EP2135071A1 - Analyse d'eau en ligne améliorée - Google Patents

Analyse d'eau en ligne améliorée

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Publication number
EP2135071A1
EP2135071A1 EP07845426A EP07845426A EP2135071A1 EP 2135071 A1 EP2135071 A1 EP 2135071A1 EP 07845426 A EP07845426 A EP 07845426A EP 07845426 A EP07845426 A EP 07845426A EP 2135071 A1 EP2135071 A1 EP 2135071A1
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EP
European Patent Office
Prior art keywords
cod
sample
working electrode
water
photocurrent
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP07845426A
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German (de)
English (en)
Inventor
Huijun Zhao
Shanqing Zhang
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Aqua Diagnostic Pty Ltd
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Aqua Diagnostic Pty Ltd
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Filing date
Publication date
Priority claimed from AU2006907133A external-priority patent/AU2006907133A0/en
Application filed by Aqua Diagnostic Pty Ltd filed Critical Aqua Diagnostic Pty Ltd
Publication of EP2135071A1 publication Critical patent/EP2135071A1/fr
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
    • G01N33/18—Water
    • G01N33/1806—Biological oxygen demand [BOD] or chemical oxygen demand [COD]

Definitions

  • This invention relates to a new method for determining oxygen demand of water using photoelectrochemical cells.
  • the invention relates to an improved direct photoelectrochemical method of determining chemical oxygen demand of water samples using a titanium dioxide nanoparticulate semiconductive electrode. It is particularly adapted for use in an online continuous measurement environment.
  • the present invention provides a method of determining chemical oxygen demand (COD) of a water sample, comprising the steps of 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 produced at the working electrode from the supporting electrolyte solution; c) adding a water sample, to be analyzed, to the photoelectrochemical cell; d) illuminating the working electrode with a light source and recording the hydro dynamic photocurrent produced under continuous flow of the water to be analyzed; e) determining the chemical oxygen demand of the water sample using the formula
  • i peak is the photocurrent peak height and i sp is the saturated photocurrent.
  • the applied potential is preferably from -0.4 to + O.8V more preferably about +0.3V.
  • the method is applicable to water samples in the pH range of 2 to 10.
  • a slow flow rate is preferred in order to achieve indiscriminate oxidation of organic compounds. However too low a flow rate may lead to lower sensitivity.
  • a preferred flow rate is 0.3mL/min.
  • the present invention provides a second method of measuring
  • COD for online monitoring comprising the steps of 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 produced at the working electrode from the supporting electrolyte solution; c) adding a water sample, to be analysed, into the photoelectrochemical cell; d) illuminating the working electrode with a light source and recording the hydro dynamic photocurrent produced under continuous flow of the water to be analysed; e) determining the Chemical Oxygen Demand of the water sample using the formula
  • Qn e t is the amount of electrons captured during the continuous flow detection
  • Qtheore t icai refers to the theoretical charge required for mineralization of the injected sample ni, is the oxidation number namely the number of electrons transferred for an individual organic compound during the photoelectrocatalytic degradation
  • Cj is the molar concentration of individual organic compound
  • V is the sample volume
  • K is the slope, which can be obtained by calibration curve method or standard addition calibration method. These methods are useful in online analysis .
  • this invention provides an online analyser for analyzing water quality on a continuous basis which includes a) an electrochemical cell containing a photoactive working electrode and a counter electrode, b) a supporting electrolyte solution chamber; c) a light source to illuminate the working electrode d) continuous flow injection means to provide a sample solution to the cell e) control means to i) actuate the light source and record the background photocurrent produced at the working electrode from the supporting electrolyte solution; ii) control the flow rate of the water sample, to be analysed, to the photoelectrochemical cell; iii) actuate the light source and record the hydro dynamic photocurrent produced under continuous flow of the water to be analysed; iv) determine the chemical oxygen demand of the water sample using any of the formula given above.
