Article pubs.acs.org/IECR
Fast Degradation and Biodegradability Improvement of Reactive Brilliant Red X‑3B by the Cobalt(II)/Bicarbonate/Hydrogen Peroxide System Zhen Yang, Hong Wang, Min Chen, Muxi Luo, Dongsheng Xia, Aihua Xu,* and Qingfu Zeng* Engineering Research Center for Clean Production of Dyeing and Printing, Ministry of Education, Wuhan Textile University, Wuhan 430073, China ABSTRACT: The cobalt(II)-bicarbonate (HCO3−) complex is a very efficient catalyst for organic dye decolorization with H2O2 as an oxidant, but its mineralization degree is very low. One interesting alternative is the combination of the system with a subsequent biological step. With the aim of employing the Co2+-HCO3− system as a pretreatment method to dye wastewater, a typical azo dye, reactive brilliant red X-3B (X-3B), was selected as the model compound, and the biodegradability enhancement of the dye was investigated. The results showed that X-3B was effectively degraded by the Co2+-HCO3− system under various mild conditions, and the BOD5/COD ratio of the dye solution could markedly increase from 0.046 to higher than 0.46. The effluent treatment by the system was efficiently post-treated in a batch biological reactor with the COD removal around 44% after 8 h aeration, while a little decrease of the COD value of the raw solution was observed. be nearly completely decolorized by the Co2+-HCO3− system under mild conditions, while control experiments with HCO3− in the absence of Co2+ ions, or with Co2+ ions in the absence of HCO3−, implied no degradation of MB.18 For the tested other organic pollutants including methyl orange, rhodamine B, sulforhodamine B, nitrophenol, and 2,4-dinitrophenol, the observed reaction rates varied very much, probably due to their different coordination abilities with Co2+ ions.19 The Co2+HCO3− system has been offering many advantages over other Fenton-like systems. For example, compared with the complex organic ligands used in many Fenton-like systems (oxalate, EDTA, etc.), HCO3− is relatively nontoxic, cheap, and simple; in addition, the degradation with the Co2+-HCO3− system can be carried out under very low concentrations of the reagents Co2+ ions (1−5 μmol L−1) and HCO3− ions (1−5 mmol L−1), while the dosages of Fe2+ ions and ligands in Fenton-like systems are usually larger. Moreover, the pH in most of printing and dyeing wastewaters ranges from neutral to alkaline, and the fast degradation of organic dyes by the Co2+-HCO3− system between pH 6.5 and 10 has been observed; however, many of the Fenton-like systems can only function in acidic environments. Despite the high decolorization efficiency of organic dyes by the Co2+-HCO3− system, the degree of mineralization was very low. After reaction, only about 19% and 42% of TOC and COD removal were obtained, respectively.19 Hence, it is not affordable to use the system as the stand-alone treatment process for mineralization of organic compounds, due to the extensive operating conditions and consequently high cost required for complete degradation of the pollutants. According to some literature,20,21 one interesting alternative is the
1. INTRODUCTION In recent decades, advanced oxidation processes (AOPs) such as photocatalysis, catalytic wet air oxidation, catalytic wet peroxide oxidation, catalytic ozonation, and sonocatalysis, depending on the type of oxidant (O2, O3 and H2O2), are becoming more important technologies for wastewater treatment.1−5 H2O2 is one of the most commonly used oxidants owing to its eco-friendly nature. AOPs that decompose H2O2 to produce highly reactive species and degrade a broad range of organic pollutants quickly and nonselectively have been considered to be the most