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Hydroxypropyl-β-Cyclodextrin-Mediated Iron-Activated Persulfate Oxidation of Trichloroethylene and Tetrachloroethylene Chenju Liang,*,† Chiu-Fen Huang,† Nihar Mohanty,‡ Chih-Jen Lu,† and Rama Mohan Kurakalva† Department of EnVironmental Engineering, National Chung Hsing UniVersity, 250 Kuo-Kuang Road, Taichung City 402, Taiwan, and Massachusetts Department of EnVironmental Protection, 205B Lowell Street, Wilmington, Massachusetts 01887
Trichloroethylene (TCE) and tetrachloroethylene (PCE) are commonly found contaminants in soil and groundwater. However, the persulfate anion (S2O82-) is an oxidant; when activated with ferrous cation (Fe2+), it generates a stronger oxidant (known as a sulfate free radical, SO4-•), which may be used for the destruction of contaminants. Hydroxypropyl-β-cyclodextrins (HP-β-CDs) are environmentally benign glucose-based molecules that have the ability to increase the solubility of contaminants such as TCE and PCE and make them more amenable to degradation by chemical oxidation. The ultraviolet (UV) and hydrogen nuclear magnetic resonance (1H NMR) spectra of various inclusion complexes of HP-β-CD, Fe2+ cations, and TCE showed that HP-β-CD forms inclusion complexes with organics, Fe2+ cations, and combinations thereof. The apparent solubilities of TCE and PCE in aqueous solutions that contain HP-β-CD were observed to increase linearly with concentration. When persulfate was activated by the continuous addition of Fe2+ cations in a system where TCE and PCE were present as dense nonaqueous phase liquids (DNAPLs), the presence of HP-β-CD increased the dissolved contaminant concentrations and the contaminants were attacked preferentially, most likely because of a controlled and slower rate of generation of the SO4-• species. 1. Introduction Soil and groundwater contamination by dense nonaqueous phase liquids (DNAPLs) has become an issue of concern in the industrialized countries. Chlorinated organic compounds (COCs) such as trichloroethylene (TCE) and tetrachloroethylene (PCE), which are characterized as DNAPLs, are common pollutants in groundwater1 and could be significant components of hazardous waste streams. The toxic and persistent nature of TCE and PCE poses a serious health threat to humans and ecological receptors. The maximum allowable contaminant levels of TCE and PCE in water are 5 µg/L, in accordance with the drinking water standards of the U.S. Environmental Protection Agency.2 Among the various treatment technologies, advanced oxidation processes (AOPs) are becoming increasingly popular as an alternative for the treatment of organic contaminants in soils, groundwater, and industrial wastewater. AOPs rely on the use of highly reactive oxidizing radicals to oxidize organic contaminants. The application of AOPs to groundwater remediation includes the use of oxidants such as the persulfate (PS) anion (S2O82-) (with a potential of E° ) 2.01 V) activated either thermally or chemically to generate the sulfate free radical (SO4-•) (E° ) 2.6 V).3-7 Chemical oxidation methods that are used to treat chlorinated solvents are usually most effective when contaminants are present in the dissolved phase. Therefore, effective oxidation of chlorinated solvents in DNAPL phases are highly dependent on the mass-transfer mechanism between aqueous and DNAPL phases. Accelerated oxidation of a contaminant in the aqueous phase (e.g., reducing the aqueous contaminant concentration) could lead to an increase in the concentration gradient for the * To whom correspondence should be addressed. Tel.: 886-422856610.Fax: 886-4-22862587.E-mailaddress:
[email protected]. † Department of Environmental Engineering, National Chung Hsing University. ‡ Massachusetts Department of Environmental Protection.
