Migration And Transformation Of Chromium(Ⅵ) From Landfill Leachate In Groundwater

As one of the characteristic pollutants in the leachate of domestic garbage,Cr⁶⁺has strong carcinogenic and mutagenic toxicity.Once it leaks into groundwater and other environments,it will pose a great threat to the ecological environment and human health.Taking Jiaozuo municipal waste landfill site as the study area,the garbage leachate as the research object,and the sandy soil in the site as the geotechnical medium,the column simulation experiment and numerical simulation method were used to study the migration and transformation of Cr⁶⁺in groundwater under the condition of accidental leakage of landfill leachate and different seepage velocity on the basis of monitoring and evaluation of environmental quality,and then carried out the environmental risk assessment of Cr⁶⁺to groundwater.The research results can provide scientific basis for landfill pollution prevention and control and environmental risk management.The research results were as follows:（1）The mineral composition of sand in the study area was quartz,calcite and feldspar measured by X-ray diffractometer.Quartz with stable chemical properties accounted for 74%,and calcite with good adsorption properties accounted for 17%.The surface structure of the sand was observed by scanning electron microscopy had a large difference.Some surfaces had more viscosity particles and uneven fine pores,and some were smooth and less pores.The specific surface area of sand measured by specific surface area-hole volume analyzer was 3.9051 m²/g,and the studied sand had a certain adsorption capacity for Cr⁶⁺.（2）Under 25℃,the migration simulation experiments of Cl^-and Cr⁶⁺were carried out at pore velocity of 0.88 cm/h and 1.76 cm/h,respectively.The diffusion coefficients were 0.318 cm²/h and 0.525 cm²/h respectively by using CDE model,indicating that the capacity of Cl^-migration diffusion in the soil column as the flow rate increased.The increase time of Cr⁶⁺under 0.88 cm/h（66.5h）was late from 1.76cm/h（30h）.The adsorption amounts of the unit mass sand on the Cr⁶⁺at different flow rates was 0.117 mg/kg and 0.035 mg/kg,respectively,indicating that the landfill sand had a weak adsorption,and the adsorption capacity of sand on Cr⁶⁺at high flow rate was weakened.The transport process of Cr⁶⁺in the studied sand was in accordance with first-order reaction kinetics in convection-dispersion equation,and the fitting correlation coefficients were above 0.97.The retardation coefficients of Cr⁶⁺were1.572 and 1.174,and the attenuation coefficients were 0.00333/h and 0.00104/h,respectively.（3）The soil column leaching experiment of landfill leachate Cr⁶⁺was carried out at 25℃and pore flow rate 0.88cm/h.The concentration of effluent Cr⁶⁺accounted for38.54%～41.92%of the leaching solution,indicating that the transformation of Cr⁶⁺occurred in the process of migration.The correlation coefficients of effluent Cr⁶⁺with p H,Fe³⁺and DOC were 0.775,0.902 and 0.842,respectively,indicating that the migration of Cr⁶⁺was closely related to the three.The decrease of p H in the early stage of the experiment improved the adsorption capacity of the studied sand,and the adsorption capacity of sand per unit mass to Cr⁶⁺in the landfill leachate leaching experiment（0.161 mg/kg）was slightly higher than that in section 4.2（0.117 mg/kg）,so the blocking ability of sand to Cr⁶⁺was enhanced.The C/C₀of DOC ranged from89.94%to 93.64%in the stationary fluctuation stage,which might be due to the redox reaction between a small amount of organic matter and Cr⁶⁺.（4）Based on groundwater and soil monitoring data of Jiaozuo Municipal landfill site from 2019 to 2021,the environmental quality status and risk assessment of the landfill site were analyzed by using Nemerow composite index method and potential ecological risk index method.The results showed that the single factor index of groundwater and soil was less than 1.0,and the Nemerow composite index method was less than 0.7.At present,the operation of the landfill had no significant impact on groundwater and soil.Potential ecological risk results showed that the groundwater point RI value was much lower than 150,indicating a low level of potential ecological risk.The RI of topsoil increased year by year,and the RI of all points reached more than 300 in 2021,which was at the high risk level（one point was extremely high risk）.