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Gaseous Mercury Uptake on Fly Ash and Iron Oxide Particles: Role of Oxygen, Humidity and Radiation

Despite the proposed importance in Hg cycling, little is known about heterogeneous Hg chemistry, specifically on ubiquitous particle surfaces in the environment. To address this gap in knowledge, we performed systematic physical and chemical characterization of three types of coal combustion fly ash and naturally ubiquitous γ-Fe2O3, α-FeOOH and Fe3O4  nano and microparticles, employed as proxies for dust aerosols, using SMPS, OPS, NTA, GC-MS, TEM and BET analysis. Size distributions of fly-ash was measured in the 10-2 – 101 µm size range, while individual iron oxides particles were found to be < 50 nm. We studied kinetics of Hg0(g) uptake on these particles at ambient pressure (760 +/- 5 Torr) and temperatures (295 +/- 2  K) in air and N2 diluents, with 0 % ≤ RH ≤ 100 % and UV-visible irradiation (290 nm ≤ λ ≤ 700 nm). The apparent pseudo first order rate constant of Hg0 uptake increased rapidly from (9.5 ± 0.7) × 10-4 min-1 for dark reactions to (7.5 ± 2.5) × 10-2 min-1 for irradiated experiments on γ-Fe2O3, and from (3.0 ± 0.4) × 10-4  min-1 to (9.6 ± 2.1) × 10-2 min-1 on α-FeOOH. The apparent rate constants for Hg0 uptake on Cumberland, Nanticoke and Shawnee fly-ash samples are found to be (1.2 ± 0.2) ×10-3 min- 1, (0.4 ± 0.04) ×10-3 min-1 and (4.4 ± 0.9) × 10-2 min-1 respectively, under our experimental conditions. We observed that humidity and O2 promoted Hg0 capture on fly-ash increasing reaction kinetics by c.a. 1.5 - 8 fold