A comprehensive review of heavy metals (Pb²⁺, Cd²⁺, Ni²⁺) removal from wastewater using low-cost adsorbents and possible revalorisation of spent adsorbents in blood fingerprint application

ABSTRACT

An increasing amount of water pollution is being caused by an increase in industrial activity. Recently, a wide range of methods, including extraction, chemical coagulation, membrane separation, chemical precipitation, adsorption, and ion exchange, have been used to remove heavy metals from aqueous solutions. The adsorption technique is believed to be the most highly effective method for eliminating heavy metals from wastewater among all of them. However, it generates secondary waste that can pose a risk to the environment. Agricultural waste has potential to be collected and converted into carbon nanomaterials, then coated with metal oxides for the removal of Pb²⁺, Cd²⁺, and Ni²⁺ ions and then the reuse of heavy metal spent adsorbents in blood fingerprint detection (BFP) can be studied. This review highlights the eco-friendly nature and abundant availability of these materials while advocating for their integration into mainstream wastewater treatment practices. It explores the prospect of revalorizing spent adsorbents in blood fingerprint applications, demonstrating a dual-purpose utilisation that bridges environmental remediation with advancements forensic sciences. Different method of removal of Pb²⁺, Cd²⁺ and Ni²⁺, removal technique as well as other reuse applications of spent adsorbents are also discussed.

GRAPHICAL ABSTRACT

KEYWORDS:

• Blood fingerprint detection
• pollution
• secondary waste
• spent adsorbent

1. Introduction

In recent years, the number of industries manufacturing products containing toxic heavy metals has been increasing rapidly, and their environmental mobilization and transport rates have risen significantly since the 1940s [1]. An unintended consequence of industrialization is environmental pollution due to the emission of materials from industrial processes into the environment. Water resources are a priority recipients of industrial pollutants, predominantly effluents, surface runoff, and incidental spills; this compromises water resources’ quality/health. These growing industries may pollute the water that is essential for our living. South Africa is regarded as a water-scarce country with a high rate of population growth, industrialization, civilization, urbanization, and agriculture. Toxic heavy metals are released from industries such as textile, mining, fertilizer, tanneries, and metal plating into water resources [2]. Other heavy metal releases may also increase from paints, paper, photography, printing, petroleum refining, printing, and dyes into water streams [3]. Such heavy metals include lead (Pb), iron (Fe), copper (Cu), cobalt (Co), cadmium (Cd), nickel (Ni), arsenic (As), hexavalent chromium Cr (VI) [4], manganese (Mn), aluminium (Al), gold (Au), zinc (Zn), and silver (Ag) are used due to their properties.

Contamination of water bodies by heavy metals poses a critical environmental challenge, necessitating effective and sustainable remediation strategies. Among the numerous methods explored, the use of low-cost adsorbents has emerged as a promising avenue for the removal of heavy metals such as Pb²⁺, Cd²⁺, and Ni²⁺ from wastewater. Wastewater may contain components such as inorganic and organic waste, nutrient-suspended solids, and pathogens with biological, physical, and chemical characteristics. Concerns have also been raised regarding adverse side effects on human health. Such effects include reduced haemoglobin, damage to the nervous system and mental retardation [5]. The World Health Organization (WHO) has estimated the maximum permissible limit of Pb²⁺, Cd²⁺, and Ni²⁺ in wastewater to be 0.01, 0.003, and 0.02 mg/L, respectively.

Regarding the removal of pollutants from wastewater during treatment, researchers have developed effective methods for removing various impurities from wastewater. Such pollutants have been considered to lead to secondary pollution. This review paper intends to present a motivation for the use of low-cost adsorbents from agricultural waste (mesoporous carbon, carbon nanorods, carbon nanoparticles) coated with metal oxides (TiO₂, ZnO, Al₂O₃) for the uptake of toxic heavy metals from wastewater and the reuse of the metal-loaded spent adsorbents for fingerprint detection as a viable future research perspective. It is worth mentioning that the reuse of spent adsorbent can enhance latent fingerprint images [6,7]. Besides solving secondary pollution, the reuse of spent adsorbents (carbon nanomaterials, metal oxides, nanoparticles) in latent fingerprint detection has drawn the interest of researchers because they exhibit properties such as good optical stability, excellent affinity, low emission peak, sensitivity, high fluorescent intensity, and selectivity [8,9]. As a result, there is a need to develop cheap, robust, and effective methods of removing heavy metals in wastewater to meet environmental regulations.

This article outlines a comprehensive review of the diverse array of low-cost adsorbents, ranging from agricultural by-products to industrial wastes, to evaluate their efficacy in heavy metal sequestration. and the reuse of spent adsorbents in forensic applications. Specifically, the focus is on Pb²⁺, Cd²⁺, and Ni²⁺ ions, adsorbed on low-cost adsorbent materials. By reviewing current knowledge on low-cost adsorption techniques and their applicability, this research aims to provide insights into sustainable water treatment practices while paving the way for interdisciplinary applications in cutting-edge forensic sciences. Additionally, the forensic applications by reuse of other metal-loaded spent adsorbents are also reviewed.

