Residues Of Pentachlorophenol In Soils And Its Effects On Rhizosphere Soil Bacterial Composition

Pentachlorophenol?PCP?is widely used in fungicides,bactericides,insecticides,herbicides,pesticides and wood-preserving agents throughout the world.In China,PCP is also frequently used to kill Oncomelania snails,the intermediate host of schistosomiasis?commonly known as bilharzia,bilharziosis or snail fever?,in traditional epidemic areas.Although several countries,including China,have banned or restricted the use of PCP,trace amounts of PCP still can be detected in the soil environment,threatening agricultural and natural ecosystems,and harming human health.We therefore investigated the natural degradation of PCP in zonal soils of China.We also studied the characteristics of extractable and bound residues of PCP in soils,and their relationships with soil properties.The residues and distribution of PCP and microbial community structure were further studied in the rhizosphere of Lolium perenne?Perennial Ryegrass?.Our research will provide a theoretical basis for assessing the environmental hazards and potential ecological threat of PCP and phyto re mediation of organic pollutants in the environment The main results were summarized as follows:?1?An analytical procedure was developed for the determination of water-and methanol-extractable pentachlorophenol?PCP?in soils based on vortex-assisted liquid-liquid extraction?VALLE?and gas chromatography?GC?.Significant extraction parameters such as vortex speed and liquid-liquid volume ratio were optimized for extracting PCP from solution.The recovery of recently added PCP was the highest?97.4%?with good reproducibility,when the vortex speed was at 2000 rpm.When the volume ratio of derivatization solution to n-hexane was at 10:4,the recovery of PCP was 103%with a RSD of 0.7%.Compared to traditional extraction methods,the VELLE procedure can achieve higher recoveries?mean of 96.8%?and smaller RSDs?mean 3.7%?,with less extractant and fewer analytical steps.The reliability of VALLE was verified by extracting PCP from four distinct types of soil samples spiked with 1 and 10 mg kg-1 PCP.The total recoveries of PCP in the soil samples were in the range of 89.5%-98.9%.Our study shows that this method is suitable for extracting and determining water-and methanol-extractable PCP in soils polluted by PCP at varying concentrations.?2?The mineralization and residual fractions of ¹⁴C-PCP were investigated in long-term incubation experiment with 13 zonal soils in China.The mineralization rates of ¹⁴C-PCP were significantly different among soils,the total mineralization rates were between 6.4%and 54.6%after 210 days incubation.The mineralization rate of PCP were positively correlated with soil microbial carbon content?MBC?and soil pH,and negatively correlated with soil surface area,soil pore,and clay content.The PCP extracted with water?WE?and water methanol mixed solution?V:V=1:1?extraction?WME?were dissipated rapidly,whereas the residual concentrations of methanol extraction?ME?residues increased at the beginning,then decreased slowly.The bound residues in the soil accumulated with time and reached 39.8?87.5%of the spiked PCP at 210 days.The regression equations between PCP residues and properties were established.It showed that soil pH,microbial biomass,fulvic acid content,humin content,soil surface area?SA?and soil pore volumes were the most significant factors controlling the degradation rates and residual forms of ¹⁴C-PCP in soils.A logistic model can better simulate the degradation,and half-life?T1/2?of ¹⁴C-PCP in soils,compared to a first-order kinetic model.The half-lifes of extractable ¹⁴C-residues of ¹⁴C-PCP in the soils were 7.1?111.8 days and the half lifes of total residues were hundreds of days to decades,indicating that the stability and ecological security of the total residues are of more environmental concern.?3?The distribution and dynamic changes of PCP bound ¹⁴C-residues in soils were studied.The results showed that the bound residues of ¹⁴C-PCP increased first and then decreased in the fulvic and humic acid,while they accumulated in the humin fraction.The content of bound residues of ¹⁴C-PCP in the humus fractions was in the order:humin>fulvic acid>humic acid after 210 days.The fraction of ¹⁴C-PCP bound residues in the fluvic fraction was correlated with soil pH,soil organic matter content,and field water hold capacity?WHC?;the fractions in the humic acid was correlated with humin,clay content and amorphous iron?Feo?;the residues in the humin fraction was correlated with soil organic matter content,soil surface area and field water holding capacity?WHC?.The accumulated trend of bound residues in the humic fraction could be simulated with Components models,which are always used in the simulation of the degradation of organic pollutant by microorganisms.With aging,PCP had the tendency to increase in the humin fraction,which had the lowest microbial biomass and chemical activity,both of which retard further degradation.?4?The extractable and bound residues of PCP,and its final distribution with stable isotope labeling technology(¹³C-PCP)in the rhizosphere and non-rhizosphere of ryegrass soils were investigated using rhizoboxes.The study showed that PCP contaminated soil could be better remediated in the ryegrass rhizosphere compared with unplanted soil.The remediation process may include two mechanisms,"activation" and "stabilization".The extractable?water,methanol?and total residues of PCP were both significantly lower in the rhizosphere than the non-rhizosphere soil,at approximately 91%and 30%,respectively.The bound residues of PCP in the rhizosphere was higher than in the non-rhizosphere soils.This suggests that the rhizosphere could not only increase the degradation of PCP but also enhance the formation of bound residues.Fertilization increased the dissipation of water extractable residues,but had no significant effect on methanol extractable and bound residues of PCP.High-throughput DNA sequencing results showed that PCP pollution changed the microbial community structure in soils.The relative abundance of some of the pollution tolerant bacteria was increased,and plants and fertilization had the ability to decrease pollutant induced stress.The changes of the microbial community structure also affected the distribution differences of extractable and bound residues of PCP.