The influence of temperature on the adsorption of mellitic acid onto goethite

The adsorption of mellitic acid (benzene-1,2,3,4,5,6-hexacarboxylic acid) onto goethite was investigated at five temperatures and 70 degrees C. Mellitic acid adsorption increased with increasing ternperature below pH 7.5 but at higher pH the effect of increasing temperature was to reduce the amount adsorbed. Potentiometric titrations were conducted and adsorption isotherms were measured over the same temperature range, and the data obtained were used in conjunction with adsorption edge data to develop an Extended Constant Capacitance Surface Complexation Model of metallitic acid adsorption. A single set of reactions was used to model the adsorption for the three different experiment types at the five temperatures studied. The adsorption reactions proposed for mellitate ion (L6-) adsorption at the goethite surface (SOH) involved the formation ot' two outer-sphere complexes: SOH + L6- + 3H(+) reversible arrow [(SOH2)(+) (LH2)(4-)](3-), 2SOH + L6- + 2H(+) reversible arrow [(SOH2)(2)(2+)(L)(6-)](4-) This rnechanism is consistent with recent ATR-FTIR spectroscopic measurements of the mellitate-goethite system. Thermodynamic parameters calculated from the temperature dependence of the equilibrium constants for these reactions indicate that the adsorption of mellitic acid onto goethite is acconipanied by a large entropy increase. (c) 2005 Published by Elsevier Inc.