Different activated carbon xerogels (ACX) and ACX–chitosan composite materials were tested for the removal of the azo dye Orange II (OII) in aqueous solutions, either by pure adsorption or by catalytic wet peroxide oxidation (CWPO). The ACX materials were produced by activation of an organic resorcinol formaldehyde xerogel (RFX), considering different activation procedures: chemical impregnation with H3PO4 at 773 K (ACX-P), chemical impregnation with monoethanolamine at 773 K (ACX–MEA) and alkali activation with KOH at 1073 K using a 1:1 mass ratio of KOH/RFX (ACX-K). The ACX–chitosan composites were produced by oxidation of ACX with oxalic acid followed by treatment with chitosan gel.
During screening studies in adsorption and CWPO tests, the composites ACX-K–chitosan and
ACX-P–chitosan revealed the best performances among all the tested materials, namely OII removals between 69 and 73% were respectively obtained in CWPO after 150 min (pH 3.0, T = 323 K, material load of 0.2 g L-1, OII concentration of 100 mg L-1 and hydrogen peroxide concentration of 1.18 g L-1). In process intensification conditions, the CWPO process performance increased remarkably when using the ACX-P–chitosan composite. Complete OII removal in 90 min and a TOC removal of 55% in 24 h was achieved by CWPO, while less than 10% of OII was removed by pure adsorption (pH 3.5, T = 353 K, material load of
2.5 g L-1, OII concentration of 4.5 g L-1 and, in CWPO, hydrogen peroxide concentration of 25 g L-1).
The superior performance of the ACX–chitosan composite at intensified process conditions was more likely related with the high pollutant/catalyst ratio, which favors a more controllable H2O2 decomposition near the adsorbed pollutant species, thus avoiding parallel parasite reactions involving hydroxyl radicals and leading to a consequent higher efficiency of its usage. These conditions are of major interest in the treatment of highly polluted waste waters.