effective, simple, and economical methods.4−6 Among them, activation of H2O2 by transitionmetal ions has been explored actively.7−9 For example, under acidic conditions, Fenton’s reagent (Fe2+/H2O2) is useful to achieve considerable reductions in the concentrations of dyes in wastewaters.7 After coordinating with complexes, iron ions were also highly efficient in the degradations of organic dyes with H2O2. Collins and co-workers have described a novel and highly effective method based on tetraamido macrocylic ligand iron to destroy Orange II into small biodegradable and nontoxic organic products by H2O2.8 A metal organic complex, β-cyclodextrin-hemin, was found to be able to activate H2O2 efficiently to oxidize and mineralize Rhodamine B and 2,4dichlorophenol in aqueous media over a wide pH range under visible light irradiation.9 Besides iron complexes, various other metal complexes including manganese, cobalt, copper, molybdate, and cerium have also been reported as Fentonlike catalysts for the degradation of the organic dye pollutants at room temperature.10−17 However, cost effectiveness and robustness are often the major obstacles associated with the application of these metal complexes in industrial processes. We have recently found that cobalt(II)-bicarbonate (HCO3−) complexes are efficient catalysts for organic dyes degradation with H2O2.18,19 It was demonstrated that HCO3− ions are an important ligand and lead to significant changes in the reactivity of Co2+ ions. The dye methylene blue (MB) could © 2012 American Chemical Society
Received: Revised: Accepted: Published: 11104
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was measured by a Multi N/C TOC Analyzer (Analytik Jena AG, Jena, Germany). The biochemical oxygen demand for 5 days (BOD5) of samples was performed in accordance with National Standard for Environmental Protection of the People’s Republic of China (HJ505-2009). The analysis of small organic acids and inorganic ions after the degradation was performed with ionic chromatography (ICS-9000). Mass spectrometry experiments were performed in the negative mode on an Esquire LC−ion trap mass spectrometer (Bruker Daltonics, Bremen, Germany) equipped with an orthogonal geometry ESI source. Nitrogen was used as the drying (3 L/min) and nebulizing (6 psi) gas at 300 °C. The spray shield was set to 4.0 kV, and the capillary cap was set to 4.5 kV. Scanning was performed from m/z 70−800 in the standard resolution mode at a scan rate of 13 kDa/s. Before analysis, samples were purified by solid phase extraction and then diluted five times.
combination of the system with a subsequent biological step. By coupling the two processes, maybe there is a great advantageous potential to attain more efficient and cost-effective technology for treating low biodegradable compounds. The production and consumption of azo dyes account for over 50% of the total dyes around the world. They are resistant to biodegradation under aerobic conditions, and it also is not proper to be treated by an anaerobic process because the breakdown of azo dye can lead to the formation of aromatic amines, which may be more toxic than the dye molecules themselves.22 Thus it is necessary to carry out a preconditioning step to the dye solution prior a biological treatment process. In this study, with the aim of employing Co2+-HCO3− as a pretreatment system to dye wastewater, the biodegradability enhancement of Reactive Brilliant Red X-3B (X-3B), a kind of typical azo dye, under different H2O2 concentrations were investigated. The influence of initial solution pH, reaction temperature, and concentrations of HCO3−, Co2+, X-3B, and H2O2 on decolorization efficiency was also discussed.
