Environ. Sci. Technol. 2007, 41, 4240-4245
Removal of Polycyclic Aromatic Hydrocarbons from Contaminated Soil by Aqueous DNA Solution RONALD R. NAVARRO, HIROYASU ICHIKAWA, YOSUKE IIMURA, AND KENJI TATSUMI* National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki, 305-8569, Japan
An aqueous DNA solution was applied for the extraction of polycyclic aromatic hydrocarbons (PAHs) from a spiked soil. Solubilities of 0.56, 11.78, and 11.24 mg L-1 for anthracene, phenanthrene, and pyrene, respectively, were achieved after 1 day equilibration in 1% DNA. Using a spiked soil that contained 72 mg kg-1 anthracene, 102 mg kg-1 phenanthrene, and 99 mg kg-1 pyrene, extractions of close to 88, 78, and 94%, respectively, were attained with 5% DNA at a 1:50 soil/extractant ratio. Maximum PAH dissolution occurred after 4-6 h. Comparative tests showed the main advantage of DNA over methyl-β- and γ-cyclodextrins and Tween 80 for pyrene removal. An ionic strength of 0.1 M NaCl was found necessary for maximum PAH dissolution and extraction. The performance of hexane regenerated DNA also remained stable after three stages of recycling. These results show the huge potential of DNA as an aqueous washing agent for PAH-contaminated soil.
Introduction Polycyclic aromatic hydrocarbons (PAHs) are highly toxic pollutants of major concern in many contaminated industrial sites throughout the world (1). Their hydrophobicity as well as strong affinity for organic matter leads to their persistency and removal difficulty from the soil environment (2). The most straightforward approach for PAH removal is through soil flushing using a combination of water miscible organic solvents such as ethanol, 2-propanol, 1-pentanol, and acetone (3). But, the exhaustive nature of solvent extraction has repercussions on important physical as well as chemical characteristics of soil. The same limitation may be said of surfactant-based extraction that utilizes micelle-forming compounds such as Tween 80, Tritron X-100, and Igepal (4-9). Bioremediation, which relies on microorganisms for PAH uptake and subsequent mineralization, has become a popular alternative for soil treatment (1, 10). Unfortunately, low aqueous solubility limits PAH bioavailability so that solubilizing agents are also essential for the effectiveness of this process (11, 12). Biosurfactants that enhance PAH solubilization have been studied (12). Recent works have focused on the use of cyclodextrins for PAH solubilization (2, 11, 13-16). With its hydrophilic shell and hydrophobic cavity, cyclodextrins have been found to effectively enhance the solubility of certain of PAHs such as phenanthrene through * Corresponding author phone: +81-298-61-8325; fax: +81-29861-8242; e-mail: [email protected]
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the process of intercalation. But, the fixed size of the cyclodextrin cavity, which is only slightly bigger than anthracene for the methyl-β type (2), has limited their application to a narrow range of PAH molecular sizes. Larger compounds could only achieve partial inclusion and dissolution by cyclodextrins (2, 13-15). Among the naturally occurring and readily available biomolecules, deoxyribonucleic acid (DNA) is known to exhibit a high affinity for toxic organic compounds such as PAHs (17, 18). In fact, the toxicity of such organics is often associated with their high affinity for DNA, which can induce mutations in living systems (18, 19). Binding is generally attributed to the intercalation of planar PAH molecules in the hydrophobic spaces between adjacent base pairs of the DNA molecule (17, 19). Recently, utilization of DNA in the field of environmental cleanup has been studied. To date, it has been successfully employed in the removal of some toxic aromatic organics such as PAHs from wastewaters through ultrafiltration and adsorption (20-23). DNA has also been used as a biosensor component for the environmental monitoring of these toxic compounds (24-26). In this paper, an extension of the application of DNA in the field of environmental remediation was evaluated. Specifically, aqueous DNA was used as a solubilizing agent for common PAHs such as anthracene, phenanthrene, and pyrene from contaminated soil. Interesting merits as well recommendations for efficiently utilizing DNA for this purpose were established.
