Degradation of Organic Pollutants in Wastewater by Bicarbonate

Mar 14, 2013 - Yong Feng , Deli Wu , Yu Deng , Tong Zhang , and Kaimin Shih. Environmental Science & Technology 2016 50 (6), 3119-3127. Abstract | Ful...
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Degradation of Organic Pollutants in Wastewater by BicarbonateActivated Hydrogen Peroxide with a Supported Cobalt Catalyst Li Zhou, Wei Song, Zhuqi Chen, and Guochuan Yin* School of Chemistry and Chemical Engineering, Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China S Supporting Information *

ABSTRACT: Developing novel technologies to cleanup wastewater has attracted attention for a long while in academic and industrial communities not only for environmental issues but also for recycling water sources. This work demonstrates that bicarbonate-activated H2O2 can be applied as a novel oxidant source in pollutant degradation. Using a supported cobalt catalyst, bicarbonate-activated H2O2 can efficiently degrade various dyes and phenol at ambient temperature. Because the reaction media remains weakly basic during degradation, the cobalt leaching from the solid catalyst has been efficiently avoided and the lifetime of the catalyst can be extended to above 180 h without significant activity loss in a fixed-bed test. Different scavengers, including ascorbic acid, t-butanol, sodium azide, benzoquinone, and tiron, have been tested to identify the active species, which may be involved in pollutant degradation, and it was found that singlet oxygen and the carbonate radical may play a key role in the degradation process.

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INTRODUCTION With the scarcity of fresh water resources, degradation of the pollutants in wastewater for its recycling has been more and more crucial than ever.1 Among versatile biological, physical, and chemical technologies in pollutant treatments, the advanced oxidation technologies are of the most attractive because they are particularly suitable for degrading concentrated wastewater from whatever industrial and living sources.2−5 Up to now, a variety of advanced oxidation technologies, including wet air oxidation, catalytic wet air oxidation, Fenton oxidation, photocatalytic oxidation, ozone oxidation, electrocatalytic oxidation, etc., have been developed and extensively studied.6−13 However, these technologies have not been widely applied in wastewater treatments because of the fact that there always exist some drawbacks for their applications. For example, in catalytic wet air oxidation using heterogeneous catalysts, in addition to the fact that it needs to be performed at a relatively high temperature with a pressure that generates high capital costs, the leaching of the redox metal ions from catalysts also limits its lifetime and causes the second pollution of toxic heavy metal ions.14 Therefore, exploring simpler, lower cost, and safer technologies is still needed for practical applications. Recently, one technology using bicarbonate-activated H2O2 was developed in these laboratories for dye decolorization, chlorophenol degradation, and even practical landfill leachate treatments.15−19 Traditionally, bicarbonate and carbonate are believed to function as hydroxyl radical scavengers, because they would slow the degradation.20−23 Unexpectedly, in this bicarbonate/H2O2 system, the presence of bicarbonate can efficiently accelerate the degradation of dyes, whereas under the similar pH condition adjusted by NaOH, the degradation is negligible. Particularly, adding trace redox metal ions, such as cobalt, to the bicarbonate/H2O2 system would greatly © 2013 American Chemical Society

accelerate its degradation rate. The drawback is that these redox metal ions are actually toxic in flash water and need to be strictly regulated to the parts per million (ppm) level. For example, the emission limit for cobalt pollutant is 1 ppm according to GB 25467-2010 in China. Therefore, to avoid the potential toxic metal pollution, it is essential to explore the heterogeneous catalysts for this bicarbonate/H2O2 method. This work introduces the diatomite-supported cobalt catalysts into the pollutant degradation with bicarbonate-activated H2O2 oxidant. Unlike the common catalytic wet air oxidation, here, the pollutant degradations are treated in basic conditions, which inhibit the solution acidification because of the organic acid formation. Thus, the metal ions leaching from the catalysts have been efficiently prevented. Also, it is less environmental impact and more friendly for the equipment of pollutant treatments than original Fenton oxidation.

