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in soils was 48-70% after 30 days and 64-95% after. 270 days. Significant increases in adsorption capacity with aging suggest that accessibility of ph...
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Environ. Sci. Technol. 2006, 40, 5409-5414

Mechanistic Characterization of Adsorption and Slow Desorption of Phenanthrene Aged in Soils ABDUL ABU* AND STEVE SMITH Department of Biochemistry, School of Biomedical and Health Sciences, King’s College London, London SE1 9NH, United Kingdom

Long-term adsorption of phenanthrene to soils was characterized in a silt-loam (LHS), a sandy soil (SBS), and a podzolized soil (CNS) by use of the Polanyi-Manes model, a Langmuir-type model, and a black carbon-water distribution coefficient (KBC) at a relative aqueous concentration (Ce/Sw) of 0.002-0.32. Aqueous desorption kinetic tests and temperature-programmed desorption (TPD) were also used to evaluate phenanthrene diffusivities and desorption activation energies. Adsorption contribution in soils was 48-70% after 30 days and 64-95% after 270 days. Significant increases in adsorption capacity with aging suggest that accessibility of phenanthrene to fractions of SBS soil matrix was controlled by sorptive diffusion at narrow meso- and micropore constrictions. Similar trends were not significant for LHS silt-loam or CNS podzol. Analysis of TPD profiles reveal desorption activation energies of 35-53 kJ/mol and diffusivities of 1.6 × 10-7-9.7 × 10-8 cm2/s. TPD tests also indicate that the fraction of phenanthrene mass not diffusing from soils was located within micropores and narrow width mesopores with a corresponding volume of 1.83 × 10-5-6.37 × 10-5 cm3/g. These values were consistent with the modeled adsorption contributions, thus demonstrating the need for such complimentary analytical approach in the risk assessment of organic contaminants.

Introduction The kinetic and thermodynamic principles governing the interactions of hydrophobic organic contaminants (HOCs) with a variety of natural solid matrixes have been the focus of significant research effort for the past two decades. Many previous papers emphasized the importance of soil and sediment organic matter (SOM) heterogeneity in establishing the nature of sorption-desorption behavior and equilibrium isotherm character of HOCs (1-4). More recent investigations have suggested the existence of a diversity of reactive domains in soils and sediments that, in general, comprise absorption or partitioning and adsorption reactions (5-9). Partitioning processes are presumed to occur by dissolution of HOCs within humic material or amorphous organic matter, and they manifest linear, low capacity, noncompetitive, and rapid behavior in the sorption and desorption phases (4). Adsorption reactions, on the other hand, are generally presumed to comprise interfacial accumulation on relatively rigid, planar, aromatic surfaces and adsorption within micro-and nanopores of carbonaceous materials such as kerogens, coal * Corresponding author phone: (44) 207-8484446; fax: (44) 2078484500; e-mail: [email protected]. 10.1021/es060489h CCC: $33.50 Published on Web 07/11/2006

 2006 American Chemical Society

particles, black carbon, and soot (5, 9). Adsorption, slow or limited desorption, and the time-dependent decline in bioavailability of organic contaminants such as polycyclic aromatic hydrocarbons (PAHs) can be significantly rate limiting for in situ biodegradation, remediation, and subsurface transport, resulting in overestimation of exposure and risk from these compounds (2, 10). Extensive sorption capacities, nonlinear isotherms, sequestration, and slow desorption of PAHs have been attributed to the presence of a variety of adsorption domains within geosorbent matrixes. Hence mechanistic characterization of the distribution and reactivity of these domains is essential for the design and implementation of effective remediation schemes in contaminated soils. In our earlier paper (11), we were able to show that heterogeneous sorbents such as soils can exhibit an increasing capacity for the adsorption of PAHs over extended time scales and that the concept of a maximum adsorption capacity may lack mechanistic basis. Thus the slow process of attaining adsorption saturation should be the mechanistic basis for the sequestration or aging of organic contaminants such as PAHs in soils, as well as accounting for observed limited desorption effects. The slowly adsorbing or sequestered fraction was shown to exhibit high desorption activation energies, while sorption onto humic materials was associated with lower desorption activation energies during temperature programmed desorption (TPD) analysis of amended soil samples. However, these characteristics could not be attributed to any fundamental differences in the SOM of the soils studied. The growing evidence of the dominant role of carbonaceous materials in the retention of PAHs cast doubt on current methods for assessing bioavailability and risks from these contaminants. There is increasing interest for information on adsorption and sequestration of PAHs onto carbonaceous geosorbents to be used to refine a risk assessment as these processes control the formation of desorption-resistant fractions, bioavailability, natural attenuation, and the effectiveness of onsite remediation. In this paper, we report improved mechanistic characterization of phenanthrene adsorption onto soil organic components over extended periods. The rates and extent of phenanthrene desorption from soils with increasing adsorption was also characterized using a suite of tests which included TPD analysis of the microscale diffusivity of this contaminant through soil organic matrixes.

