Environ. Sci. Technol. 2001, 35, 3707-3712
Artificial Aging of Phenanthrene in Porous Silicas Using Supercritical Carbon Dioxide R O B E R T G . R I L E Y , * ,† CHRISTOPHER J. THOMPSON,† MICHAEL H. HUESEMANN,‡ ZHEMING WANG,† BRENT PEYTON,§ TIM FORTMAN,‡ MICHAEL J. TRUEX,† AND KENT E. PARKER† Pacific Northwest National Laboratory, Richland, Washington 99352, Marine Sciences Laboratory, Pacific Northwest National Laboratory, Sequim, Washington 99382, and Chemical Engineering Department, Washington State University, Pullman, Washington 99164
Expedited artificial aging is described and demonstrated using a novel system that circulates a solution of supercritical carbon dioxide and a hydrophobic organic sorbate (phenanthrene) through a closed loop containing a porous substrate. Unlike traditional methods used to simulate the natural aging process, our approach allows for realtime monitoring of sorption equilibria, and the process is highly accelerated due to the unique physical properties of supercritcal carbon dioxide. The effectiveness of the system to simulate aging was demonstrated with a series of experiments in which three silicas with varying particle and pore sizes were loaded with phenanthrene. Batch aqueous desorption experiments were used to evaluate the extent of the aging process. For the two types of particles containing the largest pores (i.e., mean diameters of 202 and 66 Å), 95% and 86%, respectively, of the phenanthrene was released to the aqueous fraction within 3 h. In contrast, only 16% of the phenanthrene was released from particles having a mean pore diameter of 21 Å after 24 h. These results were confirmed by the results from an aqueous column desorption experiment. Confounding factors that might contribute to slow aqueous desorption such as the hydration state of the particles’ surfaces, the chemical form of the loaded phenanthrene, and the organic carbon content were investigated and/or normalized for all three particle types. Consequently, we were able to attribute the slow desorption behavior and the presence of the resistant fraction in the 21 Å silica to pore effects. With properly designed experiments, the results of this study suggest that the supercritical fluid system could be extended to the study of contaminant aging and bioavailability in natural soils and sediments.
Introduction Aging is a term used to describe the process by which lipophilic organic chemicals are incorporated into natural * Corresponding author phone: (509)376-1935; fax: (509)372-1704; e-mail:
[email protected]. † Pacific Northwest National Laboratory, Richland. ‡ Marine Sciences Laboratory, Pacific Northwest National Laboratory, Sequim. § Washington State University. 10.1021/es0020613 CCC: $20.00 Published on Web 08/18/2001
2001 American Chemical Society
solids (e.g., soils and sediments). The magnitude of the process is influenced by the length of the contact time of the organic compound with the natural solid. Extensive aging results in the accumulation of a resistant fraction of chemical that exhibits decreased bioavailability and slow release to the environment. Recent review articles have summarized what is known about the important properties of natural solids and the mechanisms that contribute to the aging process and the environmental fate and effects of chemicals influenced by the aging process (1-5). Diffusion into mineral micropores and microcrystalline and amorphous natural organic matter of natural solids are considered important components of the aging process (316). The relative contributions that competing mechanisms (i.e., diffusion into organic matter vs diffusion into intraparticle micropores) make to a resistant fraction that is characterized by slow desorption behavior are not well understood. Based on the results of soil or sediment desorption experiments only, it is difficult to distinguish between the different processes and the magnitude of their effects in pores and organic matter. Thus, there is a need for experimentation at the microscale level with intact soils/ sediments, isolated soil/sediment fractions, and model materials (e.g., silicas) that have been artificially aged in a way that quantifies and simulates the natural aging process. A number of studies have been performed in an effort to better understand the contribution made by the mineral and organic phases on slow sorption/desorption of contaminants using model sorbents (6-16). Others have examined the effect of contact time and spiking procedure on the extractability of contaminants from soils (17, 18). Common to some of these studies was contaminant adsorption from water or a solution of water and an organic solvent. Equilibrium status of the spiked materials was not explicitly determined in some of these studies (7, 17, 18). Based on a previously reported conceptual model (11), for sorption in micropores to occur in natural soils and sediments, contaminants must move from the bulk fluid into the water of mesoporous structures and ultimately into the water of microporous structures. The time frame over which this process occurs in nature is likely to require many years as opposed to the minutes, weeks, or months commonly used to prepare artificially aged materials for laboratory experiments. As a result, past laboratory experiments based on these short contact times may not have been able to measure the full influence of pore effects in mineral phases on aqueous slow desorption of a hydrophobic organic compound. The purpose of this study was to develop a rapid loading methodology that can be used to prepare materials for laboratory study of the aging process. The unique physical properties of supercritical carbon dioxide (i.e., high diffusivity, low viscosity and inertness) (19) make supercritical carbon dioxide a promising carrier solvent for rapid penetration of hydrophobic organic compounds into porous media. Therefore, a system that employs circulating supercritical carbon dioxide and inline UV detection was constructed, and the system was demonstrated by loading phenanthrene onto three porous silicas. The physical and chemical properties of the silicas were carefully controlled to encourage the formation of a resistant fraction and to help elucidate the process responsible for slow release. Aqueous desorption experiments were used to observe slow release of phenanthrene and determine whether artificial aging had actually occurred. VOL. 35, NO. 18, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. A schematic of the supercritical fluid loading system. Arrows indicate the flow direction in the closed-loop mode.
