CHAPTER 3 Oxidative removal of bisphenol A, triclosan, and nonylphenol by FeIII-TAML/H2O2: competition in mixtures
This chapter describe investigation on the competitive oxidation of bisphenol A (BPA), triclosan (TCS) and nonylphenol (NP) by FeIII-TAML /H2O2 (TAML = tetra-amido-macrocyclic ligand) catalytic oxidation system. The catalyst system was applied to oxidize BPA (43.8 µM), TCS (34.5 µM) and NP (34.5 µM) in single, binary and ternary mixtures of substrates within the pH range 6 -11. Results show that oxidation of the three substrates are pH dependent, maximum degradation occurs at pH close to the pKa of the substrate (for single systems).The most suitable pH for oxidation of the three chemicals is around pH8.5-9.0. During reactions in this pH range, BPA was preferentially degraded over TCS and NP in binary and ternary mixtures. Reducing initial BPA to half (22.7 µM) did not change the preferential selectivity for BPA by FeIII-TAML /H2O2. Oxidation energies for the transformation pathways obtained using density functional theory (DFT) suggest that phenolates of BPA, TCS and NP are more susceptible to oxidation than their phenol counterparts. DFT calculations show that BPA and NP require similar oxidation energy, while TCS is the most difficult. The energy calculations were observed to be consistent with the selective nature of FeIII-TAML/H2O2, which suggest that easier to oxidize substrate was selected over harder to oxidize ones. The lower pKa of BPA (pKa 9.6) as compared to NP (pKa 10.2) favored its selection over NP by the oxidant system at pH 8.5 and 9.0. The study provides new insight into the degradation of phenols under different pH conditions and the competition of oxidant for substrate present in mixtures.
As a result of widespread usage, phenols such as bisphenol A (BPA), triclosan (TCS) and 4-nonylphenol (NP) are common contaminants of surface water and wastewater (Arboleda et al. 2013, Cabana et al. 2007). BPA is used to produce polycarbonates and epoxy resins incorporated in household plastics and food packing (Cabana et al. 2007, Kabiersch et al. 2011a), while TCS is an antibacterial agent used in personal care products (PCP) such as soap and hand wash (Murugesan et al. 2010). NP arises from the degradation of nonylphenol ethoxylates used as surfactant in detergents (Cabana et al. 2007). BPA is a known endocrine disrupting chemical (EDC) with the potential to cause low sperm count, breast and prostate cancer, type 2 diabetes, and other medical disorders (Gould J.C. 1998, Krishnan et al. 1993, Maffini et al. 2006). Studies have reported TCS to possess intrinsic estrogenic and androgenic activity with the potentials to cause human breast cancer and other medical disorders (Fang et al. 2010). NP is capable of interfering with the proper function of androgens that are essential for the normal development of males and their reproductive systems (Robert J. Gilliom 2007).
Phenolic compounds are resistant to physical/chemical treatment processes in water and waste water treatment plants, which result in untreated phenol compounds in their effluents (Huang and Weber 2005, Liu et al. 2009). Phenols in water have been treated by various chemical oxidants including chlorine (Deborde and von Gunten 2008), ozone (Broséus et al. 2009), Fenton’s reagent (Georgi et al. 2007) and iron (III) tetra-amido-macrocyclic ligand (FeIII-TAML) /H2O2 (Gupta et al. 2002). Among these oxidants FeIII-TAML/H2O2 stands out as an alternative green technology (Ellis et al. 2010) which has the potential of degrading EDCs with concomitant removal of estrogenicity (Shappell et al. 2008). In addition, FeIII-TAML self-destroy after degrading pollutants, resulting in environmentally benign effluent (Chanda et al. 2006, Ghosh et al. 2008). The FeIII-TAML system functions by activating the peroxide to form a reactive intermediate which then oxidise substrates (Popescu et al. 2010). The reactive intermediate is very reactive even at low catalyst concentration (nM to µM) and more selective than hydroxyl radicals in its reactivity towards organic compounds (Georgi et al. 2007, Gupta et al. 2002).
