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ISSN: 2766-2276
Environmental Sciences . 2023 February 27;4(2):281-296. doi: 10.37871/jbres1673.

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open access journal Mini Review

Toxic Heavy Metals Elimination from Contaminated Effluents Utilizing Various Adsorbents: Critical Mini-Review

Waleed Jadaa1* and Hamad K Mohammed2

1Previously: University of Western Ontario, London, Ontario, Canada
2Petroleum and Gas Refinery Engineering, College of Petroleum Process Engineering, Tikrit University, 34001, Iraq
*Corresponding author: Waleed Jadaa, Previously: University of Western Ontario, London, Ontario, Canada E-mail:
Received: 06 February 2023 | Accepted: 25 February 2023 | Published: 27 February 2023
How to cite this article: Jadaa W, Mohammed HK. Toxic Heavy Metals Elimination from Contaminated Effluents Utilizing Various Adsorbents: Critical Mini-Review. 2023 Feb 23; 4(2): 281-296. doi: 10.37871/jbres1673, Article ID: jbres1673
Copyright:© 2023 Jadaa W, et al. Distributed under Creative Commons CC-BY 4.0.
n ="center" class="thumbnail">Keywords
  • Toxic metals
  • Wastewater
  • Biosorption
  • Bioadsorbents
  • Toxicity

The release of massive pollutants amounts continuously because of urbanization and industrialization has caused a big ecological problem worldwide. Due to their activities, effluents of many industries: mining operations, paper/pulp, and batteries, release different heavy metals, including Copper (Cu), Lead (Pb), and Nickel (Ni), into the environment. Heavy metals are of big concern due to their high toxicity, big bioaccumulation susceptibility, and serious threat to humans and ecosystems. Compared to organic pollutants, which are highly influenced by biological and chemical degradation, heavy metals have no degradation into end products using these methods. Therefore, the removal of such metals is considered a big challenge in water purification. For metals removal, different techniques have been applied, such as precipitation, ultrafiltration, and coagulation. However, those applications have many drawbacks: low-efficiency, high consumption of reagents, and generation of toxic sludge. In contrast, adsorption considers an effective method for metal removal, owing to the method’s simplicity, economic and versatility, making it the most convenient way for toxic metals removal. Many conventional adsorbents, such as activated carbon and alumina, have been effectively employed. Nevertheless, the biggest disadvantages of using such adsorbents appear to be their price due to high activation costs and limited reusability. Biosorption has lately emerged as a method with several advantages, including minimal cost, ease of use, and great efficacy, even for trace amounts of metallic ions. This paper aims to review the relevant literature regarding the adsorption method for heavy metals removal from wastewater. Different treatments of heavy metals from wastewater and their related features are highlighted. The metals’ toxicity and hazards to health and the environment are also discussed. The application of various materials as bioadsorbents is explored, such as natural adsorbents and industrial and agricultural wastes.

Urban development, modernization, industrialization, and fast population growth are essential for economic development. However, the mentioned elements negatively impact different ecosystems: water, air, and earth [1,2]. Different water resources, including lakes and rivers, are continuously contaminated by various industries: refineries, plastics, chemicals, and batteries [3]. Such industries, as well as tanneries, electrical, and electroplating industries, typically require the addition of metallic compounds as part of their process applications [3-7]. Thus, massive amounts of metals-containing wastewater have been discharged into the environment due to the mentioned industries’ activities [1,8-12].