Experimental Section Silica Particle Synthesis, Treatment, and Analysis. Commercial silica gel (Davisil) was purchased from Supelco, Bellefonte, PA. Mesoporous silica particles with a particle size range of 1-10 µ and mean pore diameters of 21 Å and 66 Å were synthesized using adaptations of the methods described in Bruinsma (20) and Beck (21). Synthesized particles were calcined by heating from room temperature to 540 °C under nitrogen purge. Prior to use in experiments, cooled particles were ground lightly with a mortar and pestle to break up large aggregates. The low organic carbon content of silica type particles was determined by gas chromatographic analysis of methane generated from the catalytic conversion of carbon dioxide. The carbon dioxide was released from the particles following placement in a platinum crucible and heated to 550 °C. The surface area and porosity of the silica particles was determined by nitrogen BET and Barrett et al. (22) (BJH) methods, respectively, and measured on a Micrometrics surface area analyzer (Model 2010 Micrometrics Instrument Corp., Norcross, GA). Hydration of Silica Particles. Five to 10 gram quantities of silica particles in a 250 mL Erlenmeyer flask were equilibrated with 150 mL of Milli-Q water over a period of 3-4 days at room temperature and subsequently filtered and dried under house vacuum in a desiccator containing Drierite for 5-6 days. Additional water (physisorbed) was removed by subjecting the silica particles to high vacuum (4-5 × 10-6 Torr) for a period of 5-6 days (23). Particles undergoing supercritical carbon dioxide loading of phenanthrene were placed in the high-pressure stainless steel vessel prior to exposure to high vacuum. Supercritical Carbon Dioxide Loading System. Phenanthrene (Aldrich, zone-refined) was used for all experiments. Loading of phenanthrene into silica particles was accomplished using a Dionex model SFE-703 supercritical extraction instrument that had been modified to circulate supercritical CO2 in a closed loop (Figure 1). Included in the closed loop system was a 10-mL high-pressure stainless steel vessel (Keystone Scientific) used to dissolve the phenanthrene in supercritical CO2. A second 10 mL vessel in the system was used to contain the silica particles that were to receive the phenanthrene. An Eldex model B-100-S HPLC pump was used to circulate the supercritical solution through the closed loop system, and a Shimadzu UV-2401PC spectrophotometer equipped with a custom mounted high-pressure flow cell (Shimadsu SPD-M6A) was used to monitor phenanthrene concentrations in CO2. The flow cell had an optical path length of 6 mm, internal volume of 3 µL, and a pressure tolerance of approximately 6000 psi. All UV measurements 3708
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were made at 250 nm, the wavelength of maximum absorbance for phenanthrene. A Druck PTX 500 Series pressure transmitter and YSI thermistors were interfaced to a digital display panel and a Campbell Scientific CR10 datalogger to facilitate real time pressure and temperature readings during system operation. Seven Rheodyne model 7000 valves were used to direct the flow of supercritical solution through the system, and two on/off SSI needle valves were used to isolate the pressure vessels from the SFE system pump. Heated flow restrictors that were mounted in a separately thermostated cavity were used to depressurize the system at the conclusion of loading experiments. The following procedure was used to conduct loading experiments. Silica particles were packed into the silica vessel, and the vessel was weighed to determine the mass of silica added. To help facilitate rapid dissolution of the phenanthrene in CO2, a small volume (i.e., < 200 µL) of a hexane solution containing phenanthrene was pipeted into the phenanthrene vessel, and the hexane was allowed to evaporate before the vessel was sealed. During evaporation, the vessel was slowly rotated to thoroughly coat the vessel’s internal surfaces with phenanthrene. Amounts of phenanthrene introduced into the system for each loading experiment were adjusted so that the ratio of moles of phenanthrene to available surface area of the silica was constant. Both vessels were mounted in the loading apparatus, and the system valves were switched in a manner so that both vessels were isolated from each other and the main flow path (bypass tubing and valves are not shown in Figure 1). The system was then heated and pressurized with CO2 to the desired loading conditions (typically 30 °C and 4500 psi). After the system temperature