TW200842354A - Improved water analysis - Google Patents

Abstract

A method of determining chemical oxygen demand (COD) of a water sample which is useful in a probe configuration includes the steps of (a) applying a constant potential bias to a photoelectrochemical cell, having a photoactive working electrode optionally a reference electrode and a counter electrode, and containing a supporting electrolyte solution; (b) illuminating the working electrode with a light source and recording the background photocurrent produced at the working electrode from the supporting electrolyte solution; (c) adding a water sample, to be analysed, to the photoelectrochemical cell; (d) illuminating the working electrode with a light source and recording the steady state photocurrent produced with the sample; (e) determining the chemical oxygen demand of the water sample using the formula [COD]=δ/FAD8000iss where δ is the Nernst diffusion layer thickness, D is the diffusion coefficient, A is the electrode area, F the Faraday constant and iss the steady state photocurrent. The method can accommodate a broad range of light intensity and pH.

Description

200842354 IX. INSTRUCTIONS OF THE INVENTION: TECHNICAL FIELD OF THE INVENTION The present invention relates to a new method for determining the oxygen demand of water using a photoelectrochemical cell. In particular, the present invention relates to a direct photoelectrochemical method for determining the chemical oxygen demand of a water sample using a manganese dioxide particle semiconductor electrode. This method is especially useful in detector configurations. [Prior Art] All domestic and industrial wastewater effluents contain organic compounds which may cause harmful hypoxia (or oxygen) in the leeches from which the effluent is released. The oxygen system is mainly due to the naturally occurring microorganisms utilizing organic shellfish as a food source for oxidative biodegradation of organic compounds. In the process, organic slaves oxidize to carbon dioxide while consuming oxygen and reducing it to water. Oxygen demand testing based on the principle of photoelectrochemical degradation has been previously disclosed in Patent Specification No. W02004088305, which is based on the principle of thorough degradation. The object of the present invention is to improve an analyzer according to the principle of non-complete degradation. Another object of the invention is to develop a detector type C〇D analyzer. SUMMARY OF THE INVENTION The present invention provides a method of determining the chemical oxygen demand (COD) of a water sample comprising the steps of: (a) applying a constant bias voltage to a photoelectrochemical cell having a photosensitive working electrode and a counter electrode, and containing a supporting electrolyte solution; 127655.doc 200842354 b) irradiating the working electrode with a light source and recording the background photocurrent generated by the supporting electrolyte solution at the working electrode; 〇) adding the water sample to be analyzed In a photoelectrochemical cell; d) illuminating the working electrode with a light source and recording the steady state photocurrent generated from the sample; e) determining the chemical oxygen demand of the water sample using the following formula;

δ FAD

[COD ]= x 8000 z:

5 is the thickness of the Nernst diffusion layer, D is the diffusion coefficient, A is the electrode area, F is the Faraday constant and the iss steady state photocurrent. The light intensity on the photoelectrode affects the linear range of the instrument. However, increasing the light intensity to an excessively high value may cause stability problems in the corrosion of the light from the source or electrode of the instrument. The preferred light intensity is in the range of 3 to 10 W/cm2, and preferably 6 to 7 W/cm2. The pH of the solution also affects the signal and is preferably operated at 3 to 1 Torr. The working electrode can be regenerated by exposure to ultraviolet light, light and the working electrode has an effective working life. In addition to the counter electrode, a reference electrode is also preferably used. The method of the present invention is particularly applicable to an analyzer which is configured to be a detector for field testing of water samples on a discontinuous basis. The invention further provides a detector for measuring water quality, and the detector comprises: t electrochemical H which contains a photosensitive electrode, a counter electrode and a reference electrode selected as appropriate; b) - support Electrolyte solution chamber; 127655.doc 200842354 C) A light source for illuminating the working electrode; d) a sample collection component that provides a large sample of the battery; e) a control component for: i) starting the light source and recording support The electrolyte solution is generated from the background photoelectric of the working electrode; ii) the water sample to be analyzed is added to the photoelectrochemical cell; 111) the light source is activated and the steady state photocurrent generated by the water sample is recorded; iv) the chemical requirement of the water sample is determined by the following formula Oxygen content:

[COD ]= δΎαώ x 8000 ζ: where 5 is the thickness of the Nernst diffusion layer, D is the diffusion coefficient, Α is the electrode area, and F is the Faraday constant. Is the steady state photocurrent. [Embodiment] Raw material and sample preparation Indium tin oxide (ITO) conductive glass sheets (8 Ω/square) were commercially supplied by Deha Technology Co., Ltd. Titanium oxide (97%, Aldrich) and 10 sodium nitrate (which were not further processed before use) were purchased from Aldrich. All other chemicals have analytical grades and are commercially available from Aldrich unless otherwise stated. Deionized water (MiUip〇re, 18 ΜΩ cm) is used for solution preparation and dilution of actual wastewater samples. The actual samples used in this research are collected in Queensland, Australia, from different industrial sites, including wastewater treatment plants, sugar factories, breweries, canned plants and milk production plants. All samples were stored according to the specifications of standard methods. If necessary, dilute the samples to 127655.doc 200842354 to a suitable concentration prior to analysis. After dilution, the same sample was analyzed by standard COD method and photoelectrochemical COD detector. A 0.1 Μ NaC104 solid was added to the photoelectrochemical measurement sample as a supporting electrolyte.

Preparation of Ti02 film electrode: the same as described in the applicant's prior patent application No. W02004088305. ^ Instruments and Methods ' All photoelectrochemical experiments were performed at 23 t in a three-electrode electrochemical cell with a window for illumination (see Figure 1). A saturated φ Ag/AgCl electrode and a platinum mesh were respectively used as reference electrodes and auxiliary electrodes. The voltammogram (CV-27, BAS) was used to apply a bias voltage in a photoelectrolysis experiment. The potential and current signals are recorded by a computer coupled to the Maclab 400 interface (AD instrument). Irradiation was performed using a 150 W xenon arc lamp source with a focusing lens (HF-200W-95, Beijing Optical Instruments). In order to prevent the temperature of the sample solution from rising due to the heating of the infrared light, the beam passes through an ultraviolet bandpass filter, UG5 (Avotronics Pty Co., Ltd.), before it is irradiated onto the electrode surface. The standard COD values (dichromate method) of all samples were measured using a COD analyzer (NOVA 30, Merck). During the oxygen-dependent experiment, the oxygen concentration was monitored by an oxygen electrode (YSI) and a '90 FLMV microprocessor field analyzer (available from T.P.S. Pty., Inc.). Analytical Signal Measurements Figure 2 A and B show a set of typical photocurrent-time curves' obtained in the presence of an organic compound in an optochemical cell. At a constant applied potential of +0.3 V, the dark current is approximately zero when the light is cut. When illuminated, the current increases rapidly before it drops to a stable value. For the blank (virtual 127655.doc -10- 200842354 line), the photocurrent is mainly generated by the oxidation of water, and the photocurrent observed by the sample solution of the organic matter is the total current of the two currents. Among them, it is derived from the oxidation of water (which is the same as the blank photocurrent (lblank)) and the other is derived from photoelectrocatalytic oxidation of organic compounds. The diffusion limit current of SS from the oxidation of organic matter can be obtained by subtracting the iblank photocurrent in the absence of organic compounds from the total photocurrent in the presence of organic compounds (see Figure 12).

i M ^ i total - i blank ( 1 · 1 ) It has been confirmed that all the organic substances transported to the surface of the Ti〇2 electrode can be unselected and subjected to total oxidation. Therefore, the net current (iss) is directly proportional to the electron transfer ratio (the amount of electron transfer per early time). When the COD system is defined as the oxygen demand of a fully oxidized organic compound, then the net current (id can be used to quantify the COD value of the sample. Analyze the signal quantification) Under the non-complete photocatalytic oxidation model, develop & Quantitative relationship between COD: (1) The concentration of the bulk solution remained substantially constant (not completely degraded) before and after the experiment; (ii) all organic compounds on the surface of the electrode were stoichiometrically oxidized to their highest oxidation state (complete oxidation (1) 丨) The photocatalytic oxidation rate is controlled by transporting organic matter to the surface of the electrode and can reach steady state (steady-state mass transfer limiting procedure) within a suitable time range; (iv) Fully applying a bias voltage to remove all photocatalysis by organic matter Photoelectrons generated by oxidation (100% photoelectron collection efficiency). The steady-state mass transfer rate (dN/dt) of the counter electrode can be provided by a semi-experimental treatment of the well-known steady-state mass transfer mode 127655.doc 200842354 :

