Influence of pH on Initial Concentration Effect of Arsenate Adsorption on TiO2 Surfaces: Thermodynamic, DFT, and EXAFS Interpretations

Under the same thermodynamic condition where the total mass of arsenate was fixed, when the initial arsenate was added to TiO2 suspension by multiple batches, adsorption isotherms declined as the multi-batch increased, which was termed initial concentration (C-0) effect. The extent of C-0 effect decreased gradually as pH decreased front 7.0 to 5.5. Extended X-ray absorption line structure analysis of 1-batch and 3-batch isotherm samples showed that the relative proportion of bidentate binuclear (BB) and monodentate mononuclear (MM) complex was rarely affected by pH change from 5.5 to 7.0, indicating that the dependence of C-0 effect on pH was not due to inner-sphere chemiadsorption. The influence of pH oil adsorption was simulated by density functional theory through changing the number of H+ in model clusters. Calculation of adsorption energy showed that BB surface complex was the most thermodynamically favorable mode (-244.5 kJ mol(-1)) at low pH, but MM surface complex was the most thermodynamically favorable mode (-135.6 to 27.5 kJ mol(-1)) at intermediate and high pH, which indicated the influence of surface functional groups (-H2O and -OH) on adsorption reaction pathways. As pH decreased, C-0 effect weakened gradually because outer-sphere H-bond adsorption became thermodynamically favorable (-203.1 kJ/mol). The dependence of C-0 effect on pH showed that traditional equilibrium adsorption constants, could not accurately describe the real adsorption equilibrium at solid-liquid interface, because real equilibrium adsorption state is generally a Mixture of various outer-sphere and inner-sphere metastable-equilibrium states.