and pressure stabilized, the main system pump was valved off to place the system in closed-loop mode. The HPLC pump was turned on, and the CO2 in the closed loop was circulated through the system at a flow rate of approximately 2.5 mL/min. The UV baseline was monitored for approximately 10 min, after which time the phenanthrene vessel was valved into the flow path. Due to the relatively large volume of the phenanthrene vessel, it was necessary to circulate the fluid for approximately 30 min to ensure complete mixing of the phenanthrene solution with the neat CO2 that was in the flow path. After mixing was complete (as indicated by an elevated, stable UV absorbance reading), the pressurized vessel containing the silica particles was valved into the flow path. Circulation of the supercritical fluid through the packed silica column was allowed to continue until the UV signal had stopped decreasing (indicating that an apparent equilibrium had been established) or until the silica had been exposed to the circulating solution for the desired length of time. For each loading experiment, an estimate of the amount of phenanthrene loaded onto the silica was calculated from on-line absorbance data. The calculation involved three steps. First, the molar absorptivity of phenanthrene in CO2 at the prescribed operating conditions was determined using Beer’s law and the following experimental parameters: the flow cell’s path length, the stabilized absorbance reading prior to valving the silica vessel into the flow path, the mass of phenanthrene added to the system, and the volume of the flow path including the phenanthrene vessel. Next, the concentration of phenanthrene remaining in solution at the end of the loading process was determined from the molar absorptivity and the measured absorbance at the end of the experiment. We found that it was necessary to account for the increase in system volume associated with valving the silica vessel into the flow path, because the pressurized vessel initially contained a significant amount of neat CO2 in the particle pores and in the spaces between the particles. This increase in volume was estimated offline for each particle type by gravimetrically determining the volume of hexane
required to fill the vessel when packed with a particular particle type. Finally, the concentration of phenanthrene loaded onto the particles was determined by subtracting the mass of phenanthrene remaining in solution at the end of the experiment from the mass initially added to the system and dividing the result by the mass of silica packed into the silica vessel. Batch Desorption Experiments. Batch desorption experiments were performed in 30 mL amber centrifuge glass tubes with screw caps and Teflon-lined silicone septa. Prior to use, the glass tubes were heated at 450 °C for 4 h. To initiate desorption experiments, 20 mL of buffer (pH 7) solution (i.e., containing 5 mg/L sodium bicarbonate and 100 mg/L sodium azide dissolved in deionized water) was added to a known mass of phenanthrene-containing silica particles that had been placed inside the glass tube. The solids-to-water ratio (S/W - g dry silica/g buffer solution) for each experiment was 105. Control tubes did not contain any silica particles, i.e., S/W ) 0. During the desorption experiments, the centrifuge tubes were tightly capped, covered with paper towels to exclude light, and placed on a modified rock roller (Model NF-1, Lortone Inc.) at 100-250 rpm for mixing. At specified sampling times, the tubes were taken from the rollers and centrifuged at 4000 rpm (2960 g) for 5 min. A supernatant sample (0.1 mL) was taken from each tube and analyzed for phenanthrene. The glass tubes were again tightly capped and placed back on the roller until the next sampling event. For the “time zero” measurements, the glass tubes were briefly mixed manually (i.e., they were not put on the roller) and placed immediately in the centrifuge. In this case, the total time for mixing, centrifugation, and sampling took ca. 15 min. At the termination of the experiments, the contents of the glass tubes were filtered using a porous glass frit to recover all of the silica particles. A subsample of the wet silica particles was dried at 105 °C for 24 h, and the dry weight of silica (Msd) was determined gravimetrically. The volume of interstitial water (Viw) was calculated using the difference in the dry and wet weight masses of the silica. The concentration of phenanthrene (Csd) on dry silica was then calculated according to
Csd )
Mp - CsupViw Msd
where Mp is the total mass of phenanthrene extracted from the subsample, and Csup is the aqueous phenanthrene concentration in the last supernatant sample (i.e., it is assumed that the phenanthrene concentration in this sample was similar to the one in the interstitial water). For the analysis of sorbed-phase phenanthrene, approximately 50 mg of the recovered silica particles was extracted and subjected to sonication for 40 min in 10 mL of acetonitrile. A 0.5 mL aliquot of the extract was forced through a 0.2 µm Teflon syringe filter and diluted with 0.5 mL of deionized water. A 100 µL sample was analyzed using the HPLC method described in the next section. Analysis of Phenanthrene in Supernatant Samples from Batch Experiments. Quantitation of phenanthrene in supernatant samples was performed on a system consisting of a Perkin-Elmer biocompatible Model 250 binary HPLC pump, a Perkin-Elmer Model LC-101 column oven, a Supelco Supelcosil LC-PAH HPLC column, and a Waters Model 474 fluorescence detector. A 0.5 mL sample of supernatant from the desorption experiment was diluted with 0.5 mL of HPLC grade acetonitrile and a 100 µL aliquot of the mixture introduced into the system using a Spectra Physics AS 3000 HPLC Autosampler. The system was operated using a 75:25 acetonitrile:H2O mixture at a flow rate of 1.5 mL/min. The column temperature was set at 30 °C. Under these conditions,
the retention time of phenanthrene was approximately 6 min. The fluorescence detector’s excitation wavelength was set at 260 nm, and the resulting signal was measured at an emission wavelength of 380 nm. Based on calibration results using known phenanthrene standards in 50:50 acetonitrile: H2O, this method yielded a linear response for phenanthrene concentrations ranging from 0.4 µg/L to 500 µg/L. For aqueous samples containing phenanthrene concentrations greater than 500 µg/L, the sample was further diluted with acetonitrile to ensure that phenanthrene quantification occurred within the linear range of the detector response. Column Aqueous Desorption Experiment. Three Alltech 78 mm × 7.8 mm stainless steel high-pressure liquid chromatography (HPLC) columns were loaded with 21 Å silica particles to within approximately 5 mm of the top of the column, resulting in masses of 0.3114, 0.3250, and 0.3309 g, respectively. The columns were equipped with a 0.5-micron frit on each end to prevent the washout of the silica particles. Ultrapure water with a 1% solution of sodium azide was continuously pumped through the columns at a flow rate of 0.2 mL/min using Eldex Laboratories single piston Model A-30-S positive displacement pumps to eliminate microbial degradation of phenanthrene. The pumps were equipped with Eldex Laboratories Piston Wash Systems and highpressure Kel-F backup washers, which were necessary to maintain piston seal integrity under the high pressures encountered in this experiment. The solution was pumped through an Alltech inline high-pressure 0.2 micron filter and a Nupro 25 psi check valve prior to entering the column from the bottom. Five milliliter fractions of phenanthrenecontaining eluent were collected in glass vials every hour for the first 5 h and then subsequently every 8 h. Phenanthrene concentrations in the fractions were measured on a HewlettPackard series 1100 high-pressure liquid chromatograph equipped with a Vydac reversed-phase C18 column (25 cm × 2.1 mm). Analysis of Phenanthrene on Particle Surfaces. Fluorescence excitation and emission spectra of phenanthrene in solid crystals, in dilute methylene chloride solution, and adsorbed on pretreated (i.e., rehydrated and evacuated) silica particles were recorded on a SPEX Flourolog II fluorimeter. The fluorescence spectra of silica particles not containing phenanthrene were used to remove the effects of background fluorescence. The fluorimeter was equipped with a 450 W xenon arc lamp, double monochromators (SPEX 1680) for excitation and emission, and a cooled photomultiplier in photon counting mode. To correct for minor variation of the lamp intensity, light from the excitation monochromator was reflected by a thin quartz plate into a Rhodamine dye cell. Red emission from this “quantum counter” was detected by a second photomutiplier and recorded by the fluorimeter in an analogue reference channel. Fluorescence intensity from the sample was normalized at each time to the corresponding excitation intensity in the reference channel.
Results and Discussion In repeated tests, the supercritical fluid system (Figure 1) was operated at pressures of up to 4500 psi for up to 10 h without significant loss in pressure (