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EXPERIMENTAL SECTION Methyl blue, cobalt nitrite, diatomite, hydrogen peroxide, sodium bicarbonate, and other chemicals were purchased from local Sinopharm Chemical Reagent. Kinetic data were collected from Analytik Jena SPECORD 205. Chemical oxygen demand (COD) analysis was performed with HACH DR1010 (note that a novel, portable instrument has been reported for rapid COD analysis24). The metal ion leaching was detected by atomic absorption spectroscopy (AAS) analysis with AAnalyst 300 Perkin Elmer. X-ray diffraction (XRD) analysis was performed on X’Pert PRO, and infrared (IR) analysis was conducted on Equinox 55 Bruker. Received: Revised: Accepted: Published: 3833

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Preparation Procedure of Diatomite-Supported Cobalt Powder Catalyst for Bench Test. In 12.5 mL of water containing 0.41 g of Co(NO3)2, 10 g of diatomite was added with stirring. After the resulting mixture stood at ambient temperature for 24 h, it was dried at 70 °C in an oven. Then, the resulting powder was calcinated in a muffle furnace at 450 °C for 3 h to obtain the diatomite-supported cobalt catalyst (cobalt content of 1.58%). The catalyst was further characterized by Brunauer−Emmett−Teller (BET), XRD, and IR analyses. Similar preparation procedures have been conducted for iron-, copper-, and manganese-supported catalysts. Preparation Procedure of Diatomite-Supported Cobalt Catalyst for Fixed-Bed Test. A total of 33.5 mL of water was added to the mixture of 5 g of (NH4)2CO3, 2.5 g of K3PO4·3H2O, and 50 g of diatomite with stirring, and then the resulting sample was shaped into strips and calcinated in a muffle furnace at 60 °C for 2 h and then at 1000 °C for 3 h to obtain the cylindrical catalyst support. Then, 24 g of the prepared diatomite support was added to 20 mL of aqueous solution containing 3.02 g of Co(NO3)2·6H2O. After the resulting mixture stood at ambient temperature for 24 h, it was dried at 70 °C in an oven and finally calcinated at 450 °C in a muffle furnace for 3 h to obtain the supported cobalt catalyst for fixed-bed test (cobalt content of 1.11%). General Procedure for Pollutant Degradation in Bench Test. A total of 4 mL of aqueous solution containing 300 mg/L of methylene blue was added to 10 mL of aqueous solution containing 0.01 g of diatomite-supported cobalt catalyst. The resulting solution was further stirred for 1 h at 25 °C to achieve absorption equilibrium between the dye and catalyst. Then, 10 mL of H2O2 oxidant solution was added to the above solution containing substrate and catalyst (the oxidant solution was prepared by adding 0.36 mL of 30% H2O2 to 10 mL of aqueous solution containing 0.126 g of NaHCO3 and then calibrated to 25 mL with water). The resulting reaction mixture was stirred at 25 °C for 5 h, and periodically, 1 mL of the reaction solution was taken out, diluted to 10 mL with water, and detected by an ultraviolet−visible (UV−vis) spectrometer at 664 nm for determining the dye conversion. General Procedure for Pollutant Degradation for Fixed-Bed Test. In a double-jacketed glass reactor (height of 60 cm and inner diameter of 1 cm), 11.04 g of cylindrical catalyst (length of 5 mm, diameter of 4 mm, and cobalt content of 1.11%) was loaded. The dye solution containing 60 mM methyl blue and the oxidant solution containing 80 mM H2O2 with 50 mM NaHCO3 were independently but simultaneously pumped from the top of the glass reactor in a 1:1 ratio (v/v) at 25 °C. The total fluid rate was 34 mL/h, and the retention time was 47 min. The reaction solution was collected per 12 h, and analyses of dye conversion, COD removal, and cobalt leaching were conducted.

