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Critical Reviews in Analytical Chemistry
ISSN: 1040-8347 (Print) 1547-6510 (Online) Journal homepage: http://www.tandfonline.com/loi/batc20
Analytical Approaches for Determining Chemical
Oxygen Demand in Water Bodies: A Review
Ji Li, Guobing Luo, LingJun He, Jing Xu & Jinze Lyu
To cite this article: Ji Li, Guobing Luo, LingJun He, Jing Xu & Jinze Lyu (2017): Analytical
Approaches for Determining Chemical Oxygen Demand in Water Bodies: A Review, Critical
Reviews in Analytical Chemistry, DOI: 10.1080/10408347.2017.1370670
To link to this article: http://dx.doi.org/10.1080/10408347.2017.1370670
Accepted author version posted online: 31
Aug 2017.
Published online: 31 Aug 2017.
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Analytical Approaches for Determining Chemical Oxygen Demand in Water
Bodies: A Review
Ji Li
a
, Guobing Luo
b
, LingJun He
c
, Jing Xu
b
, and Jinze Lyu
a
a
School of Environment and Civil Engineering, Jiangnan University, Wuxi, China;
b
Wuxi City Water Supply and Drainage Monitoring Station, Wuxi,
China;
c
Department of Housing and Urban Rural Development of Jiangsu Province, Nanjing, China
ABSTRACT
Chemical oxygen demand (COD) is a critical analytical parameter for water quality assessment. COD
represents the degree of organic pollution in water bodies. However, the standard analytical methods for
COD are time-consuming and possess low oxidation efficiency, chloride interference, and severe
secondary pollution. Works performed during the last two decades have resulted in several technologies,
including modified standard methods (e.g., microwave-assisted method) and new technologies or
methods (e.g., electro- and photo-oxidative methods based on advanced oxidation processes) that are
less time-consuming, environment friendly, and more reliable. This review is devoted in analyzing the
technical features of the principal methods described in the literature to compare their performances (i.e.,
measuring window, reliability, and robustness) and identify the advantages and disadvantages of each
method.
KEYWORDS
Advanced oxidation
processes; assessment
methods; chemical oxygen
demand; organic pollution;
water
Introduction
Water is the source of life. The survival and production of peo-
ple cannot go on without incessant water supply. With the
development of industry and agriculture, an increasing number
of wastewater is generated and discharged. However, approxi-
mately 80% of wastewater worldwide remains uncollected and
untreated.
[1]
A considerable number of pollutants are directly
drained into water bodies and the problems of environmental
monitoring and controlling have caused global concerns. Moni-
toring pollutants in water bodies is of considerable importance
because of the impact of water quality on the environment and
public health.
The most relevant pollution impact of wastewater is organic
matter. The pollution level of organic matter in water bodies is
roughly predicted by analyzing chemical oxygen demand (COD)
or biological oxygen demand (BOD), which estimates the extent
of chemical and biological oxidation, respectively.
[2]
COD is a
more superior representative of organic matter than BOD
because BOD is difficult to standardize and requires a long time
(five days). In addition, BOD does not provide any information
regarding the oxidation state of an organic substance.
[3]
By con-
trast, COD can overcome the drawbacks obtained by BOD.
Thus, COD is considered as an important water quality parame-
ter for representing the degrees of organic pollution and strictly
controlled by environmental regulatory agencies.
[4]
COD is
defined as a measure of the oxygen equivalent of the organic
matter content of a sample susceptible to oxidation by strong
chemical oxidants (potassium permanganate [KMnO
4
], or potas-
sium dichromate [K
2
Cr
2
O
7
]). Conventionally, COD indexes
have two types, namely COD
cr
and COD
Mn
. Determining COD
using K
2
Cr
2
O
7
and KMnO
4
is called COD
Cr
and COD
Mn
,
respectively. COD
Cr
is mainly used for assessing the water quality
in moderately or heavily contaminated water bodies (e.g., sewage
and wastewater), and COD
Mn
is preferred in relatively clean
water bodies (e.g., surface water and river water) with low-level
COD values.
[5]
This paper mainly focuses on the development of
the analytical approaches of COD
cr
due to its three major appli-
cations. First, COD is an evaluation index of the wastewater dis-
charge for present regulations. Second, the ratios between BOD
for 5 days (BOD
5
) and COD
cr
represent the biodegradable frac-
tion of an effluent in wastewater treatment plants (WWTPs).
[6]
Third, a few hours (2–4 hours) are required for COD instead of
5 days for BOD
5
, thereby making this the most popular and use-
ful analytical evaluation of COD.
[7,8]
The permitted values of COD, extensively vary in different
discharge standards according to the usage and properties of
water bodies. For example, the limit values of COD in China
are 3 mg/L and 500 mg/L in standards for drinking water qual-
ity (GB 5749-2006) and wastewater quality standards for dis-
charge to municipal sewers (GB/T 31962-2015), respectively.
Therefore, accurate detection of COD in water bodies is crucial
because of its extensive detection ranges.
The standard methods for COD determination are widely
adopted by numerous industries, such as environmental moni-
toring and wastewater treatment, etc., conventionally. However,
these methods have a few disadvantages that limit their applica-
tions, such as low detection sensitivity, long reflux time (2–4 h),
expensive consuming (Ag
C
), and highly toxic reagents
CONTACT Guobing Luo gbluo324@163.com Wuxi City Water Supply and Drainage Monitoring Station, 214011 Wuxi, China.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/batc.
© 2017 Taylor & Francis Group, LLC
CRITICAL REVIEWS IN ANALYTICAL CHEMISTRY
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(Cr
2
O
72¡
and HgSO
4
).
[9,10]
These methods may also cause sec-
ondary pollution and serious waste.
[11]
In addition, the meth-
ods do not meet the requirement of current real-time
environmental monitoring. Considering these drawbacks, the
development of sensitive, simple, and “green”methods for
COD determination in water bodies is of considerable interest.
This paper aims to provide an overview of the analytical
approaches for COD determination in water bodies (e.g., sur-
face water and wastewater), including two main parts, namely,
improved and optimized methods according to the standard
ones and new technologies or methods (Table 1 and Figure 1).
Over the last two decades, research efforts have been devoted
to developing more efficient technologies for COD determina-
tion in water bodies. With the development of innovative diges-
tion methods and detection instruments, several rapid
methods, including microwave-assisted digestion methods,
[9]
ultrasound-assisted digestion methods,
[12]
spectrophotome-
try,
[13]
chemiluminescence (CL) methods,
[14]
and flow injection
analysis (FIA),
[15]
have significantly shortened the detection
time. However, none of these techniques can eliminate the pol-
lution of toxic reagents (e.g., Cr
2
O
72¡
and HgSO
4
).
[16]
In recent
years, some new approaches [e.g., ozone (O
3
) oxidation,
[17]
electrochemical oxidation,
[18]
and especially photocatalytic
[19]
and photoelectrocatalytic
[20]
determination technologies based
on the reaction between hydroxyl radicals (¢OH) and organic
compounds], have been established. These new techniques
have been developed for accurate, rapid, low-cost, and online
monitoring determination of COD. New ideas and perspectives
have been provided to environmental monitoring and control-
ling using these methods.
COD assessment
In the standard COD evaluation method, organic material is
represented using potassium hydrogen phthalate (KHP), as
well as reflux in the presence of Hg
2C
sulfate of a test portion
with a known amount of K
2
Cr
2
O
7
and silver sulfate (Ag
2
SO
4
)
catalyst in strong sulfuric acid for 2 hours, during which part of
the dichromate is reduced by the oxidizable material present.
The reaction of KHP by K
2
Cr
2
O
7
is as follows:
2KC8H5O4C10K2Cr2O7C41H2SO4!16CO2C46H2O
C10Cr2SO4
ðÞ
3C11K2SO4(1)
when K
2
Cr
2
O
7
is substituted by O
2
, the equation becomes:
2KC8H5O4C15O2CH2SO4!16CO2C6H2OCK2SO4(2)
After digestion, the remaining unreduced dichromate is deter-
mined by potentiometric titration using a Fe
2C
solution accord-
ing to the following equation:
Cr2O72¡C6Fe2CC14H C!2Cr3CC6Fe3CC7H2O (3)
Standard methods
The COD measurement is commonly performed according to
the standard methods (e.g., reflux digestion and K
2
Cr
2
O
7
titration, and sealed digestion and spectrometry) (Table 2) that
is described in the International Standards ISO 6060-1989 and
ISO 15705-2002. In the former method, the excess K
2
Cr
2
O
7
is
titrated against ferrous ammonium sulfate using ferroin as an
indicator, and the oxidation of the sample is performed by the
open reflux procedure. The former method is considered suit-
able for a wide range of samples, wherein a large sample vol-
ume (commonly 10.0 mL) is required. In the latter method, the
oxidation of the sample is performed by the closed reflux pro-
cedure, and the amount of K
2
Cr
2
O
7
used in the oxidized sam-
ple is determined by measuring the absorbance of the formed
Cr
3C
. The amount of K
2
Cr
2
O
7
used is proportional to the oxi-
dizable organic matter present in the sample. Although the lat-
ter method requires less sample volume (usually 2.00 mL) and
chemical usage, the reproducibility of this method is poor due
to less sample volume and shielding agent, as well as complex
operation in comparison to the former method.
The two types of standard methods measure the oxygen
equivalent of the organic matter consumption to estimate the
COD value. However, both methods show some limitations,
including a long reflux time, high cost of chemical reagents,
similar concentration range of chloride ion (Cl
¡
) interference,
and usage of toxic reagents. Moreover, the two approaches
demonstrate inappropriateness for the online monitoring (e.g.,
productive process monitoring in WWTPs). Given all of these
drawbacks, researchers are interested in developing new meth-
ods to determine COD values to provide alternatives to the
standard ones.
