JP2010513874A - Improved water quality analysis - Google Patents

Abstract

A method for measuring chemical oxygen demand (COD) of a water sample useful for probe construction includes: a) a photoelectrochemical cell having a photoactive working electrode, optionally a reference electrode and a counter electrode, and containing a supporting electrolyte solution B) irradiating the working electrode with a light source and recording the background photocurrent generated from the supporting electrolyte solution at the working electrode; c) light the water sample to be analyzed Applying to the electrochemical cell; d) irradiating the working electrode with a light source and recording the steady state photocurrent generated in the sample; e) the chemical oxygen demand of the water sample is represented by the formula (I) [ wherein, [delta] is the Nernst diffusion layer thickness, D is the diffusion coefficient, a is the electrode area, F is the Faraday constant, and i _ss includes determining with a stationary light current. The method can accommodate a wide range of light intensity and pH.
[Selection] Figure 1

Description

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

  Since almost all domestic and industrial wastewater contains organic compounds, harmful oxygen depletion (or demand) can occur in waterways from which such wastewater is released. This requirement is mainly due to the oxidative biodegradation of organic compounds by natural microorganisms that use organic matter as a food source. In this process, organic carbon is oxidized to carbon dioxide, while oxygen is consumed and reduced to water.

  An oxygen demand assay based on photoelectrochemical degradation principles is disclosed in the patent specification of WO2004088305, but the measurements in that patent were based on exhaustive degradation principles.

  The object of the present invention is to develop an analyzer based on limited (partial) non-exhaustive degradation principles. Another object of the present invention is to develop a probe-type COD analyzer.

To achieve this objective, the present invention provides a method for measuring the chemical oxygen demand (COD) of a water sample, the method comprising:
a) applying a constant potential bias to a photoelectrochemical cell having a photoactive working 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 generated from the supporting electrolyte solution at the working electrode;
c) adding a water sample to be analyzed to the photoelectrochemical cell;
d) illuminating the working electrode with a light source and recording the steady state photocurrent generated in the sample;
e) The chemical oxygen demand of the water sample is given by the formula:

Wherein, [delta] is the Nernst diffusion layer thickness, D is the diffusion coefficient, A is the electrode area, F is the Faraday constant, and i _ss is the steady-state photocurrent] and determining using.

The intensity of light on the photoelectrode affects the linear range of the instrument. However, increasing the light intensity to a too high value may lead to equipment stability problems due to either light source or electrode photo-corrosion. Suitable light intensity in the range of 3~10W / cm ^2, the value of 6~7W / cm ² is preferred.

  Since the pH of the solution also affects the signal, the operating pH is preferably in the range of 3-10.

  Since the working electrode can be regenerated by exposure to UV light, it has a useful working life. In addition to the counter electrode, a reference electrode is also preferably used.

  The method of the present invention is particularly suitable for analyzers configured as probes for testing water samples in the field on a discontinuous basis.

In another aspect, the present invention provides a probe for measuring water quality, the probe comprising:
a) an electrochemical cell having a photoactive working electrode, a counter electrode and optionally a reference electrode; b) a supporting electrolyte solution chamber; c) a light source for irradiating the working electrode; Collection means and e) control means,
i) To activate the light source and record the background photocurrent generated from the supporting electrolyte solution at the working electrode;
ii) to add the water sample to be analyzed to the photoelectrochemical cell;
iii) to activate the light source and record the steady photocurrent generated in the sample;
iv) The chemical oxygen demand of the water sample is given by the formula:

[Wherein δ is the Nernst diffusion layer thickness, D is the diffusion coefficient, A is the electrode area, F is the Faraday constant, and _iss is the steady photocurrent].

FIG. 1 shows a schematic diagram of a photoelectrochemical cell used in the present invention. FIG. 2 shows a typical photocurrent response graph of a 0.1 M NaClO ₄ blank solution. FIG. 3A shows the quantitative relationship between net steady-state current (i _ss ) and the molar concentration of the organic compound. FIG. 3B shows the quantitative relationship between net steady state current (in mA) and nFADC. Figure 4A shows a plot of theoretical and experimental _{i ss} against the theoretical COD value of KHP solution. Figure 4B shows a plot of experimental _{i ss} against the theoretical COD value of KHP and GGA solution. FIG. 5A shows the photoelectrochemical oxidation of glucose under different UV light intensities. FIG. 5B shows the effect of potential on i _ss (σ) and i _blank (o) by photoelectrochemical oxidation of 0.2 mM glucose and its blank solution, respectively. FIG. 5C shows the effect of pH on i _ss (σ) and i _blank (o) by photoelectrochemical oxidation of 0.2 mM glucose and its blank solution, respectively. FIG. 6 shows a typical GGA standard addition for measuring wastewater from a bakery. FIG. 7 shows the correlation between PECOD and standard dichromate COD method for real sample measurements.

