Study of sorption processes and FT-IR analysis of arsenate sorbed onto red muds (a bauxite ore processing waste)

In this study we evaluated the arsenate adsorption capacity of red muds (RM), wastes tailing from the alumina production, at different pH values (4, 7, and 10). RM samples were artificially enriched in batch tests with solutions containing increasing concentrations of As(V). The pH of the solution significantly affected the adsorption, which increased with the decrease of pH. Moreover a sequential extraction procedure [H2O; (NH4)2SO4; NH4H2PO4; NH4+-oxalate; NH4+-oxalate+ascorbic acid] was applied to RM samples exchanged with arsenate. Using this approach it was shown that low concentrations of arsenate sorbed in RM were present as water soluble and exchangeable fractions, while NH4+-oxalate and NH4+-oxalate+ascorbic acid extracted most of the adsorbed arsenate from RM at different pH values. Besides, FT–IR spectroscopy was used to better understand the nature of RM surface configuration after As(V) sorption. In the FT–IR spectra the presence of As(V) species was highlighted by a well resolved band at 865 cm−1. The intensity and broadness of this band increased at the decreasing of pH. This band could be related to v(As–O) vibration of an inner-sphere Al–O–As complex and/or due to As–O bonds of the adsorbed As(V) species on Fe oxides of RM samples.