Automated Selenium Analyzer for Water Quality Monitoring Page 1

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Automated Selenium Analyzer for

Water Quality Monitoring

VLADIMIR DOZORTSEV, WILLIAM T. DIETZE,

LI XIAO, TraceDetect Inc., Seattle, WA

DEREK A. EGGERT, Southern Research Institute,

Birmingham, AL

Paper #: 

KEYWORDS: Selenium, Waste Water, FGD, Voltammetry, Online Analyzer.

ABSTRACT:

Selenium is naturally occurring trace element which has health benefits as an essential nutrient to 

humans at low exposure levels. However, high exposure levels of selenium may severely impact human 

health. Yet, at levels which are harmless to humans (> 5 ppb) aqueous forms of selenium can pose major 

risks to aquatic and terrestrial wildlife. Pollution of water resources by selenium is a growing worldwide 

problem, resulting from human activities that range from different industrial processes to agricultural 

practices. Risk management strategies based on proper wastewater monitoring and control are critical 

for reducing environmental impacts and preserving aquatic habitats and fishery resources. 

An automated voltammetric analyzer for monitoring aqueous selenium concentrations in various 

waste streams (mining, power plants, IX concentrates, water utilities etc) has been recently developed. 

This analyzer offers performance equal to ICP-CRC-MS in both accuracy and insensitivity to 

interferences found in real samples. In addition, the voltammetric technique is amenable to automation 

and the analyzer can be installed and operated in industrial environments. Flexible analytical procedures 

developed for this measurement technique allow for rapid selenium determination (as well as speciation) 

down to low ppb levels in complex matrices. 

This paper demonstrates the performance of a new analytical technique for Se measurements in 

industrial waters, and contrasts these results to conventional methods such as ICP-CRC-MS. 
Automated Selenium Analyzer for Water Quality Monitoring Page 2

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INTRODUCTION

Combustion of coal for thermoelectric power 

production has contributed sources of selenium (Se) 

to aquatic ecosystems that have adversely affected 

biota [1]. The mass of selenium liberated from coal 

combustion and recovery of this contaminant from 

air emissions is increasing due to a greater demand 

for power and implementation of air pollution 

control systems used at coal-fired power plants for 

decreasing sulfur dioxide (SO2) [2]. The United 

States Environmental Protection Agency (U.S. EPA) 

identified SO2 as a criterion pollutant due to adverse 

effects on human health and welfare [3]. In response 

to Clean Air Act legislation (Clean Air Act, 1990), 

coal-fired power plants are decreasing SO2 emissions 

using flue gas desulfurization (FGD) scrubbers. 

FGD scrubbers typically use a wet scrubbing process 

to contact flue gases with saturated calcium water 

allowing oxidation of SO2 gases and formation of 

calcium sulfite (CaSO3) and calcium sulfate 

(CaSO4). Oxidation of SO2 can be assisted through 

introduction of force air in scrubbing towers. This 

scrubbing process produces an aqueous stream 

referred to as FGD water. In addition to the calcium 

sulfate salts, FGD waters can contain a multitude of 

problematic constituents including arsenic, boron, 

cadmium, chromium, chloride, mercury, selenium, 

and zinc [4]. Of these constituents, selenium is of 

primary concern due its concentration in FGD 

waters and ability to adversely affect aquatic 

organisms at concentrations measured in these 

waters. Selenium is also of concern in these waters 

since it can be difficult to remove, especially in high 

ionic strength waters such as FGD waters. 

Numerous approaches for selenium removal from 

FGD wastewaters have been proposed and can 

include physical treatment, chemical treatment and 

biological treatment. Of these processes, the most 

effective treatment approaches for FGD waters are 

biological (e.g. microbial, wetlands, etc.) and 

chemical (e.g. iron-based additives). FGD waters 

can present significant challenges to these removal 

technologies including frequent changes to the FGD 

wastewater composition, high levels of interfering 

constituents, and changes in selenium speciation.