Modified standard methods
In view of the drawbacks of the standard methods, efforts have
been devoted to improving and optimizing the traditional
COD determination methods. Hence, several modified meth-
ods have been proposed to circumvent these limitations. The
modified standard methods (e.g., improved digestion methods,
alternative methods of digestion reagents, optimized spectro-
photometric methods, and optimized detection methods pre-
vailing over chloride interference) are based on reflux digestion
and K
2
Cr
2
O
7
titration or sealed digestion and spectrometry.
These methods can significantly decrease the reflux time and
reduce the detection costs. However, the accuracy of the analyt-
ical results can be affected by the oxidation ability to refract
organic pollutants using the oxidants (K
2
Cr
2
O
7
or KMnO
4
).
Moreover, the secondary pollution caused by chromium salts
cannot be eliminated.
Improved digestion methods
Radiation-assisted digestion technologies (e.g., microwave and
ultrasound energy), have been applied to substitute the tradi-
tional heating tools (e.g., electric furnace and electric hot plate),
thereby minimizing the digestion time and consumption of
reagents.
[9,12]
Microwave- and ultrasound -assisted digestion
procedures are both based on radiation phenomena, wherein
the samples can be simultaneously and homogeneously heated
and various compounds (e.g., organic matter and some inor-
ganic salts) in water bodies can be oxidized and reduced. In
comparison with the classical digestion methods, the micro-
wave digestion procedure can decrease the long refluxing time
2 J. LI ET AL.
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Table 1. Comparison of different detection methods.
Types Methods Linear range (mg L
-1
)
Detection limit
(mg L
-1
) Advantages Disadvantages References
Standard methods Reflux digestion and K
2
Cr
2
O
7
titration 30–700 30 - standard quantitative procedures - long reflux time (2 h) ISO 6060-1989
- high accuracy - expensive consuming (Ag
2
SO
4
)
- good precision - highly toxic regents (HgSO4)
- good reproducibility - troublesome manual operations
Sealed digestion and spectrometry 6–1,000 6 - low detection limit - long reflux time (2h) ISO 15705-2002
(Photometric method) - good precision - expensive reagents (Ag
2
SO
4
)
15–1,000 15 - high accuracy - highly toxic regents (HgSO
4
)
(Titrimetric method)
Modified standard
methods
Ultrasound -assisted digestion procedure ——- high oxidation efficiency - expensive and toxic chemical reagents
[22]
(1 min irradiation time) (HgSO
4
,Ag
2
SO4)
- COD values are well agree with the standard
method (<3% RSD)
- can tolerate up to 2,800 mg/L Cl
¡
with an error
less than 10%
Mn
3C
as an oxidant for microwave-
assisted COD determination method
50–1,500 —- high oxidation efficiency - toxic reagents (K2Cr2O7)
[23]
(1 min irradiation time) - safety risks
- COD values are well agree with the standard
method (<4% RSD)
- avoids the use of silver and toxic mercury salts
Variable path length UV–vis spectroscopy
combined with PLS regression
112–1,872 122 - improvement of predictive accuracy - toxic reagents (K
2
Cr
2
O7)
[34]
(20–43% high than single path length
spectroscopy)
- long analytical time
- is not suitable for light polluted water
- is suitable for on-line water quality monitoring - COD values need to be verified by
standard methods
-noCl
¡
interference - high detection limit
- low cost
COD determination at high salinity and
low organic matter samples
0–200 —- provides optimized HgSO
4
:Cl
¡
ratio - toxic reagents
[39]
- is suitable for Cl
¡
<10,000 mg/L water - complex operation
sample - long analytical time
- narrow detection range
New technologies or
methods
High throughput 0.16–19.24 0.1 - relatively quick analysis speed (40 min) - toxic reagents
[14]
CL method - very narrow linear range
- high throughput (3 £96 samples in about
60 min)
- accuracy and reproducibility need to
be improve
- consumption of limited reagents
- little Cl
¡
interference
- automatic operation
FIA method based on microwave
digestion and ICP-OES
2.6–850 1.25 - high throughput (about 22 samples per 60 min) - toxic reagents
[49]
- high analytical cost
- no interference with Cl
¡
<3,000 mg/L - complex operation
- COD values are well agreed with the standard
methods
- high consumption of reagents
(Continued on next page)
CRITICAL REVIEWS IN ANALYTICAL CHEMISTRY 3
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Table 1. (Continued )
Types Methods Linear range (mg L
-1
)
Detection limit
(mg L
-1
) Advantages Disadvantages References
UV/O
3
method 1–25 0.81 - within 5% precise of the actual concentration - very narrow linear range
[17]
- complex equipments
- environment-friendly method - accuracy and reproucibility need to be
improved
- quick digestion time (15 min)
- safety risk of O3 usage
Electrochemical method using CuNi alloy
electrode
10–1,533 1.0 - wide linear range - is not reliable enough for practical use
[63]
- environment-friendly method - potential is not high enough
- high electrocatalytic activity - complex fabrication process
- long-term stability
COD determination using carbon fiber
felt/CeO
2
-b-PbO
2
electrode deposited
by cyclic voltammetry method
30–8,500 3.6 - very wide linear range - the risk of releasing toxic heavy metal (Pb)
[73]
- good performance with (3.0 §0.02) £10
¡3
mA
cm
¡2
/mg L
¡1
of sensitivity
- complex fabrication process
New technologies or
methods
Ultrasound electrochemical method using
BDD electrode
0–23,200 0.192 - wide linear range - analytical result is influenced by position and
speed of the pollutant
[78]
- environment-friendly
- COD values are well agree with standard method - is not reliable enough for practical use
- complex fabrication process
- 5 min of total detection time - high preparing cost
PcCOD method using 5-sulfosalicylic acid
in situ surface modified
titaniumdioxide
0.3–400 0.01 - very low detection limit - narrow linear range
[47]
- 30 min of irradiation time - relatively poor reproducibility
- environment-friendly method
- no sample pretreatment
- COD values are well agree with the standard
methods
PeCOD microfluidic reactors with
hierarchical TiO
2
nanotubes for COD
determination
0–300 1 - environment-friendly method - narrow linear range
[82]
- is portable and consumes few sample - accuracy and reproducibility need to be
improved
- rapid analytical speed (several minutes)
- no sample pretreatment
- COD values are well agree with the standard
methods
4 J. LI ET AL.
Downloaded by [218.206.112.202] at 16:24 30 October 2017
by up to 1 minute. By contrast, the digestion procedure is costly
and energy-consuming. In addition, test results should be veri-
fied by the traditional methods in wastewater samples using
complicated compositions. Similar to the microwave digestion
procedure, the ultrasound digestion procedure can also
improve the oxidizing efficiency and shorten the analysis time.
A COD determination method using ultrasound digestion and
titration based on oxidation reduction potential (ORP) was
reported because the ultrasound digestion method has low cost
and does not require complex equipment.
[21]
The ultrasound
time was applied for 15 minutes. However, the analytical results
obtained by this method were 5–20% less than those obtained
by the classical methods. In addition, the precisions were rela-
tively large due to the loss of the ultrasound energy with the
increasing operation time. A comparison of three types of sam-
ple digestion procedures, namely, closed microwave-assisted,
open microwave-assisted, and ultrasound-assisted showed
digestion time of 4, 4, and 1 minute, respectively.
[22]
Among
the three procedures, the ultrasound-assisted method exhibited
the highest oxidation efficiency due to the utilization of ultra-
sound radiation. Despite microwave and ultrasound digestion
techniques having several advantages over the traditional COD
determination methods, these techniques still need some
expensive and toxic chemical reagents for sample digestion.
The microwave- and ultrasound-assisted methods were investi-
gated using Mn
3C
as oxidation for COD determination to
develop more environment-friendly and simple sample
digestion procedures.
[23]
In these methods, Mn
3C
(E
0
D1.54 V)
was used for the development of an eco-friendly alternative oxi-
dant and was regarded as a more powerful one than K
2
Cr
2
O
7
(E
0
D1.36 V)
[24]
due to its high redox potential. The reactions
between Mn
3C
and KHP are as follows:
2KC8H5O4C30Mn2SO4
ðÞ
3C24H2O!16CO2C60MnSO4
C28H2SO4C2KHSO4(4)
The sample digestion time decreased significantly to 1 minute
for microwave and ultrasound. However, satisfactory results
have only been obtained using microwave radiation; low recov-
eries were obtained by ultrasound-assisted digestion method
due to less energetic conditions obtained with ultrasound
energy and different species of organic compounds in water
samples. Thus, microwave digestion procedure in this research
is still a good alternative for rapid, environment-friendly COD
determination.
The radiation-assisted COD determination approaches are
based on the standard reference methods; these approaches are
suitable for determining water samples with less refractory
organic pollutants or slightly and moderate contamination.
However, several difficulties (e.g., the insufficient oxidability of
certain organic pollutants and the inevitable use of toxic
reagents) still remain. Moreover, safety risks should be consid-
ered in utilizing radiation-assisted digestion methods.
Figure 1. Schematic illustration of different technologies or methods used for the determination of COD.
Table 2. Comparison of two standard methods for COD determination.
Standard no. Application scope
Sampling amount
(mL)
Required
time (h) Used reagents
Determination range
(mg L
-1
)
Concentration range of Cl
-
(mg L
-1
)
ISO 6060-1989 Water 10.0 2 0.040 mol/L K
2
Cr
2
O
7
30–700 <1,000
15 mL AgSO
4
–H
2
SO
4
0.4 g HgSO
4
ISO 15705-2002 Aqueous sample 2.00 2 0.10 mol/L K
2
Cr
2
O
7
6–1,000 <1,000
(Domestic sewage
Industrial wastewater)
0.0385 mol/L AgSO
4
(Photometric method)
1.8 mol/L, 4 mol/L H
2
SO
4
15–1,000
1.35 mol/L HgSO
4
(Titrimetric method)
CRITICAL REVIEWS IN ANALYTICAL CHEMISTRY 5
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Alternative methods of digestion reagents
Many efforts have been devoted to developing new COD mea-
surement methods that decrease the reagent pollution hazards
to the environment, improve the analytical efficiency, and
reduce the detection cost. Researchers have adopted the chemi-
cal substitutions or optimized reagents to replace the toxic and
expensive chemicals used in the conventional methods.
