Aerobic Biodegradation of PCBs - Paper Example

The biodegradation of PCBs aerobically has been studied extensively over the last decade. The aerobic substrates use lightly chlorinated PCB congeners which are released during the de chlorination of the highly chlorinated congeners as the substrates. On most occasions, the PCB aerobic pathway entails and incorporates a biphenyl-dioxygenase. The main function of this compound is to convert Polychlorinated biphenyls to chlorinated benzoic acids and chlorocatechols.

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Major steps in the conversion of PCBs into chlorobenzoates (Borja et al., 2005)

The figure above shows a metabolic pathway, it also indicates all the steps involved in the sequential enzymatic process. Aerobic oxidative destruction involves two main groups of genes. The function of the first gene is to ensure that the Polychlorinated biphenyls congeners are converted to chlorobenzoic acid. The function of the second gene is to ensure that the chlorobenzoic acid is fully degraded. Most commonly used growth substrate for bacteria responsible for the degradation of PCB is biphenyl or monochlorobiphenyl.

Fava et al. (1996) conducted an analysis of the de chlorination of Aroclor 1221 in a bed bioreactor which had been filled with aerobic. In another study, Borja et al. (2006) demonstrated 95% removal of mixture Polychlorinated biphenyls in a bed reactor which had been fluidized with aerobic. In the two research studies, co-metabolism of Polychlorinated biphenyls was enhanced through the use of biphenyl. Tartakovsky et al. (2001) also conducted a study on the degradation of Aroclor 1242 in a granular biofilm reactor which had limited aeration. Anaerobic and aerobic conditions were provided at the same time.

It is important to note that aerobic degradation of PCBs usually attacks congeners which have been lightly chlorinated while the increase in chlorine reduces the biodegradability of the PCB. Congeners which have double ortho substituted chlorines are not fully degraded; they are poorly and inefficiently degraded. In addition to that, and in several occasions, only the top few millimeters of sediments are aerobic. It can therefore be noted that the biggest reservoirs of Polychlorinated biphenyls in either lakes or rivers are mostly anaerobic sediments, and these are responsible for hindering the growth, and development of aerobic microorganisms. The only proper way of ensuring the microbial transformation of Polychlorinated biphenyls present in the contaminated sediments is effective, and efficient is though anaerobic mechanism or through reductive dehalogenation.

Anaerobic dechlorination of PCBs

Anaerobic dechlorination can be described as process through which halo aromatic compounds are degraded under anaerobic conditions. Anaerobic dechlorination is the most commonly known degradative technique for pollutants that are highly halogenated. The reductive dechlorination of polychlorinated biphenyls is vital since it is easier for aerobic degradation to attack these dechlorinated products, more so ring opening and mineralization, and thus lower the toxicity which is associated with polychlorinated biphenyls.

Anaerobic transformation of chlorinated organic compounds entails reductive dehalogenation. In this process, the halogenated organic compound acts as the electron acceptor. As such, hydrogen replaces the halogen substituent in the equation.

R-Cl + 2e- + H+ R-H+ Cl-

The limiting factors in anaerobic environments are the electron acceptors. Therefore, this implies that microorganisms which use polychlorinated biphenyls as terminal electron acceptors are at a selective advantage.

Brown et al. (1984) conducted the very first analysis and assessment of anaerobic dechlorination of polychlorinated biphenyls. They performed it using sediments from Hudson River. The profile that was taken from Hudson River sediment samples was studied and it was observed that the altered profile of the polychlorinated biphenyls mixture which was polluting the river, it was found to be Aroclor 1242. When a comparison was done with Aroclor 1242, the samples from Hudson River sediment showed that they contained greater amounts of ortho chlorines, had higher percentages of mono- or dichlorobiphenyls and little amounts of tri-, tetra- and pentachloro biphenyls. These differences from the original source profile were attributed to the microbial meta which acts as a reductive agent together with elimination of para in sediments. All other physical, chemical or biological transformation processes could not explain the differences in the profile. Other than this in situ research, another lab study was done to affirm environmental dechlorination. The lab affirmation of the anaerobic dechlorination of Aroclor 1242 for sediments obtained from the Hudson River was done through the use of a PCB congener profile which had same characteristics to the one obtained in the environment after sediments were incubated in microcosms for a period of sixteen weeks.

Several studies have been done on the dechlorination of polychlorinated biphenyls through the use of contaminated sediments and soils. The results have shown that there is reductive dechlorination of PCBs in the anaerobic sediments taken from polluted sites. Studies done to evaluate and assess the ability of anaerobic sediment microcosms to dechlorinate Aroclor 1242, 1248, 1254 and 1260 showed that the para and meta positions were the ones which were attacked for the PCB congeners. Through the preferential elimination of Meta and para substituted chlorines, two things happen; the amount of the highly chlorinated congeners reduces and the number of the lower chlorinated ortho substituted congeners increases. The Meta and para chlorines are eliminated from the congeners, therefore, this leads to a reduction of the dioxin-like toxity of these congeners present in the coplanar structure. The anaerobic dechlorination is responsible for the elimination of these toxins. Aerobic bacteria can also degrade the lower chlorinated congeners, and this makes it possible for the PCBs to be biodegraded completely, forming carbon (IV) oxide and water through sequential anaerobic and aerobic microbial degradation processes. Additionally, the weakening of the highly chlorinated congeners results in a reduction of the level of exposure for the polychlorinated biphenyls. Consequently, this leads to a reduction in the potential carcinogenicity and bioaccumulation of polychlorinated biphenyls. Because of all these advantages of dechlorination of PCBs, which pollute the environment and water sources, this degradation technique has a serious impact on the assessment and evaluation of risks and coming up with strategies for remediation.

A pathway of anaerobic dechlorination of a highly chlorinated congener, PCB 153

The figure below shows an anaerobic dechlorination process. It can be noted that the hydrogen atoms replace the Meta and para substituted chlorines. Through the anaerobic dechlorinated of the congeners which are highly chlorinated, there is preferential elimination of Meta and para substituted chlorines, and therefore the biphenyl ring is left intact without any alterations.

Lap experiments provide a controlled environment which is highly advantageous as it is easy to observe all the dechlorination reactions and all the parameters which have an impact on the dechlorination process. In addition to that, studies done on anaerobic microcosm have shown that sampled sediments are capable of dechlorinate spiked PCBs through reduction (Fish and Principe, 1994; Williams, 1994; Sokol et al., 1998; Wu et al., 1998; Chang et al., 2001; Bedard et al., 2005; Yan et al., 2006a, b; Zanaroli et al., 2006) in addition to aged PCBs naturally present in the sample (Zanaroli et al., 2006).

Natarajan et al. (1998) did a research on the dechlorination of Aroclor 1254 by the microbial granules. The experiment was conducted at room temperature for sediment-free and sediment-containing medium. Some of the observations made were that by the end of the twenty-fourth week, a 70% reduction of the highly chlorinated congeners had taken place. However, there was no accumulation of lightly chlorinated (mono- and di-) end products. Another observation made was that chlorines were eliminated from ortho, meta and para positions of PCBs. The third observation was that there was an accumulation of tri- and tetra-chloro biphenyl intermediates. These were however dechlorinated when incubated for a longer time. The rate of dechlorination for these congeners was a bit slower when compared to mono-, di-, penta-, hexa...