Environ. Sci. Technol. 2008, 42, 740–745
Impact of Black Carbon in the Extraction and Mineralization of Phenanthrene in Soil ANGELA H. RHODES, ALISDAIR CARLIN, AND KIRK T. SEMPLE* Department of Environmental Science and the Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, United Kingdom
Received June 15, 2007. Revised manuscript received October 22, 2007. Accepted November 2, 2007.
During the past century, increased biomass burning and fossil fuel consumption have drastically increased the input of black carbon (BC) into the environment, and that has been shown to influence the behavior of organic contaminants in soil. A study was conducted to investigate the effects of BC on the relationship between aqueous hydroxypropyl-β-cyclodextrin (HPCD) extraction and microbial mineralization (bioaccessibility) of 14C-phenanthrene (10 mg kg-1) in four soils amended with 0, 0.1, 0.5, 1, 2.5, and 5% (% dry wt soil) activated charcoal, a type of BC. Mineralisation was monitored over 20 d incubation, within respirometric assays, using an inoculum containing a phenanthrene-degrading pseudomonad and compared to HPCD extraction (24 h) using 50 mM aqueous solution; analyses were conducted after 1, 25, 50, and 100 d soil-phenanthrene contact time. Statistical analyses revealed that for each soil the addition of BC led to significant (P < 0.001) reductions in both HPCD extractability and microbial mineralization. Linear correlations for BC concentrations of 0% (r2 ) 0.95; slope ) 0.89) and 0.1% (r2 ) 0.67; slope ) 0.95) revealed a highly significant (P < 0.01) relationship between HPCD extractability and total mineralization (20 d), indicating a direct prediction of phenanthrene bioaccessibility by HPCD. However, in soils amended with 0.5, 1, 2.5, and 5% BC exhibited r2 values ranging 0.51–0.13 and slopes of 2.19–12.73. This study has shown that BC strongly sorbs phenanthrene causing reductions in extractability and, to a lesser extent, bioaccessibility to degrading microorganisms.
organic contaminants (HOCs) is thought to be of significant importance; such contaminants include polycyclic aromatic hydrocarbons (PAHs) (2, 4-7), polychlorinated biphenys (4), polychlorinated dibenzo-p-dioxins and -furans (4), polybrominated diphenyl ethers and certain pesticides (8). BC is believed to be responsible for (1) enhanced sorption phenomena for PAHs and other planar contaminants (6, 7, 9-11); (2) nonlinear sorption isotherms of organic contaminants (12); (3) very slowly desorbing contaminant fraction (13, 14); and (4) reduced bioaccessibility of certain HOCs (5, 11, 15, 16). Pure BC has been observed to sorb PAHs up to 10-1000 times stronger per weight unit than other types of organic carbon (6, 11, 17). Although BC commonly represents between 1 and 20% of total organic carbon and less than 1% of total sediment mass, it may be responsible for between 80 and 97% of PAH sequestration in soils and sediments (18, 19). The implications of such findings are that in the presence of BC, PAHs pose a reduced environmental risk subsequently leading to inaccurate estimations/prediction of ecological target values and remediation end points. Several studies have utilized cyclodextrins as a mimetic technique for the prediction of PAH biodegradation (bioaccessibility) in soils (20–23). Cyclodextrins are cyclic oligosaccharides comprising a hydrophilic exterior and a toroidal-shaped apolar interior or cavity (24). This distinctive structure allows them to form inclusion complexes with a range of HOCs, including PAHs. Hydroxypropylβ-cyclodextrin (HPCD) is a six-glucose cyclodextrin that has been chemically modified to enhance water solubility without affecting the basic structure or characteristics. The extraction of PAHs using aqueous HPCD solutions has been shown to provide a good estimate of the mineralizable or bioaccessible fraction of phenanthrene in a range of laboratory spiked soils amended with transformer oil (25), cable insulating oil (26), and cocontaminants (27). However, HPCD has not been tested in soils containing BC. The hypothesis of this study was that the addition of BC to soils amended with phenanthrene results in decreases in chemical and biological accessibility of the PAH. To test this hypothesis, the aims of this study considered the effects of adding varying concentrations of activated charcoal, a form of BC, on (i) the mineralization of 14C-phenanthrene using phenanthrene degrading inoculum; (ii) the extractability of 14C-phenanthrene using HPCD, and (iii) the correlation between the fraction of 14C-phenanthrene mineralized and the fraction of 14Cphenanthrene extracted.
