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OPEN Toxic heavy metal ions

contamination in water
and their sustainable reduction
by eco‑friendly methods:
isotherms, thermodynamics
and kinetics study
Veer Singh 1, Ghufran Ahmed 1, Sonali Vedika 1, Pinki Kumar 1, Sanjay K. Chaturvedi 2,
Sachchida Nand Rai 3, Emanuel Vamanu 4* & Ashish Kumar 1*

Heavy metal ions can be introduced into the water through several point and non-point sources
including leather industry, coal mining, agriculture activity and domestic waste. Regrettably, these
toxic heavy metals may pose a threat to both humans and animals, particularly when they infiltrate
water and soil. Heavy metal poisoning can lead to many health complications, such as liver and renal
dysfunction, dermatological difficulties, and potentially even malignancies. To mitigate the risk
of heavy metal ion exposure to humans and animals, it is imperative to extract them from places
that have been polluted. Several conventional methods such as ion exchange, reverse osmosis,
ultrafiltration, membrane filtration and chemical precipitation have been used for the removal
of heavy metal ions. However, these methods have high operation costs and generate secondary
pollutants during water treatment. Biosorption is an alternative approach to eliminating heavy metals
from water that involves employing eco-friendly and cost-effective biomass. This review is focused on
the heavy metal ions contamination in the water, biosorption methods for heavy metal removal and
mathematical modeling to explain the behaviour of heavy metal adsorption. This review can be helpful
to the researchers to design wastewater treatment plants for sustainable wastewater treatment.

Keywords Heavy metals, Water contamination, Biosorption, Eco-friendly biosorbent

The issue of heavy metal pollution is increasingly pervasive on a global scale. Heavy metals are naturally occurring
elements that are present in the earth’s crust. However, excessive amounts of heavy metals can pose a significant
­risk1. Some compounds, such as heavy metals, are resistant to decomposition and can accumulate in people and
animals when they enter the food c­ hain2. Metals can enter the environment through natural means or human
actions including waste disposal, industrial manufacturing, and m ­ ining3. Mining poses a significant danger by
potentially displacing and spreading heavy metals to surrounding regions during flooding or ­windstorms4. It
is important to acknowledge and address environmental hazards to safeguard the well-being of both humans
­ orld5.
and the natural w
Heavy metal-induced water pollution can have detrimental impacts on human h ­ ealth6,7. These metals can
enter our systems via polluted water and f­ ood8. They can bind with organic groups, resulting in the formation
of detrimental chemicals that can induce damaging effects on our c­ ells9. Multiple techniques exist for extract-
ing these metals from polluted water; however, they are accompanied by drawbacks such as the production of

1
Department of Biochemistry, ICMR-Rajendra Memorial Research Institute of Medical Sciences, Patna 800007,
India. 2Department of Microbiology, ICMR-Rajendra Memorial Research Institute of Medical Sciences,
Patna 800007, India. 3Centre of Experimental Medicine and Surgery, Institute of Medical Sciences, Banaras Hindu
University, Varanasi 221005, India. 4Faculty of Biotechnology, University of Agricultural Sciences and Veterinary
Medicine of Bucharest, 011464 Bucharest, Romania. *email: email@emanuelvamanu.ro; ashish2k8@gmail.com

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additional pollutants or exorbitant e­ xpenses10–13. Hence, it is crucial to devise appropriate biological techniques
for the remediation of heavy ­metals14.
Biosorption is an efficient and eco-friendly technology created to remove heavy metal ions from polluted
water, offering both cost-effectiveness and environmental ­benefits15. Biosorption methods can replace conven-
tional methods and can be considered as suitable alternative to existing physiochemical methods due to the
eco-friendly and cost-effective nature of biosorption techniques. The biosorption method relies on the utiliza-
tion of various types of raw materials derived from agro-waste, plant residue, and algal and microbial ­biomass16.
Biosorption is a metabolically independent method that does not require the participation of living organisms,
making it a more straightforward and user-friendly t­ echnology17. A diverse range of materials, such as rice and
wheat husks, activated carbon, agricultural waste, bananas and citrus peels, and green-synthesized nanoparticles,
­ iosorption18–20. It is important to emphasise that these materials have a distinct surface
can be effectively used for b
character that greatly enhances their capacity to absorb the heavy metal ions found in the w ­ ater21,22.
This review is focused on the heavy metals contamination sources including point and non-point sources of
heavy metal ions contamination in the water. This review also provides detailed information on the biosorption
method for heavy metal removal. In addition, the behaviour of biosorption is also described by mathematical
models including isotherms, thermodynamics and kinetics.

