| Thallium(Tl) is a highly toxic element. In the natural water, its concentration isin the range of5-20ng/L,which can reach1?g/L in polluted water. Thallium has twovalence, Tl(I) and Tl (III). Compared to Tl(I), Tl (III) is harder to migrate in theenvironment and more toxic. In the water treatment process, KMnO4and HOCl,.areusually used as preoxidants. In view of completely different nature between Tl(I)and Tl (III), this paper studies the chemical reaction kinetics of Tl(I) oxidized byKMnO4and HOCl, and and focuses on the mechanism of MnO2catalytic reaction;providing an important theoretical basis for its removal by pre-oxidation inengineering applications.In this paper, ChemEQL3.0is used to analog different species of thallium inwater. When pH is less than13, Tl+is the dominating form for Tl(I) in freshwater.Tl(III) has a low solubility, and the main forms are hydroxyl complexes withdifferent coordination number. Some organic ligands can complex Tl(III), whichwill improve its solubility in water.ABTS Difference Method is established for research on Tl(I) oxidation byKMnO4. Changes with pH, the reaction exhibits different characteristics. Underacidic conditions, apparent autocatalytic reaction phenomenon is obserbed, whichcan be divided into slow homogeneous process and fast heterogeneous process.Autocatalytic model established shows good fit, homogeneous reaction rates are5.30M-1s-1,4.56M-1s-1,0.71M-1s-1for pH4, pH5, pH6, respectively, whileheterogeneous reaction rates are14.44M-1s-1,22.49M-1s-1,20.59M-1s-1,respectively. Under alkaline conditions, KMnO4can oxidize Tl(I) quickly, and therate increases with increasing pH, the apparent rate constants are33.5M-1s-1?164.3M-1s-1and895.7M-1s-1for pH8, pH9and pH10, respectively. TlOH displays higherreactivity than Tl+, resulting in higher reaction rate at higher pH condition.An ABTS Direct Method measuring Tl(III) concentration is established tostudy Tl(I) oxidation by other oxidants. In the range of pH7-9, oxidation rate of Tl(I)by hypochlorous acid decrease with increasing pH. The reaction mechanism is verycomplex and can not be described with the concept of reaction order. Under thecondition using10mM HOCl the half-lives of Tl(I) is13.3min,51.9min and109.2min for pH7, pH8and pH9,respectively.. It was confirmed by simulationsfitting that the reactive species is protonated HOCl.The oxidation of Tl(I) with Br-can promoted by hypochlorous acid. It isconfirmed that Br-is the catalyst in this reaction. Br-is rapidly oxidized to active Br species by hypochlorous acid. The quickly Br-catalyzed oxidation withhypochlorous acid can't be explained with the rate of Tl(I) oxidation by HOBr. Inaddition HOBr, other Br active species play a much more important role. In theactual water, Tl(I) oxidation rate becomes faster. The rate of Tl(I) oxidation, beforeand after treatmeng by anion exchange column, don't change. The added HA canpromote the oxidation rate. These all prove that the organic matter can accelerateTl(I) oxidation.MnO2play a catalytic role in Tl(I) oxidation with both HOCl and KMnO4. Ni2+,Cd2+and Zn2+can inhibit MnO2catalysis, because Ni2+, Cd2+and Zn2+can competethe adsorption sites with Tl(I). Therefore, the catalytic reaction can be described byadsorption-oxidation mechanism: firstly, Tl(I) is adsorbed on the surface of MnO2;and then adsorbed Tl(I) reacte quickly with oxidants to generate Tl(III). All themetals, Tl(I), Ni2+, Cd2+and Zn2+, share the vacant sites in MnO2, indicating that thecatalytic active sites are the vacant sites in MnO2. |
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