Article pubs.acs.org/est
A New Method for Removal of Hydrogen Peroxide Interference in the Analysis of Chemical Oxygen Demand Tingting Wu* Department of Civil, Architectural and Environmental Engineering, University of Miami, 1251 Memorial Drive, MEB 314, Coral Gables, Florida 33146, United States
James D. Englehardt Department of Civil, Architectural and Environmental Engineering, University of Miami, P.O. BOX 248294, Coral Gables, Florida 33124-0630, United States ABSTRACT: Many advanced oxidation processes involve addition of hydrogen peroxide (H2O2) with the aim of generating hydroxyl radicals to oxidize organic contaminants in water. However, chemical oxygen demand, a common measure of gross residual organic contamination, is subject to interference from residual H2O2 in the treated water. A new method, involving catalytic decomposition of H2O2 with addition of heat and sodium carbonate (Na2CO3), is proposed in this work to address this problem. The method is demonstrated experimentally, and modeled kinetically. Results for 5 mM H2O2 in deionized (DI) water included reduction to below the COD detection limit after 60 min heating (90◦C) with addition of 20 g/L Na2CO3 concentrated solution, whereas 900 min were required in treated municipal wastewater. An approximate second order rate constant of 11.331 M−1·min−1 at Na2CO3 dosage of 20 g/L was found for the tested wastewater. However, kinetic modeling indicated a two-step reaction mechanism, with formation of peroxocarbonate (CO42‑) and ultimate decomposition to H2O and O2 in pure H2O2 solution. A similar mechanism is apparent in wastewater at high catalyst concentrations, whereas at low Na2CO3 addition rates, the catalytic effects of other constituents appear important.
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INTRODUCTION Advanced oxidation processes (AOPs) involve the in situ generation of the highly reactive hydroxyl radical (OH•), generally at near-ambient temperature and pressure, for the treatment of recalcitrant organic compounds and overall chemical oxygen demand (COD).1 This treatment approach has attracted increasing interest not only because hydroxyl radicals are capable of oxidizing contaminants that are refractory to attack by traditional water treatment oxidants, but also due to the “clean” nature of these processes, which do not transfer constituents to another fraction or phase and may be less subject to the production of hazardous by-products than other processes.2 AOPs used to generate OH• in aqueous systems include: O3;3 O3/H2O2;4−6 O3/UV;7,8 UV/H2O2;9,10 O3/UV/H2O2;11,12 Fenton process (H2O2/Fe2+), PhotoFenton (UV/H2O2/Fe2+), and electro-Fenton.10,13−17 H2O2 is commonly utilized in AOPs as well as in situ chemical oxidation (ISCO) because it is a strong oxidizing agent and a potent source of OH• at economical levels of addition. In addition, H2O2 is a naturally occurring substance which yields only water and oxygen upon decomposition.18 H2O2 can be applied either by dosing, such as implemented in peroxone (O3/H2O2), UV/H2O2, and Fenton processes, or through in situ generation, as implemented in electro-Fenton © 2012 American Chemical Society
methods. H2O2 dosing rates may vary from micromolar to molar concentration ranges,19−21 depending on the types of AOPs, the treatment scheme, required efficiency, and wastewater characteristics. Residual H2O2 has been found to be a problem since it will cause errors in the measurement of BOD and COD, and/or hinder bacterial action in subsequent biological treatment.21 However, very limited information about the quantity of residual H2O2, especially for full-scale applications, is available in the literature. In laboratory studies, residual H2O2 has been reported to range from less than 1 mM up to several tens of mM for various AOPs.10,14,15,17 However, in typical full-scale AOP installations employing H2O2 as part of the treatment it has been reported22 that 70−80% of the H2O2 dose can be expected to remain in the treated water as residual. In that case, residual H2O2 concentrations may range as high as molar levels. Any residual H2O2 in the treated water represents COD, and thereby interferes with COD analysis. COD is one of the most widely used parameters for wastewater characterization, Received: Revised: Accepted: Published: 2291
November 27, 2011 January 30, 2012 January 30, 2012 January 30, 2012 dx.doi.org/10.1021/es204250k | Environ. Sci. Technol. 2012, 46, 2291−2298
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hydrogen peroxide in the oxidation of sulfides, since the second order rate constants for sulfide oxidations by HCO4− were ∼300-fold greater than those for H2O2. However, the catalytic effect of carbonate ion on the decomposition of H2O2 at higher temperatures via the peroxocarbonic acid anion is little known, except that it has been reported responsible for the reduction of brightness and/or increases in H2O2 usage in pulp and paper bleaching processes operating at temperatures in excess of about 70 °C.30,31 The purpose of this work is to describe a new method for removing residual H2O2 in water and wastewater, to prevent interference in the analysis of COD, based on the addition of carbonate ion and heat. First, experiments were conducted in a deionized water (DI) matrix and in treated municipal wastewater. These results are then analyzed kinetically to examine the apparent mechanism of H2O2 decomposition. Applicability of the technique is discussed.
