Adsorption-induced molecular fractionation of dissolved organic matter (DOM) by iron (Fe) oxides affects DOM composition and properties, but how DOM fractionation depends on surface chemistry of iron oxides remains poorly understood. We quantified the degree of fractionation of Suwannee River fulvic acid by adsorption on five Fe oxide species (ferrihydrite, goethite, hematite, and Al-substituted goethite and hematite) of contrasting surface chemistry under a wide range of DOM removal ratios using electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS). The fractionation degree based on H/C ratios, O/C ratios, modified aromaticity (AI<sub>mod</sub>), or nominal oxidation state of carbon (NOSC) strongly depends on the DOM removal ratio, and their relationship follows a Langmuir-like equation parameterized with two constants pertinent to the maximum fraction degree and fractionation power, respectively. However, the fractionation is insensitive to the type of Fe oxide with ferrihydrite and the two goethite species having the same fractionation degrees that are slightly larger than those of the two hematite species at a given DOM removal ratio. The insensitivity to surface chemistry suggests that mechanisms other than ligand exchange contribute dominantly to DOM removal. Consistently, aliphatic compounds are preferentially removed from solution at high adsorption loadings, likely by organo-organic interactions. Our study suggests that the role of surface chemistry in organic matter adsorption on hydrophilic mental oxide surfaces needs to be deemphasized.

Insensitivity of molecular fractionation of dissolved organic matter to surface chemistry of iron oxides

ACS Fall 2021

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