Characterization of natural riverine colloids and their fate at increased salinity

Riverine colloids are important carriers of macronutrients, trace metals and pollutants into marine waters. These carriers span in size from molecules over colloidal, to particulate matter. Our current understanding of the different riverine carrier phases and how they respond to salinity relies on indirect assessments based on size separation. Thus a division between iron (Fe)-rich and organic carbon (OC)-rich colloids has been made, where the former are predominantly found in the larger size fractions and the latter in the smaller fractions. While these applications have significantly improved our knowledge of the physical partitioning of the two main fractions, those phases are likely to overlap in size. To gain a more comprehensive understanding of Fe and OC colloids in boreal rivers and their fate at higher salinities, X-ray absorbance spectroscopy (XAS) and dynamic light scattering (DLS) were used to explore both Fe speciation and colloidal characteristics such as size and surface charge. The presence of two Fe phases in the river waters - Fe-organic matter (OM) complexes and Fe(oxy)hydroxides - were confirmed by XAS. Further, the DLS measurements, combined with filtration, identified three different particle size distributions. Fe (oxy)hydroxides were observed both as nanoparticles (10-40 nm) with positive surface charge, and larger aggregates with OM interactions (300-900 nm). An intermediate (100-200 nm) and negatively charged distribution was inferred to contain Fe-OM complexes. After increasing salinity, the smallest Fe (oxy)hydroxide nanoparticles were no longer detected in suspension. Unexpectedly, both the intermediate and largest size distributions were still detected in suspension at high salinity. The collective results from XAS and DLS suggest that Fe (oxy)hydroxides and Fe-OM complexes are both found across the wide size range studied, and that colloidal size does not necessarily reflect neither Fe speciation nor stability towards salinity induced aggregation. From this it follows that the fate of riverine nutrients, trace elements and pollutants depend largely on the carrier phase to which they are associated and not solely on the size.