Introduction
Experimental Section
During the past century, increased biomass burning and fossil fuel consumption have drastically increased the input of black carbon into the environment (1). Black carbon (BC) is a collective term for the remnants of incomplete combustion such as unburned coal, kerogen, coke, cenosphere, soot, fly ash and charcoal (1–3). The characteristics of BC vary widely, but generally comprise a three-dimensional structure with high carbon contents and relatively few functional groups. As a consequence, BC particles are relatively inert with mean residence times in the environment exceeding 1000 years (1). BC is now ubiquitous in atmospheric aerosols, marine sediments, freshwater sediments, and soils where its influence on the transport and bioaccessibility of hydrophobic
Chemicals. Unlabeled and [9-14C] phenanthrene 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) from Perkin-Elmer Life Sciences and Combustaid from Canberra Packard, U.K. Hydroxypropyl-β-cyclodextrin (HPCD) was obtained from Fisher Scientific; U.K. Activated charcoal (mean particle diameter 21 µm; total pore volume 2.5 cm3 g-1)was obtained from BDH Chemicals. Plate count agar was supplied by Oxoid. Soils. Four uncontaminated soils were used in this study and were collected (A horizon; 5–20 cm) from fields in Lancashire, U.K (Table 1), with selection based on organic matter and clay content (28). The physicochemical characteristics of the soils are presented in Table 1. The soils were air-dried for 24 h and passed through a 2 mm sieve to remove
* Corresponding author phone: +44 1524 594534; fax: +44 1524 593985; e-mail:
[email protected]. 740
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 3, 2008
10.1021/es071451n CCC: $40.75
2008 American Chemical Society
Published on Web 01/03/2008
TABLE 1. Physico-Chemical Properties of Soils particle size analysis (%) sand soil type typical stagnogley soil (A) typical brown earth soil (B) earthy oligo-fibrous peat soil (C) tyical humic alluvial gley soil (D) a
grid ref
texture
pH
a
C
M
F
silt
clay
dH2O
CaCl2
OMb
SD496402
clay loam
1.71
15.56
38.36
24.96
19.41
6.53
5.18
4.821
SD491655
loam
7.53
11.17
36.69
26.77
17.84
5.44
5.02
9.33
SD511775
silty clay
0.96
1.8
7.69
47.37
42.18
7.50
6.01
27.15
SD447543
clay loam
0.92
1.2
36.65
34.44
27.79
6.93
5.19
10.25
Course, medium, and fine sand.
b
Organic matter content (loss on ignition) (%).
roots and stones. The organic matter content was determined by mass loss on ignition on a dry weight basis and particle size analysis by a standard method using sieves and sedimentation (29). Soil Spiking. The air-dried soils were rehydrated with deionized water to the original field moisture content (35% on dry weight basis). BC concentrations of 0, 0.1, 0.5, 1, 2.5, and 5% dry weight soil were prepared by blending specific quantities of activated charcoal with each soil using a stainless steel spoon. Soils were then spiked with 12C/14C-phenanthrene as described by Doick et al. (30) using acetone (7.5 mL per 250 g soil) as a carrier solvent to deliver a phenanthrene concentration of 10 mg kg-1 and 14C-activity of 95 kBq kg-1. Control soils that were amended with acetone, but not amended with phenanthrene were also prepared. After spiking (within 1 d) soils were sterilized by γ irradiation (32.2 kGy; Isotron plc, Bradford, U.K.) within sealed amber glass jars and then incubated in darkness at room temperature (20 ( 2 °C) for 1, 25, 50, and 100 d after which time the soils were analyzed as described in the following sections. After each aging period the sterility of the soil was tested using standard microbiological techniques (31). Determination of Total 14C-Phenanthrene-Associated Activity in Soil. The 14C-phenanthrene associated activity was determined by sample oxidation at each sampling point (period of aging). 