Dissipation Of Pentachlorophenol At The Water-soil Interface Under The Influence Of Electron Donors And Acceptors

Pentachlorophenol (PCP) is a ubiquitous, and highly persistent organic environmental pollutant. In China, the primary purpose for using PCP and its sodium salt (Na-PCP) is to kill the schistosome intermediate of host snails. PCP tends to accumulate in soils because of its slow rate of degradation. Soil contamination with PCP poses a great threat to the productivity and ecological functioning of soil, of food quality and human health. Therefore the behavior of PCP in the environment and remediation technologies have received increasing investigation. Paddy soils comprise the largest anthropogenic wetlands on earth. Redox potential fluctuations due to paddy soil management controls microbial community structure and function and thus short-term biogeochemical processes.PCP was selected as the target compound in this study. The behavior of PCP dissipation at the water-soil interface, the influence of redox potential, electron donors, electron acceptors, the aerobic-anaerobic gradient in soil profiles, and the response of microbes were studied. The main experiments and results were as follows:(1) The potential for dissipation of pentachlorophenol (PCP) was investigated in soils from four different sites in China. These were a silt clay Umbraqualf (Soil1), a silt clay Plinthudult (Soil2), a silt loam Haplustalf (Soil3) and a silt clay loam Argiustoll (Soil4) which were either flooded, to produce anaerobic conditions, or incubated aerobically at60%water-holding capacity (WHC). The molar ratio of [Cl-]/â–³[PCP] for PCP is5. Changes in the molar ratio indicated that the dissipation of PCP in different soils was caused by other processes (e.g. irreversible sorption by soil particles) in addition to dechlorination. The molar ratio [Cl-]/â–³[PCP] was:Soil1(4.86)> Soil4(2.68)> Soil3(0.29)=Soil2(0.22) under flooded conditions, and Soil1(4.93)> Soil3(4.87)> Soil4(4.60)> Soil2(1.53) at60%WHC.(2) Ionic oxidants (Fe(III), sulphate and nitrate) were applied to study their effects on the fate of PCP under flooded conditions. Dissipation of PCP was significantly inhibited by addition of Fe(III)(as FeCl313.5g kg-1), while addition of sulphate (as Na2SO4,2.8g kg-1) and nitrate (as NaNO3,1.7g kg-1) had different effects, depending upon the soil type. Extractable Fe(â…¡), sulfate and nitrate concentrations were determined to investigate interactions of redox reactions involved in the PCP dissipation. Sulfate reduction occurred in Soil1and Soil4under flooded conditions. Similarly, the redox potential decreased significantly in Soil1and Soil4, and the dissipation of PCP in the two soils was statistically significant (about96%and98%, respectively) at the end of the flooded incubation (120days). By120days of incubation, the redox potential decreased from220mV to-4mV and180mV to-50mV in soils1and4respectively. The added oxidants inhibited the dechlorination process of PCP.(3) A soil microcosm was designed to mimic the soil-water interface of three typical Chinese paddy soils, Umbraqualf (Soil1), Plinthudult (Soil2), Tropudult (Soil3). Vertical variations in the dissipation potential of pentachlorophenol (PCP) were investigated at the mm-scale in an unplanted, flooded, paddy soil. The depth of soil profiles where PCP dissipation occurred was different due to different soil characteristics. They were0to15mm,0to25mm and0to30mm with the PCP dissipation of40-93%,42-88%and16-100%for Soil1, Soil2and Soil3respectively. The dissipation rate of PCP decreased with increasing soil depth. Pentachlorophenol was significantly (p<0.05) dissipated within the10to50mm depth in Soil2, while dissipation in Soil1was weak, and did not occur in Soil3, where the PCP concentration remained constant until the end of the120day incubation. The PCP dissipation in the surface layer may have been caused by leaching, photolysis, diffusion and biodegradation, and controlled by complex factors including both soil chemical properties and environmental conditions (e.g. temperature and moisture). The vertical profiles of Fe(II) and SO42-indicated that the high SO42-concentration inhibited the dissipation of PCP and the reduction of Fe(III), and the reduction of Fe (III) also competed for electron donors with PCP. The PCP significantly inhibited the reduction of Fe(III) and SO42-and the production of NH4+. In addition, the anions in flooded soils were highly mobile in the soil profiles. (4) The effect of different crop residue additions, wheat(Triticum aestivum), rice (Oryza sativa), rape (Brassica campestris L.) and Chinese milk vetch(Astragalus sinicus) on PCP dissipation under greenhouse incubation conditions were investigated in three typical Chinese paddy soils. These were an Umbraqualf (Soil1), a Plinthudult (Soil2) and a Tropudult (Soil3). Crop residue addition rapidly increased PCP dissipation in soil profiles. PCP was dissipated completely in all crop residues addition treatments at60days incubation. The distribution of electron donors and acceptors in soil profiles was also influenced by crop residues. The distribution of electron donors and acceptors clearly differed, as indicated by Principle Component Analysis (PCA) of their concentrations in the soil profiles. The metabolites3,4,5-trichlorophenol (3,4,5-TCP) and2,3,4,6-tetrachlorophenol (2,3,4,6-TeCP) were produced during PCP dissipation in soils1and2.(5)The changes in microbial community structure were investigated in crop residues (wheat, rice, rape and Chinese milk vetch) and electron acceptors (NO3-, SO42-), in amended soils under greenhouse conditions.In all treatments, the highest concentration of total PLFAs was in the0-10mm depth. Compared with the control treatment, there were significant changes between the PLFA patterns in soils amended with crop residues and electron acceptors as indicated by Principal Component Analysis (PCA) of the PLFA signatures. The i14:0, which can represent gram-positive bacteria, had remarkable relationships with PCP dissipation (p<0.01).3,4,5-TCP was negatively related (p<0.01) to16:0(10Me),17:0(10Me), cyl7:0and cyl9:0, which represent actinomyces and anaerobic bacteria respectively. In addition,16:0(10Me) is also the signature PLFA of sulfate-reducing bacteria.