The effects of particle sizes of goethite on the adsorption and photocatalytic reduction of Cr(VI)

• Main Authors: Pin-Hsueh Wu, 吳秉學
• Other Authors: Yu-Min Tzou
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/10703581173762776559

碩士 === 國立中興大學 === 土壤環境科學系所 === 100 === This study mainly investigates the interactions of Cr(VI), a toxic anion in the environment, with goethite with different particle sizes. Several methods were used for goethite syntheses, including ferrihydrite heat transformation, ferric nitrate acidification, and ferric sulfate, which gave particle sizes of 22, 139, and 449 nm, respectively. In addition, a method adopted from Schwertmanm was employed for preparing goethite with greater particle sizes of 849 and 1207 nm. The results of powder X-ray diffraction (XRD) conformed the goethite structures for each synthesized samples. The particle sizes and morphologies of these goethite samples were further verified using a laser scattering particle size distribution analyzer and a transmission electron microscopy (TEM). Surface areas and pHzpc (zero point charge) for each particle were measured using a BET surface area meter and Zeta meter. Cr(VI) adsorption on goethite complied with Elovich kinetic model and Langmuir adsorption isotherm, and the adsorption behaviors was indifferent with the particle sizes of goethite. The maximum adsorption of Cr(VI) on goethite was in the range 0.0707-0.207 mmol/g, depending on the particle size of goethite. A liner relationship existed between the amount of Cr(VI) adsorption and the goethite particle sizes and the smaller goethite particle exhibited higher adsorption ability, corresponding to its higher surface area. A significant declination of Cr(VI) adsorption on goethite was observed at pH 6-8, related probably to the change in Cr(VI) species (i.e., converting from HCrO4- to CrO42- at the specific pH) or/and an increase of surface negative charges and OH ion concentrations. An apparent endothermic reaction was observed while adsorbing Cr(VI) on goethite. Cr(VI) adsorption on smaller particle size of goethite had higher absorption heat which may lead to a decrease of Cr(VI) desorption from the surfaces of small particles. Small particles revealed a higher efficiency for photo-catalytic reduction of Cr(VI) on goethite, attributing to the higher adsorption ability of Cr(VI) and rapid electric transferring on the surfaces of small goethite particles.