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The influence of temperature on selenate adsorption by goethite

Abstract Acid-base batch titration data up to 75 ºC were used to constrain a temperature-dependent 1-pK basic Stern model of the surface protonation reactions of goethite. Experimental data for the temperature dependence of pHPZC (as determined using the two-term Van't Hoff extrapolation) yielded a negative value of −44.9 kJ/mol for the surface protonation enthalpy, and therefore a shift of the zero point of charge towards lower pH values with increasing temperature. Batch titrations at selenate concentrations of between 10 and 100 μM showed an increased degree of adsorption in the acidic pH range, which appeared to be sensitive to the ionic strength of the solution. The selenate adsorption edges shifted towards more acidic pH values with increasing temperature. A 1-pK charge distribution multi-site surface complexation (CD-MUSIC) model was applied, assuming the formation of an outer-spheric surface complex together with an inner-spheric one, in agreement with published spectroscopic information. The temperature behaviour of the intrinsic equilibrium constants were well represented by a linear Van't Hoff log K vs. 1/T plot, from which negative enthalpy values could be derived for both adsorption reactions. The adsorption of the selenate was therefore exothermic and became weaker with increasing temperature. The bidentate inner-spheric complex was more sensitive to rises in temperature (−70 kJ/mol), compared to the outer-spheric complex (−36 kJ/mol). The latter ultimately became the dominating adsorption process at the highest temperature studied.