K
2
Cr
2
O
7
is employed as an oxidation reagent due to its strong
oxidation ability; however, the inadequate oxidation of certain
organic compounds results in the decrease of the degradation
ratio of oxidizable substances. Approaches toward greening the
traditional COD determination method by replacing the toxic
oxidant K
2
Cr
2
O
7
with the more environment-friendly oxidants,
including Mn
3C
(E
0
D1.54 V) and H
2
O
2
(E
0
D1.80 V),
[25]
have
been developed. Both modifications can reduce the use of
unsafe chromium species. In the former method, a silver cata-
lyst was not required, which made it relatively environment
friendly.
[24]
The latter method can decrease energy expenditure
through avoiding the need for heating during the 120 minutes
oxidation of the samples.
[25]
In addition, bismuth-based adsorbents instead of Hg
2C
were
used to avoid the utilization of highly toxic mercury salts.
[26]
The proposed approach not only eliminates metals, such as
Cr
6C
, Hg, and Ag but also decreases the test time. Therefore,
the proposed method can provide a rapid and environment-
friendly alternative for COD determination.
In order to avoid the health and environmental impact,
researchers have used an interesting “compromise solution”to
minimize the use of chemicals.
[27]
The downscaled COD proce-
dure reduced up to 67% of digestion time and up to 80% of
toxic residues generation compared with the standard method.
The linear range and detection limit were 30–600 mg/L and
3.6 mg/L, respectively. The results of various municipal or
industry wastewater samples tested via downscaled COD
method showed good agreement (the % standard deviation is
below 1.5%) with those obtained with the standard method.
Chloride with concentrations less than 2,000 mg/L did not
reveal a significant interference.
[8]
The alternative methods of digestion reagents can reduce
test costs and decrease environmental pollution. However,
these methods only updated the traditional methods partially.
In addition, the accuracies of these modified methods should
also be verified by the standard COD methods when they are
employed for COD determination in routine monitoring.
Given the advantages of the spectrophotometric methods,
mainly including short digestion time, less reagent consump-
tion, cheap analytical cost, and low environmental pollution,
these methods have been extensively employed in environ-
mental monitoring laboratories. The COD detector, which is
based on the spectrophotometric method (made by Hach in
the USA and Merck in Germany), can determine COD in
water samples through rapid digestion process and directly
read the test results; however, the digestion reagents are
costly, which restrict the promotion of these instruments.
Some researchers investigated homemade reagents to deter-
mine COD using a Hach DR/5000U spectrophotometer
detector and a Hach DRB-200 COD digester.
[13]
The analyti-
cal results obtained could be in accordance with the COD
values obtained by the traditional methods. The proposed
method has the advantages of shorter oxidation time
(30 minutes), fewer toxic reagents, 1/10 of the consumption
of reagents compared with the conventional methods, and
simultaneous determination of multiple samples, thereby
making it a fast, economical, and accurate method. However,
chemicals, such as K
2
Cr
2
O
7
,H
2
SO
4
,andAg
2
SO
4
,werealso
used in this method; thus, environmental pollution and high
cost could not be completely avoided. In addition, the
amount of HgSO
4
was insufficient to eliminate the chloride
interference.
Optimized spectrophotometric methods
According to the correlation between the ultraviolet–visible
(UV–vis) absorbance of certain water samples and COD, a
method for determining COD using UV–vis spectrophotome-
try was first proposed in 1965.
[28]
This method only relies on
the regression model standards provided by chemical methods,
and does not require sample digestion by chemical reagents,
thereby avoiding the time-consuming process of the oxidation
in water bodies; thus, this method is an indirect COD determi-
nation method.
With the development of optical techniques, UV–vis spec-
troscopy has shown high potential and has been extensively
used for COD measurements because of its rapid analytical
speed, good correlation with COD, and no secondary pollution
(no chemical reagents added) in recent years. However, mutual
interference and cross sensitivity for the water samples with
complicated or unstable compositions can lead to errors in the
COD predictions. In addition, turbidity, low concentration (
50 mg/L) sample, and the stability of the apparatus can influ-
ence the accuracy of the analytical results. A dual wavelength
(440 nm and 560 nm) spectroscopic method for low content
COD measurement was established.
[29]
Net absorption from
dichromate ions was determined by subtracting the absorption
at 560 nm (only contributed by Cr
3C
) from the absorption at
440 nm because of the same absorptivities of Cr
3C
at 440 nm
and 560 nm. Therefore, the influence of turbidity and the sta-
bility of the apparatus could be deducted according to the
established mathematical model, and the reliable analytical
results (with 8.6 mg/L detection limit) were obtained for low
COD determination. In some surface water samples, the turbid-
ity of the reaction mixture caused by the presence of inorganic
particulate substances increased the absorbance, thereby caus-
ing a negative error in COD determination. Thus, turbidity
compensation method for COD measurements by UV–vis
spectroscopy was developed and optimized to estimate the
interference of turbidity.
[30,31]
The measurement accuracy was
improved by eliminating the blue shift and peak height reduc-
tion in baseline subtraction. Multivariate calibration meth-
ods,
[32,33]
such as multiple linear regression (MLR), partial least
squares (PLS), least squares support vector machine (LS-SVM)
and back-propagation artificial neural network (BP-ANN),
have been used for COD determination to alleviate the work-
load of the management and analysis of a large quantity of
spectral data using regression models. Among these methods,
PLS regression is extensively used in generating a regression
model based on UV–vis spectral data to estimate the COD
value.
[34]
A comparison of the preceding methods mentioned
four calibration methods (MLR, PLS, LS-SVM, and BP-ANN),
6 J. LI ET AL.
Downloaded by [218.206.112.202] at 16:24 30 October 2017
which indicated that the successive projection algorithm-BP-
ANN model with the standard normal variate spectrum pro-
vided the best performance due to the best prediction
results.
[32]
The optimized spectrophotometric methods, such as dual
wavelength spectroscopic methods and turbidity compensation
methods, can improve the measurement accuracy and elimi-
nate secondary pollution for COD determination. Calibration
methods, such as MLR, PLS, LS-SVM, and BP-ANN, have been
adopted to obtain accurate prediction results. However, some
errors exist between the mathematical prediction models and
actual samples. Therefore, the values obtained by these opti-
mized spectrophotometric methods should also be verified by
the traditional methods.
Optimized COD determination methods prevailing over
chloride interference
Chloride ion is amenable to oxidation by K
2
Cr
2
O
7
and can pre-
cipitate the silver used as a catalyst.
[23]
Saline wastewater is con-
siderably common and is known to affect COD
measurement.
[35–37]
Dichromate does not oxidize Cl
¡
in aque-
ous solutions by natural processes. However, dichromate can
react with Cl
¡
in H
2
SO
4
medium forming chromyl chloride
(CrO
2
Cl
2
, Equation 5) in the refluxing process using the stan-
dard COD determination methods
[8,24]
:
K2Cr2O7C4NaCl C6H2SO4!2KHSO4C4NaHSO4
C2CrO2Cl2C3H2O (5)
During the 2 hours digestion process at (148 §3) C, Cl
¡
is
quantitatively converted to chlorine (Cl
2
) due to the insufficient
masking of Cl
¡
according to the following equation:
Cr2O72¡C6Cl ¡C14H C!3Cl2C2Cr3CC7H2O (6)
The dichromate oxidizes organic compounds and Cl
¡
in water
samples. Therefore, chloride is the main interference in COD
analysis of water samples.
[38]
HgSO
4
is employed as a masking
agent to form unionized mercuric chloride in the K
2
Cr
2
O
7
reflux method and the spectrophotometric method (Equation
7). However, HgSO
4
cannot completely eliminate the chloride
interference when the concentration of chloride is more than
2,000 mg/L
[39]
:
Hg2CC2Cl ¡!HgCl2(7)
This finding is mainly because high temperatures result in the
insufficient masking of chloride.
[8]
Researchers investigated the
influence of the digestion temperatures for mercury-free COD
determination.
[40]
The optimum digestion temperature used
was 120 C to inhibit the oxidation of chloride selectively (with
Cl
¡
<3,000 mg/L for undiluted water samples) without the
use of mercury. The Cl
¡
interference was further decreased by
adding Ag
C
and was practically absent at Ag
C
/Cl
¡
molar ratio
over 1.7.
Conventionally, the sample dilution method has been
demonstrated to be a simple and effective technique to
decrease the concentration of chloride concentration. This
method is suitable for samples with high organic matter and
high chloride. However, this method is unsuitable for sam-
ples with low organic matter and high chloride because chlo-
ride oxidation inevitably results in difficulty of accurately
measuring diluted COD concentration. By contrast, research-
ers investigated the optimum HgSO
4
:Cl
¡
ratios according to
the chloride concentrations of the samples by calculating the
oxidant consumption through Cl
¡
.
[41]
According to the tra-
ditional COD determination methods, the use of HgSO
4
:Cl
¡
ratio is suggested to be 10:1 to avoid chloride interference
when the chloride concentrationislessthan2,000mg/L.
Nevertheless, a significant error at low and moderate CODs
was found using this ratio (10:1) in samples with chlorides
more than 1,000 mg/L. More importantly, different HgSO
4
:
Cl
¡
ratios must be applied for different chloride concentra-
tions to obtain precise results for low COD and high chloride
values.