Heavy metals refer to a class of substances that include metals and metalloids with atomic weights ranging from about 64 to 200 [10,11,13-15]. Some examples of heavy metals and metalloids are Chromium (Cr), Copper (Cu), Nickel (Ni), Zinc (Zn), Lead (Pb), and Arsenic (As); their concentrations usually range between ng L-1 and mg L-1 in various water resources and/or industrial effluents [16-19]. In ecosystems, heavy metals’ presence is a serious problem as they are considered toxic substances [20,21]. Heavy metals are very hazardous materials even at the minimum concentrations because of their high carcinogenicity and the possibility of bioaccumulation tendency [2,4,22-27]. Heavy metals possess high solubility in the water environment, making it simple for different organisms to absorb them. Thus, as heavy metals reach the food chain, they can be accumulated in living creatures, especially humans, resulting in serious effects [9,10,25,28-31]. Heavy metals also have hazardous implications for the environment related to disposal problems because of their persistence and non-degradability nature, even for biological degradation [8,32]. Based on this, it is no surprise that heavy metals-contaminated water has gained considerable attention worldwide because of its high risk to different organisms (e.g., humans) and adverse environmental impacts [22-25,33-35].

Heavy metals are mainly released into the environment from two different sources: anthropogenic activities and natural sources. The natural sources include volcanic eruptions, soil erosion, and deteriorating rocks and minerals [1,13]. River sediment and atmospheric pollution are also considered the leading causes of heavy metal contamination in coastal lagoons [36]. The anthropogenic activities comprise industrial operations such as mining, semiconductors production, metal plating, and battery and dye manufacturing, as well as runoffs, landfills, and farming activities [13,36-38]. Therefore, heavy metals represent the most common hazardous pollutants in water and soil ecosystems [39]. Heavy metals exposure could seriously damage different organism components at even their minimal levels. Hence, it is crucial to remove heavy metal pollutants from contaminated wastewater [4]. To achieve this, US Environmental Protection Agency (US EPA) has regulated standards, frequently updated, and placed the maximum allowable concentrations, known as Maximum Contamination Level (MCL), for heavy metals in drinking water [40]. Table 1 shows the MCL values set by the US EPA compared with California and World Health Organization (WHO) regulations for the most dangerous types of heavy metals.

Table 1: MCL values established by US EPA compared to California and WHO regulation values for different heavy metals [40,41].
N Contaminant US EPA (2018) California WHO (2011)
MCL (mg L-1) Effective year MCL (mg L-1) Effective year MCL (mg L-1)
1 Aluminum (Al) 0.2 1991 0.2 1994 -
2 Antimony (Sb) 0.006 1992 0.006 - 0.02
3 Arsenic (As) 0.010 2006 0.010 2008 0.01
4 Barium (Ba) 2 1991 1 1977 0.7
5 Beryllium (Be) 0.004 1992 0.004 1994 -
6 Cadmium (Cd) 0.005 1991 0.005 1994 0.003
7 Chromium (Cr) 0.1 1991 0.05 1977 0.05
8 Copper (Cu) 0.25* - 1.3 1995 2
9 Iron (Fe) - - - - a**
10 Lead (Pb) 0.006 - - - 0.01
11 Manganese (Mn) - - - - b**
12 Mercury (Hg) 0.00003 - - - 0.006
13 Nickel (Ni) 0.20* - 0.1 1994 0.07
14 Selenium (Se) 0.05 1991 0.05 1994 0.04
15 Silver (Ag) - - - - c**
16 Thallium (Tl) 0.002 1992 0.002 1994 -
17 Zinc (Zn) 0.80* - - - -
Notes: *Refer to values that were obtained from different references [4,9,13].
**There is no suggested health-based benchmark [41].
a) Fe stains clothes and water fittings at concentrations higher than 0.3 mg L-1 [41].
b) Mn gives an unpleasant flavour in drinks as well as stains sanitation and washing equipment when its amounts are higher than 0.1 mg L-1 [41].
c) Ag concentrations may be allowed up to 0.1 mg L-1 without posing risks to public health [41].      

