Research On Titanium/Dolomite-based Phosphorus Adsorbent And In-Situ Sediments Capping For Endogenous Phosphorus Pollution In Control

In-situ capping technology is one of the main technologies to prevent secondary pollution caused by the release of endogenous phosphorus from sediments.In recent years,in order to achieve a safer,more economical and applicable capping effect,materials that use natural minerals as the mainstay have been modified to strengthen their functions and engineering applicability,which has become the research focus.Dolomite is a natural mineral rich in resources with low cost.Its chemical composition is CaMg（CO₃）₂,which has large specific surface area with adsorption ability.Compared with commonly used minerals that adsorb phosphorus,there are fewer studies on the modification of dolomite.In this paper,titanium-modified dolomite（Ti/DLMT）was prepared by sol-gel method,and Ti/DLMT was loaded on the surface of ceramsite to prepare a new ceramsite material Ti/DLMT@Ceramsite.Performance of DLMT and Ti/DLMT@Ceramsite for phosphorus removal and feasibility as in-situ capping materials was discussed.Based on indoor simulation experiments,Ti/DLMT@Ceramsite was further applied to the sediment restoration project of inflow river-Xin Hu Tian Zhuang in Wuli Lake,and the restoration effect was discussed,which provided a reference for its engineering application.First,experiments have shown that titanium modification greatly improved the adsorption performance of dolomite and ceramsite.Characterizations such as SEM,XRD,FTIR,and XRF confirmed the successful loading of titanium,the increase of specific surface area,and the formation of hydrogen bonds,which provide more adsorption points.The saturated adsorption capacity of Ti/DLMT for phosphorus was 5.7352 mg/g in 500 mg/L phosphate solution and the capacity of Ti/DLMT@Ceramsite for phosphorus was 0.1805 mg/g in 50 mg/L phosphate solution.Both adsorbents had selective adsorptivity.In the range of pH 2～12,the presence of co-existing anions Cl^-,SO₄^2-and NO^3-has little effect on their phosphorus adsorption performance.After 4 times of desorption regeneration,the high removal rate of 96.7%ensured good reproducibility,so it has application prospects for treating phosphorus-containing wastewater.The study of the adsorption mechanism showed that the ligand-exchanging effect of phosphate-substituted hydroxyl groups during the adsorption was more significant than that of electrostatic adsorption.Then through 3-months of in-situ capping simulation experiments,it was found that the two materials capping on the surface layer of the sediment performed following functions in phosphorus transfer and transformation and microenvironment of the sediment-water interface:1.The diffusion rate and diffusion flux were reduced.On the one hand,adsorption effect of materials could directly remove phosphorus from the gap water and that diffused into the overlying water.On the other hand,the capping material played a role in fixing the adsorbed phosphorus and these phosphorus were mainly fixed in materials in the stable form of HCl-P,which also reduced the risk of secondary release;2.The total phosphorus content in the sediment gradually decreased during the capping experiment.NH₄Cl-P and BD-P in the surface sediments decreased and changed to more stable forms such as NaOH-P,HCl-P and extremely stable form of Res-P.The deeper sediments also showed the same trend;3.The in-situ capping increased the level of DO in the sediment-water interface and changed the abundance of some microbial flora in the surface sediment.Phosphate-accumulating bacteria with higher abundance levels could also reduce endogenous phosphorus in the sediment.All above results preliminarily indicated the possibility of titanium-modified dolomite being applied to the in-situ capping project of polluted sediments.On the basis of the above research,Ti/DLMT@Ceramsite was selected as the capping material in a 16-months field project initial test to repair heavily polluted point——Xin Hu Tian Zhuang,Wuli Lake inflow river.Results showed that the environment has generally improved:the phosphorus concentration in overlying water has significantly decreased,and the comprehensive pollution degree of nutrients（N and P）in the sediment has changed to light/moderate pollution;the average level of dissolved oxygen has increased to 5～7 mg/L and average reduction rate of turbidity was 76%.Chlorophyll-a levels have decreased to 5μg/L;the organic matter content in surface sediments decreased by 5.6%,and the potential ecological risk of heavy metals reduced too.The field trial revealed that Ti/DLMT@Ceramsite performed a significant role on the continuous and stable improvement of the environment quality of Xin Hu Tian Zhuang.Besides,it effectively reduced the possibilty of eutrophication and the ecological risk of water.Therefore,Ti/DLMT@Ceramsite is a promising in-situ capping material in engineering applications on sediments.