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Theses and Dissertations
2011
Characteristics and applications of cobalt-based
phosphate microelectrodes for internal phosphorus
loading
Xue Ding
The University of Toledo
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Recommended Citation
Ding, Xue, "Characteristics and applications of cobalt-based phosphate microelectrodes for internal phosphorus loading" (2011).
Theses and Dissertations. 559.
http://utdr.utoledo.edu/theses-dissertations/559
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A Thesis
entitled
Characteristics and Applications of Cobalt-Based Phosphate Microelectrodes
for Internal Phosphorus Loading
by
Xue Ding
Submitted to the Graduate Faculty as partial fulfillment of the requirements
for Master of Science Degree in Civil Engineering
Dr. Youngwoo Seo, Committee Chair
Dr. Cyndee L. Gruden, Committee Member
Dr. Defne Apul, Committee Member
Dr. Patricia R. Komuniecki,
Dean College of Graduate Studies
The University of Toledo
December 2011
An Abstract of
Characteristics and Applications of Cobalt-Based Phosphate Microelectrodes
for Internal Phosphorus Loading
by
Xue Ding
Submitted to the Graduate Faculty as partial fulfillment of the requirements
for the Master of Science Degree in Civil Engineering
The University of Toledo
December 2011
Monitoring phosphate concentration is very important to control eutrophication in the
water systems. In this study, cobalt-based sensors were modified, characterized and tested
to improve detection limits for phosphate. The effectiveness of surface modification on
the performance of a modified cobalt-based microelectrode was fully examined for its
characteristics, which includes: detection limit, response time, selectivity, interference
with ions (sulfate, nitrate, and nitrite) and dissolved oxygen (DO) interference. To assess
the performance of the sensors for environmental applications, emphasis was also placed
on monitoring phosphate release from Lake Erie sediments (internal SRP loading in
sediments) using the sensors. After increasing phosphate sensing area and re-modifying
the surface, phosphate sensors showed increased detection limit up to 10-8 M
concentration of phosphate ion. Re-modified phosphate sensors showed improved
sensitivity and could be applied to both water and sediment samples. However, signal
interferences (especially with oxygen) required consideration during sample analysis with
iii
phosphate sensors. The phosphate microsensor showed its ability to measure sediment
SRP profiling without disturbing sediment structure. And also, diffusion coefficients
under oxic and anoxic conditions were calculated. Experiments showed that phosphate
sensors may be an appropriate tool for measurement of phosphate in lake water and
sediment samples.
iv
Table of Contents
Abstract………………………………………………………………………………….iii
Table of Contents………………………………………………………………………...v
List of Tables………………………………………………………………………….vii
List of Figures………………………………………………………………………..viii
Chapter 1 Introduction………………………………………………………………….1
Chapter 2 Methods…………….....…………………………………………………….6
2.1 Microelectrode Fabrication…………………………….……………………6
2.1.1 Phosphate Microelectrode ……………………………………………….6
2.1.2 ORP (Redox Potential) Microelectrode ……………………………….8
2.2 Ion and DO (Dissolved Oxygen) Interference Tests………………………...9
2.3 Water and Sediment Samples…………………………………………….....11
Chapter 3 Results and Discussion……………………………………………………13
3.1 Calibration of Microelectrodes……………………………………………...13
3.1.1 Phosphate Microelectrode Calibration…………………………………13
3.1.2 ORP Microelectrode Calibration…………………………………….…15
3.2 Ion Interference………………………………………………………….. 16
3.3 DO (Dissolved Oxygen) Interference……………………………………….18
3.4 Wastewater and Lake Water Sample Measurement…....................................19
v
3.5 Position Interference during Sediment Profiling……………………………20
3.6 Reversibility of Profiling Measurement……………………………………..21
3.7 Sediment SRP Profiling Under Different Oxygen Concentrations …………22
3.8 Sediment ORP Profiling Under Different Oxygen Concentrations…………25
3.9 ORP versus SRP Concentration in Sediment……………………………..26
3.10 Diffusivity Calculations…………………………………………………..27
Chapter 4 Conclusions………………………………………………………………….33
References………………………………………………………………………………35
vi
List of Tables
2.1 Concentrations used for ion interference measurements ……………………………10
3.1 Phosphate measurement comparison………………………………………………20
vii
List of Figures
2-1 Structure of phosphate microelectrode ……………………..……………………8
2-2 Experimental setup for sediment profiling measurements……………….……..…12
3-1 Calibration curve of a phosphate microelectrode…………………………..…14
3-2 Response changes of microelectrodes over time…....………………………….……14
3-3 Calibration curve of a ORP microelectrode…...……………………………………15
3-4 Sulfate ion interference ………………………………………………………...… 16
3-5 Nitrite ion interference ...……………………………………………………………17
3-6 Nitrate ion interference…...……………………………………………………….…17
3-7 Dissolved oxygen interferenc …………………………………………………..…19
3-8 Reversibility of sediment profiling measurement …..………………………………22
3-9 Sediment SRP profiling under 0 mM DO and 0.13 mM DO concentration………25
3-10 Sediment ORP profiling under 0 mM DO and 0.13 mM DO concentration ………26
3-11 ORP versus SRP concentration in sediment …………………………………… 27
3-12 Ln [1-(C-Cs)/ (Cf-Cs)] versus (x-xs) in sediment (DO is 0.13 mM) …………… 31
3-13 Ln [1-(C-Cs)/ (Cs-Cb)] versus (x-xs) in bulk solution (DO is 0.13 mM) ……… 31
3-14 Ln [1-(C-Cs)/ (Cf-Cs)] versus (x-xs) in sediment (DO is 0 mM)………………… 32
3-15 Ln [1-(C-Cs)/ (Cs-Cb)] versus (x-xs) in bulk solution (DO is 0 mM)…………… 32
viii
Chapter 1
Introduction
As a result of anthropogenic polluted sewage effluent and agricultural fertilizers release
into the water or soil table, phosphorus is considered a major concern for universal
environmental managers as it contributes to the eutrophication of the water body (Ritter
et al. 2002, Sharpley et al. 1994). In a water system, phosphorus loading consists of two
major parts: the external phosphorus loading and the internal phosphorus loading.