The configuration of the electronic state influences such pollutants’ reactivity and the ability to generate compounds, thus, increasing their metabolic and physiologic activities, resulting in a diverse range of health and environmental effects. Consequently, handling effluents containing heavy metals has become essential before releasing them into the ecosystems to prevent harmful effects like drinkable water contamination [13]. Numerous traditional techniques, including precipitation, liquid extraction, electrodialysis, and photocatalysis, have been used to eliminate metallic ions from effluents. Nevertheless, the above methods possess drawbacks, including low removal efficacy, hazardous waste generation, significant energy needs, imperfect elimination, as well as expensive disposal costs [9]. Several strategies targeted to find less expensive and even more effective ways to enhance industrial wastewater quality have been placed to get around these limitations. A significant portion of these proposed techniques is focused on the adsorption approach because adsorption seems to greatly influence metallic ions’ movement, their toxic effect, and their bioactivity in water environments. It is also a simple process to run and an inexpensive method [42,43]. To successfully eliminate pollutants, substances utilized for adsorption require a strong interaction with the desired pollutants. Different material adsorbents, including mineral and organic substances like activated carbons and zeolites, could be applied [9,44].

The purpose of this article is to review the literature on applying adsorption techniques for heavy metal elimination from effluents. Various methods for removing heavy metals and their associated characteristics are discussed. The toxins of metallic ions and their potential risks to human health and ecosystems are also addressed. The biosorption method and its features are mentioned, and the use of different materials as bioadsorbents is highlighted, like natural materials and various waste kinds such as industrial and agricultural wastes.

Effluents from different industries, such as the electrolysis, galvanizing, and dyestuff industries, include significant quantities of hazardous metal ions released into the environment. Such hazardous metal ions are not only a potential danger to public health, but they are also a threat to other living organisms [8].

Heavy metals possess the possibility of causing genotoxicity, acute and long-term toxic effects, growth and reproduction toxicity, and carcinogenicity on living organisms [19,45,46]. Heavy metals are often poisonous towards the organism communities when their concentrations are high enough. Nevertheless, some of them, like Cu, Silver (Ag), and Cadmium (Cd), are significantly toxic at trace concentrations [21]. Among them, Mercury (Hg), Lead (Pb), and Cd are considered the most dangerous heavy metals because of their significant environmental effects [47]. Pb and Cd are highly toxic to the nervous system (neurotoxic), whereas Arsenic (As), Chromium (Cr), Cu, and Zinc (Zn) are also considered poisonous elements [48]. In addition to causing physical pain, toxic metal ions can permanently harm critical physiological systems and even cause potentially fatal sickness [9,49]. Toxicity details associated with exposure to various heavy metals are presented in table 2.

Table 2: Health hazards of exposure to heavy metals with concentrations higher than their MCL values [4,9,13,50,51].
N Metal Health Hazards of Various Heavy Metals
1 Antimony Abnormally high blood cholesterol and lowering of sugar levels.
2 Arsenic Various issues with the vascular system, damaged skin, and a high chance of developing cancer.
3 Barium Blood pressure increasing.
4 Cadmium Various kidney problems, including damage and having a high possibility of causing cancer.
5 Chromium Different health issues, including headaches, vomiting, diarrhoea, nausea, and allergic dermatitis, as well as the possibility of causing cancer.
6 Copper Abdominal distress and insomnia for the short term, while related to Wilson’s disease and liver and kidney damage after long-term exposure.
7 Lead Fetal brain damage and physical or psychological growth delays for infants and children, including weaknesses in attention span and learning abilities. It is also related to different health problems, including mental disorders, kidney diseases, and neurological and vascular systems.
8 Mercury Different health problems, such as kidney diseases, including damage, rheumatoid arthritis, and vascular and neurological systems.
9 Nickel Various problems, such as coughing, chronic asthma, and dermatitis, with a high possibility of causing cancer.
10 Selenium Associated with many health issues, including numbness in the fingers and toes, losing hair and nails, and vascular problems.
11 Zinc Different health hazards like fatigue, elevated thirst, depression, and neurological symptoms.