2.1. Occurrence and environmental release

2.1.1. Lead

Pb²⁺ is one of the first metal ions that humans discovered [10]. It has been widely employed in numerous industries because of its unique features, such as softness, low melting point, and corrosion resistance. It is generally released from industrial sources such as coal combustion, battery recycling, and lead smelting. Pb is regarded as one of the most highly toxic heavy metals to humans and the environment [11] and occurs at low concentrations in the earth’s crust [12]. The burning and mining of Pb-containing materials release Pb into the atmosphere, and humans may be exposed through inhalation, ingestion, or skin contact [13]. Exposure to Pb²⁺ poisoning may lead to kidney and liver damage, nervous system difficulties, anaemia, memory loss, and psychological and reproductive issues [14]. Human exposure can also be through the ingestion of Pb-contaminated water, soil, dust, and food.

2.1.2. Cadmium

Cd²⁺ is categorized as highly toxic and carcinogenic to humans by the International Agency for Research on Cancer [15]. It occurs naturally due to volcanic eruptions and rock and soil erosion/abrasion [16]. Cd²⁺ is considered among the most toxic heavy metals and is non-degradable and persistent [17]. Untreated wastewater containing Cd²⁺ may be released from various industrial processes such as phosphate fertilizers, mining, alloy industries, electroplating, sewage sludge, Cd²⁺ batteries, and pigments [15]. Due to inhalation, oral ingestion, and dermal absorption of Cd²⁺-containing food, water, or dust, exposure to Cd²⁺ can last for 10–35 years [18]. The toxicity of Cd²⁺ arises due to its high mobility and bioaccumulative ability. Excess Cd²⁺ consumption can result in heart disease, kidney damage, liver, and cancer [19]. According to the World Health Organization, Cd²⁺ in wastewater should not exceed 0.003 ppm [20]. The global use of Cd²⁺ is predicted to be approximately 20,00024,000 t/year [21]. Cd²⁺ in the wastewater should not exceed 2 mg L^-1 as the permissible threshold [22].

2.1.3. Nickel

Ni²⁺ can be released into the environment by volcanic eruptions, anthropogenic activities, and forest fires. Different industries, including rubber, electroplating, alloys, electroforming, plastics, aerospace industry, mining, and batteries, release Ni²⁺ compounds into the environment [23]. Depending on the exposure route, such as dermal, oral, or inhalation, Ni²⁺ can have hazardous consequences on human health. Acute and chronic exposure are two types of exposure. Ni²⁺ does not only cause allergies but is also a risk factor for several illnesses, including dermatitis of the skin, anaemia, hepatitis, renal damage, and diarrhea. WHO has recommended that the maximum permissible of Ni²⁺ should not exceed 0.02 mg/L [24].

2.2. Remediation techniques for heavy metals in wastewater

To meet the standard of maximum allowable limits industries, different methods have been used to reduce contamination to permissible limits. The advantages and drawbacks of the techniques are provided in Table 1.

Table 1. Techniques for eliminating heavy metals in wastewater [25].

Treatment technologies	Advantages	Disadvantages
Membrane filtration	Good removal of heavy metals	Sludge production and very expensive
Ion exchange	Good removal of a wide range of heavy metals	Absorbent material requires regeneration and disposal problems.
Electrochemical treatment	Rapid process but effective for particular metal ions	High energy cost and generation of by-products
Coagulation/flocculation	Economically feasible	High sludge formation
Adsorption	Low sludge; effective, Low-cost material; low metal concentration	Regeneration requires chemicals; modification required for better performance
Electrocoagulation	easy operation, can even settle small colloidal particles	Require regular replacement and maintenance of electrodes

2.3. Methods for removal of heavy metals

2.3.1. Chemical precipitation

Chemical precipitation is one of the most common technologies used by most wastewater treatment plants to remove dissolved pollutants in wastewater [26]. It is the process in which chemicals are used to convert substances in wastewater into insoluble precipitates. An impurity in ionic form is transformed into an insoluble form by a chemical reaction, and particulates are removed by settling co-filtration. The effectiveness of chemical precipitation depends on several factors, particularly the pH, solution composition, and pollutant concentration. A detailed schematic diagram of chemical precipitation processes is shown in Figure 1.

Figure 1. Schematic diagram of the chemical precipitation treatment process [27].

2.3.2. Electrochemical treatment

The electrochemical process involves the use of electricity to remove pollutants from water. During the electrolytic method of pollutant removal, an electric current is passed through the anode and cathode immersed in the solution. In between the electrodes, negatively and positively charged substances are caused to attract to each other (Figure 2). The electrochemical treatment process has proven to remove numerous pollutants that include pharmaceutical residues [28], synthetic dyes [29], inorganic ions [30], humic fulvic acids, and pesticides [31]. Currently, electrochemical treatment processes are utilized in wastewater treatment, including electrochemical oxidation, electro-flotation, and electro-coagulation.

