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Figure 2: Schematic of aqueous extraction.
The exit gas was captured onto a solid phase sorbent trap which was then analysed by combustion/Atomic Absorption Spectrometry (AAS) using a Milestones DMA-1 Mercury Analyser in accordance with ASTM D5954-98 (14) (modifi ed).
Liquid-Liquid Extractions
Samples of synthetic deoxygenated condensate (5 mL) were spiked in turn with known concentrations (50 - 350 µg/kg) of a number of inorganic and organic (salts) mercury compounds. Five aqueous solutions, L-cysteine (1% m/v), sodium chloride (saturated), sodium chloride (100 mg/L), nitric acid (1% v/v) and deionised water were used in a 1:1 ratio with the condensate to extract the respective compounds.
The fi rst series of extractions involved mixing the solution vigorously for fi ve minutes and allowing the mixture to separate over 25 minutes (Figure 2). An aliquot of both phases was then taken and the mercury quantifi ed by combustion/AAS in accordance with ASTM D7623 10 (15) (modifi ed) using a Milestone DMA-1 Direct Mercury Analyser. The mixture was then agitated for an additional ten minutes and allowed to separate for a further 80 minutes (120 minutes total contact time) before allowing the two phases to separate and redetermining the mercury in both.
A second series of extractions was conducted adopting a serial extraction (Figure 3). The spiked condensate was extracted with the aqueous solutions under test at a 1:1 ratio, mixing vigorously for three minutes and allowing to separate for seven minutes before quantifying the mercury in both phases. The organic phase was then removed and decanted into a fresh aliquot of aqueous solution and the extraction and analysis repeated twice.
Summary of Results Purge and Trap
After purging for 30 minutes with nitrogen at 500 mL/min, 100% of the Hg0
Hg was also removed, which is not unexpected as UOP 938-10 states that during the two-hour purge ‘up to 30% of the dimethyl mercury may be removed during purging’. The application of a fi ne sintered frit appears to facilitate the use of a larger fl ow rate to remove the Hg0
)2 in purge time reduces the potential for desorption of (CH3)2
was recovered, effectively reducing the time required for purging by 90 minutes (75%). Under the same conditions 9% of the (CH3
Figure 3: Schematic of serial aqueous extraction.
Figure 4: Bar chart showing the extraction of inorganic mercury salts from condensate into various aqueous solutions.
more quickly. The decrease Hg;
however, by adjusting the fl ow rate of the purge gas and the volume of liquid hydrocarbon taken, it may be possible to optimise the purge conditions to achieve 100% recovery of Hg0
of (CH3 Hg. This would provide not only a quicker method but also )2
one that quantifi es both elemental mercury and non-ionic organic mercury potentially more accurately than UOP 938-10. As a pseudo control, the removal of HgCl2
also quantifi ed and, as expected, found to be < 1%. Liquid-Liquid Extractions
The 30-minute extractions showed that for all inorganic (Figure 4) and organic (Figure 5) ionic mercury compounds tested, L cysteine (1% v/v) was the most effi cient extractant. The 100 mg/L NaCl advocated by UOP 938-10 exhibited poor extraction effi ciency for all compounds with only mercurous sulphate (Hg2
SO4 ) having an affi nity comparable with that exhibited for L-cysteine.
The data shows that there are solubility / partitioning inadequacies with many soluble ionic mercury species when 100
under the same purge conditions was with no loss Figure 5: Bar chart showing the extraction of organic mercury salts from condensate into various aqueous solutions.
mg/L NaCl is used under the modifi ed extraction conditions and elevated mercury concentrations employed in this study. The data indicated that if a NaCl solution is to be used then saturated NaCl should be adopted as this exhibited superior extraction effi ciency for all mercury species tested except Hg2
SO4 .
Extending the total contact time to 120 minutes did not signifi cantly improve the extraction effi ciency of any extractant for any species under test. Indeed, for 100 mg/L NaCl, a signifi cant decrease in extraction effi ciency was observed for HgCl2 and C2
, Hg2 H5 Cl2
HgCl which is most likely due to the extracted species migrating to the liquid-liquid interface on extended standing.
Adopting a serial extraction resulted in a signifi cant improvement
in removal effi ciency for most compounds when 100 mg/L NaCl was employed; however, for all compounds tested the performance of L-cysteine remained markedly superior (Table 1). The extraction of mercurous sulphate into 100 mg/L NaCl was found to be lower than that observed when a single/ longer extraction was employed. For the serial extractions, the agitation, separation and contact time for each extraction was reduced, although the effective total volume of aqueous extractant and total agitation and contact time was increased. This suggests that for some compounds, in particular mercurous sulphate, the rate of partitioning into the aqueous phase is slow and is largely dependent on the total contact time and the time allowed for separation.
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