Removal of sulphates from acid mine drainage using barium and organically modified clay

Abstract: Attapulgite and Mozambican bentonite (MB) were modified by using barium chloride and ammonium cations such as hexadecyltrimethylammonium bromide (HDTMA) and trimethyldecylammonium bromide (TDTMA) to enhance the removal capacity of sulphates from acid mine drainage (AMD). Through the modification process the surface properties of the clays was rendered organophilic. Batch adsorption experiments of the modified clays were done in a thermostatic shaker at different temperatures, mass loading of adsorbent, and cation exchange capacity (CEC) percentages to investigate the sorption behaviour of sulphates from AMD. Characterization of the modified clay that gave the highest removal of sulphates was done using XRF, XRD, FTIR and SEM to investigate the chemical composition of the clay, removal mechanism and structural change of the clay as a result of sorption of sulphates. Attapulgite showed higher removal of sulphates (70.8%) when modified with BaCl2 than the other two surfactants. This maximum removal was achieved at 25oC and 10% w/v clay to AMD. The reaction fitted the Temkin adsorption isotherm and the second order kinetic model. Adsorption of sulphates on attapulgite was not dependent on temperature as the difference in recovery was not statistically significant when temperature was varied. The value of activation energy (23.7kJ/mol) showed that chemisorption was the dominant mechanism of sulphate removal. MB showed higher removal of sulphates (74.0%) when modified with TDTMA. The reaction was endothermic and fitted the Temkin isotherm model and the second order kinetic model. More sulphates were adsorbed at higher temperatures, suggesting that this was an endothermic reaction, which was supported by the positive value of enthalpy. The activation energy for this reaction (-124.8 kJ/mol) showed that physisorption was the dominant mechanism of sulphate removal. Characterization of both clays showed that the sulphates were removed by adsorption and that the modifiers were only adsorbed on the surface of the clay during the cation exchange process without destroying the crystalline structure of the clay. At high solid loading, more sulphates were adsorbed from the AMD sample by both clays. This implied that at higher solid loadings, there are more sites of adsorption and thus, the higher sulphate removal...M.Tech. (Chemical Engineering