Calorimetric Investigation of Anion Adsorption onto Goethite

Titration calorimetry was employed to investigate adsorption of fluoride, iodate, phosphate, and salicylate anions onto goethite at constant temperature (25 °C), pH (4.0), ionic strength (0.05 M), and solid concentration (10 g/L). In addition, adsorption data were collected at three temperatures (10, 25, and 40 °C) and proton stoichiometry values (H⁺ adsorbed/anion adsorbed) were monitored as well. Measured adsorption enthalpies were exothermic and decreased with increasing surface coverage for all anions. Furthermore, the enthalpy curves all exhibited the same general shape. That is, at low surface coverages (<20% of maximum adsorption densities for each anion) enthalpies were highest but decreasing, at intermediate coverages (>20% and <70%) enthalpies were more constant, and at high coverages (>70%) enthalpies again decreased more rapidly and even became slightly endothermic for phosphate and fluoride. The primary reason for this similarity in shape is believed to be the surface site distribution inherent to the goethite. Except for iodate, calculated enthalpies did not agree with those measured. It is also postulated that anion adsorption free energies have a large favorable entropic component with enthalpic factors of more minor importance.