Environ. Sci. Technol. 2004, 38, 4169-4175
Desorption Kinetics of Phenanthrene in Aquifer Material Lacks Hysteresis SYBILLE KLEINEIDAM, HERMANN RU ¨ GNER, AND PETER GRATHWOHL* Geological Institute, Applied Geology Group, University of Tu ¨ bingen, Sigwartstrasse 10, 72076 Tu ¨ bingen, Germany
Desorption experiments were carried out in flow through columns following long-term sorption batch experiments (up to 1010 days at 20 °C; Ru¨ gner, H.; Kleineidam, S.; Grathwohl, P. Long-term sorption kinetics of phenanthrene in aquifer materials. Environ. Sci. Technol. 1999, 33, 1645-1651) to elucidate sorption/desorption hysteresis phenomena of phenanthrene in aquifer materials. Most of the sorbents employed in this study (homogeneous lithocomponents separated from aquifer sediments or fresh rock fragments) showed highly nonlinear sorption isotherms because of coal particles embedded inside the grains. Because sorption capacities were high, sorption equilibrium was not reached in most of the sorbents during the initial sorptive uptake experiments lasting up to 1010 days. Desorption was studied up to 90 days at 20 °C. The temperature was raised after that stepwise from originally 20 to 30, 40, 50, and finally to 70 °C for selected samples to estimate activation energies of desorption. A numerical intraparticle pore diffusion model was used to fit sorptive uptake data and subsequently for pure forward prediction of the release rates in the desorption column experiments. Desorption was initially fast followed by extended tailing which in other studies is fitted by using multirate first-order models. Our results demonstrate that the retarded intraparticle pore diffusion model can predict the desorption rates with a single diffusion rate constant obtained independently from the long-term batch sorption experiment. No evidence for hysteresis was found, suggesting that many hysteresis phenomena reported earlier are experimental artifacts resulting from nonequilibrium effects and “nonphysical” models. The different temperature steps allowed one to additionally calculate activation energies of desorption (45-59 kJ mol-1), which were in reasonably good agreement with results from earlier studies for a retarded pore diffusion process. In addition, equilibrium sorption isotherms were determined at 20 and 40 °C to compare sorption and desorption enthalpies. Both were in good agreement, confirming that desorption was not significantly different from sorption.
Introduction Fate and transport of contaminants in the environment depends largely on their interaction with soil solids. Slow desorption kinetics limits the efficiency of soil and groundwater remediation as well as the bioavailability of contami* Corresponding author phone: 49-7071-2975429; fax: 49-70715059; e-mail:
[email protected]. 10.1021/es034846p CCC: $27.50 Published on Web 06/25/2004
2004 American Chemical Society
nants (2, 3). It is now well accepted that slow sorption/ desorption rates can be attributed to diffusion through the pore spaces (meso-micropores; e.g., 4-9) and/or diffusion in natural organic matter (e.g., 10), in carbonaceous particles such as soot, coal, or charcoal (11) or a combination of all processes (12) as illustrated in Figure 1. Surface properties (e.g., hydrophobicity) within microporous solids (e.g., zeolites) have been proven to influence the sorption/desorption kinetics as well (13). Desorption experiments mostly show initially fast desorption followed by extended tailing (e.g., 14, 15). In homogeneous samples (same grain sizes and grain properties), this can be fit by a spherical diffusion model describing the physics of the desorption process accurately. To achieve the same fit with first-order models, two or more compartments are needed (multirate models; 16, 17). Additional tailing may be due to the heterogeneity of natural soil samples where different grain sizes and grain properties result in widely different rate constants. In such cases, multirate models have to be applied which account for the relevant diffusion rate constants (6, 17-21). Hysteresis termed as (1) nonsingular sorption/desorption isotherms and/or (2) different diffusion rates for sorption and desorption (e.g., slower rates and longer time scales for desorption) has been reported in several papers, but many of them lack a physical or chemical explanation for the observations (e.g., 14, 21-23). More recently, desorption data established for model substances reveal that irreversible changes in the structure of internal nanopores in the organic matrix analogous to pore deformation mechanism in glassy polymers occur if the sorbent was conditioned with a solvent at high concentrations (24, 25). For benzene sorption onto charcoal particles, strong hysteresis was found over a large concentration range, indicating a structural deformation of the sorbent (26). A similar explanation (configurational changes