Separation of palladium and silver from E-waste leachate: effect of nitric acid concentration on adsorption to Thiol scavenger

Abstract

The development of recovery techniques for metals present in low concentrations in E-waste, such as silver and palladium, is important from the aspect of the circular economy. Adsorption of palladium and silver was studied in detail in a batch process with silica-based Thiol scavenger from nitric acid leachate of waste printed circuit boards (PCBs). High adsorption efficiencies of Pd(II) and Ag(I), >97%, were reached in nitric acid concentrations below 3 mol L−1. At higher acid concentrations, adsorption efficiency of Ag(I) decreased drastically which enables the separation of Ag(I) and Pd(II) based on nitric acid concentration in sample solution. Pd(II) and Ag(I) followed pseudo 2nd order kinetical model with fast adsorption with contact times of 60 and 30 min, respectively, in all studied acid concentrations. Adsorption followed Langmuir isotherm reaching highest loading capacities of 199 mg g−1 for Ag(I) and 105 mg g−1 for Pd(II) in 1 mol L−1 nitric acid media, after which loading capacities decreased while nitric acid concentration increased. Thiourea-based desorption agents were most effective for desorption with >80% desorption efficiencies for Ag(I) and 55 - 88% efficiencies for Pd(II) in a one-step desorption. However, separation of Pd(II) and Ag(I) is not entirely possible in desorption step, hence the highest separation between Pd(II) and Ag(I) was found with two-step adsorption in which Pd(II) and Ag(I) were separated initially at adsorption step based on nitric acid concentration, followed by desorption step with acidic thiourea solution.