Tannin‐Adsorbents for Water Decontamination and for the Recovery of Critical Metals: Current State and Future Perspectives

Abstract

Biosorption is known as an effective way to clean‐up water from organic and inorganic contaminants and has also emerged as a promising technology to recover critical substances. Tannins are renewable materials, coming from multiple vegetable sources. A variety of biosorbents have been developed from tannins, including tannin resins, rigid foams, composites with mesoporous silica, cellulose, collagen, and magnetic adsorbents. These materials have shown an excellent ability to uptake heavy‐metal cations (Cd(II), Cu(II), Pb(II), Ni(II), Cr(III)), owning to the chelating ability provided by the plentiful adjacent hydroxyl groups. In addition, tannin‐adsorbents have shown exceptional ability to remove Cr(VI), and to uptake Au(III) and Pd(II) from strong acidic solutions, which has evident application in the recovery of precious metals from e‐wastes leaching. The fact that tannin‐adsorbents can reduce the oxidation state of these adsorbates to Cr(III) and to elemental species of Au and Pd is interesting. Adsorption of dyes, surfactants, pharmaceuticals and antimony is also feasible, but the removal of certain metalloid species, such as arsenic and phosphate, seems to be limited even after applying chemical modifications. This article presents a systematic review on the preparation of tannin‐adsorbents and their application in water decontamination and in the recovery of critical metals.

Tannin-Based Hybrid Materials and Their Applications: A Review

“…Tannins, which are prone to metal chelation, are widely applied in the field of metal adsorption, in particular for water and wastewater treatment as well as for the recovery of valuable metals from a fluid. In the process of metal chelation, especially the ortho-dihydroxyl groups of the tannins’ catechol ring (3′-4′ site) represent the most involved phenolic hydroxyl groups, which hence allow catechol-like coordination with metals [ 18 , 19 , 38 ]. However, transition metals can also form coordination complexes to the polyphenol not solely via its orthodihydroxy (catechol) but also via its trihydroxybenzene (pyrogallol) moieties, if a hydroxyl group at the 5′ site is present [ 39 , 40 ].…”

“…However, transition metals can also form coordination complexes to the polyphenol not solely via its orthodihydroxy (catechol) but also via its trihydroxybenzene (pyrogallol) moieties, if a hydroxyl group at the 5′ site is present [ 39 , 40 ]. The metal chelation process takes place for all classes of tannins; however, condensed tannins are most likely to form complexes with metals due to their high chemical reactivity in comparison to hydrolyzable tannins [ 19 ].…”

“…In the literature, it has been shown that tannin-based adsorbent materials possess the ability to regain gold [ 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 ], silver [ 61 ], and palladium [ 53 , 54 , 55 , 57 , 58 , 61 , 62 ] ions from aqueous solutions. The recent research progress on tannin-based adsorbent materials can be found in greater detail elsewhere [ 19 ].…”

“…Due to their multiple aliphatic hydroxyls and phenolic hydroxyl groups, tannins have the ability to form complexes with proteins and polysaccharides as well as to interact with inorganic salts [ 12 , 14 , 16 ]. In general, the significant tendency of tannins to adsorb and reduce metal ions and hence generate stable metal complexes has to be stressed [ 17 , 18 , 19 ]. Particularly, condensed tannins possess a higher reactivity compared to hydrolyzable tannins and therefore are often chemically and economically of higher interest [ 19 , 20 ].…”

“…In general, the significant tendency of tannins to adsorb and reduce metal ions and hence generate stable metal complexes has to be stressed [ 17 , 18 , 19 ]. Particularly, condensed tannins possess a higher reactivity compared to hydrolyzable tannins and therefore are often chemically and economically of higher interest [ 19 , 20 ]. In addition, condensed tannins also show a heterocyclic reactivity as well as a reactivity of the aromatic nucleophilic sites.…”