Removal of basic and industrial azo reactive dyes from aqueous solutions via Fenton-like reactions using catalytic non-magnetic Pd-flyash and magnetic Pd-Fe3O4-flyash composite particles

Palladium (Pd)-flyash composite particles have been conventionally used for obtaining copper (Cu) or silver (Ag)-coated flyash particles as fillers for manufacturing the conducting polymers. We demonstrate that the involvement of Fenton-like reactions (advanced oxidation processes (AOPs)), makes the same composite particles useful as catalyst for the decoloration of aqueous solutions containing organic synthetic-dyes. The non-magnetic Pd-flyash and magnetic Pd-magnetite (Fe3O4)-flyash composite particles have been processed for this purpose, for the first time, using the combination of electroless and inverse co-precipitation techniques. The dye-removal characteristics of the catalysts have been investigated in the dark-condition via Fenton-like reactions involving the activation of hydrogen peroxide (H2O2) to generate hydroxyl radicals ((OH)-O-center dot) in the aqueous solutions containing basic and industrial azo reactive dyes. The initial dye, H2O2, and catalyst concentrations have been varied within the range of 0.003-0.4 g l(-1), 3-50 wt%, and 40-100 g l(-1) respectively. Within the investigated test-conditions, Pd-flyash composite particles show higher dye-removal rates than those associated with Pd-Fe3O4-flyash composite particles due to higher amounts of surface-deposited Pd and reduced concentration of Fe2+ ions leached into the surrounding acidic medium from the catalyst-surface. Conversely, Pd-Fe3O4-flyash composite particles exhibit effective magnetic separation after the dye-decoloration process which is not possible without the presence of intermediate layer of Fe3O4. For both the catalysts, the chemical oxygen demand (COD) levels of decolorized aqueous solutions are observed to be increased after the catalyst-treatment of initial dye-solutions. Nevertheless, the increased COD levels are easily controlled by the post treatment with high surface-area activated carbon (AC) which strongly adsorbs the intermediate products of dye-decoloration and carbon leached out from the surface of flyash particles. Hence, the Fenton-like reactions backed up with the AC-based adsorption process appear to be effective combination for the fast and efficient removal of organic synthetic-dyes from the aqueous solutions. (C) 2016 Elsevier B.V. All rights reserved.