Efficient Formation of Light-Absorbing Polymeric Nanoparticles from the Reaction of Soluble Fe(III) with C4 and C6 Dicarboxylic Acids

Abstract

The role of transition metals in the formation and aging of secondary organic aerosol (SOA) from aliphatic and aromatic precursors in heterogeneous/multiphase reactions is not well understood. The reactivity of soluble Fe(III) toward known benzene photooxidation products that include fumaric (trans-butenedioic) and muconic (trans,trans-2,4-hexadienedioic) acids was investigated. Efficient formation of brightly colored nanoparticles was observed that are mostly rod- or irregular-shaped depending on the structure of the organic precursor. The particles were characterized for their optical properties, growth rate, elemental composition, iron content, and oxidation state. Results indicate that these particles have mass absorption coefficients on the same order as black carbon and larger than that of biomass burning aerosols. The particles are also amorphous in nature and consist of polymeric chains of Fe centers complexed to carboxylate groups. The oxidation state of Fe was found to be in between Fe(III) and Fe(II) in standard compounds. The organic reactant to iron molar ratio and pH were found to affect the particle growth rate. Control experiments using maleic acid (cis-butenedioic acid) and succinic acid (butanedioic acid) produced no particles. The formation of particles reported herein could account for new pathways that lead to SOA and brown carbon formation mediated by transition metals. In addition, the multiple chemically active components in these particles (iron, organics, and acidic groups) may have an effect on their chemical reactivity (enhanced uptake of trace gases, catalysis, and production of reactive oxygen species) and their likely poor cloud/ice nucleation properties.

Surface Water Structure and Hygroscopic Properties of Light Absorbing Secondary Organic Polymers of Atmospheric Relevance

“…The dry spectra are identical to those reported earlier by our group for thin films using ATR–FTIR. 37 The absorbance features for these organometallic polymers were assigned based on the structure of the organic precursors and data obtained from analyzing the solid particles using electron energy loss spectroscopy and X-ray absorption spectroscopy. Scheme 2 shows the suggested structure of these organometallic polymers formed in the aqueous phase reaction of FeCl 3 with fumaric and muconic acids at pH 5, respectively.…”

“…Scheme 2 shows the suggested structure of these organometallic polymers formed in the aqueous phase reaction of FeCl 3 with fumaric and muconic acids at pH 5, respectively. 37…”

“…The synthesis procedure of polycatehcol and polyguaiacol is reported in ref (36), and that for Fe-polyfumarate and Fe-polymuconate in ref (37). Briefly, aqueous phase standard solutions of iron chloride, catechol, and guaiacol were prepared by dissolving the chemicals in Milli-Q water (18.5 MΩ cm) with the solutions’ ionic strength adjusted to 0.01 M by adding potassium chloride (KCl powder, 99.5%, EM Science) in order to stabilize the pH reading.…”

“…37 The mass-normalized absorption coefficients of Fe-polyfumarate and Fe-polymuconate particles in the near UV light between 350 and 400 nm ranged from 1 to 8 m 2 g –1 , which are on the same order of magnitude as black carbon and biomass burning aerosols. 37…”

Perspective on identifying and characterizing the processes controlling iron speciation and residence time at the atmosphere-ocean interface

“…The CueFe redox coupling that has been incorporated in aqueous aerosol and cloud chemistry models (Herrmann et al, 2000) should be reevaluated as a mechanism to sustain nighttime Fe(II) and dominate Fe(III) reduction in the absence of Fe(III)-organic complexes (Mao et al, 2017). Recent studies have also shown that oxidative polymerization of polyphenols and metal-catalyzed polymerization of dicarboxylic acid could be important sources of Fe-complexing atmospheric organic ligands (Slikboer et al, 2015;Tran et al, 2017). Finally, "giant" sand-sized (> 63 μm in diameter) particles, often found thousands of kilometers away from the source (Stuut and Prins, 2014;van der Does et al, 2018), could influence Fe biogeochemistry by both increasing atmospheric fluxes of Fe and scavenging of DFe from the surface ocean.…”