| The transformation and fate of pentachlorophenol (PCP, C6Cl5OH) in paddy soils have been extensively studied. It is known that the degradation rate of PCP in paddy soils is higher under anoxic/anaerobic conditions than under aerobic conditions. In flooded paddy soils, the biogeochemical processes at the soil-water interface are more complex than those in the bulk soil due to the presence of adjacent anaerobic and aerobic zones. Thus, study of the degradation process of PCP at the soil-water interface has gained increasing attention from researchers. The aims of this study were:1) to examine the relationship and influencing factors of PCP degradation and Fe(Ⅱ) accumulation in a flooded paddy soil;2) to elucidate the promoting effect and associated biochemical mechanisms of different organic carbon sources [low-molecular-weight dissolved organic carbon (LMW-DOC) compounds and Chinese milk vetch dissolved organic matter (CMV-DOM)] as electron donors, organic ligands or electron shuttles on the reductive degradation of PCP in the flooded paddy soil; and3) to evaluate the effect of green manures (Chinese milk vetch and bird vetch) as amendment on the transformation and fate of PCP in the flooded paddy soil. The primary findings are summarized as follows:(1) The effect of different LMW-DOC compounds (fourteen organic acids and eight neutral monosaccharides) on the kinetic parameters of Fe(Ⅱ) accumulation and PCP degradation at the soil-water interface. Logistic curve fitting and cluster analysis showed that the kinetic parameters of Fe(Ⅱ) accumulation and PCP degradation significantly varied with different organic ligands and/or electron donors as well as carbon sources for Fe(Ⅲ) reduction and PCP degradation. The large variations in the number of carbon atoms per molecule, dissociation constants, and degree of reduction of the LMW-DOC led to substantial differences in the maximum capacities for accumulation, accumulation rate constants, and maximum accumulation rates of NaOAc-extractable Fe(Ⅱ)[Fe(Ⅱ)NaoAc] and HCl-extractable Fe(Ⅱ)[Fe(Ⅱ)HCl]-Different degree of reduction of the LMW-DOC also caused significant changes in the maximum capacities for accumulation of PCP. Correlation analysis demonstrated that there was a significant relationship between the maximum capacities of Fe(Ⅱ) accumulation and PCP degradation. Based on the above results, the relationship of microbial parameter (phospholipid fatty acid, PLFAs) and selected environmental variables with the kinetic parameters of Fe(Ⅱ) accumulation and PCP degradation was examined using biochemical, chemical and electrochemical analysis tools in combination with correlation analysis, regression analysis and redundancy analysis. Results showed that Fe(Ⅱ)HCl was the key environmental variable that played a decisive role in PCP degradation. The pH value, which varied with different types of LMW-DOC, was a key factor that determined the structure and distribution of soil microbial communities. Overall, the increase in pH promoted to Fe(Ⅱ)NaOAc accumulation and further significantly enhanced PCP degradation. The pH decreases and Eh increases were associated with decreases in the degradation rate of PCP. Fe(Ⅱ)/Fe(Ⅲ) was the redox couple that determined the anodic peak potential (Ep). In the presence of different LMW-DOC, the changes in pH value and/or WSOC content led to significant differences in the Ep value over time as well as different degrees of Fe(Ⅱ)HCl accumulation and PCP degradation. As compared to Fe(Ⅱ)NaOAc and Eh, Fe(Ⅱ)HCl and Ep were more indicative of the biochemical mechanism of the reductive degradation of PCP.(2) The dynamic changes in CMV-DOM production, consumption, and properties during decomposition under continuously-flooded (CF) and non-flooded (NF) conditions, and the differences in the redox capacity and redox state of CMV-DOM between fresh and CF-/NF-decomposed samples. Correlation analysis and principal component analysis showed that under different redox conditions, the changes in the redox capacity and redox state of CMV-DOM were related to large variations in the surrogate parameters (biochemical index, atomic ratio, Fourier transform infrared absorption ratio, weight-average/number-average molecular weight, ultraviolet absorbance ratio, degree of reduction, and ultraviolet spectral absorbance ratio). Thus, it is possible to indirectly predict the redox properties of CMV-DOM through a variety of instrument analyses (ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, gel permeation chromatography, and elemental analysis).(3) The effect of different CMV-DOM (fresh, NF-decomposed for7d and14d, or CF-decomposed for7d and14d) on the kinetic parameters of PCP degradation and Fe(Ⅱ)/Fe(Ⅲ) accumulation and disappearance at the soil-water interface. Addition of fresh or CF-/NF-decomposed CMV-DOM significantly enhanced the degradation of PCP and the accumulation of Fe(Ⅱ)NaOAc or Fe(Ⅱ)NaOAc+Ha (sequential extraction) in the flooded paddy soil. Results of sequential extraction showed that the degree of transformation between Fe(Ⅲ) and Fe(Ⅱ) varied with the ratio of Fe(Ⅱ)NaOAc to Fe(Ⅱ)NaOAc+HCl·In addition, most surrogate parameters of CMV-DOM properties were significantly related to the maximum capacity for Fe(Ⅱ)NaOAc accumulation. However, a single surrogate parameter could not explain the variations in the redox reactivity of fresh and CF-/NF-decomposed CMV-DOM that were jointly determined by multiple factors. Based on the above results, the relationship between selected microbial parameters as well as environmental variables and the kinetic parameters of Fe(Ⅱ) accumulation and PCP degradation was examined using biochemical and chemical analysis tools in combination with correlation analysis, regression analysis, and redundancy analysis. Results showed that Fe(Ⅱ)NaOAc+Hcl was the environmental variable that played a decisive role in PCP degradation. The consumption of WSOC and the increases in pH led to increases in the Fe(Ⅱ)NaOAc and Fe(Ⅱ)NaOAc+HCl concentrations, thus contributing to PCP degradation. As compared to Fe(Ⅱ)NaoAcl Fe(Ⅱ)NaOAc+HCl is more indicative of the biochemical mechanisms involved in the reductive transformation of PCP.(4) The effect of different dosages (1%and3%) of Chinese milk vetch (Astragalus sinicus L.) and bird vetch (Vicia cracca L.) on the kinetic parameters of PCP degradation and Fe(Ⅱ) accumulation at the soil-floodwater interface. The kinetic parameters of PCP degradation and Fe(Ⅱ) accumulation significantly varied with the dosage and type of green manures. The variations in the initial ultraviolet-visible spectral parameters of DOM led to significant changes in the maximum rates of Fe(Ⅱ)NaOAc accumulation. Based on the above results, the relationship between selected environmental variables and the kinetic parameters of Fe(Ⅱ) accumulation and PCP degradation was examined using chemical and electrochemical analysis tools in combination with correlation analysis and regression analysis. Results showed that Fe(Ⅱ)NaOAc was the key environmental variable that played a decisive role in PCP degradation. The changes in pH value and WSOC content significantly varied with the dosage rather than the type of green manure added into the soil. At a low dosage of Chinese milk vetch or bird vetch, the pH increase and WSOC consumption greatly enhanced Fe(Ⅱ)NaOAc accumulation and thus contributed to PCP degradation. The degradation rate of PCP decreased with pH decreases, especially at a relatively high dosage of Chinese milk vetch or bird vetch. Fe(Ⅱ)/Fe(Ⅲ) was the redox couple that determined Ep. At different dosages of Chinese milk vetch or bird vetch, the Ep value significantly varied over time, leading to different degrees of Fe(II)NaOAc accumulation and PCP degradation. Fe(II)NaOAc could well explain the mechanisms involved in the reductive transformation of PCP. |
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