Study On Passivation Restoration And Stability Of Cadmium-contaminated Soil By Different Functionalized Montmorillonite

With the acceleration of industrialization and urbanization,the problem of heavy metal pollution has been becoming more and more serious.The passivation remediation technology was proved to be very effective for controlling cadmium-polluted soil.The montmorillonite,a rich clay mineral in soil,was widely used to remove heavy metal cadmium.In generally,the adsorption performance of single functionalized montmorillonite using organic or inorganic molecular on heavy metals could been improved to some extent,but it could not meet the actual demand.Hence,a sulfhydryl/iron-based functionalized montmorillonite composite passivator(SH-Fe-Mont)has been designed and prepared for restoring cadmium pollution in soil.The adsorption performance and passivation effect of passivator on cadmium were discussed through a series of adsorption experiments in aqueous system and soil passivation experiments.Meanwhile,the stability of the passivator was evaluated by desorption and leaching experiments.Importantly,the passivation mechanism was preliminarily explored by combining the series characterization of passivator materials and five soil forms distribution.In addition,the adsorption behavior and passivation effect of single sulfhydryl functionalized montmorillonite(SH-Mont)and iron functionalized montmorillonite(Fe-Mont)on cadmium were compared.The SH-Mont and Fe-Mont were prepared by using montmorillonite as the base clay,3-mercaptopropyltrimethoxysilane and ferric nitrate nonahydrate as the functionalization reagent,and the SH-Fe-Mont was also prepared by introducing sulfhydryl group based on Fe-Mont.The three functionalized passivates were successfully prepared by XRD,XPS,and FT-IR.A series of XRD,BET and SEM characterization demonstrated that the layer spacing,specific surface area and total pore volume of SH-Mont,Fe-Mont and SH-Fe-Mont were increased.Meanwhile,SH-Mont exhibited a loose surface and highly porosity and the surface roughness of Fe-Mont was increased,and SH-Fe-Mont possessed a well-developed pore structure.Importantly,the inherent layer structure of montmorillonite during functionalization remained basically unchanged.The test results of adsorption performance of the passivator showed that the optimal synthetic ratios of SH-Mont and Fe-Mont were 10:1 and OH/Fe=0.6 mol,respectively,and SH-Fe-Mont was prepared by selecting the optimal synthesis ratio of SH-Mont and Fe-Mont.Batch adsorption experiments were performed when the solid-liquid ratio was 1:200 and p H was 6.At the same time,the effects of contact time,reaction temperature and initial concentration on adsorption were also explored.The kinetic and isothermal adsorption experiments showed that the adsorption of Cd(II)by SH-Mont,Fe-Mont and SH-Fe-Mont conformed to the Langmuir model and pseudo-secondary kinetic model,and the theoretical maximum saturated adsorption capacity(q_m)of SH-Mont and SH-Fe-Mont was 53.57mg/g and 55.31 mg/g,respectively,45.40 times and 46.87 times that of Mont,while Fe-Mont was 293 qm is the largest at K,which is 38.90 times that of Mont.From the formation of strongly bound species-S-Cd and-Fe OOCd~+,SH-Mont,Fe-Mont and SH-Fe-Mont had almost no desorption(1.94%,4.58%and 0.31%)and leaching(0.72 mg/g,1.65 mg/g and 0.22mg/g)under acid rain conditions,indicating that the three passivators had good stability in aqueous solution and soil system.Among them,SH-Fe-Mont has the best adsorption performance and better stability.The results of potting experiments showed that with the increase of SH-Mont,Fe-Mont and SH-Fe-Mont additions,the concentration of the available cadmium in soil decreased significantly and the p H gradually increased.On the basis of the original electrostatic adsorption,ion exchange and coordination reactions of montmorillonite,the formation of S-Cd and-Fe OOCd~+of strongly bound species of iron and sulfhydryl groups increased the transformation of cadmium morphological distribution from highly active exchangeable state and carbonate binding state to strong stable iron and manganese oxidative binding state,organic state and residue state,which effectively hindered the migration and transformation of cadmium in contaminated soil,thereby weakening the risk of migration to Brassica rapa L,resulting in a decrease in the concentration of cadmium in stems,leaves and roots,and an increasing trend in biomass(fresh weight,dry weight,root length and plant height).Among them,SH-Fe-Mont Brassica rapa L.have the best growth.A series of studies showed that compared with SH-Mont and Fe-Mont,SH-Fe-Mont not only has significantly improved adsorption performance,but also has good passivation fixation effect and stability.