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Effect of freeze–thawing cycles on soil aging behavior of individually spiked phenanthrene and pyrene at different concentrations
Science of the Total Environment 444 (2013) 311–319
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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Effect of freeze–thawing cycles on soil aging behavior of individually spiked phenanthrene and pyrene at different concentrations Qing Zhao a, b, Baoshan Xing b, Peidong Tai a, Hong Li c, Lei Song b, Lizhu Zhang b, d, Peijun Li a,⁎ a
Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA Wenzhou Vocational College of Science and Technology, Wenzhou, Zhejiang 325006, China d School of Science, Harerbin Institute of Technology, Harerbin, Heilong Jiang 150001, China b c
H I G H L I G H T S ► ► ► ►
Concentration effect on PAHs' extraction was studied. Changes in extraction efficiency rate caused by slow sorption were analyzed. Conceptual model of the interactions between SOM and chemicals was proposed. A new way on studying the aging behavior of HOCs was suggested.
a r t i c l e
i n f o
Article history: Received 24 January 2012 Received in revised form 16 November 2012 Accepted 17 November 2012 Available online 29 December 2012 Keywords: Aging Freeze–thawing Soil Pyrene Phenanthrene Concentration effect
a b s t r a c t This work was initiated to study the concentration effects on phenanthrene and pyrene extraction during process of aging in phaeozem and burozem with or without freeze–thawing cycles. 1, 10, and 100 μg g−1 phenanthrene and pyrene contaminated soils were extracted by 10,000 mg L−1 surfactant SDBS at various times. The extraction amount decreased with increasing contact time. Aging process could be divided into two stages: an initially rapid and then a slow sorption period. The amount extraction of 1 μg g−1 pyrene in phaeozem was higher at the initial sorption stage. The time for 100 μg g−1 phenanthrene and pyrene extraction efficiency to reach equilibrium in both soils were shorter than 1 and 10 μg g −1. Changes in extraction efficiency rate caused by slow sorption were affected by contact time, physicochemical properties of soils and chemicals and the applied concentration of chemicals. For 1 and 10 μg g−1 phenanthrene and pyrene contaminated soils, freeze–thawing cycle increased the extraction efficiency at the initial 1st and 8th days. The extraction efficiency with no freeze–thawing cycle was higher than that with freeze–thawing cycles at 30 and 120 days. A conceptual model was proposed to account for this process. © 2012 Elsevier B.V. All rights reserved.
1. Introduction With an increasing amount of hydrophobic organic compounds (HOCs) entering soil during their production, application, transport, handling and disposal, their environmental risks in soil have gained broad concerns. Aging of HOCs in soil, which results from the movement of compounds from accessible soil compartments into less or inaccessible compartments and exhibits reduced extractability to solvents and bioavailability to microorganisms, is one of the most important behaviors that must be taken into consideration in risk estimation (Northcott and Jones, 2001; Chung and Alexander, 2002; Abu and Smith, 2005; Song et al., 2006; Louchart and Voltz, 2007; Zhao et al., 2009). There is a virtually universal accord in the scientific community that HOCs' aging in soil plays an important role both in pollutant sequestration and removal. For ⁎ Corresponding author. Tel.: +86 24 83970367; fax: +86 24 83970368. E-mail address: [email protected] (P. Li). 0048-9697/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.scitotenv.2012.11.062
pollutant sequestration, we aim for accelerating the aging process in order to decrease the activity and toxicity of HOCs. For pollutant removal, we aim for diminishing aging process to increase HOCs' mobility, bioavailability and extractability and to maximize the remediation efficiency. Freeze–thawing cycle has great influence on HOCs' sorption and desorption in both aged and unaged soils (Zhao et al., 2009). Freeze–thawing cycles may lead to either breakdown or formation of soil aggregates (Lehrsch et al., 1991; Lehrsch, 1998). However, the mechanism of this process is still unclear. In addition, study of relevant variables, for example concentration effects, in the process is limited. As in the light of actual conditions, HOCs distribute in a wide range of concentrations in soil. Previous studies showed that aging behavior occurred at both low and high level concentrations (Robertson and Alexander, 1998; Chung and Alexander, 1999). Regarding sorption of HOCs as a mass transfer process, aging phenomenon can be explained to mass transfer resistance — the resistance of the matrix to molecular
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previous studies (Zhao et al., 2009, 2010). A certain amount of soils were added into stock solutions (0.15 g phenanthrene/pyrene in 0.8 L hexane) with continuous mixing. The slurry was then mixed with 0.02% w/w NaN3. The mixture was then stirred every 15 min to allow the solvent evaporate slowly and to make sure thorough mixing. The prepared soil samples were then moved into a bottle and kept in darkness in 25 °C thermostated containers. The extraction of phenanthrene and pyrene was determined at the following times 1, 4, 8, 13, 16, 30, and 120 days. For samples (2 g) that went through different frequencies of freeze–thawing cycles (1, 4 and 6 times), the freeze–thawing cycles were conducted to determine the day (30th and 120th days).