Alternatively, heterogeneous catalysts are very attractive for their feasible recycling and, especially, for their applications in fixed-bed technologies. The challenge is that the reaction medium gradually turns acidic, which causes serious catalyst leaching during wastewater treatments.27 Because the bicarbonate-activated H2O2 system has a weakly basic medium in which bicarbonate simultaneously serves as a buffer, it may retard the redox metal ions leaching from heterogeneous catalysts. In present studies, supported cobalt catalysts were investigated in the bicarbonate/H2O2 system for pollutant degradations. The catalyst preparations with diatomite support were described in the Experimental Section, and their characterizations were conducted with BET, XRD, and IR. As measured by physical adsorption of N2 using the BET method, the specific surface area of diatomite-supported cobalt catalyst (calcinated at 400 °C) was 52.6 m2 g−1. With the calcination temperature increasing from 400, 450, 500, 550, to 600 °C, the surface area changes from 52.6, 44.3, 43.7, 40, to 41.9 m2 g−1 gradually. After dissolution in aqueous HF solution, the AAS analysis of the supported catalyst displayed that the content of the cobalt ions is 1.58%. However, XRD studies revealed that the cobalt catalyst has an identical spectra to that of the diatomite support itself, indicating that the cobalt ions on the catalyst have been highly dispersed and no crystal Co3O4 or Co2O3 formation during the treatments. In IR spectra, a minor absorbance band at 664.5 cm−1, because of the low cobalt concentration, can be assigned to the Co−O stretching band of Co3O4, while the enhanced band at 534.2 cm−1 is possibly related to the Co−O stretching but not conclusive (Figure 1).28,29 In particular, the band at 917.3 cm−1 is normally

Figure 1. IR spectra of diatomite-supported cobalt catalyst.

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assigned to a lattice defect correlated with the presence of tetrahedral framework linkages of the type M−O−Si, which indicates the formation of the Co−O−Si bridge rather than physical absorption.30 In wastewater treatment by bench test, methylene blue was first applied as a dye model to test the catalytic efficiencies of the prepared catalysts with bicarbonate-activated H2O2, and the degradation kinetics was monitored by an UV−vis spectrophotometer. Under the bench test conditions described in the Experimental Section, 98% of decolorization of methyl blue with 70.4% of COD removal could be achieved with cobalt catalyst in 5 h, and the degradation can be illustrated by its

RESULTS AND DISCUSSION Wastewater containing dye pollutants is one of the major pollution sources in many countries, and their degradations have attracted a lot of attention in academic and industrial communities.25,26 In previous studies, we had demonstrated that bicarbonate-activated H2O2 is active for versatile wastewater degradations and adding minor redox metal ions, such as cobalt, may significantly accelerate the degradation.15−18 However, the most concern is that the added simple cobalt salts may cause the second heavy metal ion pollution. 3834

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characteristic absorbance decreasing at 245, 298, and 664 nm with the reaction proceeding (Figure 2). In the complementary

Figure 3. Degradation of methylene blue (50 mg/L) with H2O2 (60 mM) under different conditions: (a) 25 mM NaHCO3, 0.01 g of diatomite-supported cobalt, and 25 °C; (b) pH 8.2 adjusted by NaOH, 0.01 g of diatomite-supported cobalt, and 25 °C; (c) 0.01 g of diatomite-supported cobalt and 25 °C; (d) 25 mM NaHCO3; and (e) 25 mM NaHCO3 and 3.9 μM Co(NO3)2.

Figure 2. Degradation kinetics of methylene blue (5 min/scan). Conditions: 50 mg/L methylene blue, 25 mM NaHCO3, 60 mM H2O2, 0.01 g of diatomite-supported cobalt, and 25 °C.

tests, 94, 87, and 68% of decolorizations with 53.9, 34.2, and 20.7% of COD removals could be achieved for supported manganese, iron, and copper catalysts, respectively. Accordingly, the cobalt catalysts were used for all of studies below. The leaching of redox metal ions from the heterogeneous catalysts is always one of the crucial issues for their practical applications. Here, cobalt leaching from the diatomite support was investigated under the methylene blue degradation conditions, and the results are summarized in Table 1. One may see that,