External phosphorus loading refers to the phosphorus load deposited into the water
column while internal phosphorus loading refers to the accumulation of phosphorus in the
sediment (Ekholm et al. 1997). The retention of phosphorus in sediment is the difference
between the downward flux (sedimentation of particles entering the lake or produced in
lake water) and the upward flux (release of phosphorus by concentration gradients or
transport mechanisms or decomposition of organic matters). Studies show that
phosphorus becomes accumulated in sediment when external phosphorus loading is high
(Granéli 1999, Søndergaard et al. 2003). Also, the recovery periods of water quality in
lakes after the reduction of the external phosphorus loading are lengthened by release of
the internal phosphorus load (Garber and Hartman 1985, Molen and Boers 1994,
Søndergaard et al. 1999, Steinman et al. 2004). As a major influence on the duration of
recovery, the amount of phosphorus in sediment is very large compared to amount of
1
phosphorus in water column, where reports indicate that phosphorus content in sediment
can be approximately 1000 times higher than the concentration found in the water
column (Rivas et al. 2000). Thus, even a small phosphorus release from sediment may
significantly affect lake water concentration (Pettersson 1998). The surface layer of the
sediment is considered especially important in shallow lakes, as it might prevent the
internal phosphorus from being released into the water column (Søndergaard et al. 2003).
It is well accepted that the mechanism of phosphorus exchange between sediment and the
water column is highly complicated, involving mechanical, physical, chemical and
biological processes. Accordingly, numerous mechanisms have been proposed to explain
for the process, based upon different overlying conditions and assumptions. Furthermore,
mechanisms may act in concert with various environmental factors to drive the process,
where phosphorus release among different lakes or ecosystems may be governed by
different mechanisms (Pettersson 1998).
In order to prevent the eutrophication of lakes, many measures have been taken to control
external phosphorus loading, such as encouraging the use of phosphorus free fertilizes
and detergents, discouraging large flocks of waterfowl, installing new sewer systems,
building sedimentation impoundments, and diverting agricultural and urban run-off
(Carpenter 2008). However, no efficient means has been found to reduce the internal
phosphorus loading release. Various experiments have been conducted to study possible
methods to mitigate internal phosphorus loading, such as dredging (Reddy et al. 2007),
sediment mixing and oxygenation (Gächter and Wehrli 1998, Tsujimura 2004), alum and
iron amendments (Cooke et al. 1993, Deppe and Benndorf 2002, Steinman et al. 2004),
calcium salts (Cooke et al. 1993), and hypolimnetic oxygen injection technology
2
(Nurnberg 1987, Prepas and Burke 1997), etc. However, research regarding effective and
inexpensive ways to reduce phosphorus in sediment has been shown ineffective, thus the
development of new methods is needed.
Various analytical methods have been used for phosphorus determination in sediment or
soil samples. The common methods of analyzing phosphorus in sediment in previous
studies involve two general procedures. First, phosphorus is digested by oxidizing
various forms of phosphorus in a strong acidic medium in order to obtain dissolved
phosphate. The perchloric acid method, the nitric sulphuric acid method and the
persulphate oxidation method are three standard digestion methods (Colina et al. 1996).
Second, after digestion, the quantity of digested phosphate is determined using various
detection techniques such as spectrometry and ion-chromatography methods (Chen et al.
1998, Colina et al. 1996). These techniques provide fairly accurate results due to their
sensitivity and selectivity. However, such analytical methods require complex and timeconsuming sample pretreatment procedures and cannot be directly applied to the samples.
In addtion, the instruments are too expensive for field use and therefore increase the cost
of sample analysis (Kim et al. 2007).
Microelectrodes, sometimes called microsensors, are useful tools that have become
widespread for characterizing environmental samples in recent years. With a tiny
diameter, the microelectrodes are widely used in the in situ monitoring of phosphate in
many environmental applications (Štuĺk et al. 2000). The technology offers several
advantages over standard phosphorus detection methods, for example, low cost, simple
fabrication procedures, portability, and ability to be applied in both direct and indirect
detection of phosphorus. However, few microelectrodes are used for direct determination
3
of phosphorus since they are not commercially available (Carey and Riggan 1994, Chen
et al. 1998). In a previous study, a cobalt wire based phosphate sensor with a small tip
diameter (5-10 um) was successfully fabricated and characterized. The sensor was
successfully employed to directly monitor phosphate ion concentration changes in
activated sludge floc during the anaerobic phase of an enhanced biological phosphorus
removal (EBPR) process (W. H. Lee et al. 2009). However, the sensor showed limited
field application capability beyond samples from wastewater with high phosphate
concentration.
The main goal of this project was to improve the sensitivity (detection limit) of phosphate
microsensors for in-situ soluble reactive phosphorus (SRP) measurement. SRP is largely
comprised of the inorganic orthophosphate (PO4) form of phosphorus. SRP enters the
environment as a component of fertilizers, detergents, and wastewater treatment plant
discharge. This labile phosphorus variant is of great concern as it is directly ingested by
algae and accelerates eutrophication. Thus, in this study, surface modified cobalt-based
sensors were re-modified, characterized and tested to improve detection limits for
phosphate (< 10-5 M) to increase their applicability for environmental measurements.
Then the cobalt-based sensors were applied directly to environment samples to detect
SRP concentration instead of using standard methods to indirectly detect total phosphate.
