Ionic Strength and Species Drive Iron–Carbon Adsorption Dynamics: Implications for Carbon Cycling in Future Coastal Environments
Elizabeth J. Tomaszewski, Elizabeth K. Coward, Donald L. Sparks
Abstract
In coastal environments, sea level rise (SLR) will likely alter reactions between organic matter (OM) and iron (Fe) minerals. A molecular understanding of OM–Fe reactions in complex ionic matrices such as seawater is lacking. We investigated the temporal molecular composition and adsorption of coastal OM to ferrihydrite (Fh) over 24 h in low-ionic strength water (LIW) (I = 0.0009 M), mid-ionic strength water (MIW) (I = 0.003 M) (freshwater proxy), and high-ionic strength water (HIW) (I = 0.6 M) (seawater proxy). Adsorbed OM concentrations significantly (p < 0.05) decreased in HIW (2.12 ± 0.03 mg C/g Fh), compared to those in MIW (3.24 ± 0.56 mg C/g Fh) and LIW (3.74 ± 0.36 mg C/g Fh). In combination with adsorbed ions in HIW (9–195 mg/g Fh) compared to adsorbed ions in MIW and LIW (0.02–0.9 mg/g Fh), an ionic strength threshold is evident. This threshold effect was reflected in the dynamic molecular composition of OM, characterized via Fourier transform ion cyclotron mass spectrometry. In LIW and MIW, rapid sequential adsorption of polycyclic aromatic and phenolic compounds occurred, followed by increasing adsorption of highly unsaturated compounds. Conversely, OM in HIW did not exhibit observable selective adsorption. Overall, limited OM adsorption and indiscriminate fractionation as a result of SLR will likely impact carbon cycling.