providing a gross measure of organic contamination not subject to interference from inorganic carbon. The dichromate method is preferred for measurement due to its superior capacity for oxidization, wide applicability, and ease of manipulation.18 Of course, inorganic species with reductive capacity, such as nitrite, chloride, and H2O2, can interfere with analysis of organic COD. By standard methods23, the interference of chloride and nitrite is minimized by the addition of mercuric sulfate and sulphamic acid, respectively. While H2O2 can be removed from some high-strength wastewaters such as landfill leachates by heating,24 methods to address the effect of H2O2 interference in COD analysis in other matrices are not as well developed. To our knowledge, there are only three articles in the scientific literature that focus on H2O2 interference with the COD test. Talinli and Anderson25 reported an equivalent of 8.5 mg COD/mmol H2O2, and proposed a method of calculating COD in the presence of H2O2. Kang et al.18 conducted a more thorough study, proposing a quantitative correction for the effect of hydrogen peroxide on the COD analysis. They found that the ratio of equivalent COD to H2O2 was affected by the concentration of H2O2, increasing to a theoretical value of 16 mg COD/mmol H2O2. While these two studies were performed on static clean water or synthetic wastewater, Lee et al.12 investigated interference by H2O2 on COD analysis associated with advanced oxidation of livestock wastewater. They reported that the COD exerted by H2O2 was proportional to the remaining H2O2 concentration and the average ratio was 17−19.72 mg COD/mmol H2O2 in livestock wastewater, depending on the quality of wastewater. Previous studies just reviewed were intended to address H2O2 interference with COD analysis by quantitatively determining the concentration of H2O2, and correcting COD based on a best-fit linear relation between residual H2O2 and corresponding measured COD. This is primarily because reagents added to remove H2O2 oxidatively generally introduce COD error themselves.25 Although experimental data on clean water and synthetic wastewater reportedly fitted the empirical correction equations of the previously studies well,18,25 several constraints hinder wider application of such approaches. First, although these correction formulas are based on experimental results obtained with synthetic wastewater, significant differences in the COD equivalence of H2O2 were reported. Also, it has been found that in real wastewater, the extent of COD overestimation in the presence of H2O2 depends on the quality of the water treated. Even for the same type of wastewater, the COD equivalence of H2O2 can vary up to 16%.12 Unfortunately, if such methods are to be employed for the assessment of the performance of AOPs used for tertiary treatment of low concentrations of COD, such variance could result in complete misinterpretation of process performance. Hence, empirical ratios may need to be found experimentally for each treatment case. In addition, H2O2 concentration must be measured accurately, to further avoid error. Solid peroxocarbonates, such as Li2CO4, KHCO4, and NaHCO4, have long been known,26 and Adam and Mehta27 proved the existence of peroxocarbonate or hydrogen peroxocarbonate anion in saltlike compounds by structural methods. Flanagan and co-workers28 presented the first direct evidence for the formation of peroxocarbonate in the aqueous phase from the reaction of HCO3− with H2O2. Richardson et al.29 then showed that HCO3− is an effective activator for