The effectiveness of surface modification on the performance of a modified cobalt-based
microelectrode was fully examined for its characteristics, including detection limit,
response time, selectivity, reproducibility, life time, interference with ions (sulfate, nitrate,
and nitrite) and dissolved oxygen (DO). The sensor was characterized and tested on
various samples with different composition and concentration range (lake water,
4
wastewater, and sediment samples) to monitor field applicability of a sensor. To assess
the performance of the sensors for real environmental applications, emphasis was placed
on monitoring SRP release from Lake Erie sediments (internal SRP loading from
sediments) using the modified sensors then comparing acquired data to redox potential
values within sediment, which is a known mechanism controlling phosphorus release.
5
Chapter 2
Methods
2.1 Microelectrode fabrication
2.1.1 Phosphate microelectrode
The basic phosphate detection mechanism of the cobalt based phosphate sensor involves
measurement of the voltage output response from different phosphate concentrations in
presence of the sensor. When cobalt wire first contacts with water, a cobalt oxide film is
formed on surface of the sensor tip. Subsequently, multiple reactions occur between
cobalt oxide and phosphate, thus a cobalt phosphate layer is formed on the surface
(Engblom 1999).
3CoO + 2H2PO4- + 2H+
Co3(PO4)2 + 3H2O (pH 4.0)
3CoO +2HPO42- + H2O
Co3(PO4)2 + 4OH- (pH 8.0)
3CoO +2PO43- +3H2O
Co3(PO4)2 + 6OH- (pH 11.0)
In order to construct the phosphate sensor, two fabrication methods were considered.
First, glass pipette barrels were used and tested for phosphate sensor fabrication.
Alternatively, a new type of phosphate sensors was fabricated using polymer based
sealants. For glass pipette based sensor fabrication, glass pipette barrels (O.D.: 1.2mm,
6
I.D.: 0.69mm, 15cm length, Sutter instrument Co.) were purchased, heated and pulled
over the flame. After pulling barrels, a section of cobalt wire (0.1mm diameter, 99.995%
pure, Aldrich Chemical Company) was inserted into the pulled glass micropipette.
Without use of an etching process, the cobalt wire maintained its larger tip size. The
micropipette was then melted in the middle section using a trough heating filament
(Sutter instrument Co.) to completely seal the cobalt wire in the stretched glass pipette
barrel. The tip of the sensor was then beveled using a diamond abrasive plate (BV101684, Sutter instrument Co.) to hone the sensor tip to a 45°angle and expose the cobalt
surface. Then the microelectrode was connected to copper wire by melting a small
section of bismuth alloy (44.7% bismuth, 22.6% lead, 19.1% indium, 8.3% tin and 5.3%
cadmium). (W. H. Lee et al. 2009). Figure 2-1 shows the configuration of the phosphate
microelectrodes.
In addition, an alternative phosphate sensor was fabricated utilizing polymer sealants,
where cobalt wires were dipped in a polymer solution and permitted to solidify. This
polymer coated sensor was then beveled to expose cobalt surface. To measure electrical
sensor response during calibration and sensor performance the experiments, the potential
between the working microelectrode and reference electrode was monitored using a
pH/milli-volt (mV) meter (Model 250, Denver instruments). An Ag/AgCl reference minielectrode (MI-401, Microelectrodes Inc.) was used as the reference electrode. The pH/
mV meter was connected to a computer and data was acquired with a spread data logger
(Balance Talk SL TM, Labtronics Inc.) to record responses at five seconds intervals.
After sensors fabrication, the sensors were pretreated prior to calibration. In detail,
sensors were first immersed into DI water to form a cobalt oxide (CoO) layer on the
7
surface of sensor tips. After reaching a stable potential, the sensors were removed from
DI water and immersed into 10-4 M potassium phosphate (KH2PO4) solution at pH 7 until
a new steady-state potential was obtained.
After the pretreatment, phosphate sensors were calibrated. Several phosphate standard
solutions were prepared using KH2PO4 at a range of 10-1 M to 10-8 M, where solution pH
was maintained at 7. The pH of the solution was adjusted to about 7. Sensor calibration
and experiments were conducted under condition of ambient oxygen levels and room
temperature (20℃).
Figure 2-1- Structure of phosphate microelectrode: left) with glass pipette; right)
with polymer sealant.
2.1.2 ORP (redox potential) microelectrode
The redox potential microelectrode was used to measure potentiometrically the oxidationreduction potentials in a sample. As compared to the phosphate sensor, the core of ORP
microelectrode used platinum wire. A similar fabrication procedure as used for the
8
phosphate microelectrode was employed to construct the ORP microelectrode using same
fabrication devices and materials. After heating and pulling a glass micropipette, the
platinum wire (0.127mm diameter, Aldrich Chemical Company) was inserted. Other
fabrication procedures follow construction of the phosphate microelectrode described
previous.
Unlike other electrodes that measure potential responses as proportional to the
concentration of the chemical species in a solution, a redox potential electrode directly
measures the electrical potential of a solution itself (J. H. Lee et al. 2006). In order to
validate the performance of the redox potential microelectrode, three redox potential
reference solutions were prepared and the responses from the redox potential
microelectrode were evaluated. The three redox standard solutions are pH 4 quinhydrone
reference solution, pH7 quinhydrone reference solution, and ferrous-ferric solution
(Sensorex). The redox potential microelectrode was connected with the Ag/AgCl
reference millielectrode and the pH/mV meter. Data was also obtained by collected using
the spread data logger with five seconds frequency.
