| Phthalic acid diesters (PAEs), used primarily in polyvinylchloride (PVC) as plasticicers and known as endocrine-disrupting chemicals or environmental hormones, has been listed as priority pollutants by US EPA and China. Generally, discarded plastic are co-landfilled with other municipal solid waste (MSW). Therefore, landfill leachate is an important way of PAEs entering the environment. With respect to the increasingly pollution arisen by MSW, investigating the PAEs transformation and degradation in landfill and analyzing the landfill condition for accelerating their mineralization are important for controlling PAEs secondary pollution by leachate. There are few studies conducted to evaluate the behavior of PAEs in bioreactor landfills. In this study, PAEs listed as priority pollutants in water by Environmental Monitoring of China were chosen as target pollutants. Considering the refuse and leachate as one whole system, the behavior of PAEs in two simulated landfill bioreactors were investigated. A conventional landfill was set as a control. In addition, effect of general physicochemical, biochemical and microbial characterization of the landfill on PAEs biodegradation were evaluated. Finally, mechanism of bioreactor landfill on the PAEs biodegradation was also analyzed. The main conclusions of this study are list below.(1) Dimethyl phthalate (DMP), dibutyl phthalate (DBP) and dioctyl phthalate (DOP) were all detected in both leachate and refuse from conventional landfill (CL), recirculated landfill (RL) and bioreactor landfill (BL). Among the three PAEs, the DBP concentration was observed with the highest level. The stabilization process of landfill, with sequences of BL>RL>CL, play an important role on the biodegradation of PAEs in refuse. Compared to the acidic environment, the methanogenic environment is beneficial for PAEs degradation. DMP and DBP degraded rapidly, and DMP was completely degraded in landfill conditions, while the concentration of DOP decreased slowly. The residual amounts of PAEs with significant differences are well fit exponential decay models in CL, RL and BL. The operation of leachate recirculation can obviously accelerate PAEs biodegradation than conventional operation, and the introduction of methanogenic reactor can further promote the removal of PAEs in landfill.(2) Based on the real proportional characteristics of MSW, simulated MSW were loaded into the simulated leachate recirculation bioreactor landfill. The abundance of the common and the tolerant microbe of the three kinds of anaerobic microorganisms in refuse were investigated at initial phase, acidic phase and methanogenic phase, respectively. It showed that the number of bacteria was the largest while the number of actinomycetes was the smallest. The growth of microorganisms was not significantly inhibited by DBP at initial phases. However, the growth of actinomycetes and fungi was inhibited both at acidic phase and methanogenic phase, and the inhibition of actinomycetes was stronger than that of fungi. The toxicity effect of DBP on microorganisms in refuse has the ranking of actinomycetes>fungi>bacteria. In addition, the numbers of bacteria, fungi and actinomycetes significant positivly corrected with dehydrogenase activity, but negative corrected with VSS, BDM of refuse (P<0.01).(3) Two bacterial strains including T1 and T5 capable of utilizing DBP as their sole source of carbon and energy were isolated from refuse of simulated recirculated landfill. Based on their morphology, physio-biochemical characteristics, and 16S rDNA sequence, the strains were identified as Enterobacter sp. and named as Enterobacter sp. T1 and Enterobacter sp. T5, respectively. The optimal pH and temperature for their biodegradation activities were 7.0, 35℃and 7.0, 30-35℃,respectively. The degradation kinetics of DBP by Enterobacter sp. T1 fit a first-order kinetic model of ln C= -0.0359t+A with the degradation half-life of 19.32h. when the DBP concentration was lower than 1000mg/L. Similarly, degradation of DBP by Enterobacter sp. T5 fit a first-order kinetic model of ln C= -0.0332t+A with the degradation half-life of 20.88h. The degradation productions of two major metabolites were identified as phthalic acid and monobutyl phthalate. In addition, those two strains also grew in conditions of DMP, diethyl phthalate (DEP) or DOP solution as the sole source of carbon and energy, respectively. It suggested that the range of their metabolism objects was wide.(4) The degradation of DBP fit first-order kinetic models in refuse of different phases, the rate constants were 0.0140-0.0187 d-1 and the degradation half-lives were of 37.1-49.5d. The effect of DBP concentration on its degradation in refuse was not obvious. DBP was degraded fastest in the refuse of methanogenic phase, while slowest in the refuse of acidic phase. pH is the key factor of DBP biodegradation. In addition, the degradation rate of DBP obviously increased after inoculating the dominant DBP degradation bacteria in the refuse. The rate constant of inoculation was significant higher than the nonvaccinated in the refuse of methanogenic phase (p<0.05). The degradation ability of mixed bacteria was obviously higher than single strain, it suggested that the synergistic effect can promote the DBP degradation effectively in the refuse. Till day 50, the removal rate of DBP in the refuse of methanogenic phase increased from 60.3% to 74.5%, 72.4% and 87.3% when T1, T5 and mixed bacterial were inoculated, respectively.
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