| Polycyclic aromatic hydrocarbons (PAHs) can cause acute or chronic toxic effect and have carcinogenic, teratogenic, mutagenic effect on living beings. In recent years, more and more PAHs have entered environment through the release of tail gas, the agricultural application of sewage sludge, and the discharge of waste water. Red soil hilly area is large in China, and is the important area for agricultural production. But it is polluted due to the developed industry. The enrichment of PAHs in soil and water environment poses a great threat to public health and ecological security because it can migrate and transform through food chain. At the same time, modern large-scale agriculture produces a large number of agricultural plant wastes, which can not consume through traditional way, resulting in the waste of biological resource and environmental pollution.For the remediation of the soil polluted by PAHs, composting technology with the basic characteristics of strengthening biodegradation and biotransformation is lower cost and easier to realize resource recycling than the other physical and chemical methods. The degradation of PAHs during composting is limited by mass transfer due to its high hydrophobicity. The utilization of composting product containing residual PAHs may result in the re-enrichment of PAHs in environment.The method utilizing high efficient PAHs-degrading bacteria to degrade PAHs is also more efficient, lower cost than the other physical and chemical methods. But the degradation efficiency is limited by environmental conditions such as temperature, pH, and the coexistence of pollutants.Heavy metals and PAHs tend to exist at the same time in the polluted environment. They may interact and thus affect the efficency of biotransformation or biodegradation.Based on the above reasons, a method, which utilize composting technology to realize both the recycling of agricultural waste and the remediation of the red hilly soil polluted by PAHs, was presented, and futhermore two strengthening methods, by which the degradation efficiency of PAHs was increased and the residual PAHs was decreased during composting, were studied. The interaction between PAHs and heavy metals during compositng was also studied. One high efficient PAHs-degrading bacterium was isolated based on acclimatizing, screening and identifying. And both its degradation ability and the degradation ability of its enzymes were studied. The important results were listed as following:(1) PAHs in polluted soil were efficiently degraded by composting adding agricultural plant waste, which product was unharmful to plant and furthermore promoted its growth. Accordingly, both the reuse of agricultural plant waste and the restoration of PAHs contaminated soil were realized at the same time. After composting for32d, the residual rate of phenanthrene (PHE) and pyrene(Pyr) were4.6%and9.2%, respectively. When the ratio of the soil, agricultural plant wastes, leaves and hay, and wood chips was8:3:1:1, the organic matter content and C/N were respectively suitable for composting. The addition of agricultural plant waste stimulated the microbial metabolism. Bacterium was the main microbial group to degrade organic matter and PAHs. The pH ranged from6.6to7.6during composting process, which was favorable for the biodegradation of PHE and the composting process.(2) Pre-ozonation further decreased PHE in soil and thus decreased its inhibition on the composting microbes and quickened the startup of the following composting. After composting for31d, the residual rates of PHE and Pyr in surface red soil were1.1and5.0%, respectively, and the phytotoxicity of the contaminated soil disappeared. The organic content in soil remarkably decreased the efficiency of removing PAHs by ozonation.(3) Composting with inoculation of acclimatized activated sludge was also an effective method for further decreasing PAHs in composting product. The addition of acclimatized activated sludge made both the composting and the degradation of PAHs more rapid and further decreased PAHs in composting materials. The residual rate of PHE and Pyr were decreased to1.5%and7.4%, respectively, by composting for31d.(4) Composting with agricultural plant waste both decreased the residual PHE in composting product and promoted the morphological transformation of Pb. When composting was completed, the product could be reused because it was unharmful to plant, in which the percentage of PHE was decreased to9.6%and more than50%of Pb was bound to the residual fraction. During composting, pH could guarantee the composting without adjustment and the alkaline environment in the middle and the later periods of composting was helpful for the precipitation of lead ion, which reduced its inhibition on composting microbes and promoted the biodegradation of PHE. The coexistence of Pb inhibited the biodegradation of PHE to some extent by inhibiting the growth of composting microbes, decreasing the dissolved organic carbon, humic acid and fulvic acid. On the other hand, the coexistence of PHE inhibited the transformation of lead. But this effect was weak.(5) One high efficient PAHs-degrading bacterium was isolated by acclimatizing and screening. It was identified as Bacillus sp. and named Bacillus sp. PAHD3through the observation of morphology, cultural characteristics, physiological and biochemical experiments and the analysis of16S rDNA sequences. Naphthalene, fluo-ranthene, fluorene, phenanthrene and pyrene were all able to be utilized by this bacterium. And its gene for degrading PAHs existed in plasmid.(6) Bacillus sp. PAHD3was able to grow with the sole carbon source of PHE under the pH value of5to12. And the degradation was most efficient under the pH value of7.0. The concentration range of PHE which Bacillus sp. PAHD3could utiliaze was wide. After7d, PHE with the concentration of200mg·L-1,500mg·L-1and2000mg·L-1were degraded by100%, more than90%, and more than70%, respectively. Glucose showed inhibitory effect on the biodegradation of PHE while salicylic acid of20mg·L-1stimulated its degradation. Both Tween-80(50mg·L-1) and SDBS (200mg·L-1) promoted the biodegradation of PHE, and the former effect was bigger than the latter.(7) Under the concentration ranging from0mg·L-1to500mg·L-1, Bacillus sp. PAHD3was able to degrade PHE. But lead ion had certain inhibitory effect on the biodegradation of PHE, furthermore the inhibition increased with the increase of lead ion concentration. Acid environment could abate this inhibition. Lead ions affected the functional group, the extracellular enzymes, the cell morphology and the ability to produce surfacants of Bacillus sp. PAHD3. It was the internal mechanism of lead affecting the biodegradation of PHE.(8) Both intracellular and extracellular enzymes of Bacillus sp. PAHD3were efficient on degrading both PHE and pyrene. The removal rate of PHE (pyrene) reached98%(90%) by intracellular enzymes under the optimal pH value of6.0, and 88%(73%) by extracellular enzymes under the optimal pH value of7.0. The degradation of PHE by intracellular enzyme was stronger than by the extracellular enzymes. The optimum temperature for the above degradation was30℃. Both Pb2+and Cd2+inhibited the removal of PHE by extracellular enzymes. Pb2+and Cd2+of low concentration stimulated enzymatic removal of PHE, while which of high concentration had an opposite effect.(9) Compared with composting soil with the addition of activated sludge, composting soil with the addition of Bacillus sp. PAHD3was not more efficient for degrading PAHs. |
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