Stabilization Of Cadmium In Cd And Cd-Pb Contaminated Loesses By Addition Of Biochars

Biochar has great potential as a soil amendment to immobilize heavy metals, to reduce their bioavailability, and to improve soil properties. The researches on the effects of biochar on migration and conversion of Cd(Ⅱ) in Cd(Ⅱ)-or Cd(Ⅱ)/ Pb(Ⅱ)-contaminated loess are meaningful to remediation of contaminated loess.Based on the immobilization of Cd by biochar in loess, rape straw-derived biochar(BS600), chicken maunure-derived biochar(CM600) and flax straw-derived biochar(LS600) were characterized using elemental analysis, fourier transform infrared spectroscopy(FTIR), scanning electron microcopytometer(SEM), et al. The influence of biomass source on properties and structure of biochar were investigated. Metal immobilization studies were performed with Cd and Cd-Pb contaminated loess to examine the effect of biochar on the immobilization of aged metals. In this study, a 180-day incubation experiment was conducted using rape straw-derived biochar produced at 600℃with the different addition rates of 0%, 1%, 3%, 5% and 10%. The changes of available Cd contents with incubation time( in 180 days) in Cd and Cd-Pb contaminated treatments were determined. The loess samples were air-dried and ground to pass through a 60-mesh sieve for BCR sequential extraction. Furthermore, the effect of the different qualities of loess on the immobilization was also investigated. The moisture contentin and available nutrients(nitrogen, phosphorus and potassium) of loess after biochar amending treatments were investigated. The main conclusions of this dissertation are as following:(1) The structural characteristics of biochars are related to the biomass sources. BS600, CM600 and LS600 showed alkaline, with the pH values in the order of LS600, BS600, CM600; the ash contents were in the order of CM600, BS600, LS600; the highest element content of BS600, CM600 and LS600 was C, and the H, N, O and S contents were relatively lower; CM600 had the lowest nutrient content, but higher aromatization and more hydrophility; the BET specific surface area of biochars was in the order of CM600, BS600, LS600; the DRIFT spectrophotometry showed BS600, CM600 and LS600 consisted of surface functional groups, such as hydroxyl, carboxyl, and carbonyl; the SEM images showed BS600, CM600 and LS600 consisted of highly complex network of pores.(2) The stabilization of Cd in Cd and Cd-Pb contaminated loesses by addition biochar showed the overall reduction of the acid soluble fraction of Cd became slowly and then leveled off in the short term, during which a part of bioavailable fraction of Cd even exceeded those in CK-Cd and CK-Cd+Pb. With time prolonged, the acid soluble fraction of Cd decreased, and the immobilization by biochars reached steady. When the type and addition of biochar were different, the change of the bioavailable Cd was also different. The acid soluble Cd reduced and residual Cd increased in the same contaminated loess with increasing biochars dosage. When 10% of BS600, CM600 and LS600 were added respectively, the immobilization was the most significant in Cd and Cd-Pb contaminated loesses. The immobilized content of Cd accounted for 56.63%, 66.65% and 59.8% of the initial content of Cd, while 54%, 63.35% and 54.2 % in Cd-Pb contaminated loess, respectively.(3) The biochar amendment had effects on TOC, pH, EC and the CEC in Cd and Cd-Pb contaminated loesses. TOC of contaminated loess increased with increasing biochar amount, and the effects of biochars on TOC were in the order of LS600> BS600> CM600. BS600, CM600 and LS600 had different effects on pH values of loess. The pH values of Cd contaminated loess increased with increasing biochar dosage. In Cd-Pb contaminated loess, the addition of 1% and 3% CM600 had no difference effect on pH values of Cd-Pb contaminated loess. The biochars could improve the EC values of loess. The EC values of loess increased obviously with BS600 amelioration. There was no significant difference in CEC between CK and loess; the CEC values were even reduced with biochars compared to CK. Correlation analysis showed, in BS-Cd, CM-Cd and LS-Cd treated groups, the pH values of loess played a major role in residual Cd promotion; in BS-Cd+Pb, CM-Cd+Pb, and LS-Cd+Pb treated groups, TOC had the greatest impact on the residual Cd content. The residual Cd content and the values of TOC, pH and EC showed a significant positive correlation(P <0.01) respectively. TOC, pH and EC in two kinds of loesses that were added biochar had a significant effect on immobilization. In contrast, the CEC values of loess and residual Cd content were not correlated, except BS-Cd and LS-Cd+Pb.(4) Biochar had an impact on moisture content and available nutrients(nitrogen, phosphorus and potassium) in two kinds of contaminated loesses. The results showed that the addition of LS600 and BS600 increased the moisture content, and with the increasing biochar, the moisture content of air-dried soil was gradually increased. CM600 had unstable affect on the moisture content. When the species and addition of biochar were different, the change of available nitrogen was also different. The CM600 and BS600 amelioration could increase the available phosphorus content of loess, with the increasing biochars, the available phosphorus content of loess was also increasing. The available phosphorus of loess with CM600 addition was highest. LS600 amelioration had ineffective on loess. The biochars amelioration could increase the available potassium content of loess. The available potassium of Cd contaminated loess was increased with the increase of biochars. The effect was BS600> CM600> LS600; LS600 had unstable effect on available potassium in Cd-Pb contaminated loess.