Chemistry Department, Wilfrid Laurier University, Waterloo, ON N2L 3C5, Canada.
J Phys Chem A. 2012 Feb 16;116(6):1596-604. doi: 10.1021/jp210093n. Epub 2012 Feb 6.
Dimethylarsinic acid (DMA) is an organoarsenical compound that, along with monomethylarsonic acid, poses a health and an environmental risk, and a challenge to the energy industry. Little is known about the surface chemistry of DMA at the molecular level with materials relevant to geochemical environments and industrial sectors. We report herein the first in situ and surface-sensitive rapid kinetic studies on the adsorption and desorption of DMA to/from hematite and goethite at pH 7 and I = 0.01 M KCl using ATR-FTIR. Values for the apparent rates of adsorption and desorption were extracted from experimental data as a function of spectral components, flow rate of the aqueous phase, film thickness of hematite, and using chloride and hydrogen phosphate as desorbing agents. The adsorption kinetic data show fast and slow rates, consistent with the formation of more than one type of adsorbed DMA. Apparent adsorption and desorption rate constants were extracted from the dependency of the initial adsorption rates on [DMA(aq)]. Desorption rate constants were also extracted from desorption experiments using hydrogen phosphate and chloride solutions, and were found to be higher by 1-2 orders of magnitude than those using chloride. In light of the complex ligand exchange reaction mechanism of DMA desorption by phosphate species at pH 7, apparent desorption rate constants were found to depend on [hydrogen phosphate] with an order of 0.3. The impact of our studies on the environmental fate of DMA in geochemical environments, and the design of technologies to reduce arsenic content in fuels is discussed.
二甲砷酸（DMA）是一种有机砷化合物，与一甲基砷酸一起，对健康和环境构成风险，也是能源行业面临的挑战。对于在与地球化学环境和工业部门相关的材料中，DMA 的分子水平表面化学性质，人们知之甚少。本文报道了在 pH 值为 7 和 I = 0.01 M KCl 条件下，使用 ATR-FTIR，首次在原位和表面敏感的情况下研究 DMA 在赤铁矿和针铁矿上的吸附和解吸的快速动力学。通过实验数据作为光谱分量、水相流速、赤铁矿膜厚度的函数，以及使用氯化物和磷酸氢盐作为洗脱剂，从实验数据中提取出表观吸附和解吸速率的值。吸附动力学数据显示出快速和慢速速率，与形成一种以上类型的吸附 DMA 一致。从初始吸附速率对[DMA(aq)]的依赖性中提取出表观吸附和解吸速率常数。还从使用磷酸氢盐和氯化物溶液进行的解吸实验中提取了解吸速率常数，发现使用磷酸氢盐的解吸速率常数比使用氯化物的解吸速率常数高 1-2 个数量级。鉴于在 pH 7 下，磷酸物种对 DMA 的解吸的复杂配体交换反应机制，发现表观解吸速率常数与[磷酸氢盐]有关，其阶数为 0.3。讨论了我们的研究对 DMA 在地球化学环境中的环境归宿以及减少燃料中砷含量的技术设计的影响。
