KR20090101188A - 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 (I): where H 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

Methods and probes for improved water analysis {IMPROVED WATER ANALYSIS}

The present invention relates to a novel method for determining the oxygen demand of water using a photoelectrochemical cell. In particular, the present invention relates to an improved direct photoelectrochemical method for determining the chemical oxygen demand of a water sample using titanium dioxide nanoparticulate semiconductor electrodes. It is particularly suitable for use in probe configurations.

Nearly all household and industrial wastewaters contain organic compounds, which can lead to harmful oxygen depletion (or demand) in the channels where the wastewater is discharged. Most of these demands are due to the oxidative biodegradation of organic compounds by naturally occurring microorganisms using organic materials as food sources. During this process, oxygen is consumed and reduced to water while the organic carbon is oxidized to carbon dioxide.

An oxygen demand analysis method based on the photoelectrochemical decomposition principle has already been disclosed in patent application WO2004088305, where the measurement is based on the principle of exhaustive degradation.

It is an object of the present invention to develop an analyzer based on the principle of non-exhaustive degradation. Another object of the present invention is to develop a probe type COD analyzer.

For this purpose, the present invention provides a method for determining the chemical oxygen demand (COD) of a water sample, the method

a) applying a constant potential to a photoelectrochemical cell containing a photoactive working electrode and a counter electrode and containing a supporting electrolyte solution,

b) illuminating the working electrode using a light source and recording a background photocurrent generated at the working electrode from the supporting electrolyte solution,

c) adding a sample of water to be analyzed into the photoelectrochemical cell,

d) illuminating the working electrode using a light source and recording a steady state photocurrent generated by the sample,

를 이용하여 물 시료의 화학적 산소 요구량을 판단하는 단계로서, 이때, δ는 네른스트 확산 층 두께(Nernst diffusion layer thickness)이고, D는 확산 계수(diffusion coefficient)이며, A는 전극의 면적이고, F는 패러데이 상수(Faraday constant)이고, i_ss는 정적 상태 광전류(steady state photocurrent)인 단계e) formula

Determining the chemical oxygen demand of the water sample by using, wherein δ is the Nernst diffusion layer thickness, D is the diffusion coefficient, A is the electrode area, F is Faraday constant, i _ss is a steady state photocurrent

It includes. The intensity of light to the photoelectrode affects the linear range of the instrument. However, increasing the luminosity to a value that is too high can lead to stability problems of the device radiating from photo corrosion of the light source or electrode. Preferred luminosity is within 3 to 10 W / cm 2, with values of 6 to 7 W / cm 2 preferred. The pH of the solution also affects the signal, with a working pH range of 3 to 10 being preferred.

The working electrode can be regenerated by exposure to UV light and can have a useful operating life. In addition to the counter electrode, it is preferable to use a reference electrode.

The method of the invention is particularly suitable for an analyzer configured as a probe for testing a water sample discontinuously.

In another aspect, the invention provides a probe for determining water quality, wherein the probe,

a) an electrochemical cell comprising a photoactive working electrode and a counter electrode,

b) a supporting electrolyte solution chamber,

c) a light source for illuminating said working electrode,

d) sample collection means for providing a sample to said cell,

e) control means, wherein the control means

Iii) triggering the light source and recording a background photocurrent generated at the working electrode from the supporting electrolyte solution,

Ii) adding the water sample to be analyzed to the photoelectrochemical cell,

Iii) ignite the light source, record the steady state photocurrent generated by the sample,

을 이용하여, 물 시료의 화학적 산소 요구량을 판단하며, 이때, δ는 네른스트 확산 층 두께(Nernst diffusion layer thickness)이고, D는 확산 계수(diffusion coefficient)이며, A는 전극의 면적이고, F는 패러데이 상수(Faraday constant)이고, i_ss는 정적 상태 광전류(steady state photocurrent)인, 상기 제어 수단Iii) Formula

In this case, the chemical oxygen demand of the water sample is determined, where δ is the Nernst diffusion layer thickness, D is the diffusion coefficient, A is the electrode area, and F is Faraday. Said control means being a Faraday constant, i _ss being a steady state photocurrent

Characterized in that it comprises a.

1 is a schematic illustration of a photoelectrochemical cell used in the present invention.