Water quality assessment

Water quality criteria
A thorough examination of a wide range of variables that are well-known and recognized as key indicators to
­ ater23. The World Health Organization recommends the maximum
accurately characterize the overall quality of w
allowable limits for water physicochemical parameters, as shown in Table 1.
This comprehensive understanding and assessment of the various variables are based on the findings and
conclusions drawn from a meticulous study conducted by several ­researchers24. To ensure the utmost safety and
well-being of users who rely on water, whether it is for consumption, recreational activities, or any other specific
purpose, several essential water quality criteria are implemented and enforced. These criteria are meticulously
designed to regulate and control the maximum allowable concentration level of specific substances within a given
medium, be it water, sediment, or biota. The primary objective behind these criteria is to prevent and eliminate
any potential risks or harmful effects that may arise from exposure to excessive levels of such substances. It is
important to note that these water quality criteria are particularly crucial and significant when the medium,
such as water, sediment, or biota, is continuously utilized or relied upon for a specific purpose. This emphasis on
continuous usage further highlights the necessity and importance of establishing and adhering to these criteria
to ensure long-term safety and sustainability.
It is imperative to acknowledge and recognize the multifaceted nature and complexity of these physicochemi-
cal parameters, as they collectively play a pivotal role in determining and shaping the overall quality and charac-
teristics of water. Their interconnected and interdependent nature necessitates a thorough understanding of each
parameter’s influence and impact on water quality. Moreover, the presence and concentration of heavy metals
in water are of particular concern and importance due to their potential toxicity and detrimental effects on both
human health and the e­ nvironment25. The establishment and enforcement of water quality criteria, alongside
regular monitoring and assessment of heavy metals concentration, are crucial in safeguarding and preserving the
integrity and safety of water r­ esources26. The water quality criteria are vital components of ensuring the safety,
sustainability, and overall well-being of users who rely on water for various purposes. These criteria are meticu-
lously formulated based on a detailed understanding and examination of numerous variables that accurately
characterize water quality. The comprehensive assessment of physicochemical parameters, including dissolved
oxygen levels, pH, temperature, conductivity, BOD, COD, TDS, minerals, and heavy metals concentration, is
essential in maintaining and protecting the quality and integrity of water r­ esources27.

Parameters Permissible limits

pH 6–9
Temperature 25
Total solids (mg/l) 1500
Nitrate (mg/l) 50
Ammonia (mg/l) 1.5
Ni (mg/l) 0.07
Zn (mg/l) 0.05
Cd (mg/l) 0.003
Pb (mg/l) 0.01
Ti (mg/l) 0.05
Cr (mg/l) 0.05
As (mg/l) 0.01

Table 1.  The assessment of water quality parameters and their corresponding allowable thresholds in potable
water sources (source: https://​www.​who.​int/​water_​sanit​ation_​health/​dwq/​fullt​ext.​pdf).

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Following the comprehensive and exhaustive evaluation and analysis of the ambient water quality concern-
ing the implementation and execution of appropriate and effective measures and actions to adequately and
proficiently manage and control pollution for all types of discharges, including those occurring in the upstream
sections of the water b­ odies28. It is significant to acknowledge and recognize that this particular mechanism and
approach also serves and functions as a means and tool to facilitate and support the growth, development, and
establishment of various industries, thereby emphasizing and emphasizing the crucial and pivotal significance
and role it plays in the overall and comprehensive framework and structure of environmental management. It is
imperative and essential to explicitly state and specify that under no circumstances and situations are industries
permitted or authorized to release or discharge any form or type of waste or effluent materials into the river
­sections28,29.

Water quality assessment and management

There is a problem with water quality around the world. The preservation of public health, food security, bio-
diversity, and additional ecosystem services are progressively endangered by the intensifying and escalating
pollution of fresh water in both developed and developing n ­ ations30–33. A noteworthy association exists between
pollution and economic advancement, with population growth, agricultural expansion, industrial expansion,
and energy production all contributing to the discharge of untreated or uncontrolled wastewater into surface
and groundwater bodies. Despite recent preliminary evaluations of water quality worldwide, the extent of the
predicament remains u ­ ncertain34. Water quality needs to be protected and improved effectively and efficiently
with better information about the issues involved. Government and private agencies are working on water qual-
ity assessment and m­ anagement35.