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MATERIALS AND METHODS Chemical. 30% hydrogen peroxide (H2O2, BDH) solution and Anhydrous Sodium Carbonate (Na2CO3, J.T. Baker) were used in this study. All chemicals were analytical grade and used as received. Secondary Effluent (SE). Fresh secondary effluent was sampled from the Miami-Dade County Water and Sewer District, South District Wastewater Treatment Plant (SDWWTP), Miami, FL, U.S., after activated sludge aerobic biological treatment and secondary sedimentation but before chlorination in May 2011. The secondary effluent sample was stored at 4 °C in the refrigerator and filtered through filter paper (Fisher P5 medium) to minimize variability before each experiment. Characteristics of the SE sample are shown in Table 1. Table 1. Characteristics of the Sampled Municipal Secondary Effluenta parameter pH conductivity (μs/cm) alkalinity [mg/L as CaCO3] Cl− [mg/L] SO42‑ [mg/L] TKN [mg/L] TP [mg/L] TSS [mg/L] BOD [mg/L] TOC [mg/L] COD [mg/L] a
amount 7.2 760 208 77 28 22 2.1 7.8 5.5 11.0 34.8
± ± ± ± ± ± ± ± ± ± ±
0.1 35 9 6 3 8 0.9 1.9 1.2 2.2 7.5
Values are reported as Mean ±1Std.
Experimental Procedure. Experiments were carried out with both DI water and secondary effluent. First, various concentrations of H2O2 were spiked into 125 mL water samples at ambient temperature and immediately measured. For experiments in the DI matrix, Na2CO3 was added as both powder and concentrated solution (300 g/L). For the SE matrix, Na2CO3 was added solely as a concentrated solution. Final volume was maintained at 100 mL. Then the sample was covered to minimize evaporation losses and heated in a water bath at 90 °C for a predetermined period of time. Residual H2O2 and/or COD were analyzed at the end of each run. Error bars shown are ±1 standard deviations except as noted. 2292
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Table 2. Comparison of H2O2 Analysis Methods32,33 method
range
permanganate titration ceric sulfate titration iodometric titration
0.25−70% H2O2 1−30% H2O2 mg/L levels
titanium oxalate (spectrophotometric) cobalt-bicarbonate
0.1−50 mg/L
peroxidase enzyme
interference organic substances; any substance which reduces KMnO4 under acidic conditions none likely less susceptible to interference by organics and stabilizers added to commercial H2O2 solutions wastewater possessing strong yellow background
clear water matrix and free of turbidity ∼0.2 mg/L clear water matrix and free of turbidity ∼0.2 mg/L
reducing agents; any contaminate that absorbs UV light at 260 nm excessive turbidity or any contaminate that absorbs UV light at 596 nm
showed ratios of 17−19.72 mg COD/mmol H2O2.12 It was also reported that when H2O2 concentration is below 100 mg/L or above 2000 mg/L, the measured COD caused by H2O2 is not linearly dependent on H2O2 concentration.18,25 The variability in the COD equivalence of H2O2 may be explained by the possible reactions that consume H2O2 when dichromate is used as the principal oxidant for COD analysis:18,25
Analysis. Hydrogen peroxide concentration can be analyzed by several different methods (Table 2). For the concentration range utilized in this study, either the permanganate or the iodometric titration is applicable. Because the latter is less susceptible to interferences by organics and the wastewater sample was secondary effluent with low COD content (30−40 mg/L), the iodometric method was selected. The method was checked with standard solution prepared in SE and the measurement error was found to be ±2%. Specifically, 10−15 mL of 20% H2SO4 solution, 1−2 mL 10% KI solution and 2−3 drops of ammonium molybdate solution were added to 25 mL of sample; the sample was then titrated with 0.1 N sodium thiosulfate to a faint yellow end point; approximately 2 mL starch indicator was added and titration was continued until the blue color just disappeared. COD was measured spectrophotometrically (Beckman Coulter, DU720) by the K2Cr2O7oxidation method having a detection range of 3−150 mg/L (COD method 8000, HACH Co., Loveland, CO).