In order to maintain proper treatment system 

operation and meet strict EPA requirements for 

discharge, selenium concentrations will require 

monitoring on all critical stages of wastewater 

treatment processing. Common practice involving 

manual sample collection with further laboratory 

analysis can be unsatisfactory due to long 

turnaround times. Modern approach to a process 

controlled system requires real-time information 

from critical process stages in order to respond 

immediately on transient process changes. Thus, it is 

of great importance to develop a robust continuous 

selenium analyzer suitable for unattended online 

operation. 

Voltammetric detectors are among most suitable 

candidates to be implemented in automated tools due 

to their simple design, versatility and ease of use to 

the operator. As many as 20+ metal ions can be 

determined at trace and ultra-trace levels using 

advanced electroanalytical techniques [5,6]. Another 

important advantage of the voltammetric over 

competing techniques is the ability to determine 

analyte oxidation states and perform speciation [7,8]. 

However, to date voltammetric sensors have 

suffered from one primary drawback: the sensor 

lifetime is unsatisfactory due to sensor sensitivity 

degradation during operation. The poisoning of the 

electrode surface is one of most significant 

disadvantages characteristic for practically all kinds 

of voltammetric sensors. Such a “tired” sensor 

finally fails and has to be replaced with a new one or 

its surface must be regenerated. Numerous strategies 

have been proposed to maintain stability of the 

sensor during continuous operation: modification of 

the electrode surface with protective membrane, use 

of a disposable sensors and sonication of the 

electrode surface between measurements. Sonication 

has proven to be quite useful for solid electrodes, 

however, has practical limitations and causes 

complication in both sensor and system design. In 

this development, the characteristic drawbacks of 

previous voltammetric devices have been addressed. 

A new sensor has been developed that is selfrenewing and demonstrates:

• high automation and online capability

• high analytical performance and versatility

• high reliability and low maintenance

• compact and chemically inert design

This sensor can continuously measure most heavy 

metal ions of interest, such as Cu, Cd, Pb, Zn, Cr, 

Ni, Co, Se and U in varying water samples. 

Thousands of consecutive measurements can be 

performed using new self regenerated voltammetric 

sensor developed.

In present work we evaluated the applicability of 

the automated SafeGuard III

TM

analyzer developed 

for measuring inorganic selenium: total and 

dissolved in different water and wastewater samples 

and correlate results obtained to conventional 

methods such as ICP-CRC-MS. 
Automated Selenium Analyzer for Water Quality Monitoring Page 3

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EXPERIMENTAL

THE METHOD

Generally, inorganic selenium may exist in 

water in four common oxidation states: Se2-

(selenide); Se0 (elemental); Se (IV) (selenite); and 

Se (VI) (selenate). 

Both selenate and selenite are soluble species while 

selenides and elemental selenium are appreciably 

less soluble.

The method of Cathodic Stripping Voltammetry 

(CSV) can be applied for determination of inorganic 

total dissolved selenium in water samples as well as 

selenite/selenate speciation. However, only selenite 

can be detected using CSV while selenate is 

electrochemically inactive. In this method a 

selenium-copper intermetallic compound is 

quantitatively deposited onto the electrode surface at 

controlled potential (deposition step (1)).

2 Cu2+ + Se4+ + 8 e- → 2 Cu0 + Se0 (1)

Then, the intermetallic compound is reduced in a 

subsequent cathodic sweep and the resulting current 

is recorded (stripping step (2)):

2 Cu0 + Se0 + 2 e- → 2 Cu0 + Se2- (2)

The deposition time is automatically adjusted in 

order to accumulate sufficient intermetallic 

compound to provide the desired signal: lower 

concentration sample use a longer time in order to 

increase sensitivity. The stripping step is analytical 

and produces a peak in stripping current. The linear 

relationship between the voltammetric current and 

the concentration of selenite in the sample is used 

for sample calibration and quantification. To 

determine total dissolved selenium, the sample is 

pretreated to convert selenate into selenite via

reduction in highly concentrated hydrochloric acid

solutions under heating. Selenate concentration can 

be calculated by subtracting selenite amount from 

total selenium value. 

