Direct Determination of Chemical Oxygen Demand by Anodic

Jul 23, 2014 - ABSTRACT: Chemical oxygen demand (COD) was measured directly with a simple electrochemical method using a boron- doped diamond ...
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Direct Determination of Chemical Oxygen Demand by Anodic Decomposition of Organic Compounds at a Diamond Electrode Takeshi Kondo,*,†,‡ Yusuke Tamura,† Masaki Hoshino,† Takeshi Watanabe,§ Tatsuo Aikawa,†,‡ Makoto Yuasa,†,‡ and Yasuaki Einaga*,§,∥ †

Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan ‡ Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan § Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Yokohama 223-8522, Japan ∥ JST-CREST, 3-14-1 Hiyoshi, Yokohama 223-8522, Japan

ABSTRACT: Chemical oxygen demand (COD) was measured directly with a simple electrochemical method using a borondoped diamond (BDD) electrode. By applying a highly positive potential (+2.5 V vs Ag/AgCl) to an aqueous electrolyte containing potassium hydrogen phthalate, glucose, and lactic acid or sodium dodecylbenzenesulfonate using a BDD electrode, an anodic current corresponding to the electrolytic decomposition of these organic compounds was observed. No such current was seen on glassy carbon or platinum electrodes due to a significant background current caused by the oxygen evolution reaction. The electric charge for the anodic current observed at the BDD electrode was found to be consistent with the theoretical charge required for the electrolytic decomposition of the organic compounds to CO2 and was used to calculate COD. This analysis was performed by a simple I−t measurement at constant potential using a BDD electrode, and no calibration was needed. This new simple indicator, “ECOD” (electrochemical oxygen demand), will be useful for continuous monitoring of industrial wastewater with low protein concentrations and on-site instant analysis of natural water with a BDD electrode-based portable ECOD meter.

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enables precise evaluation of the organic content of the sample. The TOC measurement often requires a relatively large and expensive analyzer. If the precise, inexpensive, and rapid evaluation of water pollution levels can be achieved with a lightweight and simple system, it will be a powerful tool for managing water pollution levels (e.g., by continuous monitoring of industrial wastewater and on-site analysis of natural water). Electrochemical techniques show promise for COD measurement, and therefore, various electrode materials have been investigated.2−10 Yu et al. reported COD measurements through amperometric analysis of anodic decomposition of organic compounds at boron-doped diamond (BDD) electrodes.11,12 The BDD electrode is one of the most stable electrode

easurement of the amount of organic compounds in water (e.g., river/lake water and industrial wastewater) is undoubtedly important for management of pollution levels. Chemical oxygen demand (COD) is widely used as an indication of water pollution by organic compounds.1 In many countries, potassium dichromate (K2Cr2O7) is employed as an oxidizer for the COD test. Dichromate contains poisonous hexavalent chromium, and the wastewater produced by the COD test has to be strictly managed until it is treated. Potassium permanganate (KMnO4) can also be employed as an oxidizer for the COD test. Although it is less toxic than dichromate, it is a weaker oxidant and most organic compounds can only be partially oxidized. The COD test consists of several steps, including sampling, addition of the oxidizer, heating, and optical measurement, and thus is not suitable for a simple automatic system. Total organic carbon (TOC) measurement is also an important method for evaluating the pollution level of water. The TOC measurement involves complete combustion of the sample and infrared absorption measurement, which © 2014 American Chemical Society

Received: March 12, 2014 Accepted: July 23, 2014 Published: July 23, 2014 8066

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Figure 1. (a) CV for 3 mM glucose in 0.1 M Na2SO4 using a BDD electrode, at a potential sweep rate of 100 mV s−1. (b−d) Anodic current response to injection of glucose samples in stirred 0.1 M Na2SO4 with a BDD electrode. Initial solution volume was 2.7 mL, and a 0.3 mL sample aliquot containing 150 nmol glucose was injected at each step indicated by an arrow. The constant potentials were (b) 2.4, (c) 2.5, and (d) 2.6 V vs Ag/AgCl.

