| Chromium pollution in soil and water is a serious problem in China,which is difficult to control and remediation.There is a significant Cr(Ⅵ)reduction self-purification effect in the natural wetland downstream of a chromium residue storage site in northeast China,which is in sharp contrast with the above phenomenon.There are a large number of organic matter and minerals in wetland soil,which can reduce and adsorb Cr(Ⅵ).Meanwhile,there are also a large number of microorganisms,which can also reduce Cr(Ⅵ).However,the contribution and mechanism of chemical and biological reduction in natural wetland to the reduction and self-purification of Cr(Ⅵ)are still unclear.In this study,the column experiment method was used to simulate the dynamic migration process of Cr(Ⅵ)in natural wetland pore water,and the reduction and self-purification process of Cr(Ⅵ)in heavily polluted and lightly polluted natural wetlands,respectively,to explore the contribution and mechanism of chemical reduction and biological reduction,and to explore the influence of Cr(Ⅵ)concentration load and the alternation of reoxidation and reduction conditions.This paper mainly obtains the following conclusions:(1)When the Cr(Ⅵ) concentration was less than 10 mg/L,the reduction rates of Cr(Ⅵ)in shallow soil and deep soil were more than 78.74% and 100%,respectively,and the contribution rates of microbial reduction were 26.75% and 37.63%,respectively.When the concentration of Cr(Ⅵ)was 20 mg/L,the reduction rate of Cr(Ⅵ)in shallow soil and deep soil decreased to 35.65% and31.17%,and the contribution rate of microbial reduction decreased to 9.39% and 29.49%,but the reduction ability could be recovered with the decrease of Cr(Ⅵ)concentration.The results of FTIR and XPS characterization indicated that there were phenolic hydroxyl and other reducing functional groups in the wetland soil,which could directly reduce Cr(Ⅵ),while microorganisms could reduce Cr(Ⅵ)with polysaccharides.High-throughput 16 S rRNA sequencing showed that high concentrations of Cr(Ⅵ)reduced microbial diversity,and Thiobacillus or chrome-resistant bacteria in Proteobacteria phylum became the dominant bacteria.(2)When the concentration of Cr(Ⅵ) was less than or equal to 10 mg/L,there was an obvious phenomenon of dissimilated iron reduction in lightly polluted wetland soil.Fe(II)produced by microbial dissimilated iron reduction was indirectly reduced by Cr(Ⅵ),and both shallow soil and deep soil could completely reduce Cr(Ⅵ)at this concentration.When the concentration of Cr(Ⅵ)was 20 mg/L,the shallow soil was still able to fully reduce Cr(Ⅵ)at this concentration,while the reduction rate of Cr(Ⅵ)in deep soil was reduced to 62.92%,and the contribution rate of microbial reduction was reduced to 40.72% and 3.7%,and the microbial reduction ability failed to recover in a short time after the reduction of contamination load.By 16 S rRNA high-throughput sequencing,it was found that the proportion of Sphingomonas in Proteobacteria was significantly increased and became the dominant bacteria group,while the dissimilated iron reducing bacteria Acidothermus was significantly reduced due to poor chromium tolerance.(3)By controlling the dissolved oxygen content of influent water,the alternating REDOX process of wetland was simulated,and it was found that the chemical reduction was less affected by the dissolved oxygen content.The reduction rates of the heavily polluted shallow soil and deep soil are 8.16% and 0.71%,respectively,under anaerobic and aerobic conditions,while those of the lightly polluted soil are 5.2% and 6.09%,respectively,indicating that the reduction capacity of the anaerobic stage is stronger than that of the aerobic stage.At the same time,during the transition from aerobic to anaerobic stage,the heavily polluted shallow soil has the most significant restoring ability to the reduction,while the lightly polluted deep soil has no significant difference in the reduction ability between aerobic and anaerobic. |
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