Effect of TOC Concentration of Humic Substances as an Electron Shuttle on Redox Functional Groups Stimulating Microbial Cr(VI) Reduction.

Zhen Yang,Yi Zhou,Jie Jiang,Jingtao Duan

doi:10.3390/ijerph19052600

Abstract

Humic substances as an electron shuttle play an essential role in the biogeochemistry processes. However, the influence of total organic carbon (TOC) concentrations of humic substances on microbial Cr(VI) reduction remains unclear. In this study, the rates and extents of Cr(VI) reduction by Shewanella oneidensis MR-1 in the presence of Leonardite humic acids (LHA) and Pahokee peat humic acids (PPHA) with different TOC concentrations were evaluated. We found that the enhanced reduction in Cr(VI) was associated with TOC concentrations of 2.5–50 mg C/L of HA samples. The result shows that HA as an electron shuttle impacted both rates and extents of microbial Cr (VI) reduction, which delivered differently in terms of low TOC concentration range of 2.5 to 15 mg C/L and high concentration range of 15–50 mg C/L. The rates of Cr(VI) reduction significantly enhanced in the low TOC concentration range of HA compared to a high concentration range. The highest acceleration rate of Cr(VI) reduction was achieved at 15 mg C/L of HA. The quinone-like fluorophore was responsible for the main redox-active functional groups of HA by the three-dimensional excitation-emission spectroscopy. The fluorescence intensity of quinone-like fluorophore of HA in the low TOC concentration range was positively correlated with its acceleration coefficient, corresponding to the highest microbial Cr(VI) reduction rate obtained in 15 mg C/L of HA. These findings highlighted the effect of the TOC concentration of HA on microbial Cr(VI) reduction processes. It emphasized that the low TOC concentration of HA contributed to the high rates of Cr(VI) reduction, which is critical for better understanding the fate of Cr(VI) and evaluating the effectiveness of Cr(VI) restoration strategies in the future.

Highlights

The second aim of this study was to clarify the redox-active functional groups (RAFGs) distribution of Pahokee peat humic acids (PPHA) and Leonardite humic acids (LHA) with different total organic carbon (TOC) concentrations by the three-dimensional excitation-emission spectroscopy (3DEEM), and to exhibit its effects of TOC relating to RAFGs in HA on microbial Cr(VI) reduction processes
In order to explore the influence of HA with different concentrations on the microbial Cr(VI) reduction, we conducted microbial Cr(VI) reduction experiments by Shewanella oneidensis MR-1 in the absence and presence of LHA and PPHA with different TOC concentrations of 2.5, 5, 10, 15, 20, 25 and 50 mg C/L, respectively
For PPHA, we found the similar results as LHA, for both HA samples, up to 50% of extent of Cr(VI) reduction was observed by adding high TOC concentration range of 15–50 mg C/L of HA, whereas approximately

Summary

Introduction

College of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, China; College of Urban and Environmental Science, Peking University, Beijing 100871, China. The fluorescence intensity of quinone-like fluorophore of HA in the low TOC concentration range was positively correlated with its acceleration coefficient, corresponding to the highest microbial Cr(VI) reduction rate obtained in 15 mg C/L of HA. These findings highlighted the effect of the TOC concentration of HA on microbial Cr(VI) reduction processes. Humic substances are natural organic matter with abundant functional groups and it has been demonstrated to significantly influence transformation and transport of redoxsensitive contaminants such as Cr(VI), through sorption [13–16] and reduction [17–19], respectively. Humic substances are able to function as an electron shuttle that first accepts electrons from iron-reducing bacteria and reduces humic substances and donates published maps and institutional affiliations

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Discussion

Conclusion