U.S. patent application number 10/551689 was filed with the patent office on 2006-09-14 for photoelectrochemical determination of chemical oxygen demand.
Invention is credited to Huijun Zhao.
Application Number | 20060205083 10/551689 |
Document ID | / |
Family ID | 31500632 |
Filed Date | 2006-09-14 |
United States Patent
Application |
20060205083 |
Kind Code |
A1 |
Zhao; Huijun |
September 14, 2006 |
Photoelectrochemical determination of chemical oxygen demand
Abstract
A method for determining chemical oxygen demand of a water
sample comprises the steps of (a) applying a constant potential
bias to a photoelectrochemical cell, having a photoactive working
electrode (e.g. a layer of titanium dioxide nanoparticles coated on
an inert conductive substrate) 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 total
photoelectrocurrent produced with the sample; (e) determining the
chemical oxygen demand according to the type (exhaustive or
non-exhaustive) of degradation conditions employed.
Inventors: |
Zhao; Huijun; (Queensland,
AU) |
Correspondence
Address: |
CONNOLLY BOVE LODGE & HUTZ LLP
SUITE 800
1990 M STREET NW
WASHINGTON
DC
20036-3425
US
|
Family ID: |
31500632 |
Appl. No.: |
10/551689 |
Filed: |
April 5, 2004 |
PCT Filed: |
April 5, 2004 |
PCT NO: |
PCT/AU04/00438 |
371 Date: |
September 30, 2005 |
Current U.S.
Class: |
436/62 ;
422/79 |
Current CPC
Class: |
G01N 27/305 20130101;
H01G 9/2031 20130101; Y10S 436/905 20130101; C25B 1/55 20210101;
Y02E 10/542 20130101; Y10T 436/204998 20150115; Y10T 436/235
20150115; Y02P 20/133 20151101; G01N 33/1806 20130101 |
Class at
Publication: |
436/062 ;
422/079 |
International
Class: |
G01N 33/18 20060101
G01N033/18 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 4, 2003 |
AU |
2003901589 |
Claims
1. A method of determining chemical oxygen demand 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 analysed analyzed, to the
photoelectrochemical cell; d) illuminating the working electrode
with a light source and recording the total photocurrent produced
with the sample; e) determining the chemical oxygen demand of the
water sample according to the type of degradation conditions
employed.
2. A method as claimed in claim 1 wherein the photoactive working
electrode is a nanoparticulate semiconductive electrode.
3. A method as claimed in claim 2 in which the working electrode is
a layer of titanium dioxide nanoparticles coated on an inert
conductive substrate.
4. A method as claimed in claim 1 in which a reference electrode is
also used in addition to the working and counter electrodes.
5. A method as claimed in claim 1 in which the chemical oxygen
demand is determined under exhaustive degradation conditions, in
which all organics present in the water sample are oxidized.
6. A method as claimed in claim 1 in which the chemical oxygen
demand is determined under non-exhaustive degradation conditions,
in which the organics present in the water sample are partially
oxidized.
7. A method as claimed in claim 1 in which the background
photocurrent is deducted from the total photocurrent produced with
the sample to obtain the photocurrent due to the oxidation of
organic material in the sample.
8. A method as claimed in claim in claim 1 in which the sample is
diluted with the supporting electrode.
9. A method as claimed in claim 1 in which the chemical oxygen
demand is determined by measuring charge or current under
exhaustive degradation conditions with a stationary or flow cell
using different operational modes including batch mode,
flow-stopped mode and continuous flow mode.
10. A method as claimed in claim 1 in which the chemical oxygen
demand is determined by measuring charge or current under
non-exhaustive degradation conditions with a stationary or flow
cell using different operational modes including batch mode,
flow-stopped mode and continuous flow mode.
11. A photoelectrochemical assay apparatus for determining oxygen
demand of a water sample which consists of. a) a measuring cell for
holding a sample to be analyzed; b) a photoactive working electrode
and a counter electrode disposed in said cell; c) a light source
adapted to illuminate the photoactive working electrode; d) control
means to control the illumination of the working electrode, the
applied potential bias, and photocurrent recording; e)
photocurrent/charge measuring means to measure the
photocurrent/charge at the working electrode; f) analysis means to
derive a measure of oxygen demand from the measurements made by the
photocurrent/charge measuring means.
12. Apparatus as claimed in claim 11 in which the measuring cell is
a flow through cell.
13. Apparatus as claimed in claim 11 in which a reference electrode
is included in the measuring cell.
14. Apparatus as claimed in claim 11 wherein the photoactive
working electrode is a nanoparticulate semiconductive
electrode.
15. Apparatus as claimed in claim 111 in which the working
electrode is a layer of titanium dioxide nanoparticles on an inert
substrate.
16. Apparatus as claimed in claim 11 which also includes a
reservoir for a supporting electrolyte which is used to measure the
background photocurrent and to dilute the sample.
17. Apparatus as claimed in claim 16 which also includes a sample
supply/injection system and a supporting electrolyte
supply/injection system.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a new method for determining
oxygen demand of water using photoelectrochemical cells. In
particular, the invention relates to a new photoelectrochemical
method of determining chemical oxygen demand of water samples using
a titanium dioxide nanoparticulate semiconductive electrode.
BACKGROUND TO THE INVENTION
[0002] Nearly all domestic and industrial wastewater effluents
contain organic compounds, which can cause detrimental oxygen
depletion (or demand) in waterways into which the effluents are
released. This demand is due largely to the oxidative
biodegradation of organic compounds by naturally occurring
microorganisms, which utilize the organic material as a food
source. In this process, carbon is oxidised to carbon dioxide,
while oxygen is consumed and reduced to water.
[0003] Standard analytical methodologies for the determination of
aggregate properties such as oxygen demand in water are biochemical
oxygen demand (BOD) and chemical oxygen demand (COD). BOD involves
the use of heterotrophic microorganisms to oxidise organic material
and thus estimate oxygen demand. COD uses strong oxidising agents,
such as dichromate or permanganate, to oxidise organic material.
BOD analysis is carried out over five days and oxygen demand
determined by titration or with an oxygen probe. COD measures
dichromate or permanganate depletion by titration or
spectrophotometry.
[0004] Despite their widespread use for estimating oxygen demand,
both BOD and COD methodologies have serious technological
limitations. Both methods are time consuming and very expensive,
costing water industries and local authorities in excess of $1
billion annually worldwide. Other problems with the BOD assay
include: limited linear working range; complicated, time consuming
procedures; and questionable accuracy and reproducibility (the
standard method accepts a relative standard deviation of .+-.15%
for replicate BOD.sub.5 analyses). More importantly, interpretation
of BOD results is difficult since the results tend to be specific
to the body of water in question, depend on the pollutants in the
sample solution and the nature of the microbial seed used. In
addition, the BOD methodologies cannot be used to assess the oxygen
demand for many heavily polluted water bodies because of inhibitory
and toxic effects of pollutants on the heterotropic bacteria.
[0005] The COD method is more rapid and less variable than the BOD
method and thus preferred for assessing the oxygen demand of
organic pollutants in heavily polluted water bodies. Despite this,
the method has several drawbacks in that it is time consuming,
requiring 2-4 hours to reflux samples, and utilises expensive (e.g.