Cr2O7 2 − + 3H2O2 + 8H+ → 2Cr 3 + + 3O2 + 7H2O (1)
H2Cr2O7 + 5H2O2 → H2Cr2O12 + 5H2O
(2)
H2Cr2O12 + 8H2O2 → Cr2O3 + 9H2O + 8O2
(3)
According to the above equations, and noting that each millimole of Cr2O72‑ oxidizes 48 mg COD, the theoretical ratio of COD (mg/L) to H2O2 (mM) should be from 3.74/(eqs 2 and 3) to 16/(eq 1). Conceivably, therefore, different reactions/reaction sequences could occur under different conditions, resulting in variable COD equivalence. In addition, generally unknown, competing redox reactions and pathways may occur in real wastewater matrices, hindering quantitative analysis of organic COD. H2O2 Removal in DI Matrix. All experiments were carried out at 90 °C since preliminary study showed that temperature lower than 90 °C resulted in inefficient H2O2 removal. The effect of heating time and Na2CO3 dosage is illustrated in Figure 2. In these experiments, initial H2O2 concentration was 5 mM and Na2CO3 was added as concentrated solution. The analytical COD detection limit was 3 mg/L and the corresponding H2O2 concentration depicted in the figures for reference was assumed based on a theoretical ratio of 16 mg COD per mmol H2O2 (eq 1). In general, longer heating times and higher Na2CO3 dosages resulted in more efficient H2O2 removal. As shown, 90% of the added H2O2 was removed within 30 min at a Na2CO3 concentration of 20 g/L, and after 60 min the contribution of H2O2 to measured COD was below the detection limit. Further, at a 5 g/L Na2CO3 addition rate, 90% of H2O2 was removed within 60 min. H2O2 removal when Na2CO3 was added as dry powder versus in solution form, in otherwise identical experimental conditions, is shown in Figure 3. It can be seen that residual H2O2 concentration was always higher when Na2CO3 was added in powder form. The same phenomenon was observed in secondary wastewater (data not shown). These results suggest a homogeneous, aqueous-phase reaction, accelerated by predissolution of the catalyst. Also, the elevated temperature could hinder the dissolution of Na2CO3 powder because the dissolution of Na2CO3 is an exothermic process. Hence,
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RESULTS AND DISCUSSION The measured COD exerted by hydrogen peroxide added to secondary wastewater was shown in Figure 1. The slope of the
Figure 1. Results of initial tests of COD equivalence of hydrogen peroxide in secondary effluent. [Conditions: Medium: municipal secondary effluent; T: 25 ◦C].
fitted straight line is 17.4, i.e., 17.4 mg COD per mmol H2O2 (r = 0.946, p = 0.000 01, where p is the probability of obtaining a correlation as high as r by random chance, if the true correlation is zero). It is interesting to note among the limited literature addressing the quantitative effect of H2O2 on COD values, experiments carried out in synthetic water demonstrated ratios varying from 3.74 to 16 mg COD/mmol H2O2,18,25 while studies conducted in real wastewater, including the present one, 2293
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Figure 2. Effect of heating time and Na2CO3 dosage on H2O2 removal in deionized water. (The COD detection limit was 3 mg/L and the corresponding H2O2 concentration was determined based on 16 mg COD/mmol H2O2; error bars shown are ±2 standard deviation). [Conditions: a: [H2O2]0: 5 mM, Na2CO3: 20 g/L, T: 90 ◦C; b: [H2O2]0: 5 mM, t: 60 min, T: 90 ◦C ].
