| Sewage sludge is a byproduct of wastewater treatment. The volume of sewage sludge gradually increases with the improvement of wastewater treatment capacity and ratio year by year. Nutrients in sludge could be converted to more available for plants, and parts of pollutants could be removed by sludge composting. Therefore, sludge compost can be utilized as organic fertilizer and has a certain value of resource utilization. However, residual contaminants, such as polycyclic aromatic hydrocarbons (PAHs), in compost could have effect on the environmental media, such as soil, vegetation and water system, after land application. And the research on migration and transformation of PAHs in soil systems fertilized with compost is not perfect. In order to know the rule of PAH migration and transformation in soil applied with compost and provide reference and guidance of land application of compost, this research has been conducted.First of all, a greenhouse experiment with three plants (tall fescue, ryegrass and white clover) has been investigated. In five different amendment ratios of dry compost to soil, PAH concentrations of soil and plants were analyzed after three and nine months of experiment. And some plants have been pruned. The effect of compost dosages and pruning plant on biomass of plants, PAH concentrations in plant and PAH residual concentrations in soil have been investigated. The results indicated that the biomass of plants and PAH concentrations in plants and soils increased with the more compost dosages in the range of0to4.5kg/m2. Biomasses of plants fertilized with compost were8.17%-416.03%higher than those without compost. When compost dosage was6.0kg/m2, PAH concentrations in tall fescue, ryegrass and white clover were1.73,2.38and1.79times of the unfertilized control. Pruning plants (tall fescue and ryegrass) are conducive to plant growth and uptake of PAHs. After nine months’experiment, PAH residual rates in soils planted with tall fescue, ryegrass and white clover were20.4%-49.8%,27.2%-36.1%and22.5%-48.1%, respectively. Considering the biomass and PAH concentrations in plants and soils, tall fescue were the best plant grown in compost-amended soil. And the optimum dosage of dry compost were1.5kg/m2, which is propitious to plant growth and PAH degradation.A greenhouse experiment with tall fescue was conducted with amendment ratios of fresh sludge compost to soil of0%,10%,25%and50%(w/w) to investigate the dissipation of PAHs in soils. The PAH concentrations in soil and plant, total number of bacteria in the soil, catalase activity and dehydrogenase activity were detected every three weeks. And migrations of PAHs from soil to plant and in plant were analyzed. The results demonstrated that PAH degradation in soil followed the first-order kinetics well in the study. And the determination coefficients in unplanted soils (R2>0.93) were higher than those in the corresponding planted soils (R2>0.88). Planting tall fescue has positive effect on PAH degradation in soil. The PAH dissipation rates were in the range of34.78%-54.40%in unplanted soils and40.84%-60.74%in planted soils. And the dissipation rates of2-3ring PAHs were higher than those of4-6ring PAHs. The total number of bacteria, catalase activity and dehydrogenase activity were higher in soils amended with compost than those without compost. During PAH dissipation in soil,0.45%-1.78%PAHs was transferred from soil to roots of tall fescue. And a part of these PAHs was migrated from roots to shoots of plant.2-3ring PAHs in plant may disappear from shoots of tall fescue by phytovolatilization, which accounts for19.7%-40.0%of total PAHs in plant. In the other hand,4-6ring PAHs could accumulate in roots and shoots of tall fescue.Field experiment with three plants were conducted with amendment ratios of fresh sludge compost to soil of0kg/m2,1.5kg/m2,3.0kg/m2,4.5kg/m2and6.0kg/m2to investigate PAH degradation under natural condition. At the same time, rainfall, groundwater and simulated runoff were collected and analyzed to investigate PAH migration in soils amended with compost. The results suggested that PAH degradation rates under natural condition were in range of16.04%-47.01%which was lower than those in greenhouse experiment.2-3ring PAHs were detected in rainfall, groundwater and runoff. And PAH concentrations in aqueous sample were in the range of72.4-210ng/L. The highest PAH concentration was found in rainfall, and the lowest shown in groundwater. PAHs migrated with runoff and groundwater would decrease the amount of PAHs in soil, but PAHs brought by rainfall would increase the amount of PAHs in soil.Based on the results of above experiments, the migration and transformation routes of PAHs in soil were concluded. PAHs in compost-amended soil derived from soil and compost. With amendment ratios of fresh sludge compost to soil of3.0kg/m2, PAHs introduced by compost accounted for19.34%-24.58%of total PAHs in soil. Desorption amount of PAHs in compost affected by temperature, pH value, humic acid concentration and compost dosage, which pH value was the most important factor. PAHs in soil were mainly removed through biodegradation by microorganism, which occupied31.72%-33.20%of total PAHs. Phytoaccumulation, degradation promoted by plant, volatilization, photodegradation and migration with runoff and groundwater contributed slightly to PAH dissipation, which accounted for8.41%-12.13%of total PAHs. More than50%of PAHs remained in soil, and advanced process should be performed to remove these PAHs from soil. |
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