Driving microbial sulfur cycle for phenol degradation coupled with Cr(VI) reduction via Fe(III)/Fe(II) transformation

Microbial sulphur cycle is of great importance to anaerobic degradation of organic pollutants coupled with metal reduction, which however is usually limited by the poor oxidation of the elemental sulphur to sulphate, due to the lack of chemolithotrophic sulphur-oxidizing bacteria. The study presented here utilized magnetite as an Fe(III) source to enrich the chemolithotrophic sulphur-oxidizing bacteria to proceed the oxidation of the elemental sulphur, which further achieved the whole sulphur cycle. The results showed that, under the low-concentration sulphate conditions, the effluent concentration of both Cr(VI) (18.0 mg/L vs 187.7 mg/L) and phenol (354.2 mgCOD/L vs 1256.0 mgCOD/L) in the reactor with magnetite was significantly lower than that without magnetite. Energy-dispersive X-ray (EDX) analysis showed that the content of sulphur in the aggregates without magnetite was lower than that with magnetite (4.49% vs 6.85%). Conversely, with magnetite, the special enrichments, Thiobacillus species, proceeded the oxidation of the elemental sulphur to sulphate, lowering the loss of sulphur. Further analysis by X-ray Diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) observed the disappearance of the crystalline structure in magnetite as well as the loss of iron via the dissimilatory Fe(III) reduction in the aggregates. However, under the sulphate-reducing conditions, the evident peaks of both Fe2O3 and FeO were detected in the aggregates. Considered that the re-oxidation of the reduced Cr(III) might occur in the aggregates without magnetite, the generated Fe(II) might react with the slight dissolved oxygen (DO) to reduce the re-oxidation of the reduced Cr(III), which further achieved the transformation of Fe(III)/Fe(II).