Effect Of Hydroxyapatite On Cadmium Immobilization And Rhizosphere Microbial Diversity In Cadmium Contaminated Soil

In recent years,the rapid development of industry and the increase of human activities have resulted in heavy metal pollution in some of China’s soil.According to the"Soil contamination survey bulletin"issued in 2014-year,cadmium（Cd）is one of the most concerned heavy metals and widely distributed in China.Cd is toxic and non-essential for human beings,and may cause a variety of disease via food chain contamination.Therefore,it is urgent to remediate cadmium-contaminated soil.Wheat is one of the main food crops in north China,and the safe production of the wheat is critical.The“Soil Pollution Prevention and Control Action Plan”announced in 2016 aimed to remediate contaminated soil by heavy metals and make soils suitable for agricultural production.At present,the in-situ remediation of contaminated soil is relatively easy to conduct and has limited impact on crop production.Among various amendments,hydroxyapatite is widely used as a green passivator to control soil cadmium pollution due to its safety,outstanding passivation effect and limited secondary pollution effects.In this thesis,the adsorption of Cd²⁺by hydroxyapatite in mineral-water were studied under the impact of the particle size of hydroxyapatite,DOC and rape straw.Moreover,pot experiment was also conducted to investigated the influence of HAP on the transformation of Cd in alkaline Cd-contaminated soil and the transfer of Cd to wheat with different Cd accumulation capacities.In addition,the effect of HAP on the soil microbial communities after wheat cultivation was also studied.These studies provide theoretical and technical support for understanding the environmental behavior of hydroxyapatite and enhancing the role of hydroxyapatite in the treatment of alkaline Cd-polluted soil.The main research results are as follows:1.The adsorption capacities of Cd²⁺by hydroxyapatite increased with the increase of contact time,pH,initial concentration of Cd and particle size.The adsorption isotherms of Cd on hydroxyapatite can be fitted by Langmuir model,and the adsorption kinetics fits to the second-order kinetic model.The adsorption capacity of Cd on hydroxyapatite follows the order of ordinary hydroxyapatite>micron-level hydroxyapatite>nano-level hydroxyapatite.Elements composition and XRD analysis indicated that HAP with the lowest Ca/P mole ratio,which makes it not easy to agglomerate in solution.And the low crystallization rate makes it have larger specific surface area,increased the contact area of Cd²⁺in the solution,thereby enhancing the adsorption rate.2.The adsorption rate was different with the initial Cd²⁺concentration after adding DOC to the HAP adsorption system.The amount of DOC and the variety of rape also had effect on adsorption.The adsorption rate of HAP-straw composite system was less affected by DOC than HAP or rape single system.The adsorption of Cd²⁺in the solution by the mixture of rape straw and HAP in different proportions was mainly the sum of the adsorption of HAP and rape straw separately.3.The pot experiment was carried out to study the passivation effect of HAP on alkaline cadmium-contaminated wheat fields with different dosage.The results showed that applying HAP effectively reduced the concentrations of Cd in grains,stems and root of wheat.Cd concentrations in different tissues of wheat obviously decreased in the order:roots>stems>grains,indicating that the roots acted as a barrier to Cd accumulation or translocation and protected the edible parts from Cd contamination.The grain Cd concentrations of Zhong mai,Zhou mai-30 and Zheng mai-7698 were lower than the China national standard（GB2762-2017）when applying 2.5%and 5%HAP.The application of HAP increased soil pH value whereas reduced the percentage of acid-soluble Cd（11.54%-12.63%）and oxidizable Cd（3.68%-4.43%）in soil,increased the percentage of reducible Cd（4.87%-9.09%）.In addition,HAP also increased the proportion of residue states of Cr,Cu,Pb and Zn in soil,reduced the proportion of unstable components such as acid soluble state and reducible state,thereby reduces the bioavailability of heavy metals.4.High-throughput sequencing results showed that the application of HAP reduced the richness and diversity of soil microbial community structure.The abundance of dominant genus,bacterial evolution branch and active biomarker were different affected by the dosage of HAP and wheat cultivars.The addition of HAP resulted in the reorganization of soil bacterial community,in which bacteria treated by 1%and 2.5%HAP had similar structure,while 5%HAP was significantly different from other treatments.The correlation analysis between soil microbial community structure and environmental factors showed that soil pH value and available heavy metals were both important factors affecting soil microbial communities.