3. RESULTS AND DISCUSSION 3.1. General Observations. Figure 1 shows the decolorization of X-3B in aqueous solutions under different reaction
2. MATERIALS AND METHODS 2.1. Materials. Cobalt acetate, sodium bicarbonate, ferrous sulfate, and other chemicals were of analytical grade if not noted otherwise. Hydrogen peroxide (30% w/w) was obtained from Sinapharm Chemical Reagent Co. Ltd. The azo dye Reactive Brilliant Red X-3B (X-3B) was purchased from Tianjin Chemicals Company (Tianjin, China) and used without further purification. The sample solutions were prepared using deionized water throughout all the experiments. The solution pH was adjusted by H2SO4 or NaOH. 2.2. Experimental Procedure. Degradation reactions with the Co2+-HCO3− system were performed in a 50 mL flask. After the desired amount of Co(CH3COO)2, NaHCO3, and X-3B in 25 mL of the aqueous solution were added into the reactor and kept at a certain temperature in a water bath, the reaction was initialized by adding 0.25 mL of a diluted H2O2 solution. In a typical degradation experiment, the concentrations of the reagents in the reaction mixture were Co2+ 5 μmol L−1, HCO3− 10 mmol L−1, X-3B 67.5 μmol L−1, and H2O2 4 mmol L−1. The biological tests (activated sludge) were carried out in bath mode using a 2 L organic glass column equipped with a diffuser to distribute the air as the bioreactor. The concentrated sludge (300 mL) from a municipal wastewater treatment plant in Jiangxia district in Wuhan City was first mixed with 1 L of dye solution in the bioreactor, giving a sludge concentration of 4 g L−1, and then the suspension was aerated for a certain time. Before feeding the biological reactor, the treated dye solution with a high initial concentration of 0.34 mmol L−1 by Co2+HCO3− system was placed for 12 h at room temperature. Thus, possible interferences from any quantity of H2O2 in the biological treatment were eliminated. The pH of the final solution in the bioreactor was about 8.2 without any adjustment. After a specified aeration time, the suspension was allowed to settle for 30 min and it was then filtered with 0.45 μm filter paper. Chemical oxygen demand (COD) removal of the solution was tested over several aeration times between 1 and 8 h. 2.3. Analysis. To monitor the organic pollutants degradation process, solution samples were taken out at given time intervals and measured immediately at 540 nm on a Varian Cary 50 Scan UV−vis spectrophotometer. COD was measured by oxidation with K2Cr2O7 under acidic conditions according to the national criterion of China. Total organic carbon (TOC)
Figure 1. Degradation efficiency of X-3B under different reaction conditions. Conditions: Co2+ 5 μmol L−1, HCO3− 10 mmol L−1, X-3B 67.5 μmol L−1, Fe2+ 5 μmol L−1, and H2O2 4 mmol L−1, 28 °C.
conditions. Because of the presence of aromatic groups, X-3B is very stable and almost no decrease of its concentration was observed in the blank experiment. The result with Co2+ ions in the absence of HCO3− at a pH of 8.2 also indicates little degradation of the azo dye, for Co2+ ions do not readily generate free •OH radicals with H2O2.23 Although HCO3− can be used as an efficient activator for H2O2 to generate many active oxygen species and to degrade organic pollutants,24 the degradation of X-3B at a low concentration of HCO3− (10 mmol L−1) was very slow. However, the decolorization rate was tremendously accelerated by addition of simple cobalt salt to HCO3− solution. After a reaction of 15 min, 85% of X-3B was removed by the system. It is known that Fenton is a very effective method for activation of H2O2 and destruction of organic contaminants,7 but compared with the Co2+-HCO3− system, its reactivity is much lower. For Fenton reagent with a 5 μmol L−1 Fe2+ ion concentration at pH 3.0, little degradation of X-3B occurred after 40 min. From all the results, it is evident that the Co2+-HCO3− system is highly reactive for X-3B degradation with H2O2. 11105
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It should be noted that although Co2+ ions are toxic and it is not recommended that cobalt(II) be added to water, the dosage of Co2+ ions can be lower than 5 μmol L−1. The value can meet the national emission standard for cobalt pollutants in wastewater (lower than 16.9 μmol L−1) in China (GB 254672010). The chemical oxidation using peroxymonosulfate conjunction with Co2+ ion based on the generation of sulfate radicals is a very promising technique for organic pollutants degradation, but the concentration of Co2+ in this system was usually higher (30 μmol L−1) than that used in the Co2+HCO3− system.25 3.2. Effect of Operating Parameters. To go further in understanding the efficiency of X-3B degradation by the Co2+HCO3− system, the effect of different operating parameters, including the initial solution pH, reaction temperature, and the concentrations of HCO3−, Co2+, H2O2, and X-3B were investigated. Solution pH is one of the most important parameters that influence the catalytic efficiency of organic pollutant degradation with the use of H2O2, i.e., the Fenton reagent loses its activity owing to precipitation of Fe3+ at pH greater than 3.9 The catalytic activity of Fe-TAML complex was also pH dependent with the highest degradation rates at pH 9−11.8 As the amount of HCO3− in solution is influenced by pH, the activity of the Co2+-HCO3− system is expected to be sensitive to pH. As indicated in Figure 2, pH could affect the degradation
neutral to alkaline, the fast degradation of X-3B by the system between pH 7.5 and 10 is important for the removal of organic dyes. Figure 3 depicts the effect of temperature on the rate of X-3B degradation by the Co2+-HCO3− system. As is shown, at a low
Figure 3. Effect of reaction temperature on degradation efficiency of X-3B by Co2+-HCO3− system. Conditions: Co2+ 5 μmol L−1, HCO3− 10 mmol L−1, X-3B 67.5 μmol L−1, and H2O2 4 mmol L−1.
