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Ind. Eng. Chem. Res. 2009, 48, 1586–1592
GENERAL RESEARCH Removal of Toxic Organic Micropollutants with FeTsPc-Immobilized Amberlite/ H2O2: Effect of Physicochemical Properties of Toxic Chemicals Jae-Hyuk Kim,† Se-Joong Kim,† Chung-Hak Lee,*,† and Heock-Hoi Kwon‡ School of Chemical and Biological Engineering, Seoul National UniVersity, 56-1 Sillim-dong, Kwanak-gu, Seoul, 151-744, Korea, and Department of Chemical and EnVironmental Engineering, Soongsil UniVersity, Sangdo-5 dong, Dongjak-gu, Seoul, 156-743, Korea
The effect of physicochemical properties of organic micropollutants on removal efficiencies over irontetrasulfophthalocyanine (FeTsPc)-immobilized Amberlite was investigated with one endocrine-disrupting chemical (EDC), bisphenol-A (BPA), and three pharmaceutically active chemicals (PhACs), cefaclor, diclofenac, and ibuprofen. The electrical charge of each chemical was the most important factor in overall removal efficiency. Hydrophilicity was the second most important factor. Negatively charged diclofenac and ibuprofen were completely removed at the reaction pH of 7.5 in less than 1 hour in the absence of H2O2. The FeTsPc-immobilized Amberlite catalyst accelerated BPA and cefaclor removal in the presence of H2O2 at pH 7.5. For BPA, catalytic oxidation accounted for at least 35% of total removal. The stability of FeTsPc was greatly improved by immobilizing it on Amberlite compared to that of the homogeneous FeTsPc in an aqueous solution. 1. Introduction As the plastics and polymer-resins industries have rapidly expanded to meet increasing demands, chemicals from these industries have been released in greater quantities into the natural environment. Recent studies have revealed that some of these chemicals, even in small amounts, are extremely harmful to human health.1-4 Chemicals that adversely affect hormonal systems of humans and other mammals have been termed endocrine-disrupting chemicals (EDCs). In addition, dumping and release of pharmaceutically active compounds (PhACs) into the environment have caused contamination of drinking water and have become another health issue.5-7 Several treatment methods for the removal of some of these EDCs and PhACs have been investigated, including coagulation, adsorption, chlorination, electrochemical oxidation, ozonation, and photocatalytic oxidation.8-14 Unfortunately, however, coagulation and adsorption require additional chemical treatment processes to degrade the captured chemicals. The latter treatment methods have disadvantages such as formation of chlorinated byproduct (chlorination) and high energy consumption (electrochemical oxidation, ozonation, and photocatalytic oxidation). To address these issues, some researchers have investigated iron(III)-tetrasulfophthalocyanine (FeTsPc), a novel homogeneous oxidation catalyst, using hydrogen peroxide as the oxidant due to its high reactivity and because no additional energy consumption is required.15-18 Despite the many advantages of the homogeneous FeTsPc, its rapid deactivation19 and acidic reaction conditions and/or the requirement for an organic cosolvent for maintaining catalytically active monomeric species have been critical obstacles to overcome for purifying contaminated water. To solve these problems, Sorokin and Meunier immobilized FeTsPc onto Amberlite, an anion exchange resin, and reported * To whom correspondence should be addressed. E-mail: leech@ snu.ac.kr. Tel.: +82-2-880-7075. Fax: +82-2-874-0896. † Seoul National University. ‡ Soongsil University.
