Interaction between aqueous chromium solutions and layer silicates

The interactions between Cr in aqueous solutions and phyllosilicates were studied to determine: (a) the amount of Cr(VI) to Cr(III) reduction in aqueous solutions by Fe(II)-bearing phyllosilicates; (b) the removal of the Cr species from solution by interaction with phyllosilicates, as a function of Cr(III) concentration and anionic environment. Chlorite, corrensite and montmorillonite were reacted with solutions containing Cr(VI) (1.62 x 10(-3) N, 5.77 x 10-3 N and 1.32 x 10(-1) N, respectively). The sorption/desorption of Cr(III) by saponite was investigated in different anionic environments (Cl-, NO3- and CH3COO-) and at different initial Cr(III) concentrations (3.21 x 10(-3) N, 5.49 x 10(-3) N and 8.49 x 10(-3) N). The extent of Cr(VI) reduction and the amount of Cr removed by phyllosilicates were measured by analysis of the liquid portion separated by centrifugation after controled periods of exposure. The minerals were studied by chemical, thermal and X-ray powder diffraction analyses. The results show that: (i) Fe(II)-bearing phyllosilicates sorb Cr and reduce Cr(VI) to Cr(III); (ii) the extent of reduction depends on the solution concentration and on mineral crystal chemistry; (iii) Cr(III) sorption isotherms show that the degree of uptake depends both on the initial concentration of metal in solution and on the anionic environment, the order of effectiveness being Cl- congruent to NO3- > CH3COO-; (iii) Cr(III) is retained in the mineral substrate and its release is difficult. (C) 2000 Elsevier Science Ltd. All rights reserved.