(dN λ D ^rJ = y[c"c"x = 0)] (1·2) where 'cb and cs refer to the analyte concentration in the bulk solution and the analyte concentration on the electrode surface, respectively. 0 and 6 are the diffusion coefficient and the Nernst diffusion layer thickness, respectively. Under the steady-state mass transfer limit (hypothesis (in)), the total reaction rate is equal. Rate =~cb (1.3)

According to hypotheses (ii) and (iv), the number of electrons (η) transferred by a given analyte during photoelectrochemical degradation is a constant and thus the steady state photocurrent (iss) can be used to represent the reaction rate: • — nFAD gate hs = —δ~Cb (1.4) where A and F refer to the electrode area and the Faraday constant, respectively. Equation 1.4 defines the quantitative relationship between steady state photocurrent and analyte concentration. Converting the molar concentration to the equivalent COD concentration (mg/L 〇f 〇 2) gives:

a FAD δ~ [COD ]=

8000 δ ~FAD -[COD ] x8000z: (1.5a) (1.5b) [COD] : 8000 / Equation 1.5b can be effectively used to determine a sample containing a single organic compound (10). The c〇d of a sample with more than one organic species can be expressed as FAD ss (1.6) where is the thickness of the collective Nernst diffusion layer, which has been shown to be a constant under diffusion control conditions and There is no difference between the type of organic matter and the type of human organic matter. The composite diffusion coefficient is determined by the composition of the sample and is constant for the sample of the two samples. 127655.doc -12- 200842354 Verification of Analytical Principles Figure 3A shows a plot of a steady state photocurrent versus the molar concentration of an organic composition. Material (4) Compound M u ear, Chen degree 盥 3 White is served as the linear relationship between iss and C predicted by Equation 1.5. Figure 3A shows the data/processing of Figure 3B. It should be noted that all data in Figure 3B is suitable for a linear curve with slope = 0.0531 and R2 = 〇.995. Because the slope of the curve is equal to 趴δ, the following conclusions can be drawn: Under the special experimental conditions, there is a thickness of the diffusion layer (δ=1.86χ1 (Γ3 cm) and this is related to the organic compound. The rich yield ^ / 辰戾 and type have nothing to do. This finding also confirms the theoretical slope obtained by Equation 1.5, ' ^ ^ ^ ^ Stem represents the slope of the curve used for each compound in Figure 1.3a. In fact, pain ^ 'Bei On the above, we can't get a straight line in Figure 3B, unless all the four hypotheses listed above are approved. It is true that the same conditions are required for Equation 1 · 6 in the circumstance of 1 The following should be effective. Therefore, Figure 4ΑΛ4Β shows the graph of iss relative to the theoretical c〇d value ([COD] theory) of a synthetic sample formulated with κηρ (a test compound for standard COD methods). The equation ^ predicts that a linear relationship is obtained between k and [COD] theory. The slope of the obtained experimental curve is 2·8 χΐ (Γ3 mA (mg/L 〇f 〇2v1j_r2=〇9985. The theory calculated by the equation 丄$) The curves are also listed in Figure 4A (solid line) for comparison. The theoretical slope calculated according to Equation 1.5 is n 9 e ' ' 此The theoretical and experimental slope values for equality indicate the applicability of Equation 15 for COD determination. The suitability of Equation 1.6 is tested using a GGA synthetic sample. The gga synthetic sample is a mixture of glucose and glutamic acid, which has typically been used. 127655.doc 200842354 Standard test solution for bod analysis. As predicted by Equation 1.6, the steady state photocurrent is directly proportional to the sample [COD] (see Figure 14b). However, due to the unknown composite diffusion coefficient' It is not necessary to apply Equation 1·6 to the actual sample to be corrected. Unlike other analytes, since the COD system is a conglomerate, the positive standard for C〇D analysis is not easily defined. In fact, the COD calibration standard Can only be selected by experimental methods. The following two substantive criteria should be met by the selected calibration standard's line: (i) the calibration standard should have the same value of the original value; (Η) can be completely oxidized. Standard reflection Experimental observation: The added calibration standard causes the steady-state photocurrent to change with the same slope of the original sample. The optimization of the analytical signal examines the effect of light intensity on the steady-state photocurrent (see Figure 5 A). Note that these light intensities The change has a great influence on the linear range. The increase of the light intensity leads to the increase of the linear range. The off-linear relationship of the iss is related to the photocatalytic oxidation rate being slower than the rate at which the mass is transmitted to the electrode. Increasing the light intensity leads to an increase in the rate of photocavitation. In fact, it increases the photocatalytic oxidation rate. In other words, under the control of the quality of the car's Nanhan degree, the high light intensity can maintain the entire process. Therefore, to provide a wide linear range and good operating conditions, a relatively low (but sufficiently sufficient) light intensity (6·6 mW/cm2) should be used. For a particulate Ti〇2 semiconductor electrode, the bias voltage applied serves to collect electrons generated by an interfacial photocatalytic reaction. The efficiency of