Study On The Efficiency And Mechanism Of The Combined Use Of Ferrihydrite And Magnetite For Controlling Phosphorus Release From Sediments

At present,anthropogenic eutrophication of freshwater ecosystems characterized by harmful algal blooms（HABs）is a worldwide environmental problem.As an indispensable macronutrient for algae growth,phosphorus is considered to be the main limiting nutrient leading to freshwater eutrophication.When the input of exogenous phosphorus is effectively controlled,the continuous release of endogenous phosphorus in sediments will still lead to the increase of phosphorus concentration in overlying water.Active mulching/modification techniques are considered to be a promising sediment remediation strategy for controlling the release of phosphorus from sediments.The key of this technology is to select efficient,economical,safe and easy to prepare blunt phosphorus materials.Ferro-based materials are considered as active coating materials/amendments with great application prospects due to their widespread existence,low cost and environmental friendliness,and have attracted extensive attention from scholars at home and abroad in recent years.Ferrihydrite and magnetite are common iron oxides.If ferrihydrite and magnetite can be combined by a simple method,it is expected to build a more efficient recovery and covering system,so as to better realize the dual purposes of phosphorus release control and recovery in water bodies.However,there are few reports about the construction of a recoverable mulching system based on ferrihydrite and magnetite and the control effect and mechanism of phosphorus release in sediments.In this paper,the adsorption behavior and mechanism of ferrihydrite and magnetite on phosphate in water were studied by batch adsorption experiment in the absence and presence of Ca²⁺,and then the influence of Ca²⁺ on the adsorption of phosphate in water by ferrihydrite and magnetite in the presence of common cations was discussed.The results showed that both Langmuir and Freundlich isotherm models could be used to describe the isotherm adsorption of phosphate in water in the absence and presence of Ca²⁺.The Freundlich isothermal adsorption model is better than Langmuir isothermal adsorption model to describe the adsorption data of phosphate in water by magnetite.Compared with the quasi-first-order and quasi-second-order kinetic models,the Elovich model can be better used to describe the adsorption kinetic data of phosphate in water by ferrihydrite and magnetite in the absence and presence of Ca²⁺.When there is no Ca²⁺,the adsorption capacity of ferrihydrite and magnetite for phosphate in water decreases with the increase of pH value of solution.When there is Ca²⁺,the adsorption capacity of ferrihydrite for phosphate changes little with the increase of pH value of solution,while the adsorption capacity of magnetite for phosphate changes little or increases with the increase of pH value of solution.In the absence of Ca²⁺,phosphate is adsorbed by ferrihydrite and magnetite to form an inner iron-phosphorus complex.In the presence of Ca²⁺,the inner iron-phosphorus complex is the main mechanism of phosphate adsorption by ferrihydrite and magnetite,and Ca²⁺ is involved in the phosphate adsorption process.Ca²⁺ promoted the adsorption of phosphate by ferrihydrite and magnetite under the condition of common zwitter-ion.The promotion mechanism of Ca²⁺ is as follows: Firstly,Ca²⁺ reacts with phosphate in water to form CaHPO₄⁰,which is then adsorbed on the surface of ferrihydrite and magnetite to form a ternary complex of Fe-P-Ca with phosphate as bridge.Compared with H₂PO₄^-and HPO₄^2-,CaHPO₄⁰ is more easily adsorbable,thus promoting the adsorption of phosphate by ferrihydrite and magnetite.Secondly,the adsorption properties and mechanism of phosphate in water with the coexistence of multiple ions and anions were further investigated.The results showed that the coexistence of Na⁺,K⁺,Cl^-and SO4^2-had little effect on the adsorption of phosphate in water by ferrihydrite and magnetite,while Ca²⁺ and Mg²⁺ would form phosphate bridging ≡Fe（OPO₃H）Ca⁺ and ≡Fe（OPO₃H）Mg⁺ with ferrihydrite and magnetite,thus promoting the adsorption of phosphate by ferrihydrite and magnetite.HCO₃^-can inhibit the adsorption of phosphate by ferrihydrite and magnetite.The adsorption capacity of ferrihydrite to phosphate in water is stronger than that of magnetite.When common anions coexist,the adsorption mechanism of ferrihydrite and magnetite on phosphate is that the surface hydroxyl group is replaced by phosphate to form iron-phosphorus binary inner complex and the trivalent iron-phosphorus-calcium/iron-phosphorus-magnesium complex with phosphate as the bridge.Finally,the effects of the combined addition and mulching of ferrihydrite and magnetite on phosphorus migration and transformation of sediments were studied,and the construction method of a recoverable mulching system based on ferrihydrite and magnetite was discussed,as well as the control effect and mechanism of phosphorus release in sediments.The results showed that under anoxic condition,the combined coverage of water-iron ore/magnetite could effectively control the release of endogenous phosphorus in sediments,and the passivation of DGT（thin film gradient diffusion）available phosphorus and potentially mobile dynamic phosphorus in sediments played a key role in the combined improvement of water-iron ore/magnetite.The combined coverage of ferrihydrite/magnetite can effectively control the migration of endogenous phosphorus to the overlying water in sediments under anoxic condition,and the passivation of available DGT phosphorus in the upper sediments plays a key role in controlling the release of endogenous phosphorus to the overlying water in sediments under the combined coverage of ferrihydrite/magnetite.The stability of ferrihydrite/magnetite combined overburden to phosphorus passivation depends on its application method.In common pH（5-9）and anoxic conditions,the vast majority（about 90%）of phosphorus passivated by a mulch system based on a ferrihydrite/magnetite mixture has a low risk of release.The combined coverage of ferrihydrite/magnetite did not increase the risk of iron release from the sediments,and had little effect on the diversity and richness of bacterial communities in the sediments.Considering the control efficiency,convenience of application,environmental friendliness and sustainability,the covering system based on geotextile coated ferrihydrite/magnetite mixture is a promising method to control the release of endogenous phosphorus from sediments to overlying water.