  • Figure 1 is a schematic illustration of the detection cell used
  • Figure 2 shows a set of typical photocurrent-time profiles obtained in the presence of organic compounds under continuous flow conditions
  • Figure 3 illustrates the effect of potential on the peak response of 100 ⁇ M glucose
  • Figure 4 illustrates the effect of injection volume on the photoelectrochemical detection
  • FIG. 5 illustrates the effect of flow rate on the photoelectrochemical detection
  • FIG. 6 illustrates the effect of pH on the photoelectrochemical detection of 100 ⁇ M glucose
  • Figure 7 illustrates the effect of (a) The quantitative relationship between the peak height and concentration ( ⁇ M) of organic compounds, (b) The quantitative relationship between the peak height and theoretical COD. (c) The correlation between the PECOD and theoretical COD for the synthetic COD test samples using glucose as COD standard;
  • Figure 8 illustrates the photoelectrochemical detection of COD value using glucose as a standard
  • FIG. 10 illustrates a typical photocurrent response in continuous flow analysis
  • Figure 11 illustrates the effect of flow rate on (a) the photoelectrochemical charge and (b) the oxidation percentage
  • Figure 12 illustrates the effect of pH on the photoelectrochemical detection of 100 ⁇ M glucose
  • Figure 13 illustrates the photoelectrochemical determination of COD value of the synthetic samples: (a) Q ne t versus C ( ⁇ M) relationship and (b) the correlation between the PeCOD and theoretical COD;
  • Figure 14 shows the continuous flow-based photoelectrochemical determination of COD of a real sample using the standard addition method.
  • ITO Indium Tin Oxide
  • TiO Indium Tin Oxide
  • TiO Titanium butoxide (97%, Aldrich), sucrose, glucose, glutamic acid, and sodium perchlorate were purchased from Aldrich without further treatment prior to use. All other chemicals were of analytical grade and purchased from Aldrich unless otherwise stated.
  • High purity deionised water (Millipore Corp., 18M ⁇ cm) was used for solution preparation and the dilution of real wastewater samples.
  • the GGA synthetic samples used for this study were prepared according to the reported method. All real samples used for this study were collected from bakeries, sugar plants and breweries, based in Queensland, Australia. All samples were preserved according to the guidelines of the standard method.
  • the samples were diluted to a suitable concentration prior to the analysis. After dilution, the same sample was subject to the analysis by both the standard dichromate COD method and the flow photoelectrochemical COD detector. A certain amount of solid NaCIO 4 equivalent to 2M was added to the sample.
  • Illumination was carried out using a 150W xenon arc lamp light source with focusing lenses (HF- 200w-95, Beijing Optical Instruments). To avoid the sample solution being heated by infrared light, a UV-band pass filter (UG 5, Avotronics Pty. Limited) was used. Standard COD value (dichromate method) of all the samples was measured with an EPA approved COD analyzer (NOVA 30, Merck). Analytical Signal Measurement
  • Figure 2 shows a set of typical photocurrent-time profiles obtained in the presence of organic compounds under continuous flow conditions with a constant applied potential of +0.30 V and light intensity of 6.6 mW/cm 2 .
  • the peak-shaped photocurrent profile is the result of concentration dispersion effect of sample flow.
  • the peak in Figure 2a shows the unsaturated photocurrent profile with relatively small injection sample volume while the peak in Figure 2 b shows the saturated photocurrent profile with a large injection sample volume.
  • the baseline (i b i a n k ) for both cases resulted from the photoelectrocatalytic oxidation of water and has been electronically offset to zero.
  • both peak photocurrent (i pea k for unsaturated photocurrent profile) and saturated photocurrent (i sp for saturated photocurrent profile) have resulted from the photoelectrocatalytic oxidation of organic compounds.
  • the baseline is the blank (i b iank) for both cases and offset to zero, both i pea k and i sp are net photocurrents, originating from the oxidation of organics and so can be quantitatively related to the diffusion limiting current (i ss ), obtaining from a stationary cell. All organics transported to the T1O 2 electrode surface can be indiscriminately and fully oxidized. Therefore, both i peak and i sp can be used to quantify the COD value of a sample. Analytical Signal Quantification
  • the quantitative relationship between the net photocurrent (i pea k or i sp ) obtained under the continuous flow, non-exhaustive photocatalytic oxidation conditions can be developed based on the following postulates: (i) all organic compounds at the electrode surface are stoichiometrically oxidized to their highest oxidation state (fully oxidised); (ii) the overall photocatalytic oxidation rate is controlled by the transport of organics to the electrode surface and the bulk solution concentration- time profile follows the flow-injection dispersion profile; (iii) the applied potential bias is sufficient to remove all photoelectrons generated from the photocatalytic oxidation of organics (100% photoelectron collection efficiency).
