Key Role Of Natural Mineral Materials-humic Acid For Cd Stabilization In Contaminant Soils

Adsorption and precipitation is considered to be the main mechanisms of in-situ soil heavy metal immobilization,that reduce soil heavy metal bioavailability and mobility.Recently,application of natural mineral materials for in-situ heavy metal immobilization have gained increased attention,sinceit can decrease DTPA-Cd in soil through adsorption,precipitation,complexation,ion exchange and redox,which also elevated the concentration of cadmium in morestable fractions.In this study,we conducted trial experiment and field experiment using a combination of natural mineral materials and humic acid as curing agentand investigatived soil immobilization efficiency and related mechanisms,which is expected to provide theoretical and practicalreference cadmium immobilization.Furthermore,surface morphology?SEM-EDS?,Fourier transform infrared spectroscopy,?FITR?characterization were performed to validate the proposed immobilization mechanisms.The main conclusions of the study are as follows:?1?We applied natural mineral materials-humic acid composite curing agent at various ratio for soil Cd stabilization.The optimal strategy for Cd immobilization is 5 g/kg natural mineral materials+0.4g/kg humic acid and incubation for 50d,with a dcreased in DTPA-Cd of0.63 mg/kg by 46.13%The pH value increase from 6.0 to 6.8,and the organic matter concentration decreased to 0.3741mg/kg 1.22%,4.85%and 1.59%of the exchangeable Cd in soil transform to organic bound,Fe-Mn oxidized and residual Cd,respectively,which add up to 7.73%of exchangeable cadmium.The application of natural mineral materials-humic acid composite curing agent is the optimal Cd immobilization strategy as indicated by the highest immobilization efficiency and that a proportion of exchangeable Cd was transformed into organic bound,Fe Mn oxidized and residual Cd.?2?Our study showed that heavy metal adsorption efficiency of natural mineral materials-humic acid composite curing agent was the highest based on Elvotion model and Two-constant model fits.The a value is 1.6203 and 0.6933 respectively,which proved that composite materials had the strongest adsorption capacity.The value b is-0.1393 and-0.1589respectively,indicating transformation of heavy metals from exchangeable state into the stable state and the transform efficiency of natural mineral materials-humic acid.In order to veritied the transform efficiency of natural mineral materials-humic acid was the highest to soil heavy metal by pearson's correlation analysis.A significant positive correlation from Pearson's correlation coefficients among the parameters was found to available Cd and exchangeable state,carbonate bound state,iron manganese oxidation state.Significantly negative correlations were found between available Cd and Organic binding state,residue state heavy metal concentration.It was confirmed that the application of the material can effectively reduce the concentration of available cadmium in the soil and convert soil available heavy metal into the stable form.The mobility factor is 0.0988,which showed the highest Cd stability after natural mineral materials compound-humic acid composite curing agent treatment.?3?We used natural mineral materials-humic acid composite curing agent at various ratio for stabilization of Cd contaminated soils.The optimal effect was obtained under condition of 5 g/kg natural mineral materials+0.4g/kg humic acid,incubation 70d,with the DTPA-Cd decreased by 0.0504 mg/kg,the immobilization rate reaching 59.8%,the pH value increased from 6.0 to 6.45.Humic acid application increased the K content of soil and applying natural mineral materials improved the P content of soil.However,combined application of the two material decreased the N content of soil.The result shows that natural mineral materials compound humic acid had an effect on soil fertility.?4?Surface morphology?SEM-EDS??Fourier transform infrared spectroscope?FITR?characterizationshowed that the curing agent exhibited lose surface structure and low specific surface area.The existence of ions on the surface,which included Na²⁺,Ca²⁺,Mg²⁺,could contribute to chemical adsorption through ion exchange.Besides,functional groups such as quinone,alcohol,hydroxyl,carboxyl,carbonyl and C=C bond in humic acid could coordinate with Cd²⁺,which also constitute an important pathway for decreasing soil available Cd.