Ag.sub.2SO.sub.4), corrosive (e.g. concentrated H.sub.2SO.sub.4)
and highly toxic (Hg(III) and Cr(VI)) reagents. The use of toxic
reagents being of particular environmental concern, leading to the
Cr(VI) method being abandoned in Japan.
[0006] Titanium(IV) oxide (TiO.sub.2) has been extensively used in
photooxidation of organic compounds. TiO.sub.2 is
non-photocorrosive, non-toxic, inexpensive, relatively easily
synthesised in its highly catalytic nanoparticulate form, and is
highly efficient in photooxidative degradation of organic
compounds.
[0007] Fox M. A. and Tien, T, Anal. Chem, (60 1988) 2278-2282
investigated the development of a photoelectrochemical detector by
employing an anodically formed TiO.sub.2 electrode for use in
high-pressure liquid chromatography. This photoelectrochemical
detector is reported as being sensitive to oxidisable organics,
such as alcohols. The electrode system developed by Fox et al had
low photocatalytic efficiency of the system and is difficult to use
as it cannot discriminate between the respective currents generated
from the oxidation of water and organic matter.
[0008] Brown, G. N., et al., Anal. Chem, 64 (1992) 427-434
investigated the use of a photoelectrochemical detector by
employing a thermally formed TiO.sub.2 electrode for use as a
detector in flow injection analysis and liquid chromatography. The
detector was found to be non-selective in its response to a variety
of organic analytes. Brown et al found that the response of the
detector varied with temperature, duration of heating, oxidative
atmosphere, etching of the titanium wire electrode, amount of
doping on the TiO.sub.2 detector and solvents. Similar to Fox et al
this electrode system had low photocatalytic efficiency and cannot
discriminate between the currents generated from the oxidation of
water and organic matter.
[0009] Matthews R. W. et al., Analytica Chimica Acta 233 (1990)
171-179 (also the subject of Australian patent 597165) utilised a
TiO.sub.2 photocatalytic oxidation system to determine total carbon
in water samples, by placing TiO.sub.2 into a slurry or suspension,
photooxidising the organic material with in the sample to evolve
carbon dioxide (CO.sub.2). The evolved CO.sub.2 was measured to
predict TOC value of the sample. Matthews found that the total
organic carbon can be estimated from the total amount of carbon
dioxide purged from photocatalytic cell. Jiang D. et al., J.
Photochem & Photobio A: Chemistry 144 (2001) 197-204 also
investigated the photoelectrochemical behaviour of nanoporous
TiO.sub.2 film electrodes in the photooxidation of methanol. Jiang
found that the photocurrent response of the electrode was greatly
influenced by applied potential, light intensity, methanol
concentration and pH. A linear relationship was found to exist
between the photocurrent produced through the photo-oxidation of
methanol and the concentration of methanol in the sample. However,
as concentration of methanol increased the migration of
photoelectrons across the TiO.sub.2 film and therefore
photogenerated charge separation becomes a rate-limiting step, thus
limiting the working range in which the linear relationship between
photocurrent and concentration occurs.
[0010] Lee Kyong-Hoon et al., Electroanalysis 12, No 16 (2000)
1334-1338, investigated the determination of COD using a
microfabricated Clark-type oxygen electrode and TiO.sub.2 fine
particles suspended in a sample solution under photocatalytic
oxidative degradation conditions. The current generated from the
oxygen electrode under -800 mV applied potential was used to
indicate the oxygen concentration change before and after the
photooxidation. The change in oxygen concentration was then
correlated to COD value of the sample.
[0011] Kim, Yoon-Chang, et al., Anal. Chem, 72 (2000) 3379-3382;
Analytica Chimica Acta 432 (2001) 59-66 and Anal. Chem, 74 (2002)
3858-3864 all relate to the determination of COD using a
photocatalytic oxidative degradation of organic compounds at a
titanium dioxide particles. In Anal. Chem, 2000, Kim et al
investigated the use of translucent poly(tetrafluroethylene) (PTFE)
membrane having fine particles of TiO.sub.2 absorbed or entrapped
onto the surface of the membrane in combining with a oxygen
electrode as a possible COD sensor. The immobilised TiO.sub.2
particles serve as an oxidation reagent and the analytical signal
was based on the oxygen concentration measurements between the
working and reference oxygen electrodes.
[0012] Calibration curves where established using sodium sulfite
(Na.sub.2SO.sub.3), prior to determining COD of analytes. In this
study Kim et al reports that the membrane sensor did not show good
reproducibility.
[0013] In Analytica Chimica Acta 432 (2001) 59-66, Kim et al
investigated the use of titanium dioxide (TiO.sub.2) beads in a
photochemical column and an oxygen electrode as the sensor in
determining dissolved oxygen from the photocatalytic oxidation of
organic compound and thus the COD value of the analyte.
[0014] In Anal. Chem, 74 (2002) 3858-3864 Kim et al investigated
the use of 0.6 mm TiO.sub.2 beads in a quartz tube in the
determination of oxygen consumption from photochemical oxidation of
organic compounds and subsequent calculation of COD values from the
difference in the currents recorded at the reference and working
oxygen electrodes.
[0015] The methods described by Lee et al and Kim et al above all
utilise TiO.sub.2 as an oxidative reagent to replace the
traditional reagent used in COD such as chromate salts, with the
analytical signal being obtained via two traditional oxygen
electrodes. There are many disadvantages of their method, which
makes the practical application of the method very difficult.
[0016] To date the COD assay methodologies of the prior art are
indirect in their analysis methods requiring calibration and often
suffer from having low sensitivity, poor accuracy, narrow linear
working ranges and/or difficult to operate. More importantly, these
prior art COD assay methodologies are matrix dependent due to the
low oxidation efficiency. It is an object of this invention to
overcome these shortcomings.
SUMMARY OF THE INVENTION
[0017] To this end this invention provides a method of determining
chemical oxygen demand of a water sample, comprising the steps
of
[0018] 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;
[0019] b) illuminating the working electrode with a light source,
preferably a UV light source, and recording the background
photocurrent produced at the working electrode from the supporting
electrolyte solution;
[0020] c) adding a water sample, to be analysed, to the
photoelectrochemical cell;
[0021] d) illuminating the working electrode with a UV light source
and recording the total photocurrent produced;
[0022] e) determining the chemical oxygen demand of the water
sample according to the type of degradation conditions
employed.
[0023] The working electrode may be formed from any nanoparticulate
semiconductive material capable of photooxidation of organic
compounds. The nanoparticulate semiconductive electrode may be
selected from titanium dioxide, niobium pentoxide, strontium
titanate, indium trioxide, calcium titanate, tungsten trioxide,
barium titanate, ferric oxide, zinc oxide, potassium tantalate, tin
dioxide, cadmium oxide, hafnium oxide, zirconium oxide, tantalum
pentoxide, chromium trioxide or yttrium trioxide. Preferably the
semiconductive electrode is titanium dioxide. More preferably the
semiconductive electrode is formed by laying nanoparticles of
titanium dioxide on an inert substrate, such as conducting
glass.
[0024] This invention is partly predicated on the insight that the
methods described in the prior art, utilising photoelectrochemical
properties of TiO.sub.2 nanoparticle semiconductive electrodes
provide for the direct determination of COD. In the prior art
method, the method relies for accuracy on the two oxygen
electrodes, which have to be identical in responding to the oxygen
change. In addition, the prior art method cannot be used for low
COD samples due to the insufficient sensitivity of the method. Low
COD content is important in testing water for suitability in
drinking and cleaning applications. By using this technique the
measurement of milliamperes of current allows much greater
sensitivity in the low COD range.