contaminant (e.g., DNAPL dissolving into the aqueous phase).8 Hence, treatment of the contaminants where DNAPLs are present would be limited by low solubilities of target contaminants. In this study, TCE and PCE were evaluated as the model contaminants with aqueous solubilities of 8.37 and 0.90 mM, respectively.9 In the subsurface, where the presence of DNAPLs could provide a continuous source for groundwater contamination, the use of surface-active reagents (surfactants) and cosolvents such as alcohols or cyclodextrins (CDs) to increase the contaminant solubility and enhance the performance of the pump-and-treat method has been successfully demonstrated in the laboratory and in the field.10-14 If the introduction of suitable chemicals to increase the aqueous contaminant concentration can be combined with AOPs, the increased solubility may result in an increased effective contact between the oxidant and the contaminant within the aqueous phase and thereby accelerate the oxidation reaction. This hypothesis was recommended by LaChance et al.8 without supporting information. This combined approach would require an evaluation of issues such as the use of the oxidant for potential oxidation of the solubility-enhancing chemicals, in addition to oxidation of the DNAPL source. The introduction of solubility-enhancing chemicals such as cosolvents are expected to consume the oxidant and thereby decrease the oxidant strength. In addition, an associated problem of using solubility-enhancing chemicals (such as surfactants and co-solvents) is remobilization of DNAPLs by reducing the interfacial tension between the DNAPL and water. CDs are a class of solubility-enhancing chemicals. CDs are bucket-shaped cyclic oligosaccharides that consist of six, seven, or eight glucose rings; their corresponding CDs are classified as R-, β-, and γ-CDs, respectively. CDs are formed via the action of Bacillus macerans on starch.15 The torus-shaped CD has a hydrophobic interior and a hydrophilic exterior. Lindsey et al.16 reported that iron is likely coordinated by the hydroxyl oxygen on the rim of the cyclodextrin. The solubility of CDs in water increases with temperature.17 However, R- and β-CDs are
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insoluble in solvents such as methanol, ethanol, isopropanol, acetone, etc. The solubility of chemically modified derivatives of CD (e.g., hydroxypropyl-β-cyclodextrin (HP-β-CD)) in water is higher than the natural CD (e.g., β-CD). Moreover, chemically modified derivatives of CD are soluble in some organic solvents. Because of the structure and character of CDs, they have the ability to form an inclusion complex simultaneously with metals at the polar side while entrapping a hydrophobic compound in its internal cavity.18 Similar illustrations of a hypothesized configuration of cadmium-anthrancene-carboxymethyl-β-cyclodextrin were also proposed by Wang and Brusseau.19 CDenhanced solubility has been used in conjunction with other subsurface remediation processes to improve the efficiency and time required to remediate the contamination. For example, Wang et al.20 demonstrated that HP-β-CD significantly increased the apparent solubility of phenanthrene that resulted in its enhanced biodegradation rate. Leite et al.21 demonstrated that the inclusion compound of ferrous ion lactate and CD provided the possibility to regulate a reduced form of iron (i.e., ferrous ion) under an oxidizing condition in the bulk solution. Lindsey et al.16 investigated the application of two CDs (β-CD and carboxymethyl-β-CD) for simultaneous complexation of ferrous ion and the organic compounds (e.g., phenol, pyrene) in Fenton’s reaction. Their results indicated that the ternary hydrophobic organics-CD-iron complexes effectively direct hydroxyl radicals toward the target contaminant, which results in increased degradation rates of the target contaminants. Bizzigotti et al.22 reported that increasing the concentrations of R-, β-, and γ-CDs resulted in only slight increases in the interfacial tension between PCE and water. Wang and Brusseau23 also reported that the magnitude of the reduction of surface tension by HP-β-CD is much less than the value of typically encountered with micelle-forming surfactants. Unlike the solubility enhancement of contaminants with surfactants by the mechanism of reducing the interfacial tension, CDs have the ability to form inclusion complexes with hydrophobic compounds that are water-soluble. This characteristic of the CDs can be beneficial when used in conjunction with other soil and groundwater remediation technologies, such as in situ chemical oxidation (ISCO), to overcome the problem of contaminant remobilization and to increase contaminant solubility for oxidation to occur in the aqueous phase. ISCO with persulfate is a viable technology for remediation of TCE4-6 and PCE. The persulfate anion with a redox potential of 2.01 V is a strong oxidant24 and is capable of degrading a variety of organic compounds. At ambient temperature (∼20 °C), a transition-metal (e.g., Fe2+)-activated persulfate anion leads to the formation of the SO4-• species, which exhibits a redox potential of 2.6 V.25 The overall stoichiometric reaction between the persulfate anion and the ferrous cation is shown in the following equations:26