materials for electrolytic experiments, and its wide potential window or large overpotential for the oxygen evolution reaction of water decomposition is also favorable for efficient decomposition of organic compounds.13−15 However, it is difficult to make amperometric measurements in practical COD tests. First, the use of a reaction current as the signal for COD estimation requires a calibration curve. Furthermore, the amperometric current does not always reflect the COD value. Basically, only when the sample contains a single species can the COD value be estimated from the calibration curve. Lastly, calculating the number of electrons transferred during oxidation is not simple, especially for real sample solutions containing many chemical species. This is because the particulars of the oxidation reaction depend on the type of compounds present. A direct method for determining COD based on the complete oxidation of organic compounds in water with a photocatalytic technique has been investigated.16,17 This should be useful because COD can be calculated directly, without calibration, from the charge required for photoelectrolytic decomposition of organic compounds. For the photoelectrochemical measurement, a UV light source is necessary for excitation and this may limit the miniaturization of the measurement system. In this work, we investigated the direct determination of COD from the anodic charge of electrolytic decomposition of organic compounds at highly positive potentials using a BDD electrode. In principle, organic compounds in an aqueous electrolyte can be oxidized completely to carbon dioxide by electrochemical oxidation at a highly positive potential:

On the other hand, complete oxidation of organic compounds by reaction with oxygen can be expressed as follows: CaHbNcOd + yO2 → aCO2 + (b/2)H 2O + c NO2

From eqs 1 and 2, the relationship x = 4y (= 4a + b + 4c − 2d) can be obtained. Thus, the total charge for the electro-oxidation can be used to calculate COD (denoted here as electrochemical oxygen demand, ECOD). Practically, however, estimation of the charge is often difficult because the highly positive electrode potential causes water decomposition, resulting in a significant anodic current by the oxygen evolution reaction (OER). The OER current at a highly positive potential often fluctuates significantly due to bubble formation on the electrode surface making it difficult to separate the current due to the decomposition of organic compounds from the total current. In addition, application of a highly positive potential may cause reaction/decomposition of the electrode material itself. BDD thin films have been known to be one of the most stable electrode materials with respect to electrochemical polarization at large overpotentials for water decomposition. Thus, we expected the BDD electrode to be suitable for the electrochemical estimation of COD. In addition, it has been reported that hydroxyl radicals (•OH) can be generated at the BDD electrode during anodic water decomposition with high current efficiency:18−22 H 2O → •OH + H+ + e−

(3)

The hydroxyl radical is a strong oxidant capable of oxidizing organic compounds to carbon dioxide: CaHbNcOd + z • OH

CaHbNcOd + {(x − b)/2}H 2O → aCO2 + c NO2 + x H+ + x e−

(2)

→ aCO2 + {(b + z)/2}H 2O + c NO2

(1) 8067

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AgCl for the electrolysis processes examined in this study, since the response current was large and stable at this potential. 2. ECOD Measurements on Organic Compounds at +2.5 V vs Ag/AgCl Using a BDD Electrode. Figure 2a

In this study, we discovered that the electric charge for anodic decomposition of certain organic compounds at +2.5 V versus Ag/AgCl can be evaluated by the use of a BDD electrode. The amount of charge was found to correspond to the theoretical value, enabling direct determination of COD.

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EXPERIMENTAL SECTION

BDD electrodes were prepared by microwave plasma-assisted chemical vapor deposition (MPCVD) of a polycrystalline BDD thin film on a conductive silicon wafer substrate using a microwave power of 5000 W, a hydrogen gas flow rate of 540 sccm, a total pressure of 120 Torr, and a deposition time of 8 h. The carbon/boron source was a mixture of acetone/methanol (9:1, v/v) containing trimethoxyborane at a total atomic B/C ratio of 0.5%. Detailed MPCVD conditions are described elsewhere.23 For ECOD measurements, potassium hydrogen phthalate (KC8H5O4, KHP), glucose (C6H12O6), lactic acid (C3H6O3, LA), and sodium dodecylbenzenesulfonate (NaC18H29O3S, SDBS) were used as model organic pollutants. The BDD electrode was placed at the bottom of an electrochemical cell, contacting the electrolyte solution with an O-ring (11 mm in diameter). A magnetic stirrer was put on the bottom of the cell, parallel to the electrode, with a 4 mm thick spacer. A platinum wire and an Ag/AgCl electrode in 3 M NaCl were used as the counter and reference electrodes, respectively. A potential of +2.5 V vs Ag/AgCl was applied to 4 mL of 0.1 M Na2SO4 in the cell with stirring, and 0.3 mL of 0.1 M Na2SO4 containing the organic compound was added to the solution after the background current became stable. The charge transferred during the anodic decomposition was calculated from the area under the I−t curve after subtraction of the background. The area was calculated by integration of current with a sampling interval of 1 s. In this study, the background current was determined as a linear line passing through the current value just before addition of the sample and that after 8000 s from the sample addition. A simple COD meter (photoLab 6100, WTW) and a TOC analyzer (TOC-L, Shimadzu) were employed to estimate the COD and TOC, respectively, of the model solutions for comparison. For a standard addition study using a real sample, water was collected from the Tone canal, Nagareyama, Japan.