In earlier studies, without redox metal ions, bicarbonate at a high concentration, for example, 0.5 M, could activate H2O2 and degrade dyes directly.16 In the presence of the supported cobalt catalyst, using H2O2 in neutrally aqueous solution provides relatively poor degradation efficiency. Adding NaOH to the neutral solution to generate a similar pH condition of bicarbonate solution (pH 8.2) gives improved methylene blue degradation; however, its efficiency is still substantially lower than that of the bicarbonate-activated H2O2 system. As shown in Table 1, although there is far less than 1 ppm of the cobalt ions leaching (0.2 ppm) occurring when using supported cobalt catalyst, the minor leached cobalt ions may also have a chance to play significant roles in methylene blue degradation. Here, in the control experiment using 3.9 μM Co(NO3)2 as the catalyst (equal to 0.24 ppm), the degradation efficiency is far less than using supported cobalt catalyst. Particularly, after the reaction for 5 h, the COD removal is 64.2% when using supported cobalt catalyst, whereas it is only 10.5% in the case of using 3.9 μM Co(NO3)2 catalyst, strongly supporting that the leached cobalt ions only play a minor role in the supported cobalt catalyst mediated degradation. It is worth noting that the leaching of cobalt ions is due to its redox behavior in dye degradation; thus, its concentration is gradually accumulated to 0.24 ppm in 5 h. Thus, its contribution to dye degradation in the supported cobalt catalyst system should be less than that directly using 3.9 μM Co(NO3)2 catalyst. Although it has been widely proposed that bicarbonate and carbonate are the common hydroxyl radical scavengers in versatile advanced oxidation processes,20−23 these data clearly demonstrate that bicarbonate plays a critical role in methylene blue degradation, including both activating H2O2 and serving as a base medium to retard the catalyst leaching in this system. In consideration of the practical applications of this method in treating weak acidic wastewater (pH > 5), sodium bicarbonate may be directly used to adjust the pH of wastewater to the weak basic condition. For the wastewater having a low pH value, to control the cost, certain cheap bases, such as CaO, may be applied to adjust pH at first, and then sodium bicarbonate can be used to generate buffer conditions for degradation. The influence of the bicarbonate concentration on methylene blue degradation was also investigated. As stated above, without bicarbonate, the supported cobalt may also catalyze the degradation of methylene blue with H2O2 oxidant; however,

Table 1. Leaching of Cobalt Ions under Different Catalyst Calcination Temperatures in Bench Testa calcination temperature (°C) cobalt leaching (ppm)

400 0.22

450 0.20

500 0.27

550 0.16

600 0.19

a

Conditions: 50 mg/L methylene blue, 25 mM NaHCO3, 60 mM H2O2, 0.01 g of cobalt catalyst, 24 mL of reaction volume, 25 °C, and 5 h.

after 5 h of reaction, the leaching of these cobalt catalysts is very similar, about 0.16−0.27 ppm in solution, even though they were prepared under different calcination temperatures from 400 to 600 °C. After the reaction, the pH of the reaction medium remains unchanged because of the presence of bicarbonate. Significantly, in the control experiment without H2O2 added, there is no leaching of the cobalt ions occurring, suggesting that minor leaching of the cobalt ions comes from its redox behavior in the degradation process rather than the acidassisted dissolution. During the catalytic degradation, the oxidation state of cobalt ions changes frequently between +2 and +3, which may cause a loose coordination structure of the cobalt ions and make it possible for leaching. The influence of the catalyst calcination temperature on the degradation efficiency was also investigated. Under the identical conditions, the cobalt catalyst calcinated at 450 °C demonstrates the highest degradation efficiency, while the temperature higher or lower than that provides less efficiency (see Figure S1 of the Supporting Information). Accordingly, all of the catalysts used in the following studies were calcinated at 450 °C. Figure 3 demonstrates the degradation of methylene blue under different catalytic conditions. Without cobalt catalyst, almost no degradation of methylene blue occurs in 0.025 M bicarbonate solution, which is different from those in previous observations. 3835

