The Remediation Effect Of Modified Attapulgite Combined With Calcium Silicate On Cadmium Contaminated Farmland Soil

In recent years,heavy metal pollution of soil in China is more and more serious.Among heavy metal pollutants,cadmium（Cd）attracts more attention because of its extreme toxicity and great harm,which has brought adverse effect on economic development and agricultural planting.Excessive Cd in soil can pose terrible risks and hazards to humans through its accumulation in the food chain.Therefore,the control of Cd pollution in farmland soil is extremely urgent.Attapulgite is a kind of natural non-toxic clay mineral,which has been widely used in heavy metal contaminated soils remediation,and has been widely praised for its low price and large reserves.However,the adsorption efficiency of attapulgite clay that naturally exists in the environment is not ideal.This study aims at how to overcome the defects of natural attapulgite and improve it for practical applications.Therefore,the natural attapulgite was modified and optimized in this study.The effects of the CS content,calcination temperature and calcination time were investigated to determine the optimum preparation technology.Then it was combined with calcium silicate to determine the optimal ratio and adsorption properties through water simulation adsorption experiment,the optimum dosage in soil was determined by soil culture experiment.Finally,the actual remediation effect of modified attapulgite combined with calcium silicate on Cd contaminated farmland soil was further discussed through potted rice experiment.The main findings are as follows:（1）The orthogonal experiment results show that the order of influence of various factors on the adsorption performance of modified attapulgite for Cd²⁺was as followed:calcination temperature>mass ratio of attapulgite to chitosan>calcination time.The optimal preparation technique is calcination temperature=400°C,mass ratio of ATP to CS=1:4 and calcination time=1 h.A series of adsorption experiments showed that the Langmuir adsorption isotherm and pseudo-second-order kinetic model better corresponded with the adsorption characteristics of the adsorbent prepared under optimal conditions,and the maximum adsorption amount of Cd²⁺was 109.30 mg/g.However,its adsorption capacity was more easily affected by the solution with a lower pH value.Coexisting metal cations also inhibited the adsorption capacity of the material for Cd²⁺.In the soil culture experiment,the concentration of soil Ca Cl₂-Cd decreased gradually with the increase of the additive amount of materials,and when the additive amount of material was 2.5%,the remediation effect of farmland soil was the best and the economy was the best,the soil pH value and organic matter content were improved significantly.（2）Modified attapulgite was combined with calcium silicate to improve its adsorption performance.Through water simulation adsorption experiment,the mixed material had better adsorption properties on Cd²⁺and the best cost performance when the mass ratio of modified attapulgite and calcium silicate was 6:4.A series of adsorption experiments showed that the Langmuir adsorption isotherm and pseudo-second-order kinetic model better corresponded with the adsorption characteristics of the mixed material,and the maximum adsorption amount of Cd²⁺was 363.97 mg/g.The material was also susceptible to the solution with a lower pH value,and the coexistence of metal cations will affect the adsorption performance of the mixed material for Cd²⁺.In the soil culture experiment,the concentration of soil Ca Cl₂-Cd was significantly decreased when the addition amount of mixed materials was 2.5%or 3%,and the soil pH value was significantly increased,while the soil organic matter had no obvious effect（3）In the potted rice experiment,the combination of modified attapulgite and calcium silicate still had a better ability of remediation of Cd contaminated soil.The combination of modified attapulgite and calcium silicate significantly improved the basic physical and chemical properties of the soil,the pH value of the soil was significantly increased by18.73%-31.26%,and the content of available Si in the soil was significantly increased by90.06%-115.50%,so that rice could grow better;It could significantly reduce the availability and migration of Cd in the soil,and promote the conversion of soluble Cd to in soluble Cd.The combination of modified attapulgite and calcium silicate could significantly reduce the content of Cd in various parts of rice,reaching the national standard of contaminant content in food GB2726-2017（Cd≤0.2 mg/kg）.The experiment proved that the combination of modified attapulgite and calcium silicate could significantly reduce the concentration of Ca Cl₂-Cd in the soil,and could also effectively reduce the content of Cd in brown rice.Therefore,it is a passivating agent that can be applied to effectively remediation of heavy metal pollution soil.