Figure 2. Diagram of the electrochemical reactor (Adapted from: [32]).

2.3.3. Membrane separation-based technique

Membrane separation technology is one of the most advanced wastewater treatment technologies for removing dissolved solutes in wastewater. Membranes commonly employ pressure to remove trace levels of contaminants in wastewater. The higher membrane level tehnologies, such as reverse osmosis, remove all types of pollutants that are difficult to remove from any other system. The system is recognized as the most energy-intensive treatment option. The drawbacks of membrane technology include being expensive, not user-friendly, and high rejection efficiency, but the technology is advancing quickly, and performance is improving. Various membrane types, such as liquid, metallic, inorganic, and polymeric membranes, can be used during wastewater treatment. Additionally, various systems, namely, nanofiltration membranes, ultrafiltration, reverse osmosis, and microfiltration, can target pollutants in the membrane system. Amongst them, reverse osmosis, nanofiltration, and ultrafiltration are widely used heavy metal removal techniques. Figure 3 shows a diagram of the membrane separation-based process. In this system, the feed components permeate through the membrane; the other feed components are transported through the membrane [33].

Figure 3. Membrane-based separation process (Adapted from: [33]).

2.3.3.1. Reverse osmosis

Reverse osmosis applies pressure to the membrane, and the permeable membrane permits the water molecules to pass through and flush the dissolved inorganic minerals into the drain. It can reject dissolved inorganic minerals from the water. Thaçi and Gashi, (2018) utilized reverse osmosis for the uptake of heavy metals from synthetic aqueous solution using some agricultural waste such as wheat bran, olive waste, and maize cob. Even though high removal efficiency of ions was achieved, the rejection of heavy metals removed was found to be between 40–90% [34]. In another study, Li et al utilized nanoporous graphenes as reverse osmosis membranes for the uptake of heavy metals (Cu²⁺, Mn²⁺, Zn²⁺) in an aqueous solution, and ion rejection was 100% [35]. Disadvantages of reverse osmosis include higher operating costs, severe fouling, and lower flux [36].

2.3.3.2. Ultrafiltration (UF)

In the UF method, the permeable membrane is used as a separation technique of macromolecules, heavy metals, and suspended solids from the inorganic solution [27]. It has been demonstrated that UF can achieve above 90% removal efficiency in pH above 5.

2.3.3.3. Nanofiltration (NF)

NF is similar to other membrane processes such as ultrafiltration, microfiltration, and reverse osmosis. It has high efficiency and needs less energy to purify inorganic minerals than reverse osmosis. It consists of pore sizes ranging from 0.1–10 nm. Peydayesh et al. 2020 designed nanofiltration self-assembly of Ethylenediamine (ED) grafted multi-walled carbon nanotubes to remove Zn²⁺, Mg²⁺, Cd²⁺, Cu²⁺, Ca²⁺, Ni²⁺ and Pb²⁺ with a rejection rate ranging from 90.5–96.7% [37].

2.3.4. Ion-exchange

Ion exchange, also known as deionization or demineralization, is the physical process used to remove dissolved ions in wastewater. It is a modern technology used to convert hard water to soft water, which can be used for cleaning, drinking, and washing. Some organic compounds known as resins can exchange ions, and zeolites are among the well-known ion exchangers [38]. It is a reversible chemical reaction that replaces an existing unwanted metal iosn with environmentally friendly ions. Exchange resins can be classified as cationic or anionic, and are either naturally occurring or a synthetic organic material. For water to be free from dissolved ions or minerals, hard water has to pass through a cation exchanger followed by an anion exchanger (Figure 4).

Figure 4. A typical ion exchange ion treatment process [38].

2.3.5. Adsorption

Adsorption is a process that traps dissolved impurities in polluted wastewater onto the surface of the adsorbent. It is a promising technique for removing various contaminants, such as non-degradable organic compounds, heavy metals, and micropollutants [39]. Adsorption can occur in two ways; physisorption or chemisorption. Physisorption involves the van der Waals force for adsorbate to bind to the adsorbent surfaces (Figure 5). In contrast, chemisorption occurs when chemical bonds intermediate with the adsorbate which is to be attached to the adsorbent. The adsorption mechanism occurs in three ways: mass transfer to the adsorbent surface, intraparticle, and diffusion onto the surface of pores. The advantage of using adsorption processes to remove pollutants in wastewater include ease of operation, low cost, versatility, environmental friendliness, ease of regeneration, and higher removal efficiency of contaminants [39]. Adsorption is an effective, economical, and eco-friendly treatment technique.

Figure 5. Schematic representation of adsorption molecules onto the surface [40]

2.3.6. Phytoremediation

To treat environmental contamination, a process known as phytoremediation uses plants and the associated soil microbes [41]. It is a brand-new, economical, effective, environmentally friendly, and solar-driven remediation technique that can be used in situ [42]. Heavy metal phytoextraction has a high potential in plants that produce numerous harvests in a single growing cycle, such as Trifolium spp. Because of their high growth rate, greater capacity to adapt to stressful environments, and high biomass, grasses are preferred to shrubs and trees for phytoextraction.