removal of H2O2 are shown in Figure 5. Given that peroxocarbonate ions, HCO4− and CO42−, have been found able to oxidize sulfide derivatives,29 it is necessary to examine if the change in measured COD is caused by organic COD being oxidized by peroxocarbonate. Therefore, residual H2O2 in the final samples was also measured. As shown, the difference between COD values measured in the original samples and those measured in the corresponding catalytically treated samples were negligible. In addition, residual H2O2 in all final samples was below the COD detection limit with the exception of one with 5 mM H2O2 and 20 g/L Na2CO3 added and 360 min heating applied (5/20/360). All of these results indicate that the method can successfully purge H2O2 interference in the analysis of COD over a wide range of H2O2 concentrations in real wastewater. Mechanism and Kinetics of H2O2 Removal. The negative impact of carbonate ions on pulp and paper bleaching processes due to the consumption of hydrogen peroxide has been reported.30 Suess and Janik31 pointed out the instability of the peroxo compounds, which decompose relatively easily into carbonate and oxygen. It was speculated that the decomposition of H2O2 is catalyzed in the presence of carbonate anion via the following reactions in the presence of excess peroxide:31
Figure 3. Comparison of Na2CO3 added as concentrated solution and as dry powder on H2O2 removal in DI water. (The COD detection limit was 3 mg/L and the corresponding H2O2 concentration was determined based on 16 mg COD/mmol H2O2). [Conditions: [H2O2]0: 5 mM; T: 90 ◦C; t: 60 min].
Na2CO3 was added as concentrated solution in subsequent experiments. H2O2 Removal in Secondary Effluent Matrix. Experiments were also conducted in secondary effluent to examine applicability of the technique in a real wastewater matrix. The results are illustrated in Figure 4. With respect to the effects of Na2CO3 dosage and heating time on H2O2 removal, results in the SE matrix exhibited similar trends as observed for the DI matrix, i.e., increasing heating time and Na2CO3 dosage enhanced removal efficiency. However, it was more difficult to remove H2O2 from the more complex and heterogeneous SE matrix than from the DI water, revealing a strong effect of water sample chemistry on H2O2 removal. This effect was attributed to a masking of active catalytic sites by organic and inorganic moieties. However, as demonstrated in Figure 4c, when the Na2CO3 dosage was increased to 50 g/L, H2O2-derived COD was brought to below the detection limit after 120 min of heating, even at an initial H2O2 concentration of 50 mM (1700 mg/L). To further quantify and characterize the performance of the method in wastewater, experiments were conducted to determine removal efficiency in secondary effluent as a function of initial H2O2, Na2CO3 dosage, and heating time. Values of initial COD, COD after H2O2 addition, and COD after catalytic
k1, k−1
CO32 − + H2O2 ←⎯⎯⎯⎯→ CO24 − + H2O k2
CO24 − + H2O2 → CO32 − + H2O + O2
(4)
(5)
In that case, the H2O2 and CO42‑ reaction rate laws can be expressed as follows: d[H2O2 ] = k−1[CO24 −] − k1[CO32 −][H2O2 ] dt − k2[CO24 −][H2O2 ]
(6)
d[CO24 −] = k1[CO32 −][H2O2 ] − k−1[CO24 −] dt − k2[CO24 −][H2O2 ] 2294
(7)
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Figure 5. Comparison of COD values of secondary effluent before and after H2O2 addition/removal. Numbers on the x-axis label represent: H2O2 addition (mM)/ Na2CO3 addition (g/L)/heating time applied (min.). [Conditions: T: 90 ◦C].
H2O2 decomposition in secondary effluent as a function of time is shown in Figure 6, for a Na2CO3 dosage of 20 g/L
Figure 6. H2O2 decomposition versus time in secondary effluent. [Conditions: Na2CO3: 20 g/L; [H2O2]0: 5 mM; T: 90 °C. Symbols and lines represent the measured data and the model of eq 8 and 9, respectively.]