2.2 Ion and DO (dissolved oxygen) interference tests
Considering the co-presence of other ions in natural environment and their interference
on phosphate sensors performance, sulfate, nitrite, and nitrate ions were used to
investigate potential ion interferences on the phosphate sensor response. For each ion
interference experiment, interfering ions were kept constant as phosphate ion
concentrations were varied from 10 -3M to 10-6M. Ion interference tests were also
conducted by keeping the phosphate ion concentration constant while interfering ion
9
concentrations were varied. Table 1 shows tested ions and the ranges of tested ion
concentration. The concentrations of three interfering ions were selected based on
reported ion concentrations in sediments (Boström et al. 1988, Boström and Pettersson
1982, Cowan and Boynton 1996, Holmer and Storkholm 2001, Jensen and Andersen
1992, Lovley and Klug 1983, Søndergaard et al. 1993, Xie et al. 2003).
Typical benthic DO concentration in lakes is found to be around 0.1 mM (Archer and
Devol 1992). So, two different oxygen concentrations (0 mM and 0.13 mM) were tested
to monitor dissolved oxygen interference on the performance of phosphate sensor, while
other test conditions such as phosphate concentration, pH, temperature, and distance
between reference electrode and the phosphate sensor were fixed. To create specific
oxygen levels in the water column, 100% nitrogen gas was bubbled into the solution via a
diffuser to achieve 0 mM DO concentration. In 0.13 mM DO test, 10% oxygen and 90%
nitrogen gas was bubbled into the same solution. A micro-oxygen electrode (MI730,
Microelectrodes, Inc.) was used to monitor the dissolved oxygen levels in the solution.
Table 2.1 Concentrations used for ion interference measurements.
Ion
Reagent
SO42-
K2 SO4
0.5
5
15
30
NO3
Na NO3
0.3
0.5
1
2
-
Na NO2
0.1
1
2
4
NO2
Concentrations for ion interference measurement (mg/L)
2.3 Water and Sediment Samples
Performance of fabricated sensors was first evaluated for measurement of water samples.
Tested samples include effluent from a wastewater treatment plant and lake water
10
samples. Sensors were also tested with sediment samples. Sediment samples were
collected from the same site in Lake Erie in June by Dr. Tomas Bridgeman’s research
group. Upon collection, the sediment samples were sealed in plastic containers and
transferred directly to a laboratory, where they were stored in a refrigerator prior to
sensor tests and phosphorus analysis. The total phosphorus (TP) and soluble reactive
phosphorus (SRP) concentrations were tested as 0.9698 mg/g dry weight and 0.0959
mg/g dry weight respectively by Heidelberg Water Quality lab.
In order to monitor SRP release from sediments with the phosphate sensors and ORP
sensors, sediment samples were carefully transferred into a beaker and then saturated
with sampled lake water. The depth of the sediment sample was approximately 5 cm and
the depth of lake water was approximately 10 cm (5 cm above sediment surface). The
beaker was observed under a microscope to increase sediment surface resolution for
sensor application. The phosphate sensors and ORP sensors were connected to a
manipulator, which enhanced precise vertical movement of sensors through the depth of
the sample. After a stable potential was acquired, the position of the sensors was slowly
adjusted to avoid disturbing the sediment structure. During the profiling process, the
Ag/AgCl reference electrode was kept in a static position. Figure 2-2 shows the
experiment setup for sediment profiling measurements. The experiments were conducted
in ambient air and room temperature (20°C). The concentration of phosphorus in surface
water was also measured using ion chromatography (IC-1200, Dionex Company) to
verify the performance of the sensors.
11
Figure 2-2- Experimental setup for sediment profiling measurements.
12
Chapter 3
Results and discussion
3.1 Calibration of microelectrodes
3.1.1 Phosphate microelectrode calibration
Figure 3-1 shows the calibration curve of a well-functioning phosphate sensor. Eight
different concentrations of standard phosphate solutions were prepared in a range of 10-8
to 10-1M KH2PO4 for sensor calibration. The sensors performed with an enhanced
detection limit up to 10-8 M (typically between 10-6 and 10-7 M) concentration of
phosphate ion. Also, the phosphate microelectrodes exhibited good linear response to
phosphate concentration change with a slope of -32.9 mV/ decade. Figure 3-2 shows the
response changes of sensors over time. During the calibration, the response time of
sensors for each standard point could be as low as 5 seconds and usually with an upper
limit less than 1 min except to 10-8 M phosphate solution. The lowest test condition (10-8
M) required approximately 100 seconds for sensors stabilization.
13
M i c ro e le c tro d e re s p o n s e (m V vs . A g /A g C l)
-1 5 0
-2 0 0
Y= -3 2 .9 x - 4 6 8
2
R = 0 .9 8
-2 5 0
-3 0 0
-3 5 0
-4 0 0
-4 5 0
-1 0
-8
-6
-4
L o g [H 3 - X P O 4
-X
-2
0
](M )
Figure 3-1- Calibration curve of a phosphate microelectrode. Errors bars represent
M i c ro e le c tro d e re s p o n s e (m V vs . A g /A g C l)
1 standard deviation.
-2 0 0
-4 0 0
10
10
10
-6 0 0
10
10
10
10
-8 0 0
10
0
20
40
60
80
100
120
140
-8
-7
-6
-5
-4
-3
-2
-1
M
M
M
M
M
M
M
M
160
180
T i m e (s e c )
Figure 3-2- Response changes of the microelectrode over time
14
3.1.2 ORP microelectrode calibration
Figure 3-3 shows the calibration curve of the redox potential microelectrode against the
three redox reference solutions. The redox potential microelectrode performed linearly
with a slope of close to the theoretical value of 1.00. The response time was less than 0.5
min in ferrous-ferric standard solution and pH 4 quinhydrone reference solutions, and
approximately 5 min for pH7 quinhydrone reference solution. In comparison to nominal
redox potentials of the reference solutions, the values obtained with the redox potential
microelectrode were similar. These responses demonstrated that the characteristics of the
redox potential microelectrode meet the criteria set by the American Society for Testing
and Materials (Yu 2000).