[39]
Therefore, varying HgSO
4
amounts in relation to
the chemical ingredient of the sample is necessary to remain
within acceptable limits for the COD measurement. How-
ever, the considerable addition of HgSO
4
alone is not a solu-
tion to erroneous measurement results.
[39]
The choice of the
appropriate HgSO
4
:Cl
¡
ratio depends on both COD and Cl
¡
concentrations. A previous study suggested that water sam-
ples with Cl
¡
concentrations up to 2,000 mg/L could be ana-
lyzed as long as the Cl
¡
/COD ratio did not exceed 5:1.
[40]
Greater HgSO
4
concentrations allow for the samples deter-
mination with Cl
¡
/COD ratios up to 30, but for water sam-
ples with higher Cl
¡
/COD ratios, efficient masking of
Cl
¡
using HgSO
4
is impossible.
[8]
Therefore, Cl
¡
removal
approaches are not very practical and complex.
The methods mentioned above can effectively overcome the
Cl
¡
interference in determining COD. Although accuracy and
precision are improved, environmental pollution still exists due
to mercury salt and dichromate. In addition, a complex opera-
tion is obtained due to tedious test processes and methods.
New technologies or methods
K
2
Cr
2
O
7
is used as an oxidation reagent to oxidize the organic
compounds in water samples using the traditional methods
and optimized methods. However, oxidizing some refractory
organics in complex wastewater samples by K
2
Cr
2
O
7
is difficult
results in erroneous analytical results. Furthermore, problems,
such as environmental pollution and high test cost, still need
resolution by these techniques. CL methods, flow injection, or a
combination of several techniques have also been employed for
COD measurement in water bodies. The novelty of these tech-
niques includes wide detection limits, short detection time, and
low environmental pollution. Based on the reaction between
hydroxyl radicals (¢OH) and organic compounds, new
approaches, which are called advanced oxidation processes
(AOPs) (e.g., ozone oxidation and electro- and photo-oxidative
methods), have been recently developed. Most of the organics,
including the refractory organic matter, can be oxidized by the
extremely strong oxidation capacity; therefore, they can be
applied for determining COD with different pollution levels.
Furthermore, the environmental pollution and toxicity of
chemicals obtained by the traditional methods can also be
avoided.
CRITICAL REVIEWS IN ANALYTICAL CHEMISTRY 7
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CL methods
CL is known to be a powerful detection technique that has sev-
eral advantages, including high sensitivity, fast response time,
wide linear range, simple instrumentation and portability.
[42]
In addition, the energy required for CL emission is produced in
a chemical reaction and not come from an exciting light
beam.
[4]
This method is based on chromium, which can cata-
lyze the luminol oxidation reaction with hydrogen peroxide in
basic medium. Cr
3C
is produced during the chemical oxidation
of COD substances in the samples and measured with the assis-
tance of the luminol-H
2
O
2
system as shown as follows
[42]
:
Luminol CH2O2CCr3C!hyCproduction (8)
All the preceding advantages mentioned have allowed the CL
methods to be used conveniently for COD measurement in
water bodies through the appearance of Cr
3C
after sample
digestion with K
2
Cr
2
O
7
, which is proportional to the COD.
Based on this principle, a cost-efficient CL photodiode detector
was developed to measure CL emission produced by the lumi-
nol-H
2
O
2
-Cr
3C
reaction.
[42]
A CL combined with UV photoly-
sis method for COD measurement was also established.
[43]
With the assistance of free radicals obtained in the UV irradia-
tion process in producing luminescence, the proposed method
has been successfully applied to determine COD indirectly. The
method also significantly extended the detection limit to
0.08 mg/L and improved the detection efficiency (5–10 minutes
per sample). Furthermore, this method only required simple
instrumentation and could realize online monitoring.
CL methods combined with flow injection for automatic
COD determination were investigated using luminol-H
2
O
2
-Mn
(II) and luminol-KMnO
4
CL systems.
[44,45]
Generally, these
developed CL methods are rapid, simple, and sensitive but still
require high consumption of chemical agents and display low
throughput of the sample. To overcome these drawbacks, a
luminol-H
2
O
2
CL microflow system for rapid COD determina-
tion at room temperature was reported.
[46]
The interference of
Cl
¡
was minimal because of the short reaction time (40 sec-
onds) between the oxidizing reagent and sample. The method
was fast (40 seconds per determination), low-contaminative,
and automatic because of the use of a discrete micro droplet
sampling mode with a volume of 5 mL on a polymethyl meth-
acrylate microflow chip. The method is suitable for COD deter-
mination with various content ranges (0.27–10 g/L of the linear
range). However, the detection limit was extremely high
(100 mg/L), and accurate and representative sampling should
be ensured to obtain accurate analytical results. Based on the
KMnO
4
-glutaraldehyde CL system, a high throughput CL
method for COD determination was established
[14]
(Figure 2).
The method had a linear range of 0.16–19.24 mg/L with a low
detection limit of 0.1 mg/L COD. However, this method is only
suitable for slightly polluted water samples because of its low
linear range.
The CL technique is environment friendly, rapid, and can be
easily automated. However, the oxidative degradation must be
conducted by using strong oxidants, such as dichromate and
permanganate. In addition, the accuracy and reproducibility
are diluted by the various factors in these systems.
FIA methods
FIA has been employed as a mature detection method in envi-
ronmental analysis for about 40 years due to its improved
repeatability, rapid reaction time, easy standardization, consid-
erable reduction of reagent, and absence of secondary pollution.
Moreover, implementing online coupling of detection is
easy.
[16]
In a traditional FIA system, numerous gas bubbles that are
generated in the flow path influence the follow-up steps
because the oxidization reaction should proceed under heating
conditions.
[4]
Furthermore, pipelines are commonly corroded
by the strong acidity of the K
2
Cr
2
O
7
reaction system. There-
fore, the reaction system or medium should be modified or
optimized. FIA approaches have been combined with CL
[47]
or inductively coupled plasma-optical emission spectrometry
(ICP-OES)
[48,49]
for rapid COD determination and automation
of COD measurement, which overcome the problem of
incomplete sample oxidation and the abovementioned draw-
backs. The FIA method was first adopted for COD measure-
ment in 1980 using a spectrophotometer; this technique has
attracted significant attention.
[50]
An FIA manifold that incor-
porates online UV irradiation for COD determination in
freshwater was reported.
[15]
The combination of FIA method
to spectroscopy decreased the complexity of the instrument
and provided good reproducibility. The proposed method was
well suited for slightly contaminated water (0.5–50 mg/L)
with 30 hours
¡1
of the sample throughput. A FIA-CL system
for automatic determination of COD was investigated.
[44]
The
analytical results showed that the method is suitable for a
wide COD concentration of 4–4,000 mg/L with 2 mg/L of the
detection limit, and only requires 1.5 minutes for a complete
analysis.
[44]
A mini-cation ion-exchange column was adopted
on the sampling system to simply eliminate the interference of
excessive MnO
4
¡
with the luminol CL system and some cati-
ons in the water samples. However, researchers indicated that
the FIA-CL technique obtained lower oxidation efficiency in
comparison to O
3
/UV technique due to the pH effect on CL,
which significantly resulted in high COD values (0–20% rela-
tive errors).
[51]
To overcome the shortcomings of the above-
mentioned FIA methods, a flow injection microwave solid-
phase extraction by means of activated carbon system for
sample pretreatment was developed.
[48]
Then, the content of
COD was determined by ICP-OES (Figure 3). A large linear
range of 2.78–850 mg/L with 0.94 mg/L detection limit was
Figure 2. Schematic illustration of the CL system for the determination of COD.
[14]
8 J. LI ET AL.
Downloaded by [218.206.112.202] at 16:24 30 October 2017
obtained. In addition, Cl
¡
did not influence the COD mea-
surement with the concentration below 3,000 mg/L without
any masking agents, and 18 hours
¡1
of the sample throughput
was obtained. K
2
Cr
2
O
7
was used as the oxidizing reagent in
this method; thus, the problem of environmental pollution
was still unavoidable. Manganese was employed as an envi-
ronment-friendly oxidizing reagent to replace K
2
Cr
2
O
7
by the
FIA method based on microwave digestion and manganese
speciation (solid-phase extraction) (FI-MW-SPE),
[49]
wherein
similar analytical results were obtained. Nevertheless, the
method was of complex operation and high cost due to the
usage of ICP-OES.
Although the combinations of FIA with CL and ICP-OES
can automate COD detection, applying them on-site is difficult
due to the use of bench top laboratory instrumentation. The
effective oxidation of organic compounds by the amount of
energy is insufficient due to the short contact time (few
minutes) involved. Furthermore, the FIA method needs high
consumption of chemical reagents, and the instrumentation is
of high cost and requires careful and regular maintenance.
Ozone oxidation technique
AOPs have been employed for the COD treatment to improve
the efficiency of the oxidation and degradation of recalcitrant
compounds. AOPs based on the in situ generation of the highly
reactive hydroxyl radical (¢OH) that non-selectively reacts with
most organic substances.
[52]
¢OH has gained increasing interest
due to its strong capability of oxidizing refractory contaminants
and environment-friendly nature.
[53]
Moreover, ¢OH has a
short lifetime, which is estimated as only few nanoseconds in
water bodies; thus, they can be self-eliminated from the reac-
tion system. O
3
and O
3
-based oxidation processes (O
3
,O
3
/UV,
O
3
/H
2
O
2
, and O
3
/H
2
O
2
/UV) are the most common AOPs and
they have already been developed for COD determination. O
3
is a strong and environment-friendly oxidant. It can directly
oxidize organics by cycloaddition, as well as electrophilic and
nucleophilic reactions and indirectly oxidize organics by ¢OH
that is generated during the reaction process.
