Automated Selenium Analyzer for Wate r Quality Monitoring Page 1

Page 1 

# Automated Selenium Analyzer for W ater 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, Voltammetr y, 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 severe ly impact human health. Yet, at levels whic h are harmless to humans ( > 5 ppb) aqueous forms of selenium can pose major risk s to aquatic and terrestrial wildlife. Pollution of water resources by selenium is a growing worldwide problem , resulting from human activities that rang e from different indust rial 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 analyze r 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 i nterferences 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 f or 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 Wate r Quality Monitoring Page 2

Page 2 

## 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 a nd 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 (SO 2 ) [2] . The United States Environmental Protection Agency (U.S. EPA) identified SO 2 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 SO 2 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 SO 2 gases and formation of calcium sulfite (CaSO 3 ) and calcium sulfate (CaSO 4 ). Oxidation of SO 2 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 incl uding 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 th ese 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 physi cal 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 signifi cant 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 lon g 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. Th u s, it is of great importance to develop a robust continuous sele nium analyzer suitable for unattended online operation. Voltammetric detectors are among most suitable candidates to be implemented in automated tools due to their simpl e design , versatil ity and ease of use to the operator. As many as 20+ metal ions can b e 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 ]. How ever, 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 charac teristic for practically all kinds of voltammetric sensors. Such a “tired” sensor finally fails and has to b e replaced with a new one or it s surface must be regenerated. Numerous strategies have been proposed to maintain stability of the sensor during cont inuous 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 practica l 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 self

renewing and demonstrat es :

• high aut omation and online capability

• high analytical performance and versatility

• high reliability and low maintenance

• compact and chemically inert design
Th is sensor can continuously measur e most heavy metal ions of interest , such as Cu, Cd, Pb, Zn, Cr, Ni, Co, S e 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 measur ing 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 Wate r Quality Monitoring Page 3

Page 3 

## EXPERIMENTAL

THE METHOD Generally, inorganic selenium may exist in water in four common oxidat ion states: Se 2

- (selenide); Se 0 (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

- copp er intermetallic compound is quantitatively deposited onto the electrode surface at controlled potential (deposition step (1))

. 2 Cu 2+ + Se 4+ + 8 e - → 2 Cu 0 + Se 0 (1) Then, the intermetallic compound is reduced in a subsequent cathodic sweep and the result ing current is recorded ( stripping step (2)):
2 Cu0 + Se0 + 2 e

- → 2 Cu0 + Se2

- (2)
The deposition time is automatically adjusted i n order to accumul ate 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 l inear relationship between t he voltammetric current and the concentration of selenite in the sample is used for sample calibration and quantification. To determin e total dissolved s elenium

, the sample is pretreated to convert selenate into selenite via reduction in highly concentrate d 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 measura ble 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 tot al 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 inso luble 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 mat r ix, 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). Diff erentiation 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 assumpti on 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 acidi c 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

s ample 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. o xidizer, acid , T 1

- 3 min Se 6+ Se 0
Se 2

- Se 4+ m ineralization step: reducing, acid, T 5

- 30 min Se 4+ Se 6+ reduction step:Automated Selenium Analyzer for Wate r Quality Monitoring Page 4

Page 4 MEASUREMENT UNIT AND O PERATING PRINCIPLE
A s chematic of the flow an alysis unit is shown in Figure 1

. The m easurement 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 10 0 μl and uses three electrodes: a renewable working electrode, reference electrode and a uxiliary electrode. The w orking

- electrode is the innovation that allows for long

- term maintenance

- free operation. Prior to each measurement, the surface of the working e lectrode is renewed , creating a clean and reproducible sensor surface for each measuremen t. Detector design is simple

, robust and compatible with highly in tensi ve sample flow s.

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 c irculated 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 sca n. Each measurement is preceded b y 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 on

site quality assurance lab. F lexible 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 w ithout detector replacement or maintenance. The analyzer is economical and environmentally friendly due to very low reagent consumption

.

Figure 2 : S elenium analyzer with auto

- sampler.



## RESULTS AND DISCUSSION

CHARACTERISATION OF A NALYZER PERFORMANCE Linear range and Limit of Detection (LOD)
D etector response (peak current) was measured as a function of concentration of selenite ( Figure 3 ) to test the linearity of the analyzer

. The lowest concen tration of the linear dynamic range is the lower limit of detection (L O D) , while t he 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 1.0 μg/L to about 70μg/L. As the concentration of selenite exceeds 70μg/L the voltammetric signal (current) stops growing

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

. The linear dynamic range can be extended significantly by controlling measurement parameters (deposition time and hydrodynamicAutomated Selenium Analyzer for Wate r Quality Monitoring Page 5

Page 5 conditions during deposition step) in accordance with expected concentration o f analyte.

