Photocatalytic degradation and chlorination of azo dye in saline wastewater: Kinetics and AOX formation
Highlights
► A dual effect of Cl− on AO7 degradation efficiency in UV/TiO2 system was observed. ► Several refractory chlorinated byproducts were identified in UV/TiO2/Cl− system. ► AOX formed was found to increase with the increasing Cl− concentration. ► Possible reaction pathways involving hydroxyl/chlorine radicals were proposed.
Introduction
Dying industrial effluents often contain a large amount of non-biodegradable organic contaminants which are toxic and pose a serious risk of environmental pollution. These wastewaters are usually characterized by highly concentrated electrolytes ranging from 5 to 60 g/L, generally being NaCl due to its lower cost, which can seriously inhibit the microbiological activity and thus the treatment efficiency of traditional biological methods [1], [2]. Chloride ion in the solution can also scavenge hydroxyl radicals (, 2.8 V) rapidly to generate less reactive chlorine species (Eqs. (1), (2), (3), (4)), leading to significant influence on the kinetics of dye degradation using radicals-based AOPs as reported previously [3], [4], [5]. A dual effect of chloride (inhibitory and accelerating effect) on dye degradation was observed in cobalt/peroxymonosulfate (Co/PMS) system based on sulfate radicals, due to the secondary chlorine radicals-mediated mechanism involved under a chloride-rich environment [6], [7]. During the electrolysis treatment, the dye degradation was found to be inhibited by Cl− iron in the range of 0–10 mM, but significantly enhanced by the further addition of Cl− concentration was greater than 20 mM [8]. Both non-radicals and radicals-based reaction mechanisms may be responsible for these unexpected effects of chloride ion on degradation kinetics of azo dyes in Co/PMS or electrolytic treatment.All reactive chlorine species produced (Eqs. (1), (2), (3), (4)), including chlorine atom, dichloride radical and free chlorine, can add to some unsaturated bonds of dye degradation intermediates (Eq. (5)), generating chlorinated hydrocarbons (RCl), therefore leading to an undesirable increase of the parameter AOX (halogenated organic compounds adsorbable on activated carbon) [7], [8], [9].Former studies have confirmed the generation of some chlorinated aromatic byproducts in Co/PMS process using GC–MS measurement, including 4-chlorophenol, 2,4-dichlorophenol, 4-chlorophthalic acid, 1,3,5-trichloro-2-nitrobenzene and 2,4,6-trichlorophenol [7], [10]. The toxic and carcinogenic nature of RCl compounds warrants a further detailed investigation in aqueous organic media on the mechanism and the parameters leading to RCl formation during dye degradation [11].
It should be noted that the previous works were mainly concerned with the effect of chloride ion on dye degradation in homogeneous systems [3], [4], [5]. Few studies were focused on the more complicated heterogeneous AOPs such as UV/TiO2 [12], [13], [14]. Heterogeneous photocatalysis using TiO2 as photo-catalyst based on hydroxyl radicals ( , 2.8 eV) has been proved to be a promising and attractive treatment method for the effective degradation of various organic pollutants, as shown in previous studies [15], [16], [17]. In this UV/TiO2 system, the effects of chloride salt on dye degradation may be complicated based on the following hypothesis: (a) Cl− may competitively reduce the adsorption of the dye on the catalyst surface; (b) Cl− may be directly oxidized by the valence band holes; and c) a large amount of Cl− adsorbed on the catalyst surface may form considerable inner-sphere Ti–Cl surface complexes. The surface properties and photocatalytic activity of TiO2 are also related to the solution pH, as well as the content of chloride ion. Therefore, it seems very necessary to examine the roles of chloride in the dye degradation during TiO2-based photocatalytic treatment.
However, until now the great part of studies dealing with the degradation of specific textile dyes and regarding the importance of halides influences relied only on the monitoring of solution decoloration, and may also have contradictory results [17], [18], [19], [20]. Knowledge on both decoloration and mineralization of the dye wastewater in a wide salinity range is lacking. Moreover, little information is available on the chlorine-involved reaction mechanisms of dye degradation based on the identification of major transient intermediates.
Therefore, in this work a common nonbiodegradable azo dye Acid Orange 7 (AO7) was selected as a model. The effect of chloride ion on dye decoloration, mineralization and AOX formation in aqueous solution was investigated, considering various solution pH. Our attention has also been focused on the dye degradation mechanisms in high saline solutions based on the identification of the transformation products. These results may provide valuable insight into the area of fundamental photodegradation research regarding the role played by halide ions.
Section snippets
Materials
Acid Orange 7 (AO7: C16H11N2O4SNa) used in the present work was purchased from Sigma–Aldrich. The TiO2 photocatalyst (P25) was supplied by Evonik Corporation. NaCl was of analytical grade and used as received. Other chemicals used herein, including sulfuric acid and sodium hydroxide (obtained from Sinopharm Chemical Reagent Cooperation), were of reagent grade and were used to adjust pH. All sample solutions were prepared using deionized water from Barnstead UltraPure instrument. The salinity
Effects of Cl− concentration on dye degradation
The photocatalytic oxidation of AO7 was carried out by mixing chloride ion in a wide range, while keeping the remaining variables constant (Fig. 1(a)). It is interesting to note that Cl− ion (0–200 mM) showed an obvious enhancement for the decolorization of AO7, but such promotion effect declined significantly with the increasing addition of Cl− (50–200 mM). Higher content of chloride ion (>200 mM) was found to reduce the dye bleaching rate. This inhibitory effect of Cl− has been observed in the
Conclusions
Degradation of one model azo dye AO7 in water by UV/TiO2 process was carried out in the presence of chloride ion. The results indicated that photocatalytic decoloration and mineralization of AO7 can be influenced dramatically by sodium chloride. The retardant effect of high concentrations of chloride ion on dye degradation rate was favored in acidic pH. Compared to the homogeneous systems, the presence of chloride ion may change dye degradation mechanisms, mainly due to its effect on dye
Acknowledgments
The authors would like to acknowledge the financial assistance from the Shanghai Leading Academic Discipline Project (B604), the PhD Thesis Innovation Foundation of the Donghua University (No. 11D11311) and Shanghai Tongji Gao Tingyao Environmental Science & Technology Development Foundation (STGEF). This work was also partially supported by State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. CRRF11023).
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