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For years, personal care products have made their way into the environment raising concern about their potential harmful effects on the ecosystem and humans possibly exposed to these compounds through medium such as water (Daughton and Ternes 1999). Triclosan [TCS; 5-chloro-2-(2,4-dichlorophenoxy)phenol], a common antimicrobial agent, is a frequent PCP contaminant detected in water bodies (Kolpin et al. 2002) and its continuous release into the environment could represent a threat to water systems, including drinking water sources and human health.
TCS is used in personal care products such as toothpaste, soaps and detergents, and cosmetics (European Commission 2009). TCS is highly hydrophobic, having a fairly high log Kow of 4.76, which enables its sorption to particles leading to incomplete removal during wastewater treatment (Chu and Metcalfe 2007; Ying and Kookana 2007). TCS (pKa = 8.14) has been determined in neutral and anionic forms at a 50/50 ratio in wastewater treatment facilities functioning at a pH of around 8.0 (Ngiem and Coleman 2008). Due to its extensive use and incomplete removal, TCS can be found in nearly every aquatic environment, including wastewater treatment effluents, surface waters, lakes, and river sediments (Wilson et al. 2009; Kolpin et al. 2002; Lindstrom et al. 2002; Singer et al. 2002). TCS is reported to be extremely toxic to a variety of aquatic organisms which includes invertebrates, fish, amphibians, algae and plants (Palenske et al. 2010; Raut and Angus 2010; DeLorenzo and Fleming 2008; Dussault et al. 2008; Yang et al. 2008; Coogan et al. 2007; Ishibashi et al. 2004; Orvos et al. 2002).
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Other studies reported TCS to form toxic degradation products (Aranami and Readman 2007) and to develop microbial resistance (Heath et al. 2000; Heath et al. 1999; Heath et al. 1998; Hoang and Schweizer 1999; McMurry et al. 1998a; McMurry et al. 1998b). In addition, TCS is reported to react with free chlorine during water disinfection forming byproducts that include dioxins, chlorinated phenoxy-phenols, chlorinated phenols, and trihalomethanes (Buth et al. 2010; Buth et al. 2009; Canosa et al. 2005; Rule et al. 2005; Kanetoshi et al. 1987; Onodera et al. 1987).
Various approaches have been used to remove TCS and other contaminants from aqueous media, particularly the use of membranes for nanofiltration, ultrafiltration and reverse osmosis (Ngiem and Coleman 2008; Snyder et al. 2007; Yoon et al. 2007; Yoon et al. 2006). Other alternatives include the use of activated carbons (Behera et al. 2010; Fang et al. 2009; Snyder et al. 2007), chlorination and UV disinfection (Buth et al. 2011), sonochemical and electro-fenton degradation (Sánchez-Prado et al. 2008; Sirés et al. 2007) and activated sludge (Bester 2003). Drawbacks arising from these methods are relatively high cost of operation, carbon regeneration and disposal related to the use of membranes and activated carbons, formation of chlorinated degradation products and incomplete removal. Therefore, it is imperative to explore feasible, cost-effective alternatives for the removal of TCS and other contaminants from environmental waters.
TCR represents a viable medium for the adsorption of contaminants from aqueous solutions. The recycling of this material becomes necessary due to the increasing production and accumulation of tires worldwide (Jang et al. 1998). TCR is composed of naturally and synthetic rubbers, mostly SBP (62.1%), CB (10-45.6%), steel, and oxides such as SiO2 and ZnO (0.55-2.79%) (ISRI 2007; R.M. Association 2006; Amari et al. 1999). CB has been used as adsorbent of chlorophenols, p-nitrophenol, toluene and xylene from aqueous solutions (Alamo et al. 2011; Domínguez-Vargas et al. 2009; González-Martín et al. 1994) and as molecular sieve for sampling air contaminants (Betz and Supina 1999). Therefore, the presence of CB in TCR should contribute in the removal of contaminants through adsorption mechanisms (Alamo et al. 2011). Furthermore, TCR has been used as adsorbent of naphthalene, toluene, xylene, cadmium(II), mercury(II), lead(II) and Cu(II) (Alamo et al. 2011; Calisir et al. 2009; Entezari et al. 2006; Gunasekara et al. 2000; Rowley et al. 1984).