2.4. Adsorbents for removal of heavy metals in wastewater

2.4.1. Nanomaterial-based adsorbents

Several studies reported the use of metal oxides. Some of the conventional adsorbent examples are shown below.

2.4.1.1. Metal-based nanomaterials (MNMs)

MNMs can be produced by either destructive or constructive methods from metals down to nanomaterials diameters. The initial constituents of metal nanostructures are divalent and trivalent metal ions. Using reducing agents, metal ions are transformed into metal nanoparticles. These are good for adsorbing small molecules, have a huge surface area, and are frequently used in numerous applications [43]. Table 2 summarizes the adsorbents reported for removing Pb²⁺, Cd²⁺, and Ni²⁺ ions from wastewater by MNMs.

Table 2. Pb²⁺, Cd²⁺, and Ni²⁺ ions for removal by MNM.

Adsorbent	pH	Adsorption capacity (mg/g)	Comment(s)	References
Pb(ll)
ZnO nanoparticles	5	84.66	Regeneration, reuse of spent adsorbent, and the effect of competing ions are not reported.	[44]
Chitosan – iron oxide nanomposite	5	214.9	Regeneration reported, reuse of spent adsorbent, the effect of competing ion, real water sample not explored.	[45]
Polyaniline/Fe3O4	9.3	111.1	Real water samples explored the effect of competing ions, reuse of spent adsorbent, and regeneration studies were not conducted.	[46]
TiO2 chitosan coated	6	475.50	Regeneration studies were conducted, on the effect of competing ions, but real water sample was not studied	[47]
Cd (II)
ZnO nanoparticles	7	119.75	Regeneration studies were conducted, but the effect of competing ions and real water samples was not studied.	[48]
ZnO nanosheets	5	97.36	The effect of competing ions, a real water sample was not conducted.	[49]
γ-Fe2O3/Fe-doped hydroxyapatite	5	257.9	Desorption and regeneration studies were conducted, and the effect of competing ions was explored.	[50]
Activated/ZrO2 carbon	6	166.7	Real water samples were assessed, but regeneration studies, and the effect of competing ions were not conducted.	[51]
MnO2 loaded resin Ni(II)	6	21.5	The effect of competing ions was studied, real water samples and reuse of spent adsorbent were not investigated.	[52]
Lemon peel biomass chemically modified with TiO2 nanoparticles	6	15.36	The effects of competing ions, reuse of spent adsorbent, and real water samples were not investigated.	[53]
Fe3O4 NPs	5.5	11.3	Real water sample, reuse of spent adsorbent, and the effect of competing ions were not explored.	[55]
Fe3O4-MnO2	6	55.6	Reuse of spent adsorbent, real water sample, and the effect of competing ions was not conducted.	[56]
Graphene/δ-MnO2 composite	8	30.63	Desorption studies were explored, and real water samples and the effect of competing ions were not investigated.	[57]

2.4.1.2. Carbonaceous-based NMs

The detection and treatment of greywater derived from households are accomplished using various carbon-based materials (Table 3). Due to their advantage of being effective, inexpensive, and selective, carbon-based compounds are typically seen as excellent absorbents [57]. Graphene, carbon nanotubes (CNTs), graphite, graphite/graphene oxide, and other carbonaceous compounds are among them. More emphasis is placed on nanocarbonaceous materials because it has been found that they are more advantageous than conventionally sized carbonaceous materials [58]. These NMs including nanotubes, nanobeads, nanocomposites, nanofibers, and others, have high surface area-to-mass ratios, porosities, and high permeabilities [59]. Their sorption capacity is increased through surface oxidation, modification, or functionalization [60]. Iijima inadvertently discovered carbon nanotubes in 1991 while researching the graphite electrode surfaces utilized in an electric arc discharge [61]. Based on how their graphene cylinders are arranged, CNTs can be named either single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs).

Table 3. Adsorption capacities of carbonaceous-based NM for removing Pb²⁺, Cd²⁺, and Ni²⁺ ions from wastewater.