enhanced catalytic activity for oxidation of trichlorophenol at neutral pH even without an organic cosolvent.15 However, they focused only on the oxidation capability of the FeTsPc catalyst even though their target pollutant could be removed by the catalyst by one or a combination of three of the following processes: adsorption, ion exchange, and oxidation. In addition, the removal efficiencies of the water treatment processes can be greatly affected by characteristics of the target pollutants, such as hydrophilicity and electrical charge. For instance, two PhACs, ibuprofen and diclofenac, showed quite different removal efficiencies in several Korean wastewater treatment plants operating with membrane bioreactor (MBR); almost all ibuprofen was removed while diclofenac was nearly intact.20 The removal efficiencies of diclofenac and ibuprofen via oxidation processes such as chlorination and ozonation were also quite different: ∼100% for diclofenac and 10∼60% for ibuprofen.9,12 In this study, four different target pollutantssone EDC, bisphenol-A (BPA), and three PhACs, diclofenac, cefaclor, and ibuprofenswere employed to investigate the effect of the hydrophilicity and electrical charge of the pollutants on their removal efficiencies over FeTsPc-immobilized Amberlite. The removal efficiencies of the columns packed with the FeTsPcimmobilized Amberlite for the four pollutants were evaluated and correlated with their physicochemical characteristics. The contribution of oxidation to overall removal efficiency of the FeTsPc-Amb/H2O2 was also quantified. Furthermore, the reaction intermediates from the heterogeneous catalytic reaction were monitored for BPA and compared to those from the homogeneous catalytic reaction. Finally, the stability change of FeTsPc after immobilization on Amberlite was also evaluated. 2. Experimental Section 2.1. Chemicals. Iron(III)-tetrasulfophthalocyanine (FeTsPc), 2,2-bis(4-hydroxyphenyl)propane (bisphenol-A, BPA), diclofenac, cefaclor, ibuprofen, hydrogen peroxide (H2O2, 35%), and
10.1021/ie071412k CCC: $40.75 2009 American Chemical Society Published on Web 12/23/2008
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1587 Table 1. Physicochemical Properties and Molecular Structure of Target Pollutants
a
Octanol s water partitioning coefficient.
Amberlite IRA-400 (Amb) were purchased from Aldrich and used without further purification. Table 1 summarizes the physicochemical properties of the pollutants used. Cefaclor and diclofenac are more hydrophilic than BPA, as shown by their smaller log Kow values (octanol-water partitioning coefficient) while diclofenac is negatively charged and cefaclor is partially negatively charged at neutral pH, as shown by their pKa values. Ibuprofen is also negatively charged at neutral pH but it has hydrophobic characteristics like BPA. Stock solutions of each solute (100 mg/L) were prepared by dissolving each solute in DI water prior to performing the experiments. 2.2. Immobilization Procedure. To prepare monomeric FeTsPc, 20 mg of FeTsPc (Aldrich) was completely dissolved in 40 mL of deionized water, and then 40 mL of acetonitrile, a cosolvent, was added. After 1 h of gentle magnetic stirring, 2 g of Amberlite was added, and the mixture was gently stirred for 24 h, following previously published methods.15 The resulting FeTsPc-Amberlite (FeTsPc-Amb) was filtered using a GFC filter, washed with deionized water several times, and then dried in a 60 °C oven for 4 h. The fraction of FeTsPc immobilized on Amberlite was determined by measuring the FeTsPc concentration in the filtrate with UV-vis spectroscopy (DR4000, Hach). 2.3. Catalytic Reaction and Product Analysis. A 300 mg portion of the Amberlite or FeTsPc-Amb in granule form was packed into a 10 cm-long glass column with an inner diameter of 3 mm. This not only avoids breakage of the brittle immobilized catalyst, but also facilitates separation and reuse of the immobilized catalyst. Solutions containing each pollutant (100 mL, 2 mg-solute/L) were recirculated at room temperature through the packed column at a flow rate of 27 mL/min using a piston pump as depicted in Figure 2. Even though typical concentrations of these pollutants in the aquatic environment are far less than 2 mg/L, experiments were carried out with this concentration for convenient quantification. Nonetheless, evaluating the prepared samples with this pollutant concentration should create no problem in understanding their behavior in the oxidation reaction system with the catalyst. If needed, hydrogen peroxide (100 µL, 3.5%) was introduced to the solution before circulation. The pH of the solution was set at 7.5 with phosphate buffer. The same reaction condition
