Study On Bioremediation And Microbial Community Composition Of Pentachlorophenol-Contaminated Soil By Composting Or The Addition Of Compost

Bioremediation processes of pentachlorophenol (PCP)-contaminated soil by thermophilic composting with agricultural wastes or by the addition of compost were investigated in this paper. Those were accomplished by evaluating physicochemical and biochemical properties of composts, as well as microbial community composition using the advantage molecular biology methods. Subsequently, remediation effectiveness of inoculation with Phanerochaete chrysosporium (P. chrysosporium) during different fermentation phases of composting in treating PCP-contaminated soil waste was evaluated. The differences in activities of extracellular ligninolytic enzymes involved in PCP degradation and in bacteria community composition of composts were compared among three runs. Finally, mature compost and the compost inoculated with P. chrysosporium were used as the bioaugmentation of PCP-contaminated soil, which focused on the PCP degradation and soil microbial activity.Two composting piles were prepared by adding to a mixture of agricultural wastes including rice straw, vegetables and bran:(i) raw soil free from PCP contamination (pile A) and (ii) PCP-contaminated soil (pile B). The initial concentration of PCP in dry soil was133mg/kg. The results showed PCP removal rate of72%during composting of60days, indicating the microorganisms played a key role in this process. The changes of compost parameters such as temperature, pH, organic matter content and so on, showed the significant effects of PCP on the whole composting process, especially in mesophilous and thermophilic phases. All those parameters were subjected to discriminant analysis in order to elucidate the parameters most responsible for differentiation of the two piles, showing a clear differentiation of compost samples of different ages. The compost temperature was the most responsible for explaining the results.It was shown by the results that compost maturity characterized by water soluble carbon (WSC), TOC/TN ratio (C/N), germination index (GI) and dehydrogenase (DH-ase) activity was significantly affected by PCP exposure, which resulted in an inferior degree of maturity and compost quality for pile B. Although the present experiments showed a proper functioning of the composting mixed with PCP-contaminated soil, the addition of PCP led to an inferior degree of maturity, and accordingly a longer composting period was recommended for obtaining highly mature composts.Meanwhile, the polymerase chain reaction and denaturing gradient gel electrophoresis (PCR-DGGE) technique was conducted to assess the effect of PCP-contaminated soil on the microbial community composition during composting. Total genomic DNA was extracted from different compost samples, and then fragments of16S rDNA and18S rDNA genes were amplified with bacterial universal primers GC-338F/518R and fungal universal primers NS1/GC-Fung, respectively. Bacterial and fungal community compositions were obtained in the resulting DNA band profiles. Statistical analysis of the DGGE profiles for bacterial and fungal communities was performed by principal component analyses (PCA). The results revealed a distinct effect of PCP on compost microbial community.Both the bacterial and fungal community compositions were different between the two piles. The greatest difference of bacterial community composition occurred at the thermophilic phase due to the differentiation of the composting temperature. However, towards the end of the composting process bacterial community composition in the two piles became similar, of which the same composition could be considered as an indicator of mature composts. By contrast, fungal communities were more sensitive to PCP contamination due to the significant correlation between fungal community shifts and PCP removal. As the composting process, fungal communities in PCP contaminated pile changed substantially compared to an uncontaminated pile. Therefore, fungal communities could be more appropriate for a response to the degree of PCP contamination than bacterial communities.Additionally three composting systems consisting of several agricultural wastes and PCP-contaminated soil were set up, which aimed at increasing the efficiency of inoculation with P. chrysosporium during different fermentation phases on the remediation of PCP-contaminated soil. The results showed that the inoculations could significantly enhance composting efficiency and PCP removal. The best highest PCP removal rate of97.5%occurred in Run C that inoculated with P. chrysosporium during the second fermentation phase, while in Run B that inoculated during the first fermentation phase the PCP removal rate was88.6%. It is evident from the results that the effectiveness of remediation depends on the time when the inoculation is carried out. Although a positive effect on production of manganese peroxidase (MnP) and lignin peroxidase (LiP) was found in inoculated runs, especially in Run C, the significant correlation between the production of those enzymes and PCP removal was not found in our investigation. As a result of DGGE analysis, the compost bacterial community composition was altered by different inoculations, as indicated by the differences between the final composts. This variation was more significant in Run C than that in Run B, which suggested that the improvement of composting efficiency was resulted from shifts in microbial community composition.Composts, as the production of the composting process, provide a nutrient source for indigenous degraders and a diverse population of active microorganisms when applied to contaminated soils. The results obtained from the study on the remediation of PCP-contaminated soil with compost or the compost inoculated with P. chrysosporium showed decrement of PCP in soil with the process time. The extracted PCP from humic substances of soil samples indicated the presence of organic matter might enhance the sorption of PCP to soil. The addition of composts to soil increased the amount of organic matter, which resulted in lower PCP concentration in CS50. The lowest PCP concentration occurred in CPS50that was added by the composts inoculated with P. chrysosporium, suggesting that the presence of P. chrysosporium might have favoured the entrapment of PCP in the humic material, thus rendering PCP hard to extract. Moreover, it was also found those two remediation methods mentioned in this experiment, especially the treatments by the addition of compost inoculated with P. chrysosporium had capacity for promoting the activities of extracellular ligninolytic enzymes and soil microorganisms. Therefore, those two methods could be effectively applied for bioremediation of PCP-contaminated soil.