# P S A NALYTICAL

P S ANALYTICAL

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

Tel: +44 (0) 1689 891 211 Fa x : +44 (0) 1689 896009 E

- mail: p sa@psanalytical.com

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

Tel: +1 (954) 429 1577 Fa x : (954) 429 1601 E

- mail: usa@psanalytical.com 

### www.psanalytical.com 

# PSA 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

s crubbed 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 hydr ide 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 u sing 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, H 2 

### Se). This is then separated from the liquid phase us ing 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 Fa x : +44 (0) 1689 896009 E

- mail: p sa@psanalytical.com

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

Tel: +1 (954) 429 1577 Fa x : (954) 429 1601 E

- mail: usa@psanalytical.com 

### www.psanalytical.com 

# PSA 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 C onfigured for Selenium

### P S ANALYTICAL

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

Tel: +44 (0) 1689 891 211 Fa x : +44 (0) 1689 896009 E

- mail: p sa@psanalytical.com

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

Tel: +1 (954) 429 1577 Fa x : (954) 429 1601 E

- mail: usa@psanalytical.com 

### www.psanalytical.com 

# PSA T able 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 NaBH 4 - 0.3% m/v Na 2 EDTA

- 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

- 10 0ppb 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 sa mple 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

d epending 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 cali bration 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 generati on 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 Fa x : +44 (0) 1689 896009 E

- mail: p sa@psanalytical.com

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

Tel: +1 (954) 429 1577 Fa x : (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 0.01 0



### 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 le vels 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 determ ined 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.