Simulation of dynamic leaching characteristics of chromium (VI) from contaminated soil and its influence on groundwater
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Abstract:
Hexavalent chromium Cr(VI) is a heavy metal pollutant that has attracted much attention. Due to its strong mobility, the impact of Cr(VI) contaminated soil on groundwater is one of the key issues that need to be solved in the field of soil-water synergistic remediation. At present, soil pollution and groundwater pollution remediation are still considered separately in the remediation project of chromium-contaminated sites. Research of the target values of soil and groundwater remediation lacks correlations. Few studies focused on the correlation between soil remediation targets and groundwater remediation targets. Due to the complexity and heterogeneity of the site environment and the reactivity of pollutants, the release and transformation of residual and fixed Cr(VI) in the soil making the remediation of chromium-contaminated sites remains challenging.The contaminated soil of the original Changsha Chromium Salt Plant was taken as the research object, and the dynamic leaching test was used to study the leaching characteristics of soil Cr(VI) and its influence on groundwater under different leaching modes. Seven soil columns were set up to simulate the migration of Cr(VI) in contaminated soil in the vadose zone and saturated zone. For the vadose zone scenario, pure water was used for soil column leaching to study the migration and release of Cr(VI) in the soil. The saturated zone scenario was designed with continuous and intermittent leaching conditions (leaching for 12 h, suspension for 12 h) to simulate the pumping remediation scenario of contaminated groundwater, and to study the remediation effect of different leaching modes on contaminated groundwater. The model of Cr(VI) migration and transformation in the experiment was constructed by COMSOL Multiphysics ? 5.6, and the concentration change and cumulative leaching quality of Cr(VI) in six different scenarios under continuous and intermittent leaching modes were calculated.The results showed that the leaching flow rate would affect the time required for Cr(VI) to reach desorption stability in the soil column. For continuous leaching mode, the time to reach equilibrium at high, medium, and low flow rates was 1 d, 4 d, and 15 d, respectively. The leaching efficiency of Cr(VI) in intermittent leaching mode was slightly lower than that in continuous leaching mode, which was mainly controlled by the renewal efficiency of pore water in soil. Experimental calculation results showed that the leaching rate of Cr(VI) increased with the increase of flow rate under the same mode. Complex reactions happened during the intermittent leaching mode process, which may affect the final cumulative leaching mass. According to the calculation, the minimum leaching percentage was 33.67 % under the high flow rate scenario of the intermittent leaching column in the same period, and the maximum leaching percentage was 83.99 % under the high flow rate scenario of the continuous leaching column. Combined with the results of model calculation and column experiment, it was found that the leachate mass concentration of Cr(VI) reached the Class V water standard of groundwater ( > 0.1 mg/L ), even though the soil reached the screening value of Class II land use (5.7 mg/kg ). The model predicted the number of days required for different simulated columns. The days required for the 2-1, 2-2, 3-1, and 3-2 simulated columns leaching solutions to reach the Class IV groundwater standard were 270, 230, 265, and 293 days, respectively.The main conclusions are as follow : (1) The groundwater flow rate affects the time required for chromium to reach desorption equilibrium in the soil column. The influence of different leaching modes is limited. For the continuous column, under a greater flow rate, the column will reach the desorption equilibrium faster. The leaching efficiency of Cr(VI) in intermittent mode was slightly worse than that in continuous pumping mode, which was mainly controlled by the renewal efficiency of pore water in soil. (2) The cumulative leaching quality of Cr(VI) in continuous column was greater than that in intermittent column. In the same mode, the greater the leaching rate, the greater the cumulative leaching quality of Cr(VI). (3) A one-dimensional model of Cr(VI) migration and transformation was constructed using COMSOL Multiphysics ? 5.6. After verification, it was found by calculation that the intermittent leaching model can repair Cr(VI) contaminated soil more efficiently. (4) Through the model calculation, it is found that there are differences between the existing soil remediation targets and the soil remediation targets for protecting the groundwater environment. When the mass concentration of Cr(VI) in the soil medium reached the screening value of the second-class land, the content of Cr(VI) in the leachate reached the V-class water standard, which still posed a threat to the groundwater environment.According to the results, it is found that chromium-contaminated soil can release pollutants into groundwater for a long time. Therefore, it's necessary to increase the verification steps of pollutant release in the soil at different remediation stages to ensure that the soil remediation goal of protecting the groundwater environment is achieved. For the remediation of saturated zones, based on the characteristics of different water-bearing media, the pumping remediation technology should be combined with other technologies according to the different release stages of pollutants, to achieve the goal of water and soil co-governance by efficient and economical means.
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