Microelectrode potential (mV vs. Ag/AgCl)
500
Y= 1.04X - 6.37
400
R2= 0.99
300
200
100
0
0
100
200
300
400
500
Potential of the reference solutions (mV vs. Ag/AgCl)
Figure 3-3- Calibration curve of a ORP microeletrode.
15
3.2 Ion interference
Three ions (the sulfate, nitrite, and nitrate) commonly found in environmental samples
were tested to evaluate possible interference in the performance of phosphate
microsensors. Figures 3-4, 3-5, and 3-6 show the ion interference tests results. Compared
to calibration curves generated in the absence of an interfering ion, the presence of ions
changed the electrode signal responses for phosphate ion detection. However, all
calibration curves retained a linear response to phosphate ion concentration difference
and the signal response shift resulting from tested ion interference appeared to be minor.
M i c ro e le c tro d e re s p o n s e (m V vs . A g /A g C l)
-2 0 0
0 m g /L S u lfa te
Y 1 = -1 4 .6 X - 3 4 9 .7
0 .5 m g /L S u lfa te
2
R = 0 .9 7
-2 5 0
5 m g /L S u lfa te
1 5 m g /L S u lfa te
Y 2 = -1 2 .3 X - 3 4 3 .6
3 0 m g /L S u lfa te
2
R = 0 .9 9
Y 3 = -1 1 .1 X - 3 5 4 .2
-3 0 0
2
R = 0 .9 9
Y 4 = -1 2 .9 X - 3 8 1 .8
2
R = 0 .9 5
-3 5 0
Y 5 = -1 2 .1 X - 3 9 6 .2
2
R = 0 .7 8
-4 0 0
-8
-7
-6
-5
L o g [H 3 -X P O 4
-4
-X
](M )
Figure 3-4- Sulfate ion interference.
16
-3
-2
M i c ro e le c tro d e re s p o n s e (m V vs . A g /A g C l)
-2 2 0
Y 1 = -2 8 .9 X - 4 0 8 .3
0 m g /L N i tri te
2
-2 4 0
R = 0 .9 9
-2 6 0
Y 2 = -3 5 .3 X - 4 4 8 .1
0 .3 m g /L N i tri te
0 .5 m g /L N i tri te
1 m g /L N i tri te
2 m g /L N i tri te
2
R = 0 .9 9
-2 8 0
Y 3 = -3 3 .9 X - 4 5 3 .8
-3 0 0
2
R = 0 .9 6
-3 2 0
Y 4 = -1 5 .5 X - 3 8 8 .5
2
R = 0 .8 5
-3 4 0
Y 5 = -8 .7 X - 3 6 7 .4
-3 6 0
2
R = 0 .9 3
-3 8 0
-8
-7
-6
-5
L o g [H 3 - X P O 4
-4
-X
-3
-2
](M )
Figure 3-5- Nitrite ion interference.
M i c ro e le c tro d e re s p o n s e (m V vs . A g /A g C l)
-2 5 0
Y 1 = - 1 4 .3 X -3 4 7 .1
0 m g / L N i tra te
0 .1 m g /L N i tra te
2
-2 6 0
R = 0 .9 9
1 m g /L N i tra te
2 m g /L N i tra te
Y 2 = -1 7 .4 X - 3 6 1 .8
-2 7 0
4 m g /L N i tra te
2
R = 0 .9 6
-2 8 0
Y 3 = -9 .4 X -3 3 9 .8
2
R = 0 .9 9
-2 9 0
Y 4 = -5 .2 X - 3 2 2 .4
2
-3 0 0
R = 0 .9 7
-3 1 0
Y 5 = -6 .9 X -3 3 3 .3
R 2 = 0 .9 9
-3 2 0
-8
-7
-6
-5
L o g [H 3 - X P O 4
-4
-X
](M )
Figure 3-6- Nitrate ion interference.
17
-3
-2
3.3 DO (Dissolved Oxygen) interference
Figure 3-7 shows signal response changes of the phosphate microsensor under different
DO concentrations. From experimental results, a 40 mV overall signal response
differences was observed between 0 mM and 0.13 mM DO concentration. This potential
difference might cause the calculated phosphate concentration to differ 10 fold. While ion
interferences on phosphate sensor performance were not significant (signal shifts were
relatively large at very low phosphate concentration), phosphate sensor signals shifted for
all phosphate solution concentrations under 0.13 mM DO. These results indicated that
oxygen interferes with the binding mechanism between cobalt oxide and phosphate,
subsequently decreasing the sensitivity of sensors to phosphate ion(W. H. Lee et al. 2009).
At high phosphate concentrations, signal shift may cause a significant difference in
phosphate concentration measurement. It is suggested that the development of calibration
curves under different DO concentrations as well as DO measurements for samples are
necessary for phosphate sensor applications.
18
Microelectrode response (mV vs. Ag/AgCl)
-230
-240
- 237.0 _
+ 1.7 mV
-250
 40 m V
0.13 mM DO
0 mM DO
-260
-270
_ 1.7 mV
- 277.0 +
-280
-290
0
10
20
30
40
50
60
Time (Sec)
Figure 3-7- DO (dissolved oxygen) interference.
3.4 Wastewater and lake water sample measurement
After calibrating the phosphate microsensor, sensor measurement of phosphate
concentration was tested with various environmental samples. To validate the accuracy of
the phosphate sensor, the phosphate concentration in the samples were also measured
with ion comparison. Table 3.1 shows phosphate measurement comparisons. Some
samples possessed very high ion concentration (with 541 mg/L sulfate concentration),
which interfered with phosphate measurement using the sensors. However, obtained
results from various water samples revealed that the microsensors results correlated well
with IC determination and possess good accuracy to detect phosphate in the sample
solution, especially after DO inference was properly corrected.
