Isotopic Characterization of Mercury in Natural Gas via Analysis of Mercury Removal Unit Catalysts

Natural gas (NG) represents an important and rapidly growing global energy source, and some commercially relevant reserves of NG are reported to contain mercury (Hg) at concentrations between 0.01 and 5,000 μg/m³. The overall amount of Hg released to the atmosphere from gas production and combustion is largely unknown, but gaseous elemental Hg release is likely an increasing contribution to the global atmospheric Hg pool. However, no Hg isotopic compositions have been published for Hg entering the atmosphere from NG. In an effort to characterize the isotopic composition of Hg released from NG, we analyzed the stable isotopic compositions of mercury removal unit (MRU) catalysts that were loaded with Hg from NG production and supplied by Johnson Matthey Inc. We suggest that the bulk of Hg adsorbed to catalysts near the inlet of each MRU reactor is representative of the Hg isotopic composition of the NG source. In different gas fields, values of δ²⁰²Hg and Δ¹⁹⁹Hg range from −3.75 to −0.68‰ and −0.02 to 0.65‰, respectively. Analysis of four samples from different positions within a single MRU reactor demonstrates significant isotopic fractionation of a small fraction of Hg that is not removed at the entrance to the MRU. We suggest that this fractionation is due to sorption of Hg to the catalyst surface from the gas phase and that this process follows a Rayleigh fractionation model with ε ≈ −0.40‰. In total, these results suggest that Hg isotopic analysis may be a feasible monitoring tool for Hg emissions from NG production in some gas fields. With further analyses of NG from around the world, a global average isotopic composition of NG-hosted Hg could be estimated to characterize this input to atmospheric Hg isotope models