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

Abstract

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