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

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 S^2-, 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.