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to decrease but not completely quench the reaction, indicating the existence of the radical-mediated degradation (see Figure S3 of the Supporting Information). In comparison to bicarbonate, t-butanol is a more powerful hydroxyl radical scavenger, having the second-order rate constant of 6 × 108 M−1 s−1 (versus 8.5 × 106 M−1 s−1 for bicarbonate; see eq 1).41−43 Figure 4 displays the influence of t-butanol on

its efficiency is obviously poor (see Figure S2 of the Supporting Information). Adding minor bicarbonate, for example, 5 mM, to the solution would greatly improve the catalytic efficiency. The optimized bicarbonate concentration is 25 mM; further increasing its concentration, for example, up to 0.1 M, would reduce the degradation rate, which further supports the critical role of bicarbonate in methylene degradation. •OH + HCO3− → H 2O + CO3− •

(1)

In advanced oxidation technologies, the hydroxyl radical is one of the well-accepted key active species for pollutant degradation with a redox potential of +2.8 V (versus NHE) in acidic media and +1.5 V (versus NHE) in basic media, and it is capable of non-selectively oxidizing versatile pollutants to CO2.31 Although both bicarbonate and carbonate, as well as t-butanol, are commonly used as scavengers to test whether the hydroxyl radical occurs in reaction solution (eq 1),20−23 it has also reported that, under certain concentrations, bicarbonate or carbonate may enhance the degradation rates of pollutants. For example, Vione and co-workers found that, in sonochemical degradation of dyes, the degradation rates of acid blue 40 and methylene blue could be enhanced by adding bicarbonate, and it was rationalized by the fact that the radical−radical recombination of the bicarbonate radical (1.2 × 107 M−1 s−1) is much slower than that of the hydroxyl radical (5.5 × 109 M−1 s−1); thus, they are more available than the hydroxyl radical for pollutant degradation.32 Consistent with this, in natural surface water, it was reported that the steady-state concentration of the carbonate radical (10−13−10−15 M) is generally about 2 orders higher than that of the hydroxyl radical (10−14−10−18 M).33 In fact, the redox potential of the carbonate radical is about +1.78 eV at pH 7, which is much higher than that of the iron(TAML) catalyst, a well-known catalyst for a variety of pollutant degradations, including dyes, chlorophenols, and organophosphorus.22,34,35 As evidence, Hoffman and co-workers found that the carbonate radical is capable of hydrogen abstraction from methyl t-butyl ether;36 we have also found that bicarbonate-activated H2O2 is capable of damaging the aromatic ring of dyes to generate small organic acids.16 Thus, the relatively long lifetime and still good oxidizing power have facilitated the carbonate radical to serve as a good active species for pollutant degradation. Significantly, the presence of the carbonate radical may lead to the formation of other radicals, including the perhydroxyl radical, superoxide ion, and singlet oxygen, which are active for pollutant degradation under different conditions (eqs 2−6).37,38 H 2O2 + CO3−• → HCO3− + HO2 •

(2)

HO2 • → H+ + O2•−

(3)

O2•− + •OH → O21 + OH−

(4)

HO2 • + O2•− → O21 + HO2−

(5)

HO2 • + HO2 • → O21 + H 2O2

(6)

H 2O2 + HCO3− ← → H 2O + HCO4 −

(7)

Figure 4. Influence of t-butanol to the methylene blue degradation by bicarbonate-activated H2O2 with supported cobalt catalyst. Conditions: 50 mg/L methylene blue, 60 mM H2O2, 0.01 g of diatomitesupported cobalt, and 25 °C.

