Photochemical transformation of particulate organic carbon and its impact on mercury cycling in aqueous systems

Abstract

Mercury (Hg) is a global pollutant with high toxicity that accumulates in biota and biomagnifies, posing a risk to organisms throughout the food chain. Mercury is known to strongly associate with natural organic matter (NOM), which, in turn, affects its speciation, solubility, mobility and toxicity. Numerous studies have examined Hg coordination with dissolved organic carbon (DOC), an important aqueous constituent that alters the reactivity and fate of Hg primarily through complexation with carboxyl and reduced sulfur (e.g. sulfhydryl) groups. However, less is understood of Hg reactions with particulate organic matter (POM), an important reservoir of NOM found within soils, sediments, and suspended in aqueous systems. Moreover, the chemistry of NOM changes when irradiated by sunlight, altering the composition and distribution of functional groups available for complexation with aqueous Hg. Accordingly, we investigate Hg complexation with POM derived from a natural source, Phragmites australis ("common reed"), and examined how photochemical processes impact Hg adsorption through chemical alteration of POM. Mercury adsorption to irradiated and non-irradiated POM was examined through performing isotherm experiments, using an environmentally-relevant range of Hg concentrations (2-500 ppb). First, we find that non-irradiated POM is a powerful sorbent of Hg²+. Adsorption characteristics are indeed altered during POM photolysis, with a three-fold (based on Kd) increase in Hg adsorption observed for irradiated POM compared to dark controls. Further, we examine the speciation and oxidation state of adsorbed Hg²+, and deciphered functional groups that contribute to mercury association with POM. Measurements of C, S, and Hg speciation and oxidation state were performed using synchrotron-based X-ray adsorption spectroscopy (XAS) and scanning transmission x-ray microscopy (STXM). These revealed a marked decrease in POM-reduced sulfur following irradiation, leading to fewer R-SH groups capable of strongly binding Hg. However, decreased Hg binding to irradiated POM was not observed owing to a large increase in the quantity of carboxyl groups formed through photochemical processes, which resulted in a higher overall capacity of irradiated POM to bind Hg. The large capacity of POM for binding Hg, as well as enhanced Hg binding following irradiation illustrates that POM is a chemically dynamic pool of NOM that warrants consideration when evaluating reservoirs and processes governing the global Hg cycle.