous research studies on dye removal assisted by coagulant needed and the size and “sturdiness” of the floccs
adsorption, we have summarized the best performing literature (coagulated pollutants), which dominates their ease of
findings on AQ dye removal and degradation in Table 1, with removal.62 Table 1 summarizes some distinct examples of
associated comments, while below we discuss selected AQ dye treatment using chemical methods, while we elaborate
examples. Best performance was arbitrarily evaluated based on relevant research below. When degradation of Reactive Blue
on the amount of dye removed per liter, per hour, assuming 19 and 49, individually and in a mixture, was attempted using
continuous use of the system described at the optimal state active chlorine, it was shown that degradation was much faster
identified by the researchers. This arbitrary metric allows for a for individual dyes.63 Contradicting these findings, decoloriza-
comparison between results found in literature; given the lack tion of Disperse Blue 3 via coagulation with magnesium
of a consistent approach followed, superficial comparison of chloride or ferrous sulfate, as individual dye or in mixture with
results does not produce valuable conclusions. azo dyes, showed that there is a synergistic effect. Dye removal
2.2.1. Physical Methods. Among many adsorbents explored increased from 68% (individual dye) to up to 90% (mixture
such as activated carbon, peat, silica-based adsorbents, zeolites, with azo dyes) in the presence of ferrous sulfate, whereas for
or other naturally derived substances, activated carbon is magnesium chloride the decolorization percentage was
widely studied for dye adsorption. It is also the dominant maintained very high, at 93%, regardless the presence of
adsorbent in industry, based on its great adsorption ability, other dyes.64
high surface area, stability, and homogeneity,43 which outweigh 2.2.2.2. Advanced Oxidation Processes (AOP). The
the high cost of production and regeneration and the available oxidative methods include Fenton’s process with or
possibility of decreased efficiency due to material loss during without external energy supply or ozonation, and they operate
regeneration.8,44,45 A few examples using activated carbon for via the production of active OH− radicals that nonselectively
AQ dye adsorption showed that uptake was higher for acidic oxidize dyes.65 Their application in water treatment has been
solutions,46−48 and that pore structure of the materials could illustrated recently28,34,65,66 gaining much attention over the
facilitate46,48 or hinder47 adsorption. Another frequently past few years. Selected stellar examples of AOP applied for
discussed option in the area of adsorption is the abundant in AQ dye degradation are shown in Table 1, and some of them
nature zeolites, with substantially lower adsorption capacity are discussed below. Studies on Reactive Blue 19 conducted by
and again facing high regeneration costs.49,50 Silicon based different groups, using the same starting concentration (100
materials have been studied extensively for pollutant mg/mL), but different combinations of AOP, showed highly
adsorption as well.51,52 Their interesting properties such as different results. It was shown that using a combination of
ability for a wide range of pore size and surface areas, methods such as Fenton reaction coupled with adsorption on
durability, ease of functionalization and relatively cheaper pyrite ash,67 or ozonation coupled with UV radiation68 can be
regeneration compared to activated carbon, have made them much more efficient compared to ozonation only.69 Using the
excellent candidates for water treatment with many examples same dye at a much higher starting concentration (about 2000
of dye adsorption.53−56 However, issues such as manufacturing mg/L), and examining its decolorization by Fenton’s reaction,
and regeneration cost, as well as diffusional limitations arising photocatalysis, and UV radiation, as single methods or
from high throughput in industrial scale applications, have combined, resulted in generally very high dye removal
prevented them from being widely applied in water treatment (above 90% for a combination of Fenton reagent coupled
yet, although research is showing positive signs on their with photocatalysis).70 This shows that combination of AOP
industrial implementation.57 Newer trends in adsorption, with methods does work synergistically, and usually better than
application for AQ dyes, include the use of agricultural single methods. What is worth highlighting about AOP when
waste.58,59 As per filtration, the usually encountered textile applied in AQ dye degradation, is the very short reaction times
effluent treatments include nanofiltration (pore diameter up to required, usually few minutes, their very good efficiency and
10 nm) and reverse osmosis,60 but there was no example of mineralization of dye, but also their high cost, which poses
their application on AQ dyes found in literature. difficulties on their consideration for scale-up.71,72
Major issues about the application of physical methods for 2.2.2.3. Combination of Methods. Emerging combinations
dye removal are the relatively high required contact time, of the once very popular chemical coagulation with newer dye
hence large spaces required, as well as the need for adsorbent removal methods are implemented, in order to reduce the
(or membrane) regeneration. These are issues that are not effect of some major drawbacks such as sludge production and
usually addressed in literature, but are of great importance for need for further treatment of the effluent.61 Electrochemical
industrial implementation. coagulation producing in situ coagulants based on aluminum
2.2.2. Chemical Methods. 2.2.2.1. CoagulationFloccu- or iron, showed great dye removal potential (Reactive Blue 19
lation. The most common chemical treatment methods was used as a representative AQ dye, but other dyes were
applied to textile effluents are chemical coagulation and studied as well) and associated time.73 That work also
oxidation processes, while electrochemical methods are gaining presented an economic evaluation of some decolorization
attention as well (Table 1). Chemical coagulation or processes, which suggested that electrochemical and oxidative
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processes are advantageous to adsorption, however, biological/ Upon comparison of the throughput value calculated for the
enzymatic methods were not included. Furthermore, what was examples shown for physical/chemical and biological methods,
only acknowledged but not commented further is sludge it is evident that biological methods cannot compare in terms
production and the need to deal with it, but, it was shown of efficiency with the chemical and physical methods examined.
qualitatively that use of different conditions can have an effect This leads to the conclusion that biological methods might not
on the amount and type of produced sludge. In the area of be as effective as physical and chemical methods. However,
coupling photocatalysis with nanoparticles, a study optimized biological methods are generally recognized as more benign,
the degradation of Acid Green 25 using immobilized TiO2 environmentally friendly, and economically viable, with the
nanoparticles coupled with UV light photocatalysis, resulting in ability for in situ degradation of pollutants, compared to
an optimized system operating at a relatively low dye physical removal of the dye or transformation to other
