Environ. Sci. Technol. 2009, 43, 1935–1939
Flow Injection Analysis of Chemical Oxygen Demand (COD) by Using a Boron-Doped Diamond (BDD) Electrode HONGBIN YU, CHUANJUN MA, XIE QUAN,* SHUO CHEN, AND HUIMIN ZHAO Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, China), School of Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, China
Received November 30, 2008. Revised manuscript received January 15, 2009. Accepted January 22, 2009.
A simple, environmentally friendly and continuous flow method was developed for the determination of COD based on a flow injection analysis (FIA) system, in which a BDD electrode was employed as the detecting element. The structure and the electrochemical behavior of BDD were investigated by a scanning electron microscope, Raman spectroscopy, and cyclic voltammetry,respectively.Theresultsdemonstratedthatthehighquality BDD film prepared here was suitable to be used as an electrode, with which the COD measurement could be conducted. The effect of several important experimental parameters, such as applied potentials, pH, flow rates, and supporting electrolyte concentrations, on the analytical performance was investigated. Under optimized testing conditions, the proposed method was successfully applied in the COD analysis of synthetic samples. The linear range and the detection limit were 2-175 and 1 mg L-1, respectively. In addition, the COD values determined by the proposed method compared well with those analyzed by the conventional method as demonstrated by small relative errors.
Introduction Chemical oxygen demand (COD) is an important measure of organic pollution in water. The determination of COD can be completed conventionally by titrating the excess of dichromate that was normally used as a strong oxidant to digest organics (1). Unfortunately, this method involves several inherent disadvantages (2, 3) such as the timeconsuming process, the incomplete oxidation of volatile compounds, and the consumption of expensive (Ag2SO4) and toxic (Cr2O72-) chemicals. More importantly, the conventional method is difficult to conduct automatically. However, an in situ online COD testing system is very important for water pollution control, especially in developing countries, e.g., China, in which rapid economic development has brought on serious water pollution problems. An online system can provide real-time information, with which the emission of pollutants will be monitored and controlled efficiently. Considering the drawbacks involved in the conventional method and the need for the online monitoring system, many * Corresponding author phone: +86-411-84706140; fax: +86-41184706263; e-mail:
[email protected]. 10.1021/es8033878 CCC: $40.75
Published on Web 02/16/2009
2009 American Chemical Society
advanced methods have been developed in recent years. For example, the time-consuming process of the refluxing digestion in the conventional method could be replaced by a closed microwave digestion system (4). However, some problems including the use of dichromate and the tedious titration with no online process were not solved well. In addition, the spectrophotometry was a commonly used method for COD test in a flow injection form (5, 6), but it was susceptible disruption. Besides, many other approaches were also investigated, such as ultrasound-assisted digestion (7), single sweep polarography (8), and atomic absorption spectrophotometry (9). The problems with these methods were that dichromate was still indispensable and the operating processes were rather complex. Recently, a simple method was proposed to determine COD using oxygen electrodes (3, 10-12). This approach was environmentally friendly due to the advantages of TiO2, such as chemically stable and not toxic, and yet the linear range of detection was very narrow owing to the limit of dissolved oxygen in water. With TiO2 as a photoanode, a photoelectrochemical method was developed for COD measurements (13-16). The testing process could be conducted simply and rapidly with no secondary pollution, whereas the use of a UV light source would complex the device for COD measurements and increase the cost of analysis. Consequently, great attention had been paid to electrochemical COD testing methods (17-22), especially the amperometric method (19-22), by which COD values could be determined rapidly with the ease of operation. Furthermore, all the expensive and toxic reagents used indispensably in the conventional method were unnecessary. In tests, organics could be oxidized by hydroxyl radicals generated electrochemically on anodes, and then a proportional signal current would occurr (20-22). It was because the signals used for quantification could be obtained simultaneously in the process of oxidizing organics that an online COD testing system could be established easily. However, the electrode materials investigated in the past, such as AgO/CuO (19) and PbO2 (21, 22), had either low oxidation ability or the risk of toxic metal ion leaching (e.g., lead). The lack of proper electrode materials might limit the widespread application of the amperometric method in practice. Boron-doped diamond (BDD) is a versatile, environmentally friendly electrode material that has been widely studied in the fields of electrochemical water treatment (23-26) and electrochemical analysis (27-30). The BDD material possesses many interesting properties (26-28) such as wide working potential window, low background current, longterm response stability, high mechanical strength, and corrosion resistance. In our previous work, a novel amperometric method was explored to determine COD based on a BDD electrode in batch experiments (31). The COD testing process was simple, rapid, and environmentally friendly. In the