Perspectives and prospects of chelation extraction of heavy metals from wastewater: A review

Water is an essential component of life that covers two-thirds of the earth's surface with the human body consuming 75% of it hence making it one of the most essential elements responsible for life in the universe (Koki et al. 2015). Only 0.01% of the freshwater that is present on Earth's surface, which makes up around 3% of all water, is usable (Koki et al. 2015). Sadly, there are fewer water supplies available due to the increasing world population, industrial advancement, and long-time droughts (Azimi et al. 2017). The availability of clean drinking water is worsened even in areas with an abundance of water due to the uncontrolled discharge of industrial effluent laced with numerous organic and inorganic contaminants into water resources without sufficient treatment (Ali et al. 2016; Renu & Singh 2017).

Many countries are struggling with problems arising from microbiological contamination of water supplies, and therefore, due to much attention being directed to the pathogens, little focus is given to inorganic pollutants (Martínez-Santos 2017). Heavy metals, from both natural and anthropogenic sources, form part of the inorganic contaminants that are of the most concern (Renu & Singh 2017). In general, heavy metals are classified as metals that have a density of greater than 5 g/cm³ and are harmful even when present in trace levels (Carolin et al. 2017). Lead, zinc, copper, arsenic, cadmium, chromium, nickel, and mercury are some of the heavy metals that are of particular concern (Ahmed & Ahmaruzzaman 2016). High amounts of heavy metals are present in inadequately treated industrial and agricultural wastewater, which is frequently dumped into natural water sources (Chowdhury et al. 2016). According to studies, a large proportion of the heavy metals that find their way to the water sources are contained in untreated sewage (Rahman et al. 2020; Sayo et al. 2020).

The human metabolism depends on certain of these heavy metals, such as copper and zinc when present in trace amounts (Lamsayah et al. 2016; Magu et al. 2016), however, they are harmful if present in large amounts. Other heavy metals like cadmium, mercury, and lead are deadly even when trace amounts are ingested, in addition to having no known vital function in living things (Magu et al. 2016). These deadly heavy metals are frequently found in contaminated wastewater, together with others like arsenic, copper, chromium, nickel, and zinc (Akpor & Muchie 2010). Humans are often exposed to these heavy metals through ingestion, inhalation, and absorption. Drinking water is one of the primary ways that people are exposed to heavy metals, according to reports (Mahdavi et al. 2018).

Removal of heavy metals from wastewater is an important step toward a safe environment. Thus, as a result of the noxious effects of these heavy metals and their persistence in the environment due to their non-degradable nature (Ayangbenro & Babalola 2017), research is needed to ensure the safe discharge of wastewater. Unfortunately, many low- and medium-income countries are faced with the challenge of reducing some of the heavy metals below the proposed limits due to their limited economic capacities (Bank 2016). Among the existing technologies for the remediation of these noxious toxicants from contaminated wastewater, chelation extraction is emerging as an alternative strategy because it provides the choice of extractant type that influences the selectivity and extraction capabilities toward heavy metals even at trace levels (Karakuş & Deligöz 2015; Lamsayah et al. 2016; Sarıöz et al. 2018; Sayin et al. 2018). Thus, there is a definite need for the design and development of more stable chelating ligands that can meet site remediation and wastewater treatment requirements for heavy metals. Ideally, these ligands should work quickly, have a high binding capacity for the target analytes, and should not release their bound toxic metal ions easily. This review, therefore, will provide an overview of the advances made in chelation extraction as a remediation strategy for heavy metal-contaminated water, and then highlight the prospects of the technique.

Heavy metal toxicity occurs when undesirable levels of heavy metals on the earth, more so as a result of human activities, get absorbed by plants, animals, and human tissues through inhaling, diet, and skin through direct handling which binds and interferes with the working of essential cellular components (Asati et al. 2016). Due to their extended biological half-lives, non-biodegradability, and propensity to collect in various bodily areas, these metals are extremely dangerous (Xu et al. 2010; Zhang et al. 2011; Wamalwa 2016). Many of these metals are highly toxic due to their solubility in water. Nowadays, heavy metals are almost everywhere since they are mostly used in industries and also due to the increase in the number of garages (Mutuku 2013).