Adsorbent	pH	Adsorption capacity (mg/g)	Comment(s)	References
Pb(ll)
Acidified MWCNTs	9	166	The effect of co-existing ions was studied; real water samples and the reuse of spent adsorbent were not conducted.	[62]
CNTs	5	76.7	Desorption was investigated, Real water samples, reuse of spent adsorbent, and the effect of competing ions were not explored.	[63]
Al2O3/MWCNTs	7	27.21	The real water sample was studied, But the effect of competing ions and the reuse of spent adsorbent was not explored.	[64]
Graphene oxide		555	Desorption and regeneration studies were investigated, and real water samples and the effect of competing ions were not conducted.	[65]
Cd (II)
Carbon nanofibers	8	0.238	Desorption and adsorbent regeneration were studied, but the effect of competing ions, and real wastewater samples were not investigated.	[66]
MWCNTs	6.5	0.05	Real water sample, regeneration study, reuse of spent adsorbent, and effect of competing ions was not undertaken.	[67]
MWCNTs with 3-aminopyrazole	8–9	43.103	The effect of competing ions and real water samples was studied. Desorption studies were not undertaken.	[68]
Acidified MWCNTs	9	101	The effect of competing ions was explored, real water sample and a desorption study were undertaken.	[61]
Reduced graphene oxide	-	24.47	Real water sample, regeneration study, reuse of spent adsorbent, and the effect of competing ions was not investigated.	[69]
Ni(II)
Purified MWCNTs	6	55.55	Real water samples, regeneration studies, reuse of spent adsorbents, and the effect of competing ions were not explored.	[70]
Phosphine functionalized MWCNTs	6	100	Real water sample, regeneration study, reuse of spent adsorbent, and the effect of competing ions was not explored.	[71]
Graphene/δ-MnO2 composite	8	30.63	Desorption studies were explored, but real water samples and the effect of competing ions were not investigated.	[56]
graphene oxide	6	38.61	No real water samples, regeneration study, reuse of spent adsorbent, or effect of competing ions studied.	[72]

2.4.2.3. Polymer-based NMs

A natural polymer called alginate is obtained from brown seaweed and contains d-Mannuronate (M) and l-Guluronate (G) linear copolymers joined together by 1,4 – and 1,4-glycosidic linkages. Blocks of homopolymeric M-M, G-G atoms, and heteropolymeric M-G atoms are used to organize M and G units [73]. These blocks are distributed differently depending on where the alginate polymer came from. In fact, this polysaccharide's carboxylate functionalities are negatively charged in neutral and alkaline conditions, which makes them more apt to bind to cations. Table 4 presents polymer-based as a possible adsorbent for removing selected heavy metals.

Table 4. Summary of polymer-based NMs for adsorption of Pb²⁺, Cd²⁺, and Ni²⁺ ions.

Adsorbent	pH	Adsorption capacity (mg/g)	Comment(s)	References
Pb(ll)
Chitosan/vanillin	6	66.63	Regeneration study, reuse of spent adsorbent, the effect of competing ions, and real water samples were not conducted.	[74]
Chitin microspheres/glycine	5.5	574.70	No application of real water samples, regeneration study, reuse of spent adsorbent, or effect of competing ions was reported.	[75]
Chitosan/salicylaldehyde	7	123.67	Real water samples were evaluated, and the reuse of spent adsorbent, and the effect of competing ions were not investigated.	[76]
Chitosan/ nanofibrillated cellulose aerogel	5	252.6	Desorption and reusability were conducted, and the real water sample and competing ions were explored.	[77]
Chitin/palygorskite nanofibers (ChNFs)	7	53.7	None of the reuse of spent adsorbent, application of real water samples, effect of competing ions, and regeneration study were investigated.	[78]
Cd (II)
Attapulgite/CoFe2O4@SiO2-chitosan /EDTA	6	127.79	A reusability study was done, Application of real water samples, and the effect of competing ions was not investigated.	[79]
Chitosan/diatomaceous earth composite	6.5	108	Application of real water samples not reported.	[80]
Fe3O4/cyclodextrin polymer nanocomposites	5.5-6	27.7	Desorption and reusability were reported, but real water samples were not investigated.	[81]
modified chitosan TiO2 and SiO2 hydrogel nanocomposites	7	344 and 312	Reuse of spent adsorbent, application of real water samples, and effect of competing ions was reported.	[82]
polyhedral oligomeric silsesquioxanes/chitosan nanocomposite	7	36.2	None of the information was reported on real water samples, competing ions effect, and reuse of spent adsorbent.	[83]
Ni(II)
Alginate/polyvinyl alcohol/zeolite	5.5	81.51	The application of real water samples, and the effect of competing ions were not reported.	[84]
Nanochitin-contained magnetic chitosan microfibers	4.1	-	A real water samples experiment was not reported.	[82]
Chitosan Immobilized on Bentonite	-	15.8228	Competitive adsorption of heavy metals was reported.	[85]
Sulfated Chitosan/PVA	6	80.6451	None of the information was reported on real water samples, competing ions effect, and reuse of spent adsorbent.	[86]
EDTA – and/or DTPA-modified Chitosan		71.0	Real water samples, and competing ions effect were not explored.	[87]

2.4.2. Conventional adsorbents

Activated carbon, agricultural waste, and zeolites are some of the common adsorbents used for the uptake of heavy metal ions in solution. Below is a summary of some examples of convectional adsorbent.

