Promising Adsorbents for the Purification of Aquatic Environments from Heavy Metals: A Review

This work presents current aspects of chemical contamination of surface water environments, which serve as sources for drinking water, by heavy metals. Elevated concentrations of these metals induce toxic effects, while certain species, such as Cd(II), Pb(II), Cr(VI), and others, exhibit carcinogenic, mutagenic, and teratogenic impacts on the human organism and compromise the stability of the ecological environment. The adsorption capacity of various materials was analyzed, including natural aluminosilicates, layered double hydroxides (LDHs) with differing layer compositions and interlayer ligands, their calcined forms, potassium zinc hexacyanoferrate, and nanocomposites based on magnetite, among others, as adsorbents for the removal of heavy metals from environmental aqueous media. The results demonstrate that the efficiency of heavy metal adsorption significantly depends on their chemical speciation, which varies with pH, the macroscopic compositional characteristics of the aqueous environments, and other influencing factors. The analysis of adsorption properties of materials revealed that the efficiency of natural adsorbents is significantly lower than that of synthesized layered double hydroxides functionalized with inorganic and organic anions, potassium zinc hexacyanoferrate(II), as well as composites based on these materials with magnetite. The nature and spatial distribution of functionally analytical adsorption-active centers within their composition play a decisive role. When producing such adsorbents and determining conditions for their modification, it is crucial to select them according to the hypothesis of analogy. This hypothesis involves establishing a qualitative parallel between the conditions of reactions with organic and inorganic reagents (such as hydrolysis, precipitation, and complex formation) occurring in aqueous media and the functional groups present on the adsorbent surface. Applying this approach enables achieving high efficiency and selectivity of the adsorbents. Significant attention was also given to evaluating the effect of the pH of aqueous environments and the pH at the point of zero charge of the adsorbent surfaces, the nature and concentration of cationic and anionic forms of inorganic ecotoxicants, as well as the macroscopic composition of waters and the duration of adsorption. These factors decisively affect the degree of heavy metal removal by the studied multifunctional materials. The efficiency of adsorbents functionalized with chelating and hexacyanoferrate(II) anions, as well as calcined forms of Me(II),Me(III)-LDHs, toward cationic and anionic forms of heavy metals, predominantly depends on the differing stabilities of their complex compounds with the interlayer ligands of the multifunctional materials and the formation of precipitates such as hydroxides and hydroxycarbonates. Additionally, the physicochemical characteristics of the aqueous media, including pH, mineralization, and the concentration of natural organic substances, substantially affect this process.