Influence of Activated Charcoal on Desorption Kinetics and

Oct 23, 2012 - hydrocarbons (PAHs) to black carbon (BC) presents potential ... amended with 0, 0.1, 1, and 5% (dry wt. soil) activated charcoal (AC, a...
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Influence of Activated Charcoal on Desorption Kinetics and Biodegradation of Phenanthrene in Soil Angela H. Rhodes, Matthew J. Riding, Laura E. McAllister, Katherine Lee, and Kirk T. Semple* Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, United Kingdom S Supporting Information *

ABSTRACT: The observed strong sorption of polycyclic aromatic hydrocarbons (PAHs) to black carbon (BC) presents potential implications for PAH bioaccessibility in soils. The effects of BC on the desorption kinetics and mineralization of phenanthrene in four soils was investigated after 1, 25, 50, and 100 d soil−PAH contact time, using sequential hydroxypropyl-β-cyclodextrin (HPCD) extractions in soils amended with 0, 0.1, 1, and 5% (dry wt. soil) activated charcoal (AC, a form of BC). The rapidly (%Frap) and slowly (%Fslow) desorbing phenanthrene fractions and their rate constants were determined using a first-order two-compartment (biphasic) desorption model. A minimum 7.8-fold decrease in %Frap occurred when AC was increased from 0 to 5%, with a corresponding increase in %Fslow. Desorption rate constants followed the progression krap (% h−1) > kslow (% h−1) and were in the order of 10−1 to 10−2 and 10−3 to 10−4, respectively. Linear regressions between %Frap and the fractions degraded by a phenanthrene-degrading inoculum (%Fmin) indicated that slopes did not approximate 1 at concentrations greater than 0% AC; % Fmin often exceeded %Frap, indicating a fraction of sorbed phenanthrene (%Fslow) remained microbially accessible. Therefore, HPCD-desorption kinetics alone may not be an adequate basis for the prediction of the bioaccessibility of PAHs to microorganisms or bioremediation potential in AC-amended soils. soils.21−24 It is postulated that the slow or very slow rates of desorption are due to PAH-association with BC matrices.25 However, evidence exists to suggest that the rapidly desorbing fraction of PAHs may provide a direct measure of the microbially degradable contaminant fraction.26−28 For example, Cornelissen et al.26 studied the desorption kinetics of 15 PAHs in sediments before and after bioremediation using Tenax beads; an almost direct relationship between the rapidly desorbing fractions and the amounts removed by bioremediation was observed. Rhodes et al.29 investigated the impact of AC upon the bioaccessibility of phenanthrene in soil. This was investigated by quantifying microbial mineralization as well as extractability using hydroxypropyl-β-cyclodextrin (HPCD) aqueous solutions. This study was the first to have utilized this extraction technique for AC-amended soils, as well as unique in its perspective of quantifying the effect of AC upon microbial activity. In the present investigation we further build upon the work by Rhodes et al.29 through the use of a first-order twocompartment (biphasic) desorption model and sequential HPCD extractions to determine the desorption kinetics of 14 C-phenanthrene in soils amended with four different

1. INTRODUCTION Soils form a large repository for the majority of both legacy and recent contaminants, such as polycyclic aromatic hydrocarbons (PAHs).1 The extent of contaminant sorption to soils is a fundamental factor in the regulation of their bioavailability, and hence the bioremediation processes to which they can be subjected.2 Fully understanding sorption and desorption within soils is therefore critical to the risk assessment and remediation of contaminated land. The ability of carbonaceous materials, such as black carbon (BC) and commercially activated carbon (AC), to sorb in a nonlinear fashion to many hydrophobic organic contaminants (HOCs) such as PAHs,3−6 polychlorinated biphenyls (PCBs), 7−10 and pesticides,11 is well established. Much of the current research has emphasized the important role of BC in reducing contaminant aqueous availability and consequently bioaccumulation and/or toxicity of sediment-bound contaminants7,12−17 for the purpose of remediation.18 However, the impact of different BC concentrations on the HPCD desorption kinetics of contaminants in soils with a variety of different characteristics in direct comparison to microbial mineralization has not been investigated. The desorption of PAHs from soils and sediments often exhibits a biphasic profile, whereby an initial rapid phase of desorption is followed by slow and very slow phases of release.19,20 Several authors have substantiated a link between BC content and rapidly/slowly desorbing PAH fractions in © 2012 American Chemical Society