diffusion. In this process, concentration gradient is the diffusion driving force making HOC molecules migrate to achieve maximum entropy (Pignatello, 1999). The frequent observations of nonlinear sorption isotherms for HOCs reflect that as the concentration increases, the percentage of aged HOCs in soil will decrease. What's more, concentration affects not only the ratio of sorbed to solution-phase HOCs in short time periods but also the onset of slow kinetics of sorption and the extent of short-term desorption (Chung and Alexander, 1999; Divincenzo and Sparks, 1997). Thus, the specific objective of this study was to evaluate concentration effects on the extractability of phenanthrene and pyrene during the process of aging in two different soils with or without freeze–thawing.
2.3. Freeze–thawing technique 2. Materials and methods Soil samples were frozen in a refrigerator at −15 °C. After freezing, the samples were then kept in 25 °C incubator for thawing. A freeze– thawing cycle included 8 hour freezing and 8 hour thawing.
2.1. Sorbents and chemicals Two different types of uncontaminated soils, burozem and phaeozem, were used in this study. All soils were collected from the 0–10 cm upper layer, air-dried, sieved through a 2 mm sieve and did not contain detectable levels of pyrene and phenanthrene. The physical and chemical properties of burozem and phaeozem are described elsewhere in detail (Zhao et al., 2009, 2010). Phaeozem had 5.21 ±0.32% organic carbon, 39.8 ±2.8% sand, 24.7 ± 2.5% silt, and 34.5 ± 2.2% clay. Burozem had 3.28± 0.10% organic carbon, 45.2 ± 3.0% sand, 43.0 ± 2.2% silt, and 11.8± 1.5% clay. The organic matter aromaticity of phaeozem and burozem determined by NMR was 18.8% and 22.3% respectively. The surface area of phaeozem and burozem determined by nitrogen adsorption–desorption isotherms at 77 K was approximately 13.7 m 2 g−1 and 24.8 m2 g−1 respectively. Phenanthrene and pyrene were purchased from Sigma Chemical Co. Both of them have a purity >98%. The logKow of phenanthrene was 4.57 (Zhu and Feng, 2003), and logKow of pyrene was 4.88 (Fan et al., 2008).
2.4. Sample extraction Extraction experiments of phenanthrene and pyrene were performed using a batch equilibration technique at 25±1 °C, as described in our previous work (Zhao et al., 2010). Briefly, 2 g of contaminated soil and 20 mL of 10,000 mg L−1 sodium dodecylbenzenesulfonate (SDBS) solution were added to 25-mL screw cap vials. The vials were sealed with aluminum foil-lined Teflon screw caps and then placed on a shaker for 24 h at 150 rpm. After centrifugation (4000 rpm for 15 min), 1 mL of the supernatant was removed and analyzed by HPLC (Agilent 1200) with a flow rate of 0.7 mL min−1 and a mobile phase of 90% methanol and 10% water. HPLC analyses were performed with a C18 reverse-phase column (ODS2, 4.6 mm×250 mm), a VWD detector (G1314B) and a FLD detector (G1321A). The sorbed phenanthrene and pyrene by sorbents were then calculated by mass difference. Experimental uncertainties were evaluated in vials without soil, which showed that total uncertainty was less than 4% of the initial concentrations. Previous study (Zhao et al., 2010) showed that the extraction efficiency (ratio of extracted amount via total amount) of phenanthrene (range from 1 μg g−1 to 100 μg g−1) in phaeozem and burozem was 81.1%–91.8% and 86.1%– 94.5% respectively. With respect to pyrene, it was 73.4%–85.2% and
2.2. Aging of chemicals 1, 10 and 100 μg g −1 pyrene and phenanthrene contaminated soils were prepared and stored, following the approach described in our 1.0
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71.3%–92.2% in phaeozem and burozem, respectively. All samples were performed with duplications. Data were statistically analyzed using SPSS 13.0 for windows (SPSS Inc., Chicago, Illinois). 