1. The development and implementation of a comprehensive water resources plan, policy formulation, coor-
dination, and guidance.
2. Irrigation, flood control, and multi-purpose projects need to be closely monitored, supervised, inspected,
cleaned, and monitored for their effectiveness.
3. Groundwater development is the process of developing groundwater resources, establishing utilizable
resources, and formulating policies for their exploitation, along with the supervision of state-level ground-
water development activities and the support that is provided to them.
4. The development of a comprehensive perspective regarding the water resources of a nation and the assess-
ment of the water balance across various basins and sub-basins are key considerations in the evaluation of
inter-basin transfer feasibility.

The primary initiatives that are currently being undertaken involve a comprehensive investigation into the
management of groundwater, both at macro and micro levels. These measures play a crucial role in ensuring the
sustainable management of groundwater resources. It is of paramount importance to prioritize these initiatives
to guarantee the long-term viability of groundwater ­resources36. Furthermore, the Board, in collaboration with
concerned state government agencies, conducts periodic evaluations of replenishable groundwater resources in
the country. This collaborative approach ensures a comprehensive and informed understanding of the current
state of groundwater r­ esources37.
The Central Pollution Control Board (CPCB) of India and the Environmental Protection Agency (USA) are
authoritative bodies, that exercise their oversight over the numerous state boards by setting emission standards
and establishing ambient ­standards38. These bodies play a crucial role in mitigating the adverse effects of pol-
lution by conducting nationwide surveys to evaluate the existing state of pollution. To achieve this goal, the
Environmental Protection Agency has implemented two comprehensive monitoring programs for inland water
quality. Through these programs, a network of 480 measurement stations such as tanneries, chemical plants,
textile mills and distilleries has been established across the primary river basins in the c­ ountry39. These measure-
ment stations serve as crucial points of data collection and analysis, enabling a comprehensive understanding
of water q­ uality40–43.
Moreover, it is essential to recognize the significance of the field of International Environmental Law (IEL)
in safeguarding our planet’s environment, which is a shared resource. At AIDA, it is necessary to actively engage
with this field daily, utilizing its principles and frameworks to support individuals and communities in their
efforts to protect the environment. Preserving the environment is closely intertwined with the protection of
foundational human rights. Therefore, our work in the field of IEL not only seeks to safeguard the environment
but also aims to uphold and promote these fundamental rights that are inextricably linked to the environment.
Through our commitment to the principles and practices of IEL, to strive to contribute to a sustainable and
equitable future for a­ ll44.

Occurrence of heavy metals in the environment

For each 10% increase in the usage of pesticides, this phenomenon can be observed. Many investigations have
been carried out on the subject of wastewater and its impact on human health. A study examining the influ-
ence of irrigation water quality on human health discovered higher rates of illness in the villages that employed
wastewater for irrigation in comparison to the control v­ illage45. A study conducted by B­ artone46 observed that
water pollution serves as both a cause and an effect in the connections between agriculture and human ­health46.
The contamination of heavy metals in water is also influenced by natural factors such as volcanic activity, metal
corrosion, metal evaporation from soil and water, soil erosion, and geological ­weathering47. In comparison to
the global average level, the concentration of trace elements (> 0.05 mg/L hexavalent chromium and > 0.01 mg/L
arsenic) in water quality on the Child Loess Plateau is found to be higher. The quality of river water, when poor,

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is associated with high levels of sodium and salinity hazards. In the case of surface water bodies, a wide range
of pollution sources, including both point and non-point sources, have a significant impact on water q ­ uality48.

Point source
Point sources of toxic heavy metal ions contamination in water are defined as particular types of pollution that
cause high amounts of heavy metal ions contamination in water. It is important to release contaminants from
the sources and directly inject them into the nearest water bodies or environmental s­ ources49. Industrial units
situated on the banks of the rivers serve as major heavy metal contamination sources in the water. The point
sources of heavy metals contamination are described into two main c­ ategories49.

