Bioreduction performance of Cr(VI) by microbial extracellular polymeric substances (EPS) and the overlooked role of tryptophan

• Published in: Journal of hazardous materials 2022-07, Vol.433, p.128822-128822, Article 128822
• Main Authors: Luo, Xianxin, Zhou, Xiaoyu, Peng, Chengyi, Shao, Penghui, Wei, Feng, Li, Shujing, Liu, Ting, Yang, Liming, Ding, Lin, Luo, Xubiao
• Format: Article
• Language: English
• Subjects: aquatic environment; Bioreduction; chromium; Chromium - chemistry; Cr(VI); electron paramagnetic resonance spectroscopy; Extracellular polymeric substance; Extracellular Polymeric Substance Matrix - chemistry; gold; heavy metals; liquid chromatography; Metals, Heavy - analysis; polymers; reducing agents; silver; Tandem Mass Spectrometry; Tryptophan; X-ray photoelectron spectroscopy
• ISSN: 0304-3894; 1873-3336
• DOI: 10.1016/j.jhazmat.2022.128822

Extracellular polymeric substances (EPS) have exhibited promising advantages in mitigating heavy metal contamination, e.g., single-valent silver (Ag(I)), trivalent gold (Au(III)), and hexavalent chromium (Cr(VI)). However, knowledge of the specific substrate in EPSs that supports Cr(VI) reduction has remained elusive. Here, we isolated a novel Cr(VI)-reducing strain with self-mediating properties in an aquatic environment with various pH values to investigate the mechanisms. After analysis by a batch assay coupled with X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and scanning electron microscopy–energy dispersive spectrometry (SEM–EDS) spectroscopic techniques, it was found that Cr(VI) was reduced by the strain and soluble-EPS (S-EPS), and then, organo-trivalent chromium (organo-Cr(III)) was successfully formed. In addition, compared with other components of the strain, the strain and S-EPS completely removed Cr(VI), and the S-EPS exhibited a positive effect on Cr(VI) reduction with a strong monotonic correlation (R2 = 0.999, p = 9.03 × 10−5), indicating that the reduction is an EPS-dependent process. Specifically, the Cr(VI) reduction efficiency was enhanced to 48.85% and 99.4% after EPS and EPS plus tryptophan were added; their respective efficiencies were 3.94 and 8.02 times higher than that of the control assay in which the reductant was depleted. High-performance liquid chromatography–tandem mass spectrometry (HPLC–MS/MS) analysis showed that the tryptophan concentration concomitantly decreased by 61.54%. These findings highlighted the importance of S-EPS and tryptophan and improved our understanding of EPS for Cr(VI) reduction, which might provide a novel strategy for decontaminating targeted heavy metals in future applications. [Display omitted] •The strain shows a self-mediating property among various pH.•Cr(VI) was largely reduced, accompanied by forming the organo-Cr(III).•S-EPS can trigger Cr(VI) reduction and tryptophan presented a facilitation effect.