2Fe2+ + S2O82- f 2Fe3+ + 2SO42-
(1)
through the steps
Fe2+ + S2O82- f Fe3+ + SO4-• + SO42-
(2)
SO4-• + Fe2+ f Fe3+ + SO42-
(3)
The persulfate anion-ferrous cation reaction results in a rapid generation of sulfate free radicals. A half-life of 4 s was reported for the SO4-• species. at 40 °C when the persulfate and Fe2+ concentrations were 10-3 M.27 The sulfate free radical oxidizes the Fe2+ cation through eq 3 to the Fe3+ form of iron. The
reaction coefficient (at a diffusion-controlled rate) for eq 3 has been reported to be 1 × 109 M-1 s-1.28 A review of the reaction between the persulfate anion and the Fe2+ cation has been described by Liang et al.5,6 They have suggested that increasing the concentration of Fe2+the Fe2+ cations would accelerate the reactions of eqs 2 and 3, leading to completion of the overall reaction shown in eq 1. Therefore, the fast reaction between the SO4-• species and excess Fe2+ cations could possibly result in cannibalization of the SO4-• species (see eq 3), resulting in a reduction in degradation efficiency of the target contaminants. Degradation efficiency can be defined as the requirement of persulfate anions per mole of removal of the contaminant: ∆[S2O82-]/∆[contaminant]. In other words, a lower degradation efficiency means a higher value of ∆[S2O82-]/∆[contaminant]. As discussed previously, elevated levels of Fe2+ cations in solution would result in a preferential reaction of the Fe2+ cation with the SO4-• species. To optimize Fe2+-activated persulfate oxidation of a target organic contaminant, it is necessary to slow or control the reaction shown in eq 3. This process could possibly be accomplished with a controlled release of Fe2+ cations as an activator, thereby preventing the rapid conversion of the Fe2+ cation to the Fe3+ state by the SO4-• species. Because CDs have the capability of complexing organic contaminants while binding Fe2+ cations, CDs exhibit several potential advantages when used in conjunction with the Fe2+activated persulfate system: (1) CDs can increase the contaminant solubility; therefore, it would enhance the reactivity of sulfate free radicals toward the dissolved target contaminant in the aqueous phase. (2) CDs would not significantly reduce the interfacial surface tension; therefore, it could prevent further migration of contaminants from the source zone, in comparison to other solubility-enhancing chemicals when treating subsurface contamination. (3) While complexing both organic contaminants and the Fe2+ cation, the sulfate free radical formed by the Fe2+ activation near the CD’s cavity would be in close proximity to the complexed contaminants. (4) Because CDs are originally formed from starch and are biodegradable, nontoxic, and environmentally benign, they should be acceptable for release at hazardous waste disposal sites.15 The objective of this research was to evaluate the treatment of aqueous TCE and PCE contamination by means of cyclodextrin-mediated Fe2+-activated persulfate oxidation at a laboratory scale. A derivative cyclodextrin HP-β-CD was used in this study. To explore the treatment performance, this study experimentally tested the feasibility of this oxidation system in an aqueous system at 20 °C with a goal to understand (1) the effect of the Fe2+ cations on the solubility enhancement of the target contaminant; (2) the decomposition of persulfate when activated by HP-β-CD/Fe2+; and (3) the effectiveness of HP-β-CD/Fe2+ in mediating oxidation of TCE and PCE by persulfate, when TCE and PCE are present as nonaqueous phase liquids (NAPLs). This was evaluated by studying oxidation at two different concentrations: one indicative of the presence of NAPL, and the other at a lower concentration. In the low-concentration experiments, the contaminant concentrations were below their solubilities at 20 °C. 2. Experimental Methods 2.1. Chemicals and Materials. The HP-β-CD (C51H88O38, 97%, average molecular weight of MW ) 1420) and TCE (C2HCl3, g99.5%) were obtained from Acros Organics. Ferrous
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Figure 1. Ultraviolet (UV) spectra of cyclodextrins inclusion formation; the hydroxypropyl-β-cyclodextrin (HP-β-CD) and Fe2+ concentrations are 17.9 mM, and the TCE concentration is 16.3 mM. Inset shows the UV spectra for TCE alone (0.36 mM).
