Speciation transformation and release characteristics of mercury during the improvement of saline-alkali soil with sludge compost.

The application of sludge compost to saline-alkali soil is a key strategy for sustainable sludge management and degraded land improvement. Sewage sludge is rich in soil-enhancing nutrients but contains high levels of mercury. However, the environmental behavior of mercury during sludge compost-mediated saline-alkali soil amelioration, remain poorly understood. Therefore, this study selected the saline-alkali soil of Yellow River Delta as the test soils, and prepared co-compost from sewage sludge and corn straw as the soil amendment, to explore the variations of total mercury (THg), mercury speciation, and atmosphere release of Hg0 during the improvement of saline-alkali soil. The results showed a decrease in THg content. After improving saline-alkali soil with sludge compost, the proportion of labile Hg species and humic complexes increased. Sludge compost increased the net methylation rate in the soil near the water level under non-flooded conditions and in the surface soil under flooded conditions. The application of sludge compost under flooded conditions increased the relative abundance of mercury-methylating microorganisms in the soil. Sludge compost increased the concentration of the dissolved total mercury (DTHg) in soil pore water, thereby enhancing mercury mobility. Under non-flooded conditions, sludge compost promoted the release of Hg0 in soil. Flooded conditions are not conducive to the release of Hg0 in soil. Flooded conditions elevated the risk of methylmercury (MeHg) formation, while non-flooded conditions promoted the release of soil mercury into the atmosphere. The study revealed the pollution risk of mercury in saline-alkali soil improved by sludge under different scenarios.