2 is a graph of typical photocurrent response of 0.1M NaClO ₄ blank solution.

3A shows the quantitative relationship between net static state current (i _ss ) and molecular concentration of organic compounds.

3B shows the quantitative relationship between net static state current (mA) and nFADC.

4A shows a graph of theoretical i _ss and experimental i _ss versus theoretical COD values of KHP solutions.

4B shows a graph of experimental i _ss versus theoretical COD values of KHP and GGA solutions.

5A shows photoelectrochemical oxidation of glucose under different luminosities of UV light.

FIG. 5B shows the effect of potential on i _ss (σ) and i _blank (o) due to photoelectrochemical oxidation of 0.2 mM glucose and its blank solution.

5C shows the effect of i _ss (σ) and i _blank (o) due to photoelectrochemical oxidation of 0.2 mM glucose and its underlying solution.

6 shows a conventional GGA standard addition for determining waste water in bakery.

Figure 7 shows the correlation between PECOD and standard dichromatic COD method for actual sample measurement.

Examples of the Invention

Preparation of Materials and Samples: Indium tin oxide (ITO) conductive glass slides (8 ms / square) are commercially provided by Delta Technologies Limited. Titanium butoxide (97%, Aldrich) and sodium nitrate were purchased from Aldrich and there was no further treatment before use. All other chemicals had an analytical grade and were purchased from Aldirich unless otherwise noted. When preparing the solution, and when diluting the actual wastewater sample, high purity deionized water (Millipore Corp., 18 cc) was used.

The actual samples used in this study were collected at various industrial sites in Queensland, Australia (eg, wastewater treatment plants, sugar plants, brewing plants, canning plants, dairy plants). According to the standard method guidelines, all samples were preserved. When necessary, the samples were diluted to a suitable concentration prior to analysis. After dilution, the same samples were analyzed by both standard COD methods and photoelectrochemical COD detectors. To the sample for photoelectrochemical determination, NaClO solids equivalent to 0.1 M were added as a supporting electrolyte.

Preparation of TiO ₂ Film Electrode: Same as described in Applicant's previous patent application WO2004088305.

Device and method

All photoelectrochemical experiments were performed at 23 ° C. in a three-electrode electrochemical cell with a window for illumination (see FIG. 1). Saturated Ag / AgCl electrodes and platinum mesh were used as reference and auxiliary electrodes, respectively. In order to apply the potential bias in the photoelectron analysis experiment, a voltage current graph (voltammograph) (CV-27, BAS) was used. Using a computer connected to the Maclab 400 interface (AD Instruments), potential and current signals were recorded. Illumination was done using a 150W xenon arc lamp light source, including a focusing lens (HF-200w-95, Beijing Optical Instruments). To prevent the sample solution from being heated by infrared light, the light beam passed through a UV-band pass filter, ie Avotronicx Pty. Limited (UG5), prior to illuminating the electrode surface. . Using a COD analyzer (NOVA 30, Merck), the standard COD values (bichromic acid method) of all samples were measured. During the oxygen dependency experiment, oxygen concentration was monitored by an oxygen electrode (YSI) and a 90 FLMV microprocessor field analyzer (T.P.S. Pty. Ltd.).

Analytical Signal Measurement

2A and B show one set of conventional photocurrent-time graphs obtained with and without organic compounds in photoelectrochemical cells. Under a constant applied potential of +0.30 V, the dark current was nearly zero when the light was switched off. After being illuminated, the current rapidly increased before decelerating to a steady value. The photocurrent ( _blank ) for the blank experiment (dotted line) derives primarily from the oxidation of water, while the photocurrent (i _total ) (solid line) observed in the sample solution containing the organism is two current components (ie The total current from the oxidation of water equal to the background photocurrent (i _blank ) and the photoelectrocatalytic oxidation of the organic compound).

The current i _ss , i.e. the diffusion limiting current originating from the oxidation of the organism, is obtained by subtracting the photocurrent i _blank of the background experiment in the absence of an organic chemical from the total photocurrent in the presence of an organic compound, May be obtained (see FIGS. 1 and 2).

i _ss = i _total -i _blank (1.1)

It has been demonstrated that all organisms transported to the TiO 2 electrode surface can be oxidized randomly and completely. Thus, the net current (i _ss ) is directly proportional to the rate of electron transfer (number of electrons transferred per unit of time). COD is defined as the amount of oxygen required for complete oxidation of organic compounds, so net current (i _ss ) can be used to quantify the COD value of a sample.