concentrations (18 mg/mL).74 The same group examined substances requiring further treatment.82,83
the importance of the chemical structure of various dyes in 2.3. Challenges with Current Methods. Based on the
degradation through the same method.28 Their critical analysis data discussed and presented in Table 1, it is clear that AQ dye
showed that degradation of AQ dyes was more difficult removal is highly specific to the dye and the method used. It
compared to azo dyeswithout further elaboration on the has been shown that different degrees of decolorization are
differences between the dye structuresand also that the observed for a single dye when using different treatment
presence of sulfone groups reduces the efficiency of dye methods.84,85 It can be quite difficult to choose an appropriate
degradation. The application of other nanomaterials for AQ method among the available conventional methods to
dye degradation from effluents is gaining more and more decolorize AQ dyes, due to their fused ring structure that
attention, although not yet thoroughly developed, but their enhances their stability.86 Although anthraquinone on its own
potential advantages lead to an increased interest for their is not toxic,23 research has shown that some AQ dyes as well as
industrial application.75 A study coupling the use of zinc and their degradation intermediates (often unidentified), can be
titanium oxides with photocatalysis by irradiation, showed that potentially toxic, few of them are mutagenic and/or potentially
nanoparticles can be quite effective in degradation of a model carcinogenic.26,87−90 Also the lack of data on the intermediate
AQ dye, but their efficiency depends on the type of the dyes.25 degradation products91 makes it difficult to speculate any
A more recent study examined the degradation of Reactive residual toxicity, as well as to find appropriate degradation
Blue 4 based on the coupled use of copper nanoparticles and pathways.26 There are research examples where a degradation
showed that initially the dye gets adsorbed onto the pathway for a specific AQ dye has been proposed, based on
nanoparticles and then is oxidized based on the production collected data and in few cases the pathway has been fully
of hydroxyl radicals from added mediators and the action of determined, usually using analytical techniques to identify the
monovalent copper.76 byproducts. This indicates that methods which can result in
2.2.3. Biological Methods. The available biological methods complete destruction of the dye structure, rather than isolation
can be divided based on whether they are performed inside a or coupling with other chemicals, are in need. In these
cell (of bacteria, fungi, yeasts, algae), or using isolated examples, anthraquinone dyes were degraded using ozona-
enzymes. The challenge is to create methods based on tion,68,69,92,93 electrochemical methods, and their combina-
bioremediation that can bypass the disadvantages of conven- tions94 or via enzymatic degradation.95−97 A discussion on the
tional methods, yet be efficient, cost-effective, and environ- degradation pathways of a model AQ dye by various
mentally benign.77,78 techniques is available in Section 4.
Degradation of AQ dyes using aerobic and anaerobic We note that most studies examined a lower dye
cultures has been reviewed recently,40,41 so in Supporting concentration spectrum, sometimes coupled with a quite low
Information (SI) Table S1, we report some newer examples of concentration of adsorbent. These scenarios may be ideal for
biological degradation of AQ dyes. Some of the clear scanning a method or optimizing the process conditions, but
differences between research examples of biological and they do not give information on realistic conditions of
nonbiological methods include the usually low starting industrial applications and may cause barriers during
concentration of dyes examined and the longer time needed commercialization.
for decolorization in biological methods. Although usually When using physical and chemical methods for dye removal,
individual cultures are examined in research papers, a recent there is sludge generation, which can be difficult to handle, as
study79 showed that when the microorganisms were acting in a well as materials used cannot be regenerated easily, if at all. In
consortium, the decolorization of Reactive Blue 4 and 19 was addition, some of these methods are not very efficient due to
dramatically improved. Another study80 examined Escherichia the large cost, time, and space requirements.10
coli cultures for degradation of AQ dyes at higher Existing literature on decolorization of AQ dyes from water
concentrations and found that dye decolorization occurred effluents, acknowledges the problem and explores potential
primarily due to microbial induced precipitation, followed by solutions; however, the lack of consistency on the way the issue
adsorption on cells and cell metabolism. A very interesting is approached highlights the need for an evaluation of
observation was that the dye structure affected the proposed solutions on a consistent basis, on the merit of the
decolorization mechanisms and the kinetics, indicating that potential of industrial implementation and socially acceptable
this method might not be applicable in real effluents where a practices.
mixture of dyes is present. When the degradation of a mixture
of dyes was examined, (including Acid Blue 350 of AQ 3. FOCUS ON ENZYMATIC DECOLORIZATION
structure) using a specific strain of Trametes Versicolor, over Isolated enzymes are very effective as they are highly specific
90% degradation could be achieved after 48 h of treatment, catalysts that produce byproducts of lower toxicity and volume.
that being slightly lower to the almost complete decolorization The overall process is considered environmentally friendly and
achieved for the individual dyes.81 less intrusive. The enzymes responsible for dye degradation
E DOI: 10.1021/acs.est.9b03737
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 Table 2. Decolorization of Anthraquinone Dyes by Isolated Enzymes
dye name/initial
decolorization dye concentra- % max decolorizatio- throughput
method tion (mg/L) na/time scale (h) (mg/L/h)b comments ref
horseradish Reactive Blue 96%/0.1 115.2 better results shown for anthraquinone dyes compared to other types, reduction of toxicity after degradation 106
peroxidase 19/120
horseradish Remazol Blue/ 35%/9 38.8 inactivation of enzyme due to dye concentration, precipitation can occur depending on dye 164
peroxidase 1000
horseradish Lanaset Blue 90%/0.033 270−2700 very good decolorization of single anthraquinone dye, examination of enzyme performance on real effluents (see discussion), examination of 2 152
peroxidase 2R/10−100 bioindicators on toxicity of effluents before and after treatment
horseradish Acid Blue 225, 83%, 70%/0.5, 0.25 53, 113 different anthraquinone dye structures lead to different decolorization degrees under the same conditions, decolorization of AB225 is affected more by 122
peroxidase Acid Violet (for AB225 and temperature and concentration of enzyme, but dye concentration affects AV109 decolorization more
109/30 AV109 respectively)
laccase Reactive Blue 89%/ 0.5 178 laccase showed better decolorization performance on anthraquinone dyes, compared to azo, triphenylmethane or indigo dyes, no mediators were necessary 153
19/100
laccase Reactive Blue 100%/ 72 100 laccase showed better decolorization performance on anthraquinone dyes compared to other types of dyes. Furthermore, activity of purified enzyme was
Environmental Science & Technology

