Research On The Remediation Of Phosphorus-Contaminated Black Smelly Substrates By Layered Double Hydroxide Composites

The release of endogenous phosphorus from black smelly substrates has become the main source leading to the phenomenon of black odor in urban water bodies.In this study,we prepared calcium/iron layered double hydroxide-natural zeolite composites（CFL-Z）,explored the performance and mechanism of phosphate adsorption,clarified the remediation effect of the materials on phosphorus-contaminated black smelly sediment,analyzed the remediation effect of different application methods（covering/addition）on phosphorus-contaminated black smelly sediment and the migration and transformation of phosphorus in the sediment during the process,and investigated the microbial community in the sediment under different application methods.The microbial community of the bottom sediment under the different application methods was also investigated.Finally,we investigated the phosphate adsorption behavior of the CFL-Z-improved substrate and conducted ecological safety evaluation.A comprehensive and systematic investigation of the remediation process of phosphorus-contaminated black smelly sediment by layered double hydroxide composites from four aspects:material preparation,remediation effect,behavioral mechanism,and ecological assessment was realized,which provides methodological reference and scientific theoretical support for the remediation of phosphorus-contaminated black smelly sediment.The specific contents and innovative results of this study mainly include:（1）Using co-precipitation method,the composite material CFL-Z was successfully prepared by combining calcium/iron layered double hydroxide（Ca/Fe-LDH）and natural zeolite（NZ）,and characterized and analyzed.The results showed that after the Ca/Fe-LDH composite,the surface of NZ showed a lamellar structure with the characteristics of layered double hydroxide,and was uniformly dispersed on the surface and inside the pores of the material.The adsorption experiments showed that the maximum adsorption capacity of CFL-Z was 46.7 mg/g,which significantly improved the binding ability of NZ to phosphate.When the pH value in the solution was 3-6,the adsorption effect of CFL-Z on phosphate was better than that of pH 8-11,and the adsorption capacity decreased with the increase of pH value.In addition,Na⁺,K⁺,and Cl^-had no significant effects on phosphate adsorption by CFL-Z,Ca²⁺promoted phosphate adsorption by CFL-Z,while Mg²⁺,SO₄^2-,and HCO₃^-negatively affected the adsorption process.The adsorption isotherm and kinetic results indicate that there is a physicochemical multilayer adsorption mechanism in the adsorption process of CFL-Z,and there are two active sites,hydroxyl and interlayer anions,on the material surface.（2）CFL-Z composites were applied to the remediation of phosphorus-contaminated black smelly substrates.The results showed that under anoxic environment,the pH value in the overlying water continued to increase with the increase of CFL-Z injection ratio.In the substrate incubation experiment,the water-soluble phosphorus（SRP）concentration in the overlying water was reduced by 40.8%in the experimental group with 10%NZ addition compared with the blank group,while the experimental group with 6%CFL-Z addition had the best effect on SRP remediation in the overlying water,reaching 97.3%.In order to investigate the stabilization effect of CFL-Z on endogenous phosphorus in the substrate,the remediation effect was measured at 6%and 10%CFL-Z dosing rates,respectively.In order to investigate the effect of CFL-Z on the stabilization of endogenous phosphorus in the substrate,the changes of phosphorus morphology in the substrate were measured at 6%and10%CFL-Z dosing rate for 35 days.The results showed that the content of unstable phosphorus fractions（NH4Cl-P and BD-P）in the substrate gradually decreased with time,while the content of stable phosphorus fractions（HCl-P and Res-P）gradually increased,which indicated that the introduction of CFL-Z helped to reduce the release of unstable phosphorus from the substrate,thus reducing the risk of eutrophication in the water column.（3）To evaluate the restoration effect of the actual project,a microenvironment-substrate incubation experiment was set up and two application modes of mulching/addition were adopted to explore the effect of CFL-Z on phosphorus-contaminated black smelly substrates.The results showed that the phosphorus concentration in the overlying water in the cover group was lower than that in the addition group,which was due to the fact that the cover layer formed by CFL-Z could effectively intercept the phosphorus inside the substrate below the substrate-water interface.In addition,both application modes reduced the DGT unstable phosphorus concentration in the overlying and interstitial water,with 93.6%and 57.9%reduction in the overlying and interstitial water,respectively,in the cover group,and 87.3%and 73.6%reduction in the addition group,respectively.Thus,from the perspective of vertical profiles,CFL-Z achieved effective stabilization of bottom sediment phosphorus.Finally,there were some differences in the effects of the cover group and the addition group on the abundance as well as the structure of the microbial community in the substrate.Although the dominant species at the phylum and genus levels were basically similar,the different application modes changed the structure and components of some microbial communities,which also affected the phosphorus migration and transformation within the substrate to some extent.（4）The changes in the physicochemical properties of the substrate itself and the interaction with phosphate after the addition of CFL-Z were analyzed.The results showed that the phosphate adsorption capacity of the CFL-Z-amended substrate increased by 42.35mg/kg compared with that of the original substrate,and the Qmax and Kp values of the improved substrate were found to increase by using the modified Langmuir and Freundlich isotherm model,which implied that the improved substrate could be transformed from a"source"to a"sink"of phosphorus pollution.This implies that the modified substrate can be transformed from a"source"to a"sink"of phosphorus pollution.The desorption experiments also further indicated that the CFL-Z modified substrate enhanced the stability of phosphorus bound to the substrate.In addition,the CFL-Z-amended substrate increased the diversity of microorganisms in the substrate and also changed the relative abundance of different genera,but did not change the main composition of the microbial community structure.