Radiation enhanced uptake of Hg 0(g) on iron (oxyhydr)oxide nanoparticles

Abstract

Despite the proposed importance of atmospheric mercury (Hg) cycling, little is known about its heterogeneous chemistry, specifically on ubiquitous dust particle surfaces in the environment. To address this gap in knowledge, we herein report the uptake coefficients for the uptake of Hg⁰_(g) on iron (oxyhydr)oxides (γ-Fe₂O₃, α-FeOOH, α-Fe₂O₃ and Fe₃O₄) nanoparticles, employed as proxies for reactive components of mineral dust. Hg⁰_(g)-particle interactions were studied in a batch set-up, at ambient pressure (760 ± 5 Torr) and temperatures (295 ± 2 K) with UV and visible irradiation (290 nm ≤ λ ≤ 700 nm). γ-Fe₂O₃, α-FeOOH and α-Fe₂O₃ demonstrated a ca. 40–900-fold increase in uptake kinetics upon irradiation, under our experimental conditions. In contrast, uptake kinetics on Fe₃O₄'s surface displayed little dependence on irradiation. Relative humidity was shown to inhibit the effect of radiation on the uptake of Hg⁰_(g) by α-Fe₂O₃. Size distributions, electronic properties, surface area and phase characterization of the iron(oxyhydr)oxide particles were studied to explain the uptake kinetics, and to provide insights into the mechanism of Hg⁰_(g) loss. The adsorption capacity of Hg⁰_(g) on α-Fe₂O₃ was determined from the adsorption isotherm fitted with Langmuir, Freundlich and Elovich adsorption models. The implications of the results to atmospheric chemical processes are herein discussed.