sulfate (FeSO4‚7H2O, 100.8%), PCE (C2Cl4, 99.0% min), n-pentane (C5H12, g99.0%), and potassium iodide (KI, 100.3%) were purchased from J.T. Baker. Sodium persulfate (Na2S2O8, g99.0%) and sodium hydroxide (NaOH, 99.0% min) were purchased from Merck. Methyl alcohol (CH3OH, 99.0% min) was obtained from Mallinckrodt. Sodium thiosulfate (Na2S2O3‚ 5H2O, 99.5% min) was obtained from Riedel-de Hae¨n. Sulfuric acid (H2SO4, g99.8%) was obtained from Fluka. D2O (99.9%) was purchased from Aldrich. Water was purified using a Millipore reverse osmosis (RO) purification system. 2.2. Experimental Procedure. For solubility enhancement experiments, depending on the experimental designs, solutions that contained various concentrations of HP-β-CD or HP-βCD/Fe2+ were filled in 130-mL glass serum vials with crimptop polytetrafluoroethylene (PTFE)/rubber-lined aluminum seal septa. The pH of RO water was adjusted by adding appropriate volumes of 0.1 N H2SO4 or 0.1 N NaOH to a solution of pH 3.0 ( 0.2. Triplicate vials were prepared for each concentration. Thereafter, 1 mL of pure TCE or PCE was added to each vial, to have excess levels of contaminants. For instance, if the contaminants are completely dissolved, TCE and PCE concentrations would be 85.4 mM and 75.3 mM, which exceed the solubility limits by ∼10-fold for TCE and ∼80-fold for PCE. All vials were placed in a temperature-controlled chamber at 20 °C and mixed for 48 h using a magnetic stirrer. Thereafter, all samples were allowed to equilibrate for 24 h without mixing and then a quantity of 2 mL of solution was withdrawn through the septa using a 5-mL gas-tight syringe for TCE or PCE analysis. For experiments that evaluated the influence of the Fe2+ cation and pH on solubility, solutions were adjusted to pH 3.0 ( 0.2 and other desired pH levels of 5, 7, and 10, within a pH unit variation of 0.2. In the experiments related to degradation of persulfate in the absence of contaminants, solutions were prepared in a 1-L volumetric flask, based on the desired PS/HP-β-CD/Fe2+ molar ratios. The solution in the flask was continuously mixed on a magnetic stirrer to ensure complete mixing. At desired sampling intervals, an aliquot of the solution was withdrawn and analyzed for persulfate content, pH, and Fe2+ cation content. The experiments related to the oxidation of TCE and PCE using persulfate activated by HP-β-CD/Fe2+ were divided into two parts: oxidation of TCE or PCE at low and high concentrations. Low and high concentrations for TCE were 0.36 and 10.7
mM, respectively; for PCE, the concentrations were 0.36 and 2.0 mM, respectively. For the low-concentration tests, contaminant solutions were prepared in a 2-L borosilicate reservoir (Schott Puran) equipped with a Teflon stopper and valved bottom outlet. Based on the experimental designs, the contaminants (TCE or PCE), HP-βCD, and/or Fe2+ cations were added to the reservoir and the resulting solution was left mixing overnight. At the beginning of each experiment, a predetermined amount of sodium persulfate was added to the reservoir and mixed for a few minutes. Thereafter, a series of 60-mL amber glass reaction bottles were filled fully with the mixed solution. At each designated time interval, reaction bottles were removed for analysis of the contaminants and persulfate anion concentrations. This part of the experimental procedure was in accordance with the method described by Liang et al.4 For the high-concentration tests, the oxidation reaction was conducted in a 1.3-L heavy-wall plain pressure reaction flask (ACE Glass) that was placed in a temperature-controlled chamber at 20 °C and the top of the flask was covered with a flat Teflon reaction head that was sealed with a stainless steel clamp. The effect of single dosing versus continuous dosing of Fe2+ cations on the oxidation reactions was assessed by supplying the same amount of Fe2+ cations to the reaction vials in a single dose or continuously. The contaminant solution was prepared with a HP-β-CD concentration of 3.58 mM and predetermined amounts of TCE (1.24 mL) or PCE (0.26 mL) were added. Note that the ratio and amounts of HP-β-CD and TCE or PCE were estimated based on the solubility tests. In tests where the Fe2+ cation was added in a single dose, the Fe2+ cation was added initially when the contaminant solution was being prepared. In tests where the Fe2+ cation was added continuously, a Fe2+ stock solution was prepared (95.3 and 158.8 mM) and pumped into the reaction flask at a rate of 2.5 mL/ min, using a peristaltic pump. Note that the pH of the Fe2+ stock solution was adjusted to 3.0, to maintain the reduced form of iron (i.e., Fe2+). At the beginning of each test, 100 mL of solution was removed from the vessel, to analyze for the initial TCE or PCE concentrations and pH. The required amount of persulfate was predissolved in 100 mL of RO water and then was added into the flask to ensure there was no head space in the flask. However, for the tests that involved the continuous addition of Fe2+ cations, a 125-mL aliquot of solution was
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Figure 2. 1H NMR in D2O of (a) FeSO4, (b) TCE, (c) HP-β-CD/TCE, (d) HP-β-CD, (e) HP-β-CD/Fe2+/TCE, and (f) HP-β-CD/Fe2+. The concentrations used for FeSO4 and HP-β-CD were 17.9 mM, and, for TCE, the concentration was 8.3 mM (for spectrum b only), whereas in the presence of HP-β-CD, the TCE concentration used was 16.3 mM for spectra c, e, and f, because of enhanced solubility.
removed for analysis and the sodium persulfate was added directly into the vessel. After the removal of the 125-mL sample for analysis, a volume of 125 mL was available in the flask and the reaction flask could be continuously dosed with Fe2+ cations for 50 min at a pumping rate of 2.5 mL/min. Note that dilution by adding persulfate solution was corrected for subsequent data analysis (variation of