3. Results 3.1. Aging process of phenanthrene and pyrene in soils The extraction amount of phenanthrene and pyrene in phaeozem and burozem by SDBS decreased with increasing contact time and could be divided into two stages: initially rapid and then slow sorption periods (Fig. 1). For example, the extraction amount decreased from 0.860 and 0.904 μg g−1 on the 1st day to 0.782 and 0.801 μg g−1 on the 30th day, which was 9.1% and 11.4% reduction in 1 μg g−1 phenanthrene contaminated phaeozem and burozem, respectively. On the 120th day, the extraction amount in phaeozem and burozem decreased to 0.620 and 0.759 μg g−1, which was 27.9% and 16.0% reduction with respect to the 1st day, respectively. The reduction in extraction amount indicates the increase in the amount of less extractable, more desorb-resistant fraction of compound which might probably be attributed to the aging behavior of compounds in soils. Generally speaking, the extraction amount of phenanthrene and pyrene in phaeozem was lower than that in burozem, however, the amount of extraction of 1 μg g−1 pyrene in phaeozem was higher than that in burozem. Soil organic carbon content is one of the most important factors that affect organic compounds' sorption in soils. Many studies had pointed out that sorption amount of compounds especially nonpolar ones in soil
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had a positive relationships with soil organic carbon content (Feng et al., 2000; Yang et al., 2005). As phaeozem had higher organic carbon content (5.21± 0.32%), it had a higher adsorption capacity than burozem, and hence in most cases the extraction amount of phenanthrene and pyrene on phaeozem was lower than that on burozem. However, when the initial compound concentration was low, as there are enough sites for organic compounds' sorption, other effects such as the component of soil organic matter and sorption energy of sorption sites must be taken into consideration. Chemical molecule prefers to sorb on high energetic sites at low concentration (Zhang et al., 2012). Those sites may include absorption into condensed or “hard” organic polymeric matter or combustion residue and adsorption into microvoids or microporous minerals with porous surfaces (Luthy et al., 1997). The Freundlich exponential coefficient n can be regarded as an index of site energy distribution, the smaller the n, the greater contribution of high energetic sorption sites make (Xing and Pignatello, 1997). From Table S1, we can see that n value of pyrene sorption on burozem is smaller than phaeozem, indicating that there were more high energetic sites in burozem than in phaeozem. Hence, the amount extraction of 1 μg g−1 pyrene in phaeozem was higher. In order to compare the concentration effects of the samples, we selected extraction efficiency instead of extraction amount as ordinate (Fig. 2). The time for 100 μg g−1 phenanthrene and pyrene extraction efficiency to reach equilibrium in both soils were shorter than 1 and 10 μg g−1. With our experimental mixing time, 1 μg g−1 of phenanthrene and pyrene in both soils had not reached equilibrium yet. The extraction efficiency of 1 μg g−1 phenanthrene in both soils was higher
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than other concentration levels, while for pyrene, the extraction efficiency at 100 μg g−1 was the highest which was consistent with Chung and Alexander (1999). Also it is noteworthy that the change in extraction efficiency of phenanthrene and pyrene in both soils between 1 day and 120 days followed the order: 1 μg g−1 >10 μg g−1 >100 μg g−1, which might possibly arise from the onset of the slow kinetics of sorption. According to Xing and Pignatello (1996) and our recent research (Zhao et al., 2010), we assumed establishment of equilibrium for the fast state at 1 d. Then the extraction efficiency change rate caused by slow sorption, Rt (% d −1), can be estimated by the following equation: Rt ¼
Et −E1 t−1
where Et is the extraction efficiency on the t day (%), E1 is the extraction efficiency on the 1st day and represents as the fast sorption fraction. This equation can well reflect the slow sorption process because it shields the effects caused by fast sorption. The results were summarized in Fig. 3. Rt value of 1 μg g −1 phenanthrene and pyrene in both soils was higher than other concentration levels indicating that the proportion of extraction efficiency caused by slow sorption decrease with the increasing concentration. Rt decreased as the time increased. However, at the beginning of 4 or 8 days, Rt increased with increasing time in some cases under low concentration levels. This is because at the initial stage, there may be sufficient accessible sorption sites in glassy phase. The number of sites will decrease as the time increases,