In general, toxic metals inactivate enzymes and damage cells since they act as antimetabolites or form precipitates with important metabolites (Adelekan & Abegunde 2011). Depending on the type of heavy metal, exposure can have several long and short-term negative health impacts, including renal failure, breathing difficulties, brain damage, nervous system failure, damage to the skin, teeth, and bones, and various malignancies (Chowdhury et al. 2016). Mercury, cadmium, lead, and arsenic exposure are associated with the main health risks posed by heavy metals. For example, copper, cadmium, and lead could lead to upper gastrointestinal cancer which is responsible for about 25% of worldwide cancer-related fatalities (Chowdhury et al. 2016).

The United States (US) Government Agency for Toxic Substances and Disease Registry (ATSDR) ranks mercury (Hg) third among the elements that are most harmful to human health (Budnik & Casteleyn 2019). There are three basic types of mercury: organic mercury (methyl mercury (MeHg)), inorganic mercury (mercury salts), and elemental mercury (Hg⁰). MeHg is regarded as the most dangerous type of these because of its high bioavailability and neurotoxicity (Hu et al. 2021). The average person is only moderately exposed to inorganic mercury, mostly via dental amalgam, mining and smelting of metals, burning of fossil fuels, and burning of municipal waste (Council 2000). Gold miners, dentists, mercury miners, and those who receive dental amalgam all have higher levels of inorganic Hg exposure (Chan 2011).

Ingestion, skin absorption, and inhalation are some of the common ways through which people might be exposed to mercury, and the effects on the body depend on the species of mercury (Bjørklund et al. 2017). It is absorbed during pregnancy into the fetal brain and other tissues. The most hazardous form of exposure is transplacental because the embryonic brain is so delicate. Mental retardation, fetal deformities, developmental delays, language impairments, and loss of vision and hearing are among the neurodevelopmental impacts of in-utero mercury exposure (Budnik & Casteleyn 2019). It has been shown that even 30 years after chronic Hg poisoning patients stopped being exposed to MeHg, they still experience distal paresthesias in their limbs and lips, underscoring the long-term detrimental health effects of Hg poisoning (Ha et al. 2017).

The acceptable concentration levels of copper metal, a transitional element, are necessary for plants, people, and animals (Bost et al. 2016). It is crucial for humans because it helps red platelets produce hemoglobin. Typically, it is utilized in food additives and copper salts in water supply systems to prevent algae growth in water distribution pipes (Pandey & Madhuri 2014; Nzeve et al. 2018). It gets into water bodies from insecticides, fungicides, and algaecides which contain copper compounds (Akan et al. 2010). Other sources of copper include alcoholic beverages from copper equipment, copper jewelry, and emissions from smelting and casting industries (Lakherwal 2014; Nzeve et al. 2018). Copper levels in soils are mostly impacted by crop and soil treatment, such as fungicides, fertilizers, and chicken manure (Wamalwa 2016). Additionally, depending on the plumbing system and groundwater, copper content levels in drinking water may vary. Soft acidic water may cause corrosion in copper pipes leading to an increase in tap water concentration (Zietz et al. 2003).

Despite copper metal being a vital micronutrient for human beings, it is usually harmful at higher concentration levels. A high concentration of the metal may lead to the damage of cells or even cause the death of cells (Bost et al. 2016). High copper doses can also cause hemolytic anemia, hepatic putrefaction in the liver, severe tubular rot in the kidney, epigastric pain, gastrointestinal bleeding, diarrhea, vomiting, tachycardia, and hematuria (Lakherwal 2014; Pandey & Madhuri 2014; Vardhan et al. 2019). Since the homeostatic process regulates copper's absorption and excretion, its toxicity as a result of consumption through a regular diet is viewed as a health problem (Turnlund et al. 2004).

According to the ATSDR list, cadmium is the seventh most toxic heavy metal (Jaishankar et al. 2014). Cadmium is not an essential element that occurs naturally in the air; it is emitted into the atmosphere by mines and businesses that use cadmium compounds for textiles, pesticides, solders, television sets, metallurgical industries, synthetic chemicals, and photography (Morais et al. 2012; Vardhan et al. 2019). Given that cadmium is an impurity, it is typically found in a variety of items, including phosphate fertilizers, detergents, and refined petroleum products (Pandey & Madhuri 2014). Cadmium, being non-biodegradable, remains in soils and sediments for a long time. Since cadmium is highly soluble in water and highly bioavailable, plants including food crops slowly absorb this metal which builds up in them and concentrates along the food chain hence extending to human beings and other animals (Mutlu et al. 2012).