Figure 2. (a) I−t curve for anodic decomposition of 200 nmol KHP in 0.1 M Na2SO4 at a BDD electrode. The constant potential was +2.5 V vs Ag/AgCl. The KHP sample was injected at 1000 s. The shaded area was integrated to calculate ECOD. (b) Q−t curve created from the data of (a).

shows the I−t curve for the electrochemical decomposition of 200 nmol of KHP (12.0 mg O2 L−1) in 4 mL of 0.1 M Na2SO4 at +2.5 V vs Ag/AgCl. After a stable background current was obtained with the blank solution (0.1 M Na2SO4), a sample solution containing KHP was added to the solution, resulting in an increase of the anodic current. The current then decreased gradually to the initial background level within 10000 s. The background current was attributed to the anodic decomposition of water and was basically constant over the course of each electrolysis experiment. The increase in current upon addition of the KHP sample solution was thus due to the anodic decomposition of KHP. As the electrolysis continued, this current decreased as the KHP concentration in the cell dropped. The electric charge transferred during KHP decomposition, estimated by integrating the backgroundsubtracted anodic reaction current, was 580 mC (Figure 2b). The overall electrochemical oxidation reaction of the hydrogen phthalate ion is expressed as follows:

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RESULTS AND DISCUSSION 1. Optimization of Electrode Potential. In general, BDD electrodes exhibit large overpotentials for the anodic decomposition of organic compounds due to their poor surface adsorption (or catalytic effect). Figure 1a shows a cyclic voltammogram (CV) of 3 mM glucose in 0.1 M Na2SO4 at the BDD electrode. From the CVs, it was determined that the onset of the OER current was at around +1.8 V vs Ag/AgCl, and no obvious anodic current for glucose oxidation was observed at less positive potentials. Next, we investigated the operating potential for glucose oxidation at more positive potentials. At a constant electrode potential below +2.3 V vs Ag/AgCl with stirring, no obvious current response was observed after addition of the glucose solution (150 nmol). At potentials from +2.4 to 2.6 V vs Ag/AgCl, however, a clear current response was observed upon addition of the glucose sample (Figure 1, panels b−d). At potentials higher than +2.6 V vs Ag/AgCl, the current was not stable due to a significant increase in the background (OER) current. We ultimately decided on an optimal electrode potential of +2.5 V vs Ag/

C8H5O4 − + 12H 2O → 8CO2 + 29H+ + 30e−

(5)

In accordance with Faraday’s law, using the Faraday constant (96485 C mol−1), the total number of moles of electrons required for KHP decomposition resulting from 580 mC of charge passing through the cell was calculated to be 6.01 μmol. The amount of KHP reacted was then estimated from eq 5 to be 200 nmol, identical to the actual value. This confirmed that the reaction current was based on the total decomposition of KHP. The anodic charge Q [C] could be converted to ECOD [mg O2 L−1] from the following relationship derived from eqs 1 and 2: ECOD[mg O2 L−1] = (8 × 106 /Fv)Q 8068

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Figure 3. (a) Estimated ECOD for KHP, glucose (G), LA, and SDBS as a function of the number of moles of each compound. (b) Experimental ECOD (ECODexp) vs theoretical ECOD (ECODtheor) for KHP, glucose (G), LA, SDBS, and an equimolar mixture of KHP and glucose. The solid lines indicate theoretical values.

