| As essential source of Mn element for the nutrition of animals and plants,one of important and adsorbents,redox agents,catalysts and carriers for environmental information in soil and aquatic system,Mn oxide minerals have been attracting more and more concerns of researchers on their resource and environmental properties. Investigations on the syntheses,transformations and properties of them could shed light on geochemistry behaviors of Mn,soil Progress,and relations between soil quality and environments,they are also of great significance to promote exploration and utilization of Mn oxide minerals.As/Cr are naturally occurring toxic elements,widely known for their toxicity even at low concentrations.Developing efficient treatment and understanding As/Cr behavior in environment are therefore both major public health issues gaining increasing attention in recent years.Manganese oxide is known to oxidize elements with variable valences to impact and determine their soil environmental behavior,such as As and Cr.Mn oxide minerals are important component in soils.Investigation on the oxidization characteristics of their associations will provide some insights into predicting the chemical reactions and removal of As/Cr in soils,yet the detailed mechanism remains incompletely understood.For explaining the environmental behavior of As/Cr,it is significant to conduct extensive investigation of As(III)/Cr(III) oxidation by mixed valence manganese oxides to make clear of the dynamic process,reaction pathway, oxidation mechanism,the vital roles Mn(III) might play and its environmental impacts.In this paper scanning electron microscope(SEM),X-ray diffraction(XRD),X-ray Photoelectron Spectroscopy(XPS) and Batch kinetic method were used to determine the surface properties and kinetics of Cr(Ⅲ)/As(Ⅲ) oxidation by synthesized birnessites with various Mn average oxidation states(AOSs).The conclusions of the present study were listed as follows:1.Syntheses of Bir-OHs with various AOSs and characterization.XRD analyses demonstrated that the synthesized alkaline birnessites(OH-birnessites) had good characteristic diffraction peaks.AOS of Bir-OH1 is 4.02,it is average composition was K0.04Na0.02MnO2.04(H2O)1.22,and surface area of 76.14 m2·g-1.AOS of Bir-OH2 is 3.85,it is average composition was K0.02Na0.02MnO1.95(H2O)1.11,and surface area of 81.63 m2·g-1 AOS of Bir-OH3 is 3.70,it is average composition was K0.02Na0.02MnO1.87(H2O)1.04,and surface area of 79.38 m2·g-1.2.Determination of the zero point of charge(PZC) for Bir-OHs.PZC of the synthetic bimissite Bir-OH1 determined by fast potentiometeic titration(FPT) was 1.49.Because OH-bimessites are not stable in acid media,it is difficult to determine the accurate PZC of OH-birnessites with conventional methods.There were no point of interaction between ApH of synthetic birnissites and x-axis by salt titration(ST) method.The PZCs of the OH-birnessites are within a rough range of 1-2.In order to overcome the above problem, a modified ST method measured at the higher pH was used to determine the order in magnitude of PZCs for OH-birnessites with various AOSs.Results showed that PZCs increased with increase of AOS.Then prolonging the curve with the reverse direction or via the theoretical calculation,we might get the accurate PZC of OH-birnessites 3.Kinetic description of As(Ⅲ)/Cr(Ⅲ) oxidation by Bir-OHs.Under experimental conditions,oxidation of As(Ⅲ) and Cr(Ⅲ) in the initial stage of reaction complys with the pseudo first order kinetics.The reaction rate constants(Kobs) of the initial reaction stage increases with increase of AOS.The Kobs of As(Ⅲ) oxidation by the tested samples in order were:Bir-OH1(0.0951 min-1)＞Bir-OH2(0.0396 min-1)＞Bir-OH3(0.0071 min-1).Those of Cr(Ⅲ) oxidation were:Bir-OH1(0.0342 min-1)＞Bir-OH2(0.0178 min-1)＞Bir-OH3(0.0148 min-1).The Kobs also increased as the increase of AOS.They indicated that Mn(Ⅳ) and Mn(Ⅲ) have different oxidation kinetic characteristics,the AOS of OH-birnessites which is the ratio of Mn(Ⅳ) and Mn(Ⅲ) plays an important role in affecting oxidation kinetics.4.Comparison of kinetics of As(Ⅲ) and Cr(Ⅲ) oxidation by Bir-OHs.The results of XPS analyses showed that,Mn(Ⅱ) on the surface of OH-birnessites increased in amount during the reaction and Mn(Ⅳ) decreased with the reaction time.Great difference could be seen through the release of Mn(Ⅱ) by the two processes.In the beginning a part of Mn(Ⅱ) was tested in the process of Cr(Ⅲ) oxidation by OH-birnessites,the whole Mn(Ⅱ) was released to the solution later.In the initial stage,the release of Mn(Ⅱ) lagged behind the release of Cr(Ⅵ),more and more Mn(Ⅱ) were released to the solution as the reaction went on,the ratios of Mn(Ⅱ) to Cr(Ⅵ) were closed to the theoretical value of the reaction stoichiometry.While besides birnessite with Bir-OH3,nearly no Mn(Ⅱ) was detected in the whole process of As(Ⅲ) oxidation by OH-bimessites,Mn(Ⅱ) were adsorbed to the surface of OH-bimessites and then formed precipitate,preventing the reaction from going on.Initial stage of As(Ⅲ) oxidation by Bir-OHs complys with first order kinetics and then the Kobs decrease with the reaction time until it is close to zero,indicative of reverse reaction and surface precipitation.For Cr(Ⅲ) oxidation,Bir-OH1 exhibits the first order reaction in the whole reaction process.While Bir-OH2 and Bir-OH3 also possessed an decreasing Kobs during the reaction.However the rate of Kobs decrease is less than those for As(Ⅲ) oxidation.They implied that Cr(Ⅲ) and As(Ⅲ) oxidation have different kinetic processes on the surface of OH-bimessites.5.Comparison of As(Ⅲ)/Cr(Ⅲ) oxidation kinetics by Mn oxides with different structures. As(Ⅲ) oxidation by manganite(γ-MnOOH),only containing Mn(Ⅲ) with a tunnel structure,complys with a first order reaction,comprising several first order reactions with a decreading Kobs.In the initial reaction stage the Kobs is 0.0029 min-1.However even Bir-OH3,with the lowest AOS,exhibits the much higher Kobs(0.0133 min-1) than mangnite.Furthermore acidic birnessites(H-bimessites),being synthesized in acid media, with AOS of 4.00 exhibits much higher Kobs(0.0448 min-1) than Bir-OHs under the same reaction condition.Therefore,besides compositon of Mn(Ⅲ)/Mn(Ⅳ),the structural form of Mn(Ⅲ)/Mn(Ⅳ) can be considered as another important factor affecting the oxidation kinetics of Mn oxides.
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