Arsenite adsorption on goethite at elevated temperatures

Experimental closed-system ΔT acid–base titrations between 10 °C and 75 °C were used to constrain a temperature-dependent 1-pK basic Stern model of the goethite surface complexation reactions. Experimental data for the temperature dependence of pH_PZC determined by the one-term Van’t Hoff extrapolation yield a value for goethite surface protonation enthalpy of −49.6 kJ mol⁻¹ in good agreement with literature data. Batch titration data between 10 °C and 75 °C with arsenite concentrations between 10 μM and 100 μM yield adsorption curves, which increases with pH, peak at a pH of 9, and decrease at higher pH values. The slope of this bend becomes steeper with increasing temperature. A 1-pK   charge distribution model in combination with a basic Stern layer option could be established for the pH-dependent arsenite adsorption. Formation of two inner-sphere bidentate surface complexes best matched the experimental data in agreement with published EXAFS spectroscopic information. The temperature behaviour of the thus derived intrinsic equilibrium constants can be well represented by the linear Van’t Hoff $\log K_T^{\text{int}}$ vs. 1/T plot. Adsorption of arsenite on the goethite surface is exothermic (negative Δ_rH₂₉₈ values) and therefore becomes weaker with increasing temperature. Application of the new constants with the aqueous speciation code VMINTEQ predicts that the As(III) concentration in presence of goethite sorbent decreases by 10 times once the hydrothermal solution is cooled from 99 °C to 1 °C. The model curve matches data from a natural thermal water spring system. The increase of adsorption efficiency for As along the temperature gradient may well serve as an additional process to prevent ecosystem contamination by As-rich water seepage from geothermal energy generation facilities.