Industrial sources
Industrial wastewater plays a major role in the heavy metals contamination in the water. Industrial wastewater
contains several hazardous chemicals including heavy metal ions that are directly or indirectly released into the
environment. These heavy metal ions accumulate in the food chain and affect human beings including terrestrial
and aquatic ­animals50. Based on a survey by the central pollution control board, 260 million litres of industrial
wastewater daily released into the Ganga River in ­India51. According to a report released by the Ministry of
Ecology and Environment of China, the nation released a total of 25.02 billion tons of industrial wastewater in
the year 2019, which is equivalent to approximately 68.5 billion litres per d­ ay52. As stated in a report published
by the World Bank, industries operating in Bangladesh discharge an approximate amount of 1.5 million cubic
meters (equivalent to 1.5 billion litres) of untreated or partially treated wastewater into rivers and other water
bodies ­daily53. There are several industries which cause heavy metal contamination in water paper, sugar, textiles,
steel, battery, leather, chemicals, pharmaceuticals, metal works, and food industries discharge their wastewater
into the ­environment54,55.

Domestic sources of pollution

The domestic source of water contamination is the second major part of a point source. Domestic sources also
depend on the collection of waste and their ­dumping56,57. Domestic pollution can be reduced if wastewater is
properly treated before discharging into the e­ nvironment58. The main components of domestic sources are
microbes and organic matter. Domestic waste also contains a large amount of metals and salt including chlorides,
detergents, oils and grease. The Yamuna River in India is highly polluted by domestic sources, about 85% of the
other sources of c­ ontamination59. In China, the sources of water contamination from within the country com-
prise industrial emissions, untresated domestic sewage, and agricultural overflow. Based on current information,
industrial wastewater is a significant contributor to water pollution, as more than 60% of China’s underground
water and a third of its surface water are classified as unsuitable for human use due to ­contamination60. In Bang-
ladesh, various factors like insufficient sewage treatment, industrial wastage, and agricultural runoff contribute
to water pollution. Studies suggest that a significant percentage of surface water in Bangladesh, around 85%, is
contaminated. This contamination mainly stems from domestic and industrial sources, resulting in severe health
problems for millions of people who depend on polluted water ­sources53.

Nonpoint or diffused source of pollution

The contributions originating from sources that are spread out and not concentrated in one specific location are
deemed to be of lesser significance when compared to the contributions from sources that are concentrated in
one specific location. This is primarily because these diffused sources lack specificity in terms of their charac-
teristics and attributes, and also due to their sheer abundance in n ­ umber61. When pollutants, such as harmful
substances or contaminants, are discharged and flow into a body of water, they are categorized as nonpoint
sources. These nonpoint sources can arise from various activities or areas, without a specific source or location
to attribute them to. For instance, runoff from a field has the potential to carry fertilizers and pesticides into a
nearby stream, thereby contributing to the pollution of the water. The fertilizers and pesticides used in agriculture
contain several types of metal ­compounds62. These metals cause several types of contamination in the water. The
occurrence of monsoon, which is a period characterized by heavy rainfall, plays a significant role in the process
of leaching, drainage, and surface water r­ unoff63,64. These processes serve as mechanisms or pathways through
which pollution is transported from the catchment area of a river to the river itself. It is important to note that
the pollution being transported in this manner is predominantly diffused in nature, meaning that it is made up
of various components that are not concentrated in one specific location. These components include but are not
limited to topsoil, organic matter, plant residues, nutrients, toxicants, and microorganisms. Thus, the diffused
pollution being transported in this manner encompasses a diverse range of substances and ­materials62,64.

Agricultural sources of pollution

The pollution of rivers caused by agricultural activities is linked to a variety of crucial elements, specifically,
the remnants left behind from agricultural practices, the utilization of fertilizers and pesticides, the rearing of
livestock, and the excessive accumulation of salts that arise as a consequence of the implementation of irrigation
­water65. The waste generated from agricultural activities within the watershed of the river undergoes a natural
process of decomposition, ultimately culminating in the contamination of the r­ iver66. The agricultural residues
are also part of the food chain specially utilized by bacteria and fungi. Their microorganisms break down agri-
cultural waste and these degraded waste materials are responsible for water ­contamination67.