[4]
However, O
3
cannot independently oxidize certain organic matters. There-
fore, a combination of O
3
with UV irradiation or H
2
O
2
is con-
sidered to be more effective in oxidizing organic matters
because of its higher degradation efficiency in comparison to
O
3
oxidation alone.
In this process, O
3
is generated by water electrolysis and can
be degraded by UV irradiation under ambient temperature and
pressure to generate ¢OH, which is a strong oxidant that can
decompose organics within a short period of time. The chemi-
cal process can be summarized as follows:
O3CH2O!O2CH2O2(9)
O3CH2O!O2C2¢OH (10)
H2O2!2¢OH (11)
Based on the above chemical processes, ¢OH was adopted to
generate UV photolysis of O
3
as the oxidant, and the amount
of consumed O
3
was then used for COD determination
[17]
(Figure 4). The method did not require any additional chemi-
cals, thus making it environment friendly. The linear range was
1–25 mg/L with 0.81 mg/L detection limit, and approximately
four samples per 60 minutes of the sample throughput were
obtained. Researchers found that residual H
2
O
2
and acidity are
the main probable problems in COD measurement using AOPs
(i.e., the mixture of O
3
and H
2
O
2
).
[54]
They used Na
2
CO
3
as a
catalyst to eliminate the interference of residual H
2
O
2
success-
fully. However, controlling the pH of the strong acidic reaction
solution to neutrality is important to obtain accurate results.
O
3
and O
3
-based oxidation approaches have extensive appli-
cation prospects not only for COD determination but also for
wastewater treatment due to their rapid reaction time, absence
of chemical pollution, and easy implementation of online mon-
itoring.
[55]
Nevertheless, some limitations still exist in these
techniques, such as narrow linear range and low degradation
efficiency, which restrict its widespread application.
Electrocatalytic detection methods
Many efforts have been devoted in developing a simple, fully
automated, reliable, and cheap approach for COD measure-
ment to provide real-time information of water pollution. Elec-
trocatalytic oxidation method is an ideal alternative that fulfills
Figure 3. Schematic illustration of the FI-MW-SPE system for the determination of
COD.
[48]
S, sample; B, buffer; E, eluent; W, waste; P1 and P2, peristaltic pump; MW,
microwave oven; B, 4C-ice bath; T, sample and buffer line connector; C, minicol-
umn packed with activated carbon;V1 and V2, injection valves. Valve 1 positions:
(a) loading; (b) injection.
Figure 4. Schematic illustration of the UV/O
3
measurement system for the deter-
mination of COD.
[17]
CRITICAL REVIEWS IN ANALYTICAL CHEMISTRY 9
Downloaded by [218.206.112.202] at 16:24 30 October 2017
the abovementioned requirements. The basic principle is to
degrade organics into CO
2
and H
2
O eventually through direct
electrolysis or electro-catalytic oxidation using the electrochem-
ical oxidation method under high potential. Thus, the COD
values can be rapidly calculated from the charge that was con-
sumed in the electrochemical degradation of organics accord-
ing to the correlation between the variable quantities of
potential generated in the process of oxidation and COD. Thus
far, many electrode materials have been successfully developed
and employed to construct COD sensors, including CoO,
[18]
Cu,
[56]
Ni,
[57]
PbO
2
,
[58]
Rh
2
O
3
,
[59]
nano-Pt,
[60]
and boron-
doped diamond (BDD).
[61]
Generally, the three types of electro-
chemical detection approaches according to the types of sensors
are Cu (or Ni) electrodes, PbO
2
electrodes, and BDD electrodes.
Electrocatalytic oxidation methods based on Cu (or Ni) electro-
des. Research indicated that some anodic materials, including
Pt, Ag, Au, Cu, PtO
x
, and PbO
2,
have demonstrated good sur-
face catalytic activity to oxidize different organic com-
pounds.
[62]
Direct oxidation of organic pollutants in aqueous
solution using metal electrodes is impossible because high
potentials required for the oxidation of organic compounds
usually result in water oxidation; however, copper electrode
can act as electro catalysts for the alkaline oxidization of
organic species, such as carbohydrates and amino acids, which
are mainly responsible for COD.
CuO was found to have good catalytic ability to provide the
formation of active sites [CuO(OH)] through the results of the
voltammetric profile of the Cu/CuO electrode in 0.1 mol/L
NaOH solution.
[56]
A wide linear range of 53.0–2,801.4 mg/L
COD with 20.3 mg/L detection limit was obtained. Compared
with other electrodes (BDD, F-PbO
2
, Ti/TiO
2
/PbO
2
, and
Cu),
[56,63,64]
the Cu/CuO electrode has higher sensitivity and
wider linear range. However, this proposed approach has high
detection limit (20.3 mg/L) and relatively long analytical time
(approximately 15 minutes; Table 3). A copper nanoparticle
(nano-Cu) was developed on the surface of the Cu electrode to
overcome the drawbacks of the Cu/CuO electrode.
[65]
The
nanoparticle showed a strong enhancement effect and consid-
erably increased the oxidation current signal (3.6 mg/L detec-
tion limit, 4.8–600 mg/L linear range, and Cl
¡
<0.02 mol/L
influence) because of its large surface area and high catalytic
activity.
Nickel (Ni) is a multifunctional and environment-friendly
metal that is extensively used as a catalyst electrode material in
the fields of wastewater treatment and electrochemical analysis.
Ni is a potential electrode material to determine COD in
water bodies due to its high oxidization capability of organic
compounds in alkaline media. A nano-nickel (nano-Ni) elec-
trochemical sensor for COD measurement was developed.
[57]
The linear range and the detection limit were 10–1,533 mg/L
and 1.1 mg/L, respectively. A 0.02 mol/L Cl
¡
did not interfere
the oxidation current signal of 107 mg/L COD. Moreover, most
metal ions (e.g., Cu
2C
,Fe
2C
,Fe
3C
,Zn
2C
, and Pb
2C
) cannot
influence the detection because they are deposited at the bot-
tom of a beaker. The catalytic activity and selectivity of metal
nanoparticles are strongly dependent on their size and shape.
Thus, a Ni-nanoparticle on the surface of a glassy carbon elec-
trode via electrochemical reduction using NiSO
4
as the precur-
sor was developed.
[66]
High sensitivity (0.14 mg/L detection
limit and 0.24–480 mg/L linear range) was obtained by the
optimized experimental conditions, such as variations of reduc-
tion potential, deposition time, pH value, and Ni
2C
concentration.
Research shows that Cu, Ni, and Co have a similar face-cen-
tered cubic structure with similar lattice parameters, thereby
resulting in the possible combination and formation of alloys,
such as Ni–Cu or Cu–Co. On the basis of this principle, a Ni–
Cu alloy electrode was used to determine COD in water sam-
ples. Although the linear range and detection limit were consis-
tent with the result obtained by the single nano-Ni electrode
approach,
[63]
the Ni–Cu alloy electrode has the advantages of
higher electrocatalytic activity, easier preparation, and long-
term stability in comparison to several electrochemical sensors,
Table 3. Analytical features of different electrochemical sensors for COD detection.
The detection electrode
Linear equation (yDI
resp
,mAcm
¡2
;
xDCOD
Th
,mgL
¡1
) Kinds of standard COD test reagent
Linearity range
(mg L
¡1
)
Detection
limit ( mg L
¡1
)
Tolerance
of Cl
¡1
(M) Reference
Rotating Pt ring-Pt/PbO
2
A: yD2.88 £10
¡4
xC1.92 £10
¡3
Glucose 20–500 15 —
[70]
disc electrode B: yD2£10
¡5
xC1.44 £10
¡1
500–25,000 15 —
Ti/TiO
2
electrode yD8.5 £10
¡6
xC5.96 £10
¡5
Potassium hydrogen phthalate (KHP) 50–2,000 16 —
[67]
Rh
2
O
3
/Ti yD2.2 £10
¡4
xC1.256 £10
¡4
KHP 50–2,000 20.0 0.017
[59]
Cu/CuO electrode yD4.717 £10
¡4
xC5£10
¡3
Glucose 53–2,801.4 20.3 —
[56]
CuO/AgO electrode yD1.16 £10
¡6
xC1.42 £10
¡3
Glucose 106–1,292 28 —
[68]
Nano-Cu electrode yD4.538 £10
¡4
x¡1.288 £10
¡3
Glycine 4.8–600 3.6 0.02
[65]
Nano-Ni electrode yD5.9 £10
¡5
xC1.32 £10
¡3
Eight types of organic compounds
(phenol, lactose, citric acid, etc.)
10–1,533 1.1 0.02
[57]
NiCu alloy electrode yD1.4 £10
¡4
xC6.83 £10
¡3
Eight types of organic compounds
(phenol, lactose, citric acid, etc.)
10–1,533 1.0 1
[63]
Cu–Co yD8.88 £10
¡4
xGlucose 1.92–768 0.609 0.02
[64]
F-PbO
2
/Pt modified
electrode
yD2.3 £10
¡7
xC3.12 £10
¡6
d-glucose 100–1,200 15 —
[69]
Ti/Sb-SnO
2
/PbO
2
composite electrode
yD2.059 £10
¡3
xC3.762 £10
¡4
Glucose 0.5–200 0.3 —
[71]
CF/CeO
2
-b-PbO
2
electrode
yD(3.0 §0.02) £10
¡3
xC
(9.0 §7.3)10
¡3
Glucose, KHP, mannitol, glutamic acid 30–8,500 3.6 —
[73]
BDD electrode yD9.09 £10
¡5
xC8.43 £10
¡5
Nine types of organic compounds
(4-hydroxybenzoic acid, glutamic
acid, etc.)