Received: Revised: Accepted: Published: 12445

June 21, 2012 October 9, 2012 October 23, 2012 October 23, 2012 dx.doi.org/10.1021/es3025098 | Environ. Sci. Technol. 2012, 46, 12445−12451

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Table 1. Physicochemical Properties of Soils A, B, C, and D particle size analysis (%)

.

sanda

pH

elemental analysis (%)

grid ref

texture

C

M

F

silt

clay

dH2O

CaCl2

OMb

A: typical stagnogley soil

SD496402

1.71

15.56

38.36

24.96

19.41

6.53

5.18

4.821

1.7 ± 0.09

0.14 ± 0.01

B: typical brown earth soil C: earthy oligo-fibrous peat soil D: typical humic alluvial gley soil

SD491655 SD511775

clay loam loam silty clay

7.53 0.96

11.17 1.8

36.69 7.69

26.77 47.37

17.84 42.18

5.44 7.5

5.02 6.01

9.33 27.15

2.99 ± 4.71 19.50 ± 6.20

0.25 ± 0.31 1.32 ± 0.42

0.92

1.2

36.65

34.44

27.79

6.93

5.19

10.25

3.48 ± 5.10

0.26 ± 0.52

soil type

a

SD447543

clay loam

TOCc

Nd

Course, medium, and fine sand. bOrganic matter content (loss on ignition) (%). cTotal organic carbon (%). dNitrogen (%).

concentrations of AC. 14C-Phenanthrene mineralization was also measured and correlated with the rapidly desorbing fractions using linear regression analysis. Such a study has never been conducted with this extraction technique to determine desorption kinetics in AC-amended systems, and would further our understanding of the interactions between contaminants and degrading microorganisms in soils.

by liquid scintillation counting (LSC) (Canberra Packard Tri Carb 2300 TR, U.K.) using standard calibration and quench correction techniques. Desorption of 14C-Phenanthrene from Soil Using Hydroxypropyl-β-Cyclodextrin (HPCD). Desorption of 14 C-phenanthrene was measured using sequential HPCD extractions as described by Reid et al.32 and Rhodes et al.28 Soil samples (1.25 g, n = 3) were weighed into Teflon centrifuge tubes and underwent continuous extraction with 25 mL of 50 mM HPCD solution prepared with deionised water. The tubes were then sealed and shaken horizontally (100 rpm) on an orbital shaker at 20 ± 2 °C for 20 h prior to centrifugation at 3000g for 50 min (Hettich Rotanta 460 Zentrifugen centrifuge). The supernatants were sampled (6 mL), added to liquid scintillation cocktail (14 mL), and 14Cphenanthrene was quantified by LSC. The remaining supernatant was discarded and the soil pellets were resuspended with 25 mL of fresh HPCD solution; tubes were resealed and returned to the orbital shaker. This process was repeated sequentially/consecutively on each pellet a total of nine times at time intervals of 0, 3, 6, 12, 18, 42, 66, 90, and 114 h for each aging time point, similar to HPCD desorption work conducted by Rhodes et al.28 After the last extraction and supernatant sampling at 114 h, the total activity remaining in the pellet was measured through combustion and LSC to determine the mass balance as described previously. Desorption Data Interpretation. Desorption of PAHs from soils and sediments has been empirically described by the following first-order kinetics:19,20,33