  Materials and Sample Preparation: Indium tin oxide (ITO) conductive glass slides (8Ω / square) were commercially available from Delta Technologies Limited. Titanium butoxide (97%, Aldrich) and sodium nitrate were purchased from Aldrich and used without further processing. All other chemicals were analytical grade and were purchased from Aldrich unless otherwise stated. High purity deionized water (Millipore Corp., 18 MΩcm) was used to prepare the solution and dilute the actual wastewater sample.

The actual samples used in this study were collected from various industrial bases in Queensland, Australia, such as wastewater treatment plants, sugar mills, breweries, canning and dairy manufacturing plants. All samples were stored according to standard method guidelines. If necessary, samples were diluted to the appropriate concentration before analysis. After dilution, the same sample was subjected to analysis by both a standard COD method and a photoelectrochemical COD detector. A NaClO ₄ solid corresponding to 0.1 M was added as a supporting electrolyte to the sample for photoelectrochemical measurement.

Manufacture of TiO ₂ thin film electrodes: as described in the applicant's prior patent application WO2004088305.

Apparatus and Methods All photoelectrochemical experiments were performed at 23 ° C. in a three-electrode electrochemical cell with an irradiation window (see FIG. 1). A saturated Ag / AgCl electrode and a platinum mesh were used as reference and auxiliary electrodes, respectively. The potential bias of the photoelectrolysis experiment was applied using a voltammograph (CV-27, BAS). Potential and current signals were recorded using a computer connected to the Maclab 400 interface (AD Instruments). Irradiation was performed using a 150 W xenon arc lamp light source with a focusing lens (HF-200w-95, Beijing Optical Instruments). In order to prevent the sample solution from being heated by infrared light, the light was passed through a UV band-pass filter, that is, UG5 (Avotronics Pty. Limited), and then the electrode surface was irradiated. Standard COD values (dichromate method) for all samples were measured using a COD analyzer (NOVA 30, Merck). During the oxygen dependence experiment, oxygen concentration was monitored with an oxygen electrode (YSI) and 90 FLMV Microprocessor Field Analyzer (from TPS Pty. Ltd).

Analytical Signal Measurements FIGS. 2A and B show a typical set of photocurrent-time profiles obtained in a photoelectrochemical cell in the presence and absence of organic compounds. When the light was extinguished under a constant applied potential of +0.30 V, the dark current was almost zero. Upon irradiation, the current increased rapidly and then decayed to a steady value. In the case of the blank (dotted line), the photocurrent (i _blnak ) mainly originates from the oxidation of water, but the photocurrent (i _total ) (solid line) observed from the sample solution containing organic matter is the two current components. Total. One was derived from the oxidation of water and was identical to the blank photocurrent (i _blnak ), and the other was derived from the photoelectrocatalytic oxidation of organic compounds.

The current i _ss , ie, the diffusion limit current resulting from the oxidation of organic matter, is determined by subtracting the blank photocurrent (i _blnak ) in the absence of organic compound from the total photocurrent in the presence of organic compound (FIG. 1.2). reference).

It has been proven that all organics transported to the TiO ₂ electrode surface are indiscriminately and fully oxidized. Accordingly, the net current (i _ss ) is directly proportional to the electron moving speed (the number of electrons moving per unit time). Since COD is defined as the amount of oxygen required for complete oxidation of an organic compound, the COD value of the sample can be quantified using the net current (i _ss ).

Quantification of Analytical Signal Under a non-exhaustive photocatalytic oxidation model, the quantitative relationship between the sample's _iss and COD is developed according to the following assumptions. (I) Bulk solution concentration remains essentially constant before and after the experiment (limited decomposition); (ii) All organic compounds on the electrode surface are stoichiometrically oxidized to their highest oxidation state (Complete oxidation); (iii) Overall photocatalytic oxidation rate is controlled by the transport of organic matter to the electrode surface and can reach a steady state within a reasonable time frame (steady-state mass transfer limited process) process)); (iv) The applied potential bias is sufficient to remove all photoelectrons generated from the photocatalytic oxidation of organics (100% photoelectron collection efficiency).

  The steady state mass transfer rate (dN / dt) to the electrode is given by a semi-empirical process of a well-known steady state mass transfer model.

In the formula, C _b and C _s represent analyte concentrations in the bulk solution and on the electrode surface, respectively. D and δ are the diffusion coefficient and the Nernst diffusion layer thickness, respectively.

  Under steady state mass transfer limited conditions (assuming (iii)), the overall reaction rate is

be equivalent to.

According to assumptions (ii) and (iv), the number of electrons transferred during photoelectrochemical decomposition (n) is constant for a given analyte, so that steady photocurrent (i _ss ) represents the reaction rate. Can be used.