19/300 higher than use of mother culture.

laccase fol- Reactive Blue 90 (61 + 29)%/12 50.8 + 24.1 reduced phytotoxicity, individually, biosorption works better, enzymatic degradation results in brown-ish products, dye fragments are more polar compared 165
lowed by bi- 4/1000 to original dye
osorption
dye-decoloris- Reactive Blue 95%/0.167 855 decolorization examined in batch reactor and fed batch reactor, examination of stepwise and continuous feed of H2O2, batch and continuous fed reactor, 107
ing peroxi- 19/150 residual activity of DyP is 80% after 80 min, increased dye concentration prolongs the decolorization time, increased addition of H2O2 deactivated the
dase enzyme, through batch fed system one dose of enzyme managed to decolorise 3,650 mg/L RB19
a
Values shown are for the optimized methods as presented by researchers and refer to removal of color unless stated otherwise. bArbitrary value calculated to show the maximum removal capacity of any
given method within an hour, based on the best results presented in each reference. In cases where the time scale of the decolorization is within a few minutes, the assumption of decolorization ability

F
over continuous use for 1 h is made.
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belong mainly to the family of oxidoreductases, including oxidoreductases applied in dye degradation and decolorization,
peroxidases, reductases, and laccases.17,101 These enzymes have some of them focusing on AQ dyes (Table 2). Focusing on
the ability to act on dyes by either creating precipitants that DyPs, there are many research examples qualitatively
can be easily removed or chemically transforming the dyes into examining their activity on AQ dyes, and a few researchers
compounds easily dealt with.102 have tried to consider an industrial implementation. For
The use of both isolated enzymes and the whole cell/micro- example, recombinant DyP was used to treat Reactive Blue 19
organism has considerable advantages and disadvantages. The in a single batch system and also in a step fed batch reactor107
use of isolated enzymes does not depend on culture/ to assess a cyclic operation, leading to a very high
microorganism’s “well-being” or growth rate.101,103,104 Also, decolorization performance, regardless of the soluble nature
diffusional limitations of substrate and/or product in-between of the enzyme. However, a major concern for industrial
the cell compartments can be avoided as well as any other application is the continuous ingress of effluents that can make
actions besides enzymatic.105−107 Isolated enzymes are batch treatment tricky. Another study tested free horseradish
relatively easier to use under harsher conditions, offer higher peroxidase on a single AQ dye and a real effluent. The results
specificity and easier regulation of catalytic activity, as well as showed that although single dye degradation was very fast and
easier handling/storage compared to whole cells.8,77,108 It is effective (90% within 2 min), only 52% decolorization was
also easier to implement the use of isolated enzymes in an achieved for the real effluent (undefined period of time).152
industrial context since their development as biocatalysts can Research conducted using, again, horseradish peroxidase for
be faster than whole cells. Further, recombinant enzymes and/ the decolorization of two AQ dyes, showed that the structure
or their immobilization is possible to improve the perform- of the dye affected its decolorization, despite the similar
ance.108−111 On the other hand, some enzymes may require optimized operational conditions and the high decolorization
cofactors or mediators17 and may be too specific/selective to degrees achieved.122 When laccase was examined for dye
degrade multiple dyes simultaneously.108,109 decolorizing potential on several types of dyes, it was shown
3.1. OxidoreductasesPeroxidases. The enzymes that there was better affinity toward the anthraquinone
responsible for dye decolorization belong to the family of Reactive Blue 19, resulting in almost 90% decolorization
oxidoreductases (EC: 1), which catalyze oxidation and over 30 min.153 The preference of laccase toward AQ dyes
reduction reactions, finding application in various domains compared to other types was also confirmed by a different
varying from diagnostics to wastewater treatment and study, where complete decolorization of the same dye
