Biodegradation

Bioremediation is considered to be a cost-effective treatment option among the conventional treatment methods. The ability of CDs to interact with HOCs and form inclusion compounds was exploited in bioremediation processes to increase the bioavailability of pollutant and/or to decrease the toxicity of substrates towards cells.

Degradation of toluene and p-toluic acid by a Pseudomonas putida strain in the presence of b-CD was investigated by Schwartz and Bar (1995). The authors reported that the toluene toxicity towards P. putida was reduced when b-CD com-plexed toluene was provided as the substrate. The latter was fully degraded at a concentration of up to 10 g/l. Degradation of toluene vapors can also be enhanced in the presence of b-CD as a result of reduced molecular toxicity and facilitated absorption of the gaseous substrate. Similarly, b-CD alleviated the inhibitory effect of p-toluic acid on P. putida. This protective effect of CD was remarkably more prominent when the microbial culture was shock loaded with an otherwise toxic dose of p-toluic acid (1.8 g/l).

An enhanced effect of RAMEB for the biodegradation of toluene by activated sludge has been also observed by Furuta et al. (2007) . The presence of RAMEB increased the activated sludge concentration and prevented toluene from evaporating during incubation.

Beneficial effect of CD on the bioremediation was also reported by Qiu et al. (2009). The biodegradation of p-nitrophenol by a soil bacterium strain (Arthrobacter) was greatly accelerated by addition of b-CD at a concentration of 0.5%. One explanation for the enhancement of biodegradation is that the soluble b-CD/p-nitrophenol complex reduces the effective concentration of p-nitrophenol, so that the bacterium is exposed to less toxic concentration of p-nitrophenol.

Interestingly, phenol derivatives can also be efficiently degraded by bacteria immobilized on supports containing CDs. Thus, Pluemsab et al. (2007) have reported that calginate beads covalently linked with a-CD can serve as a supporting matrix for bacterial degradation of nonylphenol. In fact, Sphingomonas cloacae immobilized on a-CD/alginate beads degraded nonylphenol more efficiently than that immobilized on alginate beads. In addition, the cell leakage from a-CD/alginate beads was not observed. Sevillano et al. (2008) used an insoluble CD polymer as a biomass carrier for the degradation of aqueous phenol in a fluidized bed biological reactor. The elimination capacity was about 1.0 kg-phenol/m3 day.

Effects of b-CD, HPBCD and CMCD on the solubility and biodegradation of 2-nitrobiphenyl by an Acinetbacter sp. have been reported by Cai et al. (2006). Results showed that b-CD, HPCD and CMCD cannot be utilized by Acinetbacter sp.

as sole carbon source and none of the CDs had toxic effects on the growth of the bacteria in the experiments. Interestingly, all the CDs could enhance the apparent solubility and accelerate the biodegradation of 2-nitrobiphenyl. The biodegradation-accelerating effects of CDs on 2-nitrobiphenyl were correlated with their solubility-enhancing effects.

The biodegradation of decabromodiphenyl ether in liquid culture medium by white rot fungi in the presence of b-CD has also been evaluated by the same research group (Zhou et al. 2007). Their study indicates that b-CD had positive effects on the degradation rate of decabromodiphenyl ether but the effect was less than that of Tween 80 at the same concentration. It was also found that b-CD can also enhance the growth of white rot fungi, and the enhancement was directly correlated to b-CD concentration, suggesting that b-CD may be used as a carbon or energy source in these experiments.

The ability of CD to promote the biodegradation of aliphatic and polycyclic aromatic hydrocarbons was also observed by Bardi et al. (2000). Thus, the biodegrada-tive activity of a microbial population isolated from a petroleum-polluted soil towards dodecane, tetracosane, anthracene and naphthalene was greatly enhanced in the presence of b-CD. It was also found that b-CD influenced the growth kinetics as shown by a higher biomass yield and better utilization of hydrocarbon as a carbon and energy source.

The use of CD for the intensification of bioremediation of contaminated soils has also been investigated by Fava et al. (2002). RAMEB appears as a promising bioavailability enhancing agent for the treatment of polychlorinated biphenyls contaminated soils, not only for its positive effects on the polychlorinated biphe-nyls biodegradation, but also for its biodegradability, non-toxicity and relatively low cost. RAMEB was also successfully applied as an additive in the bioremediation of different transformer oil-contaminated soils in laboratory and field experiments (Molnar et al. 2005) . The environmental risk associated to the use of RAMEB appears much lower than the risk associated to the use of synthetic surfactants or solvents for mobilizing and improving the bioavailability of hydro-phobic contaminants. The biodegradation of field-aged PAHs and phenolic compounds in municipal gas plant site soils can also be enhanced though HPBCD amendments (Allan et al. 2007). Experiments demonstrated that the biodegradation was not limited by the catabolic activity of the indigenous microorganisms but rather by processes resulting in limited availability of contaminants to degrad-ers. Interestingly, b-CD enhanced bioremediation process can further be improved by phytoremediation. Indeed, Bardi et al. (2007) have reported that the content of PAHs in soybeans growing in contaminated soils was greatly increased in the presence of b-CD.

The degradation of hydrocarbons or mixture of hydrocarbons by Pseudomonas like species isolated from bilge oil contaminated waters has been investigated by Sivaraman et al. (2010) in the presence of native CDs. The percentage of residual hexadecane remaining in a 2.5 mM b-CD supplied medium at 120 h was found to be 15% in comparison with the biotic control which was 43%. Degradation of mixture of hydrocarbons (tetradecane, hexadecane and octadecane) by Pseudomonas like species was also found to be more efficient in the presence of b-CD. So, the residual percentage of tetradecane, hexadecane and octadecane remaining in a 2.5 mM b-CD supplied medium at 120 h was found to be 32%, 43% and 61% in comparison with the biotic control 50%, 58% and 67%, respectively.

Surprisingly, Stroud et al. (2009) have reported that the introduction of HPBCD into soils did not enhance the biodegradation of hexadecane or phenanthrene. In their study, soil was spiked with [9-1 4C]phenanthrene and [1- 14C]hexadecane at 50 mg/kg and aged for 1, 25, 50, 100 and 250 days. At each time point, the microcosms were amended with aqueous solutions of HPBCD at a range of concentrations (0-40 mM). Mineralisation assays and aqueous HPBCD extractions were performed to assess the effect of the amendments on microbial degradation. The results showed that amendments had no significant impact on the microbial degradation of 1 4C-contaminants. Inefficiency of CD to accelerate bioremediation has also been reported by Purwaningsiha et al. (2004). Indeed, although mass transfer was identified as the rate limiting step, the addition of HPBCD failed to improve the bioremediation of naphthalene particles by a pure strain of Pseudomonas putida in a water.

Finally, the interest of combining chemical and biological treatments to accelerate degradation of persistent pollutants has been recently demonstrated by Rafin et al. (2009). Indeed, BaP can be efficiently degraded in the presence of HPBCD by combining Fenton's oxidation and microbial degradation by Fusarium solani. Thus, when Fenton's treatment was combined with biodegradation, a beneficial effect on BaP degradation (25%) was obtained in comparison with biodegradation alone (8%) or with chemical oxidation alone (16%) in the presence of HPBCD for 12 days of incubation.

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