The selective nature of Fe-TAML / H2O2 oxidant system was demonstrated in the degradation of benzene (5mM) and 2,4,5-trichlorophenols (5mM) at pH 10 (Georgi et al. 2007). Authors reported complete degradation of 2,4,5-trichlorophenols (TCP) in less than 5 min, while only 40 % benzene was removed in 150 min. Overall, factors that dictates substrate selectivity by FeIII-TAML / H2O2 is yet to be fully understood. This selective nature of the oxidant is a potential advantage when selective degradation of organic pollutant among co-existing chemicals is the target, especially in the natural water with a myriad of chemicals. In the present study we investigate the selective nature of oxidant by observing the oxidation of BPA, TCS and NP in isolation and in competition for available FeIII-TAML/H2O2. We evaluate properties of substrates and how changes in operation conditions cause oxidant to change targeted substrate for selective oxidation among co-existing chemicals. Variable operating conditions tested were solution pH, Fe-TAML concentration and number of substrates competing for available oxidant in solution.
Materials and Methods
Details on production of FeIII-TAML activators is in previous study by (Horwitz et al. 2006). Bisphenol A (GC grade > 99 %) was purchased from Sigma Aldrich. Triclosan (GC grade > 98 %) was obtained from Fluka Analytical, 4-nonylphenol (GC grade > 98 %) from Sigma Aldrich. Hydrogen peroxide (30 % v/w) was purchased from Ajax FineChem. Deionised Milli-Q water (Millipore) and solvents of HPLC grade were used in all preparations.
The general procedure used for the catalytic oxidation experiments described is as follows. Aliquots of stock solutions of BPA, TCS and NP (all 10,000 mg/L) in methanol were added to the required volumes of 0.01 M buffer solution (acetate buffer for pH 6.0, and sodium carbonate/bicarbonate buffer for reactions at pH 8.5 and above) to give solutions with final concentrations of 10 mg/L BPA and TCS (43.8 μM and 34.5 μM respectively), and 5 mg/L NP (22.7 μM). Aliquots of a stock solution of FeIII-TAML (40 µM, prepared in deionised water) were added to the buffer solutions containing the substrates to give the required final concentration (4 – 8 nM). The oxidation reactions were initiated by adding an aliquot of H2O2 to the buffer solution containing the substrate and catalyst so that the overall concentration of H2O2 was 4.0 mM. After the required time intervals, aliquots of the oxidation reactions were removed and the reaction terminated by the addition of catalase (12,000 units of bovine liver catalase, which is 60 times the concentration capable of destroying 2.0 mL of H2O2 at a concentration of 4.0 mM in one minute) to rapidly destroy the hydrogen peroxide.
Each experiment was performed with 120 mL solution containing 43.8 μM BPA, 34.5 μM TCS and 22.7 μM NP, in combination or in separate reactors, along with required amount of FeIII-TAML and 4 mM H2O2 at required pH. Experiments were performed in triplicate at a temperature of ～25 ℃. Reactions were initiated by adding H2O2 to mixture solution of FeIII-TAML and the substrate chemicals (BPA, TCS and/or NP). The solution was continuously agitated in a mechanical shaker (IKA KS 260) at 150 rpm. 2mL samples were withdrawn at regular intervals for 180 min. Reactions were terminated by adding catalase.
Substrate concentrations were determined using a liquid chromatograph mass spectrometer (Shimadzu LC-MS model 2020) with an electrospray ionization (ESI) source. Ten µLs samples were injected into Phenomenex MAX-RP C12 column (2.0 × 150 mm), operated at 30 °C. The mobile phase was 80% organic solvent (acetonitrile/methanol, 2/3 v/v) in deionized water at a flow rate of 0.2 mL min-1. BPA, TCS and NP were monitored at m/z 227, 288 and 219 respectively, under isocratic elution for 20 min, in the negative selected ion mode (SIM) of the LC-MS machine.
Reaction Pathway and Metabolites
For analyzing reaction pathways, the substrate in aqueous samples obtained after 180 min of reaction were concentrated by solid phase extraction (SPE) using 500-mg hydrophilic-lipophilic balance (HLB) cartridge from Waters Corp. The cartridges were preconditioned with 2 mL methanol followed by 2 mL Milli-Q water. The sample solution was loaded onto the preconditioned cartridge at a flow rate of 10.0 mL min-1. The cartridge was dried under high vacuum and then eluted with 5 mL methanol at a flow rate of 3 mL min-1. The eluate was then collected, dried, and then dissolved in 2 mL methanol for analysis with Bruker micro-ToF-QII (Bruker Daltonics, Germany) coupled with a Dionex Ultimate 3000 HPLC with autosampler (Dionex, Germany) following the procedure described previously (Chen et al., 2012). The ToF analysis provided the probable molecular formula for byproducts.
