Peroxide promoted catalytic wet air oxidation of phenolic aqueous solutions using activated carbon as catalyst

La creixent complexitat dels efluents residuals aixi com la manifesta oposicio publica cap a determinades tecniques convencionals per a la gestio de residus, com es la incineracio, ha accelerat el desenvolupament de tractaments economics i respectuosos amb el medi ambient. Hi ha pocs dubtes sobre que els processos de degradacio biologica continuin sent els mes utilitzats pel tractament daigues residuals. Malgrat aixo, quan els processos biologics sapliquen al tractament defluents industrials, no sempre arriben a nivells de depuracio satisfactoris, ja que moltes substancies organiques produides per les industries quimiques son inhibidores, toxiques o resistents a loxidacio biologica. LOxidacio Humida Catalitica (CWAO, de langles Catalytic Wet Air Oxidation) es una de les tecnologies mes prometedores per al tractament daigues que continguin una alta carrega organica o amb presencia de contaminants biotoxics. A mes a mes, el proces CWAO ha demostrat recentment la seva eficacia com a pretractament quimic, produint efluents que poden ser tractats posteriorment mitjancant un sistema biologic. Una opcio per suavitzar les condicions doperacio de la CWAO o per al tractament de contaminants resistents a aquest proces es afegir petites quantitats dun oxidant mes potent que laire, com ara el peroxid dhidrogen. Aquest proces sanomena Oxidacio Humida Catalitica Promoguda amb Peroxid dHidrogen (PP-CWAO, de langles Peroxide Promoted Catalytic Wet Air Oxidation). Per a lestudi daquest proces recentment desenvolupat, lactivitat i lestabilitat del carbo actiu (AC) shan provat a la PP-CWAO de solucions aquoses de fenol. Aquest compost sha elegit com a model degut a que els residus fenolics son uns dels contaminants mes habituals als efluents de les industries quimiques, que a mes es caracteritzen per una elevada toxicitat i una escassa biodegradabilitat. A aquesta tesi, les solucions fenoliques shan tractat en un reactor de llit fix de goteig a condicions de pressio i temperatura mitjanes (<15 bar, <150 oC), amb aire com a principal agent oxidant i peroxid dhidrogen com a promotor de la oxidacio. En primer lloc, sha estudiat lefecte sinergic o promotor de lH2O2 per a facilitat la reaccio doxidacio del fenol sobre AC. A continuacio, el proces PP-CWAO sha aplicat al tractament daltres compostos fenolics a unes condicions doperacio seleccionades. Finalment, shan preparat uns catalitzadors de ferro suportat sobre AC i shan aplicat a la oxidacio de fenol. Els resultats daquest estudi shan avaluat en funcio de la reduccio de la carrega organica i laugment de la biodegradabilitat de les solucions tractades. Es pot concloure que el proces PP-CWAO podria ser inclos entre els pretractaments quimics aplicables a aigues residuals toxiques o concentrades, que, un cop optimitzat, produiria efluents innocus per un posterior tractament biologic. Summary Thesis Alicia Rubalcaba The increasing complexity of wastewater streams as well as the unfavourable public opinion about some conventional waste management techniques, e.g. incineration, is forcing the development of cost-competitive and environmentally acceptable waste treatments. There is little doubt that biological processes will continue to be employed as a baseline treatment processes for most wastewater. However, when biological processes are applied to treatment of industrial effluents, they do not always give satisfactory results, sine many organic substances produced by the chemical and related industries are inhibitory, toxic and/or resistant to biological oxidation. The Catalytic Wet Air Oxidation (CWAO) process is one of the most promising technologies for the remediation of concentrated and/or biotoxic water pollutants, when a stable and active catalyst can be provided. In addition, CWAO has recently proved to be effective as chemical pre-treatment that could give effluents suitable for a subsequent biological end treatment. An alternative to lower CWAO operation conditions or to treat some refractory compounds is to supply little amount of a stronger oxidant such as hydrogen peroxide in a process referred as to Peroxide Promoted Catalytic Wet Air Oxidation (PP-CWAO). To study the potentialities of this newly developed oxidation process, the catalytic activity and stability of activated carbon (AC) was tested in the PP-CWAO of phenolic aqueous solutions. Phenol was chosen as model compound because of phenolic wastes are one of the most prevalent forms of chemical pollutants, characterised by a high toxicity and a poor biodegradability. In this work, phenolic water was treated in a trickle bed reactor (TBR) at mild pressure and temperature conditions (<15 bar, <150 oC), using air as main oxidant agent and hydrogen peroxide merely being a promoter. First, the synergistic or promoting ability of H2O2 to facilitate the oxidation reaction of phenol over AC was analysed. Secondly, some selected conditions were tested for the oxidation of other phenolic compounds such as o-cresol and p-nitrophenol. Finally, iron containing AC catalysts were prepared and applied to the oxidation of phenol. Results were evaluated in terms or organic load removal and biodegradability improvement. It can be concluded that this newly developed process could be included among chemical pre-treatments for toxic and concentrated wastewater that, once optimised, could lead to innocuous effluents for a subsequent biological process.