Studies On The Preparation Of Passivation Agents And Passivation Mechanism Of Typical Heavy Metal Pollutants In Paddy Soil

Heavy metal pollution in soils is one of the most important environmental problems in China.Heavy metal pollution in agricultural soils,especially in paddy soils,not only leads to a decline in soil fertility and a decrease in the quality and yield of agricultural products,but also poses a serious risk to human health through the food chain.Passivation agents can effectively reduce the activity of heavy metals and reduce the risk of heavy metals to crops,and have the advantages of fast,effective and low cost.However,there is still a need for in-depth research on the mechanism of remediation of typical heavy metal contaminated soils in agricultural fields by emerging compound passivation agents and their long-term effects.In this study,based on previous research,we used a typical heavy metal-contaminated red loam rice soil as the research object,prepared new compound passivation agents,combined with soil passivation experiments and acid rain leaching experiments,to study the effect of different compound passivation agents on the migration and transformation of heavy metals in contaminated rice soils.The long-term effects of passivated materials on the remediation of heavy metal contamination in soils.The specific research content and findings of this paper are as follows.1.modified chitosan-loaded zeolites were prepared to passivate cadmium（Cd）,a heavy metal in paddy soils,and their passivation performance and mechanism were elucidated.The passivation effect of the passivating materials on Cd was analysed by incubation experiments using the TCLP extraction method and the BCR continuous extraction method.The results showed that the four passivation agents（modified zeolite（ZEO）,N-methylene chitosan hypophosphite/zeolite（ZHC）,N-methylene chitosan phosphite/zeolite（ZPC）and chitosan（CTS））were all effective in raising soil p H and significantly reducing the Cd content in the acid extractable state in paddy soils.The effect of heavy metal passivation in contaminated paddy soils increased significantly with longer passivation times and then stabilised.The passivation effect of ZHC on Cd was the best among the four passivation agents,with a 49.9%decrease in the concentration of Cd extracted by TCLP when the treatment sample was added with 3.2%ZHC.ZHC enhanced the adsorption of Cd due to its high specific surface area,easy exchange of surface cations with Cd²⁺and increased coordination of P elements with Cd.2.Modified chitosan-loaded carbon black（CHC）and modified chitosan-loaded fly ash（FHC）were prepared and the existing carbon black（CB）and fly ash（FA）were selected for passivation of Cd in paddy soils to elucidate the excellent performance of CHC and reveal its passivation mechanism.The passivation effects of different passivation times and passivation material additions on Cd were investigated.The four passivation agents,CHC,FHC,CB and FA,were found to significantly increase the soil p H and reduce the acid-extractable Cd content in the rice soil.The longer the passivation time,the better the effect of the passivating materials.The best passivation effect was achieved with modified chitosan-carbon black,where the acid-extractable Cd content was reduced from 60.98%to 35.7%.The modified chitosan-carbon black has a large number of oxygenated phosphorus-containing functional groups that are easily coordinated with Cd,and the cations on the surface are ion-exchanged with Cd,enhancing the adsorption of Cd by the passivation material and reducing the activity of Cd.3.Dewatered sludge and biochar were selected for the passivation of metalloid arsenic（As）in paddy soils,and the combined use of dewatered sludge and biochar as a composite passivation material was investigated for the passivation of As.The results showed that the passivation effect of the composite passivation material increased with time,with the best passivation effect（maximum passivation time）achieved at 40 days;the best passivation effect of the single passivation material was achieved by the S3（dewatered sludge,20%addition）group,with a 37.6%reduction in the effective As concentration compared to the control group CK;the best passivation effect of the composite passivation material was achieved by the S+B3（dewatered sludge+biochar,10%addition of dewatered sludge and 10%addition of biochar）group.The most effective composite passivation material was the S+B3（dewatered sludge+biochar,10%dewatered sludge and 5%biochar）group,where the concentration of active As was reduced by 25%compared to the control group CK and by 11%compared to the S2 group without biochar.In contrast,the composite passivation material was significantly more effective than the single dewatered sludge passivation material due to the higher specific surface area and number of functional groups.At the same time,the passivation effect of the passivation agents did not simply increase with the amount added,and the relationship needs to be further investigated.4.The migration and transformation of heavy metals in soil after the application of passivation agents such as lime,chitosan and zeolite under different acid rain conditions were studied.Under simulated acid rain drenching conditions,different types of passivation agents were effective in resisting heavy metal activation,with lime being the most effective,reducing the amount of Cd by 54.1-65.2%and Zn by 96%-97.4%,and reducing the amount of Pb leached by TCLP from 2.3 mg/L to 1.0 mg/L and Zn from 10.8 mg/L to 7.4 mg/L.In contrast,lime,being more alkaline,was able to increase the soil p H and thus had the strongest acid rain buffering capacity,which was more conducive to reducing the leaching of heavy metals from the soil.5.The migration and transformation of heavy metals from dewatered sludge and biochar after passivation were investigated under different acid rain conditions.The effects of acid rain leaching on the combined dewatered sludge-biochar passivation material and the heavy metal binding pattern in the soil after remediation of dewatered sludge with a single passivation material were investigated by passivation incubation and multiple acid rain leaching experiments.The results showed that both passivation agents were effective in reducing the activity of heavy metals in the soil.The p H of all soil samples after five drenchings was lower than the p H before drenching,with the largest decrease in p H in the original soil and the smallest decrease in p H in the soil sample with the 2%dewatered sludge-biochar composite passivation material.As the number of acid rain drenches increased,the effectiveness of the passivation material decreased.The dewatered sludge-biochar composite passivation material was more resistant to heavy metal activation than the dewatered sludge single passivation material due to its stronger acid rain buffering capacity and more organic ligand sites for binding heavy metals.In summary,the modified chitosan-loaded zeolite(with a high specific surface area,easy exchange of surface cations with Cd²⁺and increased coordination of P elements with Cd)and the modified chitosan-loaded carbon black（with a large number of oxygenated phosphorus-containing functional groups for easy coordination with Cd and ion exchange of surface cations with Cd）have a better passivation effect on the heavy metal Cd in paddy soils.The dewatered sludge-biochar complex passivation material has a better passivation effect on metalloid As due to its higher specific surface area and number of functional groups than the single passivation material it consists of.Under acid rain leaching conditions,both the conventional single passivation material and the composite passivation agents,dewatered sludge-biochar,increased the resistance of soil p H to acid rain and reduced the mobility of heavy metals.The results of this study provide insight into the effects of passivation agents on changes in the binding patterns of heavy metals in paddy soils and have practical implications for assessing and predicting the application of passivation agents under actual acid rain conditions,as well as providing an important reference for the remediation of heavy metal-contaminated paddy soils.