2. EXPERIMENTAL SECTION Chemicals. Unlabeled and [9-14C]-phenanthrene (specific activity = 55.7 mCi mmol−1) were obtained from Sigma Aldrich Co, Ltd. U.K. Liquid scintillation cocktail (Goldstar) was obtained from Meridian, U.K., sample oxidizer cocktails (Carbosorb-E and Permafluor-E) were from Perkin-Elmer Life Sciences, and Combustaid was from Canberra Packard, U.K.. Hydroxypropyl-β-cyclodextrin (HPCD) was obtained from Fisher Scientific; U.K. AC was obtained from BDH Chemicals; BET surface area (as determined by nitrogen adsorption) was 980 m2 g−1, the average particle size was 3.7 ± 0.2 μm, and porosity and pore volumes were 75.74% and 1.43 cm3 g−1, respectively.30 Soils. Soils were collected from the A horizon (5−20 cm) in four different locations in Lancashire, England, and classified as a typical stagnogley soil, a typical brown-earth soil, an earthy oligo-fibrous soil, and a typical humic alluvial gley soil (Table 1). The soils were air-dried for 48 h, passed through a 2-mm sieve, and rehydrated with deionized water to original field moisture (∼30% dry weight). Each soil type was then divided into divided into 200-g batches. AC concentrations of 0%, 0.1%, 1%, and 5% (of dry soil wt.) were then blended into the rehydrated soil sample batches using a stainless steel spoon. The AC was added dry, directly to the soil. Immediately after AC amendment, the soils were then spiked with both 12C and 14 C-phenanthrene using acetone as a carrier solvent; the method is described elsewhere31 to achieve a final concentration of 10 mg kg−1. The final activity of 14C-phenanthrene spiked into soil was 2500 DPM g−1. Within 1 d of spiking, soils were sterilized by γ-irradiation (32.2 kGy, Isotron plc, Bradford, U.K.) and stored in sealed amber glass jars in darkness at room temperature (20 °C) for 1, 25, 50, and 100 d. Determination of Total 14C-Phenanthrene Activity in Soil. The soils were analyzed for total 14C-phenanthrene associated activity by combustion. Soil samples (1 g soil; n = 3) were weighed into cellulose combustion cones and combusted (3 min), with the addition of combustaid (200 μL) (Packard 307 Sample Oxidizer). Carbosorb-E (10 mL) was used to trap the CO2 and Permafluor-E (10 mL) as a scintillation cocktail. The trapping efficiency, as determined prior to sample combustion, was >90%. The resultant solutions were quantified

St /S0 = Frap·exp( −k rap·t ) + Fslow ·exp( −kslow ·t )

(1)

where St corresponds to the amount of 14C-phenanthrene sorbed to the soil at desorption time t (h) and S0 is the soilsorbed amount of 14C-phenanthrene at the start of the experiment (0 d aging) (obtained by combustion and LSC as described previously). %Frap and %Fslow are the rapidly and slowly desorbing fractions, respectively, and krap (% h−1) and kslow (% h−1) are the corresponding rate constants of rapid and slow desorption expressed in % h−1. The model assumes that kslow (% h−1) is significantly less than krap (% h−1). Values of % Frap, %Fslow, krap (% h−1), and kslow (% h−1) were determined by exponential curve fitting with Excel Add-In XLfit using a nonlinear least-squares method. Preparation of Phenanthrene-Degrading Inoculum. A bacterial inoculum (identified as a Pseudomonas sp.),34 able to utilize phenanthrene as a sole carbon source for growth, was cultured on 0.1 g phenanthrene L−1 in 300 mL of a minimum basal salts (MBS) solution at 20 ± 2 °C, at 100 rpm.35 After 4 d incubation (late exponential phase of growth), the culture was 12446