Experimental Section Sorbents and Chemicals. Three different soils were used for these studies: a silt-loam designated LHS, a sandy soil designated SBS that was obtained from an uncontaminated area of a former coal treatment facility in the north of England, and a podzolized brown earth designated CNS. For sorption experiments, bulk samples of soil were ground, air-dried, and passed through a 2 mm sieve. Detailed characteristics of soils are listed in Table 1. Additional information on soil properties, determination of total organic and black carbon contents (TOC, BC), cation exchange capacity (CEC), and inorganic carbon and nitrogen analysis can be found in the Supporting Information. Unlabeled phenanthrene with a purity of >96% as well as the radiolabeled compound [9-14C]phenanthrene (12.4 mCi/mmol) with >98% purity were obtained from Sigma-Aldrich Co. (Dorset, U.K.). Selected physicochemical properties of phenanthrene are provided in Table 2. VOL. 40, NO. 17, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Characteristics of Soils LHS silt-loam TOC content (%) BET-N2 SSA (m2/g)a CEC (mequiv/100 g) total pore vol (cm3/g)b mesopore vol (cm3/g) micropore vol (cm3/g) av. pore width (nm)d av. pore diameter (nm)e C-N ratio BC content (%)f focg

1.50 10.28 12.40 0.020 0.017 ndc 4.3 8.6 10.49 0.16 0.013

SBS sand

CNS podzol

0.24 2.40 1.33 4.84 2.67 8.55 0.003 0.015 0.003 0.013 1.1 × 10-5 3.3 × 10-5 4.0 5.8 8.1 11.6 12.25 9.30 0.03 0.21 0.002 0.022

a Brunauer Emmett Teller surface area. b Calculated from volume of nitrogen adsorbed at a relative pressure (p/p°) of 0.98. c Not explicitly determined. d The hysteresis loops have the general form of ‘Type B’ (14), suggesting a dominance of slit shaped pores in all three soil samples. e Cylindrical shaped pores. Microporosity was determined by the t-plot method. f Black carbon content was determined by the CTO-375 method (32). g Fraction of organic carbon, calculated as the difference between the TOC and BC contents (31).

TABLE 2. Some Physicochemical Properties of Phenanthrene MW (g/mol) solubility, Sw (mg/L) log Kow molar volume (cm3/mol) molecular density (g/cm3)

178.2 1.29a 4.57 174.7b 1.02

a Obtained from ref 15. b Estimated as the ratio of the molecular weight and density of phenanthrene.

