Quantification of selenium species in petroleum refinery wastewaters using ion chromatography coupled to post-column isotope dilution analysis ICP-MS

H Laura Reyes,Silvia García-Ruiz,B Gisele Tonietto,J Marcus Godoy,J Ignacio García Alonso,Alfredo Sanz-Medel

doi:10.1590/s0103-50532009001000016

Abstract

A post-column Isotope Dilution Analysis (IDA) methodology has been used to carry out the quantitative speciation of selenium (Se) in industrial wastewaters by ion chromatography coupled to ICP-MS with an octapole reaction system (ORS). The interfering argon dimers on the 78Se and 80Se isotopes were suppressed by pressurizing the octapole chamber with 4 mL min-1 of hydrogen. The inorganic Se species selenite [Se (IV)], selenate [Se (VI)] and selenocyanate [SeCN-] were separated by ion chromatography (IC) using isocratic elution with 20 mmol L-1 Na2CO3 / 2 mmol L-1 NaHCO3 as eluent. After the separation, the eluent carbonates were converted to CO2 on-line by a membrane self-regenerating suppressor, in order to avoid clogging problems in the nebulizer or in the cones of the ICP-MS interface. Detection limits (as 3σ of the baseline noise) of 0.18, 0.13 and 1.0 ng g-1 were achieved for Se (IV), Se (VI) and SeCN- respectively. Quantification of selenium in each separated species was performed using post-column isotope dilution analysis by continuous mixing of an enriched 77Se spike solution with the effluent from the suppressor column. Finally, the 78Se/77Se or 80Se/77Se isotope ratios were monitored and the concentration of the corresponding selenium species was evaluated. The proposed methodology has been successfully applied to study the quantitative speciation of selenium in petroleum refinery wastewaters.

Highlights

Sour crude oils produced from geological formations containing seleniferous marine shale often contain high levels of selenium.[1]
As inorganic forms of Se are potentially toxic at high concentrations in aqueous samples,[4] the removal of selenium in industrial wastewaters is mandatory before discharging them into the environment
Most of such treatments for selenium removal depend on the selenium species, e.g. adsorption methods are effective to remove selenite anions but not so effective for SeCN, in such a way that this compound has to be oxidized before the adsorption step

Summary

Introduction

Sour crude oils produced from geological formations containing seleniferous marine shale often contain high levels of selenium.[1]. A significant concentration of selenocyanate (SeCN-), formed in the reducing environment, may be present.[3] As inorganic forms of Se are potentially toxic at high concentrations in aqueous samples,[4] the removal of selenium in industrial wastewaters is mandatory before discharging them into the environment. Most of such treatments for selenium removal depend on the selenium species, e.g. adsorption methods are effective to remove selenite anions but not so effective for SeCN-, in such a way that this compound has to be oxidized before the adsorption step. An interesting alternative to overcome those selenium interferences is the use of ICP-MS instruments equipped with collision/reaction cell, as they are able to remove some spectral interferences by pressurizing the cell chamber with gases such as He, H2, O2, NH3 and CH4.15

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Conclusion