Exploring the potential of the Cu corrosion product malachite for uranyl and selenite adsorption under alkaline conditions and in the presence of carbonate

Malachite, a copper hydroxycarbonate, was investigated as a potential sorbent for selenium ( 75 Se(IV)) and uranium ( 233 U(VI)) by batch sorption experiments, electrophoretic and spectroscopic techniques. These radionuclides are usually weakly adsorbed onto natural materials under alkaline pH, oxidizing conditions or in the presence of carbonate. Tests were conducted at pH 7.5–8.5, using NaCl and NaHCO 3 solutions at two different ionic strengths. In NaCl solutions, the retention of both elements was significant, with maximum distribution coefficients (K d ) of ∼4.5·10 3 mL g −1 for Se and of ∼1.0·10 4 mL g −1 for U. No significant effect of NaCl concentration was observed. Lower retention was observed in NaHCO 3 , being the maximum K d values of ∼1.6·10 3 mL g −1 for Se and ∼6.5·10 2 mL g −1 for U at an ionic strength of 0.01 M. A significant reduction in K d values (more than an order of magnitude) was observed at 0.1 M. The presence of NaHCO 3 produced a large shift of the malachite isoelectric point (from pH ∼ 9 to 6) and a decrease in the electrokinetic potential. Carbonates in solution strongly interact with the malachite surface and can be competitive with Se and U adsorption. Different possible interaction mechanisms of the sorbates with the malachite and linearity/non-linearity of adsorption in the presence of different electrolytes are discussed. The retention capacity of the material was also evaluated after gamma irradiation at 140 kGy, which did not significantly affect the retention.