Macadamia nutshells-derived activated carbon and attapulgite clay combination for synergistic removal of Cr(VI) and Cr(III)

A physical mixture of Macadamia-derived activated carbon and cationic attapulgite clay was investigated for total chromium removal in aqueous solution. The parameters influencing the adsorption of chromium on the sorbents were investigated, and it was shown that pH 3, contact time 2 hours, concentration 50mgL(-1) and calculated adsorption capacity of 96.28mgg(-1) were the optimal parameters. The process of adsorption was better described by Freundlich adsorption isotherm, and the kinetic modelling data suggested a chemisorption mechanism described by pseudo-second-order (PSO) rate model. Ionic strength studies demonstrated that the removal of anionic Cr(VI) species was mostly affected by the presence of anions like Cl- and NO3- 3, while the removal of the cationic Cr(III) species was affected largely by cations NH4+ > Na+ > K+. Overall, the removal mechanism involved adsorption, reduction and ion exchange processes.