Abstract

The aim of this thesis was to study the interaction mechanisms between an inorganic sorbent, red mud (RM), and two different anions, arsenate and phosphate. Adsorption isotherms and sequential extraction procedures were firstly used to investigate these interactions. Sorption isotherms of arsenate and phosphate on RM, at pH 4.0, 7.0 and 10.0, were obtained while a sequential extraction procedure using H2O, (NH4)2SO4, NH4H2PO4, NH4 +-oxalate and NH4 +- oxalate + ascorbic acid, was applied to saturated RM samples to determine the interaction strength between respective anions and sorbent. Moreover, a physical, chemical and mineralogical characterization of not treated RM (alumina industry by-product) and RM exchanged with anions was carried out using spectroscopic techniques (FT–IR spectroscopy and X-Ray diffraction) and thermal analysis (TG and DTG) to evaluate the general suitability of RM as sorbent of phosphate and arsenate.
Results showed that the RM was an effective sorbent of both arsenate and phosphate at all the pH values tested even if, as expected, the sorption capacity increased as the pH decreased. However, it was worth noting the different retention behaviors of the two anions on RM. Particularly, phosphate adsorption on RM at pH 4 was higher 2.6 times than arsenate adsorption. A different reactivity of the anions considered, likely due to different interaction of phosphate and arsenate with the mineralogical phases of RM, was also observed. In particular, the phosphate uptake capacity of RM was much greater than that shown for arsenate. X-ray diffraction pattern of RM exchanges with phosphate at pH 4 and 7, revealed additional phase, berlinite [AlPO4].
Overall, the results from this study suggest that RM could be a useful and environmentallyfriendly alternative for the “in-situ” and time-effective immobilization of arsenic in contaminated soils.

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