was applied to the removal experiments for other target compounds, other than the target chemical itself. The concentrations of pollutants were quantified using high performance liquid chromatography (HPLC): a 250 mm × 4.6 mm, OPTIMAPAK C18 column connected to a UV detector (2487 dual γ absorbance detector, Waters). The mobile phase, a mixture of water/acetonitrile (50/50, v/v, 0.1 vol % phosphoric acid) was pumped at a flow rate of 1 mL/min. The detection wavelength was set at 228 nm. A LC-MS (QUATTRO LC Triple Quadrupole Tandem Mass Spectrometer, UK) was used to identify byproducts from the BPA oxidation reaction using the same mobile phase and column as described for the HPLC analysis. 3. Results and Discussion 3.1. Characterization of Amberlite after FeTsPc Immobilization. After immobilization, the fraction of FeTsPc immobilized on Amberlite, defined by the mass ratio of immobilized FeTsPc to Amberlite, was 0.7 wt %. The color of the Amberlite changed from pale yellow to dark blue after immobilizing FeTsPc, as shown in the inner boxes in Figure 1. Once the FeTsPc-Amb was prepared, FeTsPc did not leach into the water because of the strong electrostatic interaction between FeTsPc and Amberlite, verified by the filtrate after 2 h of stirring of the prepared FeTsPc-Amb in water. The composition of the FeTsPc-Amb is shown in Figure 1, as determined by scanning electron microscope-energy dispersive spectroscopy (SEM-EDS). Before immobilization, carbon, oxygen, and chlorine were detected, whereas additional iron and sulfurs elements comprising the FeTsPc moleculeswere identified after immobilization of FeTsPc. The atomic percentage of chlorine decreased from 6.07% to 4.94% after FeTsPc immobilization, indicating that some of the chloride was ion-exchanged by FeTsPc. 3.2. BPA Removal via FeTsPc-Amb Packed in a Column. Since each experiment was performed with or without H2O2, a total of four systems were tested: Amberlite alone (Amberlite), Amberlite with H2O2 (Amberlite + H2O2), FeTsPc-Amb (FeTsPc-Amb), and FeTsPc-Amb with H2O2 (FeTsPc-Amb + H2O2). The results are shown in Figure 2. Interestingly, Amberlite itself removed approximately 70% of
1588 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009
Figure 1. The atomic composition of the Amberlite surface (a) before and (b) after FeTsPc immobilization. The composition was identified with scanning electron microscope-energy dispersive spectroscopy (SEM-EDS).
Figure 2. Comparison of BPA concentration changes in Amberlite with/ without H2O2 (2/∆) and FeTsPc-immobilized Amberlite with/without H2O2 (b/O). A 300 mg portion of each sample was packed in glass columns. The pH was set at 7.5 with phosphate buffer.
BPA in 2 h. Given that the pKa1 and pKa2 of BPA are 9.6 and 10.2, respectively,21 most of the BPA should be protonated at the pH of the reaction system, ∼7.5. Therefore, the removal of
BPA by Amberlite via the ion-exchange mechanism can be excluded at this neutral pH condition, and this result was verified experimentally (not shown). Since Amberlite is a porous material that has a hydrophobic matrix (styrene-divinylbenzene), the principal mechanism for BPA removal by Amberlite should be adsorption. A similar effect of pH on adsorption and ion exchange of Amberlite has been reported by other researchers. Carmona et al. investigated adsorption and the ion exchange equilibrium of phenol on Amberlite IRA-420 and proposed that phenol uptake onto Amberlite IRA-420 occurs by adsorption at an acidic pH, and by both adsorption and ion exchange at an alkaline pH.22 There was almost no difference in BPA removal rates among the first three reaction systems, implying that H2O2 alone can neither directly oxidize BPA, nor affect the adsorption capacity of Amberlite. Immobilizing FeTsPc onto Amberlite did not affect the adsorption capacity of Amberlite either. The FeTsPc-Amb revealed its oxidation ability only when H2O2 was added to the system. When H2O2 was introduced into the FeTsPc-Amb system, BPA was removed more quickly and almost completely after 2 h of operation. This is because, in addition to the adsorption of BPA by Amberlite, BPA was oxidized by FeTsPc in the presence of H2O2. This result makes the FeTsPc-Amb catalytic oxidation system very attractive as
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Figure 3. Changes in concentration of the target pollutants as a function of reaction time in the presence of 300 mg of FeTsPc-Amb packed in a glass column that treated a 100 mL solution of each chemical (2 mg/L). The pH was set at 7.5 with phosphate buffer.