Treating effluents contaminated by heavy metals effectively and for a reasonable cost is indeed a challenging problem for researchers and scientists [8,51]. To protect public health and achieve environmental sustainability, numerous approaches have been employed to treat heavy metals-polluted effluents [2]. Examples of such methods included membrane filtration [8,52,53], ion exchange [13,54], chemical precipitation [5,13,53,55], electrochemical treatment [8,9,13,56], chemical oxidation [8,51], coagulation [13,53], reverse osmosis [8,51,53], and chelation [57,58]. Table 3 presents various methods and their characteristics available in the relevant literature which have been used for heavy metals removal. Nevertheless, the mentioned approaches have several drawbacks, including their need for big-budget regarding facilities installation, operation, and chemicals consumption. Other drawbacks include excessive energy demands, critical operational conditions, and low-efficiency removal methods, especially for the lower concentration range of heavy metals below 100 mg L-1. In addition, they are related to the formation of hazardous biosolid waste; disposal of such sludge makes those methods more costly and environmentally unfriendly [8,9,59-64]. In detail, the main drawbacks of the chemical coagulation method for heavy metals removal include big expenses due to excessive chemicals use.

Table 3: Various methods and their benefits and drawbacks for treating heavy metals-contaminated effluents [4,8,9,72-74].
N Treatment Approach Method Benefits Method Drawbacks
1 Adsorption An effective method that can remove the majority of metals easily; high performance with efficiency reaches to up 99%. The adsorbent type governs performance; high expenses due to the cost of adsorbent (activated carbon) with no regeneration possibility.
2 Biosorption Cheap price bioadsorbents with high effectiveness and the possibility of regeneration; generation of small sludge amounts; metals recovery with no need for additional nutrients. Materials used could be saturated early; little chance of advancement in biological process applications; no chance of changing the valence level of removed metals biologically.
3 Chemical precipitation A simple method with low-cost and; ability to remove the majority of metals effectively. Generation of big sludge quantities; sludge disposal requires additional operating expenses; poor settling; method efficiency is influenced by a lower pH range and additional ions.
4 Coagulation Settling of sludge; dewatering High expensive method; consumption of big chemical amounts
5 Electrochemical method Metals removal selectivity; no requirements for chemicals consumption; achieving pure metals is highly possible Big initial and operational costs; consumption of large energy amounts; efficiency is affected by electric current and its density and pH value.
6 Ion-exchange Capable of reaching ppb concentrations; high rate of material recovery; metals removal selectivity High expensive method; fewer metal ions are eliminated; the method is quite sensitive to the pH value.
7 Membrane filtration The process requires low-pressure, limited space, and small chemicals amounts; generation of low solid waste amounts; high separation effectiveness that reaches over 95%. The method is considered complicated with small flow rates; removal efficiency is related to other metals’ existence; big initial and operational expenses.
8 Photocatalysis Heavy metals and organic contaminants are removed together; a lower number of hazardous by-products are produced. The method requires long reaction times and has only a few applications.

Meanwhile, the major drawbacks of the electrochemical technique involve significant energy usage and large installation and operating expenses. In contrast, the chemical precipitation technique releases massive sludge quantities, and their removal is considered a significant issue. Although the ion exchange method is highly effective, polymer-based resin regeneration leads to significant additional environmental contamination [65-68].

Based on much research, adsorption has become an alternative way and has shown to be a successful approach for treating metals-contaminated effluents [5,20,51,69-73]. Due to the process characteristics, including cheap, simple, and safe, adsorption is considered the most inexpensive approach for treating wastewater contaminated with heavy metals. Other major advantages of the adsorption technique include low initial process setup and operating expenses, a simple layout, and a limited need for process control functions. In metals-contaminated effluents, such metals are usually found in small amounts, and adsorption works effectively even though those metals exist at low levels, about 1 mg L-1. Adsorption is thus a practical and affordable method for removing metals from polluted wastewater [51,74,75]. For water decontamination, including heavy metals removal, several conventional materials have been successfully applied, like activated carbon and alumina, zeolites, and different resins [76-86]. The majority of those adsorbent materials have outstanding properties, including high effectiveness, polarity, and large surface area, in removing contaminants like organic dyes, different inorganic ions, and heavy metals [78,87].

