Selective mesoporous adsorbents for Cr2O72- and Cu2+ separation

NH2–MCM-41 and COONa–MCM-41 were prepared by grafting aminopropyls and propionates on the pore wall of the mesoporous silica MCM-41 for the separation of Cr2O72- and Cu2+. Single and binary component adsorptions show that NH3+–MCM-41 has a 100% selectivity for Cr2O72- adsorption at low pH (<3.5 i.e., point of zero charge, p.z.c.), while COO−Na+–MCM-41 adsorbs only Cu2+ over the entire pH range of the study (i.e., pH 1.5–5.5). Adsorption uptakes were rapid on both adsorbents reaching equilibrium in less than 3 h and in most cases less than 10 min. The adsorption capacity of NH3+–MCM-41 for Cr2O72- is high, approaching the theoretical values of two aminopropyls for one Cr2O72-, the Cu2+ adsorption on COO−Na+–MCM-41 was less efficient requiring five propionates to adsorb one Cu2+. High purity hydrogen dichromate and copper chloride solutions were respectively recovered by a simple acid wash and the regenerated adsorbents were reused without a loss of performance.