Batch Experiments. All sorption experiments were conducted in glass Pyrex tubes fitted with silicone septa, which were also lined with poly(tetrafluoroethylene) (PTFE) tape, and cleaned according to standard procedures (see Supporting Information). Ten primary stock solutions of phenanthrene were prepared in methanol (HPLC grade) according to procedures described previously (12). Radiolabeled phenanthrene (8-110 µg/L) was added to 1-5 mL of methanol at a ratio of 1:10 to the unlabeled compound before addition to background stock solutions which comprised distilled deionized water, CaCl2 (0.01 M) as mineral source, and HgCl2 (200 mg/L) to inhibit microbial activity. Volumes of methanol transferred were designed to deliver a maximum concentration of 0.2% v/v, a level at which methanol is reported to have no significant effect on sorption (13). One gram of soil was mixed with 16 mL of stock solution in glass tubes, leaving a headspace of ∼0.5 mL. Samples were then incubated at 24 °C for 30, 60, or 270 days in darkness on a horizontal shaker (Model G25, New Brunswick Scientific Co. Inc., NJ). After incubation, tubes were removed from the incubator and centrifuged at 2000 rpm for 30 min, and a 2 mL sample of the aqueous supernatant was analyzed by liquid scintillation counting (LSC) on a Canberra Packard 2500TR scintillation system (Pangbourne, U.K). The solid-phase phenanthrene concentration was estimated by difference following determination of the aqueous-phase concentration. The remaining aqueous supernatant was pipetted off, and tubes were refilled with 16 mL of solute free background solution, shaken for 60s on a vortex mixer, and placed again in the incubator for a 21 day desorption kinetic study. Tubes containing stock solutions but no soil were also included to account for losses to glass walls or cap assemblies. Phenanthrene loss was found to be generally less than 4% and so were not included in mass balance calculations. Temperature-Programmed Desorption. Details of the thermal analysis systems used as well as initial sorption 5410

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experiments can be found in our earlier paper (11) and Supporting Information. Analysis of Data. The composite adsorption-partitioning model proposed by Xia and Ball (16, 17) was used to fit all sorption data of phenanthrene. This dual-mode modeling approach with the Polanyi-Manes model for describing the adsorption component has been explained in detail by Xia and Ball (16) and references therein.

qe ) Qo′ × 10a′(sw/Vs) × F + KpCe b′

(1)

where qe is the total mass of solute sorbed (mg/g), Qo′ is the adsorption volume capacity at saturation (cm3/kg), sw is the effective adsorption potential (J/mol) of the solute from aqueous solution calculated using the aqueous solubility (Sw) as opposed to the subcooled liquid solubility which we used in our previous report (11), Vs is the bulk molar volume (cm3/ mol) of solute at the temperature of adsorption, a′ is a fitted parameter, the exponent b′ in our application was set to 2, which corresponds to a log-normal distribution of sorption site energy for organic molecules on heterogeneous surfaces (18, 19), Ce is the equilibrium aqueous concentration of solute (mg/L), and F is the molecular density (g/cm3). The partition coefficient (Kp) was estimated from the Koc-Kow relationship provided by Karickhoff (20) for HOCs such as phenanthrene. Soil sorption data were also fitted with the Langmuir model and an expanded black carbon-water distribution model. These models have been summarized in the footnote to Table 3. Phenanthrene desorption kinetic studies were conducted over a 21 day period with two (2 mL) analysis of the aqueousphase concentrationson day 1 and 21, respectively. A firstorder two-compartment model (eq 2) was used to operationally describe desorption kinetic data of phenanthrene in the three soils.

qt/qo ) f e-kft + (1 - f)e-kst

(2)

where qt is the sorbate concentration in soil after time t, qo is the initial concentration of sorbate, and kf and ks are the first-order rate constants for the fast and slow desorbing pool, respectively. In our earlier work (11) we used an activated first-order rate model of the form described by the Arrhenius equation to derive activation energies associated with phenanthrene release from LHS silt-loam and SBS sand during TPD analysis. Here, the effect of temperature on diffusivity was also approximated to a first-order rate process by use of the relationship between a first-order rate constant and the diffusion coefficient presented by Glueckauf (21) in early work on application of spherical diffusion models to chromatography. This approximate relationship has also been used to describe diffusive mass transfer in natural sorbents by others (22-24).

k ) 15(Da/a2)

(3)

where Da is the apparent diffusion coefficient (cm2/s) and a is the indeterminate diffusion length scale which in natural solids cannot be separated from Da. Values of Da/a2 estimated from eq 3 were then factored into an Arrhenius expression to model the change in diffusivity with rise in temperature (24).

D ) (Da/a2)e(Ea/R)(1/T1 - 1/T2)

(4)

where D represents diffusivity (cm2/s), Ea is the activation energy (kJ/mol) for diffusion, R is the molar gas constant (kJ/mol), and T is the temperature of desorption in degrees Kelvin.