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centrifuged at 10 000g for 30 min (Beckman J-6M, Beckman Coulter, USA). The supernatant was discarded, and the cells were resuspended in MBS. This procedure was repeated twice to ensure thorough washing of the cells and removal of any residual phenanthrene. Mineralization of 14C-Phenanthrene in Soil. Mineralization assays were performed in modified 250-mL Schott bottles (“respirometers”) using the method of Reid et al.34 Each respirometer incorporated a Teflon-lined screw cap and a 14 CO2 trap containing 1 M sodium hydroxide (NaOH) (1 mL) within a suspended 7-mL glass scintillation vial. Each respirometer was prepared in triplicate and contained 10 ± 0.2 g soil (wet wt.), 25 mL of MBS, and 5 mL of degrading bacteria (107 cells g−1 soil). Respirometers were stored on an orbital incubator shaker (100 rpm) at 20 ± 2 °C over a period of 20 d. On the basis of previous investigations,36 the 20 d incubation period allowed sufficient time for phenanthrene mineralization to both peak and plateau. Any 14CO2 evolved as a result of the mineralization of 14C-phenanthrene was trapped in the NaOH and assessed at regular intervals using LSC. Statistical Analysis. Blank corrected readings for both mineralization and HPCD extractions were statistically analyzed using Sigma Stat for Windows (Version 2.03, SPSS Inc.). The statistical significance of BC addition, aging, and variation in soil type were determined using a General Linear Model (ANOVA, Tukey Test). Figures were produced in Sigma Plot for Windows (2000).

3. RESULTS Desorption Kinetics of 14C-Phenanthrene. The cumulative desorption of phenanthrene, plotted as the natural log of St/S0 versus time, is displayed in Figures 1 and 2 for soils A, B, C, and D over 1, 25, 50, and 100 d soil-PAH contact time. The solid lines were obtained by exponential curve fitting using a two-compartment model represented by eq 1. Fitting the data using a three-compartment (triphasic) model resulted in a higher sum of squared deviations (data not shown); this may be attributed to the lack of a very slowly desorbing fraction becoming established in freshly contaminated and AC-amended soils over the time period considered. Statistical analyses at the 90% confidence level revealed that the two-compartment model provided a good fit of all the desorption kinetic data (r2 range 0.996−0.998). The biphasic model of desorption provided estimates of the rapidly and slowly desorbing phenanthrene fractions (%Frap and %Fslow) and their rate constants (krap (% h−1) and kslow (% h−1)); the data are presented in Supporting Information Tables S1−S4. An initial rapid drop in the amount of phenanthrene sorbed was followed by a slower decrease in sorbed concentrations (Figures 1 and 2). In control soils (0% AC), the rapidly desorbing fraction generally occurred from 0 to 6 h, thereafter from 6 to 20 h, and >20 h desorption was dominated by the slowly desorbing fraction. Overall, with AC amendments between 0.1% and 5%, %Frap decreased between 1 and 100 d soil−PAH contact time by approximately 2−30%, with a comparable increase in %Fslow (Tables S1−S4). In control soils, the greatest extents of increase in %Fslow took place between 50 and 100 d for soils A and B and between 25 and 50 d for soils C and D. At 0% AC, relatively large decreases of 18.96%, 12.55%, and 26.77% were observed in %Frap between 1 and 100 d for soils B, C, and D, respectively (Tables S2−S4), while soil A recorded a much smaller decrease of 6.77% (Table S1). However, %Frap values at

Figure 1. Desorption of 14C-phenanthrene from soils A and B amended with 0 (●), 0.1 (○), 1 (▲), and 5 (△) % AC, after 1, 25, 50, and 100 d soil PAH contact time; plotted as St/S0 versus time. Solid lines are obtained by a two-compartment exponential curve fitting.