Analyze Signal Quantification

Under the non-exhaustive photocatalytic oxidation model, the quantitative relationship between the iss and the COD of the sample develops according to the following criteria: (i) The bulk solution concentration remains essentially constant before and after the experiment ( Non-complete degradation); (Ii) all organic chemicals at the electrode surface are quantitatively oxidized to their highest oxidation state (completely oxidized state); (Iv) The overall photocatalytic oxidation rate is controlled by transporting organisms to the electrode surface and can reach a steady-state within a reasonable time (steady-state process limited by mass transfer); (Iii) The potential bias applied is sufficient to transfer all photoelectrons resulting from photocatalytic oxidation of the organism (100% photoelectron collection efficiency).

The steady state mass transfer rate to the electrodes is determined by semi-empirical processing of the well-known static state mass transfer model.

(1.2)

(1.2)

Can be given as In this case, C _b and C _s refer to the concentration of the analyte on the bulk solution and the electrode surface, respectively. D and δ are diffusion coefficient and Nernst diffusion layer thickness, respectively. Under conditions limited by static state mass transfer (rate), the overall rate of reaction is

(1.3)

(1.3)

Same as

According to the standards (ii) and (iii), the number n of electrons transferred during the photoelectrochemical decomposition is constant for a given analyte, so that the static state photocurrent i _ss can be used to express the reaction rate. .

(1.4)

(1.4)

In this case, A and F refer to electrode area and Faraday constant, respectively. Equation 1.4 defines the quantitative relationship between the steady state photocurrent and the concentration of the analyte. When converting molecular concentrations to equivalent COD concentrations (mg / l of O ₂ ),

(1.5a)

(1.5a)

(1.5b)

(1.5b)

Use

Equation 1.5b is valid for COD determination of samples containing single organic compounds. The COD of a sample containing two or more organic species is

(1.6)

(1.6)

는 총체적 네른스트 확산 층 두께이며, 상기

는 상수이며, 유기체의 타입에 독립적이라고 증명되었다. 확산에 의해 제어되는 상태 하에서,

는 시료의 조성에 따라 달라지는 복합 확산 계수(composite diffiusion coefficient)이며, 주어진 시료에 대해서는 상수이다.Can be expressed as

Is the overall Nernst diffused layer thickness, and

Is a constant and has been proven to be independent of the type of organism. Under conditions controlled by diffusion,

Is a composite diffiusion coefficient that depends on the composition of the sample and is constant for a given sample.

Validation of the analysis principle Validation of Analytical Principle )

3A shows a graph of static state photocurrent versus molecular concentration of organic compounds. For all compounds tested, a linear relationship between iss and C was obtained, predicted using Equation (1.5). Further processing of the data of FIG. 3A provides FIG. 3B. All data in FIG. 3B fits one linear curve with slope = 0.0531 and R ² = 0.995. Since the slope of the curve is equal to δ ⁻¹ , under this experimental condition, there is a stagnant diffusion layer thickness (δ = 1.86 × 10 ⁻³ cm), which is independent of the concentration and type of organic compound. Can be derived. This finding also confirms that the theoretical slope given by equation (1.5) represents the slope of the curve for each compound of FIG. 1.3a. In fact, if all four previous criteria are not satisfied, the linear line of FIG. 3B will not be obtained.

In theory, equation (1.6) should be verified under the same conditions as required by equation (1.4). 4A and 4B thus show graphs of i _{ss versus} theoretical COD values ([COD] _theoreetical ) of synthetic samples prepared using KHP and test compounds for standard COD methods. As predicted by equation (1.5), a linear relationship between i _ss and [COD] _theoretical was obtained. The slope of the experimental curve obtained was 2.8 × 10 ⁻³ μs (mg / L of O ₂ ) ^{− 1} with R ² = 0.9985. The theoretical curve calculated from equation (1.5) is provided (in solid line) in FIG. 4A for comparison. Nearly identical theoretical and experimental slope values demonstrate that Equation (1.5) can be applied for COD determination.