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Landfill and dumping of toxic waste

The dumping and landfill of hazardous materials are done carefully and follow the guidelines of CPCB, India and
other environmental protection agencies. The scope of Municipal Solid Waste (MSW) is wide-ranging, encom-
passing not only household waste but also healthcare and industrial ­refuse68. However, it is concerning that there
is a lack of adequate categorization for these different types of waste, resulting in their indiscriminate deposition
into a single landfill. This indiscriminate disposal practice has significant consequences for the environment and
public health, as it leads to severe pollution of both the immediate and surrounding a­ reas69. The landfill, being
the primary location for the disposal of solid waste, plays a central role in these detrimental effects. The repercus-
sions of such disposal methods are far-reaching, with environmental pollution and the spread of diseases being
particularly severe o­ utcomes70. A specific concern in the context of open dumping sites is the transportation of
leachate, which serves as a prominent source of heavy metals in various environmental compartments such as
surface and groundwater, soil, and v­ egetation71. The heavy metals that are of particular concern in this regard
include Cd, Cr, Cu, Pb, Ni, and Zn, as they pose significant issues due to their presence and potential for h ­ arm72.
Furthermore, the impurity of wastewater has emerged as a direct cause of the contamination of food crops,
further exacerbating the overall issue at h ­ and73.

Other sources of water pollution

There exist additional origins of water contamination, including but not limited to the excessive utilization of
water for bathing and clothes washing, the practice of cattle wading, and the act of open ­defecation74. Bathing
and cloth washing in the river are among the most prevalent activities that are closely associated with water pol-
lution. In various towns and villages located along rivers, it is customary to lead cattle to the river for drinking
and ­bathing75. The impact of cattle-related activities on water quality cannot be underestimated. This is evident
through the direct release of urine, dung, and both organic and inorganic matter that gets washed off from the
cattle. These activities not only contaminate the water but also have a significant influence on its overall quality.
Moreover, when cattle wade through rivers, they disturb the sediments present at the riverbed, further exacerbat-
ing the situation by introducing additional pollutants into the w­ ater76. It is important to note that these issues are
not limited to rural areas alone. Even in urban areas, especially in slum clusters where proper sanitation facilities
are lacking, open defecation is rampant. This leads to a surplus of waste being dumped into open spaces. Conse-
quently, a considerable portion of the population resorts to using either the catchment area or the river itself as
a means of waste disposal. This further contributes to the introduction of organic pollution and pathogens into
the river water, exacerbating the already compromised q ­ uality77–80.

Biosorption of heavy metal ions

There exist various methodologies by which wastewater may be cleansed of hazardous compounds, including but
not limited to heavy metals. One such technique, referred to as biosorption, involves the utilization of expired
microbial biomass for the express purpose of extracting these aforementioned m ­ etals81. This particular approach
is further elucidated and visually demonstrated within a schematic representation, as denoted by Fig. 1.
It is widely believed that dead plant material can be used to remove heavy metals from polluted water. This
process, called biosorption, happens when the metal ions stick to the surface of the dead plant m ­ aterial82. Inter-
estingly, living plants can also do this. In living plants, the metal ions can stick to the surface of the cells or get
absorbed through the plant’s metabolic processes. This is an important process to help clean up polluted ­water83.
In the process of removing harmful heavy metal ions from water, a natural and cost-effective solution is to
use ­biosorbents83. These are materials that contain certain functional groups, such as amino, amide, imidazole,
sulfonate, and carboxyl groups, that can bond with heavy metal ions to remove them from ­water84. The effective-
ness of biosorbents depends on the variety and concentration of functional groups present on their surface, as
well as their surface s­ hape85. The rough and porous surface of biosorbents is better at binding heavy metal ions
and removing them from ­water86. Scientists use various methods such as FT-IR, SEM, EDX, NMR, and XRD to
analyze the surface shape and functional groups of biosorbents and ensure their q ­ uality87.
The process of biosorption can be influenced by numerous factors, including the utilization of specific micro-
organisms, the existence of various metal ions (including those that contend with the target metal), temperature,
and ­pH88,89. If the pH level decreases, the competition among metal ions that possess a positive charge can
­intensify89. Conversely, if the pH level rises, a greater number of surface binding sites become accessible. The
mechanism of biosorption for hexavalent chromium ions (Cr (VI)) is rather i­ ntricate90. These ions possess the
ability to adhere to groups on the surface that are positively charged and subsequently undergo a transforma-
tion into trivalent chromium ions (Cr (III)) via diverse pathways. Ordinarily, this process transpires in three
distinct ­stages91.
The initial step in the biosorption process involves the attachment of positively charged surface functional
groups to negatively charged Cr (VI) ions. The subsequent stage of the biosorption process is the reduction pro-
cess. The conversion of Cr (VI) to Cr (III) is facilitated by electron donor g­ roups17,92,93. Heavy metals biosorption
capacity of different adsorbents are mentioned in the Table 2, 3, 4 and 5.