(0.189 M) and initial H2O2 concentration of 5 mM. The Na2CO3 concentration at a zero addition rate represents native carbonate in the sample, after conversion of bicarbonate to carbonate at 90 °C. The heterogeneous nature of real wastewater notwithstanding, the rate law of eq 8 fits the observed H2O2 concentration profile well (r = 0.998, p = 0.000 006), with K1 = 0.0127 M·min and K2 = 11.573 M·min. Because rate constants such as k1 are dependent on the concentration of a homogeneous catalyst, values found for K1 and K2 are specific to a concentration of 20 g/L Na2CO3. Results of Figure 6 support the proposed mechanism of eqs 4 and 5, suggesting possible applicability at other rates of Na2CO3 addition, and consequent prediction of H2O2 concentration versus heating time for different Na2CO3 dosages. The results of eqs 8 and 9 are plotted versus the experimental data at an initial H2O2 concentration of 5 mM and 90 min heating, assuming the same values of K1 and K2 found at 20 mg/L added Na2CO3 in Figure 7a. It can be seen that within the Na2CO3 dosing range 0−50 g/L, the model tends to under-predict H2O2 removal assuming constant K1 and K2. Of course, the mechanism of eqs 4 and 5 suggests increasing k1 with increasing catalyst dosage up to an effective saturation point, and this effect would explain the increased removal at
Figure 4. H2O2 removal from secondary effluent. (The COD detection limit was 3 mg/L and the corresponding H 2 O 2 concentration was determined based on 16 mg COD/mmol H2O2).
Applying a pseudosteady-state approximation for peroxocarbonate (CO42−) and some algebra, gives: d[H2O2 ] =− dt
[CO32 −]T K1 K2 + [H2O2 ] [H2O2 ]2
(8)
in which K1 = (k−1)/(2k1k2); K2 = (k1 + k2)/(2k1k2)The relationship between [H2O2] and time, t, can be obtained by solving eq 8: ⎛ 1 ⎞ 1 − K1⎜ ⎟ − K2(ln[H2O2 ] − ln[H2O2 ]0 ) [H2O2 ]0 ⎠ ⎝ [H2O2 ] = t[CO32 −]T
(9) 2−
in which [H2O2]0 and [CO3 ]T are the initial molar H2O2 concentration and Na2CO3 dosage, respectively. 2295
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moieties and reactions was beyond the scope of this study. While increasing Na2CO3 concentration can enhance H2O2 removal in both regimes A and B of Figure 7b, the mechanism of eqs 4 and 5 apparently dominates only in regime B. However, eq 9 may still be applied over a wide range of Na2CO3 concentrations, by fitting of and K1 and K2 to experimental data. It is possible that multiple pathways beyond those considered here are involved in hydrogen peroxide decomposition. For example, if sufficient Fe2+ were present, then peroxide might react to produce OH•, which would then likely be scavenged efficiently by the carbonate present. However, iron and other transition metals are not typically present in concentrations approaching those of the peroxide and carbonate used in this study. Alternatively, Xu et al.34 reported the formation of O2−• during the activation of H2O2 with bicarbonate. However, results presented in Figure 5 suggest that superoxide is not an important intermediate in this case, because if any radicals were involved as key intermediates, organics in the wastewater would likely be attacked with consequent changes in COD. In any case, given the complexity and variability in wastewater from site to site and in time, more detailed mechanistic development would likely be specific to the wastewater sample considered. While eq 9 cannot be solved explicitly for H 2 O 2 concentration, a simpler approximate form can be found by assuming a one-step mechanism as follows:
Figure 7. Effect of Na2CO3 dosage on reaction kinetics. a. Residual H2O2 concentration versus Na2CO3 dosage in secondary effluent, and residual H2O2 concentration predicted by eqs 8 and 9 assuming no change in rate equation constants with Na2CO3 dosage; b. Rate equation constants computed as a function of Na2CO3 dosage, assuming eq 8 and 9. [Conditions: [H2O2]0: 5 mM; t: 90 min; T: 90 °C. Symbols and lines represent the measured data and model extrapolation, respectively.]
