Interactions between dissolved organic matter (DOM) and different physical-chemical forms of iron (Fe) represent important biogeochemical processes in the organic carbon cycle. Due to the effect of climate change and anthropogenic activities such as land-use change, the loading of terrestrial DOM into aquatic systems is increasing, and thus, enhancing the organic matter-based acidity in aquatic ecosystems. While complexation of Fe with DOM and the sorption of DOM on iron oxides and (oxy)hydroxides have been reported, less is known about how co-precipitation processes might affect by DOM composition. Here the co-precipitation of two DOM standards, namely the Suwannee River Standard Humic Acid Standard II (SRHA) and Nordic Aquatic Fulvic Acid Reference (NAFA), with Fe was investigated in a pH range of 4.0–8.0. The DOM remaining after co-precipitating with Fe was systematically characterized by various analytical methods to reveal the molecular fractionation of DOM. The co-precipitation of SRHA or NAFA with Fe was enhanced by decreasing the pH, where at pH 4.0, about 70~80% DOC of SRHA or NAFA was removed at Fe(III)/C ratios higher than 0.12. The decrease in SUVA254 and the humification index (HIX) during the co-precipitation process suggests that DOM with high aromatic character was preferentially co-precipitated with Fe. DOM molecular weight influenced the selectivity to co-precipitation, with high molecular weight (HMW) DOM showing a stronger affinity. DOM co-precipitation with Fe was clearly dependent on DOM composition, showing an affinity order of terrestrial humic-like> ubiquitous humic-like> microbial humic-like components. The difference in the reactivity and the relative abundances of excitation emission matrix fluorescence combined with parallel factor analysis (EEM-PARAFAC) components explained the difference in DOC removal efficiency between SRHA and NAFA. This study provides direct insights into the effects of DOM composition on its fractionation specifically through co-precipitation with Fe, and suggests that for aquatic systems rich in iron-based (oxy)hydroxides, and for environmental redox interfaces, co-precipitation of DOM with iron might affect optical properties of the aqueous phase, and represent an important sink for terrestrially-derived organic matter.
