Preparation Of Three Modified Bentonite And Passivation Of Cadmium Contaminated Soil

Heavy metal pollution is currently the main environmental problem that needs to be solved in China,of which cadmium pollution is the most harmful and widely distributed.In-situ passivation remediation technology is an economical and efficient treatment technology for heavy metal non-point source pollution in soil.Passivators can reduce the content of available heavy metals and biohazards,but the selection of materials,high efficiency and longevity,and the presence or absence of secondary pollution are the research difficulties.Natural clay minerals are the main substrates for passivators due to strong adsorption,polarity and ion exchange properties,but modification is necessary to improve the capacity and efficiency of heavy metal adsorption/passivation.In this study,the thiol-modified bentonite（SH-Bent）,Fe-modified bentonite（Fe-Bent）and thiol-Fe-modified bentonite（SH-Fe-Bent）were prepared with Bentonite as the substrate,cysteamine hydrochloride as the thiol group donor source and ferric nitrate as the zero valent iron donor source.The structure of modified bentonite was characterized by XPS,FTIR,XRD,BET,SEM,etc.The passivation effect of Cd and its stability were investigated by Cd²⁺adsorption-desorption behavior of Cd²⁺in aqueous solution and a series of isothermal adsorption and kinetic experiments on Cd content and occurrence form,plant growth and leachate concentration in contaminated soil,and the passivation mechanism was discussed.The thiol-modified bentonite（SH-Bent）was prepared by the solution method using cysteamine hydrochloride as the thiol group donor source;XPS and FTIR patterns showed the presence of sulfur elements and thiol groups,XRD and BET showed that the thiol group modification resulted in the decrease of layer spacing,specific surface area,pore size and pore volume of the bentonite,SEM showed that the surface was denser.However,all the characterizations showed that the modification did not change the basic layered structure of the bentonite;the optimum ratio of acidified bentonite and cysteamine hydrochloride for SH-Bent was 10:1,and the series of adsorption experiments of cadmium in aqueous solution were carried out at p H 7 and solid-liquid ratio of 1:200,and the isothermal adsorption experiments showed that the adsorption behavior was more consistent with the Langmuir model of monolayer adsorption,and the kinetic experiments were more consistent with the pseudo-second-order model.SH-Bent was 37.69 mg/g,which was more than 36 times the adsorption capacity of Bent and the desorption rate under simulated acid rain was only 13%.In the passivation experiment of Cd-contaminated soil,when the added amount was 20 g/kg,the passivation efficiency increased2.1 times,and the fresh weight,dry weight,plant height and root length of Chinese cabbage all showed an increase,and the application of passivator reduced the water-soluble and ion-exchange states with mobility in the soil.The concentration of Cd leaching solution under acid rain infiltration was only 9.5%.Combined with the characterization and passivation effect can be found that the thiol modification in the case of bentonite laminar structure without changing the layer spacing,specific surface area and pore volume,thereby reducing the diffusion channel,but the adsorption performance,passivation capacity and binding stability was greatly improved.In addition to the original bentonite electrostatic adsorption,ion exchange adsorption,the thiol group complexed and formed stable-S-Cd.Fe-Bent was successfully prepared by ion-exchange method with ferric nitrate as the zero valent iron donor source.The series characterization proved the successful addition of iron element,but iron group modification resulted in the increase of interlayer spacing,specific surface area,pore diameter and pore volume of bentonite and the more ordered surface.The optimal OH/Fe formulation ratio of Fe-Bent was 2.0mol,and a series of adsorption experiments of cadmium in aqueous solution showed isothermal adsorption is more in line with the Langmuir model,kinetic experiments are more in line with the pseudo-second-order model,the maximum saturation adsorption capacity was 31 times that of bentonite,while the desorption rate under simulated acid rain was 3.5%.In the passivation experiment of cadmium contaminated soil,the passivation efficiency,and the growth of Chinese cabbage were similar to thiol group modification,but the carbonate-bound form of cadmium also decreases obviously,and the concentration of Cd leaching solution is 3.3%under the acid rain infiltration of cadmium.The results showed that the iron group modification exerted its column support role,increased the layer spacing,specific surface area and pore volume,enlarged the diffusion channels,increased the amount of-O-Cd by the polyhydroxy polymer,and generated the strongly bound-Fe-Cd by the chemisorption behavior-complexation reaction between the iron group and cadmium,all of which led to the effective improvement of adsorption performance,passivation ability and binding stability.Based on preparing SH-Bent with acidified bentonite and cysteamine hydrochloride at the ratio of 10:1,SH-Fe-Bent was successfully prepared by ion exchange method with the addition of 2.0 mol poly-hydroxyl iron solution.A series of characterization proved the successful addition of iron and sulfur elements.The Fe-SH-Bent just integrates the structural changes of single modification,and the data of interlayer spacing,specific surface area,pore diameter and pore volume were between SH-Bent and Fe-Bent,and the surface was compact and orderly.In adsorption experiments of cadmium in aqueous solution show that isotherm adsorption is more in line with Freundlich model,and the kinetics is in line with pseudo-second-order model.The maximum saturated adsorption capacity was 18 times that of bentonite,but there is almost no desorption behavior under simulated acid rain.Passivation experiments in cadmium-contaminated soil show that the passivation efficiency and the growth of Chinese cabbage were slightly better than single modification,and the occurrence form of cadmium was like thiol group modification.The concentration of Cd leaching solution under acid rain infiltration is 1.3%.The results showed that the composite modification was the comprehensive performance of the thiol group and iron group,and the removal efficiency of SH-Fe-Bent was lower than the single modification because of the masking behavior.The simultaneous addition of both caused a more complex interlayer filling situation,resulting in less easy desorption of adsorbed Cd in the form of-O-Cd,-S-Cd,and-Fe-Cd,so that SH-Fe-Bent is more stable in water and soil.The composite modification exhibited weaker adsorption performance and extreme stability compared with the single modification.