Physical separation In water treatment plants, organic halides are adsorbed using
GAC or
PAC in agitated tanks. A two step filtration process consists of two GAC filters in series. The first filter is loaded with exhausted GAC, while the second filter is loaded with fresh GAC. This set up is preferred for its increased efficiency and higher throughput capacity. The GAC is replaced cyclically and the extracted organic halide-carbon mixture is then sent for subsequent biological or chemical treatment such as ozonation to regenerate the GAC. Note that enzymatic or microbial action could be regulated through feedback inhibition-the final product in the series inhibits a reaction in the process. An example of a microbe that can degrade AOX is shown below in Figures 1 A sample dechlorination of chlorinated aliphatic hydrocarbons (CAHs) such as
perchloroethylene (PCE) by
Dehalococcoides ethenogenes has been illustrated above. PCE is one of the highly chlorinated CAHs with no known microorganisms capable of aerobic degradation. The high
electronegative character of PCE renders oxidizing agent capabilities through accepting electrons by co-metabolism or dehalorespiration. In a co-metabolism, the reduction of PCE is made feasible by the utilization of a primary metabolite for carbon and energy source. In dehalorespiration, the electron transfer from oxidation of small molecules (H2 is the major source; but, glucose, acetate, formate, and methanol can also be used) to PCE generates energy required for the bacterial growth. The hydrogen involved in this mechanism is often a product of another process such as fermentation of simple molecules like sugars or other complex molecules like fatty acids. While several enzymes and electron carriers are involved in process, two enzymes perform the dechlorination reactions–PCE reductive dehydrogenase (PCE-RDase) and TCE reductive dehydrogenase (TCE-RDase). The PCE-RDase is normally found freely in cytoplasm while the TCE-RDase is found attached to the exterior cytoplasmic membrane. These enzymes normally utilize a metal ion cluster like Fe-S cluster to complete electron transfer cycle. While little is known about the dehalogenation mechanisms of polyhalogenated phenols (PHPs) and polyhalogenated benzenes (PHBs), regioselectivity for halide location on the aromatic ring was observed. This regioselectivity is however dominated by both redox potentials for the reaction and the microbe's familiarity to the reaction. Moreover, due to the specificity of most microbes along with complex aromatic structures, in order to achieve a complete dehalogenation, a mixture of more than one species of bacteria and/or fungi (often known as a consortium) is utilized. The reaction in figure 2 shows the reductive debromination of
2,4,6-tribromophenol (2,4,6-TBP) by
Ochrabactrum. Based on the relative degradation of the molecule along with analytical results, it has been postulated that degradation of 2,4,6-TBP proceeds through debromination of
ortho-bromine in the first step by a dehalogenase to yield
2,4-dibromophenol (2,4-DBP). Since there are two
ortho bromines, debromination of either
ortho carbons would yield the same product . Other species such as
Pseudomonas galthei or
Azotobacter sp. showed preference for
para-halide over the
meta-
or ortho -halides. For example, the
Azotobacter sp. degrades
2,4,6-trichlorophenol (2,4,6-TCP) into 2,6-dichlorohydroquinone due to TCP-4-monooxygenase selectivity differences between
ortho- and
para-halide. These differences in regioselectivity between the species can be attributed to the specificity of the 3-dimensional enzyme structure and its hindrance from steric interactions. It has been postulated that a proton lost by the phenol group of 2,4,6-TBP resulting in the formation of a negatively charged halo-phenolate ion. Subsequent attack of the
para-carbon with a hydride anion from NAD(P)H in a nucleophilic attack manner and resonance rearrangement results in substitution of bromine with hydride and formation of 2,4-DBP. Subsequent steps in a similar pattern yield 2-bromophenol, and
phenol in the final step.
Phenol can be metabolized by microorganisms to make
methane and
carbon dioxide or can be extracted easier than AOXs. == Related terms ==