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The P adsorption-desorption characteristics on ferrihydrite and crystalline Fe oxides suspension

WANG Xiao-ming,SUN Shi-fa,LIU Fan,TAN Wen-feng,HU Hong-qing and FENG Xiong-han(Key Laboratory of Subtropical Agriculture Resource and Environment,Ministry of Agriculture,College of Resources and Environment,Huazhong Agricultural University,Wuhan 430070,China)  
The retention and release of P onto iron oxides influence and determine the bioavailability and eutrophication of P.We study the adsorption-desorption characteristics of P onto ferrihydrite and explore the relevant mechanisms compared with two common crystalline Fe oxides(goethite and hematite),using the techniques of X-ray diffraction(XRD),transmission electron microscopy(TEM),thermal gravimetric analysis(TGA),porosity structure analysis and the methods of kinetics and adsorption-desorption equilibrium.Results show a fast initial adsorption took place in a few minutes followed by a slow reaction process,both of which can be well fitted by first order kinetics.During the reaction,the release of OH– lags behind P adsorption and increases step by step,indicating that P adsorption may go through complexation from outer-sphere to inner-sphere,from monodentate to bidentate coordination.Compared with goethite and hematite,ferrihydrite has higher amount of P adsorption and OH– release,faster slow reaction process,and exhibits slow diffusion reaction stage.The maximum adsorption capacities are in order of: ferrihydrite(4.36 μmol/m2) goethite(2.62 μmol/m2) hematite(2.28 μmol/m2);adsorption isotherms of goethite and hematite agree well with Langmuir model while ferrihydrite exhibits an adsorption isotherm of Freundlich type.Desorption percentage of adsorbed P by neutral electrolyte at maximum adsorption capacities is in order of: ferrihydrite(8.5%) goethite(10%) hematite(12.5%).Relatively,citric acid significantly promotes the desorption reaction.Desorption percentages at the maximum adsorption capacity are in order of: goethite(25%) ferrihydrite(32%) hematite(50%).
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