| Municipal sewage has been identified to be an important source of mercury to the environment.However,as the major sink of sewage-borne Hg,s the the geochemical characteristics and the environmental effects of mercury in sewage sludge?SS?remain unresolved.We presented here,a national-wide survey on the geochemical characteristics of Hg in SS of China.De-watered SS samples?containing80%moisture?were collected from 40 MSTPs located in 40 cities of China in the 2016sampling campaign.Total Hg?THg?and Methylmercury?MeHg?in the SS samples were analyzed,and mass loadings and fate of THg and MeHg carried by SS in each city were calculated.The release characteristics of mercury were studied by leaching experiments,and the potential ecological risk of mercury in sewage sludge was evaluated.The main results of this study are as follows:?1?THg and MeHg in SS of China followed log-normal distributions.The geometric mean of THg and MeHg were 2.02±2.84 mg·kg-1 and 3.07±5.32 ng·g-1,respectively.Based on those values,the mass loadings and fate of mercury in sewage sludge were estimated in China.In general,THg in the SS of China showed a descending trend from north to south.After logarithm transformation,MeHg in the SS of China was positively correlated to THg.The ratios of MeHg/THg also followed a log-normal distribution,with a geometrical mean of 0.15%±0.21%obtained.?2?Analysis of geochemical speciation of Hg in SS revealed that majority of Hg in SS occurred in mercury sulfide?F5?and organo-chalated fractions?F3?.The sum of F3and F5 accounted for 86%of THg.Bioavailable Hg,i.e.,water soluble fraction?F1?+“human stomach acid”soluble fraction?F2?,only accounted for less than 1%of THg in the SS.Positive relationships were found for F3 vs.THg,F4?strongly-chelated praction?vs.THg,and F5 vs.THg.F2 fraction was positively correlated to F4,while negtive correlations were found for F3 vs.F5 and F4 vs.F5?after logarithm transformation?,indicating the possibility of transformation of Hg between those fractions.It is noteworthy that MeHg positively correlated to F1?water soluble fraction?and F3?organo-chelated fraction?.?3?The concentrations of THg in the leachate ranged from 12.02 to 718.6 ng·L-1?simulated rainwater,SRW?and 5.49 to 260.4 ng·L-1?de-ionized water,DIW?,and dissolved THg varied in the ranges of 1.45-360.6 ng·L-1?SRW?and 1.03-71.71ng·L-1?DIW?,respectively.The concentration of MeHg in the leachate ranged from 0.01to 8.22 ng·L-1?SRW?and 0.01 to 15.08 ng·L-1?DIW?,and DMeHg ranged from 0.01 to3.03 ng·L-1?SRW?and 0.01 to 7.92 ng·L-1?DIW?,respectively.There were significant positive correlations between DTHg and THg,and DMeHg and MeHg in the leachate.The range of MeHg/THg in leaching solution was 0.01%-44.58%?mean value 5.02%±9.98%?and 0.01%-50.80%?10.37%±11.47%?for SRW and DIW,respectively.The leaching dynamics of THg and MeHg from SS could be simulated by using the parabola equation?diffusion equation?.The mass balance closure of MeHg was 135?leached by SRW?and 137%?leached by DIW?,indicating the possible occurrence of Hg methylation during the leaching processes.?4?Referring to the soil background values of Hg in each capital city,the enrichment factor calculations showed that about 90%of the SS reached a moderate pollution level.The geo accumulation index calculation reviewed that Hg in SS showing slight to extremely strong pollution,and the potential ecological risk index demonstraed that high potential ecological risk of mercury in 87.5%of SS.Referring to the soil background values of Hg in the capital city of each province/district,calculations of the enrichment factors of THg in SS revealed that about 90%of the SS samples were at a moderate enrichement level.The geo accumulation indices showed that slight to extremely strong pollution of SS by Hg,and the potential ecological risk indices indicated that 87.5%of SS were poesed at a high potential ecological risk. |
PDF Full Text Request