the removal efficiency of target organic pollutants can easily be boosted by simple injection of H2O2. 3.3. Effect of Hydrophilicity and Electrical Charge of Organic Pollutants. To investigate the effect of chemical properties of micropollutants such as hydrophilicity and electrical charge on removal efficiencies over Amberlite and FeTsPc-Amb, three pharmaceutically active chemicals, cefaclor, diclofenac, and ibuprofen were used. In our preliminary experiment, we found the oxidative degradability of each chemical via homogeneous FeTsPc/H2O2 to be ranked in the following order: BPA ≈ diclofenac > cefaclor > ibuprofen. The differences in oxidative degradability should affect the overall removal efficiency of the FeTsPc-Amb/H2O2 system. The changes in concentration of these organic pollutants using FeTsPc-Amb without H2O2 were monitored and are presented in Figure 3. Only ion-exchange and adsorption of the chemicals can occur in the system. As expected, hydrophilic cefaclor was removed more slowly and in lower quantities onto Amberlite than BPA due to its weak interaction with the hydrophobic Amberlite matrix. Given its pKa, cefaclor should be partially negatively charged at the reaction pH. Thus the initial concentration drop was more likely to be due to ion-exchange than adsorption. At a low pH of less than 2, cefaclor was not removed by FeTsPc-Amb, implying that hydrophilic cefaclor cannot be noticeably adsorbed onto FeTsPc-Amb (data not shown). On the other hand, at pH 7.5, negatively charged diclofenac and ibuprofen were readily removed by Amberlite within 1 hour regardless of their hydrophilicity, indicating that the major removal mechanism was fast ion-exchange. The prepared FeTsPc-Amb played its role as an oxidation catalyst again when H2O2 was added as an oxidant (Figure 4). For cefaclor, the most difficult compound to remove, an additional decrease in concentration was observed with the FeTsPc-Amb when H2O2 was supplied. Compared to the case of FeTsPc-Amb without H2O2, it is obvious that this enhancement was from chemical oxidation by the activated FeTsPc catalyst. Diclofenac and ibuprofen were removed so fast, however, with FeTsPc-Amb, regardless of the presence of H2O2, that the role of FeTsPc as a catalyst could not be assessed under this experimental condition. 3.4. Removal Capacity of FeTsPc-Amb with and without H2O2. To determine the numerical removal capacities of FeTsPc-Amb for these organic pollutants with and without
Figure 4. Changes in concentration of (a) cefaclor, (b) diclofenac, and (c) ibuprofen in the presence of 300 mg of the FeTsPc-Amb catalyst packed in glass columns with and without H2O2 addition, respectively. Other reaction conditions were the same as those shown in Figure 3.
H2O2, a series of experiments was carried out with higher initial concentrations of the solutes (∼50 mg/L) and varying amount of FeTsPc-Amb to avoid solute depletion (shown in Supporting Information Figures S1-S4 and summarized in Table 2). BPA removal by adsorption of FeTsPc-Amb became saturated at approximately 20 h and reached removal capacity of 40.8 mgBPA/g-FeTsPc-Amb. In the presence of H2O2, however, the concentration of BPA decreased continuously even after 20 h
1590 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 Table 2. The Removal Capacity (Mass of the Solute Removal/Mass of Solid, mg/g) and Removal Efficiency (%, in Parenthesis) of FeTsPc-Amb with and without H2O2a BPA FeTsPc-Amb 40.8 (49%) FeTsPc-Amb + 58.6b (70%) H2O2
cefaclor
diclofenac
ibuprofen
5.9 (21%) 17.1 (66%)
190.2 (81%) 234.6 (99%)
>224 (>89%)
a Initial loadings for BPA, cefaclor, diclofenac, and ibuprofen are 83, 26, 235, and 251 mg-solute/g-solid, respectively. b The BPA removal increased with reaction time and the reported number was measured at 45 h. The other figures in this table were obtained when removal efficiencies reached their asymptote.
