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

Does anoxia promote the mobilization of P-bearing colloids from dam reservoir sediment?

In the context of a reservoir, the anoxia that develops in the bottom sediment induces the release of phosphorus (P) into the overlying water, thus supporting eutrophication. Most studies focusing on P dynamic in an aquatic environment fail to consider the "truly" dissolved and colloidal fractions, hence the colloidal P has gone largely unexplored. The aim of this study was to investigate the release of sedimentary P under oscillating aerobic, anoxic and aerobic conditions, in taking into account the colloidal (10 kDa-1 µm) and truly dissolved (< 10 kDa) fractions. Laboratory incubations of wet sediment originating from a dam reservoir were performed over 63 days, consisting of 25 days of aerobic conditioning (lasting 2 periods) and 38 days of anoxia. Results showed that oxic conditions induced a very limited release of phosphorus, both in truly dissolved and colloidal forms. In turn, the development of anoxic conditions caused a large release of P, mainly in the colloidal fraction, representing about 90 % of the total water-mobilizable P (PWM < 1 µm). The initial release of truly dissolved P during the anoxic stage gradually diminished over time, possibly due to the formation of secondary minerals or re-adsorption processes. Approximately half of the PWM released during anoxia persisted under subsequent oxic conditions and consisted solely of colloidal P. The dynamics of PWM were primarily influenced by two main factors: (i) the reductive dissolution of iron, which released both dissolved and colloidal P, and (ii) the release of indigenous organic matter, which impacted the stability of the released colloids through bridging mechanisms.

Bioavailability of Colloidal Iron to Heterotrophic Bacteria in Sediments, and Effects on the Mobility of Colloid-Associated Metal(loid)s

The submicrometric fraction of surface sediments that accumulate in the bottom of dam reservoirs represent important sources of nutrients and contaminants in freshwater systems. However, assessing their stability in the presence of sediment bacteria as well as their bioavailability in the sediment remains poorly understood. We hypothesized that sediment’s bacteria are able to extract nutrients from sedimentary colloids (<1 µm fraction) and thus contribute to the release of other colloid-associated elements to water. Experiments were performed under laboratory conditions, using the submicrometric fractions of sediments recovered from two dam reservoirs (in calcareous and crystalline granitic contexts) and two heterotrophic bacteria (Gram-negative Pseudomonas sp. and Gram-positive Mycolicibacterium sp.). The results demonstrated that bacteria were able to maintain their metabolic activity (the acidification of the growth medium and the production of organic ligands) in the presence of colloids as the sole source of iron (Fe) and regardless of their chemical composition. This demonstrates that bioavailable Fe, aside from ionic forms, can also occur in colloidal forms. However, the bacteria also catalyzed the release of potentially toxic metallic elements (such as Pb) associated with colloids. These results help improve our understanding of the processes that influence contaminants’ mobility in the ecosystems as well as provide an important insight into current research evaluating the bioavailability of different forms of nutrients.

Effect of extraction methods on mobilizable colloids and associated phosphorus from reservoir sediment

Mobilizable colloids from reservoir sediment contain nutrients and contaminants, thus may affect water quality once being released. A major obstacle to evaluate the quantity and quality of mobilizable colloids in natural system is the using of appropriate method for colloid extraction from sediment and their separation from dissolved and particulate phases. This work evaluates the role of different extraction methods (agitation, sonication at sediment pH, and sonication at alkaline pH) on the characteristics (mass, size, shape and composition) of water-mobilizable colloids from sediment of Champsanglard dam reservoir (France). Attention has been paid to phosphorus (P), an important element in controlling eutrophication. Recovered colloids were highly affected on both quantity and quality according to the different applied protocols. The less aggressive agitation liberated low-energy water-dispersible colloids without physical damage and with less modification in colloidal chemical composition and shape, whereas sonication released 10-20 times higher colloid quantity but in lower size, due to physically disruption of fragile sediment structure or aggregated/chained colloids. In contrast, alkaline pH intensified colloid release by fortified repulsive forces between colloids and dissolution of organic coat. Concerning phosphorus, competition with hydroxide ions for sorption site or dissolution of phosphate minerals in alkaline pH caused release of dissolved P to solution and decrease of P content in recovered colloids. A special care should be paid to method selection according to the aim of the study and when comparing data from experiments conducted with different colloid extraction methods.