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), and Permafluor-E (10 mL) were used to trap 14CO2. The trapping efficiency was found to be >90%. 14C activity was then quantified by liquid scintillation counting (LSC) (Canberra Packard Tri Carb 2300 TR, U.K.) using standard calibration and quench correction techniques. Extraction of 14C-Phenanthrene-Associated Activity by Hydroxypropyl-β-Cyclodextrin (HPCD). Determination of 14C-phenanthrene extractability using HPCD was carried out at each time point, as described by Reid et al. (20). HPCD solutions (50 mM) were prepared using deionized water (MillQ). Soils (1.25 g) were weighed into 30 mL Teflon centrifuge tubes (n ) 3) and 25 mL HPCD solution added to each. The tubes were the placed onto an orbital shaker at 100 rpm for 24 h. The tubes were then centrifuged at 3000g for 1 h (Beckman Centaur 2 centrifuge) and supernatants pipetted (6 mL) into 20 mL glass scintillation vials containing Goldstar scintillation cocktail (14 mL) which were analyzed using LSC as described previously. A mass balance was determined on completion of the extraction by combustion of the soil pellet (sample oxidizer, model 307, Packard.). Preparation of the Phenanthrene-Degrading Inoculum. A bacterial, phenanthrene-degrading inoculum (identified as a Pseudomonas sp.) (32) 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 (32). After 4 d incubation (late
exponential phase of growth), the culture was centrifuged at 10000g for 30 min. (Beckman J-6M, Beckman Coulter, U.S.). The supernatant was discarded, and the cells resuspended in MBS to obtain approximately 108 cells mL-1. This procedure was repeated twice to ensure thorough washing of the cells and to remove any residual phenanthrene. Respirometry. Mineralisation assays were conducted in modified 250 mL Schott bottles (‘respirometers’) to assess catabolism of 14C-phenanthrene using a phenanthrenedegrading inoculum (25, 32). Respirometers were prepared in triplicate, with 10 ( 0.2 g soil (wet wt), 25 mL MBS, and 5 mL phenanthrene-degrading bacteria (107 cells g-1 soil). Cells were enumerated by measurement of colony forming units (CFUs) on plate count agar, following standard microbiological techniques. The respirometers incorporated a Teflon lined screw cap and a CO2 trap containing 1 M NaOH (1 mL) within a suspended 7 mL glass scintillation vial. The respirometers were placed in a SANYO Gallenkamp orbital incubator set at 100 rpm and 25 °C over a period of 20 d. Evolved 14CO2, as a result of 14C-phenanthrene catabolism, was trapped in 1 M NaOH with 14C-activity assessed daily by LSC, as described previously. Statistical Analysis. Following blank-correction, statistical analysis of the results was performed in Sigma Stat for Windows (Version 2.03, SPSS Inc.). The statistical significance of black carbon addition was determined using a general linear model (ANOVA, Tukey Test) at the 95% confidence level (P < 0.05). Comparisons between HPCD-extractability and mineralization were performed using Student t test (p < 0.05) and linear regression modeling.
