Study On The Regularity And Mechanism Of The Interaction Between Thallium(Ⅰ) And Typical Metal Oxides

Thallium（Tl）is a highly toxic heavy metal.In recent years,due to the smelting of Tl-bearing minerals and the combustion of fossil fuels,the release of Tl into the environment has led to the increase of Tl pollution in aqueous water.In the 13th five-year plan for ecological environment protection issued by the State Council,it is clearly stipulated that the investigation of Tl pollution emission in typical areas should be organized and the prevention and control scheme of Tl pollution should be formulated.At present,compared with other toxic heavy metals such as Hg,Cr and Pb,the research on the removal technology and mobility behavior of Tl in aqueous water is less,and the basic data obtained is limited.Adsorption has been proved to be an effective method to remove Tl.As a major category of adsorbents,metal oxides are abundant and widely distributed,and play an important role in controlling the bioavailability of Tl in environments.In this paper,based on the existing technology of Tl adsorption,the differences of crystal structure in the process of Tl（I）adsorption and oxidation by manganese dioxide（Mn O₂）in a wide range of p H are investigated.Moreover,the common metal oxides in nature,such as manganite,goethite and rutile,are selected to explore the influence of p H,I and other aqueous conditions on their adsorption of Tl（I）.Surface complexation model is used to predict the adsorption process of Tl（I）under different aqueous conditions.The basic data obtained in this study are helpful to guide the optimization of Tl（I）removal in the wastewater and predict the mineral-dominated Tl（I）mobility behavior in environments.The adsorption and oxidation of Tl（I）by three crystal structured Mn O₂（α-,β-andλ-Mn O₂）over a wide p H range（5,7 and 12）were investigated.The Tl（I）adsorption isotherms of there Mn O₂ were fitted to Freundlich model.The results showed that the uptake of Tl followed the order ofα-Mn O₂>β-Mn O₂>λ-Mn O₂,and the increase of p H was beneficial to the removal of Tl（I）.Furthermore,XPS results showed thatα-Mn O₂ had the highest Tl（I）oxidation ability at all p H values,which might be related to the large tunneled structures and high content of Mn（III）.In contrast,Tl（I）was scarcely oxidized byβ-Mn O₂ because of its small tunneled structures.However,forλ-Mn O₂,the extent of Tl（I）oxidation byλ-Mn O2 at p H=12 was much higher than that of p H=5 or 7,which is due to the active species Tl OH involved in the redox process.Besides,XRD results showed that Tl（I）is oxidized to T_l2O₃,and the reflection of diffraction peaks of Tl₂O₃ was also consistent with the rule of oxidation of Tl（I）by Mn O₂ with different crystal forms obtained by XPS.In addition,the adsorption of Tl（I）on rutile,goethite and manganite reached equilibrium within 10 minutes.However,Tl（I）adsorption capacities of these three metal oxides were much lower than that of Mn O₂.High p H was more conducive to the adsorption of Tl（I）,and the increase of ionic strength inhibited the uptake of Tl（I）.By adding CO₃^2-in the adsorption experiment,it was proved that CO₂ in air had no effect on the adsorption process.In addition,XPS results showed that the redox reaction degree of the metal oxides with Tl（I）was negligible in the adsorption process.By assuming three complexation ways,≡SWOTl,≡SWOTl OH^-and≡SWOHTl⁺（≡SWOH represents chemical formula of the metal oxides）,the process of Tl（I）adsorption by the three metal oxides under different aqueous conditions（p H,initial Tl（I）concentration）was successfully simulated by using the diffuse two-layer model.The model parameters,including the density of hydroxyl sites on the surface of three metal oxides and the surface complexation constants with Tl（I）,were obtained.It can be extended to predict the contribution of different complexes to the Tl（I）adsorption at equilibrium,and then to know the distribution of Tl（I）in the solid-liquid phase.