temperature of 15 °C, the removal of X-3B was 86% after 40 min reaction, and as expected by the Arrhenius law a higher reaction rate was observed when the temperature increased to 40 °C. In a previous paper, it has been shown that the Mn2+HCO3− system, a fast and environmental benign method for the oxidative degradation of organic pollutants, is unstable and inactive for organic dyes degradation at a high temperature of 40 °C with formation of insoluble manganese species.26 Compared with the Mn2+-HCO3− system, the Co2+-HCO3− system is much more stable. Similar to other metal complexes in oxidation reactions, the concentrations of reagents were essential parameters for the degradation of X-3B by the Co2+-HCO3− system. Figure 4
Figure 2. Effect of initial solution pH on the degradation efficiency of X-3B by the Co2+-HCO3− system. Conditions: Co2+ 5 μmol L−1, HCO3− 10 mmol L−1, X-3B 67.5 μmol L−1, and H2O2 4 mmol L−1, 28 °C.
efficiency of X-3B. At acid conditions, HCO3− will react with H+ to form H2CO3 which then decomposes to CO2 and H2O, thereby decreasing the concentration of HCO3−. Thus, the observed degradation reaction at pH 6.5 was very slow, and an increase in pH could result in a significant increase in the reaction rate. The highest reactivity was observed at a pH of 9.0. When further increasing the solution pH, the reaction rate slowly decreased, presumably due to the dissociation of HCO3− to CO32−, which may form more stable complexes with Co2+ ions. In addition, at higher pH values, the decomposition of H2O2 is accelerated and may reduce the oxidation rate. As the pH in most of printing and dyeing wastewaters ranges from
Figure 4. Effect of HCO3− concentration on degradation efficiency of X-3B by the Co2+-HCO3− system. Conditions: Co2+ 5 μmol L−1, X-3B 67.5 μmol L−1, H2O2 4 mmol L−1, 25 °C. 11106
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shows the effect of HCO3− concentration. It can be seen that with the increase of HCO3− concentration from 0 to 50 mM, the degradation efficiency increased and then decreased. The maximum rate was obtained at 10 mM, but even at a low concentration of 5 mM, fast degradation of the dye was still observed. The initial enhancement of X-3B degradation efficiency was attributed to the high concentration of active Co2+-HCO3− complex generated at a high concentration of HCO3−, producing more active oxygen radicals in the system. However at higher concentration, the number of HCO3− complexed to Co2+ ions increased and there were no equatorial positions available to bind the dye and H2O2, leading the efficiency to decrease. In addition, the free HCO3− in solution at higher concentrations may act as an HO• radical scavenger. The effect of Co2+ concentration on treatment of X-3B by the Co2+-HCO3− system is shown in Figure 5. A marked
Figure 6. Effect of X-3B concentration on degradation efficiency by the Co2+-HCO3− system. Conditions: Co2+ 5 μmol L−1, HCO3− 10 mmol L−1, and H2O2 4 mmol L−1, 25 °C.
of X-3B, 0.34 mmol L−1 (250 mg L−1), 80% of the dye was still decolorized. Figure 7 presents the degradation of X-3B with the Co2+HCO3− system in the presence of various concentrations of
Figure 5. Effect of cobalt concentration on degradation efficiency of X3B by the Co2+-HCO3− system. Conditions: HCO3− 10 mmol L−1, X3B 67.5 μmol L−1, and H2O2 4 mmol L−1, 25 °C.
