Transportation And Transformation Of Mercury In Landfill Disposal Of Municipal Sewage Sludge

Mercury is a global heavy metal pollutant.Municipal sewage has been considered as an important anthropogenic source of mercury in the environment.Sewage sludge（SS）can be also considered as a potential source of mercury pollution,due to the reseasons:large amount of sludge production,and the great quantity of total mercury（THg）and methylmercury（Me Hg）existed in SS.However,landfill disposal in China is is the most widely used method of final disposal.Therefore,it is necessary to study the transformation and release of mercury in landfill disposal.In this study,sewage sludges from sewage treatment plants in Zhengzhou city and Wuhan City were used as the study system.The research contents mainly included:（1）distribution and release characteristics of mercury;（2）speciation transformation of mercury;（3）changes of mercury methylation microbial community;（4）the effects of geochemical factors and microbial communities on the release and transformation of mercury.The main results were shown as follows:（1）The release kinetics and release mechanisms of THg and Me Hg were studied.The leaching process of THg and Me Hg in leachate could be divided into two distinct stages:the initial stage of rapid decline and the second stage of gradual decline to stable equilibrium.By fitting the release kinetics model equation,it was concluded that the release of leachated THg conformed to the heterogeneous mechanism,and the release of leachate Me Hg was the liquid film diffusion mechanism.Movereover,the variation trend of THg content in gas absorption solution was as follows:THg content maintained at～100 ng/L in winter and spring,increased significantly（about 6-12 times）in summer,and decreased to about 200 ng/L in autumn.This change was almost consistent with the temperature change,so it can be speculated that higher temperature promotes the release of Hg.By calculating the mass balance of mercury,the leaching rate of THg in leaching solution was 0.48%（Wuhan）and 0.47%（Zhengzhou）,and the release rate of THg in gas adsorption solution were 0.09%and 0.08%respectively.It revealed that the leaching process was the main release pathway of THg in sludge under rainfall conditions.（2）The isotope double tracers ¹⁹⁹Hg²⁺and Me¹⁹⁸Hg were used to study the transport and transformation of mercury during the landfill process.It found that the transformation of mercury in SS was firstly dominated by the methylation of inorganic mercury,and finally converted to the demethylation of Me Hg.In the early stage of landfill,the rate of methylation/demethylation fluctuates greatly,indicating methylation/demethylation was more active at this stage.It may be because the drastic changes of physical and chemical properties in the initial stage of landfill have changed the microbial composition and activity.It verified the formation and degradation of methylmercury hanppened in landfill.（3）The microbial community structure and diversity were analyzed by using Illumina Mi Seq high-throughput sequencing system.The microorganisms of SS mainly consisted of Bateria and Archaea,of which bacteria account for more than 97%.There were 5 kinds of mercury-methylated microorganisms at phylum level,including Chloroflexi,Frimicutes,Proteobacteria,Cyanobateria and Euryarchaeota.Among them,Chloroflexi,Frimicutes and Proteobacteria accounted for more than 98%.Therefore,bacteria played a key role in the methylation of Me Hg.Moreover,mercury methylated microorganisms in SS were mainly distributed inδ-Proteobacteria and Firmicutes,e.g.,Bacillaceae,Syntrophomonadaceae,Desulfovibrionaceae,Syntrophobacteraceae,Geobacteracese,Ruminococcaceae,Peptocoocacsae and Syntrophaceae.In addition,the community structure of methylated microorganisms has changed significantly during the landfill process.（4）Correlation analysis showed that organic matter and microorganism played an important role in the transportation and transformation of mercury.And F1 and TOC were the main control factors of methylation rate.Moreover,microorganisms with hgc A gene had a significant effect on the process of methylation and demethylation.There were five class-level microorganisms that were significantly correlated with the transformation of mercury in sludge,including Clostridia,Bacteroidia,Chloroflexia,Anaerolineae and Thermoleophilia.Besides,gene-level microorganisms such as Geobacteracese,Syntrophomonadaceae,Desulfovibrionaceae,Syntrophaceae and Syntrophobacteraceae were related to the rate of methylation/demethylation.43 figures,16 tables,184 references.