methylene blue degradation. Obviously, t-butanol significantly decreases the degradation rate, and the inhibition effect is concentration-dependent upon a wide range (0.22−3.3 M), indicating that the hydroxyl radical does occur in the reaction and plays a role in the degradation. The hydroxyl radical can be produced by decomposition of H2O2 or percarbonate by the cobalt catalyst, where percarbonate is formed through the equilibrium between H2O2 and bicarbonate (eq 7).44 However, it also demonstrates that t-butanol cannot completely inhibit the degradation. Sodium azide is a well-used scavenger for the singlet oxygen.45−47 Here, in the presence of sodium azide, the methylene blue degradation was also significantly compressed, suggesting the occurrence of singlet oxygen in the degradation (Figure 5). Benzoquinone is the scavenger of the superoxide radical.48−50 As demonstrated in Figure 6, adding benzoquinone does not affect methylene blue degradation at all, indicating that superoxide is not involved in the degradation here. Similarly, no inhibition effect was observed with tiron,

To test whether these radicals occur in this bicarbonate/H2O2 system with cobalt catalyst, first of all, ascorbic acid, a popular radical scavenger,39,40 was added to the reaction mixture to investigate whether the degradation could be retarded. As observed, adding ascorbic acid does cause the degradation rate

Figure 5. Influence of sodium azide (NaN3) to the methylene blue degradation by bicarbonate-activated H2O2 with supported cobalt catalyst. Conditions: 50 mg/L methylene blue, 60 mM H2O2, 0.01 g of diatomite-supported cobalt, and 25 °C. 3836

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recombination to generate singlet oxygen could be much faster than its participation in degradation or trapping by benzoquinone or tiron. Thus, there was no inhibition effect observed for the superoxide radical. On the other side, none of the scavengers, including ascorbic acid, t-butanol, sodium azide, benzoquinone, and tiron, can completely quench the reaction, indicating that there exist other active species beyond singlet oxygen to participate degradation. As stated above, in the control experiment without cobalt catalyst, bicarbonate plus H2O2 does not degrade methylene blue under the described conditions, indicating that neither H2O2 nor percarbonate (HCO4−) directly degrade methylene blue. Consequently, the carbonate radical, generated by trapping the hydroxyl radical with bicarbonate,20−23 possibly serves as a second key active species responsible for methyl blue degradation. In fact, it has been reported that the carbonate radical is capable of degrading versatile pesticides, dyes, and even methyl t-butyl ether.32,36,52 Because its radical−radical recombination (1.2 × 107 M−1 s−1) is much slower than that of the hydroxyl radical (5.5 × 109 M−1 s−1), its concentration in the reaction medium could be much higher than that of the hydroxyl radical.33,41−43 Thus, even though its oxidizing power is lower than that of the hydroxyl radical, the carbonate radical may play a significant role in dye degradation. Besides methylene blue, the bicarbonate-activated H2O2 with supported cobalt catalyst is also capable of degrading other dyes, such as methylene orange and rhodamine B (see Figures S5 and S6 of the Supporting Information). After 5 h of reaction, 90% of decolorization with 60.7% of COD removal can be achieved for methylene orange, while it is 70% of decolorization with 47.2% of COD removal for rhodamine B. Under identical conditions, 98% of methyl blue decolorization with 70.4% of COD removal could also be achieved. Phenols are another category of pollutants widely occurring in wastewater.53,54 Here, the bicarbonate-activated H2O2 with cobalt catalyst also demonstrates considerable activity in its degradation (Figure 8).

Figure 6. Influence of benzoquinone (BQ) to the methylene blue degradation by bicarbonate-activated H2O2 with supported cobalt catalyst. Conditions: 50 mg/L methylene blue, 60 mM H2O2, 0.01 g of diatomite-supported cobalt, and 25 °C.

which is another superoxide scavenger (see Figure S4 of the Supporting Information).51 The above inhibition tests reveal that both the hydroxyl radical and singlet oxygen possibly occur and participate in the degradation process, whereas the superoxide radical does not. However, as shown in Figure 4, the inhibition effect of tbutanol is apparently concentration-dependent, and the maximum of its inhibition effect is equal to that of using sodium azide to trap singlet oxygen (see Figure 5). Furthermore, using the combination of t-butanol and sodium azide to trap both the hydroxyl radical and singlet oxygen in a single run reveals a similar inhibition effect of using either the hydroxyl radical or sodium azide alone (Figure 7). These data

