[Interactions between Goethite and Humic Acid and the Stability of Goethite-Humic Acid Complex]

Goethite-humic acid complex was prepared in a suspension containing goethite and humic acid. X-ray diffraction (XRD) results showed that the crystal structure of this complex had no obvious changes compared to pure goethite, but the peak intensity of the complex was slightly reduced. Transmission electron microscopy (TEM) images indicated that the surface of the goethite was coated by particles of humic acid. Compared to the infra-red (IR) spectra of goethite and humic acid, the anti-vibrational frequencies of COO^- and the vibrational frequencies of ≡Fe-OH decreased by 20 cm^-1 and 9 cm^-1, respectively, while the vibrational frequencies of the associated hydroxyls increased by 10 cm^-1 and the absorption band of carboxylic C-O and free hydroxyls almost disappeared. This indicates that the interactional mechanisms between goethite and humic acid include the monodentate coordination of Fe(Ⅲ) -carboxylate and hydrogen-bonds. Thermogravimetry/differential thermogravimetry (TG/DTG) analysis showed that the temperature of the weight loss peak for ≡Fe-OH in goethite and the complex were 258℃ and 276℃, respectively. This indicates that the coating of humic acid enhances the heat stability of ≡Fe-OH in goethite. Compared with humic acid, the temperature of the weight loss peak for aliphatic organics and aromatic organics in complex decreased by 60℃ and 26℃, respectively and the ratio of weight loss from aliphatic organics to aromatic organics in complex increased. This indicates that organics with a lower heat stability may be more easily adsorbed onto goethite and the affinity to goethite was higher for the aliphatic organics than for aromatic organics. After ultrasonic dispersion, the content of large particles (≥ 2 μm) decreased significantly for both goethite and humic acid, but the content and the size of large particles in the complex changed only slightly.