2.4.2.1 Activated carbon

Table 5 lists forms of activated carbon adsorbents prepared from agricultural wastes. The carbonization of the carbon-based agricultural waste with lower temperatures of 800 °C in an inert environment and the activation of the carbon material are the two primary phases in preparation [88]. Thus, any carbonaceous materials may be converted into activated carbon. However, the characteristics of the end product will vary based on the kind of waste utilized, the type of activating agent employed, and the circumstances surrounding the processes of activation, and carbonization. Using hot gases or steams of water vapour results in the physical activation of carbonized material [89]. This is typically done by using one or a combination of activation/oxidation, which requires high temperatures in the presence of carbon dioxide and steam, and carbonization, which turns this organic precursor into any type of primary carbon like tars, ash, and amorphous carbon. Unfortunately, during the carbonization process, certain tars or decomposition products are trapped in the pores [90]. Contrarily, chemical activation prevents the production of tar, allowing for the one-step production of a carbonized product with a highly developed porosity [91]. Prior to carbonization, chemical activation occurs when agricultural waste is modified with certain chemicals. These chemicals are often an acid like H₃PO₄, a strong base like NaOH, and KOH or a salt like ZnCl₂. Then, at lower temperatures (450–900 °C), the agricultural precursor is carbonized. Therefore, activated carbons are regarded as good adsorbents due to its high surface area and porosity.

Table 5. Remediation of Pb²⁺, Cd²⁺, Ni²⁺ ions by activated carbon.

Adsorbent	pH	Adsorption capacity (mg/g)	Comment(s)	References
Pb(ll)
Activated carbon pistachio wood	6	190.2	The reuse of spent adsorbent, application of real water samples, and effect of competing ions was not investigated.	[92]
Date Pit	5	34.18	Application of real water samples, reuse of spent adsorbent, and effect of competing ions were not investigated.	[93]
Albizia lebbeck	5	4.38	Application on real water samples, reuse of spent adsorbent, effect of competing ions was not conducted.	[94]
Prawn Shell	8	256	No information was investigated on real water samples, regeneration studies, reuse of spent adsorbent, and the effect of competing ions.	[95]
Wheat straw	6	74.63	The effect of competing ions was not investigated.	[96]
Cd (II)
Pomegranate Peel	6	53.8	Application of real water samples, reuse of spent adsorbent, and effect of competing ions were not investigated.	[93]
Leucaena leucocephala	7	70.423	No information was reported on the reuse of spent adsorbent, its application on real water samples, and the effect of competing ions.	[97]
Oil palm shell	6	227.27	Reuse study not reported	[98]
Sugarcane bagasse	4	2.0140	The reuse of spent adsorbent, the effect of competing ions, and real water samples were not reported.	[99]
Melia azedarach	5	4.20	Application on real water samples, reuse of spent adsorbent, the effect of competing ions was not reported.	[94]
Ni(II)
Activated carbon Mespilus germanica Leaf	7	13.08	Reuse of spent adsorbent, the effect of competing ions, and real water samples were not conducted.	[100]
palm kernel chaff	9	120.6	None of the information on the effect of competing ion, reuse of spent adsorbent, or real water sample were conducted.	[101]
Mangosteen shell	5	57.14	The effect of competing ions, real water samples, and reuse of spent adsorbent was not investigated.	[102]
doum-palm seed	7	13.51	The effect of competing ions, real water samples, and reuse of spent adsorbent was not reported.	[103]
Sawdust	7	6.6	Regeneration studies were reported, but the effect of competing ions, and real water samples were not investigated.	[104]

2.4.2.2 Agricultural waste

Numerous reviews have been published on removing heavy metals using agricultural waste products as adsorbents [105,106]. Due to their ease of availability, low cost in significant quantities as agricultural wastes, agricultural wastes have gained popularity among biosorbents. They typically consist primarily of cellulose and lignin, with several hydroxyl and carboxylic groups that can provide an electron pair to bind metal ions and create complexes [107]. There is evidence that certain vegetable wastes which can filter out heavy metals from water. Heavy metal removal using mushroom residues has been successful [108]. The Cu²⁺, Zn²⁺, and Hg²⁺ removal efficiency ranged from 39.7% to 81.7% based on an analysis of four different kinds of mushroom residues. Potatoes, after rice and wheat, are the most essential foods in developing countries because they include minerals, vitamins, antioxidants, and polysaccharides such as cellulose, hemicellulose, lignin, and starch [109]. Table 6 lists the maximal adsorption capacities of various agricultural wastes for removing Pb²⁺, Cd²⁺, and Ni²⁺ ions, along with the ideal pH to consider for biosorption.

Table 6. Adsorption of Pb²⁺, Cd²⁺, and Ni²⁺ ions by agricultural wastes.

