Simultaneous nitrate and chromium removal mechanism in a pyrite-involved mixotrophic biofilter

Microbially mediated NO₃^--N and Cr(VI) reduction is being recognized as an eco-friendly and cost-effective remediation strategy. Iron sulfide mineral, as a natural inorganic electron donor, has a strong influence on NO₃^--N and Cr(VI) transformation, respectively. However, little is known about the simultaneous nitrate and chromium removal performance and underlying mechanism in an iron sulfide mineral-involved mixotrophic biofilter. This study demonstrated that the NO₃^--N and Cr(VI) removal efficiencies were stable at 62 ± 8% and 56 ± 10%, and most of them were eliminated in the 0-100-mm region of the biofilter. Cr(VI) was reduced to insoluble Cr(III) via microbial and chemical pathways, which was confirmed by the SEM-EDS morphology and the XPS spectra of biofilm and pyrite particles. SO₄^2- was as a main byproduct of pyrite oxidation; however, the bacterial SO₄^2- reduction synchronously occurred, evidenced by the variations of TOC and SO₄^2- concentrations. These results suggested that there were complicated and intertwined biochemical relations between NO₃^--N/Cr(VI)/SO₄^2-/DO (electron acceptors) and pyrite/organics (electron donors). Further investigation indicated that both the maximal biomass and greatest denitrifiers' relative abundances in microbial sample S₁ well explained why the pollutants were removed in the 0-100-mm region. A variety of denitrifiers such as Pseudoxanthomona, Acidovorax, and Simplicispira were enriched, which probably were responsible for both NO₃^--N and Cr(VI) removal. Our findings advance the understanding of simultaneous nitrate and chromium removal in pyrite-involved mixotrophic systems and facilitate the new strategy development for nitrate and chromium remediation.