State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, No. 68 Jincheng Street, East Lake High-Tech Development Zone, Wuhan 430078, P. R. China.
Hubei Key Laboratory of Yangze Catchment Environmental Aquatic Science, School of Environmental Studies, China University of Geosciences, No. 68 Jincheng Street, East Lake High-Tech Development Zone, Wuhan 430078, P.R. China.
Environ Sci Technol. 2021 Oct 5;55(19):13366-13375. doi: 10.1021/acs.est.1c02461. Epub 2021 Sep 22.
Hydroxyl radical (•OH) production by electron transfer from Fe(II)-bearing clay minerals to oxygen has been increasingly reported. However, the influence of ubiquitous coexisting humic acid (HA) on this process is poorly understood. Here, we investigated the effect of different HA on •OH production during the oxygenation of reduced nontronite NAu-2 (rNAu-2), montmorillonite, and sediment. Results showed that HA could enhance •OH production, and the enhancement was related to the content of reactive Fe(II) in rNAu-2 and the electron-accepting capacity of HA. Coexisting HA leads to a new electron-transfer pathway from Fe(II) in rNAu-2 to HA (instead of the HA-Fe complex) and then to O, changing the first step of O reduction from one- to two-electron transfer process with HO as the main intermediate. Reduced HA decomposes HO to •OH at a higher yield (13.8%) than rNAu-2 (8.8%). Modeling results reveal that the HA-mediated electron-transfer pathway contributes to 12.6-70.2% of HO generation and 13.2-62.1% of •OH formation from HO decomposition, with larger contributions at higher HA concentrations (5-100 mg C/L). Our findings implicate that HA-mediated electron transfer can expand the area of •OH production from the mineral surface to the aqueous phase and increase the yield of •OH production.
羟基自由基（•OH）可通过铁（II）载粘土矿物向氧的电子转移产生，这一现象已被越来越多地报道。然而，普遍存在的腐殖酸（HA）对此过程的影响还知之甚少。在这里，我们研究了不同 HA 对还原钠蒙脱石（rNAu-2）、蒙脱石和沉积物中氧还原过程中•OH 产生的影响。结果表明，HA 可以增强•OH 的产生，这种增强与 rNAu-2 中反应性 Fe（II）的含量和 HA 的电子接受能力有关。共存的 HA 导致了一个新的电子转移途径，即 rNAu-2 中的 Fe（II）与 HA（而不是 HA-Fe 配合物）然后与 O 发生反应，从而将 O 还原的第一步从单电子转移过程改变为两电子转移过程，HO 是主要的中间体。还原的 HA 将 HO 分解为•OH 的产率（13.8%）高于 rNAu-2（8.8%）。模型结果表明，HA 介导的电子转移途径对 HO 生成的贡献为 12.6-70.2%，对 HO 分解生成•OH 的贡献为 13.2-62.1%，在较高的 HA 浓度（5-100mgC/L）下贡献更大。我们的研究结果表明，HA 介导的电子转移可以将•OH 产生的区域从矿物表面扩展到水相，并提高•OH 产生的产率。