In the present work, the use of TCR for the removal of TCS from aqueous solution was studied in batch mode. CB and SBP were also evaluated separately to compare their sorption capacity with that of TCR, and asses their contributions in the overall sorption process. Adsorption isotherms were determined and fitted by the Langmuir and Freundlich models. The effect of solution pH on the adsorption process was investigated as well. Using TCR as adsorbent material contributes by expanding the recyclability options of waste tire rubber, in addition to its extremely low cost ($0.15/lb in Puerto Rico), and ease of handling as a granular sorbent material.
MATERIALS AND METHODS
TCR was provided by REMA Inc., a tire rubber recycling company located in Caguas, Puerto Rico. TCR mesh 30 (average diameter 0.67 mm) was washed with deionized water for 24 hours and dried at room temperature. CB N330 (average particle diameter 46 nm; surface area 80 sq m/g, Othmer 1992) was produced by Sid Richardson Carbon Company (CAS 1333-86-4). SBP was purchased from Sigma Aldrich (CAS 9003-55-8), and was trimmed to a similar size as TCR (Alamo et al. 2011).
Triclosan [TCS; 5-chloro-2-(2,4-dichlorophenoxy)phenol] (≥97%) (CAS # 3380-34-5) was purchase from Fisher Scientific, USA. TCS stock solutions were prepared using amber volumetric flasks at concentrations of 1000 mg/â„“ in HPLC grade methanol (Fisher Sci., USA). Solutions were transferred to amber bottles and stored at 4 -C and used within two weeks of preparation. Methanol was added to the solutions (20% v/v) as co-solvent to assure TCS stability during sorption tests. Ultra High Quality water (Barnstead, 18.2MΩ_cm) were used to prepare the aqueous solutions for the adsorption tests.
Adsorption experiments were carried out in batch mode using 120 mâ„“ amber bottles containing 100 mâ„“ of 60 mg/â„“ TCS solution. Predetermined concentrations of TCR (1 g/â„“), CB (3 g/â„“) and SBP (6 g/â„“) were added to the bottles and placed in a thermostat controlled shaker (INFORS AG HT) at a constant temperature of 25 -C. Agitation speed was set at 200 rpm for 9, 6 and 24 hours for TCR, CB and SBP respectively as determined by previous kinetic experiments. CB and SBP concentrations were calculated taking into account their concentration in the TCR matrix (30% and 60% w/w, respectively (Alamo et al. 2011). Samples were analyzed at given time intervals and experiments were performed in triplicate.
The amount of TCS adsorbed was calculated from the difference between the initial TCS concentration and the one remaining in solution after equilibrium was reach by using the following equation:
where qe is the concentration of TCS adsorbed (mg/g), C0 and Ce are the initial and equilibrium liquid-phase concentrations of TCS in the solution (mg/â„“), respectively, V is the volume of the solution (â„“) and W is the amount of adsorbent used (g).
Determination of the pH of the point of zero charge (pHpzc).
The pHpzc of TCR, CB and SBP was determined using the batch equilibrium technique (Faría et al. 2004). 50 mL of 0.01M NaCl solutions were placed in closed amber bottles. The pH was adjusted to a value between 2 and 12 by adding HCl or NaOH 0.1M solutions. Then, 0.15 g of TCR, CB and SBP were added and the final pH measured after 48 hours of continuous agitation at room temperature. The difference between the initial and final pH value was plotted against the initial pH value. Thus, the pHpzc is the point where the curve intersects the abscissa (Rao et al. 2011). Figure 1 show the pHpzc determined for TCR (7.01), CB (8.03) and SBP (4.99).
All analyses were performed with an Agilent 1200 Series HPLC system (Agilent Technologies, USA) equipped with a Zorbax Eclipse XDB C-8 column (4.6 mm-150 mm, 5µm), a binary pump (model G1312A) and a diode array detector (model G1315D) at a detection wavelength of 280 nm. The mobile phase used was 70% acetonitrile and 30% deionized water, flow was set at 1 mâ„“/min, sample injection volume was 20 µâ„“ and column temperature was maintained at 25 -C throughout the analyses.
