Supported photocatalysis as a pre‐treatment prior to biological degradation for the removal of some dyes from aqueous solutions; Acid Red 183, Biebrich Scarlet, Methyl Red Sodium Salt, Orange II

Abstract

International audienceBACKGROUND: The aim of this study was to assess the feasibility of coupling photocatalysis and a biological treatment for the removal of azo dyes from aqueous effluents. Biological processes do not always appear relevant for dyes removal, owing to the low or total absence of biodegradability of this class of pollutants. RESULTS: During photocatalysis pre-treatment, a decrease in the chemical oxygen demand (COD) indicated oxidation of the target compound and thus a change in the chemical structure; better biodegradability or less toxicity could then be expected. However, the concomitant decrease in dissolved organic carbon (DOC), characteristic of a high mineralization yield, led to nearly constant COD:DOC ratios, which was unfavorable for an increase in biodegradability. It was confirmed by the low values found for the ratios biological oxygen demand (BOD5) to COD, which remained in the range 0.09-0.19, namely below 0.4 after photocatalytic reaction. Moreover, toxicity increased or remained at a high level after irradiation of the azo dyes for 3 h, and decreased only for Orange II, from toxic (EC50 = 53%) to moderately toxic (EC50 = 76%). CONCLUSION: An integrated process involving photocataysis and biological treatment to treat azo dyes appeared unsuitable under the conditions tested and may only be considered for Orange II among the four dyes tested. Copyright © 2010 Society of Chemical Industr

Photocatalytic Water Treatment by Titanium Dioxide: Recent Updates

“…Similar observations were reported for the degradation of dyes [141], cyproconazole [142], and a tetracycline/tylosin mixture [143], in which the TiO 2 photocatalytic pre-treatment eradicated the activity of biological oxidation. In this case, some of the intermediates generated during photocatalysis were not biodegradable.…”

“…These technologies include electrocatalysis [35,[136][137][138], sonocatalysis/Fenton process [139], biodegradation [140][141][142][143][144], and wetland technology [145]. The combination of NTO photocatalysis with any of these techniques can not only improve the total efficiency of the degradation but also has the advantage of treating large quantities of wastewater (in real systems), especially with electrocatalysis, biodegradation, and wetland technology.…”

Relevance of Photocatalysis prior to Biological Treatment of Organic Pollutants – Selection Criteria

“…In such integration, the advanced oxidation process (AOP) constitutes a pretreatment in order to increase the biodegradability of the effluent degrading recalcitrant pollutants and/or to reduce toxicity [17,18].…”

“…Nowadays, the most widely physicochemical treatments studied are photo-Fenton and photocatalytic ones. Previous works indicated that the considered integrated process did not seem to be relevant to treat the pesticide cyproconazole [23] and some azo dyes [18], and its use appeared limited in the treatment of antibiotic wastewater (57 % chemical oxygen demand (COD) removal) [24]. In contrast, photocatalytic degradation of sulfamethoxazole resulted in an increase of biodegradability [25], and an integrated photocatalytic-biological reactor led to 92 % mineralization during phenol degradation [26].…”

“…Limited mineralization is required to ensure sufficient residual organic carbon for a subsequent microbial culture. Consequently, a favorable trend is a decrease of the ratio COD/TOC [18] or an increase of the average oxidation state (AOS) [16,30] during the photocatalytic degradation of the target compound. BOD 5 on COD ratios were also examined, since a value of 0.4 is considered by several authors as the boundary of biodegradability [16,30].…”