present work, a continuous flow method was developed for determining COD based on a flow injection analysis (FIA) system, in which a BDD electrode was employed as the detecting element. As compared with our previous batch systems (31), the combination of a flow injection system and the BDD electrode might promote the development of an environmentally friendly, in situ, and online COD testing system. Meanwhile, the application of FIA system could not only shorten the analysis period but also reduce the dosage of reagents used, and a better reproducibility could also be expected. VOL. 43, NO. 6, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Experimental Section Materials and Sample Preparation. Nine organic compounds were used to prepare standard samples with known COD values and synthetic samples with unknown COD values: 4-hydroxybenzoic acid, glutamic acid, acetic acid, glucose, ethanol, acetone, sucrose, phenol, and potassium hydrogen phthalate (KHP). All the reagents purchased from Tianjin Bodi Chemicals Co., China were of analytical reagent grade and used without further purification. The synthetic solutions were prepared using water from a Milli-Q purification system. The real samples tested were collected from various industrial sites including sewage treatment plants and chemical manufacturers. When necessary, both synthetic and real samples were diluted to a proper concentration prior to being analyzed. As supporting electrolyte, Na2SO4 solid equivalent to a certain concentration should be added into the sample tested for COD by the proposed method. Deposition of BDD Film. A microwave plasma chemical vapor deposition system was used to prepare BDD films. The pretreatment of a titanium sheet was similar to that described in the literature (31), and then the titanium sheet was put into a microwave plasma chemical vapor deposition reactor with 2 kW power for the depositing of BDD films. The source gas (flow rate 100 sccm) was a mixture of methane and hydrogen (0.9% CH4 in H2), and B2H6 was employed as the boron doping agent. The substrate temperature was 800 °C. The typical growth rate was 1 µm h-1. Instruments and Methods. The general morphology of the prepared BDD film was characterized using a scanning electron microscope (ESEM Quanta 200 FEG) with an accelerating voltage of 25 kV. The Raman spectrum was recorded on a Renishaw Micro-Raman 2000 Spectrometer operated at He-Ne laser excitation (wavelength 623.8 nm; laser power 35 mW) with a beam spot size of about 2 µm. COD measurements were performed on an electrochemical working station (CH Instruments 650B, Shanghai Chenhua Instrument Co. Ltd., China) in an electrochemical flow cell (seen in Supporting Information (SI) Figure S1). It consists of a three-electrode system with a BDD working electrode, a platinum counter electrode, and a saturated Ag/AgCl reference electrode. The effective area of the BDD electrode was 0.196 cm2. The flow cell was made of Perspex, and the inner diameter of inlet pipe was 1 mm. The flow injection apparatus (IFIS-D, Xi’an Remex Analysis Instrument Co. Ltd., China) used in this work was equipped with a set of peristaltic pumps and eight-way valves, through which water samples could be injected automatically. The injector volume was 100 µL. Each COD test was repeated six times. Additionally, the electrochemical behaviors of the BDD electrode were studied with the same three-electrode system in a 10 mL of 1 M KCl or 0.5 M H2SO4 solution. All the electrochemical experiments were conducted at room temperature (around 20 °C).
Results and Discussion Characterization of BDD Film. SI Figure S2 displays a typical SEM image of the prepared BDD film, and the inset is the corresponding Raman spectrum. Well faceted crystals as well as the sharp and narrow Raman peak at 1334 cm-1, which is slightly away from the characteristic peak of diamond (1332 cm-1), indicate the presence of high quality diamond films (31). Electrochemical Behavior of BDD Electrode. SI Figure S3 displays the cyclic voltammetric behaviors of the BDD electrode in a 0.5 M H2SO4 solution and a 1 M KCl solution containing 1 mM Fe(CN)63-/4-. As seen in SI Figure S3(A), there was a wide working potential window (the potential where the current density was 0.5 mA cm-2 geometric 32, 33) of over 3.7 V with a high onset potential for oxygen evolution. The working potential window observed here was comparable 1936
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FIGURE 1. Effect of applied potentials on the signal current (rectangle) and the background current (triangle). Standard sample (a mixture including 4-hydroxybenzoic acid, glutamic acid, acetic acid, glucose, ethanol, acetone, sucrose, phenol, and KHP) COD 50 mg L-1, flow rate 2.5 mL min-1, carrier 0.25 M Na2SO4, pH 6.5. with the typical values of 3-4 V for the BDD films deposited on Si substrates and titanium sheet (33). Within the potential between hydrogen and oxygen evolution, the voltammetric current was very small and featureless, reflecting the ideally polarizable nature of the solid-electrolyte interface (34). SI Figure S3(B) shows the electrochemical responsiveness of the BDD electrode toward the redox couple of Fe(CN)63-/4-. The separation between anodic and cathodic peak potentials was 0.095 V, and comparable with the values of 0.071-0.097 V obtained by using high quality BDD films (33, 35), indicating the BDD film presented here was suitable to serve as an electrode. Effect of Applied Potential. Figure 1 shows the effect of applied potentials on the signal current and the background current. The signal current was defined as the peak height arising from the injection of water samples. It could be observed that the background current at 2.0 V was very low (
