P S ANALYTICAL

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Tel: +44 (0) 1689 891 211 Fax: +44 (0) 1689 896009 E-mail: psa@psanalytical.com 

USA: 1761 W. Hillsboro Blvd., Suite 318, Deerfield Beach, FL 33442, USA

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Application Note 142

ONLINE DETERMINATION OF SELENIUM IN FGD WASTEWATER

Introduction

Selenium is naturally present in coal in trace amounts. During the combustion of coal 

selenium is released into the flue gas mainly as a gaseous species because of its high 

volatility. The gas phase Selenium then becomes associated with the fly ash or is 

scrubbed during flue gas desulphurization. Waste water streams from coal fired power 

station often have high levels of Selenium and therefore need to be treated and 

measured. 

In June 2013, the EPA proposed a new rule “40 CFR Part 423” which relates to effluent 

limitation guidelines to limit the amount of toxic metals being discharged to surface 

waters. Selenium is one of the main elements of concern because of its high toxicity and 

potential impact to the environment and wildlife. The proposed effluent limitation 

guideline is based on the treatment technology utilized on site and is typically in the low 

ppb range.

The reliable determination of Selenium below 10ppb in a FGD wastewater m matrix is 

very difficult using ICP-MS because a relatively large dilution is required to lessen the 

total dissolved solid content of the sample. In addition to this there are isobaric 

interferences which have to be overcome.

In this application note we describe the use of a newly developed online analyser based 

on hydride generation atomic fluorescence spectrometer to monitor online Selenium in 

FGD Wastewater. Samples from a UK thermal power station were tested. All samples 

were spiked with selenite (Se IV) and selenate (Se IV) to confirm the accuracy of the 

technique. The samples were also analysed using the ISO 17378-1 reference laboratory 

method and the results were compared. 

Instrumentation and Conditions

The 10.255 Online Analyser is shown in Figure 1. The system offers multiple stream 

analysis so in principle it is possible to monitor the inlet and outlet of water treatment 

technologies. The sample is injected using FIA with a water carrier. This is then mixed 

with an acidified oxidant and passed through a heater/UV photolysis unit. This converts 

all forms of Selenium in the sample to selenite (Se IV). This is an important step of the 

analysis as only Se (IV) can generate hydride to provide accurate determination. The 

acidified sample is then mixed with sodium tetrahydroborate (III) which reacts with the 

selenium to form the gaseous hydride (hydrogen selenide, H2Se). This is then separated 

from the liquid phase using a gas liquid separator and then delivered to the detector 

using an argon carrier gas. The hydrogen selenide is atomized using a miniature 

hydrogen flame and the selenium atoms are excited using a boosted discharge hollow
P S ANALYTICAL 

UK: Arthur House, Crayfields Industrial Estate, Main Road, Orpington, Kent BR5 3HP, UK

Tel: +44 (0) 1689 891 211 Fax: +44 (0) 1689 896009 E-mail: psa@psanalytical.com 

USA: 1761 W. Hillsboro Blvd., Suite 318, Deerfield Beach, FL 33442, USA

Tel: +1 (954) 429 1577 Fax: (954) 429 1601 E-mail: usa@psanalytical.com

www.psanalytical.com

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cathode lamp. The detector then monitors the atomic fluorescence signal. A schematic diagram is 

shown in Figure 2. The instrumental conditions used are summarized in Table 1.

Figure 1 Photograph of the 10.255 Online Analyser

Figure 2 Schematic of the 10.255 Online Analyser Configured for Selenium

1 2

3 4 5

6 LOADING

Carrier Sample Out To UV Pump

Cooling

Pre-reductant Reductant

To GLS

UV

Cooling Water In

Cooling Water To Waste

10-PORT VALVE To WASTE Ar H2

Perma Pure Dryer

To AFS

INJECTING

SAMPLE LOOP

Heater
P S ANALYTICAL 

UK: Arthur House, Crayfields Industrial Estate, Main Road, Orpington, Kent BR5 3HP, UK

Tel: +44 (0) 1689 891 211 Fax: +44 (0) 1689 896009 E-mail: psa@psanalytical.com 

USA: 1761 W. Hillsboro Blvd., Suite 318, Deerfield Beach, FL 33442, USA

Tel: +1 (954) 429 1577 Fax: (954) 429 1601 E-mail: usa@psanalytical.com

www.psanalytical.com

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Table 1 Instrumental Conditions

