In Situ and Real-Time ATR-FTIR Temperature-Dependent Adsorption Kinetics Coupled with DFT Calculations of Dimethylarsinate and Arsenate on Hematite Nanoparticles.

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 in situ 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 (ΔEa), and pre-exponential factors (A). 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 ΔEa and A 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 ΔEa barriers are consistent with the experimental data and provide a mechanistic explanation for the low ΔEa values observed. The presence of defect sites with under-coordinated Fe atoms or exchangeable surface water (i.e., Fe-OH2 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.