[0025] The method of determining the chemical oxygen demand value
of a water sample may be determined under exhaustive degradation
conditions, in which all organics present in the water sample are
oxidised. Under exhaustive degradation conditions the chemical
oxygen demand value can be obtained according to the steps of;
[0026] a) integrating both the background photocurrent and total
photocurrent (i.sub.l(1)) to give the background charge and total
charge, and subtracting the background charge from the total charge
to determine the net charge Q.sub.net; for the water sample; and
[0027] b) calculating the chemical oxygen demand value utilising
formula (I); COD .function. ( mg .times. / .times. L .times.
.times. of .times. .times. O 2 ) = Q net 4 .times. FV .times. 32000
( I ) ##EQU1## [0028] wherein [0029] Q.sub.net=net charge [0030]
F=Faradays constant [0031] V=sample volume
[0032] The method of determining the chemical oxygen demand value
of a water sample may be determined under non-exhaustive
degradation conditions, in which the organics present in the water
sample are partially oxidised. Under non-exhaustive degradation
conditions, all photocurrent are measured under the diffusion
controlled conditions and the method of determining oxygen demand
value may further include the steps of;
[0033] adding a standard solution, having known organic
concentration or chemical oxygen demand value, to the
photoelectrochemical cell containing the water sample to be
analysed;
[0034] illuminating working electrode with a UV light source and
recording the limiting photocurrent produced from the partial
oxidative degradation of the standard solution (i.sub.l(2));
and
[0035] determining the chemical oxygen demand value by;
[0036] a) calculating the current for the water sample by
subtracting the limiting background photocurrent (i.sub.background)
from the limiting photocurrent of the sample (i.sub.l(1)) at a
predetermined illumination time according to the formula
i.sub.lsample=i.sub.l(1)-i.sub.background;
[0037] b) calculating the limiting photocurrent for the standard
solution by subtracting the background photocurrent
(i.sub.background) from the limiting photocurrent of the sample and
standard (i.sub.l(2)) at the predetermined illumination time
according to the formula:
i.sub.lstandard=i.sub.l(2)-i.sub.background
[0038] c) determining the chemical oxygen demand value of the
sample solution according the following sequence of computations:
i.sub.lsample=nFAk[COD].sub.sample=K[COD].sub.sample
i.sub.lstandard=nFAk{[COD].sub.sample+[COD].sub.standrad}=K{[COD].sub.sam-
ple+[COD].sub.standard} where [0039] K=nFAk is a constant for given
experimental conditions [0040] n=number of electrons transferred
during the photoelectrochemical degradation, [0041] F=Faraday
constant, [0042] A=active electrode area, [0043] k=mass-transfer
coefficient; and [ COD ] sample = i lsample i lstandard - i lsample
.times. [ COD ] standard ##EQU2##
[0044] The method steps for both the exhaustive and non-exhaustive
degradation condition may be repeated as many times as required to
analyse all necessary water samples.
[0045] The potential bias applied to the electrodes is preferably
between -0.1V and +0.5V. More preferably the potential difference
is between approximately +0.15V and +0.35V.
[0046] A supporting electrolyte is used to determine the background
photocurrent and to dilute the water sample to be tested. The
determination of the background photocurrent measures the oxidation
of water and this can be deducted from the sample reading to give
the photocurrent due to the oxidation of organic material in the
sample. This measurement may be made as a separate measurement to
the sample reading or when conducting an exhaustive degradation the
final steady current after the oxidation is completed is a measure
of the background photo current. The supporting electrolyte may be
selected from sodium nitrate, sodium perchlorate or any other
electrolytes that are electrochemically and photoelectrochemically
stable under the experimental conditions and do not absorb UV
radiation in the range being used. The dilution of the samples
enables the method to have a wide linear range while still keeping
the test duration to a relatively small period.
[0047] The water sample is preferably illuminated by a light source
having a photo intensity of between between 1 and 100 mWcm.sup.-2.
More preferably the frequency of the light source is between
approximately 6-9.5 mWcm.sup.-2.
[0048] In another aspect the present invention provides a
photoelectrochemical assay apparatus for determining oxygen demand
of a water sample which consists of
[0049] a) a measuring cell for holding a sample to be analysed
[0050] b) a photoactive working electrode and a counter electrode
disposed in said cell,
[0051] c) a light source, preferably a UV light source, adapted to
illuminate the photoactive working electrode
[0052] d) control means to control the illumination of the working
electrode, the applied potential and signal measurement
[0053] e) current measuring means to measure the photocurrent at
the working and counter electrodes
[0054] f) analysis means to derive a measure of oxygen demand from
the measurements made by the photocurrent measuring means.
Preferably a reference electrode is also located in the measuring
cell and the working electrode is a nanoparticulate semiconductive
electrode preferably titanium dioxide.
In other embodiments the measuring cell may be a stationary cell
with different cell geometry and volume, or a flow through cell
with different cell geometry and volume, and with a flow rate
adjusted to optimise the sensitivity of the measurements.
BRIEF DESCRIPTION OF DRAWINGS
[0055] To assist in understanding the invention preferred
embodiments will now be described with reference to the following
figures in which:
[0056] FIG. 1. is a schematic illustration of the analytical signal
generation for use in the chemical oxygen demand method of this
invention;
[0057] FIG. 2 Schematic of the instrumentation of
photoelectrochemical detection system;
[0058] FIG. 3. Schematic diagram of the photoelectrochemical batch
(stationary) cell of this invention;
[0059] FIG. 4. is a schematic cross section of a thin-layer
photoelectrochemical flow cell according to this invention;
[0060] FIG. 5 Schematic diagram of the thin-layer
photoelectrochemical flow cell.
[0061] FIG. 6. FIA Manifolds for sample and supporting electrolyte
injection in an automatic COD photoelectrochemical detection
system.
[0062] FIG. 7. Graphical representation of non-exhaustive
degradation, photocurrent/time profiles of supporting electrolyte,
sample and standard solutions.
[0063] FIG. 8. Photocurrent response of a solution containing 40
.mu.M of potassium hydrogen phthalate and 0.1M NaNO.sub.3. Photo
intensity: 9.1 mWcm.sup.-2; Applied potential bias: +0.20V vs
Ag/AgCl.
[0064] FIG. 9 i.sub.l-C curves for a range of organic compounds,
namely;
[0065] 1=p-chlorophenol; 2=potassium hydrogen phthalate;
[0066] 3=methanol; 4=d-glucose;
[0067] 5=malonic acid; 6=succinic acid;
[0068] 7=glutaric acid; and 8=glycine.
[0069] Photo intensity: 9.1 mWcm.sup.-2; Applied potential bias:
+0.20V vs Ag/AgCl.
[0070] FIG. 10 Photocurrent response of (a) 0.10M NaNO.sub.3 and
(b) a solution containing potassium hydrogen phthalate and 0.1M
NaNO.sub.3. Photo intensity: 9.1 mWcm.sup.2; Applied potential
bias: +0.20V vs Ag/AgCl.
