Investigating effect of salinity on methylation of exogenous mercury of soil in wastewater-irrigated area by labeling with stable isotopically enriched tracers
1. Rural Energy & Environment Agency, Ministry of Agriculture, Beijing 100125, China;
2. Agro-Environmental Protection Institute, Ministry of Agriculture, Tianjin 300191, China;
3. Key Laboratory of Technologies and Models for Cyclic Utilization from Agricultural Resources, Ministry of Agriculture, Beijing 100125, China
This study is designed to pinpoint the effect of salinity (NaCl and Na2SO4, add at salinity levels of 0%~5%, respectively) on methylation of exogenous Hg (Ⅱ) in wastewater-irrigated areas of Tianjin City. Solutions containing stable isotope tracers of 199Hg2+ were spiked into the soils at the concentrations of 80% of the total soil mercury. The formation of CH3199Hg+ (MeHg) was measured in time series experiments using gas chromatographic separation and isotope-specific detection by inductively coupled plasma mass spectrometry. The results manifested that, generally, changes in concentrations of CH3199Hg+ in soil with incubation time were similar in direction and pathways, and three regions (retarded-sharp-stable) could be distinguished. Logistic equation can well fitted kinetic experimental data of concentrations of CH3199Hg+ formed during the incubation time. According to Logistic model, the maximum amount of formed MeHg of control soil (without salinity addition) was 0.698μg/kg, while the maximum methylation rate of exogenous Hg was 0.217μg/(kg·d). In NaCl spiked soils, methylation rate of exogenous Hg increased first and then decreased with the increasing salinity level of NaCl. It found that the maximal amount of formed MeHg and the methylation rate of exogenous Hg were significantly higher in 0.2%~0.6% NaCl spiked soils by comparison with control soil. The highest amounts of them were in 0.4% NaCl soil, which accounted for 3.589μg/kg and 0.415μg/(kg·d), respectively. The maximal amount of formed MeHg over incubation time and the methylation rate of exogenous Hg in 1%~2% NaCl spiked soils were close to them in control soil. It also found that high salinity level of NaCl (5%) inhibited the methylation of exogenous Hg. In Na2SO4 spiked soils, salinity (>0.2%) constrained the availability of mercury for methylation. The results of these studies demonstrated higher MeHg production potentials in comparatively lower salinity environments, which may enhance Hg bioavailability in the soil and cause a hazard to human body through the food chain.
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