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A Polar-Nonpolar, Acetic Acid/Heptane, Solvent Medium for Degradation of Pyrene by Ozone P. K. Andrew Hong* and Jiun-Chi Chao Department of Civil and Environmental Engineering, University of Utah, Salt Lake City, Utah 84112
Pyrene of natural and anthropogenic sources is one of the toxic, mutagenic polycyclic aromatic hydrocarbons (PAHs) listed as priority pollutants. The recalcitrant nature of pyrene and other PAHs lies in part in their low solubility in water, rendering them less susceptible to chemical or biological degradation. This work investigates the use of a polar-nonpolar (PNP) solvent system, in conjunction with ozonation, to overcome this remediation obstacle. The PNP solvent system consists of equal portions of polar acetic acid and nonpolar heptane. The heptane component enables high concentrations of pyrene to be dissolved, while the acetic acid keeps subsequently formed polar intermediates and byproducts in solution. This PNP solvent system maintains effective exposure of all compounds to aqueous ozone throughout the course of reaction and prevents the formation of solid precipitation. Following ozonation, the solvent system is added with a small amount of water that results in the formation of two distinct phases. The lighter upper heptane phase contains any remaining parent pyrene and little if any hydrophilic intermediates, whereas the heavier lower aqueous acetic acid solution accommodates a plethora of polar intermediates formed during ozonation. The intermediates-laden acetic acid solution (e.g., 95% water) is further subject to biochemical oxygen demand (BOD) and Escherichia coli toxicity tests to assess the inhibitory effect of the intermediates. The results suggest that for a heavy load of pyrene (e.g., 600 mg/L) in the PNP solvent system, brief ozonation treatment (e.g., 10-20 min) removes virtually all pyrene and significantly decreases the toxicity of the intermediates, as evidenced by increased BOD measured in the effluent; this is further corroborated by a decrease in E. coli inhibition by the intermediates from -15.8% in the toxic range to -3.27% in the nontoxic range. The PNP solvent proves useful as a medium to effectively facilitate ozone attacks on hydrophobic, recalcitrant contaminants as well as any concomitant, polar intermediates throughout the entire course of reaction. Introduction Polycyclic aromatic hydrocarbons (PAHs) such as pyrene are priority pollutants because of their toxic, mutagenic properties. The sources, detrimental effects, and degradation studies employing chemical, biological, or a combination of both means have been reviewed previously.1-3 This paper highlights mainly studies utilizing ozonation in an organic cosolvent system as either a homogeneous phase or a two-phase system. The main impetus for using cosolvents, which typically involve an organic solvent in addition to water, in contaminant treatment studies lies in the enhanced solubility of the contaminants afforded by the organic solvent. Early studies of pyrene degradation by O3 were carried out in different solutions including acetic acid,4 methanol,5 and tert-butyl alcohol.6 Kefely et al.7 studied the kinetics and mechanism of ozone with polystyrene using CCl4 as a solvent. Lugube et al.8 identified byproducts from ozonation of a concentrated naphthalene solution using H2O/CH3OH (50:50, v/v) as the solvent and found a reaction stoichiometric ratio of 2 mol of O3/mol of naphthalene removed. Heterogeneous cosolvents were also used in ozonation of PAHs and other hazardous contaminants. Kornmuller et al.9 used dodecane as an aliphatic cosolvent to promote an oil/ water emulsion that allowed the delineation of relative reaction rates of PAHs with ozone. Two-phase solvent * To whom correspondence should be addressed. Tel.: (801) 581-7232. Fax: (801) 585-5477. E-mail:
[email protected].
