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

The production of secondary precious and rare-earth metals is a key topic for sustainable development, due,to the increasing demand and the lowering of natural resources. Palladium is one of the most largely used noble metals, and there is an increasing interest toward its recovery from waste materials. Processes for secondary palladium production often include refinery processes in which the metal must be recovered from an acid leachate. Adsorption and ion-exchange are among the most reliable refinery methods. In the past years, several interesting studies deeply investigated the palladium capture capacity of ionic exchangers or functionalized biomasses. Thanks, to its mechanical properties, market size, industrial experience in process design and relatively low cost, the simple, non-impregnated, activated carbon may occupy a niche market in the production of secondary palladium. 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 HC1 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. (C) 2016 Elsevier B.V. All rights reserved.