  • C 0 and C t are the original concentration and the concentration at a given time, respectively.
  • the dispersion coefficient (Y) is a constant for any given system setup and can be experimentally measured.
  • Rate - — C x (-4) ⁇ D is the diffusion coefficient and ⁇ is the concentration diffusion layer thickness.
  • ⁇ is a constant under a given hydrodynamic condition (i.e. flow rate).
  • the number of electrons transferred (n) during photoelectrochemical degradation is constant for a given analyte and the maximum photocurrent (i peak or i sp ) can, therefore, be used to represent the maximum rate of reaction.
  • the peak photocurrent can be given as:
  • Equations.5 and .6 define the quantitative relationship between the maximum photocurrent and the concentration of analyte. Convert the molar concentration into the equivalent COD concentration (mg/L of O 2 ), we have:
  • Equations 7b and 8b are valid for determination of COD in a sample that contains a single organic compound.
  • the COD of a sample contains more than one organic species can be represented as:
  • the photocatalytic degradation efficiency at TiO 2 depends on the degree of recombination of photoelectrons and holes. The recombination will lead to the disappearance of holes; therefore, the recombination needs to be suppressed.
  • the photoelectrons are "trapped" by electrochemical means rather than oxygen. The photoelectrons are subsequently forced to pass into the external circuit and to the auxiliary electrode, where the reduction of oxygen (or other species) takes place.
  • Figure 3 shows the effect of applied potentials where 100 ⁇ M glucose was tested. In the region between -0.4V and OV, the photocurrent resulting from the oxidation of the glucose increased almost linear with the increase of potential.
  • FIG. 4 shows the effect of injection volume on the photoelectrochemical detection of glucose at a flow rate of 0.3 mL/min.
  • Figure 4 clearly indicates that a larger injection volume results in higher sensitivity, such a larger injection volume also suffers from a narrower linear range.
  • the detection limit could be as low as 0.1 ppm COD, while the linear range was only up to 100 ⁇ M glucose (19.2 ppm COD).
  • the detection limit was about 1 ppm COD and the linear range continued up to 100 ppm COD.
  • a 1 ppm detection limit is likely to be sufficient, while an upper linear range of only 20 ppm COD will normally be impractical.
  • An upper linear range of 100 ppm COD is desirable.
  • a smaller sample volume also has an advantage in terms of higher sample throughout. Note that a 13 ⁇ l_ injection volume has a sample throughout of 60 per hour while a 262 ⁇ L injection volume has a throughput as low as 10 per hour. Therefore, in this work, a standard injection volume of 13 ⁇ L was established.
  • Figure 5 shows the effect of flow rate of the analytical signal. It was found that a slower flow rate (i.e. 0.3 mL/min) offers a higher sensitivity and wider linear range. The lower flow rate favors a longer contact time, and therefore allows a more complete equilibration and more sensitive response. Also, at a slower flow rate, less oxidation intermediates will be removed before further oxidation. However, while a low flow rate is essential to achieve indiscriminative oxidation of organic compounds, too low a flow rate (e.g., 0.2ml_/min) may lead to lower sensitivity due to dispersion of the analyte in the flow tubing. Thus a flow rate of 0.3mL/min was set as a standard for further experimentation.
  • a flow rate of 0.3mL/min was set as a standard for further experimentation.
  • Variation of pH causes change in the band edge potential of the TiO 2 electrode due to the flat band potential and the band edge potential of oxide semiconductors which have a Nernstian dependence on the pH of the solutions .
  • speciation of the TiO 2 surface is pH dependent , and so can affect the level of photoelectrochemical oxidation of water and organic matters in the photoelectrochemical system.