systems consisting of a fluorocarbon (FC77) and water were used to study the degradation of phenol and naphthol10 and other chlorinated phenolic compounds.11,12 Freshour et al.13 employed a two-phase system that consisted of a contaminant-laden aqueous solution in contact with a fluorinated hydrocarbon solvent (FC40) saturated with dissolved ozone to study the degradation of contaminants including pentachlorophenols, oxalic acid, chlorendic acid, 1,3-dichlorobenzene, and trichloroethylene. Yao et al.14,15 mimicked the degradation of pyrene and benz[a]anthracene in ozonated aqueous environment by exposing dissolved pyrene and benz[a]anthracene, respectively, in 90% acetonitrile:water (v/v) homogeneous mixture to varying dissolved ozone concentrations. This study demonstrates a miscible cosolvent system consisting of heptane and acetic acid (1:1, v/v) for the ozonation treatment of pyrene. The solvent system contains both the polar acetic acid and the nonpolar heptane solvents in a single, homogeneous phase. The premise of this polar-nonpolar (PNP) solvent system being particularly effective for recalcitrant, hydrophobic compounds such as PAHs is its ability to maintain all compounds in solution during the entire course of reaction. Another unique feature of this solvent system is that the two organic solvents can be readily separated by addition of a small amount of water, resulting in a recoverable, reusable heptane phase and an acetic acid solution containing the polar reaction intermediates, with the former lighter phase on top of the latter denser
10.1021/ie049490k CCC: $27.50 © 2004 American Chemical Society Published on Web 10/22/2004
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phase. The latter phase with reaction intermediates is amenable to further ozonation or biological treatment. Using pyrene as the study compound, this paper explores the beneficial use of this PNP solvent system for the treatment of hydrophobic chemicals by ozone. The solvent system would be useful for cleaning up PAH residues and bottom sludge in old or abandoned storage tanks especially in developing countries. The solvent system would be more amenable to ex situ treatment of extracted PAH contaminants, which are common in contaminated soils. Materials and Methods Chemicals. Pyrene was purchased from Aldrich Chemical Co. (99%). Acetic acid (99%, Mallinckrodt) and n-heptane (Fisher Scientific) of HPLC grade were used as the reaction medium in a batch reactor. Stock and working Indigo Blue solution were prepared from potassium indigo trisulfanate (C16H7N2O11S3K3, Aldrich Co.) for ozone concentration measurements per Standard Methods.16 Polyseed (HACH Co.) was used in dilution water for biochemical oxygen (BOD) measurements per Standard Methods.17 Inoculums for toxicity tests were prepared according to an HACH method (HACH, Toxicity Method 10017). ToxTrak reagent pillows and ToxTrak accelerator solution (HACH Co.) were used according to the manufacturer’s methods without further processing. Activated sludge of secondary effluent was obtained from the Central Valley Water Reclamation Facility of Salt Lake County, Utah. Low-organic ( 18 MΩ‚cm), and nonpyrogenic (up to 4 log reduction with reverse osmosis pretreatment) distilled-deionized water was used in all procedures (four-stage Milli-Q Plus system, Millipore Co.). Other chemicals used in this research were of reagent grade. Analytical Methods and Equipment. Ozone was generated by an ozone generator (model T-816, Polymetrics Corp.) from dry and filtered air at an applied voltage of 65 V and an air flow rate of 2 L/min. The concentration of ozone in the PNP solvent was determined by absorbance at 270 nm with a spectrophotometer (HP 8452 UV-vis spectrophotometer, HewlettPackard Co.) using a predetermined extinction coefficient of 1955 M-1 cm-1. This extinction coefficient was obtained by correlation with actual ozone concentrations in the PNP solvent, which were measured by contacting 10 mL of O3-saturated PNP solvent with 50 mL of standard Indigo Blue solution in a separatory funnel, following calibration