A closed-loop sustainable scheme for silver recovery from water by reusable thiol-grafted graphene oxide

Abstract

Two-dimensional graphene oxide was modified with mercaptoethylamine and then treated with sodium dithionite to produce a 3D thiol-grafted graphene oxide (TGO) composite, which was investigated for the highly selective recovery of silver ions from water. Scanning electron microscopy, powder X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy were used to characterize the material before and after Ag+ adsorption. Batch adsorption experiments were carried out as a function of the initial Ag+ concentration, agitation time, temperature, and pH. The maximum adsorptive capacity of Ag+ by TGO, obtained from the Langmuir model, which fit the isotherm data well, reached 134.1 mg/g at 25 °C and a pH of 5.0. In addition, the adsorption was an endothermic process, and the adsorption kinetics followed the pseudo-second-order model. TGO was highly selective for Ag+; the selectivity factor of Ag+ reached 445 compared to Cu2+ ions and exceeded 1900 compared to Zn2+, Mg2+, Ca2+, Na+, and K+ ions. The adsorption of Ag+ on TGO induced the release of H+, and increased adsorption with increasing pH was observed. The adsorbed Ag+ can be proportionally desorbed using 1% HNO3. The reusability of TGO was confirmed by five cycles of adsorption-desorption-regeneration. The adsorption column studies indicated that TGO was stable in water and can be used as a filtering material for Ag+ recovery. The binding mechanism of Ag+ by TGO was proposed as the coordination of Ag+ with the sulfur atom of the mercapto groups attached to TGO followed by the reduction of Ag+ to Ag0.