Characteristics Of Mercury Change In Sludge Compost And Application Process And Its Effects On Soil And Crops

With the improvement of wastewater treatment capacity of sewage treatment plants in China,the amount of sludge produced by urban sewage treatment plants has exceeded 40 million tons every year.So a lot of sludge has become a major environmental problem that can not be avoided,if disposed of properly,it will cause serious harm to the environment and human health.The implementation of the new environmental law and the implementation of the water pollution control action plan emphasize the importance of sludge disposal.High temperature aerobic composting is one of the main ways of sludge stabilization.Sludge compost is rich in a variety of nutrients and a large amount of organic matter in the plant to make it have high agricultural value.Therefore,land use after sludge compost has gradually become one of the main ways of sludge disposal and resource utilization.However,sludge composting and land use process may also cause secondary contamination risk,in which heavy metals in the sludge is a key factor limiting its use.As mercury has high volatility,high toxicity,high bioaccumulation,if mercury transfers to the soil through the application of compost,the risk of exposure to mercury is high.At present,there are few studies on total mercury,methylmercury and mercury emission flux in sludge composting and its utilization.The studies of environmental behavior and bioaccumulation characteristics of mercury in composting and soil-plant system during composting are also relatively less.Therefore,in this study,a high-temperature aerobic composting and its utilization experiment were carried out to analysis and discuss the changes in total mercury and methylmercury content,total mercury and methylmercury migration and plant enrichment during composting,soil-atmospheric mercury release characteristics during composting,to assess the mercury environmental risks in the composting process of sludge to provide basic data for the environmental risks of heavy metals（especially mercury）for the development of municipal sludge.The results showed that:（1）.The total mercury content before and after two high temperature aerobic composting did not change significantly,but all were lower than the total mercury content of the original sludge.The contents of methylmercury during the two high-temperature aerobic composting expressed as the beginning of composting > the end of composting,indicating high-temperature aerobic composting did not utilize mercury methylation.The addition of biomass carbon had no significant effect on the total mercury content of the composting process,but had some inhibitory effect on the methylation of mercury.（2）The application of sludge compost will significantly increase the total mercury and methylmercury content.The total mercury content of treated soils increased by 124.32%-264.73% with different dosage of sludge composting（first composting）and the content of methylmercury increased by 38.66%-151.33%.The total mercury content in soils treated with non-biomass composting and biomass composting increased by 43.83% and 28.48%,respectively,during the application of different fertilizers（second composting）and the content of methylmercury increased by 236.36% and 215.15%,respectively,but the total soil mercury content was lower than that of the national soil environmental quality in the two composting processes.Application of different amounts of sludge compost（first composting）experiment shows that,the content of methylmercury(0.28-0.40 ng·g^-1)in soils composted with biomass composting was higher than that of soils containing no biomass composting(0.22-0.38 ng·g^-1),indicating that biomass carbon may be beneficial to the production of soil methylmercury.Application of different fertilizers（second composting）experiment shows that,the content of methylmercury in the soil treated with composting increased significantly,while the content of methylmercury in the soil treated with chemical fertilizers decreased,indicating that the different fertilizers may affect the methylation of soil mercury.（3）The content change and enrichment characteristics of total plant mercury and methylmercury in two composting using experiments were the same,the total mercury content showed that the harvest time of plant was less than that of seedling stage,the methylmercury content behaves just the opposite.The total mercury enrichment capacity of plant was less than that of methyl mercury.Application of different amounts of sludge compost（first composting）experiment shows that,the content of methylmercury in the biomass composting treatment group(0.21-0.54 ng·g^-1)was significantly lower than that in the non-biomass carbon composting treatment group(0.28-1.13 ng·g^-1),indicating that the addition of biomass carbon compost could be harmful to the migration of methylmercury in soil and accumulation of methylmercury in plants.Application of different fertilizers（second composting）experiment shows that,the content of total mercury in the mature plants was mainly distributed in roots and leaves,and the respective proportions were 32.77%-50.89%,41.21%-55.48%.The content range of methylmercury in the roots of each treated plant was 2.25-9.26 ng·g^-1,and in the stem was 1.33-2.28 ng·g^-1,and in the leaves was 0.47-0.90 ng·g^-1.The order of methylmercury content in different tissues was root> stem> leaves.The methylmercury transfer coefficients were small（TFs<1）,which indicated that the the ability to transport methylmercury to the ground parts of plant is weak.（4）The mercury flux in the soil-gas interface was higher in the initial stage of sludge composting,the range of mercury flux for each treatment were A1（application of non-biomass carbon sludge composting）: 49.16¹10.09 ng·（m²·h）^-1,A2（application of biomass carbon sludge composting）: 38.89⁸3.35 ng·（m²·h）^-1,A3（application of chemical fertilizers）: 31.00⁵1.12 ng·（m²·h）^-1,CK（control）: 20.29³9.99 ng·（m²·h）^-1.The emission of mercury for each treatment was mainly concentrated in the germination period,and after 5 days the mercury emission was significantly reduced.The cumulative emission flux of mercury in the soil treated without biomass carbon compost(379.89 ng·（m²·h）^-1)was higher than that in the treatment with biomass carbon compost(302.29 ng·（m²·h）^-1),indicating that biomass carbon on the emission of mercury.The main environmental factors affecting the emission flux of has some inhibitory effect mercury from soil-air interface were light intensity,air temperature,atmospheric humidity,which showed a positive correlation with light intensity,air temperature,and a negative correlation with atmospheric humidity.（5）The total mercury content of the two high-temperature aerobic composting processes were lower than that of the control standard for agricultural sludge,and there was no risk of environmental mercury pollution.Based on the cumulative index method,it is shown that the accumulation index（Igeo）of soil mercury in the two processes of composting of sludge is less than 1 and basically does not cause soil mercury pollution.Based on the potential ecological risk index method（Er）,the results showed that the sludge composting process has moderate to strong ecological risk of mercury pollution and the factor influencing the ecological risk of mercury pollution was fertilizer mercury content.In order to carry out the utilization of municipal sludge for science,we need to study the reasonable fertilization and the removal mechanism of mercury in composting process.