Modelling approaches for heavy metals biosorption

Isotherm models
Isotherms, which are indispensable tools in the realm of adsorption investigations, are primarily concerned
with the meticulous and intricate analysis of the multifaceted and convoluted correlation existing between the
adsorption capacity of a given substance and the residual concentration of heavy metal ions that are inherently
present therein, all while ensuring that the temperature remains constant. In the vast field of adsorption, an
abundance of isotherm models has been introduced and extensively employed, encompassing, yet not limited to,

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Figure 1.  Several processes involved in the biosorption of heavy metal ions.

Biosorbent Biosorption capacity (mg/g) pH Temperature (°C) References

Bacillus salmalaya139SI 20.35 3 25 94

Opuntia biomass 18.5 2 20 95

Opuntia biomass 16.5 2 20 96

Dictyota dichotoma biomass 9.02 4 27 97

Table 2.  Cr (VI) biosorption capacity of different biosorbents.

Biosorbent Biosorption capacity (mg/g) pH Temperature (°C) References

98
Okara waste 14.80 6.2 70
Maize corncob 105.6 6 – 99

Sugarcane bagasse 69.06 6 – 99

100
Wheat straw biochar 69.80 5 25
Klebsiella sp. biomass 170.4 5 30 100

Alga Anabaena sphaerica biomass 111.1 5.5 25 100

Table 3.  Cd (II) biosorption capacity of different biosorbents.

the renowned Freundlich, Langmuir, Temkin, Halsey, Harkin-Jura (H-J), D-R, Redlich-Peterson, and Jovanovic
isotherm models, all of which possess their own distinct merits and drawbacks when it comes to the accurate
­ ehaviour113.
prediction of adsorption b

Langmuir isotherm
The fundamental principle that forms the basis for the Langmuir isotherm is founded on the concept of mon-
olayer adsorption, which takes place exclusively on a homogenous adsorbent. This phenomenon is achieved by
disregarding any potential surface interaction that may occur between two molecules that have been absorbed
into the adsorbent ­material114. The mathematical expression is shown in Eq. (1).

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Biosorbent Biosorption capacity (mg/g) pH Temperature (°C) References

Citrus grandis peels 2.13 3 50 101

Pea (Pisum sativum) peels 140.84 6 30 102

Gingelly oil cake (thermally activated) 105.26 – – 103

104
Meranti sawdust 34.24 6 30
Solanum melongena leaves 71.42 5 40 105

Araucaria heterophylla (green plant) biomass 9.64 5 30 106

Azadirachta indica A. Juss seeds 17.96 5.5 – 107

Table 4.  Pb (II) biosorption capacity of different biosorbents.

Biosorbent Biosorption capacity (mg/g) pH Temperature (°C) References

108
Watermelon peel waste 0.00242 5.5–7.5 –
Moringa oleifera seeds 99.9% 4 – 109

Chemically modified Chlorella vulgaris biomass 20.9 6 25 110

Chemically modified Spirulina platensis biomass 24.8 6 25 111

Dried microalga Chlamydomonas sp. biomass 53.8 4 25 112

Table 5.  As (III/V) biosorption capacity of different biosorbents.

Ce 1 Ce
= 0 + 0 (1)
qe Q b Q
Recent investigations have employed the Langmuir isotherm to explore adsorption phenomena in diverse
domains, including environmental r­ emediation115,116.

