Selective mesoporous adsorbents for Cr₂ O₇^{2 -} 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 Cr₂ O₇^{2 -} and Cu²⁺. Single and binary component adsorptions show that NH₃⁺ s(-) MCM- 41 has a 100% selectivity for Cr₂ O₇^{2 -} 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⁺ s(-) MCM- 41 for Cr₂ O₇^{2 -} is high, approaching the theoretical values of two aminopropyls for one Cr₂ O₇^{2 -}, 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.