where F is the Faraday constant and v is the solution volume in milliliters. The fluctuation of the current in the I−t curve (Figure 2a) due to stirring of the solution and O2 gas evolution was ±0.008 mA, leading to a variation of the charge to be ±64 mC. This corresponds to a variation of the estimated ECOD value to be ±1.3 mg O2 L−1. When limit of detection is defined as three times this value, it can be estimated to be 3.9 mg O2 L−1, which is in the range of those reported for other electrochemical methods.11,12 Reproducibility of the ECOD for 200 nmol of KHP was confirmed to be 11.6 ± 1.6 mg O2 L−1 (n = 5). For various concentrations of KHP, glucose, LA, and SDBS, the ECOD values obtained from the I−t curves were found to be close to the respective theoretical values (Figure 3a). Similarly, the ECOD values for glucose, LA, and SDBS obtained at +2.5 V versus Ag/AgCl were consistent with the theoretical values. The experimental ECOD values for KHP, glucose, LA, SDBS, and mixtures of KHP and glucose were also found to be identical to the corresponding theoretical values (Figure 3b). From the results above, it was evident that the anodic decomposition of organic compounds at highly positive potentials at a BDD electrode is useful for the direct determination of COD. COD may be estimated from the anodic current by the use of a calibration curve. By referring to the electric charge passed, on the other hand, COD could be calculated rather simply, obviating the measurement of calibration curves. 3. Necessity of Using a BDD electrode. Figure 4 shows scanning electron microscopy (SEM) images of BDD and glassy carbon (GC) electrode surfaces before and after application of a potential at +2.5 V versus Ag/AgCl in 0.1 M Na2SO4 for 150 h (BDD) or 3 h (GC). No obvious change in morphology was observed in the BDD electrode surface even after applying such a high potential for such a long time. The surface morphology of the GC electrode, however, was changed greatly by corrosion, indicating that GC was too unstable to be used as an electrode material for ECOD measurements. In the I−t curves for the anodic decomposition of 300 nmol of KHP at +2.5 V vs Ag/AgCl at GC and platinum electrodes, the current for the anodic decomposition of KHP could not be recognized due to the large background current (Figure 5, panels a and b). As can be seen in the CV curves (Figure 5c), GC and platinum electrodes exhibited smaller overpotentials for water decomposition (i.e., narrower potential windows) than the BDD electrode, leading to the large background currents observed at highly positive potentials (more positive

Figure 4. SEM images of (a and b) BDD and (c and d) GC electrode surfaces. (a) Before and (b) after application of +2.5 V vs Ag/AgCl for 150 h; (c) before and (d) after application of +2.5 V vs Ag/AgCl for 3 h.

than +1.5 V vs Ag/AgCl). The foregoing results demonstrated that BDD was the most suitable electrode material of the three examined for ECOD measurements involving anodic decomposition of organic compounds at highly positive potentials, based on its high dimensional stability and low background current (OER current). Another advantage of the BDD electrode for electrolysis at highly positive potentials is the efficient generation of hydroxyl radicals (•OH).15,20−22,24−26 It is known that this radical can be generated by the anodic decomposition of water (eq 3). For example, anodic water decomposition at an active electrode such as Pt or RuO2 generates oxygen species that are chemically adsorbed on the electrode surface, and these species can oxidize organic compounds in the electrolyte. On the other hand, at an inactive electrode material with poor adsorption of the oxygen species, such as PbO2 or BDD, the hydroxyl radical can be generated efficiently.19,27 This radical is a strong oxidant and can oxidize low molecular weight organic compounds 8069

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Figure 5. Anodic decomposition of 300 nmol KHP at +2.5 V vs Ag/AgCl at (a) GC and (b) Pt electrodes. The KHP sample was injected at 1000 s (arrows). (c) CVs in 0.1 M Na2SO4 using BDD, GC, and Pt electrodes at a potential sweep rate of 100 mV s−1.

Figure 6. (a) COD values measured with a commercial COD meter and (b) TOC values measured with a commercial TOC analyzer, and the corresponding values calculated from the anodic charge, along with theoretical values, for a KHP solution. Solid lines indicate the theoretical values.

completely to CO2 during anodic decomposition.28 Direct anodic oxidation of 1 mol of glucose to carbon dioxide requires 24 mol of electrons: xC6H12O6 + 6x H 2O → 6xCO2 + 24x H+ + 24x e−

per mol of glucose. Thus, the number of electrons transferred, and hence the electric charge, can be directly used to calculate COD. In this regard, this method is fundamentally different from the electrochemical estimation of COD based on the measurement of amperometric current, which requires an experimental calibration curve. 4. Comparison with Results from a Commercial COD Meter and TOC Analyzer. The ECOD value obtained with a BDD electrode was compared to COD data measured with a COD meter. Figure 6a shows the experimentally measured COD versus the theoretical value for a KHP solution. The values obtained with the COD meter were somewhat smaller than the theoretical values. The conventional COD test includes multiple steps, namely, addition of reagent (potassium dichromate), heating (for the oxidation reaction to occur), and an absorbance measurement, thus requiring considerable time and often resulting in data scattering. On the other hand, the electrochemical method proposed in this study is more precise, can be performed more easily and is suitable for automation with a simple instrument. Figure 6b shows the experimental TOC value obtained with a commercial TOC analyzer and by calculation from the anodic charge. When the chemical composition of the sample is known, TOC can be calculated from the charge transferred using eq 1. TOC measurements, based on the complete combustion of organic compounds in