Experimental Procedures Materials. Anthracene, phenanthrene, and pyrene (purity >98%) were purchased from Nacalai Tesque (Kyoto, Japan) and used for all solubility and PAH spiking experiments. Salmon DNA powder (>90% purity) of a molecular weight ranging from 50,000-200,000 was purchased from Nippon Chemical Feeds (Hakodate, Japan). DNA solutions were prepared by dissolving the powder in distilled water. All other chemicals were purchased from Wako Chemicals (Tokyo, Japan). A PAH-free gardening soil purchased from a local home center was used for spiking purposes. It had a particle size distribution of approximately 13.6, 79.1, and 7.3% for 0-0.005, 0.075-0.25, and 0.075-0.25 mm, respectively. The average pH at a 1:2.5 soil/water ratio was around pH 6.05. The organic carbon content of the soil (fOC) was 9.67 ( 0.22%. Spiking of Soil with PAHs. Spiking of soil was made following protocols available in the literature (27). The soil was combined with PAHs dissolved in acetone. The mixture was then subjected to rotary evaporation at 35 °C under vacuum until relatively dry (as indicated by the nonadherence of soil particles to the inner walls of the rotating flask). The spiked soil was transferred in a wide rectangular pan and then left under the fume hood for 3 days to evaporate any traces of acetone. The final samples were stored in glass bottles and then kept in the refrigerator. Analysis indicated a composition of 487 mg kg-1 for pyrene-spiked soil and 72, 102, and 99 mg kg-1 for anthracene, phenanthrene, and pyrene, respectively, for the soil spiked with a mixture of PAHs. Evaluation of PAH Solubility. Approximately 5 mg of individual PAH crystals (anthracene, phenanthrene, and pyrene) were combined with 5 mL of DNA, methyl-βcyclodextrin (M-β-CD), and γ-cyclodextrin (γ-CD) solutions of different initial concentrations (0.1, 0.5, and 1%). The mixtures were shaken at 125 rpm for 1 day using a reciprocating shaker (Taitec Personal II, Tokyo, Japan). The 10.1021/es0624523 CCC: $37.00
2007 American Chemical Society Published on Web 05/11/2007
supernatants were recovered and transferred to glass centrifuge tubes. The samples were centrifuged at 3500 rpm for 15 min. The PAH contents of supernatants were analyzed as described in Analytical Methods. The effect of ionic content (as NaCl) on the dissolution of pyrene crystals in 0.5% aqueous DNA was also made. The initial pH was adjusted to pH 7, and the final pHs and pyrene concentrations of the supernatants were determined. Extraction of PAHs from Spiked Soil. A typical extraction procedure involved combining exactly 0.50 g of PAH-spiked soil with 5 mL of an aqueous 1% cyclodextrin, 1% Tween 80, and 1% DNA solution. This corresponds to a 1:10 soil/ extractant ratio. During the initial runs, the mixtures were shaken for 1 day. The samples were centrifuged, and the PAH content of the supernatants was analyzed. For kinetics analysis, exactly 10 g of pyrene-spiked soil was combined with 100 mL of 1% DNA solution and then shaken. Three milliliters of the sample were taken at regular time intervals and then centrifuged. The DNA and pyrene concentrations of the supernatants were analyzed. During equilibrium experiments for maximum PAH extraction, 0.20 g of PAH-spiked soil was combined with 10 mL of aqueous DNA of different initial concentrations. The use of a lower soil/extractant ratio of 1:50 was considered in this case due to the high viscosity of the DNA solution beyond 5%. After 4 h of shaking, the samples were centrifuged, and the PAH concentrations in the supernatants were analyzed. The effect of ionic strength was evaluated by combining 0.20 g of pyrene-spiked soil with 10 mL of 0.5% DNA solution at different NaCl concentrations. The initial pH was adjusted to pH 6.5. After shaking for 4 h, the final pHs and pyrene concentrations of the supernatants were analyzed. The isotherm for DNA uptake by soil was obtained by combining 0.5 g of PAH-spiked soil with 5 mL of DNA solutions of different concentrations. After shaking for 4 h, the DNA concentrations of the supernatants were analyzed. Finally, the extraction stability of recycled DNA was evaluated by combining 1 g of PAH-spiked soil with 10 mL of a 0.97% DNA solution of 0.1 M NaCl concentration. The mixture was shaken for 4 h and then centrifuged. The supernatant was recovered and then regenerated by vortex mixing with an equal volume of hexane. Exactly 9 mL of recycled DNA solution was taken and used for PAH removal from a fresh soil sample. During the second stage, only the volume was adjusted to 10 mL by the addition of 1 mL of 0.97% DNA. Hence, the resulting initial DNA concentration in this stage was below 0.97%. During the third stage, however, both the volume and the DNA concentration were adjusted to 10 mL and 0.97%, respectively. Analytical Methods. PAHs analyses were conducted by combining 3 mL of supernatant with 2 mL of hexane in a glass test tube. Standard samples were prepared by diluting a 1000 mg L-1 PAH-acetone stock solution in distilled water. The PAH-hexane mixture was subjected to vortex mixing for 1 min. The two layers were separated by centrifugation, and the PAHs in the organic layer were analyzed using a Shimadzu GC-MS (GC MS-QP 2010, Kyoto, Japan) equipped with a mass spectrometric detector and auto-injector (AOC-20i). A 0.25 mm × 25 m × 0.25 µm Quadrex MS capillary column made of 100% dimethyl polysiloxane was utilized. The GC mass conditions during analysis were as follows: injector, interface, and ion source temperatures: 280 °C and injection volume at splitless mode: 1 µL. Helium (99.9999%) was used as the carrier gas. The temperature program for the GC oven was as follows: initial oven temperature at 50 °C for 1.0 min, heating to 180 °C at 18 °C min-1, and then heating to 250 °C at 12 °C min-1 with a final holding time of 2 min. Mass data analysis was conducted at a range of m/z 60.00-650.00.
