Chemical Modification Of Natural Magnetite And Its Performance In Arsenic Removal From Water

High arsenic groundwater is widely distributed in the world,which has seriously affected health of the residents.Iron oxides have positive surface charges and large specific surface area,and show strong binding capacity to arsenite and arsenate under specified conditions.It has become one of the research hotspots to use ferric oxides as the adsorbent for high-arsenic water treatment.As the main constituents of natural magnetite,Fe3O4 has weak activity,and shows low arsenic removal efficiency.Prior to the practical application of natural magnetite for arsenic removal,it is most important to develop the modification technology of natural magnetite and to optimize arsenic removal by the modified magnetite.This study has adopted different methods including acidification,alkalization,and calcination to modify natural magnetite for selecting optimal modification conditions and to evaluate the influencing factors and mechanism of arsenic adsorption,which is of theoretical significance and practical values.The optimum conditions for the modification of natural magnetite for arsenic removal are to immerse magnetite in 0.5 mol/L hydrochloric acid and then to calcine for 10 min in a muffle furnace at 150°C.It shows that arsenic adsorption capacity is obviously higher than that of unmodified natural magnetite.The removal efficiency for As(V)reaches 98%,and dissolved Fe concentration is around 50 ?g/L.As(III)concentration reduces from 1.0 mg/L to 328 ?g/L,with a removal efficiency of 67%.Adsorption of As(III)and As(V)on the optimal modified natural magnetite follows the pseudo-second order adsorption kinetics,showing that the higher adsorption rate constant of As(III)than that of As(V),but the higher adsorption capacity of As(V)than that of As(III).Isothermal adsorption model shows that the As(III)and As(V)adsorption isn't in the form of monolayer adsorption.According to the value of n obtained from Freundlich isothermal adsorption model,both As(III)and As(V)adsorption on modified natural magnetite is favorable.At the initial pH values between 2 and 5,the modified natural magnetite shows weak adsorption property.At the initial and stable pH values between 6 and 9,it shows strong adsorption property.Competitive adsorption between phosphate,sulfate and As(III)is observed,while phosphate show competitive adsorption with As(V).Cations slightly increase adsorption capacity.After modification,the structure of the crystal surface of the magnetite changes.Bi-minerals being composed of active magnetite and iron oxides are formed as well,which increase the surface area,decrease the pore sizes,and increase the surface pore volume.Goethite and hematite are newly produced on the mineral surfaces.These characteristics greatly enhance the adsorption capacity of the modified material to arsenic.Therefore,modified natural magnetite can be used as a cheap and widely used environmental adsorbent for removing arsenic from groundwater.