19
Table 3.1 Phosphate measurement comparison.
Sample
Source
Ion concentration (mg/L)
(IC generated)
DO
(mM)
Phosphate concentration (mg/L)
Ion Chromatography
Phosphate
sensor
0.18
1.26
1.12*
74.3
0.15
0.2
0.1*
1.8
2.7
0.16
2.66
2.58*
0
541.3
0.16
3.57
2.27*
Nitrite
Nitrate
Sulfate
Wastewater
Plant
0
0
6.6
Surface water
0
30.8
Lake water 1
1.8
Lake water 2
0
* Without data conversion for ion interference
3.5 Position interference during sediment profiling
Before the measurement of sediment profiling using phosphate microelectrodes and ORP
microelectrodes, influence of location of reference electrode on phosphate measurement
was determined.
Distance between phosphate microelectrode and Ag/AgCl reference electrode, and the
location (in sediment or in bulk solution) of the Ag/AgCl reference electrode were
evaluated. The signal changes of the phosphate microelectrodes were tested under these
different conditions. According to the result of experiments, different distances between a
phosphate microsensor and a reference electrode resulted in different responses (±30
mV) during sediment profiling. However, sensor signal shift was negligible (±2 mV)
whether a reference electrode was dipped in sediment samples or in bulk solution phase.
In order to obtain reproducible and consistent sensor responses, the position of the
reference electrode was kept secure at the same position during all the sediment profiling
20
experiments. Also, the phosphate microelectrode was only permitted vertically movement
in order to keep a fixed positional the distance between the Ag/AgCl reference electrode
and the phosphate microelectrode during SRP profiling.
3.6 Reversibility of profiling measurement
To determine the versatility and reproducibility of phosphate sensors measurement for
SRP profile measurement in sediment, measurements were conducted during both
penetration and withdrawal of the sensors from the sample, where signal responses were
then compared. Figure 3-8 shows the measured phosphate concentration profile in
sediment. The potential trends of two processes (insertion and withdrawl) are similar,
especially as the sensor moved through the depth of the sediment. Overall, there were no
significant difference between the inserting process and withdrawing process, which
indicated phosphate microsensors are able to monitor local phosphate concentration
changes in sediment samples without disturbing sediment structure.
21
Microelectrode response (mV vs. Ag/AgCl)
-420
Bulk
-440
Sediment
-460
-480
-500
-520
Insertion
Withdrawl
-540
-560
-580
-6
-4
-2
0
2
4
6
8
10
Depth (mm)
Figure 3-8- Reversibility of sediment profiling measurement.
3.7 Sediment SRP profiling under different oxygen concentrations
Sediment SRP profiling experiments were conducted under controlled DO conditions (0
mM DO and 0.13 mM DO concentration). The profiling experiment collected data from 5
mm above the sediment surface to 25 mm within the sediment sample as shown in Figure
3-9.
First, SRP profile was generated under 0.13 mM DO concentration. The SRP
concentration was 10-7.8 M in bulk solution, which was the same as the concentration
detected by ion chromatography. Before the sensor reached the sediment surface, there
was diffusive boundary layer (DBL) where SRP concentration slightly increased. The
flux of the SRP across this area is considered to be dominated by diffusion (House and
Denison 2002). The thickness of the diffusive boundary layer was estimated to be
22
approximately 0.3 mm. Then, the sensor reached the surface of the sediment where the
phosphate concentration was 10 -7 M. As depth increased, the concentration of phosphate
increased sharply. At a depth of approximately 3 mm from the surface, the concentration
of phosphate was stable and determined as approximately 10-5.7, which was nearly 20 fold
the surface concentration.
The upper 3 mm layer of the sediment where SRP concentration changed sharply is the
oxygenated layer which is called the surface oxic layer and contains dissolved oxygen in
the pore water. According to previous studies, oxygen only penetrates 3-5 mm depth in
sediment (Jorgensen and Revsbech 1985, Revsbech et al. 1980). Accordingly, numerous
studies have shown that the surface oxic layer plays an important role in preventing
phosphorus from penetrating into bulk water. Under oxic conditions, phosphorus is
insolubly bound in sediment by two dominant mechanisms. Chemically, phosphorus is
retained in sediment by fixation to Fe (III) which absorbs phosphorus in an oxidized form.
Biologically, some polyphosphate accumulating bacteria groups, like Acinetobacter spp.,
will store phosphorus as polyphosphate under oxic conditions, while using oxygen as
terminal electron acceptor. Thus, the surface oxic layer of the sediment acts like a
boundary to phosphorus penetration into the water column. Under oxic conditions, the
profile generated during this experiment exhibited a good agreement with the theoretical
profile generated using a Boundary-Layer model and a Triple Zone Model proposed in a
previous study (House and Denison 2002).
Similarly, a SRP profile was then generated under 0 mM DO concentration. With 0 mM
DO concentration in the bulk water, there was not sufficient oxygen to generate a surface
oxic layer. Therefore, the surface of sediment and pore water was anoxic. Accordingly,
23
phosphorus was released from the sediment as a result of Fe (III) reduction and
subsequent dissolution of iron-bound phosphorus complex. In conjunction, the reduced
iron reacts with H2S as the product of simultaneous sulfate reduction in the sulfur cycling
process, and results in the formation of insoluble iron sulfide under anoxic conditions. In
this case, chemical reduction of iron complexes and sulfate in the sediment result in a
more favorable end product. Therefore, the established competition between sulfide and
phosphorus results in the indirect release of phosphorus (Holmer and Storkholm 2001). In
the biological processes, the stored polyphosphate in oxic conditions was used by
polyphosphate bacteria to provide energy in order to store organic carbon as
polyhydroxybutyrate (PHB), and therefore phosphorus was released (Davelaar 1993).
