P.C.C. Faria

Decolourisation of dye solutions by oxidation with H2O2 in the presence of modified activated carbons

The decolourisation of dye solutions by oxidation with H(2)O(2), using activated carbon as catalyst, is studied. For this purpose, three different samples, mainly differing in the respective surface chemistries, were prepared and characterized. Moreover, this work involved three pH levels, corresponding to acid, neutral and alkaline solutions, and six dyes belonging to several classes. The catalytic decolourisation tests were performed in a laboratorial batch reactor. Adsorption on activated carbon and non-catalytic peroxidation kinetic experiments were also carried out in the same reactor, in order to compare the efficiencies of the three processes. The non-catalytic reaction is usually inefficient and, typically, adsorption presents a low level of decolourisation. In these cases, the combination of activated carbon with hydrogen peroxide may significantly enhance the process, since the activated carbon catalyses the decomposition of H(2)O(2) into hydroxyl radicals, which are very reactive. Based on the experiments with the different activated carbon samples, which have similar physical properties, it is proved that the surface chemistry of the catalyst plays a key role, being the basic sample the most active. This is discussed considering the involvement of the free electrons on the graphene basal planes of activated carbon as active centres for the catalytic reaction. Additionally, it is shown that the decolourisation is enhanced at high pH values, and a possible explanation for this observation, based on the proposed mechanism, is given.

Cerium, manganese and cobalt oxides as catalysts for the ozonation of selected organic compounds

Several metal oxides, as well as metal oxides supported on activated carbon, were assessed as ozonation catalysts for the removal of selected organic compounds. Two transition metals (Mn, Co) and one rare earth element (Ce) were chosen for the preparation of the two series of catalysts. These materials were used in the ozonation of two aromatic compounds (aniline and sulfanilic acid) and one textile azo dye (CI Acid Blue 113). The results were compared with those obtained with non-catalytic ozonation. All the tested materials were found to be effective ozonation catalysts. Among the metal oxides, those containing mixtures of cerium and manganese or cerium and cobalt enabled the highest mineralisation degrees. After 120 min of reaction the TOC removal achieved with Ce-Mn-O was 63% for sulfanilic acid and 67% for aniline, while Ce-Co-O allowed TOC removals of 58 and 66%, respectively. With single ozonation, the mineralisation of sulfanilic acid and aniline solutions was 34% and 40% after identical reaction period. Regarding the metal oxides supported on activated carbon, cerium and manganese oxides were, in general, the most active for the degradation of the studied compounds.

Ozonation of Textile Effluents and Dye Solutions in the Presence of Activated Carbon under Continuous Operation

Abstract Ozonation was found to be effective for the decolorization of solutions, but has only a slight effect on TOC removal. On the other hand, adsorption on activated carbon improves the TOC removal, but the progressive uptake of the organic contaminants during the adsorption process decreases its removal efficiency. Decolorization, mineralization, and ozone consumption of colored solutions were evaluated under continuous operation in a column by three different processes: ozonation, adsorption on a fixed activated carbon bed, and ozonation in the presence of the activated carbon bed. The introduction of an ozone flow in a fixed activated carbon bed enhances both the decolorization of the solutions and mainly the mineralization of the organic matter, even when the activated carbon was previously partially saturated. Activated carbon acts both as an adsorbent and as a catalyst in the ozonation of colored solutions. The column configuration plays an important role in the performance of this system. The configurations in series and with activated carbon in the upper part of the column showed the highest colour and TOC removal for dye solutions. The results obtained clearly show that the combination of ozone and activated carbon is a promising technique for the final treatment of colored effluents. Practical applicability of this process was validated by treating two industrial textile effluents, collected after two different biological treatments.