Lower biodegradation pathway

Fig. 2 Reported aerobic biodegradation pathways to convert HIBA to CO2 (lower pathway) Suggested but not detected, S Carbon-skeleton rearranging reaction...

Due to the sequential biodegradation pathway (Fig. 19.1), a sequential electrolytic production of hydrogen and oxygen holds the opportunity to stimulate the reductive dechlorination of the higher chlorinated compounds (PCE, TCE) down-gradient the cathode in the first step and subsequently the oxidative degradation of the lower chlorinated metabolites (cA-DCE, VC) down-gradient the anode. The... 

Figure 2. Lower j-triazine biodegradation pathway. Ring-cleavage is catalayzed by cyanuric acid amidohydrolase encoded by r D, followed by ...

In the study of Pettigrew et al. (1990) a bacterial consortium was shown to mineralize 4-CB and dehalogenate 4,4 -CB. It included three isolates a Pseudomonas testosteroni which catalyzed the breakdown of the chlorinated biphenyls to 4-chlorobenzoic acid (the so-called upper pathway ) an Arthrobacter species that mediated 4-chlorobenzoic acid mineralization (the so-called lower pathway ) and a third strain from the consortium with a role that has not been determined. This pattern of co-culture degradation for upper and lower pathway degradation has been observed generally in the field of PCB biodegradation. Few strains have been shown with the capability to catalyze both upper and lower pathway degradation. 

According to one dictionary definition, the term biodegrade means "to reduce to a lower organic type". As used by us in this text, to be biodegraded means that organic molecules are broken apart by normal reaction pathways present in the environment to smaller molecules that can be assimilated into the environment without harm to the environment. 

All these observations emphasize that tests for biodegradability carried out at high substrate concentrations may not adequately predict the rates of degradation occurring in natural ecosystems where only low concentrations of xenobiotics are encountered (Alexander 1985). This phenomenon is therefore of enormous environmental importance since it would imply the possibility of extreme persistence of low concentrations in natural ecosystems. The further exploration of this phenomenon is probably only limited in practice by the access to analytical methods for measuring sufficiently accurately substrate concentrations at the level of ng/1 or lower. Most studies that have been carried out have therefore used 14C-labeled substrates which necessarily limits the range of compounds accessible and restricts the elucidation of metabolic pathways in which only biotransformation or partial mineralization has occurred. 

The chief pathways to account for the disappearance of MTBE in the environment are atmospheric reactions and biodegradation. By virtue of its sterically hindered structure, MTBE has lower reactivity than other hydrocarbons in gasoline. The atmospheric half-life of MTBE has been estimated at 4 toll days (Carter et al., 1991). MTBE will react with hydroxyl (OH) radicals in the atmosphere. The rate of atmospheric reactivity of hydroxyl radicals with MTBE has been determined to be close to 2.8 x 10 cm molecule sec . While the ultimate product of degradation of MTBE is carbon dioxide and water, in laboratory experiments, observed products are tertiarybutyl formate, formaldehyde, methyl acetate, and acetone. Organic nitrate has also been noted when nitrogen oxides are present. 

The esterquats discussed are readily biodegradable and effects are found only at concentrations greater than their water solubility. The (bio)degradation pathway demonstrates that formation of persistent toxic compounds is circumvented. The main degradation products, i.e., polyalcohol quaternary ammonium salts, are not toxic. The ready biodegradability and determined effect concentration strongly indicate that esterquats are safe at the intended maximum usage volumes. PEC/PNEC ratios are lower than 1 for all esterquats examined. 

More recently, because of the availability of potential natural resources for the manufacture of improved environment-friendly materials, and reduced dependency on petroleum resources, bio-based biodegradable plastics have garnered attention from researchers, industries, and governments as possible substitutes for conventional plastics. PHAs are one such class of biopolymers. The LCA apphed to a potential pathway for PH B production, from the ceUulosic fraction of organic residuals, normally disposed of by landfill (Figure 15.11) [175], shows GHG emissions lower than a PHB produced from a dedicated agricultural feedstock. 

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