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and the rest of chemical molecules have to diffuse into interior area or pores. Hence after 8 days, Rt decreased with the increasing time. For example, Rt increased from 0.193% d −1 on the 4th day to 0.412% d −1 in 10 μg g −1 pyrene contaminated phaeozem. The Rt value of phenanthrene and pyrene in burozem was higher than that in phaeozem. For example, Rt of 10 μg g −1 pyrene in burozem on the 16th day was 0.412% d −1, while in phaeozem, it was 0.208% d −1. The Rt value of pyrene in both soils was a bit higher than that of phenanthrene. For example, Rt of 1 μg g −1 pyrene in phaeozem on the 13th day was 0.962% d −1 compared to 0.646% d −1 of 1 μg g −1 phenanthrene in phaeozem on the 13th day. We also assumed establishment of equilibrium for the fast state at 8 d (Table S2). The result was consistent with 1 d assessment. Hence we can safely draw the conclusion that slow sorption processes of organic compounds in soil were affected by contact time, physicochemical properties of soils and chemicals, and the concentration of chemicals. 3.2. Effects of freeze–thawing cycles on the extraction efficiency The extraction efficiencies by 10,000 mg L−1 SDBS of 1, 10, and 100 μg g−1 phenanthrene and pyrene with or without freeze–thawing cycles in phaeozem and burozem during various time periods are shown in Figs. 4 and 5, respectively. For 1 and 10 μg g−1 phenanthrene contaminated soil samples, freeze–thawing cycles increased extraction efficiency at the initial 1st and 8th days, However, for pyrene in burozem, an increase in extraction efficiency following a freeze–thawing cycle was
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Fig. 4. Extraction efficiency by 10,000 mg L−1 SDBS of 1, 10, and 100 μg g−1 phenanthrene with or without freeze–thawing cycles in phaeozem and burozem during various time periods. Values are the means of the duplicates. Error bars represent the maximum and minimum. Values among different freeze–thawing treatments followed by the same letter (a–b) were not significantly different at p b 0.05.
observed only after day 1. While for pyrene in phaeozem, a decrease was found from the 1st day. According to their Freundlich exponential coefficient n (Table S1), the values of phenanthrene in phaeozem and burozem
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Fig. 5. Extraction efficiency by 10,000 mg L−1 SDBS of 1, 10, and 100 μg g−1 pyrene with or without freeze–thawing cycles in phaeozem and burozem during various time periods. Values are the means of the duplicates. Error bars represent the maximum and minimum. Values among different freeze–thawing treatments followed by the same letter (a–b) were not significantly different at p b 0.05.
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decreased after freeze–thawing. For 100 μg g−1 phenanthrene and pyrene contaminated soils, the extraction efficiency decreased except in burozem at the initial 1st and 8th days. The different observations between the low concentration and high concentration might probably be attributed to the onset of the slow kinetics of sorption, which was caused by concentration effects (Divincenzo and Sparks, 1997). The lower the concentration is, the later the slow kinetics starts. The effects of the frequency of freeze–thawing cycles on the extraction efficiency of 1, 10 and 100 μg g−1 phenanthrene and pyrene in soils on the 30th and 120th days were investigated (Figs. 6 and 7) because it is common that soils experience several freeze–thawing cycles during winter. It is difficult to draw a general conclusion concerning the relationships with the extraction efficiency and the frequency of the freeze–thawing cycles because the extraction efficiency seems to fluctuate among different frequency cycles. However, in most cases, the extraction efficiency of no freeze–thawing cycle was higher than the other frequency cycles. Furthermore, similar to the extraction efficiency change, fluctuation extents of phenanthrene and pyrene in both soils also followed the order: 1 μg g−1 > 10 μg g−1 > 100 μg g−1.
