Removal of heavy metal ions from wastewater: a comprehensive and critical review | npj Clean Water
The adsorption mechanism is defined by the physicochemical properties of adsorbent and heavy metals and operating conditions (i.e., temperature, adsorbent amount, pH value, adsorption time, and initial concentration of metal ions). Generally, heavy metal ions can be adsorbed onto the adsorbent’s surface, as shown in Fig. 1a. This method was reported to have low operating costs, high removal capacity, easy implementation, and simple treatment by regenerating the adsorbed heavy metal ions7. Different types were developed for wastewater remediation, as discussed in the following sections.
Fig. 1: Adsorption process used for water treatment.
a Heavy metal ions adsorption process; the metal ions of wastewater adhere to the surface of nanoporous adsorbents, which has a high surface area due to its porosity. The adsorption process could be selective for one or more metals than others. The regeneration process could be achieved using a desorbing agent. b Various modification techniques (i.e., nitrogenation, oxidation, and sulfuration) are used to functionalize carbon with different functional groups. Functionalization enhances adsorption capacity and stability.
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Carbon-based adsorbents
Carbon-based nanoporous adsorbents, especially activated carbons (ACs), carbon nanotubes (CNTs), and graphene (GN), are extensively used in the applications of heavy metal removal owing to their tremendous surface area (500–1500 m2/g)8. The carbon surface charges can be enhanced by surface functional groups (such as carboxyl, phenyl, and lactone groups, as shown in Fig. 1b) to improve the heavy metal uptake9. Among various modification methods, nitrogenation, oxidation, and sulfuration are the most commonly employed techniques to enhance the specific surface area, pore structure, adsorption capacity, thermal stability, and mechanical strength10. However, they depend mainly on the adsorbent materials, which sometimes are very expensive11. Subsequently, adsorbent’s cost should be considered in choosing the most suitable adsorbents.
Surface modification often reduces its surface area and, in turn, increases the content of surface functional groups. Consequently, more metal ions can be adsorbed12. Supplementary Tables 1 and 2 summarize the removal capacity and characteristics of carbon-based adsorbents and composite adsorbents. The adsorption uptake increases by increasing the adsorbent surface area, adsorbent dose, initial concentration of metal ions, and contact time. Although the multi-wall carbon nanotubes (MWCNTs) have received particular interest for heavy metal removal13, they are highly hydrophobic and suffer from rapid aggregation in aqueous solution due to large Van der Waals forces, decreasing the adsorption potential.
There is a lack of literature in quantitative assessment of functional groups’ role in heavy metal ions sorption. Moreover, the current surface modification techniques demand high heat/pressure, strong acid/base, or intensive oxidation/reduction reactions. This complex preparation process makes the carbon-based adsorbents expensive, burdening their widespread use in industrial applications. Thus, researchers should propose innovative, low-cost, and environmentally friendly surface modification techniques.
Chitosan-based adsorbents
Chitosan (CS) is a natural adsorptive polymer that has an affinity toward pollutants in wastewaters because it has amino (–NH2) and hydroxyl (–OH) groups14. Despite its unique features, it suffers from low mechanical strength and poor stability15, making the regeneration inefficient. Also, it is challenging to use CS in its powder or flake form because of its low porosity, low surface area, resistance to mass transfer, and high crystallinity15. Consequently, structural and chemical modifications have been proposed to overcome these drawbacks. Cross-linking chemical modification imparts strength to CS by bridging between polymer chains and the functional groups. However, this approach reduces the uptake16.
Grafting is another chemical modification method that involves the covalent bonding of functional groups (like amine and hydroxyl) on the backbone of CS, leading to a remarkable increase in the adsorption capacity17. Combining CS with other adsorbent materials has also been proposed to enhance CS’s adsorption capacity, mechanical strength, and thermal stability18. The ion-imprinting technique was followed to prepare adsorbents which high selectivity for target metal ions19.
Supplementary Table 3 lists the uptake of different CSs for heavy metal ions removal from wastewater. Generally, the uptake of CS depends mainly on the presence of protonation or non-protonation of amine (–NH2) and phosphoric (H3PO4) groups, which affect the pH value of the wastewater. In the absence of the modifications, CS-based shows low reusability. This behavior might be attributed to the strong bond (between the metal ions and adsorbent surface), low thermal/chemical stability, low mechanical strength, incomplete desorption, declination in the effective adsorbate-adsorbent interaction, and unavailability of adsorption sites20. So, alternative regeneration methods and modifications should be proposed to enhance the reusability of CSs.
Mineral adsorbents
Mineral adsorbents such as zeolite, silica, and clay are considered good candidates for water purification with low operating costs21. Clay has extraordinary cation exchange capacity (CEC), cation exchange selectivity, surface hydrophilicity, high swelling/expanding capacity, and surface electronegativity22. In addition, acid washing, thermal treatment, and pillar bearing could enlarge the pore size, pore volume, and specific surface area, leading to a remarkable increase in the adsorption efficiency22. Research studies (listed in Supplementary Table 4) showed that physical adsorption, chemical adsorption, and ion exchange are the most common mechanisms controlling wastewater treatment using mineral adsorbents. Besides the mentioned parameters, the pH, temperature, adsorption time, and adsorbent dosage are also considered vital parameters controlling the adsorption process. The adsorption removal efficiency increases when the pH increases and the initial concentration decreases23.