Adsorbent	pH	Adsorption capacity (mg/g)	Comment(s)	References
Pb(ll)
Yam peels	6	93.36	Regeneration and desorption study, the effect of competing ions, and real water samples were not explored.	[110]
Walnut shell	4	9.9	None of the regeneration and desorption studies, the effect of competing ions, and real water samples were explored.	[111]
Lentil husk	5	81.4	Desorption studies were explored, but real water samples and the effect of competing ions were not conducted.	[112]
Soya bean	4	0.9	None of the reuse of spent adsorbent, the effect of competing ions, and real water samples were explored.	[113]
Banana Peels	5.5	66.7	The effects of competing ions were explored.	[114]
Cd (II)
Lentil husk	5	107.31	The effect of competing ions was investigated	[115]
Apple peels	5.5	0.8	No reuse of spent adsorbent, Effect of competing ion, and the real water sample was investigated	[116]
Orange peels	7	140	Desorption and reusability studies were conducted.	[117]
Sawdust	7	21.3	None of the effects of competing ions, real water samples, reuse of spent adsorbent, regeneration study, and desorption were explored.	[115]
Mandarin peels	5.5	3.33	No reuse of spent adsorbent, Effect of competing ion, and the real water sample were investigated	[118]
Ni(II)
Sugarcane bagasse	6	3.008	Real water sample, reuse of spent adsorbent, and effect of competing ions, was not explored.	[119]
Raw hemp felt	5	2.4	Application on real water samples and the effect of competing ions were investigated.	[114]
Corncob	8	0.0517	Effect of competing ion, real water sample, reuse of spent adsorbent, regeneration study and desorption were not conducted.	[120]
Cassava husk powder	6	1.388	Regeneration study and desorption, real water sample, and Effect of competing ions were not explored.	[121]
Cashew nutshell	5	18.868	A desorption study was investigated.	[122]

2.4.2.3 Zeolites

A class of hydrated aluminium-silicates of the alkaline earth metals known as zeolites are distinguished by their availability, cheap cost of mining, high bulk density, and strong resistance to modification [123]. Magnesium, potassium, calcium, and sodium are such classes of aluminium-silicate earth metals. The main component of zeolites is a tetrahedral complex made up of four oxygen ions coordinated tetrahedrally with the Si⁴⁺ cation. A negative charge density is produced for the zeolite lattice by the isomorphic substitution of Al³⁺ for Si⁴⁺. Alumosilicates, known as zeolites, are composed of three-dimensional SiO₄ and AlO₄ tetrahedron networks connected by a common oxygen atom [124]. Zeolites are highly porous crystalline aluminosilicates that have extra-framework charge-balancing cations and tetrahedrally linked three-dimensional frameworks. These frameworks feature pores that can accommodate molecules as small as 1 nm in size [125]. The pore space's dimensions, geometry, and connectivity are frequently used to define zeolite formations. The surface area plays a crucial role in the efficacy of zeolites and zeolite-carbon composites as adsorbents. Zeolites are microporous crystalline materials with a well-defined structure, characterized by a three-dimensional network of channels and cages. Zeolites have been used in various applications, including ammonia removers from urban, chemical sieves, agricultural, and industrial wastewaters, water softeners, toxic gas removers from gaseous emissions, herbicide, odour control filters, animal nutrition, chemical fertilizer, and pesticide carriers. Table 7 lists the zeolites as adsorbents for removing Pb²⁺, Cd²⁺, and Ni²⁺ ions.

Table 7. Summary of removal of Pb²⁺, Cd²⁺, and Ni²⁺ ions by zeolites.

Adsorbent	pH	Adsorption capacity (mg/g)	Comment(s)	References
Pb(ll)
Zeolite	6	45.0	Effect of competing ion, real water sample, reuse of spent adsorbent, regeneration study and desorption were not investigated.	[126]
Zeolite based on oil shale ash	5	224.72	No spent adsorbent and application on real water samples were studied.	[124]
calcium treated bentonite clay	4	0.203	Reuse of spent adsorbent, the effect of competing ions, and a real water sample was not conducted.	[127]
Calcium Alginate Beads	7		Reuse of spent adsorbent, effect of competing ion, and real water sample was not included.	[128]
Cd (II)
Zeolite based on oil shale ash	5	118.34	No spent adsorbent and application on real water samples were studied.	[124]
calcium treated bentonite clay	4	32.17	Reuse of spent adsorbent, Effect of competing ion, and a real water sample was not conducted.	[127]
Calcium Alginate Beads	6	27.4	The effect of competing ions, reuse of spent adsorbent, and real water samples was not investigated.	[128]
natural and Fe(III)-modified zeolite	9.8 and, 8.0	66, and 133	Effect of competing ion, regeneration study, desorption, and real water sample was not investigated.	[129]
Ni(II)
Jordan natural zeolite	4	153.846	No regeneration and desorption, study effect of competing ions, and application on real water samples were investigated.	[123]
Zeolites	7	28.57	Effect of competing ions, real water samples, reuse of spent adsorbent, regeneration study, and desorption were not investigated.	[126]
Zeolite based on oil shale ash	6	53.02	No spent adsorbent and application on real water samples were studied.	[124]
silica xerogel	10–11	50	Reusability studies were conducted, Effect of competing ins, and real water samples were not assessed.	[130]
lower globular magnesium hydroxide	6	287.11	Recyclability study was assessed, and no effect of co-existing ion, real water sample was investigated.	[131]

Despite all the adsorbents reported on the removal of heavy metals and their better adsorption capacity, reports on the heavy metal-loaded adsorbent are still lacking. Hence the reuse of heavy metal loaded spent adsorbent to protect the environment should be investigated.

