Soldoozy Sara, Trinh Anthony, Kubicki James D, Al-Abadleh Hind A
Department of Chemistry and Biochemistry, Wilfrid Laurier University, Waterloo, Ontario N2L 3C5, Canada.
Department of Geological Sciences, The University of Texas at El Paso, El Paso, Texas 79968, United States.
Langmuir. 2020 Apr 28;36(16):4299-4307. doi: 10.1021/acs.langmuir.0c00252. Epub 2020 Apr 15.
Temperature-dependent kinetic studies of the adsorption of critical pollutants onto reactive components in soils and removal technologies provide invaluable rate information and mechanistic insight. Using attenuated total internal reflection Fourier transform infrared spectroscopy, we collected spectra as a function of time, concentration, and temperature in the range of 5-50 °C (278-323 K) for the adsorption of arsenate (iAs) and dimethylarsinate (DMA) on hematite nanoparticles at pH 7. These experimental data were modeled with density functional theory (DFT) calculations on the energy barriers between surface complexes. The Langmuir adsorption kinetic model was used to extract values of the fast (<5 min) and slow (6-10 min) observed adsorption rate, initial rate constants of adsorption and desorption, Arrhenius parameters, effective activation energies (Δ), and pre-exponential factors (). The trend in the kinetic parameters correlated with the type of surface complexes that iAs and DMA form, which are mostly bidentate binuclear compared to a mix of outer sphere and monodentate, respectively. The observed initial adsorption rates were found to be more sensitive to changes in the aqueous concentration of the arsenicals than slow rates. On average, iAs adsorbs 2.5× faster and desorbs 4× slower than dimethylarsinate (DMA). The Δ and values for the adsorption of iAs bidentate complexes are statistically higher than those extracted for outer-sphere DMA by a factor of 3. The DFT results on adsorption energies and Δ barriers are consistent with the experimental data and provide a mechanistic explanation for the low Δ values observed. The presence of defect sites with under-coordinated Fe atoms or exchangeable surface water (., Fe-OH groups) lowers activation barriers of adsorption. These results suggest that increasing organic substitutions on arsenate at the expense of As-O bonds decreases the effective energy barrier for complex formation and lowers the number of collisional orientations that result in binding to the hematite surface.
对土壤中反应性成分及去除技术上关键污染物吸附的温度依赖性动力学研究，提供了宝贵的速率信息和机理见解。利用衰减全内反射傅里叶变换红外光谱，我们在5 - 50 °C（278 - 323 K）范围内，针对pH值为7时砷酸盐（iAs）和二甲基砷酸盐（DMA）在赤铁矿纳米颗粒上的吸附，收集了作为时间、浓度和温度函数的光谱。这些实验数据通过对表面络合物之间能垒的密度泛函理论（DFT）计算进行建模。使用朗缪尔吸附动力学模型来提取快速（<5分钟）和慢速（6 - 10分钟）观察到的吸附速率值、吸附和解吸的初始速率常数、阿累尼乌斯参数、有效活化能（Δ）和指前因子（）。动力学参数的趋势与iAs和DMA形成的表面络合物类型相关，与分别为外球和单齿混合类型相比，iAs和DMA形成的表面络合物大多为双齿双核。发现观察到的初始吸附速率比慢速速率对砷化物水溶液浓度的变化更敏感。平均而言，iAs的吸附速度比二甲基砷酸盐（DMA）快2.5倍，解吸速度慢4倍。iAs双齿络合物吸附的Δ和值在统计学上比外球DMA提取的值高3倍。关于吸附能和Δ能垒的DFT结果与实验数据一致，并为观察到的低Δ值提供了机理解释。具有低配位铁原子或可交换表面水（如Fe - OH基团）的缺陷位点的存在降低了吸附的活化能垒。这些结果表明，以As - O键为代价增加砷酸盐上的有机取代基会降低络合物形成的有效能垒，并减少导致与赤铁矿表面结合的碰撞取向数量。