2–175 1.0 —
[77]
10 J. LI ET AL.
Downloaded by [218.206.112.202] at 16:24 30 October 2017
including F-PbO
2
, nano-Cu, Cu/CuO, Ti/TiO
2
,
[66]
and BDD.
Nevertheless, Cl
¡
and dissolved oxygen (DO) had no intensive
interference with this determination because of its good resis-
tance to surface fouling. All these advantages meet the require-
ments for extensive practical perspective on monitoring the
organic pollutants in water bodies. A micro-nano Cu–Co com-
posite electrode using Co(NO
3
)
2
and CuCl
2
as the precursor
was adopted to determine COD in water bodies because Cu
and Co can strongly bind and easily reduce the oxygen.
[64]
This
method is suitable for COD determination of 1.92–768 mg/L
concentration range with 0.609 mg/L detection limit due to the
high surface roughness of Cu and Co and numerous micro-
and nano-particles of the alloys, as well as the synergistic effects
of oxygen–oxygen bond breaking between Cu and Co.
Nanocomposite-derived sensors based on four inorganic
electro catalysts (i.e., Ni, NiCu alloy, CoO nanoparticles, and
CuO/AgO) were fabricated for the COD analysis in influents of
WWTP to further verify the sample application of these meth-
ods in actual water.
[68]
Despite obtaining good analytical per-
formance in the laboratory with the four electrodes, only the
approach using the CuO/AgO electrode showed a good correla-
tion between the sensor response and the COD values provided
by the standard method. Therefore, online COD determination
using electrochemical methods in actual water samples remains
a challenge. In addition, the available potential for the above-
mentioned electrodes is not sufficiently high to oxidize all
organic pollutants.
Electrocatalytic oxidation methods based on PbO
2
electrodes.
The techniques of electro-generated hydroxyl radical (¢OH)
using PbO
2
-based electrodes
[69,70]
have been used to degrade
the organic pollutants in water bodies. PbO
2
exhibited the
advantages of high conductivity, high peroxide potential, easy
preparation, good corrosion resistance capability, and low
cost.
[58]
However, these electrodes have some limitations,
including complex fabrication process, high cost, and loss of
adhesion force. Moreover, dual-slope electrodes, such as modi-
fied nano-PbO
2
electrode and rotating Pt ring-Pt/PbO
2
disc
electrode are unfavorable for practical application due to the
complexity of the current signal transformation system. A
novel Ti/Sb-SnO
2
/PbO
2
composite electrode was fabricated to
determine COD. Compared with Ti/F-PbO
2
electrode,
[71]
the
composite electrode configuration significantly improved the
sensitivity of the amperometric method. A low detection limit
of 0.3 mg/L with 0.5–200 mg/L linear range was obtained; how-
ever, Cl
¡
had interferences on the COD measurement when its
concentration exceeded 100 mg/L. Two methods were sug-
gested to eliminate the interference of Cl
¡
; one is dilution
before COD determination for the samples with large contents
of Cl
¡
and high COD value, and the other is precipitation of
Cl
¡
by adding Ag
C
before COD determination for the samples
with large amounts of Cl
¡
and low COD value.
A highly sensitive sensor is important to reduce the require-
ment for the detection equipment and fabricate a COD sensor
with high performance, such as excellent reproducibility, low
cost, and good stability. This requirement is important because
the detection sensitivity (represented by the slope of the linear
equation) is less than 10
¡3
mA cm
¡2
/mg L
¡1
in several
reported electrocatalytic sensors (Table 3), including Cu/CuO
electrode, Ni–Cu alloy electrode, Ti/TiO
2
electrode, modified
nano-PbO
2
electrode, and modified F-PbO
2
/Pt electrode. By
contrast, a wider linear range is provided to obtain better
adaptability for the detection of various water samples. Based
on the two principles, a 3-D structured CF/b-PbO
2
electrode
for COD determination in wastewater was developed.
[72]
The
unique structure and high compactness of b-PbO
2
coating of
this approach resulted in the following advantages: a relative
standard deviation (RSD) within 5% compared with the conven-
tional methods, a high sensitivity of 2.05 £10
¡3
mA cm
¡2
/mg
L
¡1
, and a wide linear range of 50–5,000 mg/L with 3.9 mg/L
detection limit. Although no coating fell off, the perfect crystal-
line was susceptible to mechanical damage during COD mea-
surement. Efforts have been devoted by some researchers to
avoid the mechanical damage of PbO
2
crystalline.
[73]
They added
CeO
2
to improve the mechanical stability and electrocatalytic
activity of the electrode, and then a smaller CF/CeO
2
-b-PbO
2
electrode compared with their previous work was then employed
as an online sensor in COD determination. The modified
method showed good performance with (3.0 §0.02) £10
¡3
mA cm
¡2
/mg L
¡1
sensitivity, wide linear range of 30–8,500 mg/
L with 3.6 mg/L detection limit (Table 3).
Despite the low cost, high sensitivity, and easy operation of
PbO
2
electrodes, the inability of its widespread application can
be attributed to the risk of releasing toxic heavy metal (Pb) that
is generated during their preparation, determination, and
disposal.
[63]
Electrocatalytic oxidation methods based on BDD electrodes.
BDD is an environment-friendly and versatile electrode mate-
rial used in the fields of electrocatalytic water treatment and
electrochemical analysis due to its good conductivity.
[74]
Com-
pared with Cu (or Ni) and PbO
2
electrodes, BDD possesses a
substantial amount of interest in COD measurement because
of its extensive electrocatalytic working potential window, high
mechanical strength and corrosion resistance, high resistance
to deactivation by impurities and long term response stability,
as well as low and stable background current.
[75]
An ampero-
metric method in COD detection was first reported using BDD
electrode with satisfied potential and good performance for its
wide electrocatalytic potential window; moreover, a wide linear
range of 20–9,000 mg/L COD with 7.5 mg/L of the detection
limit was obtained.
[74]
However, conducting amperometric
measurements in practical COD tests is difficult.
[76]
In addition,
the particulars of the oxidation reaction depend on the type of
compounds present in this approach. An FIA method for COD
detection using a BDD electrode was developed to attain a
rapid, environment friendly, in situ, and online COD testing
system.
[77]
The linear range and detection limit of this approach
is 2–175 mg/L and 1 mg/L, respectively. Compared with previ-
ous work, the modified method is well suited for COD determi-
nation in slightly polluted water bodies. The former method
can detect COD in sewage and wastewater with relatively high
concentrations of organic pollutants because of its wide linear
range. Ultrasound electrocatalytic and amperometric determi-
nation methods of COD using BDD electrodes were succes-
sively optimized.
[78,79]
Based on the advantages of ultrasound
digestion in the previous section Improved digestion methods,
the ultrasound electrocatalytic determination method could
CRITICAL REVIEWS IN ANALYTICAL CHEMISTRY 11
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extend the linear range of 19.2–11,600 mg/L obtained by the
amperometric method to 0–23,200 mg/L; however, the detec-
tion limit remained constant (0.192 mg/L). The wide linear
ranges resulted in possible COD determination with various
levels of water samples. Although the above new methods
showed good accuracy, stability, and reproducibility compared
with the standard methods, these new methods still have some
drawbacks, including the detection instability for the ultra-
sound electrocatalytic determination method influenced by the
position and speed of the pollutant and the position of the
ultrasonic source. Nevertheless, a BDD film was able to be syn-
thesized using a hot filament chemical vapor deposition
(HFCVD) technique. The fabrication process of the BDD film
was complicated, and the quality of the diamond film was not
remarkably high. In addition, the elimination method of the
interference of Cl
¡
, as the main interference in COD determi-
nation was not discussed.
The researchers fabricated the BDD electrodes using micro-
wave approach with further improvement to overcome the
shortcomings of BDD films synthesized by the HFCVD tech-
nique.
[61]
They successfully synthesized the BDD films through
microwave plasma enhanced chemical vapor deposition (MW
PE CVD)
[61]
and microwave plasma assisted chemical vapor
deposition (MW PA CVD),
[80]
and COD was then determined
using the amperometric technique. These new approaches sig-
nificantly decreased the detection limits (0.3 and 0.9 mg/L,
respectively); however, their fabrication processes required spe-
cial instruments thereby resulting in high costs. A new COD
determination approach using a TiO
2
/BDD heterojunction
photoanode electrode with n-type TiO
2
was reported due to the
inherent photoelectric characteristics of TiO
2
.
[81]
This approach
could achieve 5 minutes of analysis time and 0–300 mg/L linear
range with 0.12 mg/L detection limit. In addition, the working
life of this electrode was 2 months (for 500 measurements)
before maintenance. Therefore, the proposed approach was
well suited for online COD monitoring with slightly or moder-
ately polluted water bodies. A direct COD determination
approach by anodic decomposition of organics at a BDD elec-
trode prepared by MW PA CVD technique was proposed to
improve the oxidation capability of organic compounds and
the efficiency of electrodes.
[76]
A high positive potential
(C2.5 V vs Ag/AgCl) was applied to an aqueous electrolyte that
contains KHP, glucose, and lactic acid or sodium dodecylben-
zenesulfonate at the electrode. A consistent electric charge was
obtained for the electrocatalytic oxidation of the organic com-
pounds to CO
2
. The proposed method was proven to be fairly
simple. Moreover, no calibration was necessary for online mon-
itoring of wastewater with low COD concentrations and on-site
instant analysis of natural water with a portable COD
instrument.
With the development of anode materials, porous electro-
des, especially 3D network electrodes, have gained considerable
interest due to their excellent properties that are inherent to the
nature of the original format along with high surface areas and
grid fabrics. A hydrophobic 3D network BDD electrode on
mesh titanium substrate via chemical vapor deposition (CVD)
technique was fabricated for COD measurement.
[82]
Neverthe-
less, efforts should be provided to determine COD in actual
water samples.
