Impact of redox fluctuations on microbe-mediated elemental sulfur disproportionation and coupled redox cycling of iron

Ke Zhang; Shaojian Zhang; Peng Liao; Yuanxin Zhao; Min Gan; Jianyu Zhu

doi:10.1016/j.watres.2023.120589

Impact of redox fluctuations on microbe-mediated elemental sulfur disproportionation and coupled redox cycling of iron

Water Res. 2023 Oct 15:245:120589. doi: 10.1016/j.watres.2023.120589. Epub 2023 Sep 10.

Authors

Ke Zhang¹, Shaojian Zhang², Peng Liao³, Yuanxin Zhao², Min Gan⁴, Jianyu Zhu⁵

Affiliations

¹ School of Minerals Processing and Bioengineering, Key Laboratory of Biohydrometallurgy of Ministry of Education, Central South University, Changsha 410083, PR China; State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, PR China.
² State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, PR China.
³ State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, PR China. Electronic address: liaopeng@mail.gyig.ac.cn.
⁴ School of Minerals Processing and Bioengineering, Key Laboratory of Biohydrometallurgy of Ministry of Education, Central South University, Changsha 410083, PR China.
⁵ School of Minerals Processing and Bioengineering, Key Laboratory of Biohydrometallurgy of Ministry of Education, Central South University, Changsha 410083, PR China. Electronic address: zhujy@csu.edu.cn.

PMID: 37708773
DOI: 10.1016/j.watres.2023.120589

Abstract

Elemental sulfur (S⁰) plays a vital role in the coupled cycling of sulfur and iron, which in turn affects the transformation of carbon and various pollutants. These processes have been well characterized under static anoxic or oxic conditions, however, how the natural redox fluctuations affect the bio-mediated sulfur cycling and coupled iron cycling remain enigmatic. The present work examined S⁰ disproportionation as driven by natural microbial communities under fluctuating redox conditions and the contribution of S⁰ disproportionation to ferrihydrite transformation. Samples were incubated at either neutral or alkaline pH values, applying sequential anaerobic, aerobic and anaerobic conditions over 60 days. Under anaerobic conditions, S⁰ was found to undergo disproportionation to sulfate and sulfide, which subsequently reduced ferrihydrite at both pH 7.4 and 9.5. Ferrihydrite promoted S⁰ disproportionation by scavenging biogenic sulfide and maintaining a suitable degree of sulfate formation. After an oxic period, during the subsequent anoxic incubation, bioreduction of sulfate occurred and the biogenic sulfide reduced iron (hydr)oxides at a rate approximately 25 % lower than that observed during the former anoxic period. A 16S rDNA-based microbial community analysis revealed changes in the microbial community in response to the redox fluctuations, implying an intimate association with the coupled cycling of sulfur and iron. Microscopic and spectroscopic analyses confirmed the S⁰-mediated transformation of ferrihydrite to crystalline iron (hydr)oxide minerals such as lepidocrocite and magnetite and the formation of iron sulfides precipitated under fluctuating redox conditions. Finally, a reaction mechanism based on mass balance was proposed, demonstrating that bio-mediated sulfur transformation maintained a sustainable redox reaction with iron (hydr)oxides under fluctuating anaerobic-aerobic-anaerobic conditions tested in this study. Altogether, the finding of our study is critical for obtaining a more complete understanding of the dynamics of iron redox reactions and pollutant transformation in sulfur-rich aquatic environments.

Keywords: Elemental sulfur; Iron transformation; Microbial respiration; Redox fluctuation; Sulfur cycling.