the 1〇〇❶/❸ photoelectron collection (assumed (IV) - see section on analysing the signal) is only achieved when the bias voltage is fully applied. Figure 5B shows the effect of the bias voltage on L and both. When the applied bias voltage is more positive than -〇·〇5 V vs Ag/AgCl, indicating 127655.doc 14 200842354 100°/c photoelectron collection efficiency, it shows that both iss and ibiank become constant. To ensure that the selected bias voltage is suitable for different situations and at the same time to avoid direct electrochemical reactions, select a standard bias voltage of +0.30 v vs Ag/AgCl. The pH of the solution affects the flat band and band edge potential of the Ti〇2 semiconductor in a Nernst manner. The pH of the solution also affects the surface functional groups of the semiconductor electrode and the species form of the chemical form of the organic compound in solution. These pH dependent factors may affect the analytical signal. Figure 5C shows the effect of 01^ on both 丨^ and heart. In the pH range of 2 to 3, with the solution? 11 increases, L and ibiank both increase slightly. Both the iss and ibi magnets are insensitive to solution pH changes in the 3 to 1 〇 2 pH range. When the pH of the solution was above 1 Torr, it was observed that the is is extremely insensitive to the pH change, but since the water oxidation rate was greatly increased in the high school, the ibIank was observed to increase significantly with the pH of the solution. The sensitivity of ibUnk to the pH of the solution may lead to the problem of precise measurement of the iss. Therefore, a pH range of 3 to 1 Torr is preferred. This pH range is suitable for most environmental samples (pH 3 to 10) and can be used without adjusting the pH. Actual sample analysis Perform an analysis of the actual sample. These actual samples were collected from different industrial locations. The pH of the actual samples tested in this document is in the range of 6 to 8, i.e., in the pH independent zone. For the analysis of very high c〇D samples, dilution with a NaCl〇4 or NaNCh solution will typically result in a pH in the range of 5 to 8 and an A concentration in the range of 5 to 9.5 mgL-1. To minimize any matrix action, this standard addition method can be used in photoelectrochemical measurements of the COD values of actual samples, if desired, thus ensuring that their values are constant and consistent during calibration and measurement. The results shown in Figure 6 127655.doc -15- 200842354 demonstrate that Equation 1.6 can be used to determine the c〇D value of the actual sample. Figure 7 shows the correlation between experimental COD values and standard COD values. The standard c〇D value was determined by the conventional COD method (dichromate method). The Pearson Correlation coefficient is used as a measurement tool for the correlation between the values obtained from the photoelectric chemical COD method and the conventional COD method. Obtain a highly significant correlation between the two methods (r = (K988 ' Ρ = 0.000 ' n = 18), indicating that the two methods are extremely consistent. The slope of the graph is 1 _02. This slope close to 1. It is shown that the two methods accurately measure the same COD value. Considering a 95% confidence interval, this slope is between 〇·96 and 1 ·11', which means that the true slope is 95% confidence between the two values. There is a correlation error between the chemical COD and the standard method measurements, and the errors contribute to the dispersion on the two axes. The strong correlation and slope obtained provide the suitability of the photoelectrochemical C〇D method for determining the chemical oxygen demand. Strongly supported. It was found that under the above optimized experimental conditions, the detection limit of 〇·8 mgL-i COD and the linear range of up to 7〇mgi^COD can be achieved. The range of this detection/shell J is appropriately diluted as described above. Extending. The reproducibility of 2.2% RSD was obtained from a 19 KHP analysis. It can be seen from the above that the present invention provides an improved method and a detector for performing incomplete COD analysis of water samples. Understand that the present invention can be implemented differently than this BRIEF DESCRIPTION OF THE DRAWINGS The present invention does not depart from the spirit of the present invention. FIG. 1 is a schematic view of a photoelectrochemical cell used in the present invention; 127655.doc -16- 200842354 FIG. 2 is a typical Photocurrent reaction diagram of 0.1MNaC104 blank solution; Figure 3A shows the quantitative relationship between net steady state current (iss) and molar concentration of organic compound; Figure 3B shows net steady state current (expressed in mA) and quantification between nFADC Figure 4A shows the theoretical and experimental iss vs. the theoretical COD value of the KHP solution; Figure 4B shows the experimental iss vs. the theoretical COD_ value of the KHP solution and the GGA solution; Figure 5A shows Photoelectrochemical oxidation of glucose at different UV intensities; Figure 5B shows the effects of photoelectrochemical oxidation of 0.2 mM glucose and its blank solution on potentials and iblank (〇); Figure 5C shows the effect of 0.2 mM glucose and Photochemical oxidation of the blank solution, pH effect on iss(cj)*ibiank(〇); Figure 6 shows a typical GGA standard addition for the determination of bakery wastewater; and Figure 7 shows the actual sample measurement PECOD with a standard dichromate correlation between • COD method. 127655.doc -17-

