Study On The Immobilization Effect Of Nano-hydroxyapatite On Cadmium In Contaminated Soil

In recent years,with the rapid development of the industry and agriculture,heavy metal pollution has cut agricultural production and the degradation of products,especially heavy metal cadmium pollution.Therefore,How to reduce the toxicity heavy metals effectively in soils and improve the yields and the quality of crop is indispensable for controlling heavy metal pollution and producing safety crops.After a lot of literature survey and simulated experiment,we explored the influences of NHAP on the leaching characteristics of cadmium in soil.And specified the transformation of the cadmium's content and forms in soil with different ratio NHAP application.The cadmium's distribution in soil-plants and the dynamics of microbial population in the rhizosphere soil with the NHAP and microorganisms treatment.The main results were as follows:1.The adsorption rate and desorption rate of Cd on NHAP increased with increasing time and temperature,and then tends to stabilization.The adsorption rate of Cd on NHAP increased with increasing pH,and then tends to stabilization.The desorption rate of Cd on NHAP decreased with increasing pH,and equilibrium is approximately reached at pH of 3.2.Leaching experiments were carried out to estimate Cd transformations and transportation with 0.5% NHAP treatment.NHAP application reduced the Cd concentration of the leachate by 56.45%,which fits the parabolic diffusion model(R2 =0.998).The addition of NHAP can increase the soil pH and residual cadmium.After leaching,the Cd concentration of the contaminated soil decreased,meanwhile,the Cd concentration in the uncontaminated soil increased.The results of using the proposed model indicate that the cadmium's concentration in the polluted soil leaching solution will not do harm to human's health within 50 years with acid rain interval leaching condition after the passivation of NHAP.3.The pot experiment demonstrate that NHAP also effectively increased the plant height and the biomass compared with the control and the content of Cd in rape decreased significantly with the increase of NHAP application ratio The treatment of 0.2%,0.5% and 1%(content)with NHAP in soil was decreased by 22.38%,36.25% and 50.2%,respectively compared with the control.Moreover,compared with the control,the catalase activity,urease activity and alkaline phosphatase activity in soil were significantly increased,moreover the catalase activity and alkaline phosphatase activity increased significantly with the increase of application amount.Through analysis of Cd concentration in soil and plant,enzyme activities of different treatments,it was found that the ratio of 0.5%(W/W)was better than others.4.The application of microorganisms and nano-hydroxyapatite(NHAP)in remediation of Cd in soil are a good choice for solve the contradiction which the arable land reducing and Cd pollution area expanding.A pot experiment was carried out to investigate the microorganisms and NHAP on growth of rape and the soil Cd content in Cd contaminated soil.The results showed that the microorganisms and NHAP could reduce concentation of exchangeable cadmium and increased concentration of residual cadmium.The addition of microorganisms and NHAP increased the soil Cd content and decreased the Cd content of rape,and the co-application amendments more effectively increased than with single amendment alone.The addition of microorganisms and NHAP treatments reduced Cd content of rape by 37.99-57.04%.NHAP combined with microorganisms increased the catalase activity,urease activity and alkaline phosphatase activity in cadmium contaminated soil.In addition,compared with Bacillus subtilis and white rot fungi,NHAP application had significant effect on the abundance and diversity of bacteria in soil.After adding NHAP to soil,the abundance and bacterial diversity of the soil were significantly increased.The growth of bacteria such as Paenibacillus(Bacillus sp.)increased significantly,and the harmful bacteria Gemmata(Phytophthora)were significantly reduced,and that will make soil develop in a healthier direction.In summary,this study shows that NHAP has a better passivation effect on cadmium contaminated soil,partly due to its increase in soil pH,phosphate precipitation with cadmium ions,and reduced bioavailability of cadmium concentration.Further more application of NHAP and microorganisms can bring a synergistic effect in remediation of Cd contaminated soil.It suggesting an increased transformation of cadmium from the labile into the stabile poor.And nano-hydroxyapatite as a natural soil passivation agent can significantly increase soil microbial population diversity,reduce plant Cd content,improve soil quality,low-concentration in cadmium contaminated soil which could offer material and technique supports for medium and slight pollution load soils.