Na2CO3
2H2O2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2H2O + O2 k
(10)
In that case, the reaction can be approximately modeled as a simple second-order process: 1 1 − = kt [H2O2 ] [H2O2 ]0 (11)
concentrations higher than 20 g/L. Therefore, the values of k1 needed to fit eq 9 to the experimental data of Figure 7a, as a function of Na2CO3 dosage, were computed, holding k‑1 and k2 constant at the values obtained for Figure 6. The resulting variation in K1 and K2 with Na2CO3 concentration is shown in Figure 7b. As seen, k1 did not exhibit a simple saturation relation as might be expected from eq 9. In fact, k1 decreased with increasing catalyst dosage at low catalyst concentration, indicating a more complex mechanism involving additional reaction pathways in this regime in this real wastewater. With respect to the unexpectedly high reaction rate at low catalyst dosage, it is interesting to note that although H2O2 was removed much faster in DI water than in secondary effluent at the same dosage of Na2CO3, the opposite phenomenon was observed when no Na2CO3 was added and water sample was only treated with heat. That is, 30% of H2O2 was removed after 90 min heating without Na2CO3 addition in secondary effluent (Figure 4a), while loss of H2O2 was almost undetectable in DI water using the same treatment. These observations demonstrate that: (a) spontaneous decomposition of H2O2 without catalyst in DI water was very slow even at elevated temperature; and (b) although some active catalytic sites may be masked by organic and inorganic moieties, some other moieties, such as antioxidant enzymes, transition metals (typically low in concentration in municipal wastewater), or other impurities in the secondary effluent can cause the loss of H2O2. These moieties, including native carbonate (about 20 mg/L as Na 2 CO 3 ), may also have a catalytic effect on H 2 O 2 decomposition at high temperature. The rate of decomposition then depends on the types and amount of catalyst or enzyme available, as well as on temperature. Due to the heterogeneity and high variability of real wastewater, identification of such
The fit of the simplified model to the measured kinetic data is illustrated in Figure 8 (r = 0.991, p = 0.0001). The
Figure 8. Residual H2O2 concentration versus time in secondary effluent, versus residual H2O2 concentration predicted assuming simple second order kinetics. Symbols and lines represent the measured data and the fit of the second order model, respectively. [Conditions: Na2CO3: 20 g/L; [H2O2]0: 5 mM; T: 90 °C.]
corresponding value of k was found for this secondary effluent as 11.331 M−1·min−1 at a Na2CO3 dosage of 20 g/L. Discussion and Recommendations. There are several methods that can be used to quench H2O2 in solution, such as catalase enzyme, elevated pH and temperature, chemical neutralization and decomposition by activated carbon.33 2296
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Environmental Science & Technology
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However, in the evaluation of advanced oxidation methods employing concentrations of H2O2 that may be somewhat high relative to levels of organic contamination, a method that does not introduce COD to the effluent and can be accomplished within an acceptable time scale is needed. In this light, the methods enumerated above may not be entirely appropriate. The proposed method was found to be effective and accurate with regard to the elimination of H2O2 interference in the COD test. In addition, the method was advantageous due to simplicity and ease of operation, involving only heating and the addition of a nontoxic, readily obtained, and economical reagent, Na2CO3. One possible shortcoming might be the relatively long heating time required for certain wastewater samples. However, this time can also be reduced by increasing catalyst (Na2CO3) dosage. In fact, in practice the residual H2O2 in treated water is commonly lower than the concentrations tested in this work. Therefore, the heating time as well as Na2CO3 dosing can be expected to be less. In addition to the removal of residual H2O2 in the effluent, another possible application of the proposed method would be for process start-up or tuning in the field. Some AOPs, including H2O2/UV and Fenton, depend strongly on an optimal oxidant dosage for process efficiency, and facile measurement of peroxide residual may aid in this optimization. More generally, kinetic study is always of interest for both practical installations and lab investigations, whereas the interference of H2O2 in the COD analysis may hinder such analyses, or result in much longer analysis period. For example, in studying the removal of low level organics via the electroFenton process, Alverez-Gallegos and Pletcher16 had to allow samples to stand for 2−4 days prior to COD determination, awaiting decay of H2O2 concentrations to below a significant value. In addition, COD below 10 mg/L was reported only as