TABLE 3. Fitted Adsorption Parameters, BC-Water Distribution, and Partitioning Coefficient Values for the Sorption of Phenanthrene to Soils soil

Qo′ (cm3/kg)

Qmaxa (mg/g)

Kd (L/kg)

Kpb (L/kg)

LHS loam (30 d) LHS loam (60 d) LHS loam (270 d) SBS sand (30 d) SBS sand (60 d) SBS sand (270 d) CNS podzol (30 d) CNS podzol (60 d) CNS podzol (270 d)

0.043 ( 0.027e 0.062 ( 0.043 0.081 ( 0.034 0.020 ( 0.035 0.046 ( 0.026 0.124 ( 0.026 0.110 ( 0.026 0.095 ( 0.032 0.147 ( 0.054

0.059 ( 0.006 0.080 ( 0.002 0.071 ( 0.004 0.034 ( 0.003 0.057 ( 0.004 0.045 ( 0.003 0.110 ( 0.002 0.084 ( 0.004 0.152 ( 0.005

369.18 568.83 775.17 89.39 265.44 699.27 870.47 935.57 1378.2

200.17 31.37 328.64

log KBCc

packingd efficiency

4.02 4.36 4.56 5.29 5.89 6.35 4.43 4.48 4.83

0.30 0.44 0.57 0.14 0.33 0.88 0.78 0.67 1.04

a Langmuir monolayer adsorption capacity estimated from the model q ) Q e maxbCe/(1 + bCe), where b represents the adsorption site affinity. Partition coefficient estimated from the relationship: log Koc ) 0.99(log Kow) - 0.35 (20), where Kp ) Kocfoc. c KBC values were estimated from the expanded distribution coefficient described in refs 32 and 34 [Kd ) focKoc + fBCKBC], Koc is the organic carbon normalized partition coefficient, and fBC is the fraction of BC. d Packing efficiency (ηp), is the ratio of the adsorbed volume of the solid adsorbate to that of a reference liquid compound (16). e At the 95% confidence interval. Other parameters have been provided in Table S1 of the Supporting Information. b

Results and Discussion Polanyi-Manes and Langmuir Modeling of Phenanthrene Adsorption. The Polanyi-Manes modeling for describing the adsorption component of phenanthrene sorption showed significant time-dependent increases in the adsorption volume capacity and the adsorbed solute mass for SBS sand at the relative aqueous concentration of Ce/Sw ) 0.002-0.32 after 60 and 270 days. But observed increases in the adsorption parameters were not significant for LHS silt-loam or CNS podzol (Table 3). Using partition fractions derived from the Koc-Kow correlation of Karickhoff (20), the contribution of adsorption during 30-270 days of sorption was estimated to be 48-75% for LHS silt-loam, 70-95% for SBS sand, and 59-64% for CNS podzol (Figure S1, Supporting Information). Adsorption of organic solutes has been reported to account for 37-68% of total sorption at Ce/Sw ) 0.1-0.35 in peat and mineral soils (3), but less than 40% at Ce/Sw > 0.1 in an aquitard (16). Significant increases in the adsorption capacity with aging suggests that accessibility of phenanthrene molecules to fractions of SBS pores may have been initially limited by an adsorption retarded pore diffusion process (2, 25). Estimation of the micropore volume of soils from nitrogen adsorption isotherms yielded negligible microporosities, significantly less than the adsorbed volumes of phenanthrene at all periods of analysis. Interestingly, when these values were added to a fraction of the mesopore volumes ( 0.98). c Diffusivity at 24 °C. d Diffusivity at peak temperature of phenanthrene desorption during TPD. e Desorption rate constant for initial phenanthrene released from SBS sand during TPD.