production of chemicals or potentially biofuels.112−118 They concentration was achieved in 72 h.154 However, whereas in
have been studied extensively for dye decolorzation and the aforementioned cases laccase did not need a mediator, this
bioremediation, with much research focusing on the oxidative has not been always the case. Soares et al.103 used laccase for
action of laccases and peroxidases as well as the reductive the decolorization of Reactive Blue 19 and reported that in the
action of azoreductases (azo dye specific enzymes), with many absence of a mediator, almost no decolorization was observed,
review papers available targeting dye degradation in gener- but upon the use of mediators, decolorization was able to reach
al104,110,119−121 or focusing on azo dyes,6,38,39 but none 100% success. The different results obtained by the same
focusing specifically on anthraquinone dyes. combination of enzyme and dye allows us to understand the
Peroxidases catalyze the reduction of peroxides simulta- complexity of decolorization and the difficulty in generalizing
neously with the oxidation of various organic and inorganic results and expectations. When degradation of a mixture of 3
substrates. This “dual action” mechanism has been named azo and 1 anthraquinone dyes was examined using isolated
ping-pong bi bi mechanism due to the fact that the electrons enzyme extracts from Funalia Frogii, it was shown that
liberated by the enzyme from the reduction of peroxides are degradation of dyes in the mixture followed a pattern based
recovered through the oxidation of the main substrate, with the on the ease of structure breakdown. Once the Reactive Blue 69
aid of the intermediate enzymatic compounds.122 anthraquinone dye (easiest to degrade) was almost fully
Recently, peroxidases from white-rot fungi (WRF) have degraded, degradation of the other dyes would occur, leading
attracted interest in the general area of bioremediation,84,104 as to a time dependent decolorization and resulting to 84% color
actions such as lignin degradation and dye degradation are removal after 48 h.155
dominated by similar mechanisms around structurally similar Although enzyme use in dye degradation specifically can be
substrates.123,124 The advantage of nonspecific binding of WRF quite effective under laboratory conditions, their application to
peroxidases allows them to act on a wide range of an industrial scale has many limitations, mainly due to the
substrates.125,126 Enzymes secreted from WRF include various production cost (culture, isolation, equipment) and opera-
known peroxidases like manganese, lignin, and versatile tional cost (use/reuse, downstream processing) of the
peroxidase and a less known category of enzymes, dye enzymes.29,112,156−158 Also, some of the byproducts can inhibit
decolorizing peroxidases (DyPs). DyPs were first reported the enzymatic action,146,159,160 thus limiting potential reus-
almost 20 years ago, showing a great activity over the ability of the enzymes. Furthermore, as the pH for enzymatic
decolorization of AQ dyes,97,127 followed by lignin-like action is important, difficulties can occur when treating real
compounds.128 Although their action mechanism resembles effluents as it was shown in literature.152,161,162
that of other peroxidases, AQ dye degradation is not yet fully There are mainly three ways to address the aforementioned
mapped.129−134 The characterization of DyPs can be found in limitations from isolated enzyme use, which can be applied
recent reviews.135−138 AQ dyes used as a model system to alone or together. The “invasive” way is altering the properties
examine decolorization using DyPs include Reactive Blue 19 of the enzyme via genetic engineering. The “excluding” way is
(RB19 or RBBR) 1 2 9 , 1 3 9 − 1 4 3 and Reactive Blue 5 screening for new, better enzymes. The “external improve-
(RB5).97,130,144−151 ment” way refers to the improvements and optimization of the
3.2. Oxidoreductases in Enzyme-Based Bioremedia- process and enzyme,109,163 for example, immobilization,
tion. There are quite a few examples of isolated optimization of reactor configurations, and design of effective
G DOI: 10.1021/acs.est.9b03737
Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology Critical Review

Figure 2. Brief description of immobilization methods (adapted from refs 167, 176, and 178).