procedures at 600 nm similar to the Indigo Blue method.16 Sample BOD determinations with required controls were made per Standard Methods17 using an oxygen meter/electrode system (YSI model 57 oxygen meter with oxygen electrode, YSI Co.). Sample toxicity was quantified based on a colormetric method that measured the reduction of the redox-active dye resazurin by bacterial respiration (HACH, Toxicity Method 10017) using a spectrophotometer (DR/2000, HACH Co.). Samples containing pyrene and intermediates in n-heptane, 95% acetic acid, and the PNP solvent, respectively, were analyzed using a gas chromatograph with flame ionization detection (GC/FID). GC/FID analyses were carried out using an HP 5890 (HewlettPackard Co.) fitted with a capillary column (RTX-1 nonpolar column, 30 m × 0.25 mm × 0.25 µm, Restek Co.) and interfaced with the HP Chemstation software
(Hewlett-Packard Co.) A split inlet (10:1) and 1 µL sample injection were used. The oven was held at 35 °C for 1 min and then linearly increased at 7 °C/min to 300 °C followed by a 30-min hold. All quantifications and calculations were based on an external standard and the use of a pyrene calibration curve. Compounds were identified with a comparably equipped gas chromatograph-mass spectrometer (GC-MS) of the HewlettPackard 6890 series. The instrument operated in scan acquisition from m/z 15 to 550 at 1.4 cycles/s. Batch Reactor. A glass batch reactor with a working volume of 300 mL was used. Mixing in the reactor was provided by a magnetic mixer operating at 250 rpm. After about 60 mg of pyrene solid was added and dissolved into 50 mL of n-heptane, 50 mL of acetic acid was added into the reactor (resulting in a 3 mM solution of pyrene in the PNP solvent). Ozone was sparged into the reactor near the bottom through a glass dispersion tube (ACE glass Inc.). Reaction batches were stopped after 0.17, 0.33, 0.58, 0.83, 1, 2, 3, 4, 5, 10, 20, 40, 60, 120, and 180 min of ozonation. Residual ozone was removed from solution by purging with a gentle N2 stream for 1 min. Samples were kept in 2-mL vials and preserved at 5 °C if necessary prior to GC analysis. Tests of BOD5, toxicity, and qualitative and quantitative analyses of pyrene and intermediates were performed simultaneously. Another series of batch experiments was performed by mixing 10 mL of pre-ozonated PNP solvent (full saturation with ozone) with pyrene solutions (10 mL) at different concentrations for 30 min, effecting ozone/pyrene ratios of 0.5, 1, 2, 5, and 10 in the reaction mixtures. All samples were concentrated by a gentle stream of N2 gas to best retain the intermediates with lower molecular weights. The required dosage in the ozone-to-pyrene ratio for removing all major intermediates can thus be calculated. Results and Discussion The application of the homogeneous PNP solvent in ozonation of pyrene will be first explored, followed by identification and toxicity assessment of intermediates therefrom. Ozonation of pyrene in aqueous environment is inefficient because of the limited solubility of pyrene in water, hence limited exposure of pyrene to dissolved ozone, which results in hindered degradation speed for pyrene. This limitation is particularly severe when ozone itself is readily subject to self-decomposition in water. Nonpolar organic solvents such as heptane can circumvent the problem by significantly increasing the dissolution of pyrene. However, while heptane dissolves pyrene and makes it readily susceptible to O3 attack and degradation, the nonpolar solvent when used alone fails to retain the polar intermediates in solution, resulting in the formation of a solid precipitate shortly after ozonation begins.18 This again impedes kinetics and further mineralization. The use of the PNP solvent [1:1 (v/v) acetic acid/heptane] eliminates the occurrence of a solid phase that becomes the rate-limiting step in the treatment sequence. Furthermore, the homogeneous phase makes the target compounds constantly susceptible to attack and degradation at the molecular level. To be viable, the solvent system should accommodate a reasonable amount of ozone and be relatively inert to ozone. Figure 1 shows the accumulation of O3 and approach to saturation in various solvents. Acetic acid accommodates nearly 40 mg/L O3, while water at pH 7 accommodates only about 4 mg/L after 20 min. Pure