contact time, physicochemical properties of soils and chemicals, the concentration of chemicals and the frequency of freeze–thawing cycles. As soil samples we used were air-dried and all contained soil organic matter (SOM) that was higher than 0.1 wt.%, we take SOM as the main sorption domain for phenanthrene and pyrene (Schwarzenbach and Westall, 1981; Weber and Huang, 1996). To further discuss the mechanisms of how freeze–thawing cycles affect the extractability of chemicals in soils and what roles contact time, physicochemical properties of soils and chemicals, the concentration of chemicals and the frequency of freeze–thawing cycles play in the whole sorption processes, we proposed a conceptual model (Fig. 8) of the mechanisms involved which is an expansion of the concept presented by Xing and Pignatello (1997), Lu and Pignatello (2004), Braida et al. (2003) and Lennartz and Louchart (2007). Adsorption to rubbery phase is a rapid, reversible process and the binding to the inner glassy section is a slower process that is limited by the diffusion controlled transport to the location of the reaction. Hence the extractability of phenanthrene and pyrene is mainly affected by the binding to the inner glassy sections. Freeze– thawing process mainly acts in the following ways. (1) The temperature will influence the diffusion process in SOM according to Arrhenius equation (Pilorz et al., 1999):
4. Discussion The following summarizes the findings here: (1) there were more high energetic sites in burozem than in phaeozem. (2) The sorption equilibrium time of high concentration chemical in soil was shorter than low concentration. (3) Slow sorption processes of organic compounds in soil were affected by contact time, physicochemical properties of soils and chemicals and the concentration of chemicals. (4) Freeze–thawing cycles affected extractability of chemicals in soils. The results may vary with
100%
where D is the diffusion coefficient, Ea is activation energy, R is the gaseous constant, T is the absolute temperature, and D0 is the diffusion coefficient obtained when extrapolating to very high temperatures. As freeze–thawing process contains a period that provides low temperature, hence it will decrease the
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D ¼ D0 exp ð−Ea =RTÞ
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Concentration (µg g-1) Fig. 6. Extraction efficiency by SDBS of 1 μg g−1, 10 μg g−1 and 100 μg g−1 phenanthrene in phaeozem and burozem under different frequencies of freeze–thawing cycle on the 30th and 120th days. Values are the means of the duplicates. Error bars represent the maximum and minimum.
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Concentrations ( ug g ) Fig. 7. Conceptual diagram of the proposed interactions between SOM and organic chemical molecules with or without freeze–thawing at the process of short time sorption and long time sorption, respectively. At short time sorption stage, freeze–thawing mainly decreased the diffusion rate of the molecules. At long time sorption stage, freeze–thawing processes promote chemical molecules' sorption to those sites that require high activated energy in glassy phase. During freeze–thaw process, the glassy components can swell and thus more accessible to chemicals compared to those without freeze–thawing. The transient expansions in glassy domain of SOM, will inevitably slow down the desorption of organic chemicals in soil.
diffusion rate. Thus in this aspect, freeze–thawing process will increase the extraction efficiency. (2) Sorption is an exothermic reaction. Steinberg et al. (1987) found that the release of EDB increased as the temperature increased. Piatt et al. (1996) used batch and column techniques to investigate naphthalene, phenanthrene, and pyrene on low organic carbon sediments and discovered that the desorption rate constants decreased 1.2–2.6 times with temperature decreasing from 26 °C to 4 °C. Xing and Pignatello (1997) studied the sorption of 1,2-dichlorobenzene, 2,4-dichlorophenol and the herbicide metolachlor on poly(vinyl chloride) and soil, they suggest that sorption isotherm becomes increasingly linear from 6 to 90 °C. Hence, the temperature decrease during freeze– thawing process will promote chemical molecules' sorption to those sites that require high activated energy in glassy phase. (3) SOM, as a mixture of natural macromolecules consisting of rubbery (expanded, fluid-like) and glassy (condensed, relatively rigid) components, spans a range of glass transition temperatures, Tg (Leboeuf and Weber, 1997). During freeze–thaw process, the glassy components can swell and thus more accessible to chemicals compared to those without freeze–thawing. The transient expansions in glassy domain of SOM, will inevitably slow down the desorption of organic chemicals in soil.