2.5. Hazards of heavy metal loaded spent adsorbent

Environmental issues have been a significant concern in recent decades, and environmental and health legislation have increased in significance worldwide [132]. One of the most important issues in pollution prevention is integrated and balanced water management. A number of adsorbents are developed from low-cost materials like agricultural and industrial waste, but the concern is the disposal at the end of life. When heavy metals are removed from wastewater, they are generally bound to the surface of the adsorbent, referred to as heavy metals spent adsorbent. The disposal of heavy metal-loaded spent adsorbents is considered a high concern in humans and the aquatic environment. Rufullo et al, investigated the biocidal efficiency of ZnONPs and revealed that ZnONPs hindered the growth of conidiophores and conidia of Penicillium expansum [133]. Kang et al, reported the impact of SWCNTs on microorganisms and their findings were that the size of SWCNT could thereby destroy cells [134]. When carbon NM and metal oxides are used for environmental remediation and are discarded, they interact with pollutants, and the level of toxicity may rise [135]. Carbon NMs can produce responses including lung tumours and cellular inflammation in live tissue due to their small particle size, which allows them to penetrate through cell walls and membranes. These reactions would immediately be harmful to animals and human health [135]. However, recycling the spent adsorbent is necessary to solve secondary pollution. There are various applications (photocatalysis, medical, catalysis) where spent adsorbent can be used [136], but since crime is common in South Africa, there was a necessity to explore the reuse in forensic applications, specifically BFP detection.

Due to inadequate evidence tying the offenders to the crime site during criminal behaviour (which might include physical assault, sexual violence, or murder), the perpetrators may not be prosecuted. Crime scene samples, such as saliva, blood, sperm, and fingerprints, can be collected to acquire evidence. A BFP is an important piece of evidence that has been utilized to potentially link criminals to crime scenes. Three different fingerprint categories can be collected at the crime site; these include latent (non-visible) or patent (visible) or indented [137]. The major features of the fingerprints, like ridge endings, and ridge bifurcations, are called Minutiae [138], shown in Figure 6. They can be obtained on a porous surface such as clothing items, toilet paper, raw wood, or cardboard, and non-porous surfaces such as tiles, glass, windows, knives, and others.

Figure 6. Images and minuates of latent fingerprint details [139].

Several publications have focused on developing materials for latent fingerprints [140] and carbon-coated metal oxides are one of the materials used as labelling agents to visualize the fingerprints [141]. Other materials enhancing friction ridges include carbon NMs [142] and metal oxides. Criminal investigations frequently heavily rely on fingerprints in blood, whether they are discernible or have been developed. The detection of latent BFP using heavy metals spent adsorbents has superior qualities including higher selection, better background contrast/clear images, and improved sensitivity [143]. A person whose blood-stained fingerprints are discovered at a crime scene may not immediately be the top suspect, but this evidence suggests that they were there when the blood was still fresh or was beginning to dry. Bloodstain pattern analysis and the detection of blood-contaminated fingermarks have been extensively studied over the past 40 years [144].

3. Conclusions

In conclusion, the comprehensive review of heavy metals (Pb²⁺, Cd²⁺, Ni²⁺) removal from wastewater through the utilization of low-cost adsorbents has illuminated a promising avenue for addressing environmental pollution challenges. The exploration of various adsorbents, ranging from agricultural by-products to industrial wastes, underscores the potential of sustainable and cost-effective solutions in mitigating heavy metal contamination. Adsorption has much potential for removing heavy metals from wastewater using inexpensive adsorbents, according to a review of various processes and adsorbents for heavy metal removal. To encourage the widespread use of unconventional adsorbents, more research should be done on low-cost adsorption processes from industrial waste, agricultural waste, etc. Low-cost adsorbents should be developed and used to keep costs down and increase the effectiveness of heavy metal removal. From the preliminary studies mentioned above, most studies conducted did not report the reuse of spent adsorbents. This is a fairly new area of research that has emerged. Hence more current studies focussing on closing this scientific gap are needed. The use of agricultural waste materials for Pb²⁺, Cd²⁺, and Ni²⁺ ions remediation has increased significantly. The main drawbacks, however, have been the high reagent requirements, the production of poisonous waste products, low adsorption capacity, and the inability to remove trace metals. However, in initiating such studies the reuse of spent adsorbents in blood fingerprint detection must also be explored.

Authors contribution

Yvonne Boitumelo Nthwane: Data curation, first draft writing, Conceptualization, editing, Bienvenu Gael Fouda-Mbanga: editing, conceptualization; Melusi Thwala: editing and conceptualization; Kriveshini Pillay: editing, supervision.

Acknowledgements

The authors are grateful to the University of Johannesburg and the Chemical sciences department for their facilities.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors also thank the Department of Science and Technology/Council for scientific and industrial research, South Africa, for funding the research project.