  • Levels of pH ⁇ 2 were not tested, as the pH of real samples are generally at pH>2.
  • high acidity would damage ITO sublayer of the TiO 2 electrode. pH effects therefore were investigated under experimental conditions that had been previously optimised.
  • Figure 6 shows the effect of pH on the detection of 100 ⁇ M glucose (i.e. 19.2ppm COD).
  • the peak heights shown in Figure 6 were obtained in the range of 2 ⁇ pH ⁇ 10 and were almost identical.
  • Figure 7a shows the plots of i pea k against the molar concentrations of organic compounds.
  • Equation 8a can be validated in a similar manner as the characteristics of the i sp versus COD curve are the same as those of the i peak versus COD curve shown in Figure 7b.
  • Figure 7c presents a plot of the measured COD (PeCOD) against the theoretical COD value of the samples.
  • the line of best fit with a slope of 1.0268 and R 2 of 0.9984 is obtained.
  • This near unity curve slope demonstrates the applicability of Equation 7b for COD determination.
  • the data also validate Equation 9a as the GGA sample consists of more than one organic compound.
  • Equations 8b and 9b can be validated in a similar manner as the characteristics of PeCOD versu
  • Theoretical COD curve are the same as those of the i peak versus COD curve shown in Figure 7c.
  • Figure 8 shows a set of typical photocurrent responses.
  • the calibration curve (the insert within Figure 8) was then used for real sample COD calculations, in accordance with Equation 9. COD values so obtained were subsequently plotted against the COD value determined by standard dichromate COD method, as shown in Figure 9.
  • the calibration curve (the insert within Figure 8) was then used for real sample COD calculations, in accordance with Equation 9. COD values so obtained were subsequently plotted against the COD value determined by standard dichromate COD method, as shown in Figure 9.
  • the photocurrent originating from the photocatalytic oxidation of organics can be obtained and subsequently used as the analytical signal for determination of COD, as it represents the extent of oxidation.
  • the thin- layer photoelectrochemical detector (see Figure 1) used in this work is a consumption type detector as the organic compounds in the sample are photoelectrochemically oxidized at the TiO 2 working electrode.
  • WO 2004/088305 exhaustive degradation was achieved by employing a stop-flow operation mode.
  • n refers to the number of electrons transferred for an individual organic compound during the photoelectrocatalytic degradation, C, is the molar concentration of individual organic compound; F and V represent Faraday constant and sample volume, respectively.
  • ⁇ the oxidation percentage
  • Q 1 t 1 h (eoretical)
  • Q ne t is the number of electrons captured during the continuous flow detection
  • Qtheoreticai refers to the theoretical charge required for complete mineralization of the injected sample.
  • the oxidation percentage is a constant, which is similar to the situation that occurs in a consumption-type detection in continuous flow mode.
  • the amount of electrons captured by the detector can be written as:
  • Equation 14 can be used to directly quantify the COD value of a sample when Q net is obtained, since k, the slope, can be obtained by the calibration curve method or the standard addition calibration method.
  • Figure 10 shows a typical photocurrent-time profile obtained during the degradation of organic compounds under continuous flow conditions. It can be used to illustrate how Qnet is obtained.
  • the flat baseline (blank) photocurrent (i base ime) observed from the carrier solution originates from water oxidation, while the peak response observed from the sample injection is the total current of two different components, one that originates from photoelectrocatalytic oxidation of organics (i ne t), while the other is from water oxidation, (i.e., which is the same as the blank photocurrent).
  • the net charge, Q ne t, originating from oxidation of organic compounds can be obtained by integration of the peak area between the solid and dashed line, i.e., the shaded area as indicated in Figure 10.
  • a thin-layer photoelectrochemical detector was specifically designed to suit on-line photoelectrochemical determination of COD under continuous flow conditions.
  • the thin-layer configuration is a key feature of the design. Such a configuration is essential to achieve a large (electrode area)/(solution volume) ratio that ensures rapid photodegradation of an injected sample. It also provides reliable and reproducible hydrodynamic conditions, which are crucial for accuracy, reproducibility and reliability.
