| Arsenic is an important toxic substance in the environment.The movement of arsenic in soil,sediment and groundwater is controlled by the adsorption process of iron containing minerals,and natural organic matter can also affect the adsorption process of arsenic.When carrying out arsenic pollution control and treatment,the influence of different active functional groups of organic matter on the environmental behavior of arsenic has attracted more and more attention.In this paper,tannic acid（TA）containing phenolic hydroxyl group,p-benzoquinone（PBQ）containing carbonyl group,Dodecylbenzene sulfonic acid（DBSA）containing hydroxyl group and carboxymethyl cellulose（CMC）containing carboxyl group are selected as the representatives of organic matter（OM）,to explore the adsorption isotherm and adsorption kinetics of arsenate on goethite when different organic matter coexist,the influence mechanism of different organic matter on arsenate adsorption was discussed by Zeta potential and infrared spectrum.Humic acid is widely distributed in the water environment and is a complex macromolecular substance.In this paper,humic acid is divided into humic acid with different molecular weight of<500 Da,500-14000 Da and>14000 Da through dialysis experiment,and the effect of humic acid with different molecular weight on arsenic adsorption by goethite is studied.The mechanism of arsenic adsorption by goethite was explored by fitting the data with adsorption kinetic model and adsorption isothermal model;the surface of goethite after adsorption equilibrium was characterized by infrared spectroscopy and zeta potential;humic acid was characterized by three-dimensional fluorescence spectroscopy.The results are as follows:（1）The addition of carboxymethyl cellulose,tannic acid,p-benzoquinone and Dodecylbenzene sulfonic acid inhibited the adsorption of arsenate on goethite,and the adsorption of arsenate accorded with Freundlich adsorption isothermal model.When four kinds of organic matter were added,the adsorption of arsenate on the surface of goethite accorded with the quasi second-order adsorption kinetic model.Moreover,with the change of p H,organic matter containing different functional groups has different effects on the adsorption rate of arsenate.The arsenic adsorption rate constants of tannic acid,Dodecylbenzene sulfonic acid,carboxymethyl cellulose and p-benzoquinone systems were0.37,0.03,0.02 and 0.01 mg·g-1·h-1,respectively.Therefore,tannic acid can significantly increase the adsorption rate of arsenate(K=0.37 mg·g-1·h-1),and p-benzoquinone can effectively reduce the adsorption rate of arsenate(K=0.01 mg·g-1·h-1).This is because different organic matter has different effects on As（Ⅴ）adsorption under different p H values,which is related to the change of organic matter adsorption with p H from inner layer adsorption mode to outer layer adsorption mode.Zeta potential results show that ternary complexes can be formed between arsenate,tannic acid and goethite,and the results of infrared spectroscopy also prove the formation of binary complexes.（2）The addition of medium molecular weight humic acid promoted the adsorption of arsenate on goethite,low molecular weight humic acid inhibited the adsorption of arsenate,and high molecular weight humic acid had little effect on the adsorption of arsenate.Moreover,the adsorption of arsenate conforms to the Freundlich adsorption isothermal model.In the presence of humic acids with different molecular weights,the adsorption of arsenate on goethite conforms to the quasi second-order kinetic model.The results of different p H research show that goethite binds with arsenic most firmly in the presence of humic acid at p H=9.0.Compared with p H 9.0 and 11.0,humic acid systems with different molecular weights have no significant effect on the adsorption of arsenate by goethite at p H=7.0.The surface charge results show that the negative charge on the surface of goethite increases with the increase of the molecular weight of humic acid in the arsenate‐iron‐organic matter ternary system.The low molecular weight humic acid system mainly forms As（V）inner spherical surface complex and HA outer spherical surface complex.The results of infrared spectrum and three-dimensional fluorescence spectrum showed that iron‐organic matter complex and arsenate‐iron‐organic matter ternary complex were formed after adding humic acid.（3）The addition of humic acids with different molecular weights will reduce the adsorption amount of As（III）on goethite and inhibit the adsorption of As（III）,and the adsorption of arsenite conforms to Freundlich adsorption isothermal model.In the presence of humic acids with different molecular weights,the adsorption of arsenic on goethite surface conforms to the quasi second-order adsorption kinetic model.Among them,the arsenite adsorption rate constant of medium molecular weight humic acid system is large,the adsorption rate is fast,the time to reach the adsorption equilibrium is the shortest,and the binding ability with As（III）is strong.The zeta potential results show that after the addition of As（III）,the zeta potential of low and medium molecular weight humic acid system decreases,and the zeta potential of high molecular weight humic acid system increases.With the increase of molecular weight of humic acid,the negative charge on the surface of goethite increases.Low and medium molecular weight humic acid system mainly forms HA outer sphere surface complex.Infrared spectra showed that iron‐organic matter complex was formed after adding humic acid,and three-dimensional fluorescence spectra further confirmed the formation of arsenic‐iron‐organic matter ternary complex. |
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