enhancement in X-3B degradation was observed by increasing the Co2+ dosage from 0 to 1.25 μmol L−1, since the active cobalt species were available for H2O2 decomposition into hydroxyl radicals. Though scavenge reactions may occur between free Co2+ ions and HO• radicals, which can reduce the oxidation efficiency of the system,14 the observed degradation rate increased when the Co2+ dosage further increased to 20 μmol L−1. Perhaps most of Co2+ ions were still complexed by HCO3− due to the high concentration of HCO3− ions used in this system. The effect of the initial dye concentration on the activity of the Co2+-HCO3− system was also carried out. The results given in Figure 6 demonstrated that, when increasing the dye concentration from 16.9 μmol L−1 to 0.34 mmol L−1, a slight decrease of its reduction with reaction time was observed. This finding is similar with other oxidation systems, in which the degradation rate decreased with increasing the initial dye concentration.21,27 A possible explanation might be that, under the conditions, the formed reactive oxygen species (HO• radicals) was constant, and the relative number of free radicals attacking the dye molecules decreases with increasing amount of dyes. However, even in the presence of a high concentration
Figure 7. Effect of H2O2 concentration on degradation efficiency of X3B by the Co2+-HCO3− system. Conditions: Co2+ 5 μmol L−1, HCO3− 10 mmol L−1, and X-3B 0.34 mmol L−1, 25 °C.
H2O2. It can be seen that X-3B was not damaged in the absence of H2O2, while its degradation rate greatly increased when the H2O2 concentration increased to 5 mmol L−1, which can be explained by the effect of the additionally produced OH• radicals. However, even after a long time of 30 min for this reaction, complete decolorization was not observed and there was still a significant amount of X-3B (40.8 μmol L−1) presented in the solution. A further increase in the H2O2 concentration to 100 mmol L−1 resulted in a slight improvement in the degradation efficiency but much less residual X-3B. The fact may be due to that the excess of H2O2 cannot form a complex with cobalt species and consequently does not contribute to the oxidative degradation of the organic dye. In addition, the increase in H2O2 concentration may promote 11107
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radical scavenging and the subsequent formation of HO2• radicals with an oxidation potential considerably smaller than that of OH• radicals.28 From the above studies, it can be concluded that the degradation of X-3B by the Co2+-HCO3− system can be carried out under very mild conditions: wild solution pH, room temperature, and low concentrations of reagents. However, these studies were based on the single-factor-at-a-time approach, considering the effect of each variable independently and keeping all other conditions constant. Therefore, possible interactions between variables and their potential effects on the process response are not taken into account. The modeling and optimization of the system should be considered in future work by application of statistical approaches such as Response Surface Methodogies, which allow a multivariate analysis, using a minimum number of experiments.29 3.3. Biodegradability Enhancement. The low biodegradability associated with dye wastewaters does not allow the direct application of biotreatments and, on the other hand, chemical treatments are usually expensive. It is then industrially attractive to use an AOP to first improve the effluents ability for bioremediation and afterward implement an inexpensive and effective biological process for final depuration.30 The ratio of BOD5/COD is normally used to express the biodegradability of the wastewater. When the ratio is more than 0.3, it is thought that the wastewater could be effectively degraded by biological treatment.31 Thus, COD and BOD5 determination of the treated X-3B by the Co2+-HCO3− system were carried out in order to characterize the evolution of the BOD5/COD ratio. Figures 8 and 9 show the effect of H2O2 concentration on
Figure 9. Effect of H2O2 concentration on biodegradability of X-3B by the Co2+-HCO3− system. Conditions: Co2+ 5 μmol L−1, NaHCO3 10 mmol L−1, and X-3B 0.34 mmol L−1, 25 °C.