of reaction due to catalytic oxidation of the immobilized FeTsPc, and the removal capacity improved to be 58.6 mg-BPA/gFeTsPc-Amb at 45 h of reaction. The hydrophilic cefaclor was slightly removed, as expected, by adsorption of FeTsPc-Amb (21%); contrarily, it was removed more than 64% when H2O2 was provided. The removal capacity of FeTsPc-Amb for cefaclor was 5.9 mg-cefaclor/gFeTsPc-Amb without H2O2 and 17.1 mg-cefaclor/g-FeTsPcAmb with H2O2. The negatively charged solutessdiclofenac and ibuprofenswere removed easily by FeTsPc-Amb even without H2O2 owing to rapid ion-exchange and adsorption. Interestingly, it was found that the diclofenac removal was slightly improved in the presence of H2O2 while ibuprofen removal was not, implying that diclofenac oxidation by FeTsPc/H2O2 was relatively easier than ibuprofen oxidation. The removal capacity of FeTsPc-Amb for the diclofenac was 190.2 mg-diclofenac/g-FeTsPc-Amb without H2O2 and 234.6 mg-diclofenac/g-FeTsPc-Amb with H2O2. For ibuprofen, it was >224 mg-ibuprofen/g-FeTsPc-Amb regardless of H2O2 presence. It is worth noting, comparing with other research data introduced below, that the removal capacities of FeTsPc-Amb with H2O2 for these organic pollutants having different physicochemical properties are relatively high and removal efficiencies ranged from 66 to 99%. Recently, Chen et al. developed polyethersulfone-modified montmorillonite hybrid bead to remove BPA; however, its removal capacity was lower than 13 mg/g.23 For ibuprofen and diclofenac removal by conventional coagulation process, it was less than 20%.24,25 The FeTsPc-Amb with H2O2 showed far better removal efficiencies (89 and 99%, respectively). Although adsorption with powdered activated carbon could remove hydrophobic BPA substantially (>200–400 mg/g-carbon),26 the removal efficiencies for the hydrophilic and/ or the charged compounds such as cefaclor, diclofenac, and ibuprofen were found to be low (16∼69%).27 Some researchers have tried to remove these pollutants with activated sludge by utilizing biosorption and/or biodegradation.28,29 However, the removal capacity was lower than 0.03 mg/g MLSS. 3.5. Concentration Profiles of Reaction Intermediates of BPA Oxidation via FeTsPc-Amb/H2O2. In our previous experiments with the homogeneous FeTsPc/H2O2 reaction system, 4-isopropenyl phenol and BPA-o-quinone were identified as reaction intermediates of BPA oxidation.30 In this study of BPA oxidation with FeTsPc-Amb/H2O2, these two reaction intermediates were also observed in the HPLC chromatogram. As it is hard to quantitatively measure concentrations of the intermediates due to the lack of commercially available standard chemicals, changes in HPLC peak areas of the two intermediates were plotted as a function of reaction time, as shown in Figure 5. After BPA was significantly degraded, the concentration of 4-isopropenyl phenol and BPA-o-quinone reached their maxima and then gradually decreased, which indicated that these intermediates underwent further oxidation to the final reaction
Figure 5. The concentration profiles of BPA and reaction intermediates with reaction time.
products, such as acetic, oxalic, and maleic acids, etc.17,30-32 The concentration profiles of BPA and the reaction intermediates were similar to the profiles obtained from BPA oxidation by the homogeneous FeTsPc/H2O2,30 implying that the immobilized FeTsPc oxidized BPA via an oxidation mechanism analogous to the homogeneous FeTsPc/H2O2. 3.6. Contribution of Catalytic Oxidation to Overall BPA Removal. It was noted in the previous sections that BPA removal occurred by both adsorption and catalytic oxidation when H2O2 was supplied. Therefore, it would be interesting to quantify the relative contributions of oxidation and adsorption to overall BPA removal. As adsorption is generally reversible and concentration-dependent, a simple comparison of total BPA removal in the two systems, for instance, Amberlite only and the FeTsPc-Amb/H2O2, would not accurately reflect the relative contribution of oxidation to total BPA removal. To measure the extent of BPA removal by oxidation only, the maximum amount of the reaction intermediates4-isopropenyl phenolsin the solution, produced by FeTsPc-Amb/H2O2 (at 20 min of reaction time) was compared to that produced by the homogeneous FeTsPc/H2O2 (Figure 5) because in the homogeneous FeTsPc/H2O2 system, all of the removed BPA was due to oxidation and the amount of degraded BPA was