Most of such adsorbents exhibit improved adsorption characteristics after being subjected to a surface modification, including chemical and/or physical processing [58,78]. However, the primary disadvantage of employing all those materials seems to be their significant price. It was reported that the average price per ton of activated carbon, activated alumina, and zeolite is approximately $1150, $750, and $450, respectively [78,88]. In addition, high activation costs and restricted capability to reuse are major barriers to the usage of such materials, especially activated carbon. However, it’s extensively employed as an adsorbent for various uses [51,78,89]. Various properties of an adsorbent activated carbon are presented in table 4.

Table 4: Different physical and chemical properties of an adsorbent activated carbon [90].
N Characteristic/Property Valuea Unit Notes
1 Moisture content 10.0 % aValues represent a commercially available activated carbon derived from bituminous coal. However, these values vary widely based on the precursor type and activation method kind.
bPoint of zero charge (PZC).
cThe mentioned value obtained from BET analysis.
dThis value was determined based on the BJH model.
eThis value obtained from N2 adsorption and desorption data.
2 Ash amount 14.9 %
3 Volatile material 14.6 %
4 pHpzc valueb 6.8 -
5 Carbon content 77.1 %
6 Hydrogen content 1.4 %
7 Nitrogen content 0.7 %
8 Surface areac 759.9 m2 g-1
9 Pore sized 55.8 Å
10 Pore volumee 0.4 cm3 g-1

Various types of Activated Carbon (AC) have been utilized for removing metallic ions from wastewater, including Powdered Activated Carbon (PAC), Clothed Activated Carbon (ACC), Granulated Activated Carbon (GAC), and Fibrous Activated Carbon (ACF) [91]. The relevant literature demonstrated that AC could effectively eliminate heavy metals due to its good adsorption capacity. However, such capacity is varied depending on AC type, its activation method, and heavy metal ion type. The adsorption capacity of AC types for different heavy metals is shown in table 5.

Table 5: Capacity of various activated carbon types for heavy metals adsorption. 
N Activated Carbon Type Adsorption Capacity, mg g-1 References
Cd (II) Cr (VI) Cu (II) Hg (II) Ni (II) Pb (II) Zn (II)
1 Commercial AC 0.7             [92]
2     154.0           [93]
3             5.9   [94]
4 Commercial AC (F400) 8.2             [95]
5     57.7           [96]
6 Clothed AC 3.8     65.0       [97]
7 Granular AC 1.4             [98]
8     6.8 38.0       9.9 [99]
9               18.0 [100]
10 Granular AC (F400)   145.0           [101]
11   8.0             [102]
12 Granular AC (HD 400)           30.0   [103]
13 Powder AC         31.1     [104]

To overcome adsorption restrictions, biosorption has recently risen as a technique with many benefits like cheap price, simple operation, and high effectiveness, even for lower levels of metals. Additionally, the biosorption approach has a high chance of recovering heavy metals without requiring additional nutrients. Furthermore, it has no negative ecological impact [6,105,106]. Moreover, bio-adsorbents need only a little treatment to be prepared. Such materials naturally exist in the environment and are typically either side products or waste materials generated from agricultural or industrial activities [26,88,107]. However, such materials can be categorized into three classes: natural materials, agricultural waste materials, and industrial waste materials [9,13,51,58].

Natural adsorbents are readily accessible materials with large volumes. They have many characteristics, including a large capability for exchanging cations and high surface area, which are necessary features for typical adsorbents. Additionally, their cost is significantly less expensive compared to conventional adsorbents [108]. Natural absorbents include various substances such as zeolites and clays. Clinoptilolite, one of the most researched natural zeolites, has been demonstrated to exhibit great selectivity for the removal of different heavy metals, including Zn (II), Pb (II), Cu (II) and Cd (II). In addition, it found that the pretreatment approach affects the clinoptilolite’s capacity to exchange cations, and therefore such conditioning enhances both those properties as well as elimination effectiveness [51,109-112].

