Degradation of persistent organic pollutants (pharmaceuticals & dyes) by combined dielectric barrier electrohydraulic discharge system and photo catalysts
Mouele, Emile Salomon Massima
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Water pollution problems have continued to increase not only in South Africa but worldwide due to human activities. The presence of organic toxins and bacteria in water sources is mostly due to population growth, industrial development and agricultural run-off. The accumulation of persistent organic pollutants (POPs) in water and wastewater sources has raised various questions on the safety of potable water used for drinking, households and other activities. Traditional mechanical, biological, physical, and chemical methods such as flocculation, coagulation, reverse osmosis, filtration, ultrafiltration, adsorption and active sludge treatment methods have failed to remove these new xenobiotic from aquatic media. This is due to the fact that instead of degrading the toxins, the methods listed above often transform organic contaminants from one form another. Also, the post treatment of by-products resulting from these methods is costly. In addition, this new generation of contaminants, often referred to as compounds of emerging concern (CECs), exist in tiny concentrations (ng) and conventional techniques have not been designed for these low levels of pollutants which consequently pass through during treatment processes and end up in the treated effluents at minute concentrations (ug/L to ng/L). However, complete remediation of chemical toxins in wastewater treatment plants has not been achieved. A better option involves the direct oxidation of the pollutants in the effluent but so far their complete mineralisation has not been achieved. Advanced oxidation processes (AOPs) have emerged in recent years as adequate techniques for the complete removal of POPs. AOPs focus more on the production of non-selective hydroxyl radicals (OH.) which have been considered as the most powerful oxidants (2.8 V) that directly or indirectly mineralise the organic pollutant into dissolved CO2, H2O and harmless end-products. However, the use of excessive chemicals, corrosion of catalyst supports, wasted UV, ozone escapes and the cost associated with AOPs often limit their application for the removal of POPs from water and wastewater treatment facilities. The principal aim of this study was to optimise a double cylindrical barrier discharge (DBD) system for the removal of low concentration persistent organic pollutants (POPs). The efficiency of the DBD system was initially confirmed by quantification of three main reactive oxygen species including ozone (O3), hydrogen peroxide (H2O2) and hydroxyl radicals (.OH) among others. These three active species were successfully detected and quantified using indigo, per titanyl sulphate and terephthallic acid (TA) spectroscopy methods, respectively. Thereafter, the DBD reactor was optimised by assessing the effect of electrophysico-chemical parameters on the removal efficiencies of two selected pollutants including orange II sodium salt dye (O.II) and sulfamethoxazole (SMX), a pharmaceutical, as model persistent organic pollutants.