From the figure, the SRP concentration in bulk water increase to 10-3.4 M due to the
phosphorus release from the sediment under anoxic conditions, where the results are
similar to the concentration measured using ion chromatography (10-3.5 M). As the sensor
reached the sediment surface, the SRP concentration decreased. The stable concentration
was obtained at depth of 6 mm, although the concentration was lower than the stable
concentration under oxic condition between 6 mm to 20 mm. This was because
phosphorus of these parts was released and subsequently transported to the upper layer of
the sediment. The SRP concentrations under two conditions finally reached the same
concentration at depth deeper than 20 mm.
24
-2
Bulk
Sediment
Concentration log(M)
-3
-4
-5
-6
-7
0 mM DO
0.13 mM DO
-8
-9
-10
-5
0
5
10
15
20
25
Depth (mm)
Figure 3-9- Sediment SRP profiling under 0 mM DO and 0.13 mM DO
concentration.
3.8 Sediment ORP profiling under different oxygen concentrations
The profiling experiments were conducted to measure redox potential using the ORP
sensor and collected data from about 10 mm above the sediment surface to 23 mm depth
of the sediment sample as shown in Figure 3-10. The oxidation-reduction potential
measured is the reaction potential in sediment, which varies through the sediment depth.
As previous mentioned, the release of phosphorus across the sediment-water interface
was mainly dominated by both chemical and biological processes. These processes, such
as oxidation and reduction of iron, or polyphosphate metabolism of bacteria, are all
redox-dependent processes. Studies have shown that lower potential results in increased
25
reduction of Fe (III). Accordingly, a potential below 200 mv or 230 mv indicates
conditions where Fe (III) is easily reduced and phosphorus are released (Boström et al.
1988, Hayes et al. 1958). Furthermore, redox potential alters bacterial respiration, thereby
bacteria may compete with iron for phosphorus retention (Davelaar 1993, Gächter and
Wehrli 1998). The redox potential under anoxic conditions is always lower than the
potential under oxic conditions between sediment surface and a 3 mm depth, which is the
depth of the oxic layer. Therefore, both redox-dependent chemical processes and redoxdependent biological processes promoted the release of phosphorus from the sedimentwater interface. Under anoxic condition, the potential in bulk water resultantly decreased
to 130 mV, where Fe (III) can be completely reduced.
Microelectrode response (mV vs. Ag/AgCl)
400
Sediment
Bulk
350
300
250
200
0 mM DO
0.13 mM DO
150
100
-15
-10
-5
0
5
10
15
Depth (mm)
Figure 3-10- Sediment ORP profiling
3.9 ORP vs SRP concentration in sediment
26
20
25
Figure 3-11 shows ORP versus SRP concentration in sediment. The curve performed
linearly with a slope of -30.6 M/mV, which indicated the SRP concentration in sediment
is proportional to the ORP potential. Thus, different redox potentials resulted in SRP
concentration changes in sediment. The R2 of the slope was close to 1, which indicated
that the profile generated using ORP microsensors agreed with the profile generated
using phosphate microsensors.
300
Y = -30.6 X +107
R2 = 0.93
290
ORP potential (mV)
280
270
260
250
240
230
220
-6.5
-6.0
-5.5
-5.0
-4.5
-4.0
-X
Log[H3-XPO4 ](M)
Figure 3-11- ORP versus SRP concentration in sediment.
3.10 Diffusivity calculations
The analysis of mass transport in sediment is very important to understand mechanisms
of phosphate loading in sediment, where the effective diffusion coefficient of phosphate
27
release in sediment must be determined. Here, diffusion coefficients under 0 mM DO
concentration and 0.13 mM DO concentration were both calculated and compared.
As discussed by Lewandowski, the diffusion coefficient can be determined by measuring
phosphate concentration profiles (Abrahamson et al. 1996, Lewandowski 1994). Similar
to the method described by Lee, the calculation is based on an assumption that the flux
(
J f ,Xs
) of phosphate diffusion from the sediment to the surface of the sediment is equal
to the phosphate flux (
JW , X s
) across the surface bulk solution (W. H. Lee and Bishop
2010).
In both sediment and bulk solution,
J f ,Xs  Df (
dC
dC
) f , X s  Dw ( ) w, X s  JW , X s
dx
dx
(1)
In sediment,
(
dC
) f , X s  A(Csedi  Cs )
dx
 A( x  xs )  ln(1 
C  Cs
)
Csedi  Cs
(2)
(3)
In bulk solution,
(
dC
)W , X s  B(Cs  Cb )
dx
 B( x  xs )  ln(1 
C  Cs
)
Cs  Cb
28
(4)
(5)
Where, sediment surface xs = 0 cm, x = local depth (cm), Cs = SRP concentration at the
surface (mg/L), C = local SRP concentration (mg/L), Csedi = stable SRP concentration in
sediment, Cb = stable SRP concentration in bulk water. A and B are constants, which can
be determined from the slopes of the Figures 12-15.
First, the diffusion coefficient under 0.13 mM DO concentration (
Df1
) was calculated.
Figure 3-12 shows ln[1-(C-Cs) / (Csedi-Cs)] versus (x-xs) inside the sediment. The slope is
the coefficient A in equation (3). Here, Csedi = 0.20 mg/L as Phosphate, Cs =0.01 mg/L as
Phosphate, and the calculated coefficient A = 7.3454 cm-1.
Figure 3-13 shows ln[1-(C-Cs) / (Cs-Cb)] versus (x-xs) in bulk solution. The slope is the
coefficient B in equation (5). Here, Cb = 0.002 mg/L as Phosphate, and the calculated
coefficient B = 16.146 cm-1.
dC
dC
) f ,Xs
( )W , X s
Therefore, both dx
and dx
can be calculated using equation (2) and (4). The
(
phosphate ion diffusivity (
Dw
) in water is 1.25 ×10-6 cm2/s at 20°C (Krom and Berner
1980). Therefore, the phosphate ion diffusivity in sediment under oxic condition (
Df1
)
was calculated to be 1.16 ×10-7 cm2/s.