In the formula, A and F are an electrode area and a Faraday constant, respectively.

Equation 1.4 defines a quantitative relationship between steady state photocurrent and analyte concentration. When the molar concentration is converted to an equivalent COD concentration (mg / L ₂ O ₂ ),

It becomes.

  Equation 1.5b is useful for determining COD in a sample containing a single organic compound. The COD of a sample containing two or more organic species is

It can be expressed as. Where

Is the collective Nernst diffusion layer thickness, which is constant under diffusion control conditions and is known to be independent of the type of organic matter.

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

Proof of Analysis Principle FIG. 3A shows a plot of steady state photocurrent versus molar concentration of organic compound. Linear relationship between the i _ss and C as predicted from Equation 1.5 was obtained for all compounds tested. Further processing of the data of FIG. 3A yields FIG. 3B. Note that 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 these experimental conditions there is a stagnant diffusion layer thickness (δ = 1.86 × 10 ⁻³ cm) and this is the concentration of the organic compound and It can be concluded that it is irrelevant to the type. This finding also supports that the theoretical slope given by Equation 1.5 represents the slope of the curve for each compound in Figure 1.3a. In fact, the straight line in FIG. 3B would not have been obtained if all four assumptions were not accepted.

Theoretically, equation 1.6 should be valid under the same conditions as required for equation 1.4. Thus in FIGS. 4A and 4B, shows a plot of _{i ss} against the theoretical COD value of the composite sample prepared using a test compound standard COD method _{KHP ([COD]} theoretical). As expected from Equation _1.5, a linear relationship between the _{i ss} and _{[COD] Theoretical} were obtained. The slope of the obtained experimental curve was 2.8 × 10 ⁻³ mA (mg / L O ₂ ) ⁻¹ and R ² = 0.9985. The theoretical curve (solid line) calculated from Equation 1.5 is also shown in FIG. 4A for comparison. The theoretical slope calculated according to Equation 1.5 when n = 30e ⁻ , D = 6.96 × 10 ⁻³ cm ² s ⁻¹ [ref] and δ = 1.86 × 10 ⁻³ cm. Was 2.9 × 10 ⁻³ mA (mg / L ₂ O ₂ ) ⁻¹ . These nearly identical theoretical and experimental slopes prove that Equation 1.5 is applicable to the determination of COD.

The applicability of Equation 1.6 was examined using GGA synthesized samples. The GGA synthesis sample is a mixture of glucose and glutamic acid and is usually used as a standard test solution for BOD analysis. As expected from Equation 1.6, the constant photocurrent _{i ss} is directly proportional to [COD] of the sample (see Fig. 1.4b). However, to apply Equation 1.6 to a real sample, the complex diffusion coefficient:

Is unknown and needs to be calibrated. Unlike other analyses, defining a calibration standard for COD analysis is difficult because COD is a collective quantity. In fact, the COD calibration standard can only be selected by experimental means. The chosen calibration standard should satisfy two essential criteria. (I) the calibration standard is equivalent to the original sample

Two points are that it should have a value and (ii) it can be completely oxidized. These criteria reflect the experimental observation that the added calibration standard causes a steady-state photocurrent change that follows the same slope as the original sample.

Analysis signal optimization The effect of light intensity on steady state photocurrent was investigated (see FIG. 5A). It should be noted that changes in light intensity have a dramatic effect on the linear range. An increase in light intensity results in an increase in the linear range. The deviation of i _ss from the linear relationship is related to the photocatalytic oxidation rate being slower than the mass transfer rate to the electrode. In fact, the photocatalytic oxidation rate increases because the increase in light intensity results in an increase in the photohole generation rate. That is, if the light intensity is high, the overall process can be maintained even at higher concentrations under controlled mass transfer conditions. Therefore, a relatively low (but sufficient) light intensity (6.6 mW / cm ² ) was employed to provide a wide linear range and good operating conditions.

In the case of a particulate TiO ₂ semiconductor electrode, the applied potential bias serves to collect electrons provided by the interfacial photocatalytic reaction. 100% photoelectron collection efficiency (see Assumption (iv)-Analytical Signal Quantification section) can only be achieved if the applied potential bias is sufficient. It shows the effect of potential bias on the _{i ss} and _{i blank} in Figure 5B. It is clear that both i _ss and i _blank are constant when the applied potential bias is more positive than −0.05 V vs. Ag / AgCl and show 100% photoelectron collection efficiency. A standard potential bias of +0.30 V vs. Ag / AgCl was chosen to ensure that the selected potential bias was applicable under various conditions while at the same time preventing direct electrochemical reactions.