downstream processing. In order for isolated enzymes to There is not a set combination of technique, support, and
become applicable for water treatment of industrial potential, enzyme, as immobilization highly depends on the targeted
they have to be immobilized. Even if enzymatic action has been reaction/process, the given conditions and the possible
improved via genetic engineering and screening, the interactions. Thus, for achieving a golden mean for the factors
production cost of “optimized enzyme” can be inhibitory for affecting its efficiency, approaches ranging from trial and error
application at industrial scale, without the option of reuse. to predictive designing of the targeted system are used.179 The
Hence, even after overcoming limitations through the invasive selected combination should satisfy both catalytic (productiv-
or the exclusive way, external improvementusually via ity, stability, and specificity) and noncatalytic requirements
immobilizationneeds to be applied. In the next section, we (control of the process, separation, robustness, and need for
shed light on the applicability of immobilization, focusing on further processing).180 A suitable support for enzyme
immobilized oxidoreductases and their application for dye immobilization should fulfill requirements in regards to
removal. mechanical properties, ease of synthesis and functionalization,
3.3. Immobilization. 3.3.1. General Information. The environmental friendliness, leaching prevention, toxicity,
stability of enzymes under “un-natural” conditions, enzyme loading capacity, low steric hindrance effects but high
production cost, and the need for downstream separation are availability of reactive groups.167,172,181 Also, it should fulfill
crucial for their industrial potential.166 Immobilization is an requirements about the microenvironment and mass transfer
established technique with the aim to facilitate separation and during enzymatic actions.169,182 What is aimed is to create a
reuse of enzymes as well as maintaining the most active stable and active biocatalyst that can be applied on an
conformation167,168 by “securely attaching” the enzyme onto industrial level,172,183 at an acceptable total cost.184,185
usually solid supports that offer molecular rigidity.169 Major Immobilized enzymes can be cost-effective if the cost of
advantages of immobilization include the ability for enzyme immobilization (total cost of every step of the process) is lower
reuse and the simplified downstream processing, as well as the than the cost of separation of soluble enzymes from the
enhancement of operational stability of enzymes and the product (and of further product purification if needed), in
option for cascade reactions; ultimately offering cost-effective addition to the cost of using fresh enzyme in every “catalytic
solutions.166,170 However, distinct disadvantages include the round”.186
rigorous design of the system, which has to be tailored to the 3.3.3. Anthraquinone Dye Decolorization by Immobilized
enzyme and application in mind, minimizing mass transfer Oxidoreductases. There are some reviews available on water
limitations between the enzyme and the substrates and the decontamination by immobilized enzymes, focusing either on
possibilities for enzyme deactivation.166,170 It should be noted specific pollutants or on specific enzymatic subcategories of
that immobilization does not necessarily aim to make the oxidoreductases.119,120,125,177,187,188 Generally, the operational
enzyme perform better when it is applied in its optimal stability of enzymes is enhanced by immobilization but the
operational conditions, but to maintain or ideally increase its activity of the enzyme is reduced, mainly due to the disruption
performance when the conditions are not optimal.171 of the active conformation, difficulty of the substrate to reach
3.3.2. Methods and Supports for Immobilization. There the enzyme, or deactivation of the enzyme due to
are many extensive reviews on methods and supports used for accumulation of toxic substances. The decontamination
enzyme immobilization172,173 focusing on a specific support efficiency highly depends on the combination of enzyme and
(e.g. ref 174), immobilization method (e.g., refs 175 and 176), support used, as well as on the system investigated (dye
or enzyme (e.g. ref 177). Among many available ways of structure and concentration, presence of other substances).
immobilization as shown in Figure 2, the most widely preferred Hence, enzymatic performance is typically investigated by
ones are adsorption, entrapment or encapsulation, and varying some of those parameters, as well as operational
covalent bonding.174 The typical enzyme content in the final parameters like temperature and pH (Table 3). From Table 3
product is usually less than 10% by weight, remaining being the it is clear that immobilization enhances the stability and it
support.176 facilitates enzyme reusability. Some examples include increased
H DOI: 10.1021/acs.est.9b03737
Environ. Sci. Technol. XXXX, XXX, XXX−XXX
 Table 3. Decolorization of Anthraquinone Dyes by Immobilized Oxidoreductases
substrate/dye con- throughput
enzyme method/support centration (mg/L) (mg/L/h)a comments ref
horseradish peroxidase covalent binding/ Reactive Blue 19/40 34 85% decolorization within 1h, decent reuse potential (7 times, 20% activity left by seventh), increased storage stability. 192
methacrylated Increased T stability
polysulfones
dye-decolorizing peroxidase adsorption/im- Reactive Blue 1800 100% removal within 5 min, immobilization support affects enzymatic activity hence decolorization, pH affects decolorization 190
mobilized FSM- 19/150 and enzyme leaching from support
16 and AlSBA-
15
dye-decolorizing peroxidase adsorption/meso Reactive Blue 3600 high adsorption yield but low residual activity, pH affects decolorization and enzyme leaching from support, very good reuse 191
cellular foam 19/150 potential (20 cycles) in pH4
polyphenol oxidase adsorption/celite Reactive Blue 4/50 43.5−87 immobilized enzyme shows better results than free, pH affects decolorization, immobilized enzyme treatment leads to reduced 189
545 −100 TOC post-treatment compared to free enzyme
horseradish peroxidase cross-linked en- Acid Violet 109/30 36−46 high decolorization degree (70−90%), decolorization experiments in batch/packed bed reactors (packed bed performs better), 193
zyme aggregates enhanced pH stability and higher dye and peroxide concentration tolerance, reduced toxicity after enzymatic treatment of dye
Environmental Science & Technology