[0071] FIG. 11. Q-C curves for a range of organic compounds,
namely;
[0072] 1=p-chlorophenol; 2=potassium hydrogen phthalate;
[0073] 3=methanol; 4=d-glucose;
[0074] 5=malonic acid; 6=succinic acid;
[0075] 7=glutaric acid; and 8=glycine.
[0076] Photo intensity: 9.1 mWcm.sup.-2; Applied potential bias:
+0.20V vs Ag/AgCl.
[0077] FIG. 12. Correlation between experimental COD value and
standard COD value. (a) COD standard test solution (KHP); (b) a
synthetic COD sample containing equal molar concentration of all
compounds used in FIG. 8.
[0078] FIG. 13. Comparison of PECOD and conventional COD method
(dichromate) in the detection of real samples.
[0079] FIG. 14 Photoelectrochemical detection of synthetic examples
showing peak height to concentration
[0080] FIG. 15 Photoelectrochemical determination of COD value for
the synthetic samples: (a) the quantitative relationship between
the peak height and concentration (.mu.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.
[0081] FIG. 16 Pearson correlation between PECOD and conventional
COD method (dichromate).
DETAILED DESCRIPTION OF THE INVENTION
[0082] The preferred assay method of the invention takes advantage
of the highly efficient photochemical properties of TiO.sub.2
nanoparticulate film electrodes to develop a new, rapid,
cost-effective assay for the determination of aggregate organic
properties, such as oxygen demand and in particular COD.
[0083] This embodiment is directed to a method of determining
chemical oxygen demand of water samples utilising a nanoparticulate
TiO.sub.2 film electrode. It will be appreciated by the person
skilled in the art that other nanoparticulate semiconductive film
electrode may be utilised in the method without departing from the
essence of the invention.
[0084] The assay method of the invention allows for easy
quantification of electron transfer at a TiO.sub.2 nanoparticle
film electrode during photocatalytic oxidative degradation of
organic material. This approach overcomes many of the current
problems with existing oxygen demand techniques.
[0085] The photocatalytic oxidation approach for COD determination
utilizes TiO.sub.2 particles as photocatalyst to replace the
traditional oxidizing agent, e.g. dichromate and permanganate.
Illumination of TiO.sub.2, with photons whose energy is equal to or
greater than the band-gap energy, will result in promotion of an
electron from the valence band to the conduction band (see FIG. 1).
This promotes an electron (e.sup.-) to the conduction band and
leaves a positive photohole (h.sup.+) in the valence band. The
photohole is one of the most powerful oxidizers due to its high
bandgap potential (+3.2V for anatase). The photocatalysis can lead
to stoichiometric mineralization of organic compounds due to the
strong oxidation power of photoholes.
Mathematical Derivation
[0086] The method of determining chemical oxygen demand of water
samples, according to the invention, utilises photoelectrochemical
current (or charge) generated from photoelectrochemical oxidative
degradation of organic compounds as an analytical signal.
[0087] The photocatalytic degradation efficiency at TiO.sub.2
depends on the degree of recombination of photoelectrons and holes.
With traditional TiO.sub.2 photocatalysis systems, this relies on
how fast the photoelectrons and holes are consumed by the adsorbed
species.
[0088] A TiO.sub.2 nanoparticulate film electrode is used as the
working electrode in a three-electrode photoelectrochemical cell.
By applying an appropriate potential bias to the working electrode,
it becomes more favourable for the photoelectron to be transferred
to the working electrode rather than to the adsorbed O.sub.2. The
photoelectrons are subsequently forced to pass into the external
circuit and to the counter electrode, where the reduction of oxygen
(or other species) takes place. The photocurrent (or charge) is
monitored and gives a direct measure of the oxidation of organic
compounds. In effect the assay shunts photoelectrons through the
external circuit to quantify the extent of oxidative
degradation.
[0089] Separation of the oxidative and reductive half-reactions
(Eqn.s 1 and 2, below) by imposing the electrochemical potential
suppresses the recombination of photoelectrons and holes. As a
result, the degradation efficiency is enhanced. In addition it has
been found that the rate of degradation of organic materials is
independent of O.sub.2 concentration as the rate of reduction at
the counter electrode will never be the rate-limiting step of the
overall degradation process. Thus overcoming the prior art problem
of oxygen reduction being a rate-limiting step in the
photooxidation of organic material. i . e . .times. h vb + + R - H
( ads ) .fwdarw. R ( ads ) * + H , .times. or .times. .times. h vb
+ + H 2 .times. O ( ads ) .fwdarw. OH ( ads ) * + H + ( 1 ) i . e .
.times. 2 .times. e cb - + 2 .times. H + + O 2 .times. ( ads )
.fwdarw. H 2 .times. O 2 .times. .times. or .times. .times. 6
.times. e cb - + 3 2 .times. O 2 .times. ( ads ) + 6 .times. H +
.fwdarw. 3 .times. H 2 .times. O ( 2 ) ##EQU3## Quantification of
Analytical Signal
[0090] The photoelectrochemical system described above can be used
for two different degradation models--exhaustive and non-exhaustive
degradation. With exhaustive degradation, 100% of the organic
analyte in the sample is consumed; with non-exhaustive degradation,
only a small fraction of available analyte is consumed and its
concentration in the bulk solution remains essentially unchanged.
The former is analogous to bulk electrolysis in which all of the
analyte is electrolysed and Faraday's Law is used to quantify the
concentration by measuring the charge passed; the latter is
analogous to amperometric methods where the analytical signal (i.e.
current) is dependent on the rate of mass transfer to the electrode
surface. In our case, however, the charge/current produced is the
result of photoelectrochemical processes.
[0091] When the exhaustive degradation model is employed, the
charge (O) can be measured by the integration of photocurrent
within the degradation period. The analytical principle can be
established using Faraday's Law: Q=.intg.idt=nFN=nFVC (3) where:
[0092] N=number of moles of analyte in the sample, [0093] n=number
of electrons transferred during the photo-electrochemical
degradation, [0094] F=Faraday constant, [0095] V=sample volume; and
[0096] C=analyte concentration.
[0097] Since TiO.sub.2 oxidises organic compounds to the fully
oxidised form of carbon (i.e. CO.sub.2), the value n for a given
compound will be a constant. Eqn 3 can therefore be used to
quantify the analyte concentration.
[0098] In principal, analytically useful photocurrents (or charge)
can be obtained from any photo-electrochemically degradable
species. The TiO.sub.2 system proposed is capable of oxidising
nearly any organic or low redox state inorganic species (e.g.
Fe.sup.2+, Cl.sup.-, NH.sub.4.sup.+, NO.sub.2.sup.-). In this
respect, the proposed system can be employed as a "universal"
detector capable of detecting any compounds that can be
photoelectrochemically oxidised at a TiO.sub.2 electrode. In
combination with an appropriate separation system (e.g. HPLC), the
concentration of individual analytes can be determined.
[0099] In terms of general water quality issues and pollution
control, the effect and quantification of aggregate mixtures of
organics (such as in COD or BOD analysis) are often more important
than the analysis of single species. The proposed
photoelectrochemical system is capable of determining such
aggregate properties by summing the charge measured from individual
photo-electrochemically degradable compounds within a mixture (Eqn
4). Q = .intg. i .times. d t = FV .times. i = 1 m .times. n i
.times. C i ( 4 ) ##EQU4##
[0100] The measured charge, Q, is simply the total amount of
electron transfer that results from the degradation of all
compounds in the sample. Given that oxidation by O.sub.2 can be
represented as: O.sub.2+4H.sup.++4e.sup.-.fwdarw.2H.sub.2O (5)
where one oxygen molecule is equivalent to 4 electrons, the
measured Q value can be easily converted into an equivalent O.sub.2
concentration (or oxygen demand) value: Equivalent .times. .times.
