Immobilization Of Cadmium-contaminated Farmland Soil Using Humic Acid-modified Minerals

In the process of human social development,cadmium(Cd)was produced by mining,smelting,fertilization and other human activities,and soil was contaminated by Cd when the Cd infiltrated into the soil.Cd pollution in soil can pose a great threat to animal,plant,and human health.Immobilization is a relatively simple,safe,economical,and effective remediation method.Therefore,the development of a low-cost,high-efficient,green,and safe immobilization agents to change the migration and transformation process of Cd in soil is imminent,which will be beneficial for reducing the enrichment of Cd in soil-crop system.In this paper,we prepared humic acid-modified montmorillonite(HA-Mont),humic acidmodified sepiolite(HA-Sep)and humic acid-modified albite(HA-Alb).The changes of mineral surface morphology and material composition of the mineral surfaces were analyzed by fieldemission scanning electron microscopy and energy dispersive X-ray spectroscopy(FESEMEDS),micro-Fourier transform infrared spectroscopy(FT-IR),X-ray diffraction(XRD),and N2 adsorption-desorption measurements.The effects of humic acid-modified minerals and original minerals as immobilization agents on Cd availability,Cd morphology and soil physiochemical properties were systematically studied by soil incubation experiments in the laboratory.We conducted the pot experiments to evaluate the impacts of humic acid-modified minerals and original minerals on the uptake of Cd by pakchoi and their growth after immobilization.And we investigated the effectiveness and soil physicochemical properties of humic acid-modified minerals for Cd immobilization at different incubation times in three different soils.The main conclusions are as follows:(1)The effects of humic acid-modified minerals and original minerals on the available Cd and soil physicochemical properties.The Cd contaminated soil was treated with 0.5%,1%,2%and 4%(wt/wt)original minerals and humic acid-modified minerals.After soil incubation,the changes in Cd availability,Cd speciation distribution and soil physicochemical properties with different immobilization agents were determined.The results show that the surface of minerals was covered by humic acid,the pore size of the minerals became larger and the surface became more rougher after modification.The application of humic acid-modified minerals and original minerals decreased the available Cd content by 8.16%-31.53%compared to the control.Immobilization efficiencies of 4%(wt/wt)HA-Mont,HA-Sep,and HA-Alb were 12.87%,5.86%,and 6.20%higher than those of Mont,Sep,and Alb,respectively.The immobilization effect of humic acid-modified minerals on soil Cd was better than that of original minerals.The results of the different fractions of Cd showed that humic acid-modified minerals shifted part of Cd from exchangeable and reducible fraction to residual fractions.Available Cd was negatively correlated with soil pH,electrical conductivity,organic matter,and cation exchange capacity,indicating that the immobilization effect was regulated by soil physico-chemical factors.Humic acid-modified minerals immobilized Cd by precipitation,ion exchange.adsorption,and complexation.(2)The effects of humic acid-modification minerals and original minerals on the pakchoi growth and uptake of Cd by pakchoi.We conducted the pot experiments to plant pakchoi in Cdcontaminated soils treated with original minerals and humic acid-modified minerals to investigate the subsequent changes in pakchoi growth and the uptake of Cd by pakchoi.With the decrease in Cd contents and improvement of soil quality,humic acid-modification minerals increased the biomass and SPAD of pakchoi by 56.60%-63.16%and 22.54%-34.30%.respectively.The application of HA-Mont,HA-Sep and HA-Alb decreased the above-ground Cd content of pakchoi by 36.66%,76.18%and 19.98%compared with the control,respectively.and the reduction was higher than that of the original minerals.Humic acid-modified minerals significantly reduced Cd availability in soil,increased soil pH and cation exchange capacity,which were also related to the decrease of Cd accumulation in the above-ground parts of pakchoi.(3)Effects of humic acid-modified minerals on dynamic changes of availability of Cd in three soils.Humic acid-modified minerals were added to artificially Cd contaminates farmland soil(RT soil)and naturally Cd contaminated farmland soil in Shandong province(ST soil)and Yunnan province(YT soil).The dynamic changes of Cd availability and pH as well as the final changes of soil physical and chemical properties after 120 days of culture were studied by soil culture experiment.Humic acid-modified minerals effectively immobilized Cd in three soils during 120 days of culture.In RT soil and ST soil,the humic acid-modified minerals were mainly to convert exchangeable state and reducible state to residue state,and the immobilization effect was relatively stable.The increase of pH was also one of the important mechanisms for the immobilization of Cd contaminated soil.In YT soil,adsorption was one of the main factors of the immobilization effect.The immobilization efficiencies of humic acid-modified minerals on soil Cd first showed a trend of increasing and then decreasing,mainly transforming exchangeable state Cd into reducible state Cd and residual state Cd.Therefore,the synthesized humic acid-modified minerals could immobilize Cd in soil through various immobilization mechanisms,thus decreasing the toxicity and content of Cd in plants and improve the growth of plants.All the results from this study provide the scientific support and theoretical basis for the immobilization of Cd-contaminated farmland soil by humic acid-modification minerals.