Effect Of Aged Nanoscale Zero-Valent Iron Amendment And Capping On Phosphorus Mobilization And Transformation In Sediment

Since the twenty-first century,the world has witnessed rapid economic development,increasing total population,urbanization,and explosive growth of various types of industrial and agricultural activities.These changes and activities have inevitably caused a lot of human intervention in nature.The aquatic ecosystems have been damaged to some extent,and the level of aquatic pollutants has further increased.Thus,the problem of eutrophication in freshwater lakes has been highlighted.Although the pollution control and protection of lakes in China are effective,cyanobacterial blooms still exist and there are still many lakes with degraded ecosystems.In addition,excluding the disturbance of aquatic ecosystems by human factors,the impact of climate change on the eutrophication of rivers and lakes cannot be ignored.As the climate warms and surface water temperatures increase,the redox-sensitive phosphorus fraction in surface sediments is readily released into the overlying water column,increasing the nutrient saturation in the water body.Various techniques to inhibit the diffusion of phosphorus from sediments to overlying waters have been developed,among which in situ amendment/capping techniques have received increasing attention from experts and scholars in recent years.Nanoscale zero-valent iron（nZVI）is considered as a promising amendment/capping material for contaminated sediments due to its unique properties such as high catalytic activity,high reactivity,low toxicity and high adsorption capacity.However,nZVI is highly susceptible to oxidation during preparation and application.Therefore,it is necessary to explore the removal ability and mechanism of phosphate in water by aged nZVI and its effect on phosphorus mobilization and transformation in sediments for the application of nZVI to remediate contaminated sediments.However,there are few reports on the effects of aged nZVI amendment and capping on phosphorus mobilization and transformation in sediments in China and abroad.In this study,aged nZVI（A-nZVI）was prepared by borohydride reduction in the presence of oxygen atmosphere.Characterization techniques such as XRD,SEM,XPS,EDS and specific surface area and pore size analysis were used to investigate the properties of A-nZVI such as structural composition,and the ability of A-nZVI to remove phosphate from water and the mechanism of this process.The results showed that A-nZVI consists of nZVI as well as magnetite and lepidocrocite,and the two iron oxidation products are uniformly distributed.In batch adsorption experiments,A-nZVI exhibited excellent ability to remove phosphate from water.The maximum adsorption capacity of A-nZVI for phosphate reached 53.0 mg/g at pH 4.The adsorption process was consistent with the Freundlich isothermal adsorption model（R²=0.998）and pseudo-second kinetic adsorption model（R²=0.978）.XPS results showed that the main mechanism of phosphate adsorption by A-nZVI was the ligands exchange between phosphate ions and hydroxyl groups,forming an inner iron-phosphate complex.In addition,experiments on the effect of coexisting ions in water showed that all three cations,Ca²⁺,Mg²⁺and Na⁺,could promote the corrosion of A-nZVI,thus significantly enhancing the phosphate adsorption capacity of A-nZVI to water.Since Ca²⁺and Mg²⁺carry more charge than Na⁺,Ca²⁺and Mg²⁺promote phosphate adsorption by A-nZVI more than Na⁺at the same concentration.Since both NO₃^-and SO₄^2-anions compete with phosphate anions for active adsorption sites on the surface of A-nZVI,the adsorption of phosphate in water by A-nZVI is inhibited.The fully aged A-nZVI still has a strong adsorption capacity for phosphate adsorption in water compared to A-nZVI,although its adsorption capacity for phosphate adsorption in water is significantly weaker.In this study,a sediment simulation system was also constructed to investigate the effect of A-nZVI on phosphorus mobilization and transformation in sediments.The results showed that under high A-nZVI dosage conditions（4%,8%,12%and 16%of A-nZVI to sediment）,A-nZVI effectively reduced the release of dissolved reactive phosphorus（DRP）from the sediment to overlying water under anoxic conditions and induced the conversion of reducible state phosphorus（BD-P）in the sediment to metal oxide-bound inorganic phosphorus（NaOH-IP）and residual state phosphorus（Res-P）.The mass ratio of BD-P and calcium-bound phosphorus（HCl-P）to total extractable phosphorus（TEP）decreased from 9%to 4%-5%and from 71%to 50%-68%,respectively,while the mass ratio of NaOH-IP and Res-P to TEP increased from 12%to 15%-24%and from 7%to 12%-21%.Thus,the high dose addition of A-nZVI resulted in a more stable inorganic phosphorus fraction than the control group.And for conditions at low doses of A-nZVI（A-nZVI to sediment ratio of about 0.75%）,A-nZVI amendment still reduced the risk of DRP release from the sediment to the overlying water.Furthermore,under anoxic conditions,A-nZVI capping effectively prevented the release of DRP from sediment to overlying water with control efficiencies ranging from 73.7%to 99.1%,and was higher than A-nZVI amendment（21.1%to 59.8%DRP removal efficiencies）.The uptake of DRP by the A-nZVI capping layer located at the sediment-water interface was an important reason for intercepting the diffusion of DRP from the sediment to the overlying water.In addition,under anoxic conditions,both A-nZVI amendment and capping reduced bioavailable phosphorus(P_DGT)concentrations measured by the thin film gradient diffusion technique（DGT）in the overlying water profile and the uppermost sediment profile,but neither was efficient in reducing P_DGT in the bottom sediment profile.Meanwhile,under aerobic conditions,neither A-nZVI amendment nor capping could reduce P_DGT concentrations in the overlying water profile and sediment profile at the later stage of the incubation.In this study,high-throughput sequencing revealed that A-nZVI amendment and capping not only increased the diversity and abundance of surface sediment microorganisms,but also stimulated the growth of some iron-reducing bacteria in the surface sediment.Overall,A-nZVI can be used as an in-situ active capping material to control the release of DRP from the sediment to the overlying water.