Direct Z-Scheme Tannin–TiO₂ Heterostructure for Photocatalytic Gold Ion Recovery from Electronic Waste

Abstract

Precious-metal recovery from industrial wastewater has received considerable attention because of rapidly increasing amounts of electronic waste. Existing technologies have yet to be widely applied due to their high cost and low selectivity toward precious-metal ions. Herein, we report a direct Z-scheme tannin–TiO2 heterostructure for selective gold adsorption from electronic waste under solar irradiation. The tannin-coated TiO2 nanoparticles were prepared by a simple dipping method, and under light illumination, both tannin and TiO2 can serve as photosensitive components for the reduction of metal ions, with metal-to-ligand charge transfer from TiO2 to tannin extending the lifetime of the excited electrons. Moreover, no additional electron donors are required because the tannin layer scavenges the reactive oxygen species generated by the holes from the light-activated TiO2 surface. The heterostructure allows for the highly efficient photocatalytic recovery of gold ions, with 11 times higher adsorption capacity in the light compared to the dark. High selectivity toward gold ions was also demonstrated using a metal ion mixture including nine different metal ions that are commonly found in electronic waste. Our findings suggest that the Z-scheme heterostructure of polyphenol and semiconductor provides a promising photochemical pathway for efficient and selective metal ion recovery from electronic waste.

Tannin-Based Hybrid Materials and Their Applications: A Review

“…On the one hand, it can reduce the adsorbed metal ions and thereby decrease the bandgap. On the other hand, tannin enhances the reduction reaction speed and essentially acts as a shuttle for photoexcited electrons from TiO 2 to the metal, as presented in Figure 3 g. As proposed in detail by Kim et al [ 78 ], the photoexcited electrons of tannin possess an extended lifetime because their recombination is prevented due to the unoccupied hole in the HOMO of tannin is filled by an electron stemming from TiO 2 . Photoexcited electrons can transfer from TiO 2 to tannin, in turn leaving the electron–hole in the ceramic unpaired.…”

“…Copyright 2018 Elsevier Inc. ( g – j ) Schematic representations of example applications of tannin–ceramic hybrid materials. ( g ) TiO 2 –tannin hybrids with enhanced photocatalytic metal ion reduction activity and the suggested Z-scheme of the net electron pathway [ 78 ]. ( h ) pH-responsive release of drugs from the hexagonal cavities of mesoporous particles capped with a tannin-based coating.…”

Covalent Organic Framework Isomers for Photoenhanced Gold Recovery from E-Waste with High Efficiency and Selectivity

“…41 Recently, Qiu's group has reported the photoenhanced adsorption of uranium in seawater under the irradiation of light by accelerating the generation of photoelectrons in COFs to reduce U(VI) to insoluble U(IV). 42 Considering the lower reduction potential of [AuCl 4 ] − (AuCl 4 − + 3e − = Au 0 + 4Cl − , E 0 = 1.002 V), 43,44 it is feasible to employ COFs as a platform for the photoreduction and adsorption of Au(III). However, the photoreduction and adsorption of Au(III) by COFs with photocatalytic activity have been rarely explored.…”

Photocatalytic Recovery of Gold from a Non-Cyanide Gold Plating Solution as Au Nanoparticle-Decorated Semiconductors

“…The presence of Ag­(I) and Cu­(II) decreased the photoreduction of Au­(III) due to the competition in adsorption and reduction, while the presence of Fe­(III) can enhance the photoreduction due to the reduction and quick oxidation of Fe­(II). The tannin (TA)-TiO 2 heterostructure induced a high selectivity of Au­(III) adsorption, which can prolong the lifetimes of excited electrons and then allow the adsorption/reduction of Au­(III) from the solution …”