In this study, a sequential nano-bio hybrid strategy was demonstrated for degradation of the antimicrobial compound Triclosan (TCS) using palladium-iron bimetallic nanoparticles (nFe-Pd) as catalytic reductant and diphenyl ether degrading bacteria Sphingomonas sp. PH-07 as aerobic biocatalyst. In aqueous system, TCS (17.3 µM) was rapidly and completely dechlorinated within 2 h by nFe-Pd (0.1 g/L) with concomitant release of 2-phenoxyphenol (2-POP) (16.8 µM) and chloride ions (46 µM). The mass balance of TCS and reductive product was above 97%. All possible dichoro and monochloro intermediates were clearly identified by HPLC and GC-MS analyses and based on the results the pathway for TCS dechlorination is proposed. Bacterial growth experiments revealed that strain PH-07grew well in the presence of 0.1 g/L nFe-Pd in minimal salt medium with DE as carbon source; suggesting that this strain can be used for sequential treatment system which contains iron nanoparticles. Addition DE grown PH-07 resting cells, effectively degraded the reductive product 2-POP. In control, no 2-POP removal was observed. TCS degradation in soil slurry revealed that within 24 h, complete dechlorination of TCS (5 mg/kg soil) was occurred with nFe-Pd amount 5 g/kg soil and above. 2-POP that accumulated in the soil slurry was completely degraded by PH-07 strain. Our results suggest that nFe-Pd based sequential reductive-aerobic biodegradation process could be a potential strategy for remediation of TCS contaminated aqueous, soil and sediment.
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Triclosan (2,4,4¢-trichloro-2¢-hydroxydiphenyl ether; TCS) is a synthetic antimicrobial compound that is widely used in many personal care products, in consumer products including textiles and plastics (Bhargava and Leonard, 1996; Schweizer, 2001) for disinfecting. Due to the wide use of products containing TCS, incomplete degradation of TCS in sewage treatment, TCS and its transformation product namely methyl-TCS have been detected in various environmental matrices including wastewaters, freshwater, seawater and sediments, and biotic samples such as fish and human breast milk (Okumura and Nishikawa, 1996; Adolfsson-Erici et al., 2002; Singer et al., 2002; Halden and Paull, 2005; Miller et al., 2008). TCS has also been detected in agricultural soils following land application of biosolids from a wastewater treatment plant (Cha and Cupples, 2009). In recent years, there has been increasing concern about TCS due to its toxic effects to the aquatic algae and animals (Ishibashi et al., 2004; Veldhoen et al., 2006), and its environmental fate and persistence (Miller et al., 2008). Structurally, TCS resembles other halogenated diphenyl ethers and dioxins that are known to be highly persistent in the environment. The most important concern is the formation of chlorinated dioxins from TCS upon exposure of sunlight or UV irradiation (Latch et al., 2005; Rule et al., 2005). As TCS acts on a wide range of microorganisms, presence of TCS in the environments may create possible risks including deterioration of nitrification in activated sludge systems in the presence of TCS (Stasinakis et al., 2008), and increase the levels of antibacterial resistance in bacteria (Suller and Russell, 2000). Due to a number of severe risks associated with TCS, there has been an increasing concern about TCS usage, and growing attention on monitoring its environmental levels, fate, and remediation.
Several studies have been reported in the literature for degradation or transformation or TCS. They include physico-chemical oxidation using free chlorine, ferrate (K2FeO4), permanganate (KMnO4) or manganese oxides (MnO2), ozonation, UV/TiO2 photocatalysis, Fe(III) saturated montmorillonite (Rafqah et al., 2006; Suarez et al., 2007; Yu et al., 2006; Rule et al., 2005; Zhang et al., 2008; Lee et al., 2009; Jiang et al., 2009; Zhang and Huang, 2003; Liyanapatirana et al., 2010). Most of these methods have resulted in partial degradation or generation toxic end products like chlorinated phenoxy-phenols, chlorinated phenols, trihalomethanes, and dioxins, which are known to carcinogenic. These are some drawbacks, in addition to ubiquitous distribution in various environmental media, for practical applications of these methods. Biologically, due to its strong inhibitory activity against a wide range of bacteria, bacterial degradation of TCS is also limited. The toxicity of TCS is attributed to inhibition of the bacterial fatty acid biosynthetic enzyme, enoyl (acyl-carrier protein) reductase, of both Gram-negative and Gram-positive bacteria, as well as in mycobacteria (McMurry et al., 1998; Levy et al., 1999; Rule et al., 2005). TCS is toxic to bacteria about 70,000-fold higher than its non-chlorinated form (Sivaraman et al., 2004).
Bacteria known to transform halogenated diphenyl ether compounds that are structurally similar to TCS also failed to degrade TCS (Schmidt et al., 1993). Nevertheless, some bacterial strains are able to survive in the presence of TCS due to target mutations, increased target expression, active efflux from the cell, and/or enzymatic inactivation/degradation (Schweizer, 2001; Meade et al. 2001). Our recent findings also proves that diphenyl ether degrading bacterium Sphingomonas sp. PH-07 is able to co-metabolically transform the TCS into chlorophenol and clorocatechol (Kim et al., 2008). The white rot fungi and their enzymatic system also incompletely degrade the TCS. T. versicoloar decreases the cytotoxic and microbicidal effects of TCS by converting it to methylated and glycosyl conjugated forms (Hunt et al. 2001). Laccase mediated transformation produces TCS oligomers (Cabana et al., 2008; Murugesan et al., 2010) or ether bond cleavage products such as chloropehnols in the presence of redox mediators (Murugesan et al., 2010).
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TCS degradation under anaerobic is very poor (Ying et al., 2007) thus it tends to persistent (Miller et al., 2008). The main barrier to the microbial degradation of halogenated compounds is the halogens substituent and their toxic byproducts that could potentially hinder the biological reaction. In recent years, zero valent irons (ZVI) technologies have been increasingly used for reductive dehalogenation of halogenated pollutants; particularly nano sized ZVI (nFe0) due to its extraordinary functions. Rapid and complete dechlorination of halogenated pollutants has been proved using nFe0 (Zhang, 2003). The reductive dechlorination of TCS could effectively decrease its toxicity. In our studies, however, nFe0 ions showed poor dechlorination with TCS at ambient temperature and pressure. The use of bimetallic Fe-Pd nanoparticles (nFe-Pd) has been shown in recent studies for enhanced dechlorination as nFe-Pd has galvanic cell effects which can effectively dehalogenate the persistent chlorinated pollutants (Kim et al., 2008; He et al., 2009; Nagpal et al., 2010 ). However, eventually they produce dead-end dehalogenated product that can be further biodegraded aerobically by microorganisms. The sequential reductive dehalogenation and oxidative degradation strategy might be appropriate for complete degradation of noxious halogenated pollutants.
We have recently hypothesized and proposed an integrated redox process for TCS degradation using nFe-Pd and laccase (Bokare et al., 2010), in which, however, the formation of polymeric compounds by coupling of radical intermediate is the main drawback. In this study, we demonstrate a nano-bio hybrid treatment using nFe-Pd and Sphingomonas sp PH-07 for TCS degradation in both aqueous and soil system. In our study, a rapid and complete dechlorination of TCS by nFe-Pd and biodegradation of dechlorinated product by strain PH-07 has been achieved, and dechlorination pathway has been proposed.