Asparagine anchored on mesoporous silica for Au (III) capture: Elucidation of adsorption-reduction mechanisms and their implications towards selective Au (III) recovery

[Display omitted] •Asparagine-modified silica (Asn@SBA-15) was prepared for selective Au3+ recovery.•Au3+ uptake by Asn@SBA-15 involves adsorption coupled reduction mechanisms.•Asn@SBA-15 prefers Au3+ over base- and precious metal ions in electronic wastewater.•Asn@SBA-15 is stable in acidic feed source and has competitive uptake performances.•Asn@SBA-15 can be regenerated and reused using acidic thiourea as Au stripping agent. Gold recovery from industrial wastewater requires effective materials that can selectively collect and separate Au3+ from other metal ions. Herein, amino acid asparagine (Asn) was introduced on mesoporous silica (SBA-15) through a sequential functionalization with aminopropylethoxysilane and glutaraldehyde linkers. Results from FTIR, TGA, SEM-EDS, SAXRD, N2 adsorption/desorption, EA and DLS confirm the successful fabrication of Asn@SBA-15. The material was further probed using TEM-SAED, EDS mapping, WAXD and XPS to elucidate the mechanisms involved in Au3+ capture. Results reveal that Au3+ (as AuCl4−, pH = 2) initially binds with the protonated amines of the anchored Asn and residual NH2 on the Asn@SBA-15 surface. Once captured, AuCl4− is readily reduced to Au0 and agglomerates as nano-islets on Asn@SBA-15 surface. The maximum equilibrium Au3+ uptake reached 307 mg g−1, with a rate that follows pseudo-second order kinetics. The Au3+ sequestration process is endothermic and spontaneous that is favoured with temperature increase. In a simulated acidic electronic wastewater, Asn@SBA-15 is highly selective and prefers Au3+ by α = 16 – 116,000 times over Al3+, Ni2+, Cu2+, Pb2+ and Pd2+. Asn@SBA-15 can be employed to repeatedly recover Au3+ with consistent performance through adsorption-reduction mechanisms combined with Au0 extraction using acidic thiourea.