corresponding time points demonstrated minimal variation among soil types. Across soil conditions, desorption rate constants followed the progression of krap (% h−1) > kslow (% h−1) and were generally in the order of 10−1 to 10−2 and 10−3 to 10−4, respectively. Values of krap (% h−1) in control soils were significantly greater (P < 0.05) in soils A and B when compared to soils C and D. Phenanthrene desorption kinetics were most clearly affected by the concentration of AC in soil. As AC amendment increased from 0 to 5%, there was a minimum 7.8-fold decrease (P < 0.001) in %Frap and a corresponding increase in %Fslow. After only 1 d soil−PAH contact time the values of %Frap ranged from 87.65 to 76.14%, 61.52 to 58.36%, 21.46 to 17.00%, and 9.53 to 3.72% in soils amended with 0, 0.1, 1, and 5% AC, respectively. At corresponding time points, krap (% h−1) did not vary by more than approximately 2-fold from 0 to 5% AC amendment, for each soil type. For example, in soil A after 1 d soil−PAH contact time, krap values were 0.49, 0.23, 0.11, and 0.09% h−1 for soils amended for 0, 0.1, 1, and 5% AC, respectively (Table S1). A similar trend was observed in each soil at each time point. However, kslow (% h−1) values for 1 and 5% AC-amended soils were 1 or 2 orders of magnitude lower than the same desorption rate constants measured for control 12447

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Figure 3. Linear regressions correlating the total fraction of 14Cphenanthrene mineralized by an enriched catabolic inoculum and the rapidly desorbing fraction of 14C-phenanthrene, determined using HPCD, in soils A (●), B (○), C (▼), and D (△) amended with 0, 0.1, 1, and 5% AC. Regression lines are shown as a solid line and related equations given. Error bars are SEM (n = 3).

4. DISCUSSION Desorption of 14C-Phenanthrene using HPCD. Rate constants for rapid and slow desorption in control soils which had not been amended with AC were 10−1 to 10−2 and 10−2 to 10−4 h−1, respectively. The values of kslow (% h−1) observed here from soils using HPCD extraction are in accordance with many other literature values for PAHs in sediments using a variety of other extraction methods.20,26,35,37,38 For example, Greenburg et al.37 determined desorption kinetics for fluoranthene and trifluralin in laboratory-spiked lake sediments using Tenax. The authors stated rate constants in the order of 10−1 h−1, 10−2 to 10−3 h−1 and 10−4 h−1 for krap (% h−1), kslow (% h−1) and kveryslow (% h−1), respectively; hence, a clear distinction between krap (% h−1) and kslow (% h−1) can be made. Similarly, the values of % Frap and %Fslow, for control soils are consistent with those found in previous work with sediments.19,26,35,37,39 Notably, as soil−PAH contact time increased the rapidly desorbing fractions decreased with a corresponding increase in the slowly desorbing fractions. As the size of the rapidly desorbing fraction has been linked to the labile fraction, these results suggest an important decrease in the freely available phenanthrene between 1 and 100 d. Such findings are in agreement with other studies, which have shown an increase by a factor of 2−10 in the slow desorbing fraction as a function of aging.20,40,41 In this study, the greater extent of increase in slowly desorbing fractions took place between 50 and 100 d for soils A and B and between 25 and 50 d for soils C and D. Although these results are inconsistent with other investigations,41 such as Swindell and Reid42 who found little change in the rapidly and slowly desorbing fractions between 40 and 80 d aging, the authors measured desorption of phenanthrene using butanol over a much shorter duration (1000 s) with a vortex mixer, which may account for the disparity in desorption kinetics. Overall, the difference between %Frap and %Fslow results presented here suggests that the use of HPCD extractions provides a comparable and valid method for quantifying the desorption kinetics of PAHs. Impact of AC on 14C-Phenanthrene Desorption. The results indicate AC concentrations in soil significantly affected

Figure 2. Desorption of 14C-phenanthrene from soils C and D amended with 0 (●), 0.1 (○), 1 (▲), and 5 (△) % AC, after 1, 25, 50, and 100 d soil PAH contact time; plotted as St/S0 versus time. Solid lines are obtained by a two-compartment exponential curve fitting.