Results and Discussion Determination of 14C-Phenanthrene Mineralization in Soil. The mineralization of 14C-phenanthrene was monitored over 20 d incubation, in four soils amended with 0, 0.1, 0.5, 1, 2.5, or 5% BC after 0, 25, 50, and 100 d soil-PAH contact time (see Supporting Information, Figures S1-4). The slurrying approach was used as it has been shown to determine PAH bioaccessibility as it promotes greater interaction between the soil particles, target contaminants and degrading microorganisms [25, 32]. Statistical analyses demonstrated that the presence of BC in soil had a significant effect on both the fastest rates of mineralization as determined by the steepest gradient or slope (see Supporting Information, Table S1) and total extents of 14C-phenanthrene mineralized in each soil and after each period of aging (Table 2). The total extents of mineralization after 20 d decreased by up to 50% with increasing concentrations of BC, from 0 to 5%. For example, after 1 d soil-PAH contact time, mineralization declined from 70.8 to 19.7%, 64.0 to 23%, 53.1 to 33.8%, 58.8 to 17.2% for soils A, B, C, and D amended with 0 and 5% BC respectively. Hence, the addition of 5% BC consistently resulted in statistically lower extents (P < 0.01) of 14C-phenanthrene mineralization when compared to 0% BC. However, the total VOL. 42, NO. 3, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
741
TABLE 2. Total Amounts of 14C-Phenanthrene Extracted (After 24 h) and Mineralized over 20 d during the Respirometric Assay/ Incubation in Soils A, B, C, and Da aging time
soil A
soil B
soil C
soil D
BC
extracted
mineralized
extracted
mineralised
extracted
mineralized
extracted
mineralised
1d
0 0.1 0.5 1 2.5 5
82.1 ( 0.1 41.9 ( 0.2 14.5 ( 0.3 4.5 ( 0.2 0.1 ( 0.4 0.0 ( 0.0
70.8 ( 0.7 60.0 ( 4.6 54.5 ( 3.4 52.1 ( 1.9 47.3 ( 2.4 19.7 ( 1.4
60.6 ( 0.3 39.3 ( 0.9 15.3 ( 0.4 6.1 ( 0.2 0.9 ( 0.2 0.1 ( 0.1
64.0 ( 2.4 54.8 ( 2.5 45.6 ( 2.0 45.0 ( 0.9 33.8 ( 2.9 23.0 ( 2.4
54.9 ( 1.2 37.7 ( 1.9 13.3 ( 0.3 5.5 ( 0.1 1.4 ( 0.1 0.6 ( 0.0
53.1 ( 2.9 42.4 ( 3.8 41.1 ( 1.6 35.1 ( 1.9 29.4 ( 2.7 33.8 ( 0.1
60.0 ( 1.1 26.5 ( 0.9 10.7 ( 0.3 2.6 ( 0.2 0.6 ( 0.0 0.2 ( 0.1
58.8 ( 2.2 39.4 ( 2.1 46.5 ( 1.4 35.9 ( 0.4 29.5 ( 0.7 17.2 ( 1.1
25 d
0 0.1 0.5 1 2.5 5
72.0 ( 0.4 27.0 ( 0.1 9.0 ( 0.1 3.2 ( 0.3 0.6 ( 0.0 0.1 ( 0.2
69.9 ( 2.9 54.0 ( 1.4 44.3 ( 2.4 39.2 ( 2.6 34.2 ( 1.0 16.2 ( 0.7
66.8 ( 0.4 34.5 ( 1.4 11.4 ( 0.2 3.9 ( 0.3 0.8 ( 0.1 0.6 ( 0.2
62.6 ( 1.5 62.8 ( 1.0 51.5 ( 1.1 46.4 ( 2.0 36.7 ( 2.9 28.5 ( 0.0
44.4 ( 0.7 32.0 ( 2.8 9.6 ( 0.2 4.6 ( 0.2 1.5 ( 0.1 1.0 ( 0.1
47.6 ( 3.5 43.8 ( 0.8 35.4 ( 0.5 34.6 ( 0.6 28.8 ( 1.3 21.2 ( 1.2
54.0 ( 0.7 21.9 ( 0.6 6.9 ( 1.0 3.1 ( 0.5 0.6 ( 0.1 0.3 ( 0.0
58.1 ( 2.1 42.3 ( 0.5 41.8 ( 1.6 39.6 ( 0.8 26.5 ( 1.7 12.5 ( 0.7
50 d
0 0.1 0.5 1 2.5 5
65.7 ( 0.9 23.9 ( 0.6 7.8 ( 0.1 2.1 ( 0.1 0.3 ( 0.1 0.1 ( 0.1
59.2 ( 4.4 49.4 ( 0.9 34.9 ( 2.6 28.8 ( 0.7 20.0 ( 3.4 6.3 ( 1.3
59.6 ( 0.6 32.6 ( 0.2 11.8 ( 0.2 3.5 ( 0.1 0.5 ( 0.0 0.1 ( 0.1
61.5 ( 3.0 57.8 ( 1.8 48.8 ( 0.6 31.5 ( 2.5 29.9 ( 2.7 15.3 ( 3.6
48.8 ( 0.9 25.4 ( 0.4 8.9 ( 0.2 4.3 ( 0.2 1.2 ( 0.1 0.5 ( 0.0
46.1 ( 3.3 46.0 ( 1.9 34.6 ( 1.5 35.8 ( 1.9 33.2 ( 1.6 22.3 ( 0.7
41.4 ( 0.5 20.1 ( 0.7 7.9 ( 0.6 2.6 ( 0.2 0.9 ( 0.1 0.4 ( 0.1
46.1 ( 2.3 44.6 ( 3.0 42.3 ( 1.3 38.2 ( 2.8 32.9 ( 0.4 25.1 ( 1.5
100 d
0 0.1 0.5 1 2.5 5
62.8 ( 0.7 12.1 ( 0.5 8.1 ( 0.3 2.3 ( 0.1 0.4 ( 0.0 0.1 ( 0.0
58.1 ( 5.7 29.0 ( 3.8 31.7 ( 1.5 27.0 ( 1.4 20.9 ( 2.5 12.6 ( 0.3
40.9 ( 1.0 12.2 ( 0.2 9.0 ( 0.3 3.6 ( 0.2 0.7 ( 0.0 0.1 ( 0.1
54.3 ( 2.3 27.4 ( 2.2 26.4 ( 1.2 26.0 ( 0.4 23.7 ( 2.1 14.2 ( 0.7
41.7 ( 0.6 20.7 ( 0.2 5.4 ( 0.0 2.1 ( 0.1 0.9 ( 0.1 0.4 ( 0.0
40.9 ( 1.7 41.8 ( 0.3 33.8 ( 0.5 40.9 ( 0.2 35.9 ( 0.2 21.8 ( 1.6
36.5 ( 3.1 12.2 ( 1.1 4.2 ( 0.2 1.1 ( 0.0 0.6 ( 0.1 0.3 ( 0.1
46.0 ( 0.9 49.2 ( 0.5 52.6 ( 1.6 41.1 ( 1.6 41.6 ( 2.0 21.0 ( 1.2
a
Values are the % mean (n ) 3) ( standard error of the mean (S.E.M).
