Exploring the migration and transformation behaviors of heavy metals and ammonia nitrogen from electrolytic manganese residue to agricultural soils through column leaching test.

Heavy metals (HMs) and ammonia nitrogen (AN) leaching from electrolytic manganese residue (EMR) result in the contamination of agricultural soils and water bodies. Batch and column leaching tests were conducted to simulate the release of HMs and AN in EMR during precipitation, as well as their migration and transformation in agricultural soils. The results show that Mn, AN, Cd, Ni, and Zn present in the EMR had high acid soluble fraction (un-fixed AN) content, and the leachability of Mn and AN was significantly higher than that of other hazardous elements. The cumulative release of hazardous elements in the EMR stockpile was well-fitted (R2 > 0.95) by the HILL model. Significant HMs and AN accumulated in the agricultural soils after contamination from the EMR leachate. The pollution degree of HMs in agricultural soils was ranked as Mn > Ni > Pb ≈ Zn ≈ Cr > Cd. The acid soluble fraction (un-fixed AN) content of Mn, Ni, Zn, and AN in agricultural soils increased significantly. The risk assessment code shows that the risk level of Mn in agricultural soils changed from medium to high; Ni and Zn in surface soils changed from low to medium. These results indicated that the leaching from EMR would significantly increase the ecological risk of HMs in surrounding agricultural soils, and the large release of AN would pose a great threat to aquatic systems if not properly addressed.

Laboratory model test study of the hydrological effect on granite residual soil slopes considering different vegetation types.

The problems caused by the interaction between slopes and hydrologic environment in traffic civil engineering are very serious in the granite residual soil area of China, especially in Guangdong Province. Against the background of two heavy rainfall events occurring during a short period due to a typhoon making landfall twice or even two typhoons consecutively making landfall, laboratory model tests were carried out on the hydrological effects of the granite residual soil slope considering three vegetation types under artificial rainfall. The variation in slope surface runoff, soil moisture content and rain seepage over time was recorded during the tests. The results indicate that surface vegetation first effectively reduces the splash erosion impact of rainwater on slopes and then influences the slope hydrological effect through rainwater forms adjustment. (1) The exposed slope has weak resistance to two consecutive heavy rains, the degree of slope scouring and soil erosion damage will increase greatly during the second rainfall. (2) The multiple hindrances of the stem leaf of Zoysia japonica plays a leading role in regulating the hydrological effect of slope, the root system has little effect on the permeability and water storage capacity of slope soil, but improves the erosion resistance of it. (3) Both the stem leaf and root system of Nephrolepis cordifolia have important roles on the hydrological effect. The stem leaf can stabilize the infiltration of rainwater, and successfully inhibit the surface runoff under continuous secondary heavy rainfall. The root system significantly enhances the water storage capacity of the slope, and greatly increases the permeability of the slope soil in the second rainfall, which is totally different from that of the exposed and Zoysia japonica slopes. (4) Zoysia is a suitable vegetation species in terms of slope protection because of its comprehensive slope protection effect. Nephrolepis cordifolia should be cautiously planted as slope protection vegetation. Only on slopes with no stability issues should Nephrolepis cordifolia be considered to preserve soil and water.