Human beings get exposed to cadmium metal through ingestion of contaminated food, tobacco smoking, and inhaling contaminated air. While tobacco smoking is a fundamental source of introduction for smokers, non-smokers are severely harmed by eating food products with cadmium contamination (Vardhan et al. 2019). This metal may cause both acute and chronic intoxications to human beings exposed to it even at low concentration levels (Chakraborty et al. 2013). Some of the acute effects caused by cadmium intake are nausea, vomiting, diarrhea, abdominal pains, loss of consciousness, and excess salivation (Mutuku 2013). Cigarette smokers are mostly affected by cadmium intoxication than non-smokers; indeed studies have shown that the concentration of cadmium in blood samples of cigarette smokers is four to five times higher than the concentration in non-smokers (Järup 2003). The high concentration levels of cadmium through inhalation result in severe lung damage while consuming food containing cadmium may lead to enzyme poisoning. The cadmium present in the human body displaces zinc in many important enzymatic reactions which results in disrupting activities that eventually lead to acute gastroenteritis (Mutuku 2013). Moreover, when ingested at high concentration levels, it causes stomach irritation, vomiting, and even diarrhea. Low cadmium concentrations for an extended period can cause renal and lung damage because of deposits that can form in the kidney (Jaishankar et al. 2014).

Lead is a bright silvery metal that, in a dry environment, appears somewhat bluish. It serves no vital purpose in the human body. According to the ATSDR ranking, it is the second most toxic heavy metal after arsenic (Asati et al. 2016). Lead is utilized in the production of batteries, therefore, waste from the battery industry, car exhaust, electroplating and metal finishing processes, as well as fertilizers and pesticides, and cosmetics items are all sources of lead pollution (Sharma & Dubey 2005; Martin & Griswold 2009). Lead ingestion by human beings may result from eating food contaminated with lead and also the use of vessels containing lead compounds. After ingestion, it imitates metals that are required by the body, e.g., iron and zinc, hence binding with essential molecules thus disturbing their working (Mutuku 2013).

Following the establishment of a plant that recycles and smelts lead batteries in the Owino Uhuru village in Mombasa County along the coast of Kenya, adverse health impacts, notably on children, have been documented (Etiang et al. 2018; Ericson et al. 2019). Children are most frequently exposed to lead by consumption of lead-contaminated dust, soil, and water (Etiang et al. 2018). Severe headaches and memory loss are early signs of the central nervous system's (CNS) reaction to lead exposure. Lead poisoning can even result in death in severe cases, while short-term exposure causes long-term kidney, brain, and CNS damage (Ab Latif Wani & Usmani 2015; Joel et al. 2020).

Lead metal is regarded as a carcinogen by the Environmental Protection Agency (EPA) (Brochin et al. 2014). Depending on the levels of concentration and duration of exposure to lead in a person's body, either acute or chronic lead poisoning may occur and cause a wide range of symptoms (Jaishankar et al. 2014). Acute exposure occurs mainly in the workplace and also in industries that use lead or lead compounds and may result in the deterioration of kidney function (Mutuku 2013). In addition, exposure to higher concentration levels of lead may lead to chronic nephritis, mental retardation, allergies, weight loss, and brain damage which lowers a person's IQ and may lead to death (Mutuku 2013). The unborn child and newborn babies are more affected than adults, causing a delay in speech, disability in reading, attention disorder, behavior disorder, increased risk of cardiovascular disease, retarded growth, and cognitive deficiency which continues to adulthood (Ayangbenro & Babalola 2017). Lead poisoning may also cause other effects in human beings which later cause difficulties in the formation of hemoglobin and also causes damage to the reproductive system (Järup 2003; Mutuku 2013; Pandey & Madhuri 2014; Ayangbenro & Babalola 2017).

Considering the negative effects heavy metals have on human health, it is crucial that they are removed from water and wastewater, as well as to maintain their continuous monitoring and evaluation (Ayangbenro & Babalola 2017). Several technological approaches have been explored to remove these harmful toxicants from contaminated wastewater, including ion exchange, membrane filtration, chemical precipitation, coagulation–flocculation, adsorption, and electrochemical approaches (Sayin et al. 2018). However, these methods have several drawbacks, such as insufficient metal removal, non-selectivity, the requirement for costly chemicals and significant energy usage, and the formation of hazardous sludge (Uysal Akku et al. 2015). Nonetheless, no single technology can suffice in the extraction of heavy metals from wastewater due to the variations in the physical state, aqueous state concentration and pH and conditions of the industrial effluent (non-aqueous solvents, contact time etc.) (Blue et al. 2010). Therefore, it is critical to research targeted novel and effective techniques for removing metal ions from drinking and wastewater. One such approach is the chelation extraction technique, which entails the employment of organic chelating agents to remove the heavy metal cations from water.

