Study On Adsorption Behavior And Mechanism Of Arsenic On Ti-pillared Montmorillonite

Arsenic as the toxic contaminants of the environment especially high levels of arsenic in water has been a worldwide problem. Long-term drinking water with high levels of arsenic may damage to human health. Development of high performance adsorbents stems form deep udstanding of adsorption mechanism.Adsoption mechanism of anion such as arsenic on mineral, sediment and soil is always a worldwide research facus. Adsorption of arsenic on Ti-pillared montmorillonite is the main content of this paper.Based on the adsorbent developed by the predecessors of our team, the effects of pH value, temperature, coexisting ions, ion strength on adsorption of arsenite and arsenate onto Ti-pillared montmorillonite are investigated. The adsorption kinetic of arsenite and arsenate show the adsorption equilibrium can be reached in not long time. The pseudo-second-order chemical reaction model provided good correlation of dynamics data for both arsenate and arsenite. It indicates the adsorption process might be chemisorption. The adsorption isotherm of arsenate and arsenite fits Langmuir and Freundlich adsorption isotherm model well. The optimal adsorption was obtained in the pH range 7-8 for arsenite and 3 for arsenate. As the temperatue increases the arenite adsorbed decrease, the negative values of△G,△H,△S indicate that the adsorption of arsenite onto Ti-pillared montmorillonite is spontaneous and exothermic process; the adsorption of arenate shows the same trdency with arsenite in the temperatue range of 25-35℃, while the trdency is opposite in 45-65℃, the adsorption in 45-65℃is spontaneous and endothermic process. The existence of PO43- has a noticeable effect on the adsorption of arsenate, coexisting ions have not noticeable effect on the adsorption of arsenite. The adsorption efficiency of Ti-pillared montmorillonite for arsenite and arsenate are insignificantly affected when the concentration of NaNO3 changed from 0.02 to 0.1M. It suggests the formation of inner-sphere complexation. UV light can improve arsenite adsorprtion but has no impact on arsenate adsorption.Contrasting the adsorbent and the adsorbent with arsenite and arsenate adsorbed using IR, XRD, EPR, and XPS method to investigate the adsorpotion mechanism from microcopic field. The analysis of XRD pattern show that the interlayer space and order degree of layers decrease after adsorption, due to the reaction of arenite and arsenate with active hydroxyl. Analysis of XPS and IR show that the adsorption occurs not only in the interlayer but also on the surface of adsorbent. On the adsorbent surface, arsenite and arsenate complex with surface hydroxyl. EPR analysis show that the adsorption of arsenite and arsenate results in the increase of Fe3+ isomorphism replacement amount in octahedral sheet. It suggests that the arsenic enters into the interlayer and the octahedral sheets.