Mineral clay is another natural adsorbent material. In terms of clay, it can be divided into three primary categories: kaolinite, bentonite, and mica. Of these types, bentonite has the largest capacity to exchange cations, high selectivity, and strong renewability possibility, as well as it is much less expensive than activated carbon [51,113]. In comparison with zeolites, clays have a lower capacity for removing heavy metals. Nevertheless, since they have a variety of benefits over conventional adsorbents, such materials are employed to remove various metallic ions from contaminated effluents. Such characteristics include a large surface area, suitable structure features like corrosion and chemical attack resilience, and outstanding physical and chemical qualities, including flexibility, binding force, and capacity to exchange cations [114-116]. Numerous studies have utilized different clay types: kaolinite, bentonite, and mica, without treatment for the decontamination of heavy metals from industrial effluents [117-119]. The natural clays’ capacity could be increased considerably for metallic ions removal by modifying them using a polymeric substance, obtaining what is known as clay-polymer complex materials [120-122]. In addition, many studies have used the calcination approach for clay modification before its application in heavy metals removal [123-125]. This approach includes heating clay at various temperatures, usually higher than 250°C, for a period of time prior to its usage. Clay surface modification using acidic treatment is another approach for heavy metals decontamination [122,126-128]. In this way, the clay surface is treated with an acid such as HCl and H2SO4. The clay is then treated with NaOH to neutralize its surface and remove the acid effect. Table 6 shows the adsorption capacity of various clay types to eliminate heavy metals from aqueous solutions.

Table 6: Adsorption capacity of different clay types used for heavy metals removal.
N Clay Type Adsorption Capacity, mg g-1 References
As (V) Cd (II) Cu (II) Pb (II) Zn (II)
1 Bentonite     4.0     [129]
2           5.0 [130]
3           52.9 [131]
4         20.0   [132]
5 Illite       4.3   [133]
6 Kaolinite 0.2         [134]
7       4.4     [135]
8         7.8   [135]
9           5.0 [135]
10         1.4   [133]
11 Montmorillonite   4.8     5.0 [136]

Industrial wastes could be employed to replace expensive traditional adsorbents of heavy metals decontamination from polluted effluents because such waste materials usually require minimal treatment to boost their adsorption capacities [137]. Such materials are typically generated as just by-products of various industries and are seldom employed in other applications. These materials are readily accessible and relatively affordable because of their status as by-products of industries. Industrial waste materials have been discovered to be effective as adsorbent materials. Their capabilities to adsorb heavy metals might be improved by minor physical and chemical modifications [13,51]. The possibility of using numerous materials generated from various industries has been investigated to remove hazardous heavy metals released in polluted effluents. Such materials included sludge from a blast furnace generated by the iron industry [138,139], fly ash material generated from coal combustion in power plants [140-143], black liquor generated from the pulp and paper industries [144-146], iron hydroxide (Fe(OH)3) [147,148], Red mud is a side product of the Bayer method during converting bauxite to alumina [149,150]. In addition to those, other waste types have also been evaluated for removing heavy metals but less frequently compared to the former waste types. Examples of such wastes were Areca waste [151], the coffee husk is the main by-product of coffee processing [152,153], beet pulp is a waste material (fibrous part) that remains after the sugar beet is processed to extract sugar using a water phase [154], tea waste is a fibre component generated during tea processing and consists of dust and leaves parts [155], olive oil waste is a primary side product generated during the manufacturing of olive oil and consists of fruit pulp, fragments of skin and pulp and amounts of oil [156]. Table 7 shows the chemical composition of fly ash that can be utilized in heavy metal removal applications.