SAMPLE PREPARATION FOR SELENIUM 

SPECIATION

Based on the fact that selenite is the only 

voltammetrically measurable selenium species, we 

have a developed versatile sample preparation 

procedure based on the oxidation-reduction 

(REDOX) chemistry of selenium and sample 

preparation can be specific accordingly to type of 

determination targeted. Distinguishing between total 

selenium, dissolved selenium and selenite/selenate 

speciation can be achieved by choosing appropriate 

sample preparation methods. 

Dissolved selenium: Selenite/selenate speciation

The sample is filtered through 0.45 micron filter 

in order to remove insoluble selenium species and 

then selenite is determined as described above. 

Another portion of the filtered sample was treated to 

convert (reduce) selenate into selenite in an acidic 

media and heat:

(3)

Duration of the reduction step depends on the 

sample matrix, reagent concentrations (acid

concentration) and amount of interfering compounds 

in the sample. Concentration of dissolved selenium 

in the sample was determined as amount of selenite 

found after sample treatment (3). Differentiation 

between selenite and selenate can be made by 

subtracting the selenite value from total dissolved 

selenium concentration. This form of “indirect” 

speciation assumes only selenate and selenite exist 

in the analyzed and may not be a valid assumption 

for FGD waters. 

Total selenium determination

Measuring total selenium involves both soluble 

and insoluble fractions. Conversion of insoluble 

selenium species into dissolved form (mineralization 

step) based on oxidizing the sample under heating in 

acidic conditions:

(4)

After the mineralization step (4) is accomplished, the 

sample is reduced (step 3) and finally measured. 

Concentration of selenite obtained is equivalent to 

the total selenium concentration in the sample. The 

sample preparation procedure developed at 

TraceDetect Inc. allows combining both 

mineralization and reduction steps in single stage 

which decreases the entire treatment time, labor, and 

reagents. Dilution of the sample following this 

sample preparation is 6:1.

Although, some organic compounds can be 

effectively decomposed during mineralization step 

(see Selectivity section below) evaluation of the 

possibility of their determination requires additional 

research and development efforts. 

oxidizer, acid, T

1-3 min

Se6+

Se0

Se2-

Se4+

mineralization 

step:

reducing, acid, T

5-30 min

Se4+ Se6+ reduction 

step:
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MEASUREMENT UNIT AND OPERATING 

PRINCIPLE

A schematic of the flow analysis unit is shown 

in Figure 1. The measurement is made of several 

fluidic subsystems: a sample chamber, flow-through 

voltammetric detector, circulation pump, drain valve 

and degassing coil along with chemical 

pump/chemical manifold assembly for sample and 

reagent delivery into the system. The flow-through 

voltammetric detector is made of chemically inert 

plastic with dimensions of 1”×1”×2” and internal 

volume as low as 100 μl and uses three electrodes: a 

renewable working electrode, reference electrode 

and auxiliary electrode.

The working-electrode is the innovation that 

allows for long-term maintenance-free operation. 

Prior to each measurement, the surface of the 

working electrode is renewed, creating a clean and 

reproducible sensor surface for each measurement. 

Detector design is simple, robust and compatible 

with highly intensive sample flows.

Degassing 

coil

Flow-through

detector

Sample

chamber

Chemical pump/

manifold Circulation

Pump

Drain

Nitrogen

Figure 1: Schematic of the analysis unit.

Prepared sample and buffer solution containing 

copper ions are sequentially loaded into the sample 

chamber using pump/chemical manifold assembly 

(see Figure 1). As all the chemicals are added they 

are circulated through the detector and degassing 

coil. Dissolved oxygen is removed by passing 

nitrogen gas through the degassing coil along with 

the treated sample. Next a new electrode is formed 

and accumulation step is initiated, followed by 

voltammetric scan. Each measurement is preceded 

by formation of a new electrode.

