Enhanced removal of Mn (II) from solution by thermally activated Bayer precipitates

Abstract

This study assessed the ability of thermally activated Bayer precipitates (Bppt) for manganese remediation from acidic solutions, focusing on their application in acid mine drainage water. The Bayer precipitates were formed from the addition of seawater to Bayer liquor residues, and were primarily composed of hydrotalcite and calcium carbonate. Thermal activation of these materials at 320, 380 and 440 °C showed a reduction in Bayer hydrotalcite crystallinity and increased surface area due to the decarbonation and dehydroxylation of the mixed metal hydroxide layers in Bayer hydrotalcite. At higher thermal activation temperatures (380 and 440 °C), partial decarbonation of calcium carbonate were suspected of forming calcium oxide, which aided the neutralisation process. The removal capacities for manganese were found to increase with thermal activation temperature, which corresponded with increased maximum pH (pH > 10 for 380 and 440 °C heated sample). Treatment of acidic solutions with Bayer precipitates simultaneously removed manganese and aluminium, which is not achievable in lime neutralisation technologies currently used in acid mine drainage treatment. The optimum thermal activation temperature was found to be 380 °C, showing high manganese removal performance and a ten-fold reduction in material mass required when compared to Bayer precipitate. Removal mechanisms of manganese were found to be a combination of hydrotalcite reformation (determined by X-ray photoelectron spectroscopy, X-ray diffraction and infrared spectroscopy), surface precipitation and surface complexation, as well as the formation of hausmannite and rhodochrosite.