碘离子对次氯酸氧化一价铊动力学影响及机理研究

Effects and Mechanisms of Iodide Ion on Kinetics of Tl (Ⅰ) Oxidation by Hypochlorite

铊(Tl)是一种在环境水体中广泛存在的痕量重金属,由于其对生物体毒性巨大,国家和地方高度重视铊污染防治,并将其作为优先监测的重金属污染物之一。氧化法是一种常用并且有效的除铊方法,该方法通过将一价铊(Tl(Ⅰ))预氧化为更容易沉积的三价铊(Tl(Ⅲ))来实现总铊的去除,但容易受到水体中卤素离子的影响。本研究以水处理中常用的次氯酸钠(NaOCl)作为典型氧化剂,通过采用ABTS分光光度法检测生成Tl(Ⅲ)的浓度变化,考察了环境中常见的卤素碘离子对Tl(Ⅰ)在这种典型氧化过程动力学中的影响,并结合动力学模型对其机理进行了探究。结果表明,在HOCl氧化过程中,碘离子的加入在pH为6.0、7.0和8.0时均表现出了促进Tl(Ⅰ)氧化的现象,其中碘离子浓度在高于100μmol/L时才有明显的催化氧化作用,且在100~1 000μmol/L浓度范围内反应速率随着碘离子浓度升高氧化速率加快。在酸性和中性条件下500μmol/L碘离子催化HOCl氧化Tl(Ⅰ)符合假一级动力学过程,且随着pH的升高反应速率常数逐渐降低。推测碘离子的催化氧化作用为碘离子被次氯酸氧化后,产生具有更强氧化性的次碘酸进而继续氧化Tl(Ⅰ)。本研究探究了在次氯酸氧化Tl(Ⅰ)过程中,以碘离子为代表的卤素离子的存在对氧化过程的影响,突出了碘离子在影响Tl(Ⅰ)氧化动力学中的关键作用。这对于完善铊的氧化还原性质以及评估含卤水中铊的去除策略,保障饮用水安全具有重要意义。

Thallium(Tl) is a trace heavy metal widely existing in environmental water. Due to its great toxicity to organisms, the country and local governments attach great importance to the prevention and control of Tl pollution, and regard it as one of the heavy metal pollutants monitored first. Oxidation is a common and effective method for Tl removal in waste water. This method can remove total Tl by pre-oxidizing Tl(Ⅰ) to Tl(Ⅲ), which is more easily deposited. But it is susceptible to the influence of halogen ions in water. In this study, sodium hypochlorite(NaOCl), which is commonly used in water treatment, was used as a typical oxidant. ABTS spectrophotometry was used to detect the concentration changes of Tl(Ⅲ), the effects of common halogen iodide ions in the environment on the kinetics of Tl(Ⅰ) in the typical oxidation process were investigated, and the mechanism was explored in combination with the kinetic model. The results showed that the addition of iodide ions promoted the oxidation of Tl(Ⅰ) at pH 6.0, 7.0 and 8.0 in HOCl oxidation process, and the catalytic oxidation of Tl(Ⅰ) was obvious when the concentration of iodide ions was higher than 100 μmol/L. The oxidation rate increases with the increase of iodide ion concentration in the range of 100 μmol/L-1 000 μmol/L. Tl(Ⅰ) oxidation of HOCl catalyzed by 500 μmol/L iodide ions under acidic and neutral conditions accords with pseudo first order kinetics, and the reaction rate constant decreases gradually with the increase of pH. It is speculated that the catalytic oxidation of iodide ion is as follows: iodide ion is oxidized by oxidant, which generates hypiodate acid with strong oxidizing ability and then continues to oxidize Tl(Ⅰ). In this study, the effect of the presence of halogen ions represented by iodide ions on the oxidation of Tl(Ⅰ) by hypochlorite was investigated, the results highlighted the key role of iodide ion in influencing the oxidation kinetics of Tl(Ⅰ), which is of great significance for perfecting the redox properties of thallium and evaluating the removal strategy of thallium in brine-containing water to ensure the safety of drinking water.