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Figure 1. Solubilities and stabilities of ozone in various solvents. Table 1. Intermediates Tentatively Identified and Estimated Concentrations (Assuming GC/FID Response Ratios of Compounds 2-7 Similar to That of Pyrene) after 1 min of Ozonating Pyrene in the PNP Solvent compd
concn (ppm)
compd
concn (ppm)
1 2 3 4
8.8 79 7.1 17
5 6 7
5.6 11 9.0
heptane dissolves about 8 mg/L, while the acetic acid/ heptane (1:1 by volume) dissolves about 11 mg/L. It should be noted that the rate of accumulation of O3 and its saturation concentration are subject to the specific experimental conditions employed, e.g., gas bubble size, agitation, liquid volume, and gas flow rate that influences the partial pressure of O3 (∼1%) in the influent gas, etc. The waning concentration profiles over 2 h following the cessation of ozonation indicate O3 being consumed by reaction with the solvent. As shown, the PNP solvent allows a higher and more stable concentration for O3 than is possible in water. This suggests that PNP is a better carrier of O3 than water, particularly for treatment of nonpolar pyrene that dissolves well in the former solvent but not the latter. The PNP solvent as described is readily separated into two phases following ozonation treatment. By adding a small amount of water, the two phases separate, allowing the heptane devoid of the contaminant to be reused for another treatment cycle and the acetic acid now laden with biodegradable intermediates (including acetic acid itself) to be further processed and disposed of. Figure 2 shows the degradation of pyrene (1) along with the formation of intermediates (2-7) over time when pyrene is being ozonated in the miscible PNP solvent that contains 600 ppm pyrene initially (i.e., 60 mg of pyrene/100 mL of solution). The concentrations of the parent and intermediates in the PNP solvent after 1 min of ozonation are shown in Table 1. Figure 3 shows these compounds as concluded from interpretations of the major m/z fragments. These results were obtained from a series of experiments employing 16 identical batches of pyrene solution (i.e., 60 mg of pyrene/100 mL of 1:1 heptane/acetic acid solvent) that were ozonated for varying periods from 0 to 3 h. After the predetermined duration of ozonation, each solution was added to a small amount of water (2.5% of the solvent volume), which resulted in an immediate separation of the once homogeneous, PNP solvent phase into two distinct phases, with the heptane phase on top of the acetic acid
Figure 2. Formation and degradation of intermediates during ozonation of pyrene: (a, top) pyrene and intermediates in the PNP heptane-acetic acid solvent; (b, middle) compounds (pyrene only) that partition into the heptane phase; (c, bottom) compounds (all intermediates) that partition into the acetic acid solution phase.
Figure 3. Parent pyrene (1) and intermediates (2-7) tentatively identified according to major m/z fragments in mass spectra.
phase that now contained 5% water. The contents of the homogeneous PNP solvent before phase separation as well as the two separated phases were analyzed. Figure 2a shows all the identified products and their concentration changes in the homogeneous PNP solvent throughout ozonation; given prolonged ozonation (e.g., 1 h), all compound parents and intermediates disappeared. Figure 2b shows pyrene alone remaining in the separated heptane phase with concentration decreasing with increasing ozonation duration. Figure 2c shows all compounds that preferentially partition into the separated acetic acid phase, except the major portion of pyrene that prefers the heptane phase. It should be noted that these qualitative and quantitative analyses