Chelating agents are compounds that have two or more groups that can donate the necessary pair of electrons to form a coordination bond with a metal ion to create a ring structure known as a chelate ring (Mercier & Barthés-Labrousse 2009). Potential chelating agents for trace metals include organic molecules having functional groups containing nitrogen, oxygen, or sulfur (sulfur in thiols and thiocarbamides), as in amines, amides, or nitriles (Komjarova & Blust 2006). Thus, the key element of chelation extraction is the design and development of appropriate chelating agents that can play the role of an extractant, characterized by high efficiency and selectivity for the target metal ions (Kończyk & Dlugosz 2020). The ability to design and synthesize a chelating agent able to target a specific heavy metal and hence influence its subsequent extraction capacities, even at trace levels, is what makes chelation extraction an attractive remediation technique (Karakuş & Deligöz 2015; Lamsayah et al. 2016; Sarıöz et al. 2018; Sayin et al. 2018). Huge efforts, therefore, have been put into ligand modification in the search for chelating agents to enhance their selectivity, in the hopes that the new ligand systems will have the ability to induce high extractive ability of the target heavy metals. The extraction of heavy metals from natural waters and wastewater with the aid of chelating agents is achieved mainly by solvent extraction method or solid-phase extraction (SPE).

Remarkably, solvent extraction is the technique of choice when it comes to the separation and recovery of metals from complex matrices because it is less time-consuming, has a low operational cost, and most importantly is highly selective for the target metals (Alvial-Hein et al. 2021). Moreover, the solvents used in this technique are stable, transparent to UV, not flammable, easily recoverable by distillation (low-boiling point solvents), while the high-boiling point solvents do not emulsify (Hassanzadeh-Afruzi et al. 2022). Consequently, solvent extraction is widely used for the separation and recovery of heavy metals such as Co(II) and Ni(II) from process streams and even at the industrial level (Kumbasar 2009).

Unfortunately, although solvent extraction is straightforward and quick to make, the majority of the available chelating agents are not specific enough in binding with the target metal ions and do not form stable complexes (Matlock et al. 2002c). As a result, the created complexes disintegrate and release heavy metals into the environment. Sodium dimethyldithiocarbamate (SDTC) (1) and sodium thiocarbonate (STC) (2) are two examples of often-used commercial reagents (Matlock et al. 2002c).

Unfortunately, laboratory tests have revealed that these substances have large leaching rates, can disintegrate into dangerous chemicals during chelation, need higher doses than stoichiometric values, and are useless at pH levels below 4.0 (Matlock et al. 2001a, 2002a). These compounds also raise issues due to their limited bidentate binding capacities for heavy metals, which causes them to break down and reintroduce the heavy metals to the ecosystem over a variable, but often brief, period (Matlock et al. 2001a, 2002b). The literature data on SDTC and STC show that the reagent-metal combinations can easily break down into other materials, such as HgS (Matlock et al. 2001a). For instance, the Indiana Department of Environmental Management claims that when about 1.5 million gallons of contaminated wastewater laced with SDTC were accidentally released into the city's wastewater system in December 1999, over a 50-mile span from Anderson to Indianapolis, 117 tons of fish perished as a result of it (Matlock et al. 2002a). Accordingly, there is a critical need for novel materials that can remove heavy metals from polluted media due to the difficulties that the current commercial chelating agents confront.