Claims (1)

200842354 X. Patent application scope: h A method for determining the chemical oxygen demand (COD) of a water sample, A comprises the steps of: applying a constant bias voltage to a photoelectrochemical cell, the battery has a photosensitive operation An electrode and a counter electrode, and comprising supporting electrolysis; a solution; , b) irradiating the working electrode with a light source, and recording a background photocurrent generated by the supporting electrolyte solution at the working electrode; Φ Θ adding the water sample to be analyzed to the photoelectrochemical cell d) illuminate the working electrode with a light source and record the steady state photocurrent generated by the sample; e) determine the chemical oxygen demand of the water sample using the following formula; [COD] δ FAD • x 8000 z: where 5 is the energy Nernst diffusion layer thickness, D is the diffusion coefficient, A is the electrode area, F-Faraday constant, and iss steady-state photoelectric flow. 2 'The method according to the item 1' wherein the pH of the water sample is in the range of 3 to 1 Torr. 3. The method according to claim 1 or 2, wherein the photoelectrode is a titanium dioxide nanoparticle photoelectrode. 4. A detector for determining water quality, comprising: a) an electrochemical cell comprising a photosensitive working electrode and a counter electrode; I27655.doc 200842354 b) a supporting electrolyte solution chamber; c) a light source that illuminates the working electrode; d) a sample collection component that provides a large number of samples of the battery; e) a control component that is used to activate the light source and record the background photocurrent generated by the supporting electrolyte solution at the working electrode; Add the water sample to be analyzed to the photoelectrochemical cell; ηι) start the light source and record the steady state photocurrent generated by the water sample; iv) determine the chemical oxygen demand of the water sample using the following formula: [COD] : 8000 B δ FAD 3 is the thickness of the Nernst diffusion layer, D is the diffusion coefficient, 8 is the electrode area, F is the Faraday constant, and L is the steady state photocurrent. 5. The detector according to claim 4, wherein the photoelectrode is a titanium dioxide particle photoelectrode. '6. According to the request item 4, $ 夕 拆, a 丨 mouth. The next 5th generation, the light intensity is from 3 to 1 () W/cm2 〇 10 127655.doc