Oxygen .times. .times. Concentration .function. ( mole .times. /
.times. L ) = Q 4 .times. FV ( 6 ) ##EQU5##
[0101] For exhaustive degradation, the equivalent COD value of the
sample can therefore be represented as: COD .function. ( mg / L
.times. .times. of .times. .times. O 2 ) = Q 4 .times. FV .times.
32000 ( 7 ) ##EQU6##
[0102] In the case of non-exhaustive degradation, the quantitative
relationship between the photocurrent and the concentration of the
analyte can be developed using a well-known semi-empirical
treatment of Steady-State Mass Transfer [A. J. Bard and L. R.
Faulkner, Electrochemical Methods-Fundamental and Applications.
John Wiley & Sons, Inc. New York. 2001]. Under conditions of
forced convection, the rate of mass transfer (dN/dt) to an
electrode is directly proportional to the concentration gradient at
the electrode surface. ( d N d t ) = k .function. [ C b - C s
.function. ( x = 0 ) ] ( 8 ) ##EQU7## where, [0103]
C.sub.b=concentration of analyte in the bulk solution; [0104]
C.sub.s=concentration of analyte at the electrode surface; [0105]
k=mass-transfer coefficient [A. J. Bard and L. R. Faulkner,
Electrochemical Methods-Fundamental and Applications. John Wiley
& Sons, Inc. New York. 2001] which is =D/.delta., where
D=diffusion coefficient and .delta.=thickness of stagnant
layer.
[0106] When sufficient photo intensity and adequate potential bias
are employed, and the overall process is controlled by mass
transfer, then, C.sub.s(x=0)<<C.sub.b, so that
[C.sub.b-C.sub.s(x=0)].apprxeq.C.sub.b. The maximum rate of mass
transfer, (dN/dt).sub.l is achieved and the rate of overall
reaction equals: Rate = ( d N d t ) l = kC b ( 9 ) ##EQU8##
[0107] If we again assume that after photochemical oxidation the
analyte is fully oxidised, then the number of electrons transferred
(n) during photoelectrochemical degradation is a constant, for a
given analyte. The limiting photocurrent (i.sub.l) can, therefore,
be used to represent the rate of reaction: i l = nFA .function. ( d
N d t ) l = nFAk .times. .times. C b ( 10 ) ##EQU9## where A=active
electrode area.
[0108] The development of equation 10, by the current inventors,
leads to definition of the quantitative relationship between the
limiting photocurrent and the concentration of analyte and can be
regarded as the principle of analysis.
[0109] This analytical principle can again be applied to determine
concentrations of individual analytes (and serve as a "universal"
detection system) or to aggregate mixtures of organics (to
determine properties such as COD). Whilst formula 10 above may
allow for the general determination of analyte concentration, it is
the application of the findings from formula (10) to the
determination of COD that assists in addressing one or more of the
disadvantages in the prior art methodologies. Standard analytical
and mathematical techniques may be used to calculate the COD of a
sample from the limiting photocurrent measured in a
photoelectrochemical cell utilising a nanoparticulate TiO.sub.2
semiconductive electrode in the manner described in more detail
below.
Method of Determining COD
Formation of TiO.sub.2 Electrode
A) Synthesis of TiO.sub.2 Colloid
[0110] A1) A mixture of 12.5 ml titanium butoxide and 4 ml
isopropyl alcohol was added, drop-wise, to 150 ml 0.1M nitric acid
solution under vigorous stirring at room temperature. After the
hydrolysis the slurry was heated to 80.degree. C. and stirred for 8
h to achieve peptization. The colloid is then filtered to remove
the nonpeptized agglomerates. For better crystallinity of the
nanoparticles, the colloid was hydrothermally treated in an
autoclave at 200.degree. C. for 12 h. During the autoclaving
sedimentation occurred, and the particles were redispersed by
sonication. The particle size is in the range of 8 to 10 nm as
characterised by transmission electron microscopy (TEM). Water was
used to adjust the final solid concentration to ca. 6% (wt) and
carbowax 20M (Merck) was added to the colloid in a proportion of
1-50% of the TiO.sub.2 weight. The colloid thus obtained was used
for the preparation TiO.sub.2 nanoporous film electrode.
[0111] A2) A 16.0 cm.sup.3 of isopropanol (Aldrich, AR grade) and
50.0 cm.sup.3 of titanium butoxide (Aldrich, AR grade) were
accurately measured into a 150 cm.sup.3 dropping funnel. The
resulting solution was added over 15 minutes with vigorous stirring
to 600 cm.sup.3 of ultrapure deionized water (18.2 M.OMEGA. cm) in
a conical flask. On the completion of the addition, 4.0 cm.sup.3 of
70% nitric acid (AR grade) was added into the solution as a
peptizing agent The solution was immersed in a hot water bath,
heated to 80.degree. C. and stirred continuously for 10 hours.
Approximately 400 cm.sup.3 of a white colloidal solution remained
and was stored in a dark glass vessel for use. The sizes of the
TiO.sub.2 synthesised according to this procedure were in a range
of 8 to 10 nm.
[0112] The colloidal TiO.sub.2 prepared above was placed in an
autoclave reactor (Parr bomb) and autoclaved for 12 hours at
200.degree. C. before concentrating on a rotary evaporator to 8%
(w/w), resulting in a white semi-viscous colloidal solution. 40%
TiO.sub.2 weight equivalent (e.g. 1.6 g in 50.0 cm.sup.3 of 8%
colloidal solution) of Carbowax 6,000 was added to the solution and
stirred for approximately 20 minutes.
B) Immobilisation of TiO.sub.2 Film on ITO Glass
[0113] B1) ITO (tin doped iridium oxide) conducting glass slides
were used as the substrate for immobilisation of TiO.sub.2
particles. To get a clean surface the ITO glass slide was
pretreated by washing in turn with detergent, water, and ethanol.
After the pretreatment the ITO slide was dip-coated in the above
colloidal solution from A1, above. The coated slides were then
calcined in a muffle furnace in air at 500 to 800.degree. C. for
0.5 h to 30 h. The particle size of TiO.sub.2 on the films
characterised by x-ray diffraction and scanning electron microscopy
(SEM) is in the range of 10 nm to 100 nm and the rutile/anatase
phase ratio is in the range of 0.1 to 50%.
[0114] B2) TiO.sub.2 films were prepared in a clean room
environment to minimize contamination from dust particles.
TiO.sub.2 colloidal coating solution, from A2 above, was stirred
vigorously and subjected to the ultrasonic treatment for 20 minutes
prior to a dip coating process to achieve a consistent,
reproducible homogeneous mixture. The ITO slide (conducting glass)
was used as the electrode substrate and was pre-treated by washing
in turn with detergent, water, acetone and water, and finally dried
by pure nitrogen. After pre-treatment, the ITO slide was dip-coated
with the TiO.sub.2 colloidal coating solution using a dip coating
equipment with withdrawing speeds of 0.5-1.5 cm/min. The coated
electrodes were then calcined in a muffle furnace at 450.degree. C.
for 30 minutes in air. The nanoporous TiO.sub.2 films with 1 .mu.m
thickness and anatase crystalline structure were obtained. The
films with different thicknesses can be prepared by controlling the
withdrawing speed during the dip coating.
