Immobilization Effects And Health Risk Assessment In Heavy Metal-Contaminated Soil Treated By Different Amendments

Farmland heavy metal-contaminated soil has become serious increasingly with the rapid development of industry and agriculture. There is a serious threat to environment and human health. Therefore, repairing the contaminated soil is urgent. At present, solidification/stabilization technology is an effective management method, many scholars who at home and abroad have done a lot of researches on the selection and development of ameliorants, the assessment of stabilization. But the studies on monitoring the long-term efficiency of ameliorants and human health risk assessment are lack, especially the research that taking focus on the long-term remediation effect of heavy metals is less.The long-term remediation effects of different ameliorants, including apatite, lime, charcoal was tested for the remediation of Cu and Cd contaminated soil near the Guixi smelter. The basic physical-chemical properties of the soil, leachability, availability of Cu and Cd were evaluated after the application of apatite, lime and charcoal for one and four years by TCLP, SPLP, CaCl2 leaching methods. The conditions of human stomach was simulated and the bioaccessibility, the content of dissolved Cu, Cd in gastric juice were calculated by using PBET extractions. The average daily intake, hazard quotient of Cu, Cd for adult and children were calculated, human health risk assessment was studied by using human health risk assessment model. In addition, the correlation between the content of heavy metal in gastric juice and basic physical-chemical properties of the soil, the content of leached heavy metal was analyzed. The key factor that affects the absorbed content of heavy metal in gastric juice was determined by using multiple stepwise linear regression equation, a theoretical basis to soil remediation is provided for choosing appropriate ameliorant. The main research contents and the results are as follows:(1) The pH was obviously increased after the application of ameliorants, the effect that three ameliorants increased pH followed the order:apatite>lime>charcoal, but pH decreased gradually as time goes on; the content of organic matter increased inordinately after the application of ameliorants, the effect of charcoal treatment is most obvious, had obviously different with apatite and lime. The content of exchanged acid and aluminum were decreased, there is a negative correlation between that and pH.(2) The amounts of available heavy metals in soil by three different extracting methods were compared, the result was that treatment groups were lower obviously than control group for amounts. It showed that ameliorants obtained good remediation effect, and the remediation effect for copper was consistent, the order was apatite>lime>charcoal. The repair effect of Cd by using CaCl2 extracting method had a similar rule, but there was different between Cu and Cd by using other two methods, the repair effect for Cd followed the order:lime> apatite>charcoal. In addition, the leaching contents had the increasing tendency year by year in all treatments. It showed that ameliorants had intended lifetime, it is need to add ameliorants again when their repair efficiency decreased.(3) There were different obviously about dissolved content, bioaccessibility, average daily intake between Cu and Cd, the size of these three indicators are Cu significantly greater than Cd, implying that Cu was absorbed easier. Cu had more risks to human health. In addition, from different treatments, the size of these three indicators were improved groups significantly lower than control groups, illustrating that ameliorants achieved good remediation effect for stabilizing heavy metal in soil in the short term. From different years, there was rising tendency for these indicators, implying that it was time to add ameliorants to stabilize heavy metal again. There was no health risk for people because of the hazard quotient (HQ) of the study area all below the accepted risk limit. However, it should be noted that HQ values of the children were higher than those of adults.(4) Relevance analysis showed that the contents of Cu and Cd in gastric juice was significantly positively correlative with the concentrations of exchanged acid and Al, the correlation coefficient are 0.860,0.815,0.852,0.795. there was significantly negatively correlative with pH, the correlation coefficient were-0.948 n-0.916. A result that soil pH played a key role in influencing the contents of bioaccessible Cu and Cd was studied with multiple stepwise linear regression equation. This result provided, theoretical basis for selecting appropriate ameliorant. Using stabilization method for repairing the heavy metal contaminated should choose ameliorants that can significantly increase the soil pH and reduce the activity of heavy metals in soil for longer.(5) Overall, apatite had the best longevity in decreasing the leachability, availability and bioaccessability of Cu and Cd than those of lime and charcoal. The findings of the present study provide valuable insights into the risk management during the long-term immobilization of heavy metal contaminated soil.