soils. This suggests that AC had a much greater influence upon kslow (% h−1) than krap (% h−1). Correlation between Rapidly Desorbing Fractions and Mineralizable Fractions. Figure 3 displays the correlations observed between %Frap using HPCD chemical extraction and the total mineralized fraction (%Fmin) by a phenanthrene degrading inoculum after 20 d incubation as a function of AC amendment. In control soils (0% AC) and those amended with 0.1% AC, the %Frap and %Fmin values follow a similar trend in which they both decrease with increasing aging time. Linear regression between these two parameters provided a good fit, with slopes (gradients) of 0.71 and r2 of 0.62 for 0% AC, and a weaker fit for 0.1% AC with slopes of 0.61 and r2 of 0.44. Soils amended with 1% or 5% AC showed no useful correlation between %Frap and %Fmin (r2 = 0.02 and 0.06, respectively). Tables S1−S4 also display the calculated ratios between %Frap and %Fmin. In the control soils, ratios were often close to one (range 0.73−0.99). A deviation in this relationship was evident in AC-amended soils as ratios regularly exceeded 1, as %Fmin was generally greater than %Frap. For example, in soil C (Table S3), %Fmin was 19.3−24.9% greater than %Frap, in soil B 12.6−18.5% (Table S2); however, no clear trends were apparent with soil−PAH contact time. 12448

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environmental samples is only 1−15% of the total organic content in soil (Cornelissen et al.23and references therein) and likely will not exceed 1%.52 Therefore, on the basis of the findings in this present study, at low BC concentrations (0.1%) desorption kinetics using HPCD may provide a weakly feasible method for assessing the bioaccessibility/biodegradability of PAHs associated with BC at typically occurring concentrations of BC within the field. As the mineralized fraction often exceeded the rapidly desorbing fraction for 1% and 5% AC amendments (Tables S1−S4), and no correlation between the two parameters was apparent (Figure 3), a possible explanation may be that the microorganisms were capable of adhering to, or forming biofilms on, BC particles.50,51 Although strong evidence exists to suggest that the degradation of PAHs within soil occurs predominantly from the aqueous dissolved phase, biofilm formation, and the release of extracellular enzymes, constitutes an important mechanism among bacteria to shorten the diffusion distance and attempt to overcome mass-transfer limitations in environments with poorly soluble or strongly sorbed PAHs.29,52 While we present no explicit evidence for microbial utilization of phenanthrene directly from the sorbed phase, as mineralization exceeded rapid desorption it may be tentatively hypothesized that a fraction of sorbed PAHs remained accessible. As HPCD only removes rapidly desorbing and dissolved PAHs, %Frap calculated using HPCD would largely underestimate the biodegradable or mineralizable fraction of phenanthrene in soils if substantial concentrations were bioaccessible in the solid-sorbed phase. Equally, however, it is important to note that a possible explanation for this observation is that the 100 d aging period may have been insufficient to allow the development of an equilibrium in the soil−water−phenanthrene−AC system, resulting in higher mineralization values than anticipated. The results from the present study demonstrated only limited potential in the application of HPCD solutions for characterizing desorption in relation to bioaccessibility of PAHs in AC-amended soils at low concentrations. We related for the first time the microbial accessibility of phenanthrene in soils to AC desorption kinetics. In the presence of AC, phenanthrene degradation continues at a rate faster than rapid desorption. It is therefore hypothesized that microbes may be able to utilize the sorbed PAH through mechanisms which do not involve desorption, i.e. through direct contact. Thus, although the PAH is no longer bioavailable as determined by a biomimetic chemical extraction, the phenanthrene does remain microbially bioaccessible, although additional work for longer periods of aging may be required to ensure the observed trends are not an artifact of limited equilibrium within the assay system. Although the desorption kinetics identified here highlight the importance of BC in controlling PAH bioaccessibility, it may still be difficult to estimate to what extent BC controls sorption in the field. The empirical and limited nature of Frap, Fslow, and the derived rate constants require further studies to relate the results assessed by biomimetics with other organisms and with results from the field.