C) General Setup of the Photoelectrochemical System
[0115] FIG. 2 shows the schematic of the instrumental set up of the
photoelectrochemical detection system. 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-up by infrared light, the light beam was
passed an UV-band pass filter (UG 5, Avotronics Pty, Limited) prior
to illumination of the electrode surface. A light shutter was used
to control the ON and OFF of the illumination.
[0116] Generally, photoelectrochemical experiments were performed
in a three-electrode electrochemical cell with a quartz window for
illumination. The TiO.sub.2 film electrode was installed in an
electrode holder with ca. 0.65 cm.sup.2 left unsealed to be exposed
to the solution for illumination and photoelectrochemical reaction.
A saturated Ag/AgCl electrode and a platinum mesh were used as the
reference and counter electrodes respectively. A voltammograph
(CV-27, BAS) was used for application of potential bias in steady
state photocurrent measurements. Potential and current signals were
recorded using a Macintosh computer (7220/200) coupled to a Maclab
400 interface (AD Instruments).
D) The Measurement Procedures:
D1) Exhaustive Degradation Conditions
[0117] Step 1: Once the system is set up (see FIG. 2-6), the
supporting electrolyte solution was pumped through the
photoelectrochemical cell (a thin layer cell). A bias potential of
+0.20 V vs a Ag/AgCl reference electrode was applied. Once the
stable baseline was obtained, the pump was stopped. The photo
shutter was then switched on to allow UV radiation to reach the
electrode. The photocatalytic reaction occurred and the background
current-time profile can be measured (see FIG. 10 curve (a)). The
background current (i.sub.background) was resulted from the
photocatalytic oxidation of water. Integrating i.sub.background
with time we can obtain the background charge,
Q.sub.Background.
[0118] Step 2: After the measurement of Q.sub.Background, the photo
shutter was switched off and a 5 .mu.l to 200 .mu.l of sample
solution with appropriate concentration was injected into the
photoelectrochemical cell (It is to note that the sample volume
injected is depending the volume of the cell and if the
concentration of organics in the sample was too high then an
appropriate dilution may be required prior the injection). Once the
sample injection was completed, the pump was stopped and the photo
shutter was switched on. The Current-time profile was measured (see
FIG. 10 curve (b)). The current obtained here is the total
photocurrent (i.sub.Totla) that resulted from the oxidation of
water and organics. Same to the above, by integrating i.sub.Total
with time we have the total charge, Q.sub.Total.
[0119] Step 3: Since the background charge, Q.sub.Background, is a
constant for the given experimental conditions and the total
charge, Q.sub.Total, varied with the concentration of the sample,
therefore, the net charge, Q.sub.net (the shaded area shown in FIG.
10) that resulted from the oxidation of organics can be obtained by
subtracting the background charge from the total charge, that is:
Q.sub.net=Q.sub.Total-Q.sub.Background The COD value of the sample
can then be calculated according to the equation (7) since in the
equation, COD .function. ( mg / L .times. .times. of .times.
.times. O 2 ) = Q net 4 .times. FV .times. 32000 ( 7 )
##EQU10##
[0120] F is a constant and V is known sample volume.
[0121] Repeating steps 2 and 3 to analysis next sample.
D2) Non-Exhaustive Degradation Conditions
[0122] The system set up was same as described above (see FIG. 2-6)
except that a normal flow-through cell was employed to replace the
thin layer cell. The measurement can be done by using a standard
addition method or by other calibration means.
[0123] Step 1: The supporting electrolyte solution was pumped
through the photoelectrochemical cell (a normal flow-through cell
with cell volume of 0.5 to 2.5 ml). A bias potential of +0.20 V vs
a Ag/AgCl reference electrode was applied. Once the stable baseline
was obtained, the pump was stoped. The photo shutter was then
switched on to allow UV radiation to reach the electrode. The
photocatalytic reaction occurred and the background current-time
profile can be recorded (see FIG. 7 curve (a)). The background
current (i.sub.background) was resulted from the photocatalytic
oxidation of water.
[0124] Step 2: After the measurement of i.sub.background, the photo
shutter was switched off and a 0.5 ml to 2.5 ml of sample solution
with appropriate concentration was injected into the
photoelectrochemical cell (It is to note that the sample volume
injected is depending the volume of the cell and if the
concentration of organics in the sample was too high then an
appropriate dilution may be required prior the injection). Once the
sample injection was completed, the pump was stopped and the photo
shutter was switched on. The Current-time profile was recorded (see
FIG. 7 curve (b)). The current obtained here is the total
photocurrent (i.sub.total) that resulted from the oxidation of
water and organics.
[0125] Step 3: Once the measurement of i.sub.total was completed,
the photo shutter was switched off and a 0.5 ml to 2.5 ml of sample
solution containing an appropriate concentration of standard was
injected into the photoelectrochemical cell. Once the sample
injection was completed, the pump was stopped and the photo shutter
was switched on. The current-time profile was recorded (see FIG. 7
curve (c)). The current obtained here is the photocurrent
(i.sub.standard) that resulted from the oxidation of water and the
organics in both original sample and the added standard.
[0126] Step 4: After the above measurements, the COD of the sample
can be calculated according to the equation (10).
[0127] The limiting current for each case (i.sub.l background,
i.sub.l(1) and i.sub.l(2)), can be obtained by measuring the steady
current value from each curve, for example, at 45s (see FIG. 5).
The net limiting photocurrents for the sample solution (i.sub.l(1))
and for the sample with added standard (i.sub.l(2)) can then be
calculated.
Net limiting photocurrents (or current) for the sample solution:
i.sub.l sample=i.sub.l(1)-i.sub.l background Net limiting
photocurrents (or current) for the sample with standard: i.sub.l
standard=i.sub.l(2)-i.sub.l background According to equation (10),
above, we have:
i.sub.lsample=nFAk[COD].sub.sample=K[COD].sub.sample i.sub.l
standard=nFAk{[COD].sub.sample+[COD].sub.standrad}=K{[COD].sub.sa-
mple+[COD].sub.standard} where K--nFAk is a constant for a given
experimental condition. The COD value of the sample solution: [ COD
] sample = i lsample i lstandard - i lsample .times. [ COD ]
standard ##EQU11## Repeat the steps 2 to 4 for the next sample. It
will be appreciated by the person skilled in the art that the
necessary computations set out above may be automated with the
appropriate programming of a personal computer.
[0128] There are a few operational modes with different
photoelectrochemical reactor designs (i.e. online thin-layer flow
cell, and batch cell) that utilise the assay methodology and are
demonstrated by following examples.
EXAMPLE 1
Quantification of COD Using Photocurrent
[0129] The photoelectrochemical experiment was performed in a
three-electrode electrochemical batch cell with a quartz window for
illumination as shown in FIG. 3 The TiO.sub.2 film electrode was
placed in an electrode holder with ca. 0.65 cm.sup.2 left unsealed
to be exposed to the solution for illumination and
photoelectrochemical reaction. 0.1M NaNO.sub.3 solution was used as
the supporting electrolyte. A potential bias of +0.2V was applied
at the electrode and limiting photocurrents were obtained for
different organic compound concentrations when the current reached
steady state. The limiting photocurrent differences between samples
and the blank 0.1M NaNO.sub.3 solution were taken as analytical
signals, which are directly linear to organic compound
concentrations within diffusion control. A linear relationship
between the analytical signal and COD value was then acquired after
the concentration was converted into COD value.
