Sequestration of hexavalent chromium by Fe(II)/Fe(III) hydroxides: Structural Fe(II) reactivity and PO43− effect

As Fe(II)-rich minerals, Fe(II)/Fe(III) hydroxides were intensively investigated in the sequestration of aqueous Cr(VI). Results showed that the content of Cr(VI) sequestrated ([Cr(VI)]se) was in proportional to the absolute content of structural Fe(II) (Fe(II)). Molecular ratios of [Fe(II)]co(consumed Fe(II))/[Cr(VI)]se close to 3:1, the theoretical ratio, indicated high reactivity of structural Fe(II), which could also be confirmed by the fact that 278 mg Cr(VI)/g Fe(II) was achieved. In other words, negligible inhibitory effect was deduced in the case of surface Cr(III)-solids, which was responsible for limited reactivity of previous reported Fe(II)-contained minerals. Large surface area induced adsorption, accompanied by simultaneous reduction, was supposed to govern the rapid reaction within the initial 10 min. Reaction within 10-20 min well fit a pseudo-first-order model, demonstrating high electron transfer efficiency from inner Fe(II) to adsorbed/aqueous Cr(VI), though surface Cr(III)-solids were formed. Cr(III)-solids containing end solid products were found to be goethite, CrOOH and Cr(OH)3, which was benefit to establish the interfacial reduction-precipitation mechanism of Cr(VI) sequestration. The negative effect of OH− was deduced on Fe(II) availability in the cases of [Fe(II)]/[OH−] ratios higher or lower than 1:1. Simultaneous PO43− addition caused vivianite generation, resulting Fe(II) reactivity decrease. This investigation gave well implication that Fe(II) could be applied and regulated in the reduction precipitation of Cr(VI) and possible other heavy metal ions.