THE ANALYZER

The analyzer with optional auto-sampler is 

shown in Figure 2. The analyzer is a compact bench 

top system which can be located at-line or in an onsite quality assurance lab. Flexible software package 

provides an ease-of-use operating interface, data 

logging and storage. After loading the sample and 

starting the batch by selecting the “measure” button 

automated operation reports sample results in 30 min 

on computer display. For higher sample throughput 

the auto-sampler can be easily attached to the system 

and loaded with up to 42 samples. An automated 

voltammetric detector developed is long-living and 

robust. Hundreds of tests can be performed without 

detector replacement or maintenance. The analyzer 

is economical and environmentally friendly due to 

very low reagent consumption.

Figure 2: Selenium analyzer with auto-sampler. 

RESULTS AND DISCUSSION

CHARACTERISATION OF ANALYZER 

PERFORMANCE

Linear range and Limit of Detection (LOD)

Detector response (peak current) was 

measured as a function of concentration of 

selenite (Figure 3) to test the linearity of the 

analyzer. The lowest concentration of the linear 

dynamic range is the lower limit of detection

(LOD), while the largest concentration 

measurable is that where the detector response 

becomes non linear due to sensor saturation. As it 

can be seen from Figure 3 under the chosen 

conditions the voltammetric signal increases 

linearly with concentration of selenite from 

1.0μg/L to about 70μg/L. As the concentration of 

selenite exceeds 70μg/L the voltammetric signal 

(current) stops growing. Under the chosen 

conditions the LOD estimated is about to 1μg/L. 

The linear dynamic range can be extended 

significantly by controlling measurement 

parameters (deposition time and hydrodynamic 
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conditions during deposition step) in accordance 

with expected concentration of analyte. 

0

100

200

300

400

500

600

700

800

0 10 20 30 40 50 60 70 80 90 100 110

Selenite, μg/L 

Current, nA

Figure 3: Selenite analytical signal plotted against 

concentration of selenite. Solution composition: 2M HCl; 

0.4 M copper. Deposition time 40s.

Quantification of the results by Method of Standard 

Additions

In highly contaminated and frequently changing 

FGD wastes the sensitivity of the sensor can be 

affected by factors others than concentration of 

element of interest (interferences, temperature etc). 

Thus, using a calibration curve constructed in a 

synthetic solution or sample blank matrix may cause 

significant measurement inaccuracies. However, the 

method of standard additions avoids this drawback 

by constructing a calibration curve within the actual 

sample matrix (Figure 4).

Figure 4: Voltammetric scans of determination of 

10μg/L selenite sample and two subsequent additions.

Accuracy and precision

Analytical performance of the selenium analyzer 

was evaluated by measuring synthetic test solutions 

as well as spiked real samples. Selenite 

determination results in spiked 2M hydrochloric acid 

samples are shown in Figure 5. As it follows from 

data obtained, selenite can be determined with 

accuracy and precision of 1μg/L or ±10% (whatever 

is larger) in concentration range from 1.0 to 35 μg/L 

using CSV.

y = 0.9636x - 0.1162

R2

 = 0.9921

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30 35 40

Added selenite, μg/L 

Found selenite, μg/L

Figure 5: Determination of selenite in synthetic samples 

using CSV. Concentration range: 1.0 to 35μg/L. 

Selectivity of the method

Selectivity is a highly important quality criterion 

of an analytical method reflecting its capability to

deliver signals that are free from interferences. 