EXAMPLE 2
Quantification of COD Using Charges
[0130] In this case the experiment was carried out in a thin-layer
photoelectrochemical cell as shown in FIGS. 4 and 5. A potential
bias of +0.20V was applied and 2M NaNO.sub.3 was used as supporting
electrolyte. Firstly, a 2M NaNO.sub.3 electrolyte solution was
injected into the thin-layer photoelectrochemical cell with a
syringe and a blank transient photoelectrolysis was run as a blank
sample. The photocurrent-time profile was recorded until the
photocurrent reached steady state. Then samples containing organic
compounds and 2M NaNO.sub.3 were injected into the thin-layer cell
and the sample transient photoelectrolysis was run. The
photocurrent-time profile was recorded until the photocurrent
attained steady state, indicating the organic compounds have been
exhaustively photoelectrolysed. The cell was washed with supporting
electrolyte solution between each sample injection. Integrating the
photocurrent-time profile gives the photocatalytic oxidation
charge. The charge difference between sample and blank transient
photoelectrolysis was taken as the analytical signal, which is
directly proportional to the COD value. COD value was then
determined.
EXAMPLE 3
Quantification of COD Using Charges and FIA
[0131] Besides the use of the thin-layer photoelectrochemical cell,
a flow injection analysis (FIA) system was incorporated into the
COD determination. With the combination of FIA, automatic COD
determination was realised. In this case, the injection of samples
and cell cleaning was controlled by a FIA controlling system as
shown in FIG. 6 (a). Pump 1 achieves the blank sample (R1)
injection and cell cleaning while Pump 2 does the sample injection
(R2). A potential bias of +0.20V was applied and 2M NaNO.sub.3 was
used as supporting electrolyte (blank sample). Firstly, a 2M
NaNO.sub.3 electrolyte solution was pumped into the thin-layer
photoelectrochemical cell by Pump 1 and a blank transient
photoelectrolysis was run as a blank sample. The photocurrent-time
profile was recorded until the photocurrent reached steady state.
Then samples containing organic compounds and 2M NaNO.sub.3 were
pumped into the thin-layer cell by Pump 2 and the sample transient
photoelectrolysis was run. The photocurrent-time profile was
recorded until the photocurrent attained steady state, indicating
the organic compounds have been exhaustively photoelectrolysed. The
cell was washed with supporting electrolyte solution by Pump 1
between each sample. Integrating the photocurrent-time profile
gives the photocatalytic oxidation charge. The charge difference
between sample and blank transient photoelectrolysis was taken as
the analytical signal, which is directly proportional to the COD
value. COD value was then determined.
EXAMPLE 4
Quantification of COD Using Continuous Flow Mode
[0132] Besides the use of the thin-layer photoelectrochemical cell,
a flow injection analysis (FIA) system was incorporated into the
COD determination with a continuous flow operational mode. With the
combination of FIA, automatic COD determination was realised. In
this case, the injection of samples and cell cleaning was
controlled by a FIA controlling system as shown in FIG. 6(b). The
blank sample is continuously pumping through the cell and an
injector is employed for the sample injection. A potential bias of
+0.20V was applied and 2M NaNO.sub.3 was used as supporting
electrolyte (blank sample). Recording the photocurrent from the
photocatalysis of the blank sample gives a steady baseline. An
injection of sample containing organic compounds through the
injector to allow the photocatalysis of sample takes place. The
peak shaped photocurrent-time profile can be recorded until the
photocurrent attained baseline, indicating the organic compounds
have been photoelectrolysed. The next sample can then be injected
for analysis. COD value of the sample can be determined by
measuring either peak height or peak area (by integrating the peak
photocurrent) since both peak height and peak area are directly
proportional to the COD value.
Typical Experimental Results
[0133] FIG. 8 shows a typical photocurrent-time curve obtained from
a non-exhaustive photoelectrochemical degradation process. Under a
constant applied potential, when the light was switched off, the
residual current (dark current) was approximately zero. Upon
illumination, the current increased rapidly before decaying to a
steady state value. This steady state current (limiting
photocurrent) consists of two current components. One is due to
mass transfer limited photoelectrochemical oxidation (degradation)
of the target analyte, which is directly proportional to the
concentration of the analyte. The other is due to the
photoelectrochemical oxidation (decomposition) of water, which is
constant at a given pH and supporting electrolyte concentration.
The net limiting photocurrent, i.sub.l, (resulting from analyte
oxidation) can be readily obtained by subtraction of the
photocurrent attributed to the oxidation of water from the total
photocurrent. i.sub.l can then be used for analytical purposes (Eqn
10).
[0134] Preliminary results obtained from a range of organic
compounds indicate that Equation 10 is applicable to all compounds
investigated (see FIG. 9, in which 1=p-chlorophenol; 2 potassium
hydrogen phthalate; 3=methanol; 4=d-glucose; 5=malonic acid;
6=succinic acid; 7=glutaric acid; 8=glycine). As predicted, i.sub.l
was found to be directly proportional to the concentration of the
organic compound. The slopes of the I.sub.l-C curves (sensitivity)
are determined by the mass transfer coefficient (k) and the number
of electrons transferred (n) during the photoelectrochemical
degradation.
[0135] The photocurrent-time profile of an exhaustive
photoelectrochemical degradation process was found to be similar to
that of the non-exhaustive degradation process except that a steady
state photocurrent can only be achieved when all of the organic
compounds were consumed. In this case, the steady state
photocurrent was purely due to the oxidation of water and can be
easily subtracted from the total current (FIG. 10). FIG. 10 shows
the typical photocurrent--time profiles obtained from the
electrodes during an exhaustive photoelectrochemical degradation
process in phthalic acid and in blank electrolyte solutions. It can
be seen that the photocurrent decayed with time and then reached a
steady value, which was due to the oxidation of water. It is noted
that the blank photocurrent obtained from the blank electrolyte
solution was purely due to the oxidation of water, while the
photocurrent obtained from the electrode in phthalic acid solution
consists of two current components, one is due to
photoelectrochemical oxidation of phthalic acid, and the other is
due to the oxidation of water, which is the same as the blank
photocurrent. Our experimental results showed that the blank
photocurrent was essentially constant for the given set of
experimental conditions. For a given time period, the charge passed
for both cases can be obtained by integration of the photocurrent
and blank photocurrent. The charge difference between the two cases
is the net charge, Q due to the photoelectrochemical oxidation of
phthalic acid, which is indicated as the shaded area in FIG. 10.
The net charge, Q, was measured by integration of net photocurrent
within the degradation period as described in FIG. 10. As predicted
by Eqn. 3, Q is directly proportional to concentration (see FIG.
11). In this case, the slopes of the Q-C curves (sensitivity) were
dependent only on the number of electrons transferred (n). The
results in FIGS. 9 and 11 (having conformed to theory) demonstrated
the possibility of further developing the proposed system into a
"universal" detection system for individual analytes.