Parameter Conditions

Sample and reagent flow rates 0.6 ml/min

Sample Injection Volume (FIA) 0.5 ml

Carrier De-ionized Water

Pre-reductant 50% v/v HCl – 5% m/v KBr

Reductant 0.4% m/v NaBH4 - 0.3% m/v Na2EDTA -1% m/v NaOH

Argon carrier gas 250 ml/min

Air Dryer Gas 2.5 L/min

Hydrogen Fuel Gas 80 ml/min (180ml/min for ignition)

Digestion Approach UV photolysis, heating and cooling

Lamp Conditions (Primary/Boost) 20 mA / 37.5 mA

Cycle Time per measurement 9 minutes

Integration Mode Peak Area/Height 

Calibration Range 0-100ppb in 1% HCl

Gain Range 10

Results and Discussion

A FGD wastewater sample from Europe was used for evaluation and performance testing. Relatively 

low level of Selenium was found as the samples provided were downstream of the waste water 

treatment process. To validate the method each sample was spiked with inorganic selenium in both 

the IV and VI valence states. The sample requires no sample preparation apart from filtration which 

is typically performed online. The system was calibrated using Se(IV) standards prepared in 1% HCl 

between the concentrations of 0 to 100ppb. This concentration range can obviously be varied 

depending on the expected concentration range of the samples to be determined. The calibration 

graph obtained is shown (Figure 3). The system can be configured to run higher ranges up to a 

concentration of 50ppm and also to switch automatically between calibration ranges.

Figure 3 Calibration between 0-100ppb

The results obtained for the study are summarized in Table 2. The data shows a comparison of data 

between the laboratory and online instruments for FGD wastewater samples with a without spikes. 

Excellent recovery was obtained for both the Se(IV) and Se(VI) spikes. The online measurement in 

particular is more challenging because the system is required to fully reduce the selenium prior to the 

hydride generation process and the sample was analysed directly without dilution. Laboratory 

measurements were performed by pipetting 20ml of sample into sample bottle with 15ml of HCl and 

heated at 100 ̊C for an hour. The sample was then diluted to 50 ml volume with DI water before 

analysis by hydride generation AFS (PSA Millennium Excalibur 10.055).
P S ANALYTICAL 

UK: Arthur House, Crayfields Industrial Estate, Main Road, Orpington, Kent BR5 3HP, UK

Tel: +44 (0) 1689 891 211 Fax: +44 (0) 1689 896009 E-mail: psa@psanalytical.com 

USA: 1761 W. Hillsboro Blvd., Suite 318, Deerfield Beach, FL 33442, USA

Tel: +1 (954) 429 1577 Fax: (954) 429 1601 E-mail: usa@psanalytical.com

www.psanalytical.com

PSA

Table 2 Summary of the data comparison between online and offline

Description Online (ng/ml) Lab (ng/ml)

FGD Wastewater 43.65 ± 1.03 37.08 ± 0.37

FGD Wastewater + Se(IV) spike 110% 93%

FGD Wastewater + Se(VI) spike 110% 102%

Method Detection Limit 0.044 0.010

Conclusions

The online determination of Selenium in FGD Wastewater is an important application that 

requires worldwide attention especially in those areas where strict enforcement of permitted 

levels are administrated. The online system described and demonstrated here provides real time 

measurements so that the efficiency of the water treatment processes for selenium removal can 

be monitored continuously to ensure that the permitted levels are not exceeded. The detection 

limit of the online analyser for selenium using a 0.5 ml sample volume was 0.044 ppb. The data 

for the online system compares well with the International Laboratory reference method (ISO 

17378-1) for selenium in Water. The samples were spiked with Se(IV) and Se(VI) and 

recoveries between 93 and 110% were obtained which is quite acceptable given the complexity 

of the sample matrix. Other forms of Selenium may be present in this type of sample and these 

will also be determined with this approach. Online chemistries in combination with UV photolysis 

have been developed to ensure that an accurate result is obtained irrespective of the selenium 

valence state or selenium species in the sample.