The geometry optimisation and energy calculations were performed with Gaussian 09 software suite using unrestricted DFT (Frisch et al. 2009). The non-local B3LYP functional hybrid method was employed (Becke 1988, 1993, Lee et al. 1988) with the standard 6-31+G(d,p) basis set(Hariharan and Pople 1973, Frisch et al. 1984). Zero-point energies were scaled according to Wong (0.9804) (Wong 1996). All the normal modes showed no imaginary frequencies, indicating that they represent minima on the potential energy surface. The subsequent energy calculations were performed with the larger 6-311+G (2df,p) basis set. The water simulations were performed with the same method as previously described using the polarized continuum model (IEFPCM) (Tomasi et al. 2005). The ionisation potentials and proton affinities were calculated as described in Forseman and Frisch (Foresman and Frisch 1996). The dimerization and hydrogen shifts were calculated by subtracting the ZPE corrected energies of the reactants from the products, i.e., the energy difference (∆ ) for each reaction was obtained by applying eqn (1) (Reynisson and Steenken 2002):
The results are given in Tables A3.1 – A3.3 in the Appendix. Results for calculated thermochemical energy difference (∆ ) for each step along the oxidation pathways are shown in Fig. 3.6, 3.7 and 3.8 for BPA, TCS and NP oxidation respectively. The spin densities were derived according to Mulliken (Mulliken 1962).
Results and discussion
Effect of pH on oxidative removal of the individual substrates
There was minimal change in concentration of BPA when treatment was carried out by either H2O2 or the FeIII-TAML catalyst alone (shown in A3.0), similar trend was observed for TCS and NP. Activation of H2O2 by 4 nM FeIII-TAML catalyst resulted in rapid substrates removal in alkaline solutions (at pH 8.5 and above) as shown in Fig. 3.1. As shown, the percentage BPA removal (at 180 min) increased steadily from about 5% at pH 6 to a maximum at pH 9.5 (> 50 % removal). Increasing pH above 9.5 caused reversals of this trend. A similar trend as for BPA removal was observed for TCS removal, however a more rapid increase in removal percentage was observed at pH 8.5 as compared to pH 6 removal. Optimum TCS removal was observed at pH 9 (> 90% removal) before the reversals of trend at higher pH. NP degradation increased rapidly from ~ 8% at pH 6 to ~ 85% at pH 8.5, and was relatively flat between pHs 9 and 10.5. Previous oxidation studies have shown similar pH-dependent trend for BPA removal, resulting in deceased removal at pH above 9.6 (i.e., BPA pKa) when treatment was done with chlorine, permanganate and ozone oxidants (Jiang et al. 2011, Zhang et al. 2013). Similar trend was observed during peroxidase treatment of BPA by horseradish enzyme (Sakurai et al. 2004). TCS degradation optimized around pH 8 have also been reported for treatments by permanganate (Mn (VII)) (Jiang et al. 2009) and ferrate (Fe(VI)) (Yang et al. 2011). Previously, pH trend has been observed for FeIII-TAML/ H2O2 treatment of 17α-ethinylestradiol (EE2) (Chen et al. 2012), results show optimized removal at pH 10.2 before the reversals in rate constants at higher pHs.
It is however important to note that activation of H2O2 by FeIII-TAML is a precursor for the formation of the reactive intermediate complex that oxidize substrates. Studies have shown that the deprotonated form of FeIII-TAML is the most reactive specie in activating peroxide (Collins 2011, Ryabov and Collins 2009) and for the FeIII-TAML type used here (pKa 10), reactivity is optimal at pH 10 (Ellis et al. 2010, Ryabov and Collins 2009). The reactive species has been characterized as Fe(V)=O complex in non-aqeous media, while the precise nature of the reactive complex in water is not known with certainty (de Oliveira et al. 2007, Popescu et al. 2010). Above pH 10, rate of formation of the reactive intermediate decline due to deprotonation of H2O2 (pKa 11.2-11.6 (Jones 1999)). This suggests that substrates properties and reactivity of the oxidant are a pH-dependent. Possible reason for the observed optimized removal of BPA and TCS at pH lower than the pH reported for the optimized reactivity of FeIII-TAML/H2O2 (i.e., pH 10) have been elaborated elsewhere (Chapter 2).
