Multi-Metal-Substituted-Goethite as an Effective Catalyst for Azo Dye Wastewater Oxidation

Abstract

Different samples of goethite modified with cobalt, manganese, or aluminum were synthesized and characterized by means of X-ray diffraction, chemical analysis, scanning electron microscopy, zeta potential measurements, and N2 adsorption–desorption isotherm analysis. The goethite oxides were tested as efficient catalysts for the degradation of methyl orange (MO, sodium 4-[(4-dimethylamino)phenylazo]benzenesulfonate), a sulfonated azo dye indicator, employing potassium persulfate (PS) in water, at pH 3. The Mn, Co, and Al substitution in goethite gave rise to active sites for PS activation. The most effective catalyst reached 93% of degradation at 120 min of reaction. The increase of temperature led to an expected conversion enhancement, and kinetic parameters were calculated for one of the evaluated catalysts. MO degradation curves best fitted to pseudo-second order kinetics. The extent of mineralization, measured as total removed organic carbon (TOC), was also monitored. A significant degree of mineralization was achieved and confirmed by TOC analysis. A mechanism for MO oxidation with SO•- 4 radicals was discarded and a pathway involving peroxymonosulfate species was proposed. The results indicate that the goethite-substituted oxides are effective catalysts for the studied azo dye degradation in aqueous solution.