Figure 7. Influence of t-butanol plus sodium azide to the methylene blue degradation by bicarbonate-activated H2O2 with supported cobalt catalyst. Conditions: 50 mg/L methylene blue, 60 mM H2O2, 0.01 g of diatomite-supported cobalt, and 25 °C. Figure 8. Degradation of phenol under different H2O2 concentrations. Conditions: 1.2 mM phenol, 25 mM NaHCO3, 60 mM H2O2, 0.0050 g of diatomite-supported cobalt, and 25 °C.

suggest that only singlet oxygen directly participates in the methylene blue degradation, while, because of the presence of bicarbonate, the hydroxyl radical does not directly participate the degradation but participates in the formation of the carbonate radical and singlet oxygen, as demonstrated in eqs 1−6. The disclosed inhibition effect of t-butanol could be rationalized by the fact that completely trapping of the hydroxyl radical by adding t-butanol would terminate the singlet oxygen formation; thus, t-butanol demonstrates a similar inhibition effect as that of singlet oxygen scavenger. One may suspect that the superoxide radical may occur in the reaction mixture, as demonstrated in eqs 2−6; however, the radical−radical

Increasing the oxidant dose, the conversion could be linearly improved, and COD removal of phenol was also improved as well. For example, in the presence of 0.18 M H2O2, the conversion of phenol can reach up to 71.6% with COD removal of 37.9%. To test the lifetime of the diatomite-supported cobalt catalyst, the catalyst was shaped in strips. After calcination at 450 °C, the obtained cylindrical catalysts (length of 5 mm, 3837

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cobalt catalyst (Figure S4), degradation kinetics of methylene orange in bicarbonate-activated H2O2 with supported cobalt catalyst (Figure S5), degradation kinetics of rhodamine B in bicarbonate-activated H2O2 with supported cobalt catalyst (Figure S6) and facility of the fixed-bed test in diatomitesupported cobalt catalyst catalyzed pollutant degradation (Figure S7). This material is available free of charge via the Internet at http://pubs.acs.org.

diameter of 4 mm, and cobalt content of 1.11%) were loaded in a glass reactor (height of 60 cm and inner diameter of 1 cm; see Figure S7 of the Supporting Information) for fixed-bed test, and the results are demonstrated in Figure 9. At the beginning

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 86-27-87533732. Fax: 86-27-87543632. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by Chutian Scholar Foundation from Hubei province, China. The authors also thank the Analytical and Testing Center of Huazhong University of Science and Technology for help in XRD analysis.

Figure 9. Methyl blue decoloration and COD removal in fixed-bed test. Conditions: reactor (height of 60 cm and inner diameter of 1 cm), 11.4 g of catalyst loading (length of 5 mm, diameter of 4 mm, and cobalt content of 1.11%), 60 mM methyl blue solution, 80 mM H2O2 and 50 mM NaHCO3 oxidant solution, 34 mL/h total fluid rate [1:1 (v/v) dye/oxidant], 47 min of retention time, and 25 °C.

