Suwannee River Natural Organic Matter concentrations affect the size and phosphate uptake of colloids formed by iron oxidation

Iron (Fe) oxyhydroxide colloids facilitate the transport of organic matter, oxyanions and trace metals in environment. This study was set up to compare sizes Fe-organic carbon (OC) formed by oxidation Fe(II) presence different concentrations Suwannee River Natural Organic Matter (NOM) evaluate how variable dissolved (DOC) Fe ratios impact binding phosphate. The molar DOC/Fe ratio test solutions varied cover range environmental freshwaters. Low levels phosphate (3.2 µM PO4, P/Fe 0.036) were either present during coprecipitation together with NOM or equilibrated Fe-OC post-synthesis. suspended analysed Flow Field Fractionation (FlFFF-UV-ICP-MS) radiolabeled orthophosphate 32PO4 used detection P. Decreasing consistently increased colloid size both absence PO4 formation. Over a wide (10–1400), very small (<10 nm). Further decreasing yielded larger until limit for stability reached (DOC/Fe ≤ 1). PO4/Fe factor 2–3 among colloids. Within 1–20 nm at high ratio, measured OC/Fe decreased 100 increasing size, suggesting surface area controlled sorption NOM. By contrast, dependent PO4/NOM competition on surfaces that can be explained steric interactions. Thus, enhances mobility formation OM-Fe-PO4 complexes but highest proportion adsorbed largest and, likely, least mobile particles.