extents mineralized in soils with BC concentrations of 0.1, 0.5, 1, and 2.5% were often, but not exclusively, statistically similar (P > 0.01). In addition, there were significant statistical differences between the fastest rates of 14C-phenanthrene mineralized for each BC concentration; overall, rates declined significantly (P < 0.01) with increasing BC concentration (see Supporting Information, Table S1). These results clearly show that the addition of BC to each soil caused a significant decrease in the total extent of 14C-phenanthrene mineralized. It is postulated that this reduction in mineralization may be due to sorption and a subsequent decline in the aqueous concentrations of phenanthrene. For example, Johnsen et al. (33) provided strong evidence to suggest that the degradation of PAHs by microorganisms within soil occurs more predominantly from the aqueous dissolved phase. In addition, many previous studies have demonstrated that BC materials have high affinities for PAHs and PCBs, and may be responsible for substantial reductions in the aqueous concentrations of contaminants in soils due to its strong sorptive capacity (5, 11, 15, 16, 34). Hence, much reduced desorption of 14C-phenanthrene, in soils amended BC, from solid to aqueous soil phases may have been responsible for the significant decreases in mineralization observed in this study. Such findings are consistent with those of Guerin and Boyd (35) who found complete absence of naphthalene biodegradation in soils and sediments when amended with granular activated carbon that physico-chemically resembled environmental BC. Generally, significant reductions (P < 0.05) in the extent of 14C-phenanthrene mineralized over 20 d incubation were apparent for soils A and B after different soil-PAH contact times. In soil A, the total extents of mineralization declined by approximately 20–40% between 0 and 100 d contact time for each BC concentration with the exception of 5% BC where the effects of increased soil-PAH contact time were less apparent. It is well-known that organic contaminant extraction and biodegradation in soil decreases with increased 742
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 3, 2008
soil-contaminant contact time (20, 36-39). For example, Hatzinger and Alexander (36) showed that the mineralization or biodegradation of phenanthrene reduced with increased duration of aging. More recently, Reid et al. (20) found that the extents of 14C-phenanthrene mineralization declined from 74.4% to 51.4% between 1 and 322 d soil-contaminant contact time. A decline in the amount of 14C-phenanthrene available for mineralization observed here, although often to a lesser extent, reflects the generally accepted view that with increased aging more labile compound fraction are reduced while more nonlabile fractions are formed (36, 40). In contrast, for soils C and D amended with BC no significant trends in the mineralization of 14C-phenanthrene were apparent/ observed between different soil-PAH contact times. Statistical analyses also confirmed significant differences (P < 0.05) in 14C-phenanthrene mineralization between soil types although no specific trends were observed. For 0% BC, soil A exhibited the highest total extents of mineralization with 70.8% and 69.9% for 0 and 25 d, respectively. Overall, lower soil organic matter (SOM) and clay contents equated to higher mineralization after 0 and 25 d soil-PAH contact time. Such results are in agreement with other work where it has been seen that higher total extents of mineralization related to lower SOM contents (20, 37, 41). In contrast, after 50 and 100 d soil-PAH contact time the total extents of mineralization were statistically greater (P < 0.05) in soils C and D than in soils A and B, when amended with higher BC concentrations (1, 2.5, and 5%). These findings may be a result of sorptive attenuation processes such as (i) competition between native and added/spiked PAHs for a limited capacity of environmental BC sorption sites (42) and/or (ii) blocking of BC sorption sites by SOM therefore preventing PAH molecules from becoming sorbed/sequestered (11). In this study, the latter may provide an explanation for the higher extents of mineralization observed in soils C and D, and also for the lack of statistical differences between aging times in these soils. However, it is not possible to determine whether the higher extents of mineralization observed in soils C and
D were due competitive interactions between native PAHs and spiked 14C-phenanthrene, as the levels of native PAHs were not quantified here. HPCD-Extractable 14C-Phenanthrene from Soil. HPCD is a nonexhaustive aqueous based extraction that has been shown to accurately predict the microbially degradable fraction of numerous PAHs in soils and sediments, of varying physicochemical characteristics (20-22, 27, 43, 44). Several studies have utilized HPCD as a mimetic technique for the prediction of PAH bioaccessibility (20, 21, 23, 26, 27, 43); however, HPCD has not been tested in soils containing BC. In this study, soils, freshly spiked and aged with phenanthrene, were extracted using an aqueous solution of HPCD after 1, 25, 50, and 100 days soil-PAH contact time. Statistical analyses revealed that for each soil and at each time point, the addition of BC led to significant (P < 0.001) reductions in extractability. The amounts of extracted 14Cphenanthrene decreased substantially in each soil