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(1) Levine, A. D.; Asano, T. Recovering sustainable water from wastewater. Environ. Sci. Technol. 2004, 38 (11), 201A−208A. (2) Lin, H.; Gao, W.; Meng, F.; Liao, B.; Leung, K.; Zhao, L.; Chen, J.; Hong, H. Membrane bioreactors for industrial wastewater treatment: A critical review. Crit. Rev. Environ. Sci. Technol. 2012, 42 (7), 677−740. (3) Tilak, G.; Escobar, I. C. Membranes for water treatment applicationsAn overview. ACS Symp. Ser. 2011, 1078, 155−170. (4) Hapeman-Somich, C. J. Chemical degradation of pesticide wastes. ACS Symp. Ser. 1992, 510, 157−165. (5) Ollis, D. F. Comparative aspects of advanced oxidation processes. ACS Symp. Ser. 1993, 518, 18−34. (6) Luck, F. Wet air oxidation: Past, present and future. Catal. Today 1999, 53 (1), 81−91. (7) Levec, J.; Pintar, A. Catalytic wet-air oxidation processes: A review. Catal. Today 2007, 124 (3−4), 172−184. (8) El-sousy, K.; Hussen, A.; Hartani, K.; El−Aila, H. Elimination of organic pollutants using supported catalysts with hydrogen peroxide. Jordan J. Chem. 2007, 2, 97−103. (9) Sun, C.; Chen, C.; Ma, W.; Zhao, J. Photodegradation of organic pollutants catalyzed by iron species under visible light irradiation. Phys. Chem. Chem. Phys. 2011, 13 (6), 1957−1969. (10) Deng, Y.; Englehardt, J. D. Treatment of landfill leachate by the Fenton process. Water Res. 2006, 40 (20), 3683−3694. (11) Bautista, P.; Mohedano, A. F.; Casas, J. A.; Zazo, J. A.; Rodriguez, J. J. An overview of the application of Fenton oxidation to industrial wastewaters treatment. J. Chem. Technol. Biotechnol. 2008, 83, 1323−1338. (12) Masten, S. J.; Davies, S. H. R. The use of ozonization to degrade organic contaminants in wastewaters. Environ. Sci. Technol. 1994, 28 (4), 180A−185A. (13) Martinez-Huitle, C. A.; Brillas, E. Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods: A general review. Appl. Catal., B 2009, 87 (3−4), 105−145. (14) Cybulski, A. Catalytic wet air oxidation: Are monolithic catalysts and reactors feasible? Ind. Eng. Chem. Res. 2007, 46 (12), 4007−4033. (15) Yin, G.; Xu, A.; Xiong, H. Method for oxidative degradation of organic pollutant in wastewater by using activated hydrogen peroxide. Faming Zhuanli Shenqing Gongkai Shuomingshu 2009, CN 101503241 A 20090812. (16) Xu, A.; Li, X.; Xiong, H.; Yin, G. Efficient degradation of organic pollutants in aqueous solution with bicarbonate-activated hydrogen peroxide. Chemosphere 2011, 82, 1190−1195.

of treatment, methyl blue could be completely decolorized with about 50% COD removal. After 180 h of continuous treatment, the decoloration of methyl blue still remains above 95% with COD removal of above 30%. Particularly, the catalyst leaching in the solution is always below 0.3 ppm, and it gradually drops to 0.1 ppm, which is far below the emission limit for the cobalt pollutant (1 ppm) according to GB 25467-2010 in China. However, it is worth noting that any measurable release of heavy metal ions into the reactant solution is a potential limitation for real world applications. During the treatment, the pH value of solution after treatment is always between 8.2 and 8.7, which is crucial for avoiding metal ions leaching. In the common advanced oxidation technologies, with the degradation proceeding, the pH of solution gradually become acidic because of the formation of various organic acid intermediates, which causes the metal ions to dissolve gradually from the metal oxide catalyst. Here, because of the presence of bicarbonate, the pH of solution is kept weakly basic during treatment, which apparently avoids the acid-assisted metal ion dissolution. However, in the degradation process, the oxidation state of cobalt ions would change frequently between +2 and +3, which may cause the loose coordination environment; thus, the leaching of cobalt ions still happens. Accordingly, with the loss of cobalt ions on the catalyst, its degradation efficiency may also gradually decrease with the pollutant treatment proceeding. The ongoing work is to further improve the lifetime of the catalyst to facilitate its potential application in the practical pollutant treatment.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

Influence of the catalyst calcination temperature on methylene blue degradation (Figure S1), influence of the bicarbonate concentration on methylene blue degradation (Figure S2), influence of ascorbic acid (VC) to the methylene blue degradation by bicarbonate-activated H2O2 with supported cobalt catalyst (Figure S3), influence of tiron to the methylene blue degradation by bicarbonate-activated H2O2 with supported 3838

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dx.doi.org/10.1021/es400101f | Environ. Sci. Technol. 2013, 47, 3833−3839