Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtse River) Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China.
Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China.
Water Res. 2018 Jan 1;128:92-101. doi: 10.1016/j.watres.2017.10.045. Epub 2017 Oct 23.
Arsenic is a carcinogenic element that exists primarily as arsenate [As(V)] and arsenite [As(III)] in the nature environment, with As(III) being more toxic and mobile of the two species. In addition, ferrihydrite, which is widely distributed in soils and aquatic environments, can catalyze the oxidation of Mn(II) and accelerate the formation of high-valence Mn, which can significantly influence the speciation, toxicity, and mobility of As when these species co-exist. In this context, we herein explored the mechanism of As(III) oxidation in the presence of ferrihydrite and Mn(II) using a kinetic approach combined with multiple spectroscopic techniques, including X-ray absorption near edge spectroscopy, in situ horizontal attenuated total-reflectance Fourier transform infrared spectroscopy, and in situ quick scanning X-ray absorption spectroscopy. Our results indicate that efficient As(III) oxidation by dissolved O occurs on the surface of ferrihydrite in the presence of aqueous Mn(II). Compared with As(III) oxidation in the presence of ferrihydrite and Mn oxides (i.e., Mn oxides/hydroxides), the degree of As(III) oxidation in the ferrihydrite-Mn(II) system was significantly higher, and the majority of generated As(V) was adsorbed on the mineral (i.e., ferrihydrite) surface. Furthermore, As(III) oxidation was enhanced upon increasing both the molar ratio of Mn(II)/As(III) and the solution pH. The greater As(III) oxidation by O in the ferrihydrite-Mn(II) system was mainly attributed to the formation of a strong oxidant of the instantaneous intermediate Mn(III) species via Mn(II) oxidation under catalysis by the ferrihydrite surface. Moreover, As(III) oxidation occurred mainly on the ferrihydrite surface and was accompanied by the regeneration of Mn(II), thereby rendering it recyclable. These results therefore provide new insights into the mechanism of As(III) oxidation on the surfaces of Fe oxides (i.e., Fe oxides/hydroxides) in the presence of aqueous Mn(II) as well as the new details regarding the electron transfer mechanisms between the As(III)-Mn(II, III)-O species at the ferrihydrite surface, and could lead to novel approaches for As(III) contaminant remediation in the environment.
砷是一种致癌元素，主要以砷酸盐[As(V)]和亚砷酸盐[As(III)]的形式存在于自然环境中，其中 As(III)的毒性和迁移性比两种物质都要强。此外，广泛分布于土壤和水生环境中的水铁矿可以催化 Mn(II)的氧化，并加速高价态 Mn 的形成，这会显著影响这些物质共存时砷的形态、毒性和迁移性。在这种情况下，我们采用动力学方法结合多种光谱技术，包括 X 射线吸收近边光谱、原位水平衰减全反射傅里叶变换红外光谱和原位快速扫描 X 射线吸收光谱，研究了水铁矿和 Mn(II)存在时 As(III)的氧化机制。我们的结果表明，在 Mn(II)存在的情况下，溶解氧在水铁矿表面有效地氧化了 As(III)。与水铁矿和 Mn 氧化物（即 Mn 氧化物/氢氧化物）存在时的 As(III)氧化相比，在水铁矿-Mn(II)体系中，As(III)的氧化程度要高得多，生成的大部分 As(V)都被吸附在矿物（即水铁矿）表面。此外，随着 Mn(II)/As(III)摩尔比和溶液 pH 的增加，As(III)的氧化也得到了增强。在水铁矿-Mn(II)体系中，O 对 As(III)的更强氧化主要归因于 Mn(II)在水铁矿表面的催化作用下氧化生成强氧化剂瞬时中间态 Mn(III)物种。此外，As(III)氧化主要发生在水铁矿表面，同时伴随着 Mn(II)的再生，从而使其具有可循环性。这些结果为在含有水溶液 Mn(II)的情况下，Fe 氧化物（即 Fe 氧化物/氢氧化物）表面上 As(III)氧化的机制以及水铁矿表面上 As(III)-Mn(II, III)-O 物种之间电子转移机制的新细节提供了新的认识，并为环境中 As(III)污染物的修复提供了新的思路。