as BC concentration increased from 0 to 5%. For example, at 1 d soil-PAH contact time, extractability diminished from 82.1 to 0.0%, 60.6 to 0.1%, 54.9 to 0.6%, and 60.0 to 0.2% in soils A, B, C, and D for BC concentrations of 0 and 5%, respectively; a similar trend was observed after 25, 50, and 100 d soil-PAH contact time. Such a marked reduction in extractability is thought to be a direct consequence of the BC within soils as significant differences were observed between all BC amended and unamended soils for each time point. Notably, the addition of 5% BC consistently resulted in 0.01) between HPCD extractable and microbially mineralized 14C-phenanthrene in any of the soils. Furthermore, the gradients approximated to 1 (0.89 and 0.95) for both 0 and 0.1% BC concentration, respectively, indicating a direct prediction of phenanthrene bioaccessibility by HPCD. Reid et al. [ (20)] provided strong evidence that the amount of phenanthrene mineralized by catabolically active microorganisms correlated with the HPCD extractable phenanthrene concentration in soil (r2 ) 0.964; slope ) 0.997; intercept ) 0.162). Similarly, Cuypers et al. (44) reported that residual PAH concentrations after biodegradation were similar to those following extraction and,therefore,concluded that HPCD extraction was a suitable method for the prediction of PAH bioavailability. However, in contrast, for soils amended with 0.5, 1, 2.5,and 5% BC r2 values ranged 0.51–0.13 and gradients/slopes 2.19–12.73 indicating an insignificant relationship between HPCD-extractability and mineralization. These results suggest that the phenanthrene-degrading pseudomonad actively degraded the 14Cphenanthrene to a much greater extent than estimated by the HPCD extraction. It is hypothesized that the microorganisms adhere to BC particles enabling the degradation of phenanthrene sorbed to BC surfaces. Several authors regard biofilm formation and attachment on PAH sources as a predominant mechanism among bacteria to overcome mass-transfer limitations when utilizing poorly soluble and strongly sorbed PAHs (45, 46). For example, Ehrhardt and Rehm (46) and Chang and Rittmann (45) observed that microorganisms continued to grow by utilizing phenol that had adsorbed in activated carbon micropores even though phenol was present at very low concentrations in the aqueous phase. In this case, attachment of bacteria to the activated carbon enhanced desorption rates. According to previous work, PAH sorption to BC predominantly occurs on the exterior planes of particles suggesting that PAHs may be readily accessible for microbial degradation, via sorbed phase processes, in soils amended with higher concentrations of BC (2.5 and 5%). Hence, it is assumed that although a part of the phenanthrene may indeed be sorbed to the flat surfaces of BC particles, another portion may by physically entrapped within nanopore, therefore, limiting mass transfer/accessibility to microorganisms (7) and subsequently reducing the mineralization of 14C-phenanthrene in the present study. Conversely, HPCD extractability was reduced to a greater extent in the presence of BC; this may have been because HPCD only removes phenanthrene present in the aqueous, and not sorbed, soil phase. Such results do not lessen the utility of this technique for assessing bioaccessibility as several studies have indicated that BC accounts for only 1–15% of the total organic carbon content (TOC) in soil. Therefore, it is postulated that the BC content of soil will not often exceed 1%. On this basis HPCD may provide an appropriate method for assessing PAH bioaccessibility in field contaminated soils that contain BC. Overall, the ubiquity of BC in the environment and its known propensity to reduce the bioaccessibility of PAHs may affect regulatory procedures and environmental guidelines concerned with such contaminants. Although BC has been shown to be efficient at reducing the bioaccessibility of phenanthrene, its effects at higher concentrations of contamination or with other HOCs are not known and merit further study. Further, the term BC encompasses a whole range of particle types, of which activated charcoal is only one and it may be that other types of BC may show different effects on the bioaccessibility of HOCs in soil and warrants further investigation. Ultimately, the application of BC to VOL. 42, NO. 3, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
743
FIGURE 1. Linear regressions correlating the total fraction of 14C-phenanthrene mineralized by an enriched catabolic inoculum and the HPCD extractable fraction of 14C-phenanthrene in soils A, B, C, and D. Dashed lines represent y ) correlation between the two variables. Regression lines shown as a solid line and related equations given. Error bars are the SEM (n ) 3). contaminated land could prove to be a useful remediation strategy, but the uncertainty concerning the longevity of the sequestration of HOCs requires clarification.
Acknowledgments We thank the Natural Environment Research Council (NERC, UK) for funding. 744
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 3, 2008
Supporting Information Available Cumulative 14C-phenanthrene mineralization curves and fastest initial rates of 14-C-phenanthrene mineralization for soils amended with 0, 0.1, 0.5, 1, 2.5, and 5% BC after 0, 25, 50, and 100 d soil-PAH contact time. This material is available free of charge via the Internet at http:// pubs.acs.org.