FIGURE 1. Extracted ion chromatogram of phenanthrene (m/z ) 178) released from soils during TPD analysis: (A) SBS sand after 2 days of sorption; (B) SBS sand after 270 days of sorption; (C) LHS silt-loam after 2 days of sorption; (D) LHS silt-loam after 270 days of sorption. a mechanistic process whereby slow diffusion of HOC molecules in natural sorbents is controlled by sorptive forces at hydrophobic micropore constrictions (33). Using sediment samples, log KBC values of 5.3-6.25 have been obtained along with a BC adsorption contribution of 80-90% for phenanthrene by others (31, 34), very similar to the values we obtained for SBS sand. Phenanthrene Aqueous Desorption Kinetics. For all three soils, the fraction of fast desorbing phenanthrene measured in solution represented >0.98 and was released within 1 day. Therefore, the contribution of the slow desorbing fraction in solution to the total fraction of desorbed phenanthrene was assumed to be negligible, or to comprise the ‘very slowly’ desorbing fraction which, admittedly, could not be adequately assessed during a 21 day desorption kinetic study. Reported rate constants for the fast and slow desorbing fraction of HOCs in natural sorbents are in the range of 10-5-10-6 s-1 and 10-7-10-10 s-1, respectively (24, 35-37). Our results show that the fast rate constants for phenanthrene desorption from soils were on the order of 10-6-10-7 s-1 after 30 days of sorption, but decreased by several orders of magnitude to 10-8-10-9 s-1 after 270 days (Table 4). This suggests that 5412

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initially sorbed phenanthrene increasingly diffused into inner regions of soils and smaller sized pores where sorption is much stronger than at (near) surface sites. Temperature-Programmed Desorption Results. Temperature-programmed desorption (TPD) tests were used to determine the activation energy (Ea) associated with each temperature step of phenanthrene release from LHS siltloam and SBS sand (Figure S2, Supporting Information). The change in diffusivity with rise in temperature was also evaluated as a means of characterizing diffusive mass transfer processes in the studied soils. Initial diffusion of phenanthrene from SBS sand exhibited Ea values of 35.3-38.8 kJ/ mol (Figure 1) at the programmed temperature of 135-175 °C. Activation energies for diffusion of phenanthrene out of LHS silt-loam were estimated to be 44.8 and 52.9 kJ/mol at the onset and peak desorption temperatures of 225 °C and 315 °C, respectively. Corresponding Ea values for diffusion from SBS sand were 42.9 and 51.0 kJ/mol at the onset and peak desorption temperatures of 175 °C and 260 °C, respectively. The low Ea values associated with phenanthrene diffusion from LHS silt-loam and SBS sand are consistent with values derived for diffusion of PAHs from humic and

