Elemental sulfur enhances arsenate immobilization during microbially mediated ferrihydrite transformation: Nanoscale...

Microbial phase transformation of iron (Fe) minerals coupled with sulfur (S) redox cycling plays a critical function in controlling arsenate (As(V)) partitioning in natural environments. However, interfacial Fe-S redox mechanisms controlling As(V) immobilization during reductive transformation remain insufficiently resolved. Here, S. oneidensis MR-1 mediated phase transformation of As-loaded ferrihydrite and the associated As(V) mobility were investigated at pH 6.8 in the presence of elemental sulfur (S⁰). Under anoxic near-neutral conditions, S⁰ redox reaction occurred concurrently with dissimilatory Fe reduction, producing S^2- and Fe(II). A portion of S^2- promoted ferrihydrite reductive dissolution and Fe(II) released, whereas the remaining S^2- was sequestered by Fe(II) to produce Fe sulfide minerals. Lepidocrocite was the dominant transformation product, accompanied by minor goethite and Fe sulfides. S⁰ accelerated reductive transformation, while As(V) inhibited secondary Fe minerals formation. During reductive transformation, S⁰ was conducive to nonextractable As(V) formation, and its content shown a good correlation with lepidocrocite and goethite contents. Microscopic analyses at nanoscale revealed that during microbial phase transformation, As(V) immobilization involved surface adsorption and partial structural incorporation into the lattice structure of lepidocrocite and goethite. A predictive model integrating phase transformation with As(V) binding was developed to quantify the mineral-specific contributions to As(V) immobilization. Collectively, these findings provide mechanistic and predictive insights into S-Fe redox coupling as a key control on As(V) immobilization, with implications for evaluating As mobility in S-rich, reducing soils, sediments, and aquifers.