solution
horseradish peroxidase adsorption/acti- Acid Violet 109/40 52.2 adsorption conditions examined, good decolorization (87% after 40 min), improved pH stability during decolorization, better 19
vated kaolin tolerance of high dye concentration, considerably lower substrate affinity but not very lower initial rate, high (7) reuse cycles
(35% activity left)
hematin (not enzyme, but of covalent adsorp- Alizarin red/200 97.4 for Hematin40 decolorization is based on action of hematin as peroxidase active site, comparison with immobilized horseradish peroxidase is 194
structure resembling peroxi- tion/chitosan for Horseradish taking place, about 50% efficiency on 1st cycle, after 6 cycles efficiency drops to 34%, identification of possible reasons for
dases) and horseradish perox- and APTS Peroxidase decreased activity, comparison between 2 dyes (anthraquinone and azo)
idase
laccase adsorption/mag- Reactive Blue 18−54 for Reactive decolorization experiments for the 2 dyes were under different conditions, very high loading achieved (1g enzyme/g support), 195
netic carbon 19/100- 300Acid Blue 19, 18-360 almost 80% decolorization within 1st hour, 90% within 5 h, excellent reusability (activity almost intact after 6 cycles),

I
capsules Green 25/up to for Acid Green 25 acknowledgment of dye adsorption on support, good storage stability (10% activity loss after 2 months)
2000
horseradish peroxidase adsorption/chito- Reactive Blue 17.5 use of glutaraldehyde for added functionalization, about 70% decolorization regardless of the dye concentration (25 mg/L and 196
san 19/100 100 mg/L examined), biocatalysts were reused for up to 7 cycles with more than 60% residual activity, main body of the work
was done for an azo dye
laccase adsorption/silan- Disperse Blue 3, 0.9 for Disperse Blue examination of various dye structures, decolorization of 80−90% of AQ dyes within 5 h for Reactive Blue 19 and 17 h for 197
ized silica beads Reactive Blue 3 and 3.2 for Re- Disperse Blue 3, reduction of toxicity further to free enzyme, free laccase leads to a throughput of about 34 for both AQ dyes,
19/∼20 active Blue 19
a
Arbitrary value calculated to show the maximum removal capacity of any given method within an hour, based on the best results presented in each reference. In cases where the time scale of the
decolorization is within a few minutes, the assumption of decolorization ability over continuous use for 1 h is made.
Critical Review

Environ. Sci. Technol. XXXX, XXX, XXX−XXX

DOI: 10.1021/acs.est.9b03737
Environmental Science & Technology Critical Review

Table 4. Final Identified Fragments during Degradation of Reactive Blue 19 via Various Methodsa
method final identified fragments identification methodsb and relevant comments ref
thermal-pressure hydrolysis acetic acid, oxalic acid use of GC-MS, comprehensive table with identified fragments and 212
their time occurrence during degradation
ozonation phthalic acid, carbon dioxide, water use of UV−vis, FTIR, LC-MS and GC-MS, comprehensive discussion 69
around proposed degradation pathway
photodegradation on nano-TiO2 3,6-dihydroxyphthalic acid, ethyl-sulfate- use of UPLC-MS 99
in presence of H2O2 phenyl-sulfone
Electrochemical degradation in 1,3 indanone, phthalide, phthalic anhydride, use of GC-MS, potential for further breakdown upon increased contact 213
presence of chloride phthalimide, benzoic acid time, no chlorinated byproducts detected
biological (bacterial flora benzenesulfonic acid, hexan-1-amine, 3,6- use of UV−vis, FTIR and LC-TOF-MS, comparative discussion on 215
DDMY2) dihydroxyphthalic acid, degradation products with literature
enzymatic (immobilized laccase) 5-sodium-benzenesulfonyl-ethanone, opened use of LC-MS, 2 intermediates and 2 final products identified, no 95
anthraquinone ring fragment observations for backward reactions
a
Please refer to SI Table S2 for the chemical structures of the identified fragments. bH/UPL/C: high/ultra pressure liquid chromatography, GC:
gas chromatography, MS: mass spectrometry, TLC: thin layer chromatography, UV−vis: ultra violet-visible spectrophotometry, FTIR: Fourier
transformation infra-red spectroscopy, TOF: time of flight.