(7)

On the other hand, in hydroxyl radical-mediated anodic oxidation (indirect oxidation), we have 24(1 − x)H 2O → 24(1 − x)• OH + 24(1 − x)H+ + 24(1 − x)e−

(8)

(1 − x)C6H12O6 + 24(1 − x)• OH → 6(1 − x)CO2 + 18(1 − x)H 2O

(9)

Thus, the overall reaction from eqs 8 and 9 is (1 − x)C6H12O6 + 6(1 − x)H 2O → 6(1 − x)CO2 + 24(1 − x)H+ + 24(1 − x)e− (10)

The sum of eqs 7 and 10 gives C6H12O6 + 6H 2O → 6CO2 + 24H+ + 24e−

(11)

Even when direct and indirect oxidation occur in any proportion x (0 ≤ x ≤ 1), 24 mol of electrons are transferred 8070

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complete electrolytic decomposition. This measurement time could be reduced by using an electrochemical cell with a thin electrolyte layer in order to increase the electrode area relative to the electrolyte solution volume, and this investigation will be done in a future study. Since the ECOD measurement can be performed fairly simply (I−t measurement at a constant potential) and no calibration is needed, this method should be useful for continuous monitoring of industrial wastewater with low protein concentrations and on-site instant analysis of natural water with a portable ECOD meter.

the sample solution, are known to be precise. The TOC values calculated from the charge, however, were found to be closer to the corresponding theoretical values than those obtained by a TOC analyzer. This indicated that the ECOD values found using a BDD electrode could be useful as a reliable indication of organic water pollution, measurable with a simple instrument. 5. Interference and Standard Addition Studies. Effect of the presence of proteins in the sample solution to ECOD measurement was investigated as an interference study. When a sample aliquot containing 200 nmol KHP and bovine serum albumin (BSA, 1 wt % in the sample aliquot or 0.025 wt % in the electrochemical cell) was injected in the stirred 0.1 M Na2SO4, at +2.5 V vs Ag/AgCl, the current suddenly decreased and minor current increase by electrolysis was observed (result not shown). On the other hand, in the case of lower BSA concentrations (0.001 and 0.01 wt % in the sample aliquot), no current decrease by the injection and current increase for the electrolysis of KHP was observed. However, the anodic charge was found to be smaller (7.8 and 5.4 mg O2 L−1 for 0.001 and 0.01 wt % BSA, respectively) than the theoretical one (12.0 mg O2 L−1). These results should be because of adsorption of the protein to the electrode surface, which can disturb the background current (oxygen evolution reaction current) and hydroxyl radical generation, even though the BDD surface may not adsorb proteins strongly.29,30 In addition, we have demonstrated a standard addition study using a canal water sample. 100, 200, and 300 nmol of KHP were added in the water sample, and the ECOD was estimated. When the blank canal water sample (without KHP) was injected to the electrolyte, no obvious increase of current was observed. However, in the case of KHP-containing canal water, the current increased as the sample was injected, and the ECOD calculated from the anodic charge was close to the theoretical one (Figure 7). Thus, this method should be useful for estimation of organic pollution of environmental waters with low protein concentrations.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by DKK-TOA Corporation (Saitama, Japan). REFERENCES

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Figure 7. Estimated ECOD for KHP-containing canal water samples as a function of the number of moles of KHP added. The solid line indicates theoretical values.

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CONCLUSIONS By applying a high positive potential (+2.5 V vs Ag/AgCl) at a BDD electrode, KHP, glucose, LA, and mixtures thereof, were completely decomposed in the aqueous electrolyte solution. The electric charge required for the decomposition was found to be consistent with the charge required for electrochemical oxidation of the organic compounds to CO2. In this study, we used an electrochemical cell with a solution volume of 4 mL for the experiment, requiring a long measurement time for 8071

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