RESULTS AND DISCUSSION
3.1. Effect of solution pH on TCS removal
The effect of solution pH on triclosan adsorption using TCR, CB and SBP was evaluated in batch experiments in a pH range of 3 to 9. TCS solutions of 10 mg/â„“ were placed in contact with 0.1, 0.3 and 0.6 g of TCR, CB and SBP respectively at 25 -C. Solutions pH was adjusted using 0.1M HCl or NaOH solutions. As shown in figure 2, TCS removal using TCR and CB decreased as pH increased. These results are consistent with those reported in the literature using activated carbon and clays (Behera et al. 2010). As solution pH exceeds TCS dissociation constant (pH > pKa), the anionic form of TCS is expected to be at a higher ratio than the neutral form in solution (Ngiem and Coleman 2008).
Solution pH also affects the surface chemistry of the materials. When the solution pH > pHpzc (7.01 and 8.03 for TCR and CB respectively), the net surface charge of TCR and CB surfaces are negative. Therefore, at pH 9, TCS sorption on TCR and CB is reduce due to electrostatic repulsions between the anionic TCS form and the negatively charge TCR and CB surfaces. TCS removal with TCR and CB decreased from ~ 89 and 95% in pH 3 to ~69 and 83% in pH 9 respectively (Table 1). Regarding SBP (phpzc = 4.99), no significant effect was observed on TCS removal with changes in pH (~93% removal in all pH evaluated) (Table 1), which suggest a strong absorption mechanism contributing along with the adsorption based one (Alamo et al. 2011). TCS molecules could be predominantly absorbed inside the SBP polymeric chains, whereas electrostatic repulsions are minimized, not been the case with TCR and CB where adsorption onto surface is the dominant mechanism.
3.2. Adsorption isotherms
In order to understand the adsorption mechanisms of TCS onto TCR, CB and SBP, two well known adsorption isotherm models, Langmuir and Freundlich were used to fit the adsorption experimental results. The Langmuir model assumes that adsorption takes place at specific homogeneous sites within the adsorbent. A linear form of the Langmuir equation may be written as:
where qe is the amount of TCS adsorbed at equilibrium (mg/g), Ce is the TCS equilibrium concentration (mg/â„“), b is a coefficient related to the affinity between the adsorbent and TCS (â„“/mg) and qm is the maximum adsorption capacity (mg/g). If TCS sorption onto TCR, CB and SBP are fitted by the Langmuir equation, qm and b can be evaluated from the slope and the intercept of the plot 1/qe versus 1/Ce.
In order to evaluate the favorability of the adsorption process, the separation factor (RL), a dimensionless constant based on the Langmuir equation was calculated:
where b is the Langmuir constant and C0 is the initial TCS concentration in solution. It is stated that the RL values are indicative of the type of isotherms to be irreversible (RL = 0), favorable (0 < RL < 1), linear (RL =1) or unfavorable (RL > 1). RL values calculated for TCS adsorption onto TCR, CB and SBP at various pH values and concentrations were less than 1 and greater than zero indicating favorable adsorption (Table 2).
Likewise, the Freundlich model is commonly used to describe sorption onto heterogeneous surface. A linear form of the Freundlich equation may be written as:
where KF represents the relative adsorption capacity of the adsorbent [(mg/g)(â„“/mg)1/n] and n is a constant indicative of the intensity of the adsorption process. If sorption of TCS onto TCR, CB and SBP are fitted by the Freundlich equation, KF and n can be obtained from the plot of log qe versus log Ce.
The uptake dependency (qe) of TCS on the equilibrium liquid-phase concentration (Ce), as well as the Langmuir and Freundlich isotherm plots of TCS sorption onto TCR, CB and SBP at 25 -C and pH 3 are shown in Figures 3-5 respectively. The Langmuir constants b and qm and the Freundlich constants KF and n as well as their correlation coefficients (R2) obtained from the plots of 1/qe versus 1/Ce (Langmuir model) and log qe versus log Ce (Freundlich model) for the adsorption of TCS onto TCR, CB and SBP at various pH values are listed in Tables 3-5 respectively.
In general, the regression coefficient (R2) determined from linear regression analyses is the most widely used criteria in evaluating how good the selected model fit to the experimental data. Still, the mean relative percent deviation modulus (P) is widely employed because it gives a clear idea of the mean divergence of the predicted data from the experimental data, and thus providing more precise information about the fit of experimental data to the evaluated isotherm models, particularly when there are no appreciable difference between the regression coefficients obtained for different isotherm models (Ayar et al. 2008). The P value can be calculated by using the following equation:
where qe (exp) is the experimental value, qe (pred) is the predicted value and N is the number of observations. In general, it is stated that a P value smaller than 5 indicates an extremely good fit; a P value between 5 and 10 represents a fairly good fit; and a P value greater than 10 shows a poor fit. Calculated P values used to evaluate which isotherm model best explain the sorption of TCS onto TCR, CB and SBP are listed in tables 2-4.
