Combination of ultrasonic extraction in a surfactant-rich medium and distillation for mercury speciation in offshore petroleum produced waters by gas chromatography cold vapor atomic fluorescence spectrometry

A new analytical method for mercury speciation in offshore petroleum produced water samples (PW) was developed using the combination of extraction (assisted by ultrasound in the presence of surfactant) and distillation before propylation and determination by gas chromatography cold vapor atomic fluorescence spectrometry (GC-CV-AFS). Ultrasonic (US) treatment (40 min) and the use of Triton X-114 (at 0.5% w/v at pH 3) improved recoveries during the distillation used to separate mercury species from the interfering PW matrix (rich in dispersed oil and salt). Recoveries as high as 92% (Hg2+), 87% (CH3Hg) and 86% (CH3CH2Hg) were achieved. Conditions were adjusted to minimize artifact formation during the samples treatment process in systems containing the major PW matrix components. The method detection limits were 5.0, 8.0 and 11.0 pg L-1 respectively for Hg2+, CH3Hg and CH3CH2Hg. The use of matrix matched standards (containing mineral oil at 40 mg L-1 and NaCl at 30 g L-1) improved accuracy of the method that was successfully applied for the analysis of four different PW samples, obtained from off-shore operations, with Hg2+ and CH3Hg detected at the ng L-1 level.