Threshold concentration, degradation

The effect of substrate concentration and the important issue of the existence or otherwise of threshold concentrations below which degradation is not effectively accomplished. 

There is substantial evidence from the aquatic environment for the existence of threshold concentrations below which rates of degradation of xenobiotics are slow or even negligible (Alexander 1985). A number of hypotheses have been put forward including the critical concentrations required for... 

Another research project at Colorado State University (FEDRIP 1994) will study the microbial degradation kinetics of pollutant mixtures, which will include chloroform. One aspect of this research will focus on both the degradation of chloroform as well as its inhibitory effects when present above certain threshold concentrations. 

Since the publication of the third edition, additional data have been critically reviewed. New or additional data included in this edition are bioconcentration factors, aquatic mammalian toxicity values, degradation rates, corresponding half-lives in various environmental compartments, ionization potentials, aqueous solubility of miscellaneous compounds, Henry s law constants, biological, chemical, and theoretical oxygen demand values for various organic compounds. Five additional tables have been added Test Method Number Index, Dielectric Values of Earth Materials and Fluids, Lowest Odor Threshold Concentrations of Organic Compoimds in Water, and Lowest Threshold Concentrations of Organic Compounds in Water. 

The isolation of specific compound(s) responsible for the attraction of predators to prey has been elusive. Although it is well known that specific amino acids are contained in prey items and do attract predators, field measurements have shown that fluxes of amino acids from carrion are only occasionally above the threshold concentrations required for detection by scavengers, and that fluxes from live organisms are often well below levels detectable by predators.138 Additionally, amino acids are rapidly degraded by bacteria when they are released into the water column139 and... 

Higher homologues of benzoic acid are phenylacetic and 3-phenylpropionic acids (8-73), which result from the microbial degradation of organic matter (such as lignin) and by oxidation of the corresponding aldehydes. Phenylacetic acid has a low odour threshold concentration and is reminiscent of honey. In the form of esters it is often present in some essential oils, and is also used in perfumery. 3-Phenylpropionic acid is found at low concentrations in cheeses produced in the presence of molds. 

Existence of threshold substrate concentration below which rates of degradation are low or negligible... 

BTEX bioremediation projects often focus on overcoming limitations to natural degradative processes associated with the insufficient supply of inorganic nutrients and electron acceptors. However, other limitations associated with the presence and expression of appropriate microbial catabolic capacities may also hinder the effectiveness of bioremediation. Thus, while subsurface addition of oxygen or nitrate has proven sufficient to remove BTEX below detection levels [134,145,292,315,316], it has been only marginally effective at some sites [6]. Sometimes, the concentration of a target BTEX compound fails to decrease below a threshold level even after years of continuous addition of nutrients and electron acceptors [317]. This phenomenon has also been observed for many other xenobiotic and natural substrates under various experimental conditions [327-332]. 

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