Numerous advantages, such as high oxygen evolution poten-
tial, weakly adsorption, and superior electrocatalytic stability,
make the BDD electrodes regarded as the most suitable elec-
trode materials against organic pollutants. However, complex
fabrication process, unstable background current, and high
preparation cost are the main problems for COD measurement
in practical application.
Although the electrocatalytic oxidation methods have sev-
eral advantages over the classical COD methods (e.g., fast ana-
lytical speed, the directness in the acquisition of analytical
signal, and easy incorporation into an online analysis monitor-
ing system), the electrocatalytic oxidation methods are still
insufficiently reliable for practical use mainly due to their inca-
pacity of effectively and indiscriminately oxidizing a wide spec-
trum of organic compounds.
Photocatalytic methods for COD determination
The principal disadvantages of the traditional COD methods
include long analysis time, complex handling operations, and
use of expensive and toxic reagents.
[83]
To overcome these limi-
tations, one of the AOPs, known as the photocatalytic decom-
position of organic pollutants by nano-materials (e.g., titanium
dioxide [TiO
2
],
[84]
zinc oxide [ZnO],
[85]
and tungsten trioxide
[WO
3
]
[86]
) has attracted considerable attention with the devel-
opment of material science and technique. TiO
2
is more exten-
sively applied than other semiconductors because it is
inexpensive, non-photocorrosive, highly photosensitive, and
possesses a large band gap. Moreover, TiO
2
is commercially
available in various crystalline forms and particle characteris-
tics, non-toxic, and photochemically stable among these cata-
lysts.
[87]
The strong oxidation power of illuminated TiO
2
particles results in more promising photocatalytic methods
(i.e., PcCOD) compared with the electrocatalytic degradation
methods.
In the photocatalytic process, a suspension of semiconductor
particles is irradiated with natural or artificial UV light. The
excitation transfer of an electron from the valence band to the
conduction band creates an oxidizing site (i.e., a “hole,”h
C
VB
)
and a reducing site (i.e., an “electron,”e
¡
CB
). With these holes
and electrons, organic compounds are oxidatively degraded
into harmless inorganic substances such as CO
2
and
H
2
O
[83,88,89]
(Figure 5). The chemical process can be summa-
rized by the following reactions
[86]
:
TiO2Chy!TiO2.e¡ChC/(12)
OH ¡ChC!¢OH (13)
H2OChC!¢OH CHC(14)
OH Corganics !CO2CH2O (15)
According to the DO concentration measured during photoca-
talytic oxidation of the organic pollutants correlated with COD
values obtained by the classical methods, a COD sensor using
TiO
2
for COD estimation was reported.
[90]
The results showed
that the COD values obtained by the sensor were close to those
obtained using the conventional methods. The sensor showed a
long-term stability (RSD D5.7%, 30 days) with 0–20 mg/L lin-
ear range. The proposed method is suitable for COD monitor-
ing in slightly polluted water bodies. However, developing a
12 J. LI ET AL.
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simple, fast, and “clean”sensor for in situ, online monitoring of
COD in water bodies remains a challenge. Based on the COD
sensor using TiO
2
, two oxygen electrodes coupled with a photo-
catalytic column with TiO
2
beads and FIA technique were uti-
lized to measure COD.
[91]
The modified version improved the
accuracy by utilizing the reference oxygen electrode, and accel-
erated the analysis speed through the FIA technique. Thus, this
version could be considered as an alternative approach for in
situ, real-time monitoring, and control of environmental
processes.
The works mentioned above laid a foundation for TiO
2
application among COD sensing materials,
[90,91]
wherein DO
was primarily used as the photoelectron scavengers and signal
indicators. Thereafter, efforts have been devoted to developing
new nano-materials based on TiO
2
. In recent years, the photo-
electron scavengers and signal indicators employed in PcCOD
were highly active oxidants, such as Mn
7C
,
[85,92]
Cr
6C
,
[93]
and
Ce
4C
.
[86,94]
Some synergistic systems, including nano-TiO
2
-
K
2
Cr
2
O
7[95]
and nano-TiO
2
-KMnO
4
,
[84]
were developed with
the linear ranges of 20–500 mg/L and 0.1–320 mg/L,
respectively.
The direct use of TiO
2
for COD measurement has some lim-
itations, such as inadequate UV irradiation and low oxidation
efficiency. To overcome these limitations, TiO
2
should be mod-
ified to improve surface properties, adsorption ability, and pho-
todegradation activity.
[47]
Given the improvements of the
quantum efficiency of semiconductors, surface-fluorinated-
TiO
2[92]
and nano-ZnO/TiO
2[85]
composite films have been
used as photocatalysts for COD determination with 0.1–
280 mg/L and 0.3–10.0 mg/L working calibration ranges,
respectively. Adding Ce
4C
as an electron acceptor can improve
the efficiency of holes. Based on the fluorescence spectrometric
method in determining the concentration change of Ce
3C
pro-
duced by photocatalytic reduction of Ce(SO
4
)
2
, a nano-TiO
2
-
Ce(SO
4
)
2
system was proposed for indirect determination of
COD in slightly and moderately polluted water samples,
[86]
thereby yielding linear responses for COD values between
0.9 mg/L and 100 mg/L under the optimized conditions. The
method is environment friendly and cheap because HgSO
4
and
Ag
2
SO
4
are not added (no interference when Cl
¡
<2,000 mg/L).
A molecular sieve 4A-TiO
2
-K
2
Cr
2
O
7
coexisted system as a sensor
for COD detection was reported due to its large surface for
immobilization and stable chemical properties provided by
molecular sieve compared with other catalyst carriers, such as
glass, silica gel, clay, and activated carbon. This new approach
was able to overcome the drawbacks, including photo-decay and
difficulties in recycling caused by the generated by direct use of
TiO
2
.
[96]
Nevertheless, 3–15 mg/L linear range with 0.24 mg/L
detection limit can only fit the requirements of COD measure-
ment in clean water bodies such as surface water. A 5-sulfosali-
cylic acid-TiO
2
-KMnO
4
system was proposed for COD
determination to improve the surface properties, adsorption abil-
ity, and photocatalytic activity of TiO
2
.
[47]
Based on visible light-
assisted photodegradation of organic pollutants in this system, an
extensive working calibration range of 0.3–400 mg/L was
obtained with a significantly detection limit of 0.01 mg/L. The
method overcame the shortcomings of the direct use of TiO
2
.In
addition, this method could be employed as an alternative
approach for COD determination with satisfactory results.
The researchers have made numerous efforts to modify or
replace nano materials TiO
2
to obtain semiconductors with
good performance. Quantum dots (QD) are semiconductor
crystals with nanometer-scale.
[97]
The QD core comprises ele-
ments from the II–VI (e.g., CdSe and CdTe), III–V (e.g., InP
and InAs) or IV–VI (e.g., PbSe) group. Given the high quan-
tum yields, photostability, tenability and broad absorption, and
narrow emission of QDs, a fully automated single interface
flow system coupling with an online UV photocatalytic unit,
which using CdTe QDs to promote the photocatalysis of
organic compounds for COD assessment was developed.
[97]
The proposed methodology had a linear range of 1–35 mg/L
and a sampling frequency of approximately 33 h
¡1
. However,
narrow linear scope, complex instruments, and high costs lim-
ited the development of this newly established technique,
thereby providing this work a large improvement space for
future trends and developments.
PcCOD is an effective degradation method due to its high
efficiency, simple operation, mild reaction conditions, and
slight secondary pollution by nano materials (mainly TiO
2
).
ThedirectuseofTiO
2
is limited by its wide bandgap, weak
light absorption properties, and low oxidation efficiency.
Modified TiO
2
nano materials or substitutes for TiO
2
,such
as QD nano techniques, can significantly improve the
adsorption ability and photodegradation activity, thereby
resulting in a high-quality analytical signal that is rapid,
accurate, reproducible, and reliable. Nevertheless, some
drawbacks of PcCOD, including narrow dynamic working
range and relatively poor reproducibility of COD measure-
ments, can be attributed to the easy recombination of the
photogenerated electron-hole pairs.
Photoelectrocatalytic methods for COD determination
Numerous innovative approaches have been proposed to over-
come the disadvantages of the traditional COD determination
methods. Among these techniques, PcCOD and photoelectro-
catalytic approach (PeCOD) have been demonstrated to be the
two most effective methods yet.
[19]
The principle of PeCOD is
Figure 5. Schematic illustration of TiO
2
sensor operation.
[8]
CRITICAL REVIEWS IN ANALYTICAL CHEMISTRY 13
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based on the presence of the photogenerated holes and elec-
trons, which participate in redox reactions with organic com-
pounds in a solution. When the appropriate electrode is
irradiated with UV light, decomposed substances release elec-
trons that transfer to the working electrode, which generates an
analytical signal. According to the current variations, the quan-
tification of COD values is obtained.
[98]
The superior photooxi-
dation and photomineralization abilities by the extremely
active photocarriers, which can oxidize nearly all the organic
pollutants with CO
2
and H
2
O as the final products, contribute
to the rapid and complete decomposition of the two methods
of most of organic pollutants.
As two types of photocatalytic oxidation techniques, the
main differences between PcCOD and PeCOD are the mode
for removing photogenerated electrons and the utilized analyti-
cal signal indicator.
[4]
In PcCOD, the removal mode for photo-
electrons is a redox chemical reaction with an additional
oxidant as the photoelectron scavenger, while the utilized ana-
lytical signal indicator is the concentration difference of the
photoelectron scavenger (i.e., the added oxidative reagent)
before and after the complete photodigestion of injected
organic solution. By contrast, the removal mode for photoelec-
trons is the opposite-direction driving function to the cathode
under the externally applied low bias, while the utilized analyti-
cal signal indicator is the direct collection of a quantum of pho-
toelectrons from the complete photodigestion of the injected
organic solution.