FIGURE 1. Solubility of various PAHs at different DNA and cyclodextrin concentrations after 1 day equilibriation with PAHs crystals. The absence of error bars in some data indicates very low standard deviation values. The analysis of DNA concentration was conducted by measuring the absorbance of the supernatants at a 260 nm wavelength using a spectrophotometer (Bio-rad SmartSpec TM Plus, Tokyo, Japan). The supernatants were diluted to below 150 mg L-1 with pH 7.5 Tris-HCl buffer to maintain uniform pH conditions during measurements. The carbon fraction (fOC) of the soil was analyzed by an elemental analyzer (PerkinElmer Model 2044 CHN, Waltham, MA). All solubility and extraction experiments were conducted in replicates, and the average values were reported.
Results and Discussion Comparisons of PAH Solubization by DNA and Other Agents. (A) DNA and Cyclodextrin Performance for PAH Solubilization. Figure 1 shows the solubility enhancements of different PAHs in different concentrations of aqueous DNA and cyclodextrin. The PAH solubilities increased with the extractant concentrations to confirm the interaction between these compounds. Between M-β-CD and γ-CD, the former displayed a higher solubility for both phenanthrene (8.20 mg L-1) and anthracene (0.70 mg L-1). These correspond to 7- and 17.5-fold increases relative to water solubilities of phenanthrene (1.18 mg L-1) and anthracene (0.04 mg L-1) (1). Generally, the size of the PAH molecules relative to the cavity of the cyclodextrin mainly contributes to solubility enhancements (2, 13-15). In addition, other factors such as VOL. 41, NO. 12, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
FIGURE 2. Extraction of different PAHs from spiked soil by DNA and different types of cyclodextrin. Soil/solution ratio was 1:10. The absence of error bars in some data indicates very low standard deviation values. PAH molecular surface area and orientation in the cavity relative to the hydrophobic groups of the extractant molecules may also affect solubilization (15, 18). The higher solubilities in M-β-CD indicate a better hydrophobic interaction within its intercalation cavity due to better fitting. Although the larger γ-CD cavity may also accommodate PAHs, hydrophobic interactions may be lower in this case due to a loose fit (13). The higher solubility enhancement of anthracene than phenanthrene may also suggest its better molecular fit with CD. For DNA, phenanthrene and anthracene solubilities were 11.78 and 0.56 mg L-1, corresponding to 10- and 14fold increases, respectively. This indicates that phenantherene has a slightly higher solubility in DNA, while anthracene dissolves more in M-β-CD. With the larger intercalation cavity of DNA relative to CD, the effect of the PAH orientation relative to the hydrophobic sections of the extractants may have a more significant effect on the small solubility differences in this case (15, 18). A low dissolution of pyrene has been the major limitation of cyclodextrins, and this has been attributed to its larger molecular size relative to the cyclodextrin cavity. In this work, pyrene solubility in the 1% M-β-CD solution was only 0.23 mg L-1. With the larger cavity of γ-CD, the solubility slightly increased to 1.05 mg L-1 to indicate an improved inclusion for the pyrene molecule. However, the solubility in DNA has reached up to 11.24 mg L-1, which is 90-fold higher than in water (0.126 mg L-1) (1). Clearly, the wider intercalation sites in DNA are thus also able to accommodate even larger pyrene molecules, despite its size limitation for CD. These results show that DNA is able to bind with a broad range of PAH molecular sizes. Parallel results were observed with a soil that was spiked with a mixture of the three PAHs. In all extraction runs, the equilibrium pHs were around pH 6.4. Phenanthrene removal was higher with DNA, while anthracene was higher with M-βcyclodextrin (Figure 2). A very significant advantage of DNA can be observed from the pyrene data. For M-β-cyclodextrin and γ-cyclodextrin, extractions were both below 1%. With DNA, pyrene extraction was close to 40%. Extraction data with PAH mixture not only confirmed the higher performance of DNA over CD for pyrene solubilization but also indicated the preference of DNA for this PAH. This is in contrast to the CD data where the large pyrene molecule resulted in low removal. Alhough our data may not be able to fully explain this, the larger surface area of the pyrene molecule relative to the other two PAHs, which leads to better hydrophobic interactions with the DNA bases, may be the important factor here. Indeed, our results follow the intercalation preference of DNA for 1,2- or 2,3-benzopyrene over 1,2,5,6-dibenzoanthracene. This has been attributed to the more compact ring system of benzopyrene as compared to the extended ring system of dibenzoanthracene so that a greater portion of the 4242
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FIGURE 3. Extraction of different PAHs from spiked DNA and Tween 80. Soil/solution ratio was 1:10. The absence of error bars in some data indicates very low standard deviation values.
