McClellan and Halden (2010) analysed the biosolids for TCS worldwide and suggested that the concentrations typically range between 0.6 and 58 mg kg-1. Xia et al. (2010) measured in soils for 33 years that had received biosolids annually, which was contaminated by TCS, about 48 and 67% of the extractable triclosan was located at depth (30-120 cm) indicating substantial translocation. Raisibe et al. (2017) reported that TCS concentration in biosolids is about biosolids, 2.16-13.5 mg kg-1. Pyke et al. (2014) were collected biosolid samples at 14 different sewage treatment plant and identified in every samples were ranged from 25 to 42 µg g-1. When biosolids of sewage is applied to land, the toxic metals in sewage or sludge may accumulate through the process of biodegrade or biotransform in the soil and crops and enter into food chain due to continuous land application (Garcia-Santiago et al., 2016). Lozano et al. (2012) reported the method detection limit (MDL) of TCS was 1.0 and 15.9 ng g-1 wet wt for soils and biosolids respectively and their limit of quatification (LOQ) was about 2.0 and 28.0 ng g-1 wet wt.
Table 2.2. Triclosan concentrations (?g g-1 dry weight) in national studies of biosolids produced in municipal wastewater treatment plants.
Country WWTPs examined Mean concentration
(µg g-1) Maximum concentration (µg g-1) References
USA 94 12.6 19.7 McClellan and Halden, 2010
India 5 1.2 – Subedi et al., 2015
Canada 4 4.2 – Chu and Metcalfe, 2007
Canada 6 6.8 11.0 Guerra et al., 2014
S.Korea 40 – – Subedi et al., 2014
Ireland 16 0.61 4.9 Healy et al., 2017
Although, primarily TCS is considered to be water-borne contaminant, the antimicrobial that enter into the terrestrial environment during the application of sewage sludge to agricultural and industrial land (Lozano et al., 2010; Fuchsman et al. 2010). Activated sludge concentrations of TCS are typically measured between 570 and 14,500 ?g kg?1 of dry weight, whereas concentrations in biosolids have been documented in the range of 95–32,800 ?g kg?1 (Chalew and Halden, 2009; Lozano et al., 2010). Due to lipophilic nature of TCS, the antimicrobial properties were partitioned into sediment and soil, but the transport potential from biosolids into surface runoff has been characterized as low (Al-Rajab et al., 2009).
The potential for loss via surface runoff, leaching depends on their availability in soil and their persistency in soil. It has been speculated that the persistence of TCS in the soil may be enhanced by the organic content of the soil (Fu et al., 2016). Wu et al. (2009) reported the temperature of soil (which is positively correlated to half life), the physicochemical properties of compounds, and the presence of co-contaminants, making TCS potentially more available in the soil and their surface (Walters et al., 2010). Wu et al. (2009) concluded that antibacterial agents absorbed strongly by the sandy loam and silty clay soils with and without addition of biosolids which ranged from 178- 264 Kg-1. The sorption of TCS decreased with increase in soil pH from 4 to 8. Eventhough, TCS may not be physically mobile between soil compartments but other properties that present in soil helps in transferring of TCS from soil to biota. Dann and Hontela (2008) assessed the potential for organic biosolid or manure derived soil contaminants in amended agriculture land to accumulate in biota. Hence, the wider spread of triclosan into soils through biosolids. The triclosan partitions into soil or sediment in the environment and does not degrade fast, but degrade with a half-life of week in aerobic and to month in anaerobic conditions by the process of biodegradation.