  • a thin liquid layer maximises light utilisation efficiency because the aqueous media also absorbs UV radiation.
  • a suitable TiO 2 nanoparticulate electrode was chosen that was mechanically stable, suited to a wide spectrum of organic compounds, and capable of indiscriminate organic compound photooxidation.
  • the light source is another important component, since the effective light intensity is an important parameter affecting degradation rate.
  • a modified Xenon light source was employed with an output beam regulated in terms of size and intensity of the beam by a group of quartz lenses.
  • a UV-band pass filter was used to reduce infrared radiation reaching the detector, and so prevent solution heating.
  • a potential bias of +0.3V vs Ag/AgCI was selected to ensure that maximum electron efficiency is achieved.
  • Equation 14 is further confirmed by the direct relationship between oxidation percentage and concentration (as shown in Figure 11b).
  • a low flow rate 0.3ml_/min
  • the oxidation percentage is constant throughout the concentration range investigated.
  • a constant oxidation percentage could only be maintained at higher concentrations (>40 ⁇ M glucose), and fluctuations in the oxidation percentage are noted at lower concentrations ( ⁇ 40 ⁇ M glucose).
  • the injection volume is one operational parameter that can strongly influence the detection sensitivity and linear range as it determines the sample contact time at the electrode under a constant flow rate.
  • Table 1 shows the effect of injection volume on the detection limits and linear range. It was found that when injection volume was increased from 13 ⁇ L to 262 ⁇ L, the detection limit improved from 1 ppm down to 0.1 ppm. However, despite this improvement in detection limit (sensitivity), too high an injection volume can significantly reduce the linear range, as large amounts of analytes can surpass the capacity of the photoelectrochemical detector. When this occurs, the oxidation percentage ( ⁇ ) will change with concentration and Equation 14 will become invalid. Therefore, for the work reported here, a small injection volume of 13 ⁇ L was selected to assure the validity of Equation 14. This injection volume was chosen to permit the widest linear range (1-100 ppm COD), at satisfactory sensitivity and detection limits. Additionally, such a small injection volume allows a short assay time. Table 1 Effect of injection volume on detection limit and linear range
  • Effect of pH Figure 12 shows the effect of pH on the resultant analytical signal (Q ne t), where all experiments were carried out under identical conditions except pH change.
  • the conditions for pH ⁇ 2 were not investigated here because damage of the ITO conductive layer can occur under such acidic conditions.
  • Q ne t For a given concentration, no significant changes in Q net were observed when the solution pH was varied from 2 to 10. However, a sharp increase in Q ne t was observed when the solution pH was greater than 10.
  • a question arising from this observation is whether the sharp increase in Q ne t is due to increasing oxidation efficiency towards the organics or to other factors. Therefore, to clarify this, the effect of solution pH on the blank current (baseline) was investigated. Blank solutions containing 2M NaCIO 4 with various pHs were injected.
  • Figure 13 a shows the plot of Q net against synthetic sample concentration in ⁇ M. Different slopes for different synthetic sample were observed. It revealed that the slopes decreased in the order of sucrose>GGA>glucose>glutamic acid. This is because the mineralisation of different organic compounds requires different numbers of electrons. For a given molar concentration, an organic compound having a larger n will generate more charge, hence a larger slope as shown in Figure 13 a).
  • the measured net charge should be directly proportional to the COD value of the sample.
  • the trendline of best fit has a slope of 1.0145 with a R 2 of 0.9895, which demonstrates the applicability of Equation 14.
  • a detection limit of 0.1 ppm COD and a linear range up to 100 ppm COD can be achieved depending on the injection volume and flow rate.
  • the detection limit can be further improved by increasing the sample injection volume while the linear range can be extended by a further decrease of injection volume.
  • the reproducibility is represented by RSD% of 0.8% that obtained from 12 repeated injections of 100 ⁇ M glucose. No significant change for Q net was obtained from injections of 100 ⁇ M glucose over a period of 60 days.
  • the electrode fouling caused by organic contamination and bacteria growth was not observed during the storage due to the well-known merits of self-cleaning ability of T ⁇ O 2 (24).