Although PcCOD has been demonstrated to possess several
advantages, its main limitation is low photocatalytic efficiency
which is attributed to the easy recombination of the photogen-
erated electron-hole pairs in discrete TiO
2
particles and coated
nanofilms. Research indicates that PeCOD can be a more effi-
cient and alternative method for COD determination due to
the elimination of the preceding limitation mentioned in com-
parison to PcCOD. By applying an appropriate potential bias
on TiO
2
electrodes, the recombination of electron-hole pairs
can be effectively suppressed.
[11]
An electrode comprising TiO
2
is the key factor for determin-
ing COD by PeCOD. The photoelectrocatalysis efficiency can
be improved by the techniques for the immobilization of TiO
2
nanoparticles on substrates. The immobilization of TiO
2
nano-
particles on substrates, include several techniques (e.g., sol-gel
dip-coating,
[20]
spincoating,
[99]
sputtering,
[100]
CVD,
[101]
and
electrodeposition.
[102]
) Among them, sol-gel dip-coating has
been most extensively used for sensing application.
[103]
Based
on these techniques, 1D TiO
2
nanostructured photoelectrodes,
such as TiO
2
nanotubes (TNTs),
[103,104]
TiO
2
nanorods
(TNRs),
[105]
and TiO
2
nanofibers (TNFs),
[106]
have recently
attracted significant attention due to their superior charge sepa-
ration efficiency and high photocatalytic activity properties.
Researchers fabricated TNTs on Ti sheet as the COD sen-
sor.
[103,104]
The proposed approach only requires approxi-
mately 1–5 minutes for complete oxidation of organics without
further titration. The application of TNRs in determining COD
was investigated, and the results were well consistent with those
obtained by the classical COD determination methods.
[107]
Given the difficulty in obtaining complete degradation of the
organic compounds in bulk solution in a short time, which can
be attributed to the large number of reactants in a traditional
reactor, the application of TNFs in a PDMS [(dimethyl silox-
ane) (PDMS)]-based microfluidic device to perform efficient
photoelectrocatalysis was investigated.
[106]
The practical limit
of determination of 0.95 mg/L COD with a working range of
0–250 mg/L was obtained. A wide dynamic working range and
rapid analytical speed due to high photocatalytic activity were
achieved by this method. Moreover, this method can determine
COD in surface water and slightly polluted water. However, 1D
nanostructured TiO
2
photoelectrodes, including TNTs, TNRs,
TNFs, and coated-TiO
2
nanoparticles film electrode, have two
main inevitable limitations that is, small specific surface areas
and limited use of light source for a wide band gap, thereby
leading to low degradation efficiency of photoelectrocatalysis.
From the viewpoint of solar energy (visible light) utilization,
Cu
2
O-loaded TNTs electrode was used to determine COD
using visible light, in which a detection limit of 15 mg/L and a
linear range of 20–300 mg/L were obtained.
[105]
However, the
lifetime of the electrode was insufficiently desirable because the
Cu
2
O particles suffered from photocorrosion.
TiO
2
modification can enhance the photoelectrocatalytic
efficiency in the visible range. Based on this principle, the
hydrogenated TNR (H-TNRs) arrays were used as a photoelec-
trode.
[108]
The results indicated that H-TNR photoelectrode
exhibited highly sensitive and steady photocurrent to the
organic compounds under visible light with 0–288 mg/L linear
range of COD. However, the dependence of visible light photo-
electrochemical properties of the H-TNR film on nanorod
lengths and its application in detecting COD has not been
investigated. Hierarchical TiO
2
nanotubes (H-TNTs) were fab-
ricated to improve the active area of the nanotube walls and
photocatalytic efficiency.
[11]
A linear range of 0–300 mg/L and
a detection limit of 1 mg/L were obtained. A cost-effective and
miniature photoelectrochemical sensor based on the highly
sensitive organic electrochemical transistor (OECT) using Pt
nanoparticles and modified TiO2 nanotube arrays (Pt-NPs/
TNTAs) as a gate electrode for COD measurement was
reported to significantly decrease the detection limit.
[109]
The
limitations of the conventional PeCOD methods could be elim-
inated by utilizing OECT-based sensors. Therefore, a lower
detection limit of 0.01 mg/L was obtained in comparison to
those of the conventional PeCOD methods.
PeCOD has been demonstrated to be the most promising
alternative due to its lower detection limit and wider linear
range compared with PcCOD. The application of potential bias
can further increase the photooxidation efficiency due to the
suppression of photoelectron and photohole recombination.
The current originating from the photocatalytic degradation of
organic pollutants can be directly and accurately determined as
an analytical signal for COD measurement. Nevertheless,
improvements are still required in terms of photoelectrocata-
lytic activity and stability because of the photogenerated elec-
tron-hole pairs in indiscrete TiO
2
nanoparticles and coated
nanofilms readily recombine, thereby resulting in low sensitiv-
ity and poor catalytic efficiency.
On-site (online) measurements
Developing portable, real-time, and on-site monitoring instru-
ments for determining the concentrations of the pollution
14 J. LI ET AL.
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compounds, such as COD in water bodies, is of great impor-
tance because the measurement objects of water pollution
include time variability, fluidity, and dynamic changes by time,
space, climate, and pollution source.
[16]
A typical COD online analyzer includes three parts, namely,
sample collection system, sample detection system, and data
processing system (Figure 6). In the sample collection system,
the water sample is collected, quantified, and mixed with chem-
ical reagents added in the reagent storage unit by the solenoid
valve, flow meter, and injector. Then, the water sample is trans-
formed to the reactor unit by a constant-flow pump. Based on
different principles of COD detection, data are obtained and
processed by the detection, data processing, and transforming
units.
Currently, commercial COD online monitoring instruments
have five types according to their adopted methods [i.e.,
K
2
Cr
2
O
7
or KMnO
4
oxidation methods (e.g., Hach COD
max
plus and Seres S2000), UV spectrophotometry (e.g., Thermo
Orion 3106 and Shimadzu UVM-4210), O
3
oxidation methods
(e.g., Stip Phoenix-1010), electrochemical oxidation methods
(e.g., Lar Elox100), PeCOD methods (e.g., UV-LED PeCOD
system)]. Rapid, real-time, and on-site detection can be
obtained by the above-mentioned instruments. However, some
drawbacks should also be overcome. In general, the first two
types of instruments have some limitations, including high
costs of maintenance, complex operation, and heavy workload.
In addition, the pipeline corrosion by the mixtures of several
chemical reagents and secondary pollution could not be
avoided. The instruments based on AOPs have the advantages,
including the absence of secondary pollution and rapid analysis
speed, but poor stability and anti-interference were obtained.
Figure 7 is the schematic diagram of the improvement of the
traditional COD measurement methods.
Conclusion and future perspective
This review has identified the main technological strategies of
COD measurement, which is used as an indicator of the
degradability of organic matter in water bodies. This review has
focused on the technological aspect of the evaluation methods
and their advantages and disadvantages.
In an attempt to increase the oxidation efficiency, reduce the
analytical time, and avoid environmental pollution, many alter-
native methods have been investigated and reviewed. These
methods mainly include the following: modified standard
methods (e.g., improved digestion methods, alternative meth-
ods of digestion reagents, optimized spectrophotometric meth-
ods, and optimized COD determination methods prevailing
over chloride interference) and new technologies or methods
(e.g., CL methods, FIA technique, O
3
oxidation technique, elec-
trocatalytic detection methods, PcCOD, and PeCOD).
The traditional detection methods are suitable for deter-
mining COD with the content range of 30–700 mg/L in
slightly or moderately polluted water bodies. These methods
have been extensively adopted by various environment mon-
itoring laboratories due to the use of simple instruments.
However, the limitations of long analytical time, utilization
of toxic and high-cost chemical agents, and insufficient oxi-
dation capability of organic pollutants have restricted their
practical application. To overcome the shortcomings of the
traditional methods, the improved and optimized methods
extend their detection limits, significantly shorten their
Figure 7. Development of the COD determination methods.
Figure 6. Schematic diagram of online COD analyzer.
CRITICAL REVIEWS IN ANALYTICAL CHEMISTRY 15
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detection time, and decrease their detection cost. However,
the environmental pollution caused by chemical reagents
could not be avoided, and the oxidation ability to the refrac-
tory organics was insufficient. FIA, CL, or a combination of
several techniques have also been employed to determine
COD in water bodies. Low detection limit could be achieved,
detection time could be shortened again, and environmental
pollution caused by the reagents could be dramatically
reduced. With the development of science and technology,
AOPs (e.g., O
3
oxidation, electrochemical oxidation, and
especially as PcCOD and PeCOD technologies which utilize
TiO
2
as a photocatalyst to replace the conventional oxidizing
agent) have been recently developed. Given the superior oxi-
dation abilities of illuminated TiO
2
, advantages, such as low
detection limit (i.e., 0.01 mg/L), environment friendly (i.e.,
toxic chemical reagents are not required), and rapid analyti-
cal speed (i.e., 1 minutes), were achieved by these new meth-
ods. The new techniques can provide a reference for the
accurate, rapid, low-cost, and online monitoring determina-
tion of COD. Nevertheless, the reliability and stability of
these new technologies still depend on the breakthrough of
new theories and techniques. A substantial amount of effort
should be devoted to improving the development space of
high COD concentration determination in wastewater
samples.
Funding
Financial support by National Natural Science Foundation of China
(51408264), Practice Innovation Program for University Graduate Stu-
dents of Jiangsu Province (SJLX15_0564), Fundamental Research Funds
for the Central Universities (JUSRP51512), State Key Laboratory of Pollu-
tion Control and Resource Reuse Foundation (PCRRF13022), and Jian-
gnan University Science Foundation (JUSRP11522) is greatly appreciated.
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