FIGURE 4. Time course of pyrene removal by DNA. Soil/solution ratio was 1:10. The absence of error bars in some data indicates low standard deviation values. former is available for stacking interaction with the DNA bases (18). (B) DNA and Tween 80 Performance for PAH Solubilization. Tween 80 is a well-known washing agent for a wide range of hydrophobic organic compounds such as PAHs in contaminated soil. Its outstanding performance over other PAH solubilizing agents has been well-studied (4). Our extraction results showed that DNA is comparable to Tween 80. A total PAH removal of 84% for Tween and 74% for DNA was achieved (Figure 3). The equilibrium pHs for both systems were close to pH 6.4. Although both phenanthrene and anthracene extractions were lower in DNA, this was compensated by the high value of 40% for pyrene. Pyrene extraction by Tween is only half that of DNA at 21%. This low extraction of pyrene by Tween relative to other PAHs is consistent with previous results (28) and thus further emphasizes the unprecedented performance of DNA in this aspect. Furthermore, PAH extraction by DNA was also achieved without the formation of a viscous emulsion, which is one common problem with Tween 80 (2). This property complicates both soil washing and phase separation during extraction and recycling, respectively. Even at around 0.1% Tween 80 concentration, extensive emulsion formation with hexane occurs. In the case of DNA, even at 1% concentration, phase separation was complete. Kinetics of PAH Extraction. The time course for PAH extraction using pyrene as a model indicated an equilibrium time of around 4-6 h (Figure 4). Except for the initial decline during the first hour, the DNA concentration has also remained stable throughout the extraction period. The equilibrium pHs were maintained at around pH 6.2. The
FIGURE 5. Extraction of different PAHs from spiked soil at different DNA concentrations. Soil/solution ratio was 1:50. observed rate of dissolution is comparable to systems utilizing other extractants and PAH samples. For phenanthrene extraction with hydroxypropyl-cyclodextrin (HPCD), the maximum solubilization after around 6 h was reported (14). In the case of phenanthrene, crystal dissolution by another microbially produced polymeric biosurfactant Alasan, the maximum solubility was achieved after only 3.5 h (12). Generally, the rate of dissolution or partitioning of PAHs between the aqueous solution and the soil matrix is a function of the organic content of the soil. However, kinetics data indicated a rapid partitioning of PAHs in DNA solution despite the high organics fraction of our soil sample (fOC of 9.67%). The previous equilibrium extraction data also showed high increases of individual PAH solubility to further suggest the negligible effect of soil organics in PAH partitioning in DNA solution. However, it must be emphasized that the observed kinetics for PAH removal by DNA may not be generalized for all systems, particularly for aged soils. It has been reported that PAHs become increasingly difficult to extract from soils with time exposure (28). Furthermore, soil contamination with nonaqueous phase liquids (NAPL) and PAHs may also present yet another challenge (16). In any case, past research has shown positive results with the use of cyclodextrins and Tween 80 to enhance the PAH removal in such situations (16, 28). With the additional advantages of DNA over these two extractants, its greater potential for actual applications is justified. Equilibrium Extraction of PAHs at Different DNA Concentrations. The feasibility of complete PAH removal by DNA in a single extraction stage was also evaluated. Results confirmed the dependence of PAH extraction on DNA content. At 5% DNA concentration, phenanthrene, anthracene, and pyrene extractions were satisfactorily high at 78, 88, and 94%, respectively (Figure 5). These data further support the high selectivity for pyrene, which was discussed previously. Now, the declining rate of PAH removal at an increasing DNA concentration may indicate the presence of a residual DNA-PAH complex in the soil. This will be discussed in more detail in Recycling of DNA. Effect of Ionic Strength on DNA-PAH Intercalation. With the wide array of exchangeable cations that are present in the soil, it is also important to evaluate how these species affect the performance of DNA for PAH dissolution. Using pyrene as a model, solubility and extraction were always higher in the presence of NaCl (Figure 6). However, very high NaCl concentrations also resulted in reducing PAH dissolution. Since the equilibrium pH did not change considerably, particularly for the solubility runs, these results indicate the significant function of additional ions on the structure of DNA. With regards to the positive effect of salt, the shielding of the negatively charged phosphate groups by the added cations may stabilize the double-stranded DNA
FIGURE 6. Effect of NaCl concentration on the solubility and extraction of pyrene by DNA solution. The concentration of DNA was 0.5%. During extraction, a soil/solution ratio of 1:50 was employed.