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of the three solutions were performed separately, and except for compound 6 they constituted good mass balance (i.e., for any given duration of ozonation, species concentrations in the homogeneous PNP solvent ) species concentrations in the separated heptane + species concentrations in the separated acetic acid) to within 10% error. Figure 2 suggests that while the PNP solvent was capable of maintaining all compounds, both hydrophobic and hydrophilic, in solution as they were formed and depleted, the hydrophobic nature of the solutes changed as degradation reactions proceeded. Specifically, Figure 2b,c evinced that only nonpolar pyrene compound (1) remained in the nonpolar heptane phase while all other intermediates (2-7) formed during reaction were polar, preferentially partitioning into the polar acetic acid phase. Figure 2c further shows that intermediates 3-7 were degraded more readily relative to the tetraaldehyde intermediate (2) that appeared to be most robust. However, all of them were completely removed under prolonged ozonation (e.g., 1 h). These results appear to reflect the relative speeds of their reactions with ozone. The identified products are mainly oxygenated intermediates similar to those found by Yao et al.14 As Yao et al. used a 90% acetonitrile solution as the solvent, this study employed purely organic solvents. Conditions used in neither study were conducive to formation of products from free radical pathways. Organic solvents suppressed hydroxyl and other oxygen-containing radicals either by the absence of O3 hydrolysis or by the organic molecules effectively scavenging the free radicals. When ozonation of pyrene was performed in aqueous solution without added organics as a solvent, Zeng et al.1,2 found extensive amounts of aliphatic compounds as well as branched alkenes, which were attributed to involvement of free radicals arising from O3 hydrolysis in the aqueous phase. The PNP solvent system has not been optimized with respect to the solvent ratio, contaminant loading, or other process parameters; these factors can significantly determine process effectiveness. For example, the solvent ratio may determine maximum loading of the hydrophobic contaminants, as well as the dissolved O3 concentration available for reaction, hence affecting the contact time required for degradation. The optimal solvent ratio and ozonation time prior to discharge or further biological processing are best addressed in view of specific target contaminants. The contaminant loading of 600 mg/L investigated here was much below the maximum loading capacity of the solvent. Our solubility measurements indicated that heptane alone could dissolve slightly over 10 000 mg/L pyrene and acetic acid alone could dissolve 2200 mg/L pyrene. It is expected that the maximum loading of pyrene in the PNP solvent would be between these values; thus, the viable loading for treatment would vary in the particular PNP solvent according to its solvent ratio. To examine the effect of O3 availability on the abundance and distribution of intermediates and products, a series of experiments was conducted by reacting a constant O3 level with pyrene at different concentrations (5-100 mg/L), with stoichiometric ratios of O3 to pyrene ranging from 0.5 to 10. This was achieved by adding pyrene-in-heptane solutions of varying concentration to equal volumes of acetic acid beforehand saturated with O3, and the mixtures were allowed to react for 30 min in the closed batch system before the
Figure 4. Disappearance of varying amounts of pyrene exposed to a constant dose of ozone (12 ppm) according to varying ozoneto-pyrene mole ratio. Table 2. Byproducts Identified and Estimated Concentrations (Assuming Similar Responses to Heptane with Solvent Evaporation Accounted for) during Prolonged Ozonation of the PNP Solventa concn during ozonation (ppm) compound
20 min
40 min
1h
2h
3h
4-heptanone 3-heptanone 3-heptanol 2-heptanol 2-methylheptanal heptane-2,4-dione 2-octanone 1-hexyl acetate heptanoic acid acetic acid/heptyl ester
-
340 330 260 260 -
400 350 210 260 200 -
570 580 230 290 210 -
1300 1100 480 290 260 120 160 78 87 70
a
-, not detected.
