Anoxic waters

Anoxic waters are areas of sea water, fresh water, or groundwater that are depleted of dissolved oxygen. The US Geological Survey defines anoxic groundwater as those with dissolved oxygen concentration of less than 0.5 milligrams per litre. Anoxic waters can be contrasted with hypoxic waters, which are low (but not lacking) in dissolved oxygen. This condition is generally found in areas that have restricted water exchange. In most cases, oxygen is prevented from reaching the deeper levels by a physical barrier, as well as by a pronounced density stratification, in which, for instance, heavier hypersaline waters rest at the bottom of a basin. Anoxic conditions will occur if the rate of oxidation of organic matter by bacteria is greater than the supply of dissolved oxygen. Anoxic waters are a natural phenomenon, and have occurred throughout geological history. The Permian–Triassic extinction event, a mass extinction of species from the world's oceans, may have resulted from widespre

N-removal optimization by various aeration strategies

Simon Taillard

In order to improve the purification capacity of an aerobic granular sludge, two aeration strategies were designed and tested. The goal was to optimize the nitrogen removal performances via various oxygen patterns in the aeration phase of the reactor cycle. The main process driving theses patterns is that a difference between oxygen concentrations in the reactor will modify the size of the anoxic inner zone of a granule. The first aeration was based on a succession of oxygen peaks (50%DO) and semi anoxic valleys (5%DO). The second was based on a short pulse of oxygen followed by the hermetical isolation of the reactor and a continuous recirculation of the pulse. The objective was to get an n-removal superior to 82%, results of a previous article, and if possible better than 88%, results of a previous experiment in the lab. The succession of oxygenated peaks strategy worked well and gave good results, as the n-removal was increasing with the sludge bed height and time. However, due to laboratories problem and to the shortness of the time-slots available to experiment, the system could not have the time to stabilize and no constant removal value could be found. Therefore, the average n-removal was equal to 69% for 40 days of experimentation, with some peak over 80%. The highest value observed was 88% of n-removal. The recirculation strategy gave unsatisfactory results, actually unexplained. The first recirculation led to the disappearance of high nitrification capacity, giving an n-removal percentage of 48%. The second attempt was worse, as the oxygen concentration started to behave incoherently and over-oxygenated the system. This behavior still lacks a proper explanation and only hypotheses can actually be proposed. Besides these mains results, the surveillance of the sludge in the reactors allowed the discovery of a bias in the evaluation of the suspended biomass measurement. It was explained by the fact that heavier granules tend not to mix well, and therefore the system could not be considered homogenous. The bias was created because measurements were made at different height between the two reactors. In order to remove it, a linear transformation was developed and tested. The first results seem to be consistent, but due to the lack of experimenting time, the working hypothesis could not be deepened. Another bias that was spotted is the possible influence of the operator during the aeration cycle measurement. Three of four off-trend points in the first experiment originated from a cycle measurement. The data set was too small to draw strong conclusions upon the bias. An extended survey of the cycle measurement and their supposed off-trend consequences in previous and future experiments could clarify the situation.

2009

Non-crystalline tetravalent uranium species in the subsurface: formation and stability

Luca Loreggian

Uranium (U) contamination of ground and surface waters poses an acute hazard on the ecosystem and human health. Since the discovery of microbial U(VI) reduction, U bioremediation has been explored as a promising and cost-effective method compared to traditional treatments. The speciation of the bioreduction product was originally stated to be uraninite (UO2) which is a recalcitrance crystalline U(IV) species. On the other hand, recent studies demonstrated that non-crystalline species (NCU4) are the dominant product of bioreduction. Since NCU4 species are more labile than UO2, new concerns are associated with the long-term stability of these U(IV) products. In fact, the effectiveness of bioremediation depends on the resistance of NCU4 species to oxidation and remobilization into solution. In this regard, it was previously hypothesized that aging might transform NCU4 to UO2 that would enhance the resistance of U(IV) to oxidation and release into the aqueous phase. We investigated this hypothesis by incubating NCU4 species immobilized in natural sediments under anoxic conditions for a period of 12 months. We systematically probe the speciation of U in the sediment using X-ray absorption spectroscopy. Under the investigated conditions, NCU4 does not age to UO2. Thus it is likely that NCU4 produced during bioremediation persists in the subsurface even for an extended period of time if anoxia is maintained. Therefore the re-mediated site remains vulnerable to events that bring oxygen into the reduced zone. In fact, NCU4 species are rapidly oxidized upon exposure to oxygen. Furthermore, we demonstrated that under certain conditions (i.e., high dissolved oxygen (DO) concentration), the presence of FeS accelerates the oxidation and re-mobilization of NCU4 via the production of reactive oxygen species. Since ROS production during FeS oxidation depends on the concentration of DO and the speciation of Fe(II) in the sediments, at low DO concentrations, low amounts of ROS were detected, and the enhancement of U(IV) oxidation by FeS be-came negligible. Moreover, this thesis investigates the isotopic fractionation (238U/235U) during abiotic reduction by magnetite (Fe3O4), a Fe(II) bearing mineral capable of reducing U(VI) that is commonly found in bioreduced zones. The results of preliminary experiments confirmed that U reduction by Fe3O4 has opposite fractionation than predicted by the nuclear field shift effect suggesting that a kinetic effect may drive fractionation during reduction. Preliminary XAS speciation of U immobilized on the surface of magnetite report that U(V) may occurs during reduction.

EPFL2019

N-removal optimization by various aeration strategies

Simon Taillard

2009

Non-crystalline tetravalent uranium species in the subsurface: formation and stability

Luca Loreggian

EPFL2019

N-removal optimization by various aeration strategies

The stability of non-crystalline U(IV) under oxic and anoxic conditions

Non-crystalline tetravalent uranium species in the subsurface: formation and stability

N-removal optimization by various aeration strategies

Non-crystalline tetravalent uranium species in the subsurface: formation and stability

The stability of non-crystalline U(IV) under oxic and anoxic conditions