Unveiling the potentialities of activated carbon in recovering palladium from model leaching solutions

This paper reports experimental evidences on the capture of palladium over two commercial activated carbons, one derived from bituminous coal and one from coconut shell. Model solutions of palladium in both HNO3 and HCl were adopted and pH, temperature and salinity (in NaNO3 and NaCl) were varied. The experiments revealed that palladium adsorption in HCl is about ten times higher than in HNO3. Moreover, palladium adsorption was almost insensitive to pH and salinity variation, while it reduced with temperature. A comparison with the pertinent literature was based on the values of the adsorption capacity in two reference liquid concentration, one referred to a high leachate concentration, 15 mg L−1, and one referred to a tentative final concentration of 1 mg L−1. The adsorption capacities of the activated carbons in the two cases were higher than 60% and 76% of the compared materials. The experimental results suggest that palladium capture took place over the protonated basal plane of the carbon thanks to a network of adsorption and redox reactions, which led to the formation of elemental palladium over the carbon surface. Besides, palladium micro-precipitation also took place at pH between 3 and 4.5, further increasing the overall uptake capacity on the sorbent.