Literature Cited (1) Schmidt, M. W. I. Black Carbon in soils and sediments: Analysis, distribution, implications and current challenges. Global Biogeochem. Cycles 2000, 14 (3), 777–793. (2) Cornelissen, G.; Gustafsson, O. Importance of unburned coal carbon, black carbon and amorphous organic carbon to phenanthrene sorption in sediments. Environ. Sci. Technol. 2005, 39, 764–769. (3) Koelmans, A. A.; Jonker, M. T. O.; Cornelissen, G.; Bucheli, T. D.; Van Noort, P. C. M.; Gustafsson, O., Black Carbon: The reverse of its dark side. Chemosphere 2005, In Press. (4) Lohmann, R.; Macfarlene, J. K.; Gschwend, P. M. Importance of black carbon to sorption of native PAHs, PCBs and PCDDs in Boston and New York harbor sediments. Environ. Sci. Technol. 2005, 39, 141–148. (5) Burgess, R. M.; Ryba, S. A.; Perron, M. M.; Tien, R.; Thibodeau, L. M.; Cantwell, M. G. Sorption of 2,4-dichlorobiphenyl and fluoranthene to a marine sediment amended with different types of black carbon. Environ. Toxicol. Chem. 2004, 23 (11), 2534– 2544. (6) Cornelissen, G.; Gustafsson, O. Sorption of phenanthrene to environmental black carbon in sediment with and without organic matter and native sorbates. Environ. Sci. Technol. 2004, 38, 148–155. (7) Jonker, M. T. O.; Koelmans, A. A. Extraction of polycyclic aromatic hydrocarbons from soot and sediment: solvent evaluation and implications for sorption mechanism. Environ. Sci. Technol. 2002, 36, 4107–4113. (8) Yang, Y.; Sheng, G. Enhanced pesticide sorption by soils containing particulate matter from crop residue burns. Environ. Sci. Technol. 2003, 37, 3635–3639. (9) Cornelissen, G.; Elmquist, M.; Groth, I.; Gustafsson, O. Effect of sorbate planarity on environmental black carbon sorption. Environ. Sci. Technol. 2004, 38, 3574–3580. (10) Cornelissen, G.; Haftka, J.; Parsons, J.; Gustafsson, O. Sorption to black carbon of organic compounds with varying polarity and planarity. Environ. Sci. Technol. 2005, 39, 3688–3694. (11) Jonker, M. T. O.; Hoenderboom, A. M.; Koelmans, A. A. Effects of sedimentary sootlike materials on bioaccumulation and sorption of polychlorinated biphenyls. Environ. Toxicol. Chem. 2004, 23 (11), 2563–2570. (12) Nguyen, T. H.; Sabbah, I.; Ball, W. P. Sorption nonlinearity for organic contaminants with diesel soot: method development and isotherm interpretation. Environ. Sci. Technol. 2004, 38, 3595–3603. (13) Cornelissen, G.; Gustafsson, O.; Bucheli, T. D.; Jonker, M. T. O.; Koelmans, A. A.; Van Noort, P. C. M. Extensive sorption of organic compounds to black carbon, coal and kerogen in sediments and soils: mechanisms and consequences for distribution, bioaccumulation and biodegradation. Environ. Sci. Technol. 2005, In Press. (14) Cornelissen, G.; van Noort, P. C. M.; Govers, H. A. J. Mechanisms of slow desorption of organic compounds from sediments: A study using model sorbents. Environ. Sci. Technol. 1998, 32, 3124–3131. (15) Lamoureux, E. M.; Brownawell, B. J. Influence of soot on hydrophobic organic contaminant desorption and assimilation efficiency. Environ. Toxicol. Chem. 2004, 23 (11), 2571–2577. (16) Rust, A. J.; Burgess, B. M.; McElroy, A. E.; Cantwell, M. G.; Brownawell, B. J. Influence of soot carbon on the bioaccumulation of sediment-bound polycyclic aromatic hydrocarbons by marine benthic invertebrates: an interspecies comparison. Environ. Toxicol. Chem. 2004, 23 (11), 2594–2603. (17) Accardi-Dey, A.; Gschwend, P. M. Reinterpreting Literature Sorption Data Considering both Absorption into Organic Carbon and Adsorption onto Black Carbon. Environ. Sci. Technol. 2003, 37, 99–106. (18) Middelburg, J. J.; Nieuwenhuize, J.; van Breugel, P. Black carbon in marine sediments. Mar. Chem. 1999, 65 (3–4), 245–252. (19) Vinturella, A. E.; Burgess, R. M.; Coull, B. A.; Thompson, K. M.; Shine, J. P. Importance of black carbon in distribution and bioaccumulation models of polycyclic aromatic hydrocarbons in contaminated marine sediments. Environ. Toxicol. Chem. 2004, 23 (11), 2578–2586. (20) Reid, B. J.; Stokes, J. D.; Jones, K. C.; Semple, K. T. Nonexhaustive cyclodextrin-based extraction technique for the evaluation of PAH bioavailability. Environ. Sci. Technol. 2000, 34, 3174–3179. (21) Stokes, J. D.; Wilkinson, A.; Reid, B. J.; Jones, K. C.; Semple, K. T. Prediction of Polycyclic Aromatic Hydrocarbon Biodegradation in Contaminated Soils Using an Aqueous Hydroxypropyl-β-
(22) (23) (24) (25) (26)
(27) (28) (29) (30)
(31) (32)
(33) (34)
