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环境化学

The adsorption of arsenic by ferric hydroxide and its precipitation mechanism

基金项目：国家自然科学基金(No.40773059);广西自然科学基金(桂科自0728222);广西高校人才小高地建设创新团队资助计划项目(桂教人[2007]71号);广西环境工程与保护评价重点实验室研究基金(桂科能0701K007)

刘辉利
1. 南京大学环境学院, 污染控制与资源化国家重点实验室, 南京 210093; 2. 桂林工学院资源与环境工程系, 广西环境工程与保护评价重点实验室, 桂林 541004

摘要：研究了吸附反应时间为50d和1120d时,As(V)初始浓度和pH值对氢氧化铁吸附砷的影响,并利用傅利叶红外光谱(FTIR)和粉末X射线衍射技术对吸附砷后的氢氧化铁固体进行了表征.结果表明,氢氧化铁对砷的吸附能力与pH有关,在弱酸性到弱碱性条件下,吸附砷的能力最强;在低初始砷浓度(0.01～1mmol·L^-1)和相同pH条件下,吸附率随砷浓度增高而增大.当pH=3、7和12时,吸附等温线都可用Freundlich公式来进行拟合,低初始砷浓度下(0.001～1mmo.lL^-1),R²>0.99,高初始砷浓度下(5～1000mmol·L^-1),R²>0.93,吸附反应时间对吸附能力影响不大.红外光谱分析表明,在pH=3条件下,初始砷浓度为50mmol·L^-1和500mmol·L^-1时,吸附后氢氧化铁固体的红外光谱图上As—O键的伸缩振动谱带分别位于806.11cm^-1和821.54cm^-1;表明吸附后的氢氧化铁表面有少量的砷酸铁晶体沉淀存在,砷在氢氧化铁表面的平均密度对砷在氢氧化铁表面存在形态有影响.X射线衍射分析表明,在中性和酸性条件下,砷可能是以双配位表面络合的质子化的FeO₂As(O)(OH)^-和非质子化的≡FeO₂As(O)^2-形态存在于氢氧化铁表面.

Abstract：The effect of solution pHand initial concentration of arsenic on the adsorption of As(V)onto ferric hydroxide was studied for 50 d and 1120 d.Fourier transformed infrared spectroscopy(FT-IR)and X-ray diffraction(XRD)were used to characterize the ferric hydroxide solids before and after sorption at pH3,7 and 12.The largest sorption capacity was observed at neutral pH.The adsorption rate increased with initial arsenic concentration between 0.01 and 1 mmol·L^-1.The adsorption isotherm at pH3,7 and 12 fit the Freundlich model well with R²>0.99 at lower initial arsenic concentrations and R²>0.93 at higher initial arsenic concentrations.The sorption capacity was not significantly influenced by sorption time.FT-IRanalysis indicated that the major As—Ostretching vibration was located at 806.11 cm^-1 and 821.54 cm^-1 for initial arsenic concentrations of 50 mmol·L^-1 and 500 mmol·L^-1 at pH_3.This means that some poorly crystalline ferric arsenate formed on the surface of the ferric hydroxide after sorption.The arsenate species of the sorbent were affected by the arsenic sorption density.XRDanalysis confirmed that arsenic(V)was adsorbed as a bidentate surface complex in both protonated ≡FeO₂As(O)(OH)^-and unprotonated ≡FeO₂As(O)^2-forms in neutral and alkaline media(pH₇,12).