Comparison Of Magnetite,Hematite And Goethite In Control Of Phosphorus Release From Sediment

With the continuous development of human society and economy,eutrophication of water bodies has become a major water environment problem in the world.Due to human activities and other factors,excessive phosphorus was discharged into slow-flowing water bodies,resulting in high phosphorus concentration in slow-flowing water bodies,which in turn caused eutrophication of water bodies.As algae can proliferate in eutrophic water bodies,dissolved oxygen in water was rapidly consumed,leading to deterioration of water quality,death of aquatic organisms and other problems,which negatively affected economic development and human health.The key to control the eutrophication of water bodies is to control the concentration of phosphorus in water bodies.The input of exogenous phosphorus and the release of endogenous phosphorus are the two major sources of phosphorus in water bodies.Nowadays,when the input of exogenous phosphorus has been effectively controlled,how to effectively control the release of endogenous phosphorus in sediments is the key to control eutrophication in water bodies.In order to control the release of phosphorus from sediments many researchers have developed techniques such as sediment dredging,ecological remediation,aeration and oxygenation,aluminum salt passivation,nitrate injection,and in-situ chemical passivation techniques.In-situ chemical passivation technology has received a lot of attention from researchers due to its applicability,good control and long lasting effect.This technology is used to intercept the release of phosphorus from sediments by covering the sediment surface with a passivating material or mixing it with the sediment.The key to the application of in situ chemical passivation is the selection of excellent and efficient phosphorus passivation materials.The commonly used lanthanide-based and aluminum-based materials have problems such as high cost and release of lanthanide or aluminum ions.Iron oxides are considered to be a very promising material for phosphorus passivation of sediments because of their simple availability,environmental friendliness and high affinity for phosphates.Iron oxides are ubiquitous in the natural environment,and the most abundant iron oxides include magnetite,hematite and goethite.Different types of iron oxides generally have different phosphate adsorption properties.Clarifying the differences in the properties and mechanisms of each iron oxide in controlling the release of endogenous phosphorus from sediments to the overlying water is critical for in situ passivation of phosphorus from sediments using iron oxides as passivating materials.To this end,the properties and mechanisms of phosphate adsorption in water by magnetite,hematite and goethite were comparatively studied by batch adsorption experiments.The results showed that the kinetic processes of phosphate adsorption in water by magnetite,hematite and goethite were in accordance with the Elovich kinetic model.The adsorption rates of phosphate in water were magnetite>hematite>hematite.The adsorption isotherms of magnetite,hematite and goethite on phosphate in water are in accordance with the Langmuir and Freundlich isotherm model.The ranking of the maximum monolayer unit adsorption of phosphate by magnetite,hematite and goethite was magnetite>goethite>hematite.The ligand exchange between Fe-OH and phosphate on magnetite,hematite and goethite is an important mechanism for phosphate adsorption in water.Coexistence of Ca²⁺can promote the adsorption of phosphate on magnetite,hematite and goethite.At a solution p H of 7,Ca²⁺can react with phosphate to form calcium-phosphorus precipitates that are more readily adsorbed by iron oxides.In the coexistence of Ca²⁺,the isotherms of phosphate adsorption in water by magnetite,hematite and goethite are more consistent with the Freundlich isotherm model.In the coexistence of common anions(Cl^-,HCO₃^-and SO₄^2-)and cations(Ca²⁺,Mg²⁺,Na⁺and K⁺),the rate of phosphate adsorption by iron oxides on water was ranked as magnetite>goethite>hematite.When the Ca²⁺concentration in the solution increased from 0 to 3 mmol/L,the adsorption capacity of magnetite,hematite and goethite on phosphate in water increased continuously.Under the same conditions of Ca²⁺concentration,the adsorption capacity of phosphate to water was magnetite>goethite>hematite.The ability of magnetite,hematite and goethite to adsorb phosphate decreases with the increase of solution p H.At the same p H,magnetite has a stronger ability to adsorb phosphate in water than goethite has a stronger ability to adsorb phosphate than hematite.In general,magnetite is more suitable for phosphate removal from water than hematite and goethite.The effectiveness and mechanism of magnetite,hematite and goethite in controlling phosphorus release from sediments were comparatively studied by sediment incubation experiments.The results showed that the addition of magnetite,hematite and goethite were all effective in preventing the release of phosphorus from the sediment to the overlying water.The average removal rate of DRP from the overlying water during the incubation period was in the following order:magnetite>goethite>hematite.The passivation of DRP and LP_DGT in sediment interstitial water in the presence of magnetite,hematite and goethite is important for the addition of magnetite,hematite and goethite to control the release of phosphorus from the sediment into the overlying water.The effect of magnetite,hematite and goethite additions on mobile phosphorus in sediments was limited,and the control mechanism of endogenous phosphorus release from sediments by magnetite,hematite and goethite additions was not their immobilization of mobile phosphorus in sediments.The addition of magnetite,hematite,and goethite to the sediment improved the phosphate adsorption properties of the sediment to water,with the ability to adsorb phosphate from water in the presence of sediment being magnetite>goethite>hematite.Magnetite,hematite and goethite cover all effectively reduce the risk of sediment phosphorus release to overlying water,and most of the phosphorus immobilized by magnetite,hematite and goethite is present in a relatively or very stable form.The capping mode was more efficient in removing DRP from the overlying water than the addition mode.Overall,magnetite is more suitable than hematite and goethite for controlling sediment phosphorus release in terms of its ability to adsorb phosphate.Considering the control efficiency and material recovery,magnetite fabric wrapped capping is a promising method for controlling endogenous phosphorus release from sediments.