(35) (36) (37) (38) (39)
(40) (41) (42) (43)
(44)
(45) (46)
Cyclodextrin Extraction Technique. Environ. Toxicol. Chem. 2005, 24 (6), 1325–1330. Hickman, Z. A.; Reid, B. J. Towards a more appropriate water based extraction for the assessment of organic contaminant availability. Environ. Pollut. 2005, 138, 299–306. Allan, I. J.; Semple, K. T.; Hare, R.; Reid, B. J., Prediction of mono- and polycyclic aromatic hydrocarbon degradation in spiked soils using cyclodextrin extraction. Environ. Pollut. 2006. Shieh, W. J.; Hedges, A. R. Properties and Applications of Cyclodextrins. Pure Appl. Chem. 1996, A33, 673–683. Doick, K. J.; Semple, K. T. The effect of soil:water ratios on the mineralization of phenanthrene:LNAPL mixtures in soil. FEMS Microbiol. Lett. 2003, 220, 29–33. Dew, N. M.; Paton, G. I.; Semple, K. T. Prediction of [3-14C]phenyldodecane biodegradation in cable insulating oil-spiked soil using selected extraction techniques. Environ. Pollut. 2005, 138, 316–323. Doick, K. J.; Clasper, P. J.; Urmann, K.; Semple, K. T., Further validation of the HPCD-technique for the evaluation of PAH microbial availability in soil. Environ. Pollut. 2006, IN PRESS. Mackney, D.; Hodgson, J. M.; Hollis, J. M.; Staines, S. J. The 1:250000 National Soil Map of England and Wales; Harpenden, 1983. Rowell, D. L., Soil Science: Methods and Applications; Longman: 1994. Doick, K. J.; Lee, P. H.; Semple, K. T. Assessment of spiking procedures for the introduction of a phenanthrene-LNAPL mixture into field-wet soil. Environ. Pollut. 2003, 126 (3), 399– 406. Macleod, C. J. A.; Semple, K. T. Influence of contact time on extractability and degradation of pyrene in soils. Environ. Sci. Technol. 2000, 34 (23), 4952–4957. Reid, B. J.; MacLeod, C. J. A.; Lee, P. H.; Morriss, A. W. J.; Stokes, J. D.; Semple, K. T. A Simple 14C-Respirometric Method for Assessing Microbial Catabolic Potential and Contaminant Bioavailability. FEMS Microbiol. Lett. 2001, 196, 141–146. Johnsen, A. R.; Wick, L. Y.; Harms, H. Principles of microbial PAH-degradation in soil. Environ. Pollut. 2005, 133 (1), 71–84. Cornelissen, G.; Gustafsson, O. Prediction of large variation in biota to sediment accumulation factors due to concentrationdependent black carbon adsorption of planar hydrophobic organic compounds. Environ. Toxicol. Chem. 2005, 24 (3), 495– 498. Guerin, W. F.; Boyd, S. A. Bioavailability of naphthalene associated with natural and synthetic sorbents. Water Res. 1997, 31 (6), 1504–1512. Hatzinger, P. B.; Alexander, M. Effect of ageing of chemicals in soil on their biodegradability and extractability. Environ. Sci. Technol. 1995, 29, 537–545. White, J. C.; Kelsey, J. W.; Hatzinger, P. B.; Alexander, M. Factors affecting sequestration and bioavailability of phenanthrene in soils. Environ. Toxicol. Chem. 1997, 16 (10), 2040–2045. Kelsey, J. W.; Kottler, B. D.; Alexander, M. Selective chemical extractants to predict bioavailability of soil-aged organic chemicals. Environ. Sci. Technol. 1997, 31, 214–217. Reid, B. J.; Jones, K. C.; Semple, K. T. Bioavailability of persistent organic pollutants in soils and sediments - a perspective on mechanisms, consequences and assessment. Environ. Pollut. 2000, 108, 103–112. Swindell, A. L.; Reid, B. J., Comparison of selected non-exhaustive extraction techniques to assess PAH availability in dissimilar soils. Chemosphere 2005. Macleod, C. J. A.; Semple, K. T. The adaptation of two similar soils to pyrene catabolism. Environ. Pollut. 2002, 119, 357–364. Cornelissen, G.; Gustafsson, O., Effects of added PAHs and precipitated humic acid coatings on phenanthrene sorption to environmental Black carbon. Environ. Pollut. 2005, In Press. Doick, K. J.; Dew, N. M.; Semple, K. T. Linking catabolism to cyclodextrin extractability: determination of the microbial availability of PAHs in soil. Environ. Sci. Technol. 2005, 39, 8858– 8864. Cuypers, C.; Pancras, T.; Grotenhuis, T.; Rulkens, W. The Estimation of PAH Bioavailability in Contaminated Sediments using Hydroxypropyl-beta-cyclodextrin and Triton X-100 Extraction Techniques. Chemosphere 2002, 46 (8), 1235–1245. Chang, H. T.; Rittmann, B. E. Verification of the model of Biofilm on activated carbon. Environ. Sci. Technol. 1987, 21, 280–288. Ehrhardt, H. M.; Rehm, H. J. Phenol degradation by microorganisms adsorbed on activated carbon. Appl. Microbiol. Biotechnol. 1985, 21, 32–36.
ES071451N
VOL. 42, NO. 3, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
745