Using natural minerals could be cost-effective. However, the removal efficiency might decrease after a few cycles24. Therefore, different modification methods, such as calcination and impregnation, have been proposed to enhance the removal efficiency of such adsorbents25. However, these modifications incur additional costs to the process and release new chemical agents into the environment. Grafting functional groups could synthesize eco-friendly and multifunctional adsorbents suitable for treating various types of wastewaters. The preparation of two-dimensional nanosheets and one-dimensional nanotubes-based clay adsorbents might lead to innovative low-cost and high-performance adsorbents.
Magnetic adsorbents
Magnetic adsorbents are a specific material matrix that hosts iron particles (usually magnetic nanoparticles, such as Fe3O4)26. The base material could be carbon, CS, polymers, starch, or biomass. As illustrated in Fig. 2, the adsorption process is affected by the magnetic field, surface charge, and redox activity characteristics. They showed low-cost, easy-synthesis, extraordinary surface charge, and reusability. Many magnetic adsorbents were proposed in the literature, such as zero-valent iron nanoparticles (ZVI NPs), iron oxides (hematite (α-Fe2O3), maghemite (γ-Fe2O3), magnetite (Fe3O4)), and spinel ferrites. The mechanism and kinetics of the sorption process rely on several parameters, such as surface morphology and adsorbent magnetic behavior. They are also affected by experimental conditions such as pH, irradiation time, adsorbent concentration, wastewater temperature, and the initial dosage of pollutants27. The presence of iron particles in adsorbent is very efficient in metal ions removal from effluent28.
Fig. 2: Adsorption process via magnetic adsorption.
The magnetic adsorbent particles adsorb the metal ions and sequentially accumulated; thus, the wastewater is treated.
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Some studies have focused on coating Fe3O4 particles for removing heavy metal ions. Co-precipitation, high-gravity technology, and grafting are the most commonly used methods29. The grafting method was considered a preferable choice because it is flexible and straightforward. However, it strongly depends on the active hydroxyl on the surface of Fe3O4 particles and the number of active functional groups. The produced adsorbents were not adequately cyclic stable, which is a barrier facing the commercialization of this method. Additional details about different magnetic adsorbents can be found in Supplementary Table 5.
Biosorbents
The most recent research studies in using biosorption for wastewater treatment are listed in Supplementary Table 6. The presence of numerous functional groups (i.e., carboxyl, amino, hydroxyl, phosphate, thiol, etc.) on the surface expedite the biosorption process30. Generally, the interaction between pollutants and the surface of biosorbent can occur through electrostatic interaction, aggregation, complexation/coordination, microprecipitation, ion exchange, reduction, or oxidation31. The solution pH affects the biosorbent surface charge density and ionization of functional groups located on the biosorbent surface32. When pH is low, cations are almost stable and can be bonded to the biosorbent surface. On the other hand, at higher pH values, the solubility of metal cations decreases with the possibility of a precipitation phenomenon.
The biosorbent amount is a vital factor affecting the removal efficiency due to offering more vacant biosorption sites. The biosorbent capacity could increase at higher temperatures due to decreased solution viscosity, reduction in Gibb’s free energy, and bond rupturing. These reasons increase the collision frequency (mobility and kinetic energy) between biosorbent and metal ions and enhance the biosorbent active sites, leading to a higher affinity31. In turn, the bonding force between biosorbent and pollutants could decline at higher temperatures, and thus the biosorbent sorption uptake reduces. It was elucidated that the removal efficiency increases as the mixing agitation rate increases33.
Metal-organic frameworks adsorbents
Metal-organic frameworks (MOFs) are generally synthesized via reticular synthesis in which metal ions are strongly bonded to organic linkers. Researchers proposed thousands of MOFs. It was noticed that most of the organic ligands used to form many MOFs are very expensive and toxic34. Zirconium-MOFs family (such as UiO-66) is promising nanostructure materials for sorption applications due to the easy incorporation of functional groups and hydrolytic-thermal stability such as amine, carboxylic, hydroxyl, and oxygen35 or by using the cross-linking method36. Composite-based MOF adsorbents could obtain further enhancement in the adsorption capacity of MOFs. Supplementary Table 7 lists the uptake of different MOFs towards several heavy metal ions in wastewater.
Despite the exciting features of MOFs and their good capability to remove heavy metal ions, they have micropores (i.e., tiny pores) inaccessible for some target metals. Also, most of them have low stability in water. Mn, Fe, and Cu have been used to form MOFs, but most of them resulted in poor chemical stability37,38,39,40,41,42,43,44,45,46,47,48,49,50. Therefore, further research is still needed to tune the MOFs’ structure and scale up these materials to implement them into industrial wastewater applications. Moreover, different functionalization methods should be proposed and applied to enhance MOFs’ stability and sorption kinetics.
The reported maximum uptakes of heavy metal ions for a proper adsorbent are listed in Table 2.
Table 2 Heavy metal adsorption onto nanoporous adsorbents with the highest capacity.
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