Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China; University of Chinese Academy of Sciences, Beijing, 100049, China.
Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China.
Water Res. 2019 Jun 1;156:287-296. doi: 10.1016/j.watres.2019.03.027. Epub 2019 Mar 20.
Chlorine is not effective in the oxidative removal of soluble manganese(II) ions at neutral pH. Powdered activated carbon (PAC) also has a very limited capacity for Mn(II) removal through adsorption in drinking water treatment practice. This study explored the combined use of PAC and chlorine for Mn(II) removal and found that PAC dramatically catalyzed Mn(II) oxidation by chlorine under diverse conditions. At a dose as low as 5.0 mg/L, two different commercial PACs increased Mn(II) oxidation rate by two orders of magnitude respectively and reduced Mn(II) concentration from 200 μg/L to < 10 μg/L in tens of minutes. First-order kinetics with respect to aqueous Mn(II) concentration were observed. Typically, homogeneous Mn(II) oxidation by chlorine depends strongly on alkaline pH. In the presence of PAC, however, the reaction was still rather fast at pH 6.0. Increasing PAC doses linearly increased Mn(II) oxidation rate, indicating that the reaction was highly PAC surface active sites dependent. The efficacy of PAC was further corroborated in removing Mn(II) from natural water. SEM-EDS and XPS demonstrated that a MnO coating was formed on PAC surface after reaction, which resulted from heterogeneous oxidation of Mn(II) on PAC surface rather than the precipitation of Mn oxides formed through homogeneous oxidation in solution. Adsorption of free Mn(II) ions onto PAC surface was proved to directly correlate with Mn(II) oxidation rate. Two kinds of electron transfer pathways from adsorbed Mn(II) species to chlorine, enhanced by surface-complexation and electrically-conductive carbon surface respectively, were hypothesized.
氯在中性 pH 条件下对于去除可溶性二价锰离子的氧化作用效果不佳。在饮用水处理实践中，粉末状活性炭（PAC）通过吸附去除 Mn(II)的能力也非常有限。本研究探索了 PAC 和氯联合使用去除 Mn(II)的方法，发现 PAC 在多种条件下都能显著催化氯对 Mn(II)的氧化作用。在 5.0 mg/L 的低剂量下，两种不同的商业 PAC 分别将 Mn(II)氧化速率提高了两个数量级，在数十分钟内将 Mn(II)浓度从 200 μg/L 降低至 <10 μg/L。观察到对水相 Mn(II)浓度的一级动力学反应。通常情况下，氯对 Mn(II)的均相氧化作用强烈依赖于碱性 pH。然而，在 PAC 的存在下，该反应在 pH 6.0 时仍然相当快。增加 PAC 剂量可线性提高 Mn(II)氧化速率，表明该反应高度依赖于 PAC 表面活性位点。PAC 去除天然水中 Mn(II)的功效也得到了进一步证实。SEM-EDS 和 XPS 表明，反应后 PAC 表面形成了一层 MnO 涂层，这是由于 Mn(II)在 PAC 表面的非均相氧化，而不是通过溶液中均相氧化形成的 Mn 氧化物沉淀所致。证明了自由 Mn(II)离子在 PAC 表面的吸附与 Mn(II)氧化速率直接相关。推测了两种电子转移途径，分别是吸附的 Mn(II)物种通过表面络合和导电碳表面向氯转移。