that could be oxidized became less available and the remaining inert organic compounds after oxidation were difficult to break down. Similar results have been observed in other oxidation systems. For example, an increase in ozone concentration resulted in an increase in COD removal efficiency of simulated wastewater for X-3B. However, a continuous increase of ozone did not increase the efficiency quickly and the final value achieved was less than 30%.32 From Figure 9 it can be seen that the ratio of BOD5 to COD in raw solution was below 0.1, indicating that X-3B is a recalcitrant compound. After treatment by the Co2+-HCO3− system at an H2O2 concentration of 5 mmol L−1, the BOD5 to COD ratio of the dye solution increased to 0.46. This means that the biodegradability of X-3B significantly improved at such a low concentration of H2O2. Similar to COD removal, at higher H2O2 concentrations the BOD5 to COD ratio also increased. However, compared with the former, the extent of the ratio increased was much larger, indicating more amenable products to biodegradation formed at high H2O2 concentrations. In a biodegradability study of X-3B solution treated with a microwave assisted photocatalytic process,31 it was suggested that low biodegradability of the dye was contributed to the azo linkage and decolorization can lead to the increase in biodegradability of the solution. Considering that a part of X3B still presented in solution under low H2O2 concentrations (10 mmol L−1), the same conclusion can be obtained in our study. However, at higher H2O2 concentrations, some recalcitrant intermediates can be further transformed into biodegradable products. Since a wastewater having the BOD5 to COD ratio of 0.3 and higher is considered easily biodegradable, it can be assumed from above results that after degradation with the Co2+-HCO3− system the effluent of X-3B is biodegradable enough to be easily post-treated in a biological reactor. Figure 10 gives the results of the biological experiments of raw and pretreated X-3B solution at a H2O2 concentration of 100 mmol L−1. Because many of dyestuffs are toxic to the organisms and result in sludge bulking, aerobic treatment processes are not effective for treating dyestuff wastewater and very little COD for the raw X3B solution was removed by the active sludge treatment process. The efficiency first increased only to 5% after 2 h, maybe the result of adsorption of the dye onto the biomass, and
Figure 8. Effect of H2O2 concentration on COD removal of X-3B by the Co2+-HCO3− system. Conditions: Co2+ 5 μmol L−1, NaHCO3 10 mmol L−1, and X-3B 0.34 mmol L−1, 25 °C.
COD removal and the BOD5/COD ratio for a given initial X3B concentration of 0.34 mmol L−1. As shown in Figure 8, COD removal of the solution increased with increasing the concentration of H2O2. In particular, there was a marked increase when H2O2 increased from 0 to 5 mmol L−1. However, little difference was observed when H2O2 concentration further increased from 5 to 100 mmol L−1. Note that, whereas the COD removal was 70% at the highest concentration of H2O2, only 26% TOC removal was observed under the same reaction conditions. These facts suggested that, as the reaction continued, recalcitrant organic compounds in the wastewater 11108
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Figure 10. Evolution of COD in the bioreactor for X-3B solution treated by different H2O2 concentrations. Conditions: Co2+ 5 μmol L−1, HCO3− 10 mmol L−1, and X-3B 0.34 mmol L−1, 25 °C.
