Over the next century, sea level is predicted to rise by up to 1 m, leading to the salinization of coastal environments. Salinity affects aqueous geochemistry (i.e., ionic strength, pH), which in turn influences iron (Fe) mineral reactivity, and adsorption capacity. Although Fe (oxyhydr)oxide phases are well-established adsorbents of organic matter (OM), mineral complexation of OM under different salinity regimes has not been investigated. Here, we examined the temporal adsorption and molecular fractionation of coastal dissolved OM to ferrihydrite (Fh) in deionized water (DI), freshwater (FW) and seawater (SW) matrices, using a suite of aqueous geochemical techniques. We found that salinity matrices substantially impacted the extent, rate, and composition of OM adsorption to Fh surfaces. The extent of dissolved organic carbon removal was similar in DI (46.7 ± 4.5%) and FW (40.5 ± 7.1%); however, removal decreased in SW, with only 26.5 ± 0.36%. Overall, this decreased removal in SW corresponds to a slower rate of removal (-0.004 mg/L min) over 24 hours compared to DI (-0.009 mg/L min) and FW (-0.01 mg/L min). Higher concentrations of divalent cation adsorbed to Fh (2-9 mM) in the SW system, compared to the FW system (0.04-0.1 mM), likely limited OM removal rate and extent. Fourier Transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) revealed DI and FW systems exhibited sequential adsorption of aromatic, lignin-like unsaturated, and aliphatic compounds, previously observed in goethite systems, while the SW system demonstrated rapid, unselective adsorption. These results suggest a salinity threshold effect on the adsorption of OM to Fh, in which freshwater minorly slows but otherwise does not alter OM adsorption dynamics, while seawater results in decreased, indiscriminate adsorption. This threshold is likely due to occupation of specific surface area by divalent cations, reduced ligand-exchange reactions with organic carboxyl functional groups and subsequent lack of OM-OM scaffolding. Thus, environments subject to salinization could potentially experience higher DOC concentrations, leading to elevated microbial metabolism and CO2 production than previously observed.

Organic matter adsorption and fractionation to ferrihydrite under different salinity conditions

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.