2.7. Chemistry of Triclosan degradation process
The presence of triclosan in natural water is of concern due to its potential endocrine-disrupting properties (Pothitou and Voutsa, 2008), and because it is photochemically transformed to 2, 8- dichlorodibenzo-p-dioxin (2, 8-DCDD) under natural sunlight (Aranami and Readman, 2007). Triclosan that remains in the secondary (pre-disinfection) effluent after activated sludge treatment may be chemically transformed during the final disinfection stage, generating disinfection byproducts in the final (post-disinfection) effluent. Sodium hypochlorite, which is commonly used as a source of free chlorine and as a disinfectant oxidant in sewage treatment plant of US, known to chlorinate triclosan at the ortho- and/or para-positions of its phenol ring to form chlorinated triclosan derivative (CTD) products like, 4,5-dichloro- 2-(2,4 dichlorophenoxy) phenol (4-Cl-TCS), 5,6-dichloro-2-(2,4- dichlorophenoxy) phenol (6-Cl-TCS), and 4,5,6-trichloro-2-(2,4- dichlorophenoxy) phenol (4,6-Cl-TCS) (Rule et al. 2005).
Buth et al. (2009) had explained the derivatives of 4-Cl-TCS, 6-Cl TCS, and 4,6-Cl-TCS (Fig.5.1) which undergoes photolysis process in natural waters to form the respective dioxins, such as, 2,3,7-trichlorodibenzo-p-dioxin (2,3,7-TriCDD), 1,2,8-trichlorodibenzo-p-dioxin (1,2,8-TriCDD), and 1,2,3,8-tetrachlorodibenzo-p-dioxin (1,2,3,8-TCDD). Ferrer et al. (2004) found the replacement of a chlorine atom by a hydroxyl group has been done by the degradation pathway. The chlorine atom substitution by hydrogen and the cleavage of the C–O bond have also been observed. Latch et al. (2003) investigated the reaction of pH, irradiation wavelength and its role on TCS were observed and triclosan ring closure to 2,8-DCDD in aqueous solutions buffered at pH 8 and above, suggesting that the phenolate form of triclosan (p Ka=8.1) is photoreactive, whereas triclosan and methyl ether are photostable. Tixier et al. (2002) also reported that direct phototransformation of the anionic form is the dominant photochemical degradation pathway of triclosan.
Lores et al. (2005) reported the photochemical conversion of triclosan in continental waters, into dichlorodibenzo-p-dioxin (DCDD) has been confirmed in the preliminary experiments employing photo-SPME (solid-phase micro-extraction) using 18-W UV irradiation at 254-nm wavelength. Under these conditions, triclosan is rapidly photodegraded (70% of triclosan was degraded in 2 min) to DCDD directly on the polydimethylsiloxane coating of the SPME fiber. Glaser, (2003) explained that triclosan has a very low partition coefficient (log KOW) of 4.76, hence, it is lipophilic in nature and said to be higher potential of bioaccumulation.
Fig.2.2. Degradation process of Triclosan
2.7.1. Chlorinated Triclosan derivatives (CTDs) in natural water
The toxicity of the chlorinated triclosan derivatives (i.e., dioxin photoproducts) has been estimated to be 10 to 20 times higher the toxicity of 2, 8-DCDD (Ontario Ministry of the Environment, 1984). Triclosan and CTDs were detected in every untreated samples in US, sewage treatment plant at levels ranging from 455 to 4540 and 5 to 98 ng L-1, respectively, though both were efficiently removed from the liquid phase during activated sludge treatment. Triclosan concentrations in the pre-disinfection effluent ranged from 38 to 217 ng L-1, while CTD concentrations were below the limit of quantification (1 ng L-1) for most samples.
The use of chlorine disinfection in treatment plant were decreased the concentration of , triclosan, while CTDs were formed during chlorination, as evidenced by CTD levels as high as 22-30 ng L-1 in the final effluent. No CTDs were detected in the final effluent of the treatment plant that used UV disinfection. The total CTD concentration in final effluent of the chlorinating treatment plant reached nearly one third of the triclosan concentration, demonstrating that the chlorine disinfection step played a substantial role in the fate of triclosan in this system (Buth et al., 2011). Latch et al. (2003) has observed the triclosan in an aqueous solutions by exposing it to irradiation, after exposing, triclosan disappeared from the first 5th minute and appearance of dioxin were occurred. Hence, he hypothesized that triclosan and the wastewater produced those CTDs which is important for the production of dioxin.
2.8. Degradation products of Triclosan