0 100 200 300 400 500 600 700 800

0 10 20 30 40 50 60 70 80 90 100 110 Selenite, μg/LCurrent, nA

Figure 3 : S e lenite 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 ).

Fi gure 4 : Voltammetric scans of determination of 10μg/L selenite sample and two subsequent additions.

A ccuracy 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
R 2 = 0.9921

0 5 10 15 20 25 30 35 40

0 5 10 15 20 25 30 35 40 Added selenite, μg/LFound 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 h ighly 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 lat t er or by affecting its sensi tivity (competing). The

e ffect 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 ( v ia choice of buffer, accumulation potential etc). However, eliminating interferences due to organics require s sample pretreatment or electrode protection. The influence of different metal ions, some anions

, and some organics was studied. It was found tha t 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 , e ffect 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 presen ce of high amounts of organics and surfactan ts may affect the sensitivity of the sensor , probably due to their adsorption onto the sensor surface. In order to minimize impact ofAutomated Selenium Analyzer for Wate r Quality Monitoring Page 6

Page 6 organics on sensor behavior the sample is oxidized in acidic media prior to reduction with HCl: h ighly oxidizing condition s cause effective decomposition of organic molecules and reduces their impact on the

sensor. Sensor Stability Th e 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. H igh dynamic characteristics are essential for continuous online implementation of the analyzer. The s uitability of the renewable electrode was tested by carrying out repetitive measure ments of 2 μg/ L selenite sample. Over 2 2 0 cons ecutive
measurements have been made. The results of
measurements 1 81

- 2 21 are shown in Figure 6 . The high accuracy and precision of the data ( 1. 89± 0.33 μg/l) indicates high ov erall 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 progressSelenite, 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 respon se 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

concen tration 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 simil ar 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 progressResponse, μg/L

Figure 7 : Detector response plotted due to a step change in selenite concentration between zero and 20μg/L.

T he sample throughput affects both cost ef

fectiveness of the equipment and reflects its ability to produce real

- time data , which is critical for online process control capabilities. Due to miniature f low through detector d esign and long term sensor stability the sample overall measurement time (in cluding 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 develo ped for measuring different FGD untreated and treated water samples has been evaluated versus inductively coupled plasma mass spectroscopy (ICP

MS). 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 fo r 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

ac cording to (4) and reduced according to (3). Dilutions were made to bring the total selenium concentration to the range of 5

– 50 ppb. A typicalAutomated Selenium Analyzer for Wate r Quality Monitoring Page 7

Page 7 result for treated FGD wastewater is shown in Figure 8

. Compared to synth etic 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 m easured selenium concentration i s 39ppb before dilution

.

Co mparative 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 t he Southern Research Institute. Of the voltammetric measurements, a pproximately 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 bel ow 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 agreeme nt 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 handlin g: 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 (μ g/L ).



## CONCLUSIONS

Performance of new automated selenium analyzer has been evaluat ed versus standard ICP

- MS laboratory method. Evaluation results demonstrated high applicability of the analyzer for measuring different FGD water samples in wide concentra tion 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 r obust sample preparation routine has been developed that allows reliable conversion of different selenium forms into the measu rable selenite form and suitable for automation.



## REFERENCES

1. Lemly, A. D., Aquatic Toxicology. 2002, 57, 39

- 49. 2. Feeley T. et al. 2005. Addressing the Critical Link between Fossil Energy and Water, The Department of Energy/Office of Fossil Energy’s Water

- Related Research, Development, and Demonstration Programs 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 Pollutants from FGD Wastewaters, The 1991 SO2 Control Symposi um, Washington, DC. 5. M. G. Paneli, A. Voulgaropoulos, Electroanalysis, 1993, 5, 355 6. J Barek, A G Fogg, A. Critical Reviews in Analytical Chemistry, 2001, 31, 291 7. M. G. Paneli, A. Voulgaropoulos, Electroanalysis, 1993, 5, 355 8. O. Domínguez, M. J. Arcos, Elec troanalysis, 2000, 12, 449 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 -MSS 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 500Automated Selenium Analyzer for Wate r Quality Monitoring Page 8

Page 8 9. A. P. Aldrich, C.M.G. van den Berg, Electroanalysis, 1998, 10, 369 10. R. Mohamed, L.W.Lee, Jornal Teknology, 2006, 44, 55

- 66 11. M. Korolczuk, M. Grabarczuk. Electroanalysis, 2003,15, 821