Fate of aqueous iron leached from tropospheric aerosols during atmospheric acidic processing: a study of the effect of humic-like substances

Abstract

Humic-like substances (HULIS) are complex organic molecules that can be found in the atmosphere as components of tropospheric aerosols or suspended in atmospheric water. HULIS are chelating agents and oxidation-reduction species, therefore these substances can affect the availability of aqueous iron, a heavy metal commonly leached from atmospheric particulate matter upon acidic processing. Specifically, chelating properties allow HULIS to remove aqueous iron from atmospheric water, while their redox properties can alter iron speciation. Ultimately, wet deposition of soluble iron can be influenced not only by HULIS but also by other ubiquitous atmospheric cations. In this work, we investigate the effect of HULIS on iron leached from atmospheric particles in the presence of aluminium ions, an environmentally abundant cation also chelated by HULIS. Colorimetric methods were used to examine the cation exchange (CE) of aluminium ions with both iron (II) and (III) ions in humic acids, a model system for HULIS. An effective chelation of aqueous phase iron with humic acids was observed during suspension experiments, with aqueous iron removed from aqueous phase into a HULIS complex. In addition, the redox properties of humic acids showed no oxidation of iron (II) after chelation by humic acid, but a fraction of iron (III) was reduced into the more bioavailable iron (II). Cation exchange with aluminium suggests that bioavailable iron (II) ions chelate with HULIS in a combination of exchangeable and inexchangeable iron, with a higher proportion of exchangeable iron incidence. Additionally, HULIS interaction with iron (III) ions shows chelating properties as well a reduction potential, producing aqueous and chelated iron (II) ions.

Comparative evaluation of iron leach from different sources of fly ash under atmospherically relevant conditions

“…In order to better understand the biogeochemical cycle of atmospheric iron, additional reduction mechanisms of iron should be considered. [52,53] …”

Atmospheric Processing of Anthropogenic Combustion Particles: Effects of Acid Media and Solar Flux on the Iron Mobility from Fly Ash

“…Recent studies suggest that atmospheric processing of fly ash (FA), which are combustion particles emitted from coal-fired power plants, is an important source of soluble iron and an important parameter in the Earth’s climate system. − For example, in China, increasing annual emissions of FA are currently estimated to be in the range of 31 Tg, and in Europe, FA emission is estimated to be 90 Tg per year. − Given the particle size and morphology of FA, it can be transported from urban areas to remote regions of the oceans and can undergo atmospheric processing, impacting the atmospheric balance. , Similar to mineral dust, FA interacts with the highly acidic deliquescent layer formed around its particles by the uptake of water and acidic atmospheric gases (pH ≈ 0–3) . For mineral dust, the rate of iron dissolution depends not only on the pH but also on the mineralogy of the particles, the solar flux, and the identity of the acid. ,, One important atmospheric acid is nitric acid (HNO 3 ), primarily generated from nitrogen oxide atmospheric reactions.…”

Reduction and Photoreduction of NO2 in Humic Acid Films as a Source of HONO, ClNO, N2O, NOX, and Organic Nitrogen

“…As shown in Scheme , nitrogen incorporation in the HA thin film depends on the environmental conditions, with two possible mechanisms leading to HA reactive uptake of NO 2 : (1) at relatively higher pH (pH 4.5), deprotonated or partially deprotonated species in HA can result in a direct reactive uptake of NO 2 , where the conjugate base of the humic acid reacts with adsorbed NO 2 ; conversely, at lower pH (2) protonated species in HA (pH 1.5) or species neutralized by Na + enhance the reactive uptake of NO 2 by decreasing electrostatic repulsions within the thin film . The reported pKas of HA are around 3.0–5.0, ,, suggesting that the higher pHs examined in this work correspond to a partially deprotonated HA thin film, opening pathway (1) for the incorporation of nitrogen in the condensed phase. Under these conditions, the deprotonation of phenolic and carboxylic groups within HA favors nitrogen incorporation into the organic thin film as electron donor compounds react thermally with NO 2 , an electron acceptor.…”

“…Photosensitizers contained within humic acids (HA) and marine-chromophoric dissolved organic matter (m-CDOM) are macromolecular organic compounds derived from the degradation of biological materials. − These chromophores have been observed to partition into atmospheric aerosols, suspensions of liquid or solid particles in the atmosphere with a wide variety of composition, and a highly acidic deliquescent layer. Photosensitizers contained within these aerosol particles are, therefore, in a highly acidic media (pH ≈ 1–3) .…”

“…The aromatic and carboxylic groups contained in humic substances absorb light in the UV region of the spectrum (up to 400 nm) well within the atmospheric cutoff of solar radiation, making them potential environmental photosensitizers. − In addition, it has been found that HA uptake of NO 2 increases at the daytime, through the absorption of solar radiation, − suggesting that solar radiation increases the organic chromophore–NO 2 interaction and potential photosensitization of NO 2 . The functional groups in HA are also known reducing agents, a critical property in the potential conversion of NO 2 into HONO and other reduced nitrogenous gases . In particular, the photosensitizing ability of m-CDOM within sea spray aerosols (SSA), one of the most abundant forms of natural atmospheric aerosols, has been well established, but it is also a highly complex system that can become highly concentrated in submicrometer SSA particles. ,, The role of organic chromophores can have a significant impact on the chemical balance of the lower atmosphere and their physical and chemical properties can change based on their environmental conditions.…”