Stabilization Remediation Of Cd And Zn In Soils By Using Chemicals Coupling With Organic Matter And Ecological Security Assessment

Cadmium (Cd) is an unessential heavy metal for living organisms which is applied widely in various fields, but Cd is extremely toxic and commonly found associating with other heavy metals in soil contamination. The soil contaminated by Cd was become more and more sevious result from the anthropogenic inputs. Currently, the area of farmland contaminated by Cd has exceeded0.2million hm2and the annual production of Cd contaminated agricultural products was over1.46×109kg. The high efficiency and stable treatment is in urgent needs.This study put forward and experienced an effective method that can make the acid red heavy metal contaminated soil harmlessness and resources by adding fixatives and organic fertilizer. The contamination heavy metals were Cd and zinc (Zn) in the soil sample and the total contents of them were significantly high, what’s worse was that the two heavy metals were mainly existed in the form of weak acid extraction fraction, the most activity form of heavy metals in soil. The soil single pollution index evaluation was employed to identify the pollution grade. The average pollution index of Cd was261.2and14.5for Zn, which classified the soil as extremely polluted soil. By optimizing the process parameters of the chemical fixation and organic fertilizer repair techniques, the optimal repair program was determined as follows: CaCO3and Na2S was chosen as the compound fixative which was the most effective, the mol ratio of the sum of heavy metals:CaCO3:Na2S was4:4:1; water dosage was60%of maximum field capacity; the particle size of soil was less than5mm; the reaction time was24h for Cd and360h for Zn; the mass percentage of pig manure to soil was2%. After the remediation, the removal rate of water soluble Cd and Zn reached100%,99.7%, respectively.Applied the repair program to the actual contaminated soil, after the remediation, the removal rate of water soluble Cd and Zn were reached100%and92%, respectively. The available Cd removal rate was92%, available Zn removal rate was43%. The concentration of leaching toxicity of Cd was decreased from4.12mg/L to0.02mg/L, the concentration of leaching toxicity of Zn was reduced from98mg/L to46mg/L. The soil pH was increased from around5to about6, the soil organic matter concentration increased for4.4times, the concentration of available nitrogen and available phosphorus concentrations were improved for1.2times and6.8times, respectively. Fuzzy mathematics theory was employed to the assessment of soil fertilization, the results of comprehensive evaluation showed that the soil fertility index improved slightly from0.119to0.181after the immobilization, and the whole remediation increased the soil fertility index up to0.551, indicated that the soil fertilizer was lift from low to medium fertilization level.Results of BCR sequential extraction procedure showed that the addition of fixatives made weak acid soluble fraction of Cd in soils completely transform into reducible, oxidisable and residual fractions. And the addition of pig manure made weak acid soluble and reducible fractions of heavy metals transform into oxidisable and residual fractions, which enhanced the immobilization process.Simulated acid rain leaching method was hired to evaluate the long-term stability of the repair program. The pH of the leachate was ranging from6to7, the concentration of heavy metals in unprocessed contaminated soil leachates of both water and simulated acid rain were contained large quantities of Cd, the water washed out11.31mg Cd and the simulated acid rain washed out11.16mg Cd in the first year leaching process. Through10years simulated acid rain leaching, the total content of Cd leached out from contaminated soil was up to12.39mg;843.03mg for Zn; the total content of Cd washed out from immobilization soil was0mg,290.82for Zn; the total content of Cd leaching out from soil after remediation was0mg, and196.56mg for Zn.The safety risk assessment of groundwater around the polluted soil was conducted based on fuzzy methodology. Results suggestted that the safety coefficient was improved for7stages from "Absolutely high" to "Low" after immobilization, and after the whole remediation, the safety coefficient of groundwater was improved for8stages to "Quite low". It suggests that the polluted soil treated with the method that proposed in this study has little effect on the groundwater.