Determination of ultra-low volatile mercury concentrations in sulfur-rich gases and liquids

Abstract

Determining mercury (Hg) concentrations in a wide range of naturally occurring liquids (i.e., groundwater, hydrothermal fluids, acid mine drainage, submarine groundwater discharge, etc.) and gases, (i.e., volcanic and hydrothermal emissions, flue gas, natural gas, land fill gas, etc.) has obstacles due to the presence of H2S in many of such samples. The classical approach of trapping Hg on gold traps comes up against its limits due to “poisoning” of the traps by H2S and problems for its determination by cold vapor atomic fluorescence spectrometry (CV-AFS). Due to low concentrations of Hg in these sample types it is often necessary to collect large amounts of liquid or gas in excess of 20 L, which makes transport to the laboratory difficult. With this in mind we developed a portable method for the collection of Hg from gases and liquids rich in H2S.The method uses an impinger set-up with an alkaline trap followed by two potassium permanganate - sulfuric acid traps. The potassium permanganate (KMnO4) oxidizes elemental Hg vapor to Hg2+, which remains in the KMnO4 solution and thus can be analyzed by CV-AFS. Thus, rather than 25 L of sample, only a few mL have to be transported to the laboratory. A possible caveat of this approach is that naturally occurring gases are generally a mixture of several different gases, such as H2, CH4, SO2 and H2S, which can react with and thus consume KMnO4. The influence of various gas compounds at different concentrations were tested for their effect on the trapping of Hg by KMnO4. Hydrogen and CH4 did not cause any interference, while SO2 did react with the KMnO4. When the oxidizing capacity in the first KMnO4-trap was depleted due to SO2, Hg was trapped in the second KMnO4-trap, which acted as a safety trap. Good recoveries of 99.5 % were achieved for the Hg collected in both KMnO4-traps. Nevertheless, when H2S was introduced into the system, Hg recovery dropped by almost 50 %. This observation was attributed to the formation of mercury sulfide (HgS) in the trap when the oxidation capacity of the KMnO4-trap was consumed. HgS cannot be reduced by stannous chloride (SnCl2), which is necessary for detection by CV-AFS. The problem was overcome by adding an alkaline trap with the reductant sodium borohydride (NaBH4) in front of the two KMnO4-traps. In this trap H2S was converted to S2-, which does not reach the KMnO4-trap while at the same time NaBH4 prevented the oxidation of Hg to Hg2+ followed by precipitation as HgS. Good recoveries of 98.05 ± 3.6 % (n = 3) were obtained for Hg when a volume of 1000 mL H2S was passed through the impinger train.Field testing of the method verified the effect of H2S on the trapping and ultimately the determination of Hg in the hydrothermal gas. With the alkaline trap we determined a Hg concentration of 358 ng m−3 Hg, while without the alkaline trap only 101 ng m−3 Hg. Thus, the set-up without the alkaline trap led to an underestimation of the real Hg concentration by 71.8 % and confirmed the necessity of an alkaline trap to overcome the interference of H2S.