Stream transport of iron and phosphorus by authigenic nanoparticles in the Southern Piedmont of the U.S.

Authigenic nanoparticles containing iron (Fe) and phosphorus (P) have been identified at the anoxic/oxic interface of various aquatic ecosystems, forming upon the oxidation of reduced Fe. Little is known about the prevalence of these authigenic nanoparticles in streams, their impact on biogeochemical fluxes, or the bioavailability of P associated with them. In this paper we used transmission electron microscopy to document the presence of authigenic (amorphous) nanoparticles, rich in Fe and P, in baseflow of streams in the Southern Piedmont region of the U.S. We used a simple centrifugation and ultrafiltration technique to separate authigenic nanoparticles from truly dissolved (<1 kDa) and crystalline mineral/coarse organic fractions in baseflow, employing three different quality control methods to verify a successful separation: X-ray diffraction, electron microscopy, and stoichiometry of Fe and aluminum. This allowed us to quantify the amount of Fe and P in three different fractions of baseflow: truly dissolved, authigenic nanoparticles, and crystalline mineral/coarse organic particles. For the rural and urban stream in our study, on average, authigenic nanoparticles in baseflow transport 66% of Fe, with baseflow concentrations ranging from 80 μg/L to 650 μg/L. Authigenic nanoparticles also transport an average of 38% of reactive P, depending upon seasonality and time elapsed since the last storm event.