Table 7: Chemical composition of Canadian coal fly ash [157,158]. 
N Component Value, %
1 Silica (SiO2) 35.5%-62.1%
2 Alumina (Al2O3) 12.5%-23.2%
3 Ferric oxide (Fe2O3) 3%-44.7%
4 Calcium oxide (CaO) 1.2%-13.3%
5 Potassium oxide (K2O) 0.5%-3.2%
6 Magnesium oxide (MgO) 0.4%-3.1%
7 Sulfur trioxide (SO3) 0.2%-7.8%
8 Titanium dioxide (TiO2) 0.4%-1%
9 Sodium oxide (Na2O) 0.1%-7.3%
10 Phosphorus pentoxide (P2O5) 0.1%-1.5%
11 Manganese oxide (MnO) NA*
12 LOI** 0.3%-9.7%
Notes: *Not available, **Loss on Ignition.

Agricultural waste products are a cost-effective, widely accessible, and sustainable solution for wastewater applications [159]. Agricultural wastes are generally dense with a low percentage of ash content, making them simple to transform to activated carbon, which has a superior possibility for adsorbing heavy metals [58]. Numerous investigations have revealed that agricultural wastes have strong adsorption capability for various contaminants [76,160-164]. Agricultural waste products may be utilized in their original state, with no treatment or after being modified physically and/or chemically [58]. However, the mentioned materials can be classified into four main categories: crop-related waste, fruit-related waste, fibre-related waste, and wood-related waste. The ability of different agricultural wastes to remove metallic ions from contaminated wastewater has been evaluated [8,165-167]. Examples of crop-related wastes are rice and its crop parts: bran, husk, straw, and hull [168-171], wheat and its related parts: bran and shell [172,173], and corn and its crop parts: stalks [174,175]. Meanwhile, fruit-related wastes include fruit peels such as oranges, lemons, grapefruit, and pomegranate [176-179], and fruits pomace and their waste like apples, olives, and carrots [180,181]. In addition, jute, coir, as well as cotton are examples of fibre-related wastes [182,183]. Furthermore, wood-related wastes include sawdust [184,185], tree bark [186,187], tree powder [188], tree leaves and stalks [189,190]. Table 8 presents the adsorption capacity of different agricultural wastes for removing heavy metals.

Table 8: Adsorption capacity of some available agricultural waste in removing heavy metals.
N Agricultural Waste Adsorption Capacity, mg g-1 References
As (V) Cd (II) Cr (VI) Cu (II) Ni (II) Pb (II) Zn (II)
1 Cashew nut shell   22.1           [23]
2     436.7*           [191]
3         406.6*       [191]
4           456.3*     [191]
5               455.7* [191]
6 Cassava peel         57.0     [192]
7 Cassava tuber bark waste             83.3* [193]
8 Castor seed hull   7.0           [98]
9 Cedrusdeodara sawdust             97.4 [63]
10 Cocoa pod husk           20.1   [194]
11 Coconut coir pith 13.8*             [195]
12 Coffee grounds   15.7           [95]
13 Coir pith     165.0         [93]
14       196.0*         [93]
15 Cortex banana waste   67.2           [196]
16         36.0       [196]
17 Cortex lemon waste       70.4       [196]
18 Cortex orange waste       67.2       [196]
19             76.8   [196]
20 Cotton fiber     69.2*         [197]
21 Garden grass       58.3       [198]
22 Husk of Lathyrus sativus   52.8*           [199]
23 Litchi chinensis seeds         66.6     [200]
24 Orange peel   136.1*           [201]
25     293.3*           [202]
Note: *This value was achieved by modifying the waste used chemically.

Heavy metals are dangerous to persons and ecosystems owing to their high toxicity and bioaccumulation potential. Unlike organic contaminants, heavy metals do not degrade biologically or chemically into end products. Thus, removing such metals from water is a difficult task. Precipitation, ultrafiltration, and coagulation have all been used to remove metals. But such uses are inefficient, wasteful, and produce hazardous sludge. Adsorption, on the other hand, is the most practical approach for removing hazardous metals due to its simplicity, economy, and adaptability. This article has reviewed the literature on adsorption for heavy metal removal from a contaminated environment. The metals’ toxicity and health and environmental risks were also examined. The use of diverse materials as bioadsorbents, including natural materials as well as industrial and agricultural waste substances, was addressed.

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