  • the applicability of the method for real sample analysis was examined.
  • the pH of the real samples tested in this work was within the range of 6-8 (the pH independent region).
  • the standard addition method can be used to determine the COD value in real sample to eliminate possible signal variation caused by the complex sample matrix.
  • Figure14 shows the typical photocurrent profile of the continuous flow responses, and the COD value of the real sample determined using standard addition method.

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Abstract

L'invention concerne un procédé de détermination d'une demande chimique en oxygène (DCO) d'un échantillon d'eau, utilisé dans une application en ligne, consistant a) à appliquer une polarisation de potentiel constante à une cellule photoélectrochimique comprenant une électrode de travail, éventuellement une électrode de référence et une contre-électrode, et contenant une solution électrolytique de support; b) à éclairer l'électrode de travail à l'aide d'une source lumineuse et à enregistrer le photocourant hydrodynamique de fond produit au niveau de l'électrode de travail à partir de la solution électrolytique de support; c) à ajouter un échantillon d'eau à analyser dans la cellule photoélectrochimique; d) à éclairer l'électrode de travail à l'aide d'une source lumineuse et à enregistrer le photocourant hydrodynamique produit sous un flux d'eau à analyser; et e) à déterminer la demande chimique en oxygène de l'échantillon d'eau à l'aide d'un nombre de formules différentes. Le potentiel appliqué varie, de préférence, de -0,4 à +0,8V et est, plus préférablement, de +0,3V. Ce procédé peut être utilisé avec des échantillons d'eau présentant une gamme de pH variant de 2 à 10. Le volume d'injection préféré est de 13μL. Le débit d'écoulement préféré est de 0,3mL/min.
EP07845426A 2006-12-22 2007-12-21 Analyse d'eau en ligne améliorée Withdrawn EP2135071A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
AU2006907133A AU2006907133A0 (en) 2006-12-22 Improved Online Water Analysis
PCT/AU2007/001988 WO2008077192A1 (fr) 2006-12-22 2007-12-21 Analyse d'eau en ligne améliorée

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CN (1) CN101563603A (fr)
AU (1) AU2007336707B2 (fr)
BR (1) BRPI0721041A2 (fr)
CA (1) CA2673188A1 (fr)
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WO (1) WO2008077192A1 (fr)

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WO2010077767A1 (fr) * 2009-01-02 2010-07-08 Hach Company Système de surveillance d'oxygène et procédé permettant de déterminer la charge de densité d'oxygène d'un fluide
CN101900703B (zh) * 2010-06-30 2013-03-20 宇星科技发展(深圳)有限公司 一种总砷在线分析仪
CN102331447A (zh) * 2011-04-27 2012-01-25 河北先河环保科技股份有限公司 一种用光催化氧化法测定化学需氧量的方法及设备
CN102305816B (zh) * 2011-05-23 2013-07-31 中国科学院广州能源研究所 采用pfc光电催化法测定环境气体中总有机气体浓度的方法
CN103135537B (zh) * 2013-02-04 2015-07-15 耿炜 在线水质监测仪远程质控系统
CN104165916B (zh) * 2014-08-18 2016-08-17 天津大学 用于现场光学、光力学测量的模拟电池装置
TWM496760U (zh) * 2014-11-05 2015-03-01 Univ Chaoyang Technology 化學需氧量檢測裝置
JP7118451B2 (ja) * 2017-06-29 2022-08-16 グリフィス・ユニバーシティ センサ
CN110887878A (zh) * 2019-11-04 2020-03-17 南开大学 一种微流水质cod在线检测和远程监测系统及方法
CN113281395B (zh) * 2021-04-22 2022-10-14 汕头大学 一种污染物降解、监测系统及其构建方法和应用

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AU2007336707B2 (en) 2010-06-03
TW200835912A (en) 2008-09-01
BRPI0721041A2 (pt) 2014-07-29
WO2008077192A1 (fr) 2008-07-03
AU2007336707A1 (en) 2008-07-03
CA2673188A1 (fr) 2008-07-03

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