Oxygen anoxic conditions

Aerobic bacteria complete most of the petroleum bioremediation applications, particularly those above the groundwater table. Aerobes are those bacteria that require an oxygen source as their TEA. Conversely, anaerobic species require the absence of oxygen (anoxic conditions) for their respiration. In situ anaerobic bioremediation is typically only conducted in the saturated zone because of the difficulty in maintaining a strict anaerobic environment. In some instances, facultative anaerobes are utilized because they can alter the respiration to be metabolically active under both anaerobic and aerobic conditions. As such, the type of TEA available will dictate the metabolism and subsequent degradation mode. The most common TEAs used for bioremediation are listed in Table 2. Careful selection of microbe-TEA combinations can enable a specific degradation pathway to facilitate cometabolism and prevent undesired degradation by-products. 

Entirely enclosed column systems have also been proposed to reduce contamination, personal errors, and contact of the packed sample with atmospheric oxygen (anoxic conditions), whereby undesired decomposition of reduced mineral phases and oxidation of reduced water constituents are prevented (Wisotzky and... 

Processes reported in Table 1 are typically anaerobic (AN). In agreement with the observations reported by Wuhrmann et al. [49], azo-dye bioconversion occurs with the standard organism and other facultative or obligatory aerobic bacteria in exclusively anoxic conditions. Different methods can be used to establish the required anaerobic conditions. A common procedure is simply sparging oxygen-free gas... 

The role of cell respiration has been taken into account to interpret the azo-dye conversion by particle-supported biofilm under aerobic conditions [5, 24]. The rapid depletion of oxygen expected/measured as one moves inside the biofilm promotes the establishment of the anoxic conditions needed for azo-dye conversion. 

Nitrate and oxygen also may play an important role in determining the rate of azo dyes reduction. Wuhrmann et al. demonstrated that obligate aerobes might actually decolorize azo dye compounds under temporary anoxic conditions. However, high nitrite concentrations in the mixed liquor of activated sludge plants could significantly inhibit dye removal. 

Concerning the reduction step of the redox reaction, the heterotrophic microorganisms may use different electron acceptors. If oxygen is available, it is the terminal electron acceptor, and the process proceeds under aerobic conditions. In the absence of oxygen, and if nitrates are available, nitrate becomes the electron acceptor. The redox process then takes place under anoxic conditions. If neither oxygen nor nitrates are available, strictly anaerobic conditions occur, and sulfates or carbon dioxide (methane formation) are potential electron acceptors. Table 1.1 gives an overview of these process conditions related to sewer systems. 

The microbial activity of wastewater under anoxic conditions is lower compared with aerobic conditions (Abdul-Talib et al., 2001). This is important to consider, because a low nitrate uptake rate (NUR) compared with the oxygen uptake rate (OUR) in units of electron equivalents means a reduced transformation rate of the most biodegradable fractions of the organic matter. As mentioned under the point on injection of air, this may have implications in terms of treatment. Furthermore, a relatively low NUR value also has operational advantages because of a reduced demand for nitrate to suppress sulfide formation. 

Oxide/hydroxide minerals of Mn(III,IV), Fe(III), Co(III), and Pb(IV) are thermodynamically stable in oxygenated solutions at neutral pH, but are reduced to divalent metal ions under anoxic conditions in the presence of reducing agents. Changes in oxidation state dramatically alter their solubility. Reduction of Fe(III) to Fe(II), for example, increases iron solubility with respect to oxide/ hydroxide phases by as much as eight orders of magnitude (1). 

Figure 1. Schematic diagram of Fe redox cycling through biological processes. A large number of pathways are involved in dissimilatory Fe(III) reduction, as listed in Table 2. Processes that occur under oxic conditions are placed near the upper part of the diagram, and those that occur under anoxic conditions are placed in the lower part of the diagram. Major lithologic sources of Fe are noted for high and low oxygen environments.

The potential exists for denitrification to occur in the secondary clarifier sludge bed in the refinery wastewater treatment plant under anoxic conditions. The biological reduction rate of nitrate to nitrogen in the sludge blanket is typically slow due to limited soluble Chemical Oxygen Demand (COD), the food source for denitrifying... 

Anoxic condition that exists when oxygen is absent... 

Su, J-J. Kafkewitz, D. (1994). Utilization of toluene and xylenes by a nitrate-reducing strain of Pseudomonas maltophilia under low oxygen and anoxic conditions. FEMS Microbiology Ecology, 15, 249-58. 

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