[0136] The possibility of applying the proposed method for
determining aggregate properties such as COD was also tested. We
chose the APHA COD standard test solution (potassium hydrogen
phthalate (KHP)) and a synthetic mixture with known COD values as
our test solutions. FIG. 12 shows the correlation between the
experimental COD values (according to equation 7) and standard COD
values. Excellent agreements between the two COD values were
obtained in both cases.
EXAMPLE 5
Real Waste Water Samples.
[0137] Fourteen (14) different wastewater samples were collected
from various industries in Queensland, Australia. After appropriate
dilution, all samples were subject to the COD analysis using our
method and the standard COD method. The COD values obtained from
the two methods for all samples were then correlated and shown in
FIG. 13. A correlation coefficient of 0.973 and slope of 0.992 were
obtained. This means our method predicts the same COD value as the
standard COD method. This demonstrates that our method is
equivalent to the standard method in predicting the COD values.
EXAMPLE 6
Determination of COD in Synthetic Samples
[0138] The use of flow injection (FIGS. 4 and 5) to determine COD
in aqueous solution was first tested with synthetic samples
prepared with pure organic chemicals, i.e. glucose, glutamic acid,
GGA and sucrose. The time required for a single measurement was 1-2
min. FIG. 14 shows the calibration curve of the various organic
compounds in terms of .mu.M and theoretical COD concentration
respectively. FIG. 14a shows that the photoelectrochemical detector
had different sensitivities (slope of the calibration curve) to
different organics in regards of .mu.M concentration. The
sensitivity decreased in the order of sucrose, GGA, glucose and
glutamic acid. This is because the organic compounds contribute
different number of electrons (n=4y-2j+m-3k-q) in the exhaustive
oxidation reactions. With the decrease of transferred electrons per
mole, i.e. sucrose (n=48), GGA (n=42), glucose (n=24) and glutamic
acid (n=18), the organic compounds give fewer electrons per mole
and hence the sensitivity decreases. This explained the sensitivity
order in FIG. 14a. With the transferred electron number (n), the
concentrations of the organic compounds were converted from .mu.M
to theoretical COD value in ppm. The same sensitivities, evidenced
with the same slope, were obtained for the selected organics in
FIG. 14b. This implied that the photoelectrochemical detector
oxidised the above organics to the same extent, i.e. the organic
compounds have been oxidised indiscriminately and the
mineralisation was achieved. The detection principle was therefore
validated. These is shown by plotting the PECOD values against the
theoretical COD values as shown in FIG. 14c using glucose trendline
as standard calibration curve. The line of best fit has a slope of
1.0268 and R.sup.2 of 0.9984, which directly demonstrated that
suitability to use glucose as a calibration standard to determine
COD value for the unknown sample.
[0139] It was found that the detection limit of 0.5 ppm COD with a
linear range up to 60 ppm COD can be achieved under the
experimental conditions employed using glucose as testing analyte.
The detection limit can be extended further by increasing the
sample injection volume while the linear range can be increased by
a further smaller injection volume.
[0140] Reproducibility and stability are important parameters for
the usefulness of the detector. The response reproducibility of the
sensor to 100 .mu.M glucose was studied using repeated
determinations (n=12) and RSD % was found to be 0.8%. The detector
is relatively stable. Significant baseline shift was observed for
the first two hours when the electrode was brand new due to some of
the active TiO.sub.2 particles were not attached on the electrode
surface enough firmly and was removed by the carrier. The baseline
became almost constant after these non-stable active sites were
removed. In fact all the data reported in this paper was obtained
from the same TiO.sub.2 electrode. The electrode had experience the
change of pH (from 2 to 10), the change of potential (-0.4 to
+0.8V), the change of flow rate, the change of injection volume and
analysis of real samples and finished nearly thousand of the
measurements it is still relatively sensitive and stable. When the
electrode was not being used, it is filled with Milli-Q water and
store in the light It is well known that TiO.sub.2 surface has
merits of self-cleaning and super hydrophilicity. The fouling of
electrode, which is commonly caused by adsorption of organic
compounds and growth of bacteria, was not observed after storage.
Because of this, even after a few days, it needed only about 5
minutes to regenerate the used electrode to acquire a stable
baseline to start the detection of COD again.
EXAMPLE 7
PECOD vs COD
[0141] The pH of the real samples tested in this example was in the
range of 5-9, which is the pH independent region of the
photoelectrochemical detector. Standard calibration curve method
was used to determine the COD value in real sample. FIG. 15 shows
the typical response of the flow injection response using glucose
as the standard substance. As shown in the figure, both the charge
(peak area, FIG. 15 (a)) and the peak current (peak height, FIG. 15
(b)) increased proportionally with the increase of glucose
concentration. The calibration curve (the inserts of FIG. 15) was
therefore constructed using the data from the above detection. At
the same time, the standard COD value was determined with
conventional COD method (dichromate method). FIG. 16 shows the
correlation between the experimental COD values and standard COD
values. Where valid, the Pearson Correlation coefficient was used
as a measure of the intensity of association between the values
obtained from the flow injection photoelectrochemical COD method
and the conventional COD method. This was employed for the data in
FIG. 16. A highly significant correlation (r=0.991, P=0.000, n=13)
between the two methods was obtained indicating the two methods
agreed very well. More importantly, the slope of the principle axis
of the correlation ellipse of 1.0819 was obtained. This almost
unity slope value suggests both methods were accurately measuring
the same COD value. Given a 95% confidence interval, this slope was
between 1.016 and 1.174. This implies that we can be 95% confident
that the true slope lies between these two values. Consider that
there are analytical errors associated with both the flow injection
photoelectrochemical COD and the standard method measurements and
that these errors contribute to scatter on both axes, the strong
correlation and slope obtained provides compelling support for the
suitability of the flow injection photoelectrochemical COD method
for measuring Chemical oxygen demand. The excellent agreements
between the two COD values demonstrates the suitability for the
proposed method to measure COD.
CONCLUSION
[0142] The present invention provides a COD analysis method, which
is accurate, sensitive, environmentally friendly, robust, rapid and
easy to be automated. This method in principle measures the
theoretical COD value due to the extraordinary high oxidation power
of photogenerated holes. The method described here is a direct
method and independent of sample matrix. Under exhaustive
degradation conditions, the method is an absolute method requires
no calibration. Experimentally, it correlates well with the
conventional dichromate method (Standard method). The electrode has
a very good long time-stability, without showing any decline of
photocatalytic activity. The nature of the analytical principle
employed makes the method insensitive to the change of temperature.
The method shows a good tolerance to temperature change in contrast
to Karube's method. During the experiment the temperature did not
controlled with the sample temperature ranging from 10 to
40.degree. C., no photocurrent and charge change was observed.
[0143] Whilst the description of the invention utilises a three
electrode photochemical cell it will be appreciated that the
photoelectrochemical cell may comprise a working electrode
(TiO.sub.2) and a counter electrode, wherein the counter electrode
may suitable act as both a counter electrode and a reference
electrode.
[0144] Throughout the specification the aim has been to describe
the preferred embodiments of the invention without limiting the
invention to any one embodiment or specific collection of
features.
[0145] Throughout this specification, unless the context requires
otherwise, the word "comprises", and variations such as "comprise"
or "comprising", will be understood to imply the inclusion of a
stated integer or group of integers or steps but not to the
exclusion of any other integer or group of integers.
* * * * *