Solidification/stabilization of arsenic in red mud upon addition of Fe (III) or Fe (III) and Al (III) dissolved in H2SO4

Abstract

In the aluminum industry, as an alkaline waste, red mud is composed of Fe and Al. It is difficult to safely dispose certain types of some arsenic-containing red mud owing to hazardous arsenic leaching. Hence, the method of arsenic stabilization using Fe (III) or H2SO4 to reduce arsenic leaching from red mud was investigated. The results indicated that the addition of 3 - 5% Fe2(SO4)3 or H2SO4 (ratio of H+ (mM)/red mud (g) ≈ 1.5) into red mud and maintaining the pH at 4 until dryness was achieved at room temperature, decreased arsenic solubility from 360 to 2 - 6 μg/L (< 10 μg/L: Environmental criteria in Japan) and maintained the same for several months. In the mechanism of arsenic stabilization, Fe3+ ions added or Fe3+ and Al3+ ions dissolved from red mud by adding H2SO4 reacted with arsenic and formed insoluble Fe-As and Al-As compounds. Through X-ray diffraction analysis, insoluble arsenic compounds including iron arsenate, iron hydrogen arsenate, aluminum arsenate, and aluminum arsenate hydrate were detected and considered as responsible for low arsenic leaching. Additionally, the results suggested that HCl was not appropriate for stabilizing arsenic in red mud because the high amounts of Cl- negatively influence the formation of insoluble arsenic compounds.