regarded as being closely related to the amount of the reaction intermediate produced. BPA conversion and 4-isopropenyl phenol production were found to be directly proportional to the amount of H2O2 added to the homogeneous FeTsPc/H2O2 (not shown). In addition, as shown in Figure 6, there was a linear relationship between the oxidized amount of BPA and the amount of 4-isopropenyl phenol formed (quantified by HPLC peak area). In Figure 6, the amount of BPA oxidized by FeTsPc-Amb/H2O2, which was converted to the 4-isopropenyl phenol, was calculated by substituting the highest HPLC area of 4-isopropenyl phenol (8373 µV · sec, at 20 min of reaction time) into the calibration curve, resulting in 0.7 mg/L of BPA. This result indicates that at least 35% of BPA was removed by the oxidation mechanism, given an initial BPA concentration of 2 mg/L. This percentage must be a minimum estimate as 4-isopropenyl phenol can be adsorbed by Amberlite as well. 3.7. Catalytic Stability of FeTsPc-Amb. To evaluate the long-term catalytic stability of FeTsPc-Amb, BPA and H2O2 were injected sequentially into the solution that was being recirculated through a column packed with either Amberlite or FeTsPc-Amb. BPA and H2O2 were added hourly and the BPA concentration was monitored as shown in Figure 7. For the Amberlite system, even though BPA removal took place by adsorption, the BPA concentration increased continuously
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Figure 6. Relationship between the amount of 4-isopropenyl phenol produced, given as HPLC peak area, and degraded amount of BPA in homogeneous FeTsPc/H2O2.
important factor in overall removal efficiency; hydrophilic diclofenac and relatively hydrophobic ibuprofen were both readily removed over FeTsPc-Amb owing to their negative charge. Hydrophilicity was the next most important factor; electrically neutral and hydrophobic BPA was removed more slowly by adsorption over hydrophobic FeTsPc-Amb than diclofenac and ibuprofen. Partially charged cefaclor was removed by an ion-exchange mechanism but removal efficiency was the worst because of its hydrophilicity. The prepared FeTsPc-Amb catalyst further oxidized BPA and cefaclor in the presence of H2O2 at neutral pH, with almost complete removal within 2 h. The removal capacities of FeTsPc-Amb for these organic pollutants with and without H2O2 also were determined numerically with higher initial concentrations of the solutes. For BPA, after comparing the reaction intermediate, 4-isopropenyl phenol, from the homogeneous and immobilized FeTsPc catalyst, at least 35% of total BPA removal turned out to be due to catalytic oxidation. The stability of FeTsPc was greatly improved upon immobilization as FeTsPc aggregation, which leads to catalyst deactivation, was limited. Acknowledgment This work was supported by the Korea Science and Engineering Foundation (No. R01-2005-000-10517-0 (2006). Supporting Information Available: Removal capacities of FeTsPc-Amb with and without H2O2 for BPA, ibuprofen, diclofenac, and cefaclor (Figures S1-S4). This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited
Figure 7. Consecutive BPA removal by Amberlite and FeTsPc-Amb in columns, respectively, after the sequential injection of BPA and H2O2 mixture.
as injection of BPA and H2O2 went on. The FeTsPc-Amb system, however, maintained most of its initial BPA removal efficiency, even after several injections of BPA and H2O2. It is worth noting that when BPA and H2O2 were intermittently injected into the homogeneous FeTsPc system, the BPA removal efficiency of the catalyst gradually decreased to approximately 10% of its initial value by deactivation, via for example, catalyst aggregation. In summary, immobilizing FeTsPc onto Amberlite made it possible not only to maintain its high activity even at neutral pH, but also to enhance its stability. 4. Conclusion The effect of physicochemical properties of micropollutants, such as hydrophilicity and electrical charge, on removal efficiencies over FeTsPc-Amb without H2O2 was investigated with one endocrine-disrupting chemical, bisphenol-A, and three pharmaceutically active chemicals, cefaclor, diclofenac, and ibuprofen. On the basis of the pKa values of each compound and reaction pH, a negative charge was the most
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ReceiVed for reView October 18, 2007 ReVised manuscript receiVed November 17, 2008 Accepted November 24, 2008 IE071412K