performance of the immobilized enzyme compared to free (7% It should be noted that researchers have developed artificial
higher performance was noted when polyphenol oxidase was enzyme mimicsnamely nanozymesthat mimic specific
immobilized).189 It is also worth mentioning that both the enzymatic actions. Their action is based on a chemically
available examples of immobilized DyP showed great perform- synthesized active site that is very similar to the one of the
ance of the composite, with high reusability and stabili- targeted natural enzyme, for example, enzymes that contain
ty190,191and very high throughput, comparable to the values metals or metal oxides.203 So far, there have been examples
observed for AOPs. mimicking the action of peroxidases204−208 but only two
Immobilization of laccase on silanized alumina pellets has reports of peroxidase mimetic nanozymes for dye decoloriza-
also shown to reduce the inhibitory effects of components tion exist (for an azo, a xanthene209 and a thiazine dye206), and
usually present in industrial dye-baths such as wetting, soaping none on AQ dyes. It has been reported that nanozymes are
or sequestering agents.198 When decolorization of two easy to use over natural enzymes due to lower cost, easier
structurally similar AQ dyes (Reactive Blue 19 and Acid Blue large-scale production, higher durability, and stability. How-
25) was examined using immobilized laccase in epoxy activated ever, nanozymes applications are very limited due to the lack of
Sepabeads, researchers got greatly different results (almost 0% selectivity and substrate recognition, as well as lower activity
for Reactive Blue 19 and about 40% for Acid Blue 25), compared to natural enzymes.203,210,211
indicating that structure of dyeeven if of the same general
4. ON THE DEGRADATION PATHWAY OF
typehas an important role.199 Presence of a mediator in the
ANTHRAQUINONE DYES
examined systems increased the decolorization of Reactive
Blue 19 from 0% to about 30%, whereas the effect on As it has been already mentioned, it is quite difficult to confirm
decolorization of Acid Blue 25 was negligible. Following the the degradation pathway of a dye, mainly due to the possibility
same argument, researchers195 studying the decolorization of of spontaneous oxidations and our inability to quickly isolate
two AQ dyes (Reactive Blue 19 and Acid green 25) using again fragments. Nevertheless, there are few examples where, based
laccase but immobilized on magnetic carbon nanoparticles, on the initial dye structure, some identified fragments, and
achieved highly positive results (more than 80% decolorization those identified as final products, researchers have been able to
efficiency, good reusability potential, stability) for both dyes. propose a degradation pathway for model anthraquinone dyes.
Li et al.41 discussed the degradation pathway of AQ dyes by
This shows that the immobilization support also has a great
biological methods only, and Siddique et al.42 have collected
effect on decolorization efficiency, since using the same
examples of Reactive Blue 19 degradation by different methods
enzyme (laccase) acting on the same dye (Reactive Blue 19)
but without touching on the degradation pathway. A
yield different results when different supports were examined. comprehensive discussion around the degradation pathway of
In terms of methods and matrixes used for immobilization, AQ dyes by various methods is missing. In Table 4 one can see
looking at Table 3 one can see that adsorption on inorganic a list of research examples studying the degradation of a model
matricesusually silicatesis highly favored over other AQ dye (Reactive Blue 19, structure shown in Figure 1) using
methods and matrices combinations. This is possibly due to four different methods and proposing degradation pathways
the extensive research available on those materials174,200 and based on the identified fragments. The chemical structure of
their wide industrial presence,201 thus allowing easier industrial fragments is shown in SI Table S2. By comparing the identified
implementation of the immobilized biocatalyst. An issue fragments, it is evident that each method can lead to different
usually faced with immobilization supports is adsorption of results. Degradation of Reactive Blue 19 using thermal-
dye on the actual support instead of decolorization due to pressure hydrolysis212 was the only case where the identified
enzymatic action, which might lead to false results if it is not fragments were not of aromatic structure (small carboxylic
accounted for. Indeed, another study showed an initial step of acids were detected). In every other research example studied
dye adsorption onto the carrier (silica beads), followed by the identified fragments were considerably larger, especially for
decolorization by the enzyme (laccase), allowing for fresh the examination of degradation via enzymatic action. In this
substrate to be used.202 Also, another issue is the adsorption of case, the proposed degradation pathway did not progress much
degradation products,192,202 which might lead to enzyme further than the deamination of the AQ structure and the ring’s
deactivation. rupture, as well as the deamination and desulfonation of the
J DOI: 10.1021/acs.est.9b03737
Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology Critical Review