Generally, voltammetric interferences can be divided 

into two kinds: ionic (cationic, anionic) and 

interferences due to surface active materials 

(organics, chelating agents etc). FGD wastewater 

may contain all of these interferences at very high 

levels. In some cases foreign metal ions may 

interfere with analytical signal by overlapping the 

latter or by affecting its sensitivity (competing). The 

effect of organics (surfactants) is more complex and 

may involve sensitivity degradation, signal 

overlapping, etc. In some specific cases metal ion 

interferences can be eliminated or minimized by 

optimizing measurement conditions (via choice of 

buffer, accumulation potential etc). However, 

eliminating interferences due to organics requires

sample pretreatment or electrode protection. 

The influence of different metal ions, some 

anions, and some organics was studied. It was found 

that heavy metals such as Fe, Mn, As, Zn, Cu, Hg, 

V, Ni, Mo at high concentrations have negligible

effect on selenium signal stability and sensitivity

[9,10]. In addition, effect of anions, such as sulfates, 

chlorides etc. on selenium measurement is minimal

[11]. Apparently, the well-defined selenium signal 

obtained (Figure 4) is due to the highly selective 

nature of the co-precipitation reaction with copper.

However, the presence of high amounts of 

organics and surfactants may affect the sensitivity of 

the sensor, probably due to their adsorption onto the 

sensor surface. In order to minimize impact of
Automated Selenium Analyzer for Water Quality Monitoring Page 6

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organics on sensor behavior the sample is oxidized 

in acidic media prior to reduction with HCl: highly 

oxidizing conditions cause effective decomposition 

of organic molecules and reduces their impact on the 

sensor. 

Sensor Stability

The dynamic characteristics of the analyzer such 

as long-term stability, response time and sample 

throughput reflect general reliability of the system 

and its ability to react on transient concentration 

variations. High dynamic characteristics are essential 

for continuous online implementation of the 

analyzer. The suitability of the renewable electrode 

was tested by carrying out repetitive measurements 

of 2μg/L selenite sample. Over 220 consecutive 

measurements have been made. The results of 

measurements 181-221 are shown in Figure 6. The 

high accuracy and precision of the data 

(1.89±0.33μg/l) indicates high overall system 

stability and reliable electrode behavior.

0.4

0.8

1.2

1.6

2

2.4

2.8

181 183 185 187 189 191 193 195 197 199 201 203 205 207 209 211 213 215 217 219 221

Test progress

Selenite, μg/L 

Figure 6: Fraction of the accumulative results obtained 

in synthetic solution containing 2.0μg/L of selenite. 

Dynamic characteristics of the analyzer 

The transient response of the detector after 

stepwise changes in the analyte concentration in 

concentration range from virtual zero (blank) to 

20μg/L is shown in Figure 7. Four consecutive 

measurements were performed on each sample 

concentration in following order: blank; 10μg/L; 

2μg/L; 20μg/L; 5μg/L; blank. As it is follows from 

the diagram, in first reading after switching from 

lower to higher sample concentration the detector 

reading attained about 90% of new steady-state 

value. A similar pattern was observed when sample 

was switched from 20μg/L level to blank. 

0

2

4

6

8

10

12

14

16

18

20

22

0 4 8 12 16 20 24

Step function progress

Response, μg/L 

Figure 7: Detector response plotted due to a step change 

in selenite concentration between zero and 20μg/L. 

The sample throughput affects both cost 

effectiveness of the equipment and reflects its ability 

to produce real-time data, which is critical for online 

process control capabilities. Due to miniature flow 

through detector design and long term sensor 

stability the sample overall measurement time 

(including sample pre treatment) does not exceeds 

60 min. It addition, miniature fluidics design is 

economical (50-100μ/L reagent per test) and

environmentally friendly.

