Engineered TiO₂ polymorphs with superior photocatalytic activity for silver recovery from industrial cyanide-based plating effluent

A high-surface-area mesoporous TiO₂ photocatalyst was synthesized through a sequential hydrothermal-calcination process for the photocatalytic recovery of silver from industrial cyanide-based plating effluent. Preliminary results indicated that TiO₂ synthesized at low calcination temperatures (e.g., < 500 °C) exhibited low crystallinity, high BET surface area, an anatase-brookite mixed phase, and high bandgap values. In contrast, TiO₂ synthesized at higher temperatures (650 – 700 °C) showed high crystallinity, low BET surface area, an anatase–rutile mixed phase, and lower bandgap values. Specifically, TiO₂ synthesized at a calcination temperature of 500 °C (HT50) exhibited the highest activity for silver recovery from industrial cyanide-based plating effluent. Approximately 94.13 % of silver ions were recovered within 20 min under UV light irradiation, in the presence of 3.0 vol% ethanol as a hole scavenger and a catalyst loading of 1.5 g/L. This superior performance is likely due to the optimal content of anatase-brookite polymorphs. Besides, HT50 maintained consistent activity over four consecutive uses. The spent TiO₂, found in the form of Ag/TiO₂, also exhibited excellent photocatalytic activity for H₂ production under UV light irradiation. The results obtained from this study highlight a strategy for tuning the intrinsic properties of TiO₂ photocatalysts for photocatalytic precious metal recovery, as well as for upcycling metal ions from wastewater by depositing them onto the photocatalyst surface, leading to the formation of a new type of metal-doped photocatalyst that can be further applied in sustainable green energy production.