C-phenanthrene desorption; as concentrations of AC increased, the rapidly desorbing fraction of phenanthrene decreased and the slowly desorbing fraction increased (Table S1−S4). Many previous studies have suggested strong sorption and, by inference, limited desorption from BC in field and laboratory spiked soils and sediments.3,4,6,22,23,25,44−47 In this investigation, it is postulated that the addition of AC with porosity and pore volumes of 75.74% and 1.43 cm3 g−1, respectively, substantially increased the surface area available for the sorption of PAHs. Jonker and Koelmans5 studied the sorption mechanisms between planar molecules and sediments amended with BC, concluding that pore sorption and π−π interaction processes are the most important. Our results further substantiate the extremely strong sorption of PAHs to AC sorption sites in soils, which are in abundance at AC concentrations of 1 and 5% and consequently lead to reduced phenanthrene extraction/desorption. In addition, Jonker and Koelmans5 suggested that aqueous-based extractants, such as HPCD utilized in this current study, may experience difficulties in penetrating BC matrices due to its highly hydrophobic nature, hence limiting desorption of BC-associated PAHs. Despite large variations in soil characteristics (Table 1), no consistent differences in desorption kinetics among soil types with AC amendment are apparent; hence extractabilities were often relatively comparable among soils. Such findings contradict many previous studies in which it has been found that soil type, specifically organic matter (OM), total organic carbon (TOC), and clay content, influenced the amount of extractable phenanthrene.23,29,42,43 For example, soil D amended with 0 or 0.1% AC exhibited significantly lower values of Frap and higher values of Fslow after 50 and 100 d soil PAH contact time, however, soil physicochemical characteristics offer no explanation for these results. Based on results reported in the literature,3 it would be anticipated that phenanthrene desorption from soil C, which contained the largest quantities of OM and native TOC, would be significantly higher than that from soils A, B, and D. In systems with little OM, BC represents the primary sorbent for PAHs.23 However, in OM- and TOC-rich systems, the importance of BC is relatively more limited, possibly as a result of competition between and attenuation of geosorbent pores by native TOC or OM in the soil and/or competition from native compounds for sorption sites which prevents sorption/sequestration of PAH molecules.7,23 Comparison between Rapidly Desorbing and Biodegradable Contaminant Fractions. HPCD is a nonexhaustive chemical extraction that has been confirmed as a mimetic technique for the prediction of PAH bioaccessibility and microbial degradation in a range of laboratory-spiked and field contaminated soils.32,48−51 The slope values between mineralized and rapidly desorbing phenanthrene fractions observed in the control soils (0% AC) (Figure 3) show that 71% (r2 = 0.62) of Fmin can be accounted for by Frap, confirming the findings of previous studies in sediments that it is the rapidly desorbing fraction that is primarily available for microbial degradation (e.g., Cornelissen et al.26). In 0.1% ACamended soils, slope values indicate that 61% of Fmin occurred as a result of Frap, however, no useful correlations occurred in soils amended with 1% or 5% AC. This indicates that the use of HPCD as a technique to predict bioavailability in soils containing low AC concentrations (0.1%) is substantially weakened, but is entirely unsuitable for AC concentrations of 1% or 5%. However, several studies have indicated that BC in

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ASSOCIATED CONTENT

S Supporting Information *

Desorption kinetic data of 14C-phenanthrene in soils A, B, C, and D. This information is available free of charge via the Internet at http://pubs.acs.org/. 12449

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AUTHOR INFORMATION

Corresponding Author

*Phone: +44 (0)1524 594534; fax: +44 (0)1524 593985; email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Natural Environment Research Council (NERC) and the European Commission under FP7 − Environmental Technologies (ModelProbe 213161) for funding this work.

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