Adsorption And Oxidation Of Arsenic By Iron Oxide And Manganese Oxide

It may be well known that iron and manganese are few but familiarly variable valenceelements. Natural oxides of iron and manganese with surface charges and variable valenceelements have favorable surface activities. They have strong ability of adsorption anddecontaminate for heavy metals in soils. In this study,birnessite (δ-MnO2) and ferrihydritewere synthesized and using contemporary analytical machines such as X-ray diffraction,scanning electron microscopy(SEM), potentiometric titration to determine their materialphases,appearances and superficial electron character. Then,the removal and oxidationefficiency of arsenate and arsenite by birnessite (δ-MnO2) and ferrihydrite and the factorsinfluencing the efficiency, such as adsorbent dose,treating time,pH,initial asenicconcentration,ionic strength and temperature,etc.were investigated thoroughly.The mainresults were list as follows:The reaction between the birnessite and arsenite is complicated. The results showed thatarsenite transformation occurred and was accompanied by the adsorption and fixation of bothAs(III) and As(V) on δ-MnO2. About90%of arsenite(1.0mg/L) were transformed by δ-MnO2under the conditions of25℃a nd pH6.0,49%of which was adsorbed and10%fixed byδ-MnO2. Increased δ-MnO2dosages promoted As(III) transformation rate and adsorption ofarsenic species.The experimental results at pH6indicate that the depletion of As(Ⅲ) from solution israpid,with a time scale of minutes. The oxidation product As(V) was released almost asquickly,while the release of the reduction product Mn(II) was slightly slower. Under differentinitial As(Ⅲ) concentrations of0.5-15mg.L-1,the removal rate of arsenite can reach93.59,92.56,92.31,89.15and87.17%,respectively.The effect of pH on As(Ⅲ) oxidation byδ-MnO2is obvious.When in a low pH,thetransformation rate decreased with increasing pH values of reaction solution,and theminimum amounts of As(Ⅲ) oxidized occurred. But after that,the transformation rateincreased with increasing pH values. The oxidation rate decreased and adsorbed As(III) andAs(V) increased with increasing initial arsenite concentration. High pH favored the adsorptionof Mn(II),but disfavored the oxidation of As(Ⅲ) and adsorption of As(V)．Temperature is a main factor influencing the As(Ⅲ)adsorption and oxidation by birnessite.Results suggest that high temperature favored adsorption,but disfavored oxidation．Increasing ion strength could also inhibit the oxidation process of As(Ⅲ) by manganese oxide.The remoal rate of As(Ⅲ) and As(Ⅴ) were rised as the ferrihydrite dose increasing. At thesame dose,the lower the asenic cincentration,the removal rate is higher.The As(Ⅲ)adsorption kinetics can be better fitted by parabolic diffusion equation,the As(Ⅲ) adsorptionby ferrihydrite is inclined to the mechanism of diffusion layer mode. For As(Ⅴ),theadsorption kinetics can be better fitted by two-constant equation and Elovich equation,but theparabolic diffusion equation and one-level kinetic equation are not appropriate.At different pH, the adsorption amount of As(Ⅲ) and As(Ⅴ) decreased with theferrihydrite dose increasing but increased with the asenic concentration increasing. The effectof pH on As(Ⅴ) removal is more obvious at lower adsorbent dose and higher As(Ⅴ)concentration. At the whole pH range,the adsorption amount of As(Ⅲ) was constant. As toAs(Ⅴ),the change of adsorption amount was not obvious when pH<7but sharply decreasedwhen pH>7. The adsorption of As(Ⅴ) is favorable at acidic or neutral medium,butdisadvantaged in alkaline medium.The Langmuir equation fitted the As(Ⅲ) adsorption isotherms data by ferrihydrite betterthan the Freundlich equation. In acidic or neutral medium,the As(Ⅴ) isotherms data can bebetter fitted by Langmuir equation,but less better in alkaline medium. In the whole,theadsorption mechanism of As(Ⅲ) and As(Ⅴ) by ferrihydrite at different pH can be describedby using Langmuir equation. The maximum adsorption amount of As(Ⅲ) by ferrihydrite canbe abained at pH6,but at pH7for As(Ⅴ). The maximum adsorption amount of As(Ⅲ) andAs(Ⅴ) were45.67mg/g and45.67mg/g, respectively.At the lower initial concentration of As(Ⅲ),when ferrihydrite was present in the system ofredox reaction,the amounts of oxidized As(Ⅲ) were higher that of the single birnessitesystem. But ferrihydrite itself could not oxidize As(Ⅲ). The increase in oxidizing capacitiesresulted from strong adsorption of the produced As(Ⅴ) on ferrihydrite.And at the lower pH,the promoting effect of As (Ⅲ) oxidation is more apparent.At the higher initial concentrationof As(Ⅲ),the specific adsorption of ferrihydrite is dominant,resulting in the promotingfunction of As (Ⅲ) oxidation weakened.