then was kept constant. Lu et al. also found that the COD removal of the raw textile wastewater containing X-3B was only about 30% by the up flow biological aerated filter process, and the color degree even had a slight rise.33 In their studies, a previous inoculation of the biological filter was even employed in order to obtain an adequate biological yield. During our experiments, the microbial population might be lower as the sludge collected from a wastewater treatment plant was used directly as the seeding sludge. However, for the treated X-3B solution, the COD removal continually increased to about 44% after 8 h of aeration. These observations clearly depict that, contrary to X-3B molecules, the oxidation intermediates formed during the catalytic oxidation by the Co2+-HCO3− system are readily biodegradable. In other words, the Co2+-HCO3− system could degrade the toxic dye molecules into simple intermediates, thereby overcoming their inhibitory influence on the metabolism of microorganisms so that they could be rapidly biodegraded in the biological reactor. The medium pH is an important factor with regard to the dye degradation by biological treatment, and the optimal value is often between 6.0 and 10.0.34 The pH in the effluent pretreated by advanced oxidation processes such as Fenton and photocatalysis was usually below 3.0, and then it was needed to adjust to neutral with NaOH or CaO before the solution fed to a biological reactor.33 However, for the Co2+-HCO3− system, this operation step can be left out as HCO3− can effectively buffer the solution pH changes. 3.4. Intermediate Products Analysis. The enhancement of the biodegradability can be related to cleavage of the azo bonds and aromatic rings and thus the conversion of the dye molecules into simple and degradable intermediates.35 Indeed, the formation of small organic acids and inorganic ions after X3B degradation can be detected by ESI-MS and IC. Figure 11A displays the representative ESI spectra in the negative ion mode acquired for X-3B solution before and after degradation. At the beginning of the reaction, two intense ions of m/z 592 and 284 corresponding to X-3B [M − Na] − and [M − 2Na] 2‑ were observed as expected. The RSO3Na groups in the dye will dissociate into RSO3− and Na+ ions in solution. Two other more intense ions observed at m/z 471 and 234 may be obtained by subsequent losses of SO2 and Cl in the dye
Figure 11. (A) ESI (−) mass spectra of AOII solution before and after degradation with the Co2+-MEA system at a H2O2 concentration of 100 mmol L−1, (B) expanded region from m/z 100 to 150. Conditions: Co2+ 5 μmol L−1, HCO3− 10 mmol L−1, and X-3B 0.34 mmol L−1, 25 °C.
structure. After degradation, the intensity of these intense ions decreased significantly. By expanding the two MS spectra in the range of m/z 100 to m/z 150, several additional abundant ions appeared, indicating that X-3B was degraded to some intermediate products. According to the analysis of the fate of X-3B degradation by other oxidation systems such as UV/ TiO2,36 six degradation products coming from the destruction of the azo groups and naphthalene rings were identified: (1) tartaric acid (m/z 149), (2) 6-chloro-1,3,5-triazine-2,4-diol (m/ z 146), (3) adipic acid (m/z 145), (4) nitrophenol (m/z 138), (5) pyrogallic acid (m/z 125), (6) nitropropionic acid acid (m/ z 118), and (7) succinic acid (m/z 117). From the analysis, it is understood that the Co2+-HCO3− complex is efficient to degrade the azo dye. The smaller acid products were further detected by IC. As shown in Figure 12, some measurable reaction products such as formate, acetate, oxalate, sulfate, and nitrate were identified. These breakdown molecules, known as ultimate organic products during the aromatic ring-opening, are nontoxic and biodegradable.8
4. CONCLUSIONS In this study, a simple and efficient system, Co2+-HCO3−, for the degradation of X-3B with H2O2 in aqueous solution has been developed. The main operational parameters on the catalytic activity of the system and biodegradability enhancement of the dye were investigated. It was found that the system exhibited high degradation efficiency over a wide pH range from neutral to alkaline with low concentrations of H2O2. More over, the catalytic oxidation with the Co2+-HCO3− system could significantly improve the biodegradability of the azo dye, making possible the post-treatment of the effluent in a bioreactor. Some intermediate products were also analyzed by ESI-MS and IC. 11109
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Figure 12. Products detected by ion chromatography after X-3B degradation by the Co2+-HCO3− system. Conditions: Co2+ 5 μmol L−1, HCO3− 10 mmol L−1, H2O2 100 mmol L−1, and X-3B 0.34 mmol L−1, 25 °C.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (A.X.);
[email protected] (Q.Z). Phone: (+86) 27-59367334. Fax: (+86) 27-59367334. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Science and Technology Research Project of Educational Commission of Hubei Province of China (Grant Q20121705), School Foundation of Wuhan Textile University in 2011, and the National High Technology Research and Development Program of China (Grant 2009AA063904).
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