FGD WASTEWATER SAMPLES 

MEASUREMENT RESULTS

Applicability of the selenium analyzer 

developed for measuring different FGD untreated 

and treated water samples has been evaluated versus 

inductively coupled plasma mass spectroscopy (ICPMS). FGD samples used for this study were 

collected on-site and immediately transported to a 

wet laboratory for preservation using 2% (v/v) trace 

metal grade nitric acid (Fisher Scientific). Aliquots 

from these samples were used for ICP-MS and 

voltammetric analyses with dilutions were made 

according to previous sample information. An 

Agilent 7700x ICP-MS was used for selenium 

analyses using hydrogen gas (i.e. reaction gas). The 

78Se isotope was monitored as the analytical line 

and 72Ge or 115In were used as the internal 

standard. Before analyses, the ICP-MS was 

specifically tuned for Se and other system 

parameters (e.g. oxides, etc.). Quality control 

assurances and checks followed similar procedures 

addressed in EPA’s 200.8 method. 

Sample preparation prior to voltammetric

measurement was the same for all samples. Each 

sample was preserved in acid, then mineralized 

according to (4) and reduced according to (3). 

Dilutions were made to bring the total selenium 

concentration to the range of 5 – 50 ppb. A typical 
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result for treated FGD wastewater is shown in 

Figure 8. Compared to synthetic samples, Figure 4, 

the background current is higher in the FGD 

samples, however the sensitivity is similar and peak 

shape is unchanged. It is clear from the figure that 

there are no serious interferences in actual FGD 

samples.

Figure 8: Voltammetric scans for total selenium of 

treated FGD wastewater showing the treated sample and 

two subsequent additions. This sample diluted 6:1 prior to 

measurement. The measured selenium concentration is 

39ppb before dilution.

Comparative results of over seventy different 

process water samples in wide concentration range 

(below 10 μg/L to over 4 mg/L) are shown in Figure 

9. All ICP-MS measurements were performed at the 

Southern Research Institute. Of the voltammetric

measurements, approximately half of the samples 

were analyzed at TraceDetect and the remaining 

samples were analyzed on a second unit at the 

Southern Research Institute. The samples with 

concentration below 1 mg/L represent treated FGD 

waste while samples with concentrations higher then 

1 mg/L are influent FGD waste. As it is seen from 

Figure 9, results obtained with selenium analyzer are 

in generally very good agreement with those 

obtained with ICP-MS standard method (20% 

difference or less). Apparently, higher difference 

observed in several measurements (mostly in 

samples with concentrations over 500 μg/L) is due 

to possible inaccuracies associated with sample 

handling: dilutions, particulates presence etc. Based 

on measurement results obtained, the voltammetric 

selenium analyzer detection limit can be estimated 

as 4 μg/L with accuracy of 4 μg/L or ± 20 % 

whatever is larger. This corresponds to a detection 

limit in the treated sample below 1 ug/L.

Figure 9: Correlation results between ICP-CRS-MS and 

the voltammetric selenium analyzer (μg/L). 

CONCLUSIONS

Performance of new automated selenium 

analyzer has been evaluated versus standard ICP-MS 

laboratory method. Evaluation results demonstrated 

high applicability of the analyzer for measuring 

different FGD water samples in wide concentration 

range. Detection limit as low as 4 μg/L allows 

accurate analysis of FGD effluent samples while 

robust sensor behavior provides long term stability 

of the measurements and reduces both maintenance 

and labor. A robust sample preparation routine has 

been developed that allows reliable conversion of 

different selenium forms into the measurable selenite 

form and suitable for automation. 

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3. Rubin E.S., Yeh, S., Hounshell, D .A., Inter J of 

Energy Technol and Policy.2004, 2, 52-69.

4. Mierzejewski, M.K., 1991. The Elimination of 

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y = 0.9628x + 29.393

R 2

 = 0.9891

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 500 1000 1500 2000 2500 3000 3500 4000 4500

IC P -MS

S G III T race D etect 

y = 1.2326x - 12.05

R 2

 = 0.9595

0

100

200

300

400

500

0 100 200 300 400 500
Automated Selenium Analyzer for Water Quality Monitoring Page 8

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9. A. P. Aldrich, C.M.G. van den Berg, 

Electroanalysis, 1998, 10, 369

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