It is well known that the pH of the solution affects the flat band potential and the band edge potential of the TiO ₂ semiconductor in a Nernst manner. The pH of the solution also affects the speciation of the surface functional groups of the semiconductor electrode and the chemical form of the organic compound in the solution. These pH dependent factors can affect the analytical signal. Figure 5C, shows the effect of pH is on _{i ss} and _{i blank.} Within the pH range 2-3, both i _ss and i _blank both increased slightly with increasing pH of the solution. Within the range of pH 3 to 10, both i _ss and i _blank were unresponsive to the pH change of the solution. When the pH of the solution exceeded 10, the observed i _ss was relatively insensitive to pH change, whereas i _blank was observed to increase rapidly with the pH of the solution. This is because the oxidation rate of water is greatly enhanced at high pH. _{I blank} sensitivity to the pH of the solution that could cause problems in accurate measurement of _{i ss.} Therefore, a solution pH range of 3-10 is preferred. This pH range is appropriate for most environmental samples (pH 3-10) and can be used without the need for pH adjustment.

Real sample analysis Real sample analysis was performed. These actual samples were collected from various industrial bases. The pH of the actual samples tested here was in the range of 6-8, ie in the pH independent region. When analyzing very high COD samples, dilution with NaClO ₄ or NaNO ₃ solution can usually bring the pH in the range of 5-8 and the O ₂ concentration in the range of 5-9.5 mg L ⁻¹ . In order to minimize the influence of any matrix, standard addition methods can be used for photoelectrochemical measurement of COD values of real samples if necessary. Then during calibration and measurement,

It is also guaranteed that the value is constant and consistent. The results shown in FIG. 6 confirm that Equation 1.6 can be used to determine the COD value of a real sample.

  FIG. 7 shows the correlation between experimental and standard COD values. Standard COD values were measured using a conventional COD method (dichromate method). When valid, the Pearson correlation coefficient was used as a measure of the strength of association between values obtained from the photoelectrochemical COD method and the conventional COD method. A very significant correlation (r = 0.888, P = 0.000, n = 18) was obtained between the two methods, indicating that the two methods agree very well. The slope of the graph was 1.02. This slope close to 1 indicates that the two methods are accurately measuring the same COD value. Assuming a 95% confidence interval, this slope was 0.96 to 1.11. In other words, a true slope means a 95% confidence level between these two values. Considering that there are analytical errors associated with both photoelectrochemical COD and standard method measurements, and that these errors contribute to dispersion in both axes, the strong correlation and slope obtained are This strongly supports the suitability of the photoelectrochemical COD method for measuring chemical oxygen demand.

It has been found that, under the optimized experimental conditions described above, a detection limit of 0.8 mg L ⁻¹ COD and a linear range of 70 mg L ⁻¹ COD can be achieved. The detection range can be expanded by appropriate dilution as described above. A reproducibility of 2.2% RSD was obtained from 19 analyzes of 50 μM KHP.

  From the foregoing, it can be seen that the present invention provides improved methods and probes for use in performing limited COD analysis of water samples.

  Those skilled in the art will recognize that the present invention may be practiced in other ways than described without departing from the central teaching of the invention.

Claims (6)

A method for measuring chemical oxygen demand (COD) of a water sample, comprising:
a) applying a constant potential bias to a photoelectrochemical cell having a photoactive working 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 generated from the supporting electrolyte solution at the working electrode;
c) adding a water sample to be analyzed to the photoelectrochemical cell;
d) illuminating the working electrode with a light source and recording the steady state photocurrent generated in the sample;
e) The chemical oxygen demand of the water sample is given by the formula:

Wherein δ is the Nernst diffusion layer thickness, D is the diffusion coefficient, A is the electrode area, F is the Faraday constant, and _iss is the steady photocurrent.   The method of claim 1, wherein the pH of the water sample is in the range of 3-10.   The method according to claim 1, 2 or 3, wherein the photoelectrode is a titanium dioxide nanoparticle photoelectrode. A probe for measuring water quality,
a) an electrochemical cell containing a photoactive working electrode and a counter electrode; b) a supporting electrolyte solution chamber; c) a light source for irradiating the working electrode; d) a sample collecting means for supplying a constant volume of sample to the cell; Control means,
i) To activate the light source and record the background photocurrent generated from the supporting electrolyte solution at the working electrode;
ii) to add the water sample to be analyzed to the photoelectrochemical cell;
iii) to activate the light source and record the steady photocurrent generated in the sample;
iv) The chemical oxygen demand of the water sample is given by the formula:

And a control means for determining using the following formula: where δ is the Nernst diffusion layer thickness, D is the diffusion coefficient, A is the electrode area, F is the Faraday constant, and _iss is the steady photocurrent.   The probe according to claim 4, wherein the photoelectrode is a titanium dioxide nanoparticle photoelectrode. Light intensity is 3~10W / cm ^2, a probe according to claim 4 or 5.

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