Simultaneous Speciation of Arsenic and Selenium Species in Petroleum Refinery Wastewater

Selenium is an important trace element, but it is toxic if an excess is taken. Selenosis can occur when the tolerable upper intake level of selenium exceeds 400 mg per day. Elemental selenium, as well as most metallic selenides are relatively low in toxicity due to their low bioavailabilities.

Selenites and selenates, on the other hand, are highly toxic, with a similar oxidant mode of action as that of arsenic trioxide. For humans, the chronic toxic dose of selenite is approximately 2400 to 3000 μg of selenium per day over a long period. Organic compounds such as selenocysteine, selenomethionine, methylselenocysteine, and dimethyl selenide also contain selenium. These compounds all have high bioavailabilities and are lethal in large doses (Wilber, C. G., 1980). Therefore, various environmental samples require monitoring for the presence of selenium (Tonietto, G.B., 2010).

Hyphenated techniques as modern detection systems in ion chromatography

To screen for selenite Se(IV) and selenate Se(VI) with sensitivities in the low μg/L range, a versatile approach was developed in this analysis.

Results

Ion chromatography and inductively-coupled plasma mass spectrometry (IC-ICP/MS) can be used to separate and detect Se(IV) and Se(VI). Speciation analysis is of the utmost importance, since selenium compounds strongly differ from one another in terms of toxicity. ⁸⁰Se is the main isotope of Se, but m/z 80 is also the Ar-Ar-adduct and hence, the m/z 78 isotope (23.7%) of Se was measured. It was possible to take an alternative measurement on m/z 82 (8.7%), for which equal sensitivity is predicted, as shown in Figure 1.

Figure 1. Separation and detection of Selenite Se(IV) and Selenate Se(VI) at concentrations between 0.1 and 10 μg/L. Column: Metrosep Anion Dual 3 - 100/4.0; eluent: 2.6 mmol/L Na2CO₃, 4.0 mmol/L NaHCO₃; flow rate: 0.8 mL/min; m/z 78

Simultaneous speciation of arsenic and selenium species in petroleum refinery aqueous streams

Usually, selenium exists in four oxidation states in the environment, as follows:

• Elemental selenium Se(0)
• Selenide Se(II)
• Selenite Se(IV)
• Selenate Se(VI)

Significant concentrations of selenocyanate (SeCN⁻) are present in some mining and oil refinery wastewaters. Since crude oil is processed in refinery operations, selenium is concentrated in the wastewater. It is difficult to treat waters that are polluted with selenium as different selenium species have very different mobilities. Conventional processes for treating wastewater such as coagulation with ferric salts cannot effectively remove SeCN⁻, due to its low affinity for iron hydroxide at neutral pH.

Results

In a single run, three inorganic Se species - SeCN⁻, Se(VI), and Se(IV) were isolated by IC using gradient elution (Figure 2) with an eluent containing 100 mmol/L NH₄NO₃ at pH 8.5 and adjusted by the addition of NH₃.

Figure 2. Ion chromatogram obtained from selenium species typically found at process wastewater inlet (condition as below)

Repeatabilities of peak position and of peak area assessments were better than 1% and ca. 3%, respectively. Detection limits were defined as 3× baseline noise and were 56, 75, and 81 ng/L for Se(VI), SeCN⁻, and Se(IV), respectively.

To remove selenium and arsenic interferences simultaneously, adjustments were made to key conditions of the ICP/MS octapole collision/reaction cell(C/RC), including ion lens voltages and gas flow rates. The effect on the detection limits achieved with a C/RC is greater for selenium than for arsenic.

Conclusion

The method was effective for studying the quantitative speciation of arsenic and selenium species in petroleum refinery wastewater, as shown in Figure 3.

Figure 3. Ion chromatogram obtained from selenium species typically found at process wastewater inlet. Column: Metrosep A Supp 10 - 250/4.0; eluent A: 100 mmol/L NH₄NO₃ (pH 8.5); eluent B: ultrapure water; flow rate: 1 mL/min, gradient elution; m/z 75–83

Selenium – further applications with IC-ICP/MS

Isotope Fractionation of Selenium by Biomethylation in Microcosm Incubations of Soil - Schilling, K.; Johnson, T.M.; Wilcke, W. (2013) 352, 101–107.

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About Metrohm

At Metrohm is one of the world’s most trusted manufacturers of high-precision instruments for chemical analysis. Metrohm was founded in 1943 by engineer Bertold Suhner in Herisau, Switzerland. Today, Metrohm is represented in 120 countries by subsidiaries and exclusive distributors. The global Metrohm Group also includes the Dutch companies Metrohm Applikon and Metrohm Autolab, manufacturers of online analyzers and instruments for electrochemical research, respectively. Recently, the Metrohm Group was joined by Metrohm Raman, a leading manufacturer of handheld Raman spectrometers.

Metrohm is the global market leader in analytical instruments for titration. Instruments for ion chromatography, voltammetry, conductivity, and stability measurement make the Metrohm portfolio for ion analysis complete. Instruments for Near-infrared and Raman spectroscopy are another, strongly growing segment of the Metrohm portfolio.

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