)가 알려져 있지 않기 때문이다. 그 밖의 다른 분석과는 달리, COD는 집합적 양(aggregative quantity)이기 때문에, COD 분석을 위한 교정 표준을 정하는 것은 어렵다. 실전에서, COD 교정 표준은 실험적 수단에 의해서만 선택될 수 있다. 선택된 교정 기준은 2개의 핵심적 기준(criteria)을 만족시켜야 한다, 즉, (ⅰ) 교정 표준은 원 시료와 동등한

값을 가져야 하고, (ⅱ) 완전히 산화(fully oxidize)될 수 있어야 한다. 이들 기준은, 추가된 교정 표준이 원 시료와 동일한 기울기를 따르는 정적 상태 광전류 변화를 야기한다는 실험적 관찰을 반영한다.The applicability of Equation (1.6) was examined using a GGA synthesis sample. The GGA synthetic sample is a mixture of glucose and glutamic acid and has been commonly used as a standard test solution for BOD analysis. As predicted by Equation (1.6), the static state photocurrent iss is directly proportional to the sample [COD] (see Fig. 1.4b). However, applying Equation (1.6) to the actual sample requires calibration because the complex diffusion coefficient (

) Is not known. Unlike other analyzes, since COD is an aggregate quantity, it is difficult to establish calibration standards for COD analysis. In practice, COD calibration standards can be selected only by experimental means. The calibration standard chosen must satisfy two key criteria: (i) the calibration standard is equivalent to the original sample.

Must have a value, and (ii) be able to be fully oxidized. These criteria reflect an experimental observation that the added calibration standard results in a static state photocurrent change following the same slope as the original sample.

Optimization of Analytical Signals

The effect of light intensity on the static state photocurrent was examined (see FIG. 5A). It should be noted that changes in luminosity have a significant effect on the linear range. Increasing the brightness causes an increase in the linear range. The i _ss deviation from the linear relationship is related to the rate of photocatalytic oxidation that is slower than the rate of mass transfer to the electrode. The increase in brightness leads to an increase in the rate of photohole generation, which in fact increases the rate of photocatalytic oxidation. That is, under conditions controlled by mass transfer at higher concentrations, high brightness can be maintained during the entire procedure. Thus, in order to provide a wide linear range and desirable operating conditions, a relatively low (but sufficient) brightness (6.6 kW / cm 2) was used.

For certain TiO ₂ semiconductor electrodes, the potential bias applied performs an electron collection function that is made available by interfacial photocatalytic reactions. Only if the potential bias applied is sufficient, 100% photoelectron collection efficiency (see section Analyze-Analyze Signal Quantification) can be achieved. 5B shows the effect of dislocation bias on both i _ss and i _blank . It is shown that both i _ss and i _blank are constant when the potential bias applied is more positive than -0.05 V (vs. Ag / AgCl (which represents 100% photoelectron collection efficiency)). To ensure that the chosen potential bias can be applied simultaneously under various conditions, a standard potential bias of +0.30 V (vs. Ag / AgCl) was chosen to avoid direct electrochemical reactions.

In the Nernst approach, it is well known that the pH of a solution affects the flat band potential and band edge potential of TiO ₂ semiconductors. The pH of the solution also affects the specification of the surface functional groups of the semiconductor electrode and the chemical form of the organic compounds of the solution. These pH dependent factors can affect the assay signal. 5C shows the effect of pH on both i _ss and i _blank . Within the pH range of 2-3, both i _ss and i _blank slightly increased as the pH of the solution increased. Within the pH range of 3 to 10, i _ss and i _blank did not respond to the pH change of the solution. When the pH of the solution was above 10, the observed i _ss did not respond relatively to the change in pH, but a sharp increase with the pH of the solution was observed in the i _blank . This is because the oxidation rate of water is greatly improved at high pH. The sensitivity of i _blanks to the pH of the solution can lead to problems with accurate measurement of i _ss . Therefore, the pH of the solution is preferably 3 to 10. This pH range is suitable for most environmental samples (pH 3-10), so the sample can be used without the need for pH adjustment.