Although regression coefficients obtained for TCS sorption on TCR, CB and SBP using the Langmuir model were slightly higher than those obtained with Freundlich's, (tables 2-4), calculated P values were lower for the Freundlich model in all the adsorbents at all pH values evaluated compared to those calculated with Langmuir's. These results indicate that sorption of TCS onto TCR, CB and SBP could be best explained by the Freundlich model. For TCR and CB, the effect of pH was noticeable on the calculated Freundlich adsorption capacity (KF). As pH increased, KF also decreased (table 2-3 respectively) which is in agreement with what was observed in figure 2; electrostatic repulsions affects TCS sorption on TCR and CB as pH increases. In the other hand, no drastic changes in KF were obtained for SBP as pH increased (table 4), which is in agreement with what was previously observed in figure 2, implying a predominant absorption process. Therefore, TCS sorption behavior onto SBP could depend on the polymer-water partition coefficient and not on the surface characteristics (Alamo et al. 2011).
Values of Freundlich's n parameter for TCR were over 1 in all cases, suggesting that sorption onto surface favors at lower concentrations as evidenced by the higher removal percentages shown in table 1a. For CB, n values were over 1 in a pH range of 3-7, but at pH 8 and 9 n values were below 1, suggesting that sorption onto surface at that particular pH values favors at high concentrations (table 1b). CB removal percentages in the concentration range of 30-60 mg/â„“ did not show drastic differences when varying pH values (table 1b); the same could not be said for the removal percentages for the 10 and 20 mg/â„“ solutions which removal % decreased with changes in pH. For SBP, Freundlich's n parameter was similar at all pH values studied (n ~1.3) (table 1c). In addition, no considerable differences were observed in the removal percentages at the concentrations and pH values evaluated, which suggest that absorption is the dominant mechanism in the removal of TCS with SBP. These observations are in agreement with what was seen in the literature (Alamo et al. 2011); considering that TCR is a composite material, the observed sorption capacity can be explained as the result of a combined effect of adsorption by CB nanoparticles and absorption by the SBP matrix.
3.3. Desorption experiments
Desorption studies were carried out in batch mode using same conditions as in the adsorption experiments. After adsorption equilibrium was reached, TCR with the adsorbed TCS was removed from solution and washed with deionized water and transferred to clean amber bottles containing 25 mâ„“ methanol. Methanol was choose as extractant for the desorption of TCS from TCR, CB and SBP because it has been demonstrated to have no effect on the physical structure of polymeric materials (Prpich et al. 2008). The bottles were placed in a thermostat controlled shaker (INFORS AG HT) at a constant temperature of 25 -C with an agitation speed set at 200 rpm. As with the adsorption experiments, samples were analyzed at given time intervals and experiments were performed in triplicate.
Figure 6 shows the desorption behavior of TCS from TCR using methanol as the extractant. Previous to the desorption experiments, the concentration of TCS adsorbed onto TCR was calculated (concentration range of ~1.1 to 16.5 mg/g ). Results of TCS desorption experiments are listed in table 4. As seen in figure 6, TCS desorption was achieved in 5 hours contact time with a maximum desorption of ~89% at the lowest concentration evaluated.
The capacity of TCR to remove TCS from aqueous solutions was demonstrated. TCS adsorption is pH-dependent with maximum removal occurring at pH 3 (~89%). The adsorption data was best fitted by the Freundlich isotherm model. Desorption of TCS from TCR was achieved (~89%), which suggest the viability of this material as a low cost, cheap alternative adsorbent for the removal of TCS from aqueous media.
The authors acknowledge the support Rubber Recycling and Manufacturing Company (REMA), the Puerto Rico Water Resources and Environmental Research Institute (PRWRERI), the TOYOTA Foundation, Solid Waste Management Authority of Puerto Rico (ADS) and the United States Department of Agriculture-(2008-02146 USDA-HSI Program).