Freundlich isotherm
In stark contrast, the model known as the Freundlich adsorption isotherm delves into the intricate realm of
multilayer adsorption occurring on the surface of an adsorbent that is characterized by its heterogeneity. This
particular model serves the purpose of elucidating the underlying mechanisms involved in the process of adsorp-
tion, which is fundamentally centred around the deposition of numerous layers of molecules onto the surface
of said adsorbent. This elaborate process is achieved through a meticulous examination of the heterogeneity
displayed by the surface of the adsorbent, as well as a thorough analysis of the intricate interactions that transpire
between the absorbing molecules and the material constituting the ­adsorbent117.
The linear form of Freundlich isotherm is given in Eq. (2).
1
log qe = logkf + log Ce (2)
n
The investigation of heavy metal adsorption processes, especially in environmental remediation, has been
the focus of recent studies that have utilized the Freundlich ­isotherm115,118.

Temkin isotherm
The Temkin isotherm model offers a prognostication of equivalent binding energies for the adsorption on sur-
faces, thereby enabling a more comprehensive comprehension of the process. It has been noted that the heat
associated with adsorption rises linearly alongside the number of binding sites within a given layer. This implies
that the adsorption process is predominantly influenced by the even dissemination of binding energies, albeit only
until a certain threshold, for all molecules present within said ­layer119. The Temkin isotherm is shown in Eq. (3).
RT RT
qe = ln AT + ln Ce (3)
bT bT
Recent investigations have recently employed the Temkin isotherm, a widely utilized mathematical model,
to comprehensively examine the intricate mechanisms underlying heavy metal adsorption processes, with a
specific focus on their application in the realm of environmental remediation, as explicated by the works of
Nguyen et al.115 and Raji et al.120.

Dubinin–Radushkevich (D–R) isotherm

The D–R isotherm model posits that the adsorption process of heavy metal ions is deeply contingent upon the
­ aterial121. The linear form of
intricate and nuanced characteristics intrinsic to the structure of the adsorbent m
D–R isotherm is shown in Eq. (4).

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ln qe = ln QD - R − β ε 2 (4)
where, ­QD–R (mol/g) and ꞵ ­(mol ­kJ ) are the D–R constants, calculated from the intercept and slope of the plot
2 −2

between ln qe and ɛ2. Here, ɛ is Polanyi potential and is calculated from Eq. (5).
 
1
ε2 = RTln 1 + (5)
Ce
where R is the universal gas constant (8.341 J ­mol−1 ­K−1) and T is the temperature (K).
The relationship between the free energy of adsorption and the D–R isotherm constant is established. The
free energy signifies the amount of energy required for the adsorption of one mole of adsorbate. It is possible to
determine this value by utilizing Eq. (6).
1
E= √ (6)
−2β
where E (kJ ­mol−1) is the free energy which denotes whether the adsorption system is physical or chemical.
Recent research has employed the D–R isotherm, referred to as the Dubinin-Radushkevich isotherm, as a
valuable tool in investigating and exploring the mechanisms and processes of heavy metal adsorption, especially
in the context of environmental remediation e­ fforts115,122.

Halsey isotherm
On the contrary, the Halsey isotherm model delineates the phenomenon of multilayer adsorption occurring at
a significantly greater spatial separation from the surface of the ­adsorbent123. Equation (7) exhibits the Halsey
isotherm.
1 1
qe = In KH − ln Cqe (7)
nH nH
Recent studies have applied the Halsey isotherm to investigate heavy metal adsorption processes, especially
in environmental r­ emediation124.

Harkin–Jura (H–J) isotherm

The Harkin–Jura (H–J) isotherm model talks about how adsorbents (materials used to remove pollutants from
liquids or gases) can have multiple layers of pollutants sticking to their ­surface125. H–J isotherm is shown in
Eq. (8).
 
1 B 1
= − log Ce (8)
qe2 A A
where B and A are the model constants. B and A can be calculated from the slope and intercept of the plot
between q12 versus logCe.
e
Liosis et al.126 and Czikkely et al.127 described H–R isotherm modeling in their study for heavy metal adsorp-
tion processes, especially in environmental remediation.