auxiliary structure.95 The AOPs, chemical, and biological pathway was more difficult. Research examples either did not
methods used for Reactive Blue 19 degradation (Table 4) identify further degradation past the original rupture from the
resulted in phthalic acid and its derivatives (as identified dye structure,93,95,214 or identified big fragments and even
products), with expectations for further degradation to lower polymerized by-products.97
by-products, which, however, were not confirmed by the
analytical methods used. What can be assumed based on the 5. CONCLUSIONS AND FUTURE CHALLENGES
degradation pathways proposed is that the AQ structure is
eventually broken down to simpler aromatic derivatives, Degradation of anthraquinone dyes poses an environmental
allowing for easier manipulation afterward. However, this is problem that has been ignored due to their smaller volume
not the case for the enzymatic methods, where degradation compared to azo dyes. Existing research shows that
seems to be a more lengthy procedure.95,97 Based on that, anthraquinone dyes can be sturdier when it comes to
absence of further degradation might be attributed to loss of decolorization due to their structure. Furthermore, their
enzymatic activity over prolonged time of use and/or exposure removal is highly specific to the dye and the method used,
to the reaction mixture. hence it can be quite difficult to choose a single solution
A first comparison between the identified as final fragments among the available conventional methods. Comparing the
across different methods used, show that electrochemical existing physical/chemical methods with the biological ones, it
methods lead to generally smaller fragments, with higher is clear that while every method has their distinctive advantages
mineralization potential compared to biological methods. As it and disadvantages, advanced oxidation processes and isolated
can be seen in SI Table S3, of the most common fragments enzymes stand out it terms of rate of degradation. In this
identified were phenol, phthalic acid, their derivatives, and low review we focused mainly on enzymatic decolorization of
molecular hydrocarbons. These fragments were mainly anthraquinone dyes, and showed that it has gone a long way
produced by the cleavage and subsequent degradation of the but still needs extensive research before industrial implemen-
anthraquinone ring through various steps. In some cases it was tation. Immobilization can help create powerful biocatalysts
noted that different dyes (Reactive Blue 19213 and Reactive that can be both environmentally friendly and industrially
Blue 4214) treated with the same method led to the same applicable. Currently, immobilized oxidoreductases can show
degradation products (as derived by the AQ ring). This activity comparable to free enzyme when it comes to smaller
observation could be an indication of some control over the substrates, but they sometimes suffer when it comes to dyes of
end products if a specific method is applied. However, looking larger sizes causing inaccessibility to the enzyme inside a
more in depth into a specific method, ozonation, for the porous support.
degradation of the same dye, Reactive Blue 19, results from A main challenge we identified during literature review, was
different researchers showed slightly different fragments. the lack of consistency in approaches used in various research
Identified fragments ranged from a mixture of phenol, acetic examples. This makes the comparison of the ability of
acid, and propandioic acid68 to a mixture of phenol, acetic acid, suggested methods to treat anthraquinone dyes (or pollutants
and oxalic acid,92 and to a mixture of phthalic acid and in general) quite tricky, as we showed that different conditions
unspecified carboxylic acids.69 Upon examination of the using the same method and the same dye, or using a method
conditions used, Fanchiang et al.69 used slightly higher ozone under the same conditions for different dyes, can lead to
feed rate compared to the other studies, which might have different results. A way to circumnavigate that would be to set
been responsible for the higher mineralization potential. a benchmark set of parameters per available method, making
Based on the few research examples showing a degradation comparison across methods easier. For example, setting a
pathway and fragments of AQ dyes treated using biological standard dye concentration for experiments, a fixed ratio of dye
methods as shown in the lower end of SI Table S3, we can see to enzyme, nanomaterial, oxidant, or energy used. It could also
that the identified fragments are not different from those mean setting an arbitrary unit that allows comparison of results
shown for chemical and AOP methods. However, the on a common basis, such as productivity. These approaches
identified fragments upon degradation using enzymes were would make comparison across methods easier, showing the
substantially larger compared to those identified by other strong and weak points of each method in a more comparable
methods, as shown in SI Table S3. This observation shows that manner.
although isolated enzymes can potentially be very efficient in Another challenge that became obvious across examined
decolorization, when it comes to dye breakdown and literature is the unrealistic approaches or idealistic systems
mineralization, the requirements for degradation are higher usually used. Although some level of control is needed to
than what is currently available from enzymes. That being said, define and characterize a dye degradation system, a more
combination of enzymes, as is the case in biological systems, realistic approach toward system development should be
might be a potential avenue to explore. adopted, incorporating research on real effluents or mixtures of
It should be noted that in all the research examples dyes. Although it is mainly the mixtures of dyes that are
examined, the suggested degradation pathways based on some encountered in the actual effluents from textile industry, there
identified fragments and the discussion around the specific are very limited reports examining such mixtures of dyes216−218
breakdown mechanisms, show that the initial fragments could and even fewer that include an anthraquinone dye within the
not be identified by the analytical methods used, but were dye mixture.155,219,220 The absence of extended research on
speculated retrospectively. This shows the lack of control over dye mixtures (only one review paper on dye mixture
the dye degradation reactions and the existence of spontaneous decolorization was found221) highlights the need to investigate
reactions that can lead to the same lower fragments via the ability of existing technologies to treat a more realistic form
multiple paths.69,74 With regards to the auxiliary groups of effluent, that being dye mixtures, or solutions containing
present on the dye structures (such as amino-groups, sulfone- other auxiliaries used in the textile industry. Focusing on
groups, halogen-groups), mapping down their degradation decolorization of anthraquinone dyes by immobilized oxidor-
K DOI: 10.1021/acs.est.9b03737
Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology Critical Review

eductases, the lack of information on the performance of such

systems on real (or realistic) effluents was also noted. This can
■ AUTHOR INFORMATION
Corresponding Author
be attributed to the high sensitivity of biocatalysts toward *E-mail: s.patwardhan@sheffield.ac.uk.
operational conditions, but if such methods are to be applied
ORCID
industrially, then a more realistic approach ought to be
explored. Siddharth V. Patwardhan: 0000-0002-4958-8840
Another identified challenge with respect to dye degradation Notes
The authors declare no competing financial interest.

■
is the relatively limited available information on the
degradation pathways. Research examples proposing a
degradation pathway, reached their conclusions based on a ACKNOWLEDGMENTS
few identified fragments via analytical techniques, assuming We thank the Department of Chemical and Biological
previous and further reaction steps. The existence of free Engineering of the University of Sheffield for financial support
radicals and the recalcitrant structure of anthraquinone dyes and Dr. Tuck Seng Wong for valuable discussions and
make oxidation, hence degradation, difficult to predict. In a few suggestions.
examples in literature, there were more than one pathways
proposed for the degradation of an anthraquinone dye,
indicating the need for better control over the process of
■
1
ADDITIONAL NOTE
about 500 papers on anthraquinone dye degradation
degradation. This could be achieved through more careful compared to about 8,700 for azo dyes, according to Web of
control of the reaction and the development of techniques to Knowledge search engine over the period of 1975 to 2019.
pause the reaction before progressing further, or techniques to
slow it down enough so that samples during the initial stages
can be withdrawn.
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