Next, the diffusion coefficient under 0 mg/L DO concentration (
Df 2
) was calculated.
Similar to the previous calculations, figure 3-14 shows ln[1-(C-Cs) / (Csedi-Cs)] versus (xxs) inside the sediment. Here, Csedi = 0.057 mg/L as Phosphate, Cs =5.854 mg/L as
Phosphate. The slope in figure is A =8.956 cm-1.
29
Figure 3-15 shows ln[1-(C-Cs) / (Cs-Cb)] versus (x-xs) in bulk solution. Here, Cb = 34.87
mg/L as Phosphate. The slope in figure is B = 5.427 cm-1.
Therefore, the phosphate ion diffusivity in sediment under anoxic condition (
Df 2
) was
calculated to be 3.79 ×10-6 cm2/s.
When comparing the two diffusivity values, the phosphate ion diffusivity under 0 mM
DO concentration (
concentration (
Df1
Df 2
) is larger than the phosphate ion diffusivity under 0.13 mM DO
). Thus, the release rate of SRP is larger under anoxic condition,
which confirmed the result obtained by sediment profiling. Also, the calculated
diffusivity under 0 mM DO concentration (
Df 2
= 3.79 ×10-6 cm2/s) is in good agreement
with the theoretical diffusivity in anoxic sediments (
by a previous study (Krom and Berner 1980).
30
Ds ( PO4 )
= 3.6±1.1×10-6 cm2/s) given
.2
Y = -7.345 X - 0.096
2
R = 0.95
0.0
Ln[1-(C-Cs)/(Cf-Cs)]
-.2
-.4
-.6
-.8
-1.0
-1.2
-1.4
0.00
.02
.04
.06
.08
.10
.12
.14
.16
Distance (X-Xs) (cm)
Figure 3-12- ln[1-(C-Cs)/(Cf-Cs)] versus (x-xs) in sediment (DO is 0.13 mM).
.6
Y = -16.146 X - 0.001
R2 = 0.99
Ln[1-(C-Cs)/(Cs-Cb)]
.5
.4
.3
.2
.1
0.0
-.035
-.030
-.025
-.020
-.015
-.010
-.005
0.000
Distance (X - Xs) (mm)
Figure 3-13- ln[1-(C-Cs)/(Cs-Cb)] versus (x-xs) in bulk solution (DO is 0.13 mM).
31
Y = -8.956X -0.249
R2 = 0.96
0
Ln[1-(C-Cs)/(Cf-Cs)]
-1
-2
-3
-4
-5
-6
-7
0.0
.1
.2
.3
.4
.5
.6
.7
Distance (X-Xs) (cm)
Figure 3-14- ln[1-(C-Cs)/(Cf-Cs)] versus (x-xs) in sediment (DO is 0 mM).
.8
Y = -5.428X - 0.011
R2 = 0.99
Ln[1-(C-Cs)/(Cs-Cb)]
.6
.4
.2
0.0
-.2
-.14
-.12
-.10
-.08
-.06
-.04
-.02
0.00
Distance (X-Xs) (cm)
Figure 3-15- ln[1-(C-Cs)/(Cs-Cb)] versus (x-xs) in bulk solution (DO is 0 mM).
32
Chapter 4
Conclusions
In this study, surface modified cobalt-based sensors were re-modified, characterized and
tested to improve detection limits for phosphate, considering the performance effects
caused by the detection limit, response time, selectivity, interference with ions (sulfate,
nitrate, and ammonia) and DO (dissolved oxygen). After increasing the phosphate
sensing area and re-modifying the surface, phosphate sensors possessed increased
detection capacity up to 10-8 M concentration of phosphate ions. However, signal
interferences (especially with oxygen) needed to be considered and properly addressed
for sample analysis with phosphate microsensors. The response time is less than 1 min
with a wide detection range of range 10 -8 to 10-1 M.
The measurements using various wastewater and lake water samples revealed that the
phosphate microsensors’ results correlated well with Ion-chromatography determination
and possessed good accuracy to detect phosphate in the solution. But results could be
shifted in samples with high interfering ion concentrations. Sediment SRP profiling
experiments were conducted under controlled DO conditions (0 mM DO and 0.13 mM
DO concentration). The reversibility of profiling measurement was determined. There
were no significant differences between the inserting process and withdrawing process,
with indicated the ability of phosphate microsensors to monitor local phosphate
33
concentration changes in sediment samples without disturbing sediment structure. The
different profiles of phosphate under different DO concentration confirmed the theory
that phosphorus is released from the sediment into the water column under anoxic
conditions, while phosphorus is retained in sediment by the surface oxic layer under oxic
conditions. Two diffusion coefficients under different conditions were also calculated
according to the profiling data. The calculated phosphate ion diffusivity under the anoxic
condition (3.79×10-6 cm2/s) was larger than the phosphate ion diffusivity under the oxic
condition (1.16×10-7 cm2/s), which indicated that the release rate of SRP was larger under
anoxic condition. These experimental determinations may enhance further investigations
into sediment retention and release of phosphates.
The phosphate microelectrode in this study demonstrated several advantages over current
analytical methods, e.g., low cost, simplicity, portability, wide range, quick response, and
ability to directly measure environment samples. Overall, the successful characterization
and application of phosphate microelectrode and ORP microelectrode showed that that
the cobalt-based phosphate microelectrode and platinum-based ORP microelectrode
could be very useful tools measuring phosphate in lake water and sediment samples.
34
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