aquifer materials by others (24, 38). Using the measured rates of phenanthrene diffusion from both soils at temperatures below 375 °C, Ea values of 78.5 and 83.6 kJ/mol corresponding to the thermal analysis temperature of 600 °C were estimated for LHS silt-loam and SBS sand, respectively. It is instructive to note that, although only a small fraction of initially sorbed phenanthrene was released from both soils at this high temperature (11), the estimated Ea values are indicative of phenanthrene diffusion from condensed micrporous domains of these soils (24, 33, 39). The TPD response of phenanthrene for LHS silt-loam and SBS sand exhibited different chromatographic shifts as is apparent from the TPD-MS profiles illustrated in Figure 1. This difference can be attributed to thermodynamic effects on the organic matter matrixes and differences in diffusivity through intraparticle pores of both soils (24, 40). In general, desorption rate constants and diffusivities associated with release of phenanthrene were of the same order of magnitude for LHS silt-loam and SBS sand (Table 4), although these parameters were lower for the SBS sand. Diffusivity of phenanthrene estimated at the aqueous desorption temperature of 24 °C was in the range of 7.6 × 10-8-1.6 × 10-9 cm2/s, while diffusivity during TPD analysis was in the range of 1.6 × 10-7-9.7 × 10-8 cm2/s at the peak temperature of phenanthrene release from soils (Table 4). Diffusivities of 10-9-10-12 cm2/s have been reported for desorption of HOCs from soils and sediments (33, 41). On the other hand, extremely small diffusivities in the range of 10-17-10-21 cm2/s have been estimated for PAHs (24), some field aged pesticides (2), and PCBs (40) based on parameters associated with diffusion from condensed polymeric materials. The TPDMS profiles also illustrate a bimodal diffusion of sorbed phenanthrene from the SBS sand. It is interesting to note that the area under each TPD curve, representing the total mass of phenanthrene diffused over the period of desorption, was 1.5 ( 0.2 (for four replicates) times greater for the initial diffusion of this PAH after 2 days of sorption than after 270 days. Unfortunately, we are unable to provide experimental evaluations for CNS podzol in this report as TPD data could not be obtained at the time of analysis. Relating Phenanthrene Adsorption to Diffusion Characteristics. As the adsorption potential increases with decreasing pore size (28), more phenanthrene molecules are expected to diffuse into microporous soil domains at higher concentrations. Thus, observed trends in adsorption capacities may be related to the meso-and micropore volumes of soils as well as the pore widths. Similarities in the Ce/Sw values corresponding to saturation of the adsorption potential after 270 days of sorption were observed (0.06-0.09) for all three soils. However, much smaller decreases in Ce/Sw values from 30 to 270 days suggest that adsorption of phenanthrene to CNS podzol probably occur within pores having greater widths compared to SBS sand or LHS silt-loam, where adsorption occurs essentially in narrower width mesopores (Figure S3, Supporting Information). These values also suggest a significant time-dependent sorptive diffusion of phenanthrene in SBS sand. Estimation of diffusivity at temperatures up to 600 °C indicate that above ∼400 °C, the diffusivity of phenanthrene from SBS sand would increase rapidly with temperature (Figure 2). Given the TPD curves illustrated in Figure 1 which suggest complete recovery of the phenanthrene mass in that domain of soils, we are inclined to conclude that the predicted increase in diffusivity is associated with differences in thermodynamic effects on surface regions and humic materials of both soils. Results from TPD analyses also indicate that the fraction of phenanthrene mass not recovered at the tested temperature of 75-375 °C comprised ∼2065% of the phenanthrene mass sorbed after 30-270 days. From this, adsorbed volumes corresponding to the mass of

FIGURE 2. Effects of temperature increase on diffusivity of phenanthrene from soil matrixes: (b) SBS sand; (2) LHS silt-loam. phenanthrene remaining on soils was calculated to be 1.83 × 10-5-4.13 × 10-5 cm3/g for LHS silt-loam and 3.38 × 10-5-6.37 × 10-5 cm3/g for SBS sand. These values were found to exceed the micropore volumes of soils, consistent with the modeled adsorption contributions. Furthermore, the difference between Polanyi-Manes adsorption capacity and the Langmuir monolayer coverage was found to be ∼64% after 270 days, corresponding to the fraction of phenanthrene estimated to have diffused into micropores and narrow width mesopores of SBS sand following TPD analysis. For SBS sand, (near) surface adsorption and slow diffusion into condensed carbonaceous matrixes are two distinct mechanistic processes that characterize adsorption of phenanthrene. The absence of TPD data for CNS podzol no doubt limits our ability to draw valid conclusions for this soil, although similar trends were observed as with LHS silt-loam where the implied monolayer capacities are consistent with meso- and micropore adsorption estimated from the Polanyi-Manes modeling approach. These results demonstrate the utility of adsorption modeling and TPD in providing an improved mechanistic characterization of HOC association with soil organic matrixes. Such combined analytical approach has become increasingly critical for understanding the mobility, bioavailability, and sequestration of HOCs in soils, as well as the design of appropriate remediation strategies.

Acknowledgments This work was part of a research program at King’s College London supported by the World Bank (JJWBGSP). We thank Dr. Dan Waterman and Billy Kang for assistance with GCMS analyses, Dave Thornley of the University of Reading for important work on soil pore analyses, and several anonymous reviewers for their valuable comments.

Note Added after ASAP Publication An error was present in equation 1 in the version published ASAP on July 11, 2006. The corrected version was published ASAP on July 18, 2006.

Supporting Information Available Additional information on experimental procedures, adsorption model parameter values, and the Ce/Sw values at which the adsorption potential would reach saturation capacity as well as plots of increases in Ea with rise in temperature. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Received for review March 1, 2006. Revised manuscript received May 29, 2006. Accepted May 31, 2006. ES060489H