Under a range of laboratory conditions, the ligand displays preferential binding to the mercury(II) cation in equimolar mixtures of the metal and BDET solutions by the formation of covalent Hg–S bonds (Matlock et al. 2001a). However, the kinetics of Hg binding by BDET may be hindered in solutions with low mercury contents and large concentrations of other soft heavy metals (Matlock et al. 2002c). Aside from precipitating additional divalent metals including Fe, Cu, Cd, and Pb, BDET has also been demonstrated to be successful in preventing metal leaching from coal and sulfide minerals, removing lead from lead-battery recycling effluent, removing metal from acid mine drainage, and more (Blue et al. 2010). Matlock and colleagues created a pyridine-based thiol ligand (DTPY) (3) that uses two chains at the 2,6-position and contains three carbons, one nitrogen, and two sulfur end groups to increase the stability of the precipitates that are generated (Matlock et al. 2001b). It appears that several interactions between an M²⁺ metal cation and the S and N atoms in the ligand result in a stable complex. According to literature sources, DTPY can be used to successfully precipitate cadmium and copper metals (Matlock et al. 2001b). The results revealed that 99.70% extraction of cadmium from a 50.00 ppm aqueous solution could be achieved utilizing the DTPY ligand at an 8.32% dosage increase within 1 h at a pH of 6.0, whereas >99.98% removal could be observed within 1 h at a pH of 4.5 for a 50.00 ppm copper solution.

In a related study, it was discovered that alkyl thiol chelates (4–7), which have an alkyl backbone with three or four carbons, two to four carbon alkyl arms linked to each sulfide, each arm ending in a thiol group, and sulfide groups on both ends of each arm, are efficient at precipitating mercury from water and produce stable precipitates (Hutchison et al. 2008).

Additionally, Wang and associates have created a potential chelating agent, N¹,N²,N⁴,N⁵-tetrakis(2-mercaptoethyl) benzene-1,2,4,5-tetra carboxamide (TMBTCA), (8) that may remove copper from wastewater (Wang et al. 2012). Through comparisons for heavy metal removal, TMBTCA also demonstrated a stronger affinity for cadmium and copper than commercial reagents like DTC, TBA, and TMT, and the TMBTCA precipitates have good stability.

In a related solvent extraction investigation by Ojwach et al. (2012a, 2012b), the chelating ligands 2,6-bis(pyrazol-1-ylmethyl)pyridine (9), 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)­pyridine (10), 2,6-bis(3,5-ditertbutylpyrazol-1-ylmethyl)-pyridine (11), 2-(pyrazol-1-ylmethyl)pyridine (12), and 2-(3,5-dimethylpyrazol-1-ylmethyl)-pyridine (13) demonstrated a strong preference for the zinc(II) cation and good potential as chelating ligands for the extraction of Zn(II), Pb(II), and Cd(II) from water. The architecture of the ligands and the type of metal ion had an impact on how well they could be extracted. In another related report by Njoroge et al. (2013) (3,5-dimethyl-1H-pyrazol-1-yl)ethanol (12) showed a predilection for Zn(II), Cu(II), and Pb(II), poor extractions for Cd(II), and exhibited moderate donor behavior.

In a recent solvent extraction study by Wambugu et al. (2021), phenoxy-imino ligands 2-[{(2-hydroxyethyl)imino}-methyl]phenol (14), 2-[{(2-(diethylamino)ethylimino}methyl]phenol (15), 2-[1-{2-(diethylamino)ethylimino}ethyl]phenol (16), and 2-[(2-mercaptoethylimino)methyl]-phenol (17) demonstrated excellent extraction potential for the removal of copper(II), zinc(II), lead(II), and cadmium(II) from water, with a strong preference for copper(II) cation. The outcomes illustrated the possible use of phenoxy-imino ligands in the extraction of copper(II) and zinc(II) from sewage effluent. The extraction capacity of the ligands was influenced by the type of metal ion, ligand architecture, and Irving-Williams series.

In another study, aliphatic diethyldithiocarbamate (18) and an aromatic diphenyldithiocarbamate (19) ligands were investigated as potential chelators to remove copper(II), zinc(II), lead(II), and cadmium(II) from polluted water (Abu-El-Halawa & Zabin 2017). The results showed that compared to its diethyldithiocarbamate counterparts, the diphenyldithiocarbamate ligand was better at lowering the concentration levels of the metals under investigation. Moreover, compared to activated carbon techniques, diphenyldithiocarbamate effectively extracted over 85% of the studied heavy metals.

Although solvent extraction is a dependable and effective method, the technique suffers from the requirement of a large inventory of organic solvents and is difficult to automate (Komjarova & Blust 2006). The organic solvents commonly used in solvent extraction are predominantly petroleum products, which have adverse effects on human health and the environment because of being highly volatile, flammable, non-biodegradable, and non-renewable in nature (Chang 2014). Currently, there are efforts directed toward the utilization of green organic solvents in solvent extraction processes although those efforts are hampered by their complex and costly synthesis routes (Chang 2020). Thus, there is a need for an alternative technique that can circumvent this disadvantage of solvent extraction without losing the attractive advantages of chelation extraction.

SPE presents a compelling option for green chemistry concepts, such as methods free of organic solvents and ease of reaction medium separation. Moreover, SPE has attracted a lot of interest due to its simplicity, high concentration factor, minimal sample volume needed, and greater usage of ecologically friendly chemicals (Komjarova & Blust 2006). Mesoporous silica, such as MCM-48, MCM-41, HMS, and SBA-15, that has been organically functionalized is thought to be a particularly promising adsorbent for the removal of heavy metal ions from wastewater (Kruk et al. 2000; Da'na 2017). This is because it has desirable qualities including an orderly structure, a sizable surface area, and an open porosity structure that makes it easy to reach the functional groups. This has helped the technology behind SPE advance quickly (Radi et al. 2019). The ability to modify organic ligands using SPE methods is possible because the type of donor atoms employed to immobilize the ligands has a significant impact on their ability to bind the target metal ions, to ensure robust binding and excellent selectivity toward the desired pollutant (Da'na 2017; Tighadouini et al. 2020).

In this context, several ligands bearing a variety of donor atoms have been prepared and immobilized on the silica surfaces. One of the leading researchers in SPE is Smaail Radi and co-workers who reported the synthesis of silica-immobilized [(E)-4-((pyridin-2-ylmethylene)-amino)phenol (PMAP)] Schiff's base ligand (20) that was most effective at extracting Hg²⁺, Cd²⁺, Pb²⁺, and Zn²⁺ in the pH range 6.5–8 (Radi et al. 2016b). They discovered that the coexisting alkali ions had no effect on separation or determination and that the adsorbent could be repeatedly regenerated without being damaged. Another study found that silica-immobilized bipyridine tripodal receptor (21) had outstanding Pb(II) adsorption capability in both competitive mode and natural actual water samples, as well as greater distribution coefficients for Pb(II), Cd(II), Zn(II), and Cu(II) (Radi et al. 2016a). Additionally, the material showed a high level of reusability across several cycles. These findings imply that this newly developed material has the capability of extracting heavy metals from aqueous solutions, opening up significant vistas. In a separate investigation, Radi and colleagues looked into the use of a batch technique to extract Pb(II), Zn(II), Cd(II), and Cu(II) from aqueous solutions utilizing a porphyrin receptor (22) fixed on silica by β-pyrrolic position (Radi et al. 2019). They discovered that, unlike the various previously reported sorbents, the functionalized material demonstrated outstanding adsorption ability toward Pb(II), Zn(II), Cd(II), and Cu(II). Adsorption maximums were attained in about 25 min, which suggests quick coordination.

Tighadouini et al. (2019a) has also looked at the potential use of functionalized silica for the removal of Zn(II), Pb(II), Cd(II), and Cu(II) from aqueous solutions utilizing silica-immobilized conjugated-keto-enol-pyridine-furan ligand (23). The substance was shown to be extremely stable and powerful as a remediation adsorbent for the removal of Zn(II) from real water samples from the field. Surprisingly, even after five cycles, its adsorption performance remained higher than 98%. Silica-supported amino pentacarboxylic acid (SiDTPA) (24), which has been discovered to be a potential adsorbent candidate for effective and quick extraction of Cu(II), Zn(II), Cd(II), and Pb(II) from aqueous solutions with great reusability, has also been reported to have similar stability (Radi et al. 2018). The removal of harmful metals from water has also been investigated using the silica-functionalized pyridylpyrazole-β-keto-enol receptor (25), and it has proven to be quite effective, registering a selectivity order of Pb(II) > Zn(II) > Cu(II) > Cd (II) (Tighadouini et al. 2019b).

Besides mesoporous silica, the use of magnetic Fe₃O₄ nanoparticles in the extraction of heavy metals from water has gained a lot of interest due to their distinguished properties such as high adsorption capacity, large specific area, and ease of separation from wastewater by applying a magnetic field, for further recycling or regeneration (Zhao et al. 2020). As a result of the propensity of naked magnetic Fe₃O₄ nanoparticles to aggregate and undergo oxidation which will lead to low adsorption capacity selectivity for metal ions, the surface of Fe₃O₄ is coated with silica and then functionalized with a suitable chelating ligand. For example, Wang et al. (2016) synthesized and investigated silica-coated magnetic Fe₃O₄ nanoparticles (Fe₃O₄@SiO₂) functionalized with a thiol group (26) for the extraction of Hg(II) from water and recorded a maximum adsorption capacity of 132.0 mg/g. In a related study, Nawaz et al. (2020) prepared silica-coated Fe₃O₄ nanoparticles functionalized with diglycolic acid for effective uptake of Pb(II) and Cr(VI).

Investigations on the uptake of As(V) ions from aqueous solutions using mercaptobenzothiazole-functionalized Fe₃O₄@SiO₂ (28) have been reported with a removal percentage of 93.89% (Sheikhmohammadi et al. 2018). Good adsorption of Pb(II) and Cd(II) at the optimal adsorption pH of 6 by silica-coated magnetic Fe₃O₄ nanoparticles functionalized with a Schiff-base (29), prepared using salicylaldehyde has also been reported (Zhao et al. 2018). Zhao et al. (2020) extended their investigations of adsorbent 29 to the extraction of Hg(II) and Ag(I) where an optimal adsorption pH was also 6. Moreover, the adsorbent displayed good regeneration properties with adsorption of above 92% being observed after five adsorption–desorption cycles. The effect of solution pH has also been demonstrated by a study by Bao et al. (2017) who investigated mercaptoamine-functionalized Fe₃O₄@SiO₂ nanoparticles (30) for the removal of Hg(II) and Pb(II) from water where maximum adsorptions of 10 and 12 mg at pH 5–6 and 6–7 for Hg(II) and Pb(II), respectively, were recorded. Furthermore, Kobylinska et al. (2020) have proved the ability of Fe₃O₄@SiO₂-immobilzed ethylenediaminetriacetic acid derivative (31) to extract heavy metals from environmental, tap, and wastewater samples in a wide range of pH and recyclability potential of at least six times using 0.1 M HNO₃ with high preconcentration factor.

The potential of chelation extraction for the removal of metal ions from water has been demonstrated in the research papers considered in this review. Despite the excellent extraction efficiencies reported, the requirement of organic solvents and the fear of forming unstable metal-ligand complexes that can decompose and release the heavy metals back into the environment or form toxic by-products are major drawbacks of solvent extraction (Matlock et al. 2002b). Investigations have shown the potential application of green organic solvents in solvent extraction processes (Chang 2020). Further research is, however, required to enhance their competence as heavy metal extracting solvents and optimize their manufacturing given that they are quite expensive when compared to conventional organic solvents. Moreover, despite the efforts directed toward the design and synthesis of stable and efficient biodegradable chelating agents (Pinto et al. 2014), the need for these biodegradable chelating agents and their optimized extraction processes remains.

The chelating ligand functionalized silica and magnetic nanoparticle have also shown great promise in the extraction of heavy metals from water. Most of the studies, however, only consider single heavy metal pollutants using batch and column adsorption techniques. Further work is required to validate the findings using real industrial wastewater for single and multi-pollutants.

It is urgently necessary to address the growing environmental issue of heavy metal pollution of wastewater in many developing nations. Numerous treatment systems for the extraction of heavy metals from wastewater have been developed to comply with increasingly strict environmental standards. As one of the most adaptable methods used to remove heavy metals from wastewater, chelation extraction continues to get a lot of attention from both academia and industry. The reviewed synthesized chelating ligands showed good extraction capacity for the removal of metal ions from aqueous solutions. Despite this, solvent recovery is still thought to be expensive, energy-intensive, and solvent volume-intensive. As a more useful and environmentally benign method for the retention of heavy metal ions, organically modified mesoporous silica, and organically modified magnetic nanoparticles are seen to be a very promising alternative. The functionalization of mesoporous silica and magnetic nanoparticles has not found commercial use; therefore, they are still not applicable in real environments, despite the numerous investigations and encouraging results that have been reported. Therefore, it is necessary to conduct the cost–benefit analysis of the solid–liquid extraction technique in the removal of heavy metals from wastewater based on the potential exhibited so far. Moreover, there is a need for concerted efforts toward the design of chelating agents with appropriate functionality to boost adsorbent selectivity, especially in the multi-component system, which is a key element in the operation of practical processes.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.