No abstract

No abstract

No 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.

The recovery of critical and precious metals from waste electrical and electronic equipment (WEEE) is an environmental and economic imperative. Biosorption has been considered a key technology for the selective extraction of gold from hydrometallurgical liquors obtained in the chemical leaching of e-waste. In this work, the potential of tannin resins prepared from Pinus pinaster bark to sequester and recover gold(III) from hydrochloric acid and aqua regia solutions was assessed. Equilibrium isotherms were experimentally determined and maximum adsorption capacities of 343 ± 38 and 270 ± 19 mg g−1 were found for Au uptake from HCl and HCl/HNO3 (3:1 v/v) solutions containing 1.0 mol L−1 H+. Higher levels of acidity (and chloride ligands) significantly impaired the adsorption of gold from both kinds of leaching solutions, especially in the aqua regia system, in which the adsorbent underperformed. Pseudo-first and pseudo-second order models successfully described the kinetic data. The adsorbent presented high selectivity towards gold. Actually, in simulated aqua regia WEEE liquors, Au(III) was extensively adsorbed, compared to Cu(II), Fe(III), Ni(II), Pd(II), and Zn(II). In three adsorption–desorption cycles, the adsorption capacity of the regenerated adsorbent moderately decreased (19%), although the gold elution in acidic thiourea solution had been quite limited. Future research is needed to examine more closely the elution of gold from the exhausted adsorbents. The results obtained in this work show good perspectives as regards the application of pine bark tannin resins for the selective extraction of Au from electronic waste leach liquors.