Remediation Of Pb/Cd Contaminated Sediments By Ecotype Nanochlorapatite And Its Microenvironmental Response Mechanism

Heavy metal pollution in sediments will have toxic effects on aquatic organisms and eventually spread through the food chain,endangering human health and ecological security.Therefore,it is urgent to find efficient remediation technologies for heavy metal pollution in sediments.Phosphate materials represented by chlorapatite(Cl AP)have specific stabilization effect on heavy metals,but the traditional Cl AP has poor dispersion,solubility and migration in sediments,so its application is limited.With these in mind,three ecological nano-chlorapatite materials(ENAPs)were optimized and prepared for the remediation of heavy metals lead(Pb)and cadmium(Cd)in sediments,and the stabilization efficacy and mechanisms of ENAPs for Pb and Cd in sediments were clarified.The dynamic response of microbial community structure and composition in sediments during ENAPs remediation was further investigated by high throughput sequencing,and the mechanism of microbial cooperation was revealed.Finally,the correlation between physicochemical properties and enzyme activities of sediments and the biological toxicity of heavy metals during the remediation process was further determined by the changes of physicochemical properties and typical enzyme activities of sediments during the remediation process,which provided technical support and theoretical guidance for the wide application of ENAPs in the future.The specific research content and innovative achievements of this paper included the following four aspects:In the first part,two high-performance ecological nano-chlorapatite(SOP-n Cl AP and SLS-n Cl AP)were optimally prepared and characterized by surface modification using the biosurfactant sophorolipid(SOP)and the chemical surfactant sodium lignosulfonate(SLS),which were applied to remediate sediments contaminated with a combination of heavy metals Pb and Cd.The results showed that the surface-modified SOP-n Cl AP and SLS-n Cl AP exhibit better dispersion properties relative to Cl AP,with a uniform nanosheet structure;The experimental results showed that the final stabilization efficiencies of SOP-n Cl AP for heavy metals Pb and Cd in sediment were79.94%and 68.81%,respectively,while the final stabilization efficiencies of SLS-n Cl AP for Pb and Cd were 80.31%and 76.66%,respectively.Compared with Cl AP,the surface-modified SOP-n Cl AP and SLS-n Cl AP both greatly improved the stabilization efficiency of the heavy metals Pb and Cd in the sediment;In addition,the Toxicity Characteristic Leaching Procedure(TCLP)results showed that the modified SOP-n Cl AP and SLS-n Cl AP not only significantly accelerated the conversion of heavy metals in the sediment from unstable forms to residual forms and reduced their mobility in the environment,but also reduced the biological toxicity of Pb and Cd in the sediment;The available phosphorus results showed that,compared with Cl AP,the final available phosphorus content of SOP-n Cl AP and SLS-n Cl AP after surface modification was significantly reduced,which indicated that surface modification improves phosphorus utilization and heavy metal stabilization efficiency.The change trend of available phosphorus also indicated the existence of a dissolution-precipitation mechanism during chlorapatite remediation;The changes in organic matter content indicated that the stabilization of heavy metals in sediments may be synergistically regulated by microorganisms.In the second part,based on the study in the first part,an ecological biochar-loaded nano-chlorapatite material(BC-n Cl AP)was prepared by loading modification of waste biomass and applied to sediment heavy metal remediation.The results showed that chlorapatite was successfully dispersed on the surface and in the pores of biochar(BC).The results of chemical morphology change showed that BC-n Cl AP was a more effective stabilizer for Pb,with a final residual fraction proportion up to 94.35%after remediation,but only about 55%for the heavy metal Cd.This may be since the fact that the load modified BC-n Cl AP had both the strong adsorption performance of BC and the stability properties of Cl AP for heavy metals.In this study,Pb and Cd were both divalent cations and their concentration was high,so there was a competitive adsorption between Pb and Cd,and the solubility of Pb₅(PO₄)₃Cl was significantly less than that of Cd₅(PO₄)₃Cl,so that the final Pb stabilization efficiency was much higher than that of Cd;The results of TCLP-leachable Pb/Cd showed that the removal efficiency of BC-n Cl AP towards TCLP-leachable Pb in sediments can reach 100%in a short period of time,which greatly reduced the biological toxicity of heavy metal Pb in sediments.However,the final removal rate of TCLP Cd after remediation was only77.37%.Available phosphorus results showed that due to the introduction of biochar,the final available phosphorus content can be effectively controlled,accordingly improving the utilization of phosphorus and reducing the risk of eutrophication;The changes of organic matter indicated that the introduction of biochar can significantly increase the content of organic matter in sediments and promoted the stabilization of heavy metals,but this was also related to the response of microbial community in sediments.The third part was based on the researches of the first and second parts,we further explored the response mechanism of three high-performance eco-type nano-chlorapatite(ENAPs)to the sediment microenvironment during the treatment of heavy metals by high-throughput sequencing.The results showed that the effects of abundance,richness,evenness and diversity of microbial communities in sediments remediated by Cl AP and ENAPs were different,which were closely related to the stability of heavy metals in sediments and the properties of modifiers.The dominant species at the phylum level in the sediments of Xiangjiang River were Proteobacteria and Firmicutes,while the restoration with different chlorapatite did not significantly change the type of dominant species at the phylum level,but only the relative abundance of the dominant species Different modifiers had a great impact on the microbial community structure,and the new bacterial genera that appeared after remediation in both surface-modified SOP-n Cl AP and SLS-n Cl AP treatment groups were significantly related to the properties of the surfactant,while the new bacterial genera appeared after remediation in the modified BC-n Cl AP treatment group were mostly related to the effective decomposition and utilization of phosphorus in chloroapatite.Therefore,the introduction of BC was more conducive to the decomposition and utilization of phosphorus in chlorapatite,even though BC-n Cl AP had excellent remediation effect only for heavy metal Pb and relatively poor repair effect for Cd,the modification of BC loading was a relatively superior method that can efficiently and ecologically utilize the phosphorus in chlorapatite.In the fourth part,significance analysis and correlation analysis were used to clarify the effects of Cl AP and ENAPs on the physicochemical properties and typical enzyme activities of sediments and their interactions with the biotoxicity of heavy metals Pb and Cd.The p H,cation exchange capacity(CEC),catalase and urease of the sediments after remediation all changed significantly.The physical and chemical properties and enzyme activities of sediments were significantly affected by different remediation agents,and the differences were great.The changes in physical and chemical properties and enzyme activity were not time-dependent,However,the urease activity firstly showed a downward trend,and then began to rise slowly,which was related to the release and binding of phosphorus in the process of remediation.In addition,changes of p H,CEC,catalase and urease activities in sediments were correlated with the biological toxicity of heavy metals,and this correlation was largely determined by the properties of remediation materials.In this study,the remediation efficiency of three ENAPs on Pb and Cd in sediments was comprehensively investigated in terms of material preparation optimization,heavy metal stabilization efficiency,microenvironmental response mechanism,and changes in physicochemical properties and enzyme activities of sediments.This paper aims to provide a research basis for future practical application of metal stabilization by ecological modified insoluble phosphate materials.