As the degradation of BPA, TCS and NP is high at pH 9, but decreased for TCS and BPA at higher pHs, subsequent experiments were conducted at pH 9. Increasing the catalyst concentration from 4 nM to 8 nM led to over 90% removal of the three substrates. The time course removal of individual substrate (single system) by FeIII-TAML/ H2O2 (8nM/4mm) is shown in Fig. 3.2 above. As shown, rapid removal of the three substrates were detected within the first 30 min, followed by a slower removal up to 180 minutes. Over 50% BPA was removed within the first 30 min, while over 80 % TCS and NP was removed within the same period. Reducing the BPA concentration to half (22 µM) caused complete removal by FeIII-TAML/ H2O2 (4nM/4mm) with 3 hrs (data not shown). Similar increase in removal rates of BPA, TCS and NP due to increased FeIII-TAML concentration have been reported at pH 8.5 (Chapter2), indicating the ratio significant of the catalyst to substrate ratio for complete removal of substrates under investigation.
Oxidative removal of BPA, TCS and NP in binary systems by FeIII-TAML/H2O2
Selectivity for target organic compound by FeIII-TAML/H2O2 in singles systems have previously been reported (Georgi et al. 2007). Preferential selectivity for either BPA, TCS or NP by the FeIII-TAML/H2O2 system (8nM/4nM) was investigated in binary solutions; thereby introducing competition among substrates at pH 9 for available oxidant. The time-course degradation of binary mixtures consisting of BPA/TCS, BPA/NP and TCS/NP are shown in Fig. 3.3(a) – 3.3(c). Generally, rapid removal of BPA within the first 30 min is observed in the presence of TCS (Fig. 3.3a) or NP (Fig. 3.3b). BPA removal was only slightly affected by the presence of either TCS or NP as compared to BPA removal in single system (Fig. 3.2). In both binary systems (Fig. 3.3a & 3.3b), over 50% BPA was removed within the first 30 min, which is close to what is observed in the single system. However, we observed about 10% BPA residual at 180 min in the binary systems as compared to < 5% residual in the single system of Fig. 3.2 above, suggesting some level of competition of TCS and NP with BPA for available oxidant. On the other hand, the degradation of TCS and NP was observed to be greatly affected by the presence of BPA as shown in Fig. 3.3(a) and Fig. 3.30(b) respectively. The percentage TCS and NP removed (at 180 min) was lowered by > 60% in the presence of BPA. Over the 180 min time-course, only about ~10% of TCS and ~30% of NP were degraded. These results indicate selectivity for BPA by FeIII-TAML/H2O2 over TCS and NP under the experimental conditions. In a binary mixture of TCS/NP (Fig. 3.3c) the removal of both compounds was more rapid in the first 30 min as compared to their removal in the presence of BPA. The percentage residual of TCS and NP were ~60 % and ~80 % respectively at 180 min, a significant improvement as compared to their removal in the presence of BPA. However, their individual removal rate here is clearly lower as compared to the single system (Fig. 3.2). Result in Fig. 3.3 (c) clearly show preference for TCS over NP by the oxidant system.
Table of Contents
Table of Contents
List of Figures
List of Tables
List of Schemes
Chapter 1 : Introduction
1.1 Endocrine Disrupting Chemicals Selected for this Study
1.2 Previous methods used for removal of EDCs from water
1.3 Research objectives
1.4 Thesis framework
Chapter 2 Oxidative removal of aqueous contaminats bispehnol A, triclosan and nonylphenol by H2O2 activated Fe-TAML
2.2 Materials and methods
2.3 Results and discussion
2.4 Chapter conclusion
Chapter 3 Oxidative removal bispehnol A, triclosan and nonylphenol by FeIII-TAML /H2O2 competition in mixtures
3.2 Materials and methods
3.3 Results and discussion
3.4 Chapter conclusion
Chapter 4 Oxidative removal pehnol compounds by FeIII-TAML /H2O2 in the presence of natural organic compounds
4.2 Materials and methods
4.3 Results and discussion
4.4 Chapter conclusion
Chapter 5 Summary, conclusion, recommendation and future works
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Oxidation of Bisphenol A, Triclosan and 4- Nonylphenol by Fe-B* activated peroxide