In a word, freeze–thawing cycle is a complicated process that may either increase or decrease the extraction efficiency. According to the conceptual model (Fig. 8), depending on which mechanism takes a dominant role, we need to know the exact aging extent of the chemicals in soils. At short time sorption stage, most of the chemical molecules remain non-diffused (adsorbed in outer domain), freeze– thawing mainly decreased the diffusion rate of the molecules corresponding to the increased extraction efficiency. The concentration of chemicals will affect the onset of the slow sorption. The higher the concentration is, the earlier chemicals reached to the glassy domain. Hence, for 100 μg g −1 phenanthrene and pyrene contaminated soils, the extraction efficiency decreased except in burozem at the initial 1st and 8th days. At long time sorption stage, the diffusion rate is very low, freeze–thawing processes mainly promoted chemical molecules' sorption to those sites that require high activated energy in glassy phase. The expansion in glassy domain of SOM, will also increase the sequestration of organic chemicals in soil. Thus, the extraction efficiency of no freeze–thawing cycle was higher than the other frequency cycles at 30 and 120 days. An important implication of the freeze–thawing conceptual model is that much more attention should be taken into those newly contaminated soil sites that experience freezing and thawing, because the increased extractability of HOCs will give rise to the environmental risks
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extracted
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extracted Rubbery Phase
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Fig. 8. Conceptual diagram of the proposed interactions between SOM and organic chemical molecules with or without freeze–thawing at the process of short time sorption and long time sorption, respectively. At short time sorption stage, freeze–thawing mainly decreased the diffusion rate of the molecules. At long time sorption stage, freeze–thawing processes promote chemical molecules' sorption to those sites that require high activated energy in glassy phase. During freeze–thaw process, the glassy components can swell and thus more accessible to chemicals compared to those without freeze–thawing. The transient expansions in glassy domain of SOM, will inevitably slow down the desorption of organic chemicals in soil.
after thawing. Furthermore, investigating freeze–thawing cycles seems to provide a special way to study the aging behavior of organic compounds. Further research is necessary to examine what roles other effects such as competitive sorption play during freeze–thawing process. Acknowledgments The authors gratefully acknowledge the financial supports provided by MOST (2012BAC13B03), National Natural ScienceFoundation of China (20977095) and National Science Foundation of China (41101295). Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.scitotenv.2012.11.062. References Abu A, Smith S. Assessing sequestration of selected polycyclic aromatic hydrocarbons by use of adsorption modeling and temperature-programmed desorption. Environ Sci Technol 2005;39:7585–91. Braida WJ, Pignatello JJ, Lu Y, Neimark PI, Xing BS. Sorption hysteresis of benzene in charcoal particles. Environ Sci Technol 2003;37:409–17. Chung N, Alexander M. Effect of concentration on sequestration and bioavailability of two polycyclic aromatic hydrocarbons. Environ Sci Technol 1999;33:3605–8. Chung N, Alexander M. Effect of soil properties on bioavailability and extractability of phenanthrene and atrazine sequestered in soil. Chemosphere 2002;48:109–15.
Divincenzo JP, Sparks DL. Slow sorption kinetics of pentachlorophenol on soil: concentration effects. Environ Sci Technol 1997;31:977–83. Fan SX, Li PJ, Gong ZQ, Ren WX, He N. Promotion of pyrene degradation in rhizosphere of alfalfa (Medicago sativa L.). Chemosphere 2008;71:1593–8. Feng YC, Park JH, Voice TC, Boyd SA. Bioavailability of soil-sorbed biphenyl to bacteria. Environ Sci Technol 2000;34:1977–84. Leboeuf EJ, Weber WJ. A distributed reactivity model for sorption by soils and sediments. 8. Sorbent organic domains: discovery of a humic acid glass transition and an argument for a polymer-based model. Environ Sci Technol 1997;31:1697–702. Lehrsch GA. Freeze–thaw cycles increase near-surface aggregate stability. Soil Sci 1998;163: 63–70. Lehrsch GA, Sojka RE, Carter DL, Jolley PM. Freezing effects on aggregate stability affected by texture, mineralogy, and organic matter. Soil Sci Soc Am J 1991;55:1401–6. Lennartz B, Louchart X. Effect of drying on the desorption of diuron and terbuthylazine from natural soils. Environ Pollut 2007;146:180–7. Louchart X, Voltz M. Aging effects on the availability of herbicides to runoff transfer. Environ Sci Technol 2007;41:1137–44. Lu Y, Pignatello JJ