Analytical Electron-Microscopy Fractionation of Fine and Colloidal Particulate-Phosphorus in Riverbed and Suspended Sediments

Abstract

The impact particulate-phosphorus (particulate-P) has on eutrophication of aquatic systems that greatly depends on its composition. As a result, analysis methods for evaluating particulate-P speciation must be capable of identifying and/or quantifying the wide range of forms particulate-P can take. In the present study, we compare the particulate P speciation of the fine and colloidal fractions of riverbed sediment and suspended matter from two rivers in the Lake Geneva basin (French Alps) as determined with chemical extractions to results of a combined Transmission Electron Microscopy and Energy Dispersive Detection (TEM–EDS) analysis of the same samples. TEM–EDS provides semi-quantitative information about the distribution of P throughout the solid fraction of a sample and on the diversity of carrier phases, which are identified by their morphology and stochiometry. EDS-detectable quantities of phosphorus were found in 15–35% of the particles in the samples analyzed. As expected, particulate-P existed in a wide variety of forms, mostly associated with Al, Fe, Ca and Si. Some types of particles, often well-crystallized phosphate minerals, had high P contents (10–30%), but the dominant carrier phases of P were diffuse matrices of particles with low P contents (<5%). These matrices had a wide range of chemical compositions and included clay minerals, crystallized and amorphous phases of Fe, as well as intermediary components. Classical chemical extractions showed major differences in P content and speciation between the upstream and downstream sediment samples. The downstream samples showed higher frequencies of particles containing P, a larger diversity of carrier phases, a higher contribution of clays as carrier phases of P, and the presence of a new fraction with Al and Fe amorphous minerals. The processes that create and select carrier phases of particulate-P are part of the general dynamics of P within a watershed.