How do colloid separation and sediment storage methods affect water-mobilizable colloids and phosphorus? An insight into dam reservoir sediment

Abstract

Despite the significant role played by colloids in determining the fate of contaminants in aquatic systems, obtaining knowledge about the mobilizable colloids from bottom sediments of dam reservoirs has not yet received sufficient consideration. A major obstacle to understanding the role of colloids in aquatic systems is the limited comparability of results with the literature due to the vast number of methods practiced for colloid extraction/separation and the various sample storage conditions used for colloid extraction purposes.This work presents the effects of five colloid separation methods (namely direct filtration, successive filtrations, low-speed centrifugation and filtration, high-speed centrifugation and filtration, and successive centrifugations), combined with four sediment storage modes (wet, air-dried at 20 °C, oven-dried at 40 °C, freeze-dried), on the characteristics (mass, size distribution, composition) of water-mobilizable colloids from the bottom sediment of an eutrophic reservoir (Champsanglard, France). Special attention has been paid to phosphorus (P), an element that occupies a predominant place in the study of eutrophication process.Results showed that Champsanglard sediment contained a relatively high stock of water-mobilizable colloids (over 5.4 g/kgDW-Sed), along with the associated colloidal P constituted of approximately 4 % total sedimentary P and 81 %–93 % total water-mobilizable P (fraction < 1 μm). The mass and size distribution of released colloids, as well as the amount of colloidal and dissolved P, Corg, Fe, Ca, Mn, Al and Mg, differed by a factor of up to 26 depending on colloid separation protocols. The loss in colloidal mass upon membrane clogging during direct filtration was mitigated by means of a preliminary separation of particles through filtration and centrifugation, while successive centrifugations led to the potential overestimation of colloid quantity due to incomplete separation caused by presence of organic matter. Sediment storage impacts at a greater extent on extracted colloidal mass. In comparison with fresh sediment, drying decreased up to 40 folds the amounts of colloidal and dissolved elements. Special attention should be paid to sediment storage and separation protocols. According to our results, the characterization of colloids shall be done on fresh sediment and with separation by successive filtrations or combination of centrifugation and filtration. The comparison between sediment of different reservoirs should be based on similar application of sample storage and colloid separation protocol, the definition of normalized protocol would be thus required. It is strongly advised to avoid using solely centrifugation for colloid recovery and sediment freeze-drying for long-term storage when working with sediments.