Hydrothermal synthesis of iron oxide photo-Fenton catalysts: the effect of parameters on morphology, particle size and catalytic efficiency

Iron oxide materials were synthesized by an easy and rapid hydrothermal method using different iron precursors (FeCl₃∙6H₂O, Fe(NO₃)₃∙9H₂O and K₄Fe(CN)₆∙3H₂O), solution pHs (acid and basic) and reaction times (3 h and 6 h). The synthesized materials were thoroughly characterized by transmission electron microscopy and it was found that, depending on the synthesis conditions, iron oxide samples with crystalline or with amorphous structures could be obtained. Except for the materials synthesised from K₄Fe(CN)₆∙3H₂O and FeCl₃∙6H₂O under acidic conditions all the samples show mean particle sizes below 70 nm. The materials were further tested as catalysts for the degradation of 100 mg l^-1 diphenhydramine aqueous solutions by the photo-Fenton process at 25°C, pH = 2.8, 100 mg l^-1 of catalyst and 16 mM of H₂O₂ concentration. The best results in terms of pollutant removal and mineralization extent were obtained with the catalysts prepared from Fe(NO₃)₃∙9H₂O and K₄Fe(CN)₆∙3H₂O, under basic conditions, after 3 h of synthesis reaction time (complete degradation of diphenhydramine and total organic carbon removal higher than 60% after 60 min of reaction). Leaching of iron species from these catalysts was measured below 1.5 mg l^-1, a value complying with European Union Directives.