FIGURE 7. Extraction performance of recycled DNA solution for PAH removal. Soil/solution ratio was 1:10. DNA concentration during the second stage was not adjusted. However, DNA content was adjusted during the third stage. structure for better PAH intercalation (17, 19, 29, 30). However, an extremely high ionic content may also cause deviations of DNA from the structure that is optimum for PAH intercalation (17, 18, 29). A salt concentration of 0.1 M NaCl has been reported for maximum pyrene intercalation by denatured DNA (29). Interestingly, we also observed the highest solubilization and extraction around this concentration. Since the ionic strength significantly influences DNAPAH binding, it may be important to evaluate its implications in actual applications on a case-by-case basis depending on the ion exchange characteristic of the soil. Recycling of DNA. The DNA regeneration data confirmed the stability of DNA performance even after three regeneration stages (Figure 7). During the first stage, where the DNA concentration was 0.97%, the percentage removals for anthracene, phenanthrene, and pyrene were 12.6, 25.8, and 37.1%, respectively. Now, when the regenerated DNA solution was utilized during the second stage with a fresh soil sample, anthracene, phenanthrene, and pyrene removals decreased slightly to 11.0, 22.1, and 32.1%, respectively. However, this decrease is proportional to the 15% lower DNA concentration utilized during the second stage. Expectedly, by adjusting the DNA concentration to 0.97% during the third stage, the PAH extractions returned to the first stage values. Clearly, the PAH intercalating property of regenerated DNA has remained stable. Finally, with regards to DNA losses after extraction, this may be attributed to either microbial degradation or soil binding. However, within the employed extraction time of VOL. 41, NO. 12, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
FIGURE 8. Isotherm for the adsorption of DNA by spiked soil. Soil/ solution ratio was 1:10. 4 h, microbial degradation may still be negligible. Kinetics data (Figure 4) have shown that except for the initial reduction during the first hour, the DNA concentration has remained almost constant even after 10 h. Furthermore, the pyrene concentration has also remained stable. In the event of extensive DNA decomposition, we would expect the concentrations of both DNA and PAHs to decrease after some time. Hence, soil binding may account for the DNA losses during the extraction period. With the predominantly negative charge of the DNA polymer due to phosphate groups, interaction with soil may occur through cross-linking with divalent cations, particularly Ca2+ (31). For our sample, we observed an uptake of approximately 4.5 mg DNA/g soil (Figure 8). To some extent, the nonlinear extraction curve for PAHs at increasing DNA concentrations (Figure 5) may also be a consequence of soil-bound DNA-PAH complexes. It would also be equally important to evaluate treatment steps that would minimize the amount of residual DNAPAH complexes in the soil to further improve extraction efficiency. To summarize, the potential for utilizing DNA for washing or flushing of PAH-contaminated soil has been established. It solubilizes a wider range of PAHs as compared to cyclodextrin. It was also comparable with Tween 80, without the usual problems related to the high emulsifying characteristics of the latter. As a result, it may also be regenerated quite effectively for economy of use. Now, with regards to cost concerns and the practicality of employing DNA for soil remediation, we would like to add that the current price of its commercially available material, which is obtained from waste salmon sperm, is dictated by the high purity demanded for its use. However, for environmental applications, which may impose lesser restrictions on DNA purity, this can considerably reduce production costs. With the recent successes of the use of solubilizing agents such as Tween 80 and cyclodextrin to enhance the biodegradation of PAHs in soil, DNA may even be more promising for such purposes. We are currently evaluating the enhancement of PAH uptake as well as mineralization by microorganisms in the presence of DNA in contaminated soil, and initial results are very encouraging.
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Received for review October 13, 2006. Revised manuscript received February 9, 2007. Accepted March 26, 2007. ES0624523
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