Mechanism Of Phosphate Adsorption In Seawater By Magnetite And Ferrihydrite And Their Effects On Phosphorus Transport Transformation In Seawater-Sediment System

Along with the ocean power strategy,the sustainable use of marine resources and the growth of the marine economy have increasingly become the focal point of China’s livelihood and economic development so that mariculture serving as the driving force.Because of the high density of mariculture and excessive feed input,the nutrient salt content of seawater has increased dramatically.Mariculture wastewater is directly discharged into the ocean without treatment,resulting in exogenous pollution,which has become the trigger of eutrophication in near-shore waters.As a result,removing excess phosphate in mariculture wastewater is critical to preventing and controlling marine eutrophication.As an important component of marine ecosystem,there is nutrient salt exchange between marine sediment and seawater,which may become a source of phosphorus in seawater.Managing the release of phosphorus enriched in marine or mariculture area sediments is thus critical for managing marine eutrophication.Various techniques have been used to remove phosphate from mariculture wastewater and control phosphorus release from marine sediments,the most notable of which is the use of adsorbent materials to remove phosphate from mariculture wastewater and control phosphorus release from marine sediments,due to its low cost and good treatment effect.Because of the frequency of iron oxides in nature and their high adsorption capabilities,their use for phosphate removal from water and control of phosphorus release from sediments has gotten a lot of interest from academics at home and abroad in recent years.Because the different structures and properties of different adsorbents result in significant differences in their adsorption effects and application prospects,it is critical to study the adsorption mechanism of iron oxides on phosphate in seawater as well as the influence law on the migration and transformation of phosphorus in seawater-sediment systems in order to solve problems such as marine eutrophication.The mechanism of phosphate adsorption in seawater by magnetite and ferrihydrite,two types of iron oxides commonly found in nature,needs to be studied further.As does the law of phosphorus transport and transformation in seawater-sediment system by the addition of magnetite and ferrihydrite,and the difference between them.In this study,we first synthesized magnetite and ferrihydrite and investigated the phosphate adsorption mechanism in seawater through batch adsorption experiments and material characterization.Then investigated the effect of magnetite and ferrihydrite addition on phosphorus migration and transformation in seawater-sediment system through sediment incubation experiments and batch adsorption experiments,and analyzed the differences between magnetite and ferrihydrite on phosphorus migration and transformation in seawater-sediment system.To provide factual support and theoretical foundation for the application of magnetite and ferrihydrite as environmental pollution treatment materials in order to minimize phosphorus load in the marine environment.The mechanism of phosphate adsorption from seawater by magnetite and ferrihydrite was studied and the findings demonstrated that,the kinetic processes of phosphate adsorption from seawater by magnetite and ferrihydrite better fit the Elovich kinetic model compared to the Quasi-first order kinetic model and Quasi-second order kinetic model.The adsorption data of magnetite on phosphate in seawater are better fitted with the Freundlich adsorption isotherm equation than the Langmuir adsorption isotherm equation.Both Langmuir and Freundlich models can be used to describe the isothermal adsorption behavior of ferrihydrite on phosphate in seawater.During the slow adsorption phase,the adsorption rates of magnetite and ferrihydrite on phosphate in seawater are jointly regulated by intraparticle diffusion and membrane diffusion.Regardless of the presence of magnesium ions,calcium ions increased the adsorption of phosphate by magnetite and ferrihydrite.When calcium ions were absent,magnesium ions promoted the adsorption of phosphate by magnetite and ferrihydrite.While when calcium ions were present,the coexistence of magnesium ions inhibited the adsorption of phosphate by magnetite and ferrihydrite,which indicated that the effect of coexisting magnesium ions on the adsorption of phosphate in seawater by magnetite and ferrihydrite was closely related to the coexisting calcium ions.The effectiveness of phosphate removal by magnetite and ferrihydrite in the water column depends on the amount of adsorbent dosed and p H change.Important mechanisms for phosphate adsorption from magnetite and ferrihydrite in water include complex exchange and inner complex formation.The complex exchange reaction between Ca HPO₄⁰/Mg HPO₄⁰ and hydroxyl groups and ternary complex formation with phosphate as a bridge also play an important role for phosphate adsorption from magnetite and ferrihydrite in water.According to the findings of a study on the impact of magnetite and ferrihydrite on phosphorus transport and transformation in seawater-sediment system,the addition of magnetite and ferrihydrite significantly reduced the concentration of soluble reactive phosphate in the surrounding water and effectively controlled the release of phosphorus from marine sediments to the surrounding water.The addition of magnetite and ferrihydrite decreased the chance of phosphorus release from sediments by affecting the fugitive morphology of phosphorus in sediments.The modified sediment’s ability to absorb phosphate from seawater depends on the quantity of modified sediment injected,the initial p H of the solution,and reaction temperature.The behavior of magnetite and ferrihydrite modified sediments during phosphate adsorption could be explained using both the Langmuir and Freundlich models.The Elovich model is more suitable for describing the kinetic process of phosphate adsorption from seawater by magnetite and ferrihydrite modified sediments than the Quasi-first order kinetic model and Quasi-second order kinetic model.Intraparticle diffusion and membrane diffusion together control the adsorption rate of phosphate in seawater by magnetite and ferrihydrite modified sediments during the slow adsorption phase.The ferrihydrite-modified sediment was better at adsorbing phosphate from seawater than the magnetite-modified sediment.In conclusion,both magnetite and ferrihydrite have certain adsorption and removal ability for phosphate in seawater,and can play a role in regulating phosphorus migration and transformation in marine sediments.When the two materials are compared,ferrihydrite is more promising as an adsorbent material to reduce the phosphorus load in the marine environment to solve the problem of phosphorus pollution in seawater or marine sediments.