Thermodynamics
To put it simply, we can study how certain materials interact with each other under different temperatures. If
we see a positive change in one property called enthalpy (∆H°), it means that the process needs more heat to
happen and we can make it happen faster by increasing the temperature. On the other hand, if we see a negative
change in another property called Gibbs free energy (∆G°), it means that the process can happen on its own
and will happen faster if we increase the ­temperature128. Thermodynamic parameters were calculated by using
Eqs. (9), (10) and (11).
G◦ = −RT ln kc (9)

CAe
kc = (10)
Ce

S◦_ H◦_
lnkc = − (11)
R RT
where, Cae (mg ­L−1) is the equilibrium concentration, ­Ce (mg ­L−1) denotes equilibrium metal ion concentration
in the bulk solution, T is the reaction temperature (K) and R is the universal constant (8.314 J ­mol−1 ­K−1). The
value of ΔSº and ΔHº were determined using the intercept and slope of the plot between ln kc versus 1/T129.

Kinetics
The comprehension of how metal ions adhere to the exterior of an adsorbent is of utmost significance in the
endeavour to formulate efficient wastewater treatment s­ ystems130. The influence exerted on this process by the
­ odels131. Presently, our investigation involves
attributes of the adsorbent can be assessed with the aid of various m

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the experimentation with diverse models to prognosticate the speed with which heavy metal ions will attach
themselves to the surface of a distinct material capable of eliminating them from wastewater.

Pseudo‑first order kinetics

The pseudo-first-order model refers to how certain substances attach to surfaces. It’s a way to understand how
quickly this attachment happens, and it’s often used in scientific research. The model is described by a mathemati-
cal equation, which helps researchers study these processes in more detail (Eq. 12).
ks
(12)
   
log qe − qt = log qe − t
2.303
where ­ks is the equilibrium rate constant and calculated from the slope log(qe − qt ) vs time (t). The qt and qe are
the adsorption capacities (mg/g) at time t and equilibrium, ­respectively132.

Pseudo‑second order kinetics

The process involves the absorption of a material onto a surface. It is believed that the rate at which this happens is
limited by the ability of the material to stick to the surface. This process includes a type of absorption that involves
a chemical ­reaction133. The mathematical expression of the pseudo-second-order model is shown in Eq. (13).
t 1 1
= ′ + t (13)
qt k 2 qe qe

h = k′ 2 qe2 (14)
Here, k′2 and h are constants that can be calculated from the plot between t/qt vs t.
Recent studies have successfully applied this model to study heavy metal adsorption ­processes120,133.

Significance of biosorption methods for heavy metal reduction

Biosorption, particularly the utilization of natural or modified biomaterials, presents a promising environmen-
tally friendly technique for the reduction of heavy metals. It presents several benefits, such as the utilization of
low-cost adsorbents, achieving high efficiency, and requiring minimal chemical resources. The pseudo-second-
order kinetics model, commonly employed in the explanation of biosorption, grants valuable insights into the
underlying mechanisms of the process. The comprehension of these mechanisms is of utmost importance to
optimize biosorption processes and develop effective treatment systems. In summary, biosorption makes a
significant contribution to the present understanding of environmentally friendly approaches to heavy metal
reduction, providing sustainable solutions for the remediation of the environment.

Conclusion and future prospects

Water contamination caused by heavy metals is a significant problem that affects both humans and animals.
Heavy metal ions can cause severe health problems such as liver and kidney damage, skin disorders, cognitive
impairment and even cancer. To prevent the harmful effects of these toxic metals, it is important to find an eco-
friendly and cost-effective method to remove heavy metal ions contamination from wastewater. Biosorption is
an eco-friendly method based on the biomass derived from plant, algal, and agricultural waste and microbes.
This method is environmentally friendly and does not require much investment. This review provides basic to
advanced knowledge to the research about heavy metal contamination and their eco-friendly removal process.

Data availability
All data generated or analysed during this study are included in this published article.

Received: 26 January 2024; Accepted: 25 March 2024

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Acknowledgements
It is our sincere gratitude to the ICMR-RMRIMS, Patna for providing the necessary facilities for this study. Dr.
Veer Singh thanks the Department of Health Research (DHR) for granting the Young Scientist Award (File
No.R.12014/37/2022-HR).

Author contributions
Veer Singh: Performed experiments, Writing original draft preparation including figures and Conceptualization.
Ghufran Ahmed, Sonali Vedika, Pinki Kumar, Sachchida Nand Rai, Sanjay K Chaturvedi: Reviewing and Editing.
Ashish Kumar and Emanuel Vamanu: Supervision, Writing- Reviewing and Editing.

Competing interests
The authors declare no competing interests.

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