contents of the reaction medium were analyzed. Figure 4 compares the pyrene concentration before and after ozone exposure at different stoichiometric levels. Contrary to those from continual O3 sparging, the results of these closed batches indicated no intermediates or other types of products remained after reaction save the parent pyrene, which was lowered by approximately the same amount calculated to be equivalent to one-eighth of the available O3. In other words, 8 mol of O3 was consumed in degrading 1 mol of pyrene, most likely to small fragments (e.g., compounds of less than six carbons) not readily detected in the employed analytical procedures. The absence of intermediates and the relatively constant amount of pyrene that disappeared could be explained by that O3 rapidly continued to degrade intermediates once O3 attacked and opened the rings of pyrene. New compounds could have resulted from ozonation of the organic solvents or pyrene. The stability of the solvent system itself subject to ozonation was tested. Prolonged ozonation of acetic acid alone produced little detectable byproducts, attesting to acetate’s resistance to ozone or formation of small molecules not detected. Ozonation of heptane produced 10 other compounds as identified in Table 2. Compounds 2-7 were found only when pyrene was present, suggesting that they were intermediates from pyrene. Within the initial minutes (e.g., the first minute as in Figure 2) required to remove the parent pyrene, byproducts from solvent were below detection. However, byproducts accumulated when ozonation was prolonged to 3 h, an extreme duration that
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was long past that required for removal of pyrene or its intermediates (e.g., 1 and 10 min, respectively, according to Figure 2). In such manner of ozonating primarily the solvent, the total byproduct concentration at the end of 3 h accounted for 0.5% of the initial solvent amount. This percentage represents an estimate of solvent degradation only, because while solvent evaporation was less significant in the first 10 min (at approximately 0.9 mL/min for heptane and 0.2 mL/min for acetic acid initially under the employed experimental conditions), stripping of solvents as well as low-molecular-weight byproducts would be significant by the end of 3 h. The byproducts were primarily aliphatic compounds of ketones, aldehydes, and acids, which readily partitioned to the acetic acid phase. It should be noted that while solvent degradation could occur under prolonged ozonation (e.g., hours), it did not occur to any noticeable extent in the initial minutes (∼1 min; Figure 2b) required to remove the parent pyrene. The undesirable degradation of heptane can be avoided. Once pyrene disappeared, it would be appropriate to separate heptane from the acetic acid solution; heptane can be reused by reloading with contaminants while the acetic acid now laden with intermediates (Figure 2c) can be further treated as necessary. Remediation scenarios may call for ozonation of pyrene (or other wastes) at high concentrations, which will result in an abundance of intermediates formed. To assess the level of ozonation pretreatment that would be required to render pyrene and its daughter compounds nontoxic, the intermediates from ozonation of pyrene, e.g., those shown in Figure 2c, were incubated. After varying duration of ozonation of pyrene in the PNP solvent (600 mg/L) as described previously, the separated 95% acetic acid solutions laden with intermediates were diluted to 5% acetic acid solution and were tested for Escherichia coli toxicity. Figure 5a shows toxicity of the intermediates, and Figure 5b shows the BOD5 according to ozonation duration. As shown, the E. coli toxicity of the intermediates increased from an -17% initially to +37% after 1 min ozonation and then decreased to -3% after 10 min ozonation and remained relatively stable and nontoxic thereafter. Also shown, control experiments performed in the absence of pyrene revealed inhibition within -10% to +10%, i.e., nontoxic without pyrene regardless of ozonation, thus indicating that the inhibitory effects were associated with intermediates from ozonation of pyrene. It should be noted that for the toxicity test (HACH; Toxicity Method 10017) an inhibition value between -10 and +10% is generally within the nontoxic range, while an inhibition value outside of this range indicates toxicity. The acetic acid solution with intermediates was further diluted (BODu ) 320 mg/L) and tested for BOD. The results using Polyseed show a gradual increase from little registered BOD5 to 230 mg/L (i.e., approaching 72% of the ultimate BOD) with increasing duration of ozonation. The results using municipal activated sludge as bacteria seed show a similar increase to 230 mg/L, albeit from a higher starting value. It should be noted that control experiments performed in the absence of pyrene registered relatively constant BOD values around 230 mg/L regardless of the length or whether ozonation was used, which again suggested that the inhibitory effects were associated with intermediates from pyrene. The 5-day BOD (BOD5) of 230 mg/L
Figure 5. Biological properties: (a) E. coli toxicity and (b) BOD of the effluent containing pyrene and resultant intermediates after varying durations of ozonation in the PNP solvent system.
measured for the intermediates-laden ozonated sample constituted 72% of the ultimate BOD. This ratio of BOD5 to ultimate BOD is not uncommon for readily biodegradable substances, and it indicates that the degradation of acetic acid has not been inhibited by the presence of the intermediates. The amount of intermediates present in the sample after 5 min of ozonation (e.g., 150 mg/L intermediate 2 according to Figure 2c) contributed little (