Real sample analysis

값이 상수이고 교정 및 측정 동안 유지됨을 보장하기 위해, 표준 첨가법이 사용될 수 있다. 도 1에서 나타난 결과에 의해, 수식(1.6)이 실제 시료의 COD 값을 판정하도록 사용될 수 있음이 확인된다.Analysis of the actual sample was performed. This real sample was collected at various industrial sites. The pH of the actual sample tested herein was in the range of 6 to 8, ie pH independent intervals. For analysis of very high COD samples, dilution with NaClO ₄ , or NaNO ₃ solution will usually result in a pH of 5-8 and an O ₂ concentration of 5-9.5 mgL ⁻¹ . To minimize any matrix effect, if necessary, for photoelectrochemical determination of the actual sample's COD value, and

To ensure that the value is constant and maintained during calibration and measurement, standard addition methods can be used. The results shown in FIG. 1 confirm that equation (1.6) can be used to determine the actual sample's COD value.

7 shows the correlation between experimental COD values and standard COD values. Standard COD values were determined by the conventional COD method (dichromic acid method). If valid, the Pearson Correlation coefficient was used as a measure of the magnitude of the linkage between the value obtained by the photoelectrochemical COD method and the value obtained by the conventional COD method. A high correlation (r = 0.988, P = 0.000, n = 18) between the two methods was obtained, indicating that the two methods agree very well. The slope of the graph was 1.02. This near 1 slope indicates that the two methods accurately measured the same COD value. Given a 95% confidence interval, this slope was between 0.95 and 1.11, meaning 95% confidence that the true slope lies between these two values. Given the analytical errors associated with both photoelectrochemical COD measurements and standard method measurements, and these errors contribute to the scatter plot on two axes, the strong correlations and slopes obtained measure chemical oxygen demand (COD). It provides overwhelming support for the suitability of the photoelectrochemical COD method.

Under previously optimized experimental conditions, 70㎎L ^-1 it can be seen that the detection limit of COD 0.8㎎L ^-1 with a linear range up to COD can be obtained. The detection range can be extended by appropriate dilution as mentioned above. Regeneration of 2.2% RSD was obtained from 19 analyzes of 50 μM KHP.

In view of the above, the present invention can provide an improved method and probe in performing non-exhaustive COD analysis of water samples.

Claims (6)

In the method for determining the chemical oxygen demand (COD) of the water sample, the method a) applying a constant potential to a photoelectrochemical cell containing a photoactive working electrode and a counter electrode and containing a supporting electrolyte solution, b) illuminating the working electrode using a light source and recording a background photocurrent generated at the working electrode from the supporting electrolyte solution, c) adding a sample of water to be analyzed into the photoelectrochemical cell, d) illuminating the working electrode using a light source and recording a steady state photocurrent generated by the sample, e) formula

Determining the chemical oxygen demand of the water sample by using, wherein δ is the Nernst diffusion layer thickness, D is the diffusion coefficient, A is the electrode area, F is Faraday constant, i _ss is a steady state photocurrent Method for determining the chemical oxygen demand comprising a. 2. The method of claim 1 wherein the pH of the water sample is between 3 and 10. 4. The method of any one of claims 1 to 3, wherein the photoactive working electrode is a titanium dioxide nanoparticulate photoelectrode. In the probe (probe) for determining the water quality (probe), the probe, a) an electrochemical cell comprising a photoactive working electrode and a counter electrode, b) a supporting electrolyte solution chamber, c) a light source for illuminating said working electrode, d) sample collection means for providing a sample to said cell, e) control means, wherein the control means Iii) triggering the light source and recording a background photocurrent generated at the working electrode from the supporting electrolyte solution, Ii) adding the water sample to be analyzed to the photoelectrochemical cell, Iii) ignite the light source, record the steady state photocurrent produced by the sample, Iii) Formula

In this case, the chemical oxygen demand of the water sample is determined, where δ is the Nernst diffusion layer thickness, D is the diffusion coefficient, A is the electrode area, and F is Faraday. Said control means being a Faraday constant, i _ss being a steady state photocurrent Probe comprising a. 5. The probe of claim 4 wherein said photoactive working electrode is a titanium dioxide nanoparticulate photoelectrode. The probe according to claim 4 or 5, wherein the light intensity is 3 to 10 W / cm 2.

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