in the organic matter) was given for different natural sorbents, which exhibit significantly slower desorption rates as compared to uptake rates. The slow rates were attributed to an intraorganic matter diffusion mechanism (27). Physical entrapment of native PAH on soot particles was suggested to explain a remaining fraction of 50-97% not available for partitioning into the aqueous phase (28). However, a problem in early reports on hysteresis could have been the fact that experimental artifacts and/or nonappropriate boundary conditions (e.g., in modeling) resulted in an “artificial” hysteresis. Conditions such as those listed below will lead to hysteresis in sorption/desorption rates: Desorption starting before sorption equilibrium was reached (diffusion along concentration gradients directing to the center and to the surface of the particles or aggregates) or desorption experiments performed under boundary conditions different from those of the sorption experiment (e.g., higher solid-to-liquid ratios in batch experiments as compared to desorption in continuous flow columns, i.e., finite bath vs infinite bath boundary conditions) or highly nonlinear sorption isotherms (self-sharpening front during sorptive uptake and tailing during desorption). If any of these points are not considered in modeling, different sorption/desorption rate constants may result from fitting models to experimental data, which is then often misinterpreted as hysteresis (see, e.g., ref 29). In this study, we used narrow grain-size fractions of homogeneous lithocomponents separated from natural aquifer sands and gravels or fresh rock fragments to investigate the desorption behavior of phenanthrene at ambient and elevated temperature in column leaching experiments. The well-characterized samples were used VOL. 38, NO. 15, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Scheme on the processes controlling sorptive uptake and desorption of hydrophobic organic contaminants. before in long-term sorption kinetic experiments lasting up to 1010 days (results are reported in Ru ¨ gner et al. (1) and Kleineidam et al. (30)). Therefore, the contamination history was exactly known, and complete mass balances could be obtained. The heterogeneity within the organic matter of the samples was known through intense microscopic analysis. The main objective of the study was to elucidate slow desorption processes for samples which showed extremely slow sorption kinetics to quantify hysteresis effects. To overcome complications often obscuring the relevant processes, we used (1) homogeneous samples (in terms of lithology of its components: rock fragments and minerals), (2) very narrow grain-size fractions (sands and fine gravel; see refs 1, 30), and (3) the initial conditions of desorption (after 1010 days of sorptive uptake in batch experiments) which were exactly known. In the extended tailing part during leaching of the phenanthrene, the temperature was increased stepwise to determine the desorption activation energies. This was not the main focus of our study. However, the activation energies can give insights into the nature of the diffusion and sorption processes involved. Activation energies derived from desorption column experiments using trichloroethene on model substances and natural geosorbents (silica gel, soil, sediment) were on the order of 47 kJ mol-1 up to 94 kJ mol-1 and correspond to values found for diffusion in microporous solids (see comprehensive summary in Werth et al. (13)). Cornelissen et al. (15) and Johnson and Weber (31) found in column experiments activation energies in the same range (on the order of 40-80 kJ mol-1) and argued that these values indicate a polymer diffusion process. Recently, Ghosh et al. (32) found in a coal-derived subfraction of a harbor sediment 3 times higher activation energies as compared to the clay/ silt subfraction of the same material which was on the order of 37-41 kJ mol-1. It has been under discussion that activation energies increase with decreasing solid loading (10, 31) or increasing desorption time (33).
Materials and Methods Sample Characterization and Preparation. For the physical properties of the samples investigated (lithocomponents from different gravel pits and fresh rock fragments) and their equilibrium sorption and sorption kinetics, we refer to Kleineidam et al. (30) and Ru ¨gner et al. (1). A list of the samples and the parameters of interest are given in Table 1. In brief, lithocomponents and fresh rock fragments were characterized in terms of organic carbon content and organic matter facies (30), intraparticle porosity, pore size distribution, sorption isotherms, and sorption kinetic data. Detailed descriptions of the experimental setup of the sorption equilibrium and sorption kinetic studies are given in Kleineidam et al. (30) and Ru ¨ gner et al. (1). Subsequently to 4170
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the batch sorption kinetic experiments lasting over time periods between 460 and 1010 days, column desorption experiments were carried out using the same samples. Two out of the three parallel vials from the batch experiments had to be used to establish the mass balance after the sorptive uptake by extracting the solids using the hot methanol method (34). The recovery rates after these long time periods were still between 68% and 125% (1). The third sample was immediately packed into a stainless steel column, closed with tight fittings (swagelock), and placed in a water bath at a constant temperature of 20 °C (same temperature as during the sorption experiment). The column desorption setup (Figure 2) was designed to achieve a maximum concentration gradient from the immobile (solids) to the mobile phase (water). By purging the column with water at a sufficiently high flow rate, a boundary condition of practically zero concentration was established throughout the column. The phenanthrene concentration in the column effluent never exceeded 1.0 µg L-1, which is much less than the aqueous concentration expected at equilibrium which was about 25150 µg L-1 during sorption of phenanthrene. This indicates distinct nonequilibrium and justifies the boundary condition (zero concentration outside) used in the desorption modeling. The water used for leaching was deionized, degassed, and spiked with sodium azide at a concentration level of 200 mg L-1 to inhibit bacterial growth and thus limit biodegradation of the phenanthrene. The column effluent water was trapped in 5-L brown-glass bottles already containing cyclohexane to extract phenanthrene (partitioning of phenanthrene between cyclohexane and water results in an extraction efficiency of >98%, even at solvent-to-water ratios of 500:1). Fluorene was added as internal standard. The cyclohexane was sampled over different time periods (8 h, 1, 2, 3, 5, 7, 10, 13, 17, 21, 27, 34, 42, 52, 70, 90 days). Detection and quantification of phenanthrene was done using HPLC and fluorescence detection (column: Grom PAH, 250 mm × 4 mm, 5 µm C18 silica; mobile phase: 32% water/68% acetonitrile; emission/extinction wavelengths for phenanthrene and fluorene were 249/345). A reference column containing clean industrial quartz particles of similar grain size was monitored in the same manner to account for background contamination. For four samples (DLS, DSS, MsKr, JKr), the initial isothermal desorption period of 90 days at 20 °C temperature was followed by a stepwise increase of the temperature first to 30 °C for 15 days, then to 40 °C for 43 days, to 50 °C for 21 days, and finally to 70 °C for 18 days. After the desorption experiments were finished, the solids were extracted in hot methanol according to Ball et al. (34) as described above. The recovery rates of phenanthrene for these desorbed and extracted samples were between 87% and 122%. In addition, sorption isotherms were determined at 20 and 40 °C for two samples (MsKr and DLS) following the same batch technique as described in Kleineidam et al. (30). Spherical Diffusion Model. Solute diffusion from an aqueous phase into spherical grains can be described by Fick’s second law in spherical coordinates:
[
]
∂C ∂2C 2∂C ) Da 2 + ∂t r∂r ∂r
(1)
where C, t, Da, and r denote the solute concentration in the particles pore water [µg L-1], the time [s], the apparent diffusion coefficient [cm2 s-1], and the radial distance [cm] from the center of the sphere (grain), respectively. For porous materials (all lithocomponents used in this study show a distinct intraparticle porosity; 1), Da can be defined as the
TABLE 1. Samples and Sorption Parameters from Batch Experiments at 20 °C radiusa,c lithocomponents aquifer materials light limestone (LLS) dark limestone (DLS) dark sandstone (DSS) Triassic limestone (MsK) Jurassic limestone (JK) Triassic sandstone (KS)d rock fragments Triassic limestone (MsKr) Jurassic limestone (JKr)
/a2 a
Freundlich parametersb log KFr [(µg/kg)(µg/L)-1/n] 1/n
[cm]
Da [1/s]
τfa
0.224 0.224 0.112 0.141 0.112 0.282
1.4 × 10-14 1.4 × 10-14 2.7 × 10-12 8.5 × 10-11 7.6 × 10-9 8.9 × 10-9
466 466 709 127 48 17
1.68 ( 0.07 3.69 ( 0.09 3.32 ( 0.02 2.26 ( 0.08 1.31 ( 0.08 1.67 ( 0.02
0.178 0.178
2.1 × 10-10 7.9 × 10-10
17 30
2.52 ( 0.06 2.08 ( 0.03
Mt/Meqc
CW c [µg/L]
equilibration time [days]
0.65 ( 0.04 0.39 ( 0.02 0.40 ( 0.01 0.67 ( 0.04 0.75 ( 0.02 0.76 ( 0.01
0.47 0.47 0.44 0.56 0.65 1.0
129 153 65 29 126 56
510 510 510 1010 1010 460
0.79 ( 0.04 0.72 ( 0.02
0.94 0.97
26 24
631 681
a Data from Ru ¨ gner et al. (1). b Data from Kleineidam et al. (30). c Fractional uptake (Mt/Meq) and aqueous concentration (CW) at the end of the sorption kinetic experiment (except for KS, this represents nonequilibrium conditions). d KS, comparable to SS published in refs 1, 30: grain size, 4-8 mm; ds ) 2.66 g cm-3; ) 0.08; relative amount of microporosity:
