Abstract:

Boron-doped diamond (BDD) electrodes have been widespread applied in the treatment of refractory organic pollutants for their advantages of efficient production of hydroxyl radical (·OH) and high stability. However, chlorate (ClO${}_{\mathrm{3}}^{-}$) and perchlorate (ClO${}_{\mathrm{4}}^{-}$), which are toxic by-products produced in the process of electrolysis of chlorine-containing medium, is one of the key obstacles to the application of BDD electrodes. In this work, we developed a method to use two-electron water oxidation reaction (2e^-WOR) assisted electrochemical oxidation with BDD to inhibit ClO${}_{\mathrm{3}}^{-}$ and ClO${}_{\mathrm{4}}^{-}$ formation in the water treatment, meanwhile, studied the degradation effects of atrazine (ATZ), which was one of the most popular herbicides as a model organic pollutant. The results indicated that the presence of NaHCO₃ effectively promoted the production of H₂O₂ by 2e^-WOR, which decreased the concentration of active chlorate (AC), ClO${}_{\mathrm{3}}^{-}$ and ClO${}_{\mathrm{4}}^{-}$. The concentration of AC was reduced by 60.3% in the presence of 10mmol/L NaHCO₃, the suppression rates of ClO${}_{\mathrm{3}}^{-}$ and ClO${}_{\mathrm{4}}^{-}$ were 10.2% and 39.2%. Adding 50mmol/L and 100mmol/L NaHCO₃ resulted in a decrease of 60.0% and 72.50% in the ClO${}_{\mathrm{3}}^{-}$ accumulation rate and 66.2% and 72.60% in the ClO${}_{\mathrm{4}}^{-}$ accumulation rate. The mechanism of suppression of ClO${}_{\mathrm{3}}^{-}$ and ClO${}_{\mathrm{4}}^{-}$ by 2e^-WOR was as followed. Firstly, H₂O₂ which was produced by 2e^-WOR decreased the concentration of the key intermediate—AC by reacting with it, which reduced the concentration of ClO${}_{\mathrm{3}}^{-}$ and ClO${}_{\mathrm{4}}^{-}$. Secondly, HCO${}_{\mathrm{3}}^{-}$ competed with ClO${}_{\mathrm{3}}^{-}$ for ·OH, then reduced the conversion rate of ClO${}_{\mathrm{3}}^{-}$ to ClO${}_{\mathrm{4}}^{-}$ . The appropriate concentration of NaHCO₃ was conducive to the degradation of ATZ. The addition of NaHCO₃ was improved by 27.2% and 53.8% in the presence of 10mmol/L and 50mmol/L NaHCO₃, Scavenging experiment indicated that the addition of NaHCO₃ was beneficial to the improvement of degradation of ATZ in the homogeneous solution.

Key words: electrochemistry, electrolysis, waste water, degradation, boron doped diamond electrode, chlorate, perchlorate

摘要：

掺硼金刚石（BDD）具有羟基自由基（·OH）产率高、稳定性好的优点，作为电极材料被广泛应用于难降解有机物的处理。但BDD电极在处理含氯介质时产生的高毒性副产物——氯酸盐（ClO${}_{\mathrm{3}}^{-}$）和高氯酸盐（ClO${}_{\mathrm{4}}^{-}$）是限制其广泛应用的关键障碍之一。本文通过两电子水氧化反应（2e^-WOR）来控制BDD电极氧化过程中ClO${}_{\mathrm{3}}^{-}$和ClO${}_{\mathrm{4}}^{-}$的产生。同时，以一种广泛使用的除草剂——阿特拉津（ATZ）为模型污染物，探究2e^-WOR对有机物降解效果的影响。研究表明，NaHCO₃作为电解质可以有效催化BDD电极上的2e^-WOR，进而降低电解过程中活性氯、ClO${}_{\mathrm{3}}^{-}$和ClO${}_{\mathrm{4}}^{-}$的浓度。添加10mmol/L NaHCO₃后，活性氯浓度下降了60.3%，ClO${}_{\mathrm{3}}^{-}$和ClO${}_{\mathrm{4}}^{-}$的生成速率常数分别下降10.2%和39.2%。当添加50mmol/L和100mmol/L NaHCO₃时，活性氯的生成被完全抑制，ClO${}_{\mathrm{3}}^{-}$的生成速率常数分别降低了60.0%和72.5%，ClO${}_{\mathrm{4}}^{-}$的生成速率常数分别降低了66.2%和72.6%。2e^-WOR对ClO${}_{\mathrm{3}}^{-}$和ClO₄^-抑制作用的机理有两个方面：①2e^-WOR产生的H₂O₂与活性氯反应，降低了关键中间物的浓度，进而控制了ClO${}_{\mathrm{3}}^{-}$和ClO${}_{\mathrm{4}}^{-}$的生成；②HCO${}_{\mathrm{3}}^{-}$与ClO${}_{\mathrm{3}}^{-}$竞争·OH反应，降低了ClO${}_{\mathrm{3}}^{-}$向ClO${}_{\mathrm{4}}^{-}$的转化速率。研究发现，适量添加NaHCO₃有利于ATZ的降解。添加10mmol/L和50mmol/L NaHCO₃后，ATZ的降解动力学常数分别提高了27.2%和53.8%。甲醇猝灭实验表明，NaHCO₃的添加促进了溶液本体中ATZ的降解。

关键词: 电化学, 电解, 废水, 降解, 掺硼金刚石电极, 氯酸盐, 高氯酸盐