Effects of adsorbed phosphate on jarosite reduction by a sulfate reducing bacterium and associated mineralogical...

Jarosite is one of the iron oxyhydroxysulfate minerals that are commonly found in acid mine drainage (AMD) systems. In natural environments, phosphate and sulfate reducing bacteria (SRB) may be coupled to jarosite reduction and transformation. In this research, the effect of phosphate on jarosite reduction by SRB and the associated secondary mineral formation was studied using batch experiments. The results indicated that Fe³⁺ is mainly reduced by biogenic S^2- in this experiment. The effect of PO₄^3- on jarosite reduction by SRB involved not only a physico-chemical factor but also a microbial factor. Phosphate is an essential nutrient, which can support the activity of SRB. In the low PO₄^3- treatment, the production of total Fe²⁺ was found to be slightly larger than that in the zero PO₄^3- treatment. Sorption of PO₄^3- effectively elevated jarosite stability via the formation of inner sphere complexes, which, therefore, inhibited the reductive dissolution of jarosite. At the end of the experiment, the amounts of total Fe²⁺ accumulation were determined to be 4.54 ± 0.17^a mM, 4.66 ± 0.22^a mM, 3.91 ± 0.04^b mM and 2.51 ± 0.10^c mM (p < 0.05) in the zero, low, medium and high PO₄^3- treatments, respectively, following the order of low PO₄^3- treatment > zero PO₄^3- treatment > medium PO₄^3- treatment > high PO₄^3- treatment. PO₄^3- loading modified the transformation pathways for the jarosite mineral, as well. In the zero PO₄^3- treatment, the jarosite diffraction lines disappeared, and mackinawite dominated at the end of the experiment. Compared to PO₄^3--free conditions, vivianite was found to become increasingly important at higher PO₄^3- loading conditions. These findings indicate that PO₄^3- loading can influence the broader biogeochemical functioning of AMD systems by impacting the reactivity and mineralization of jarosite mineral.