Immobilization mechanisms of Hexavalent Chromium When Reduced by Fe2+-Bearing Clay Minerals Depending on solution pH

 Structural iron (Fe³⁺)-bearing clay minerals, when they are reduced, can mediate electron transfer through the Fe³⁺/Fe²⁺ coupling reaction and transform hexavalent chromium (Cr⁶⁺) into less toxic Cr³⁺, which in turn can be removed from the solution by the clay minerals. Two types of clay minerals with different structural iron (Fe³⁺) contents, montmorillonite (2.3 wt%) and nontronite (22.3 wt%), were subjected to reaction with 50 mM dithionite at pH 9 for 48 hours, resulting in Fe²⁺ bearing clay minerals, with measured Fe²⁺ ratios of 0.68 and 0.49, respectively. Subsequently, the Fe²⁺ bearing clay minerals were reacted with Cr⁶⁺ solution with varying pH ranging from 2.5 to 11 in an anaerobic chamber. Results show that the reduction of Cr⁶⁺ was observed at all pH conditions, consistent with the stoichiometric ratios with structural iron (Cr⁶⁺:Fe²⁺/1:3). At pH 7 and below, over 99% of the structural iron (Fe²⁺) participated in the reduction reaction. At pH 9 and 11, however, the reaction exhibited a shortfall, with approximately 7-27% and 20-32% of unutilized structural iron remaining in montmorillonite and nontronite, respectively. According to the Visual MINTEQ model and DTPA extraction experiments conducted on solids obtained, Cr³⁺ is immobilized through sorption onto the clay mineral surface at pH 4.5 and below, and through precipitation and deposition on the clay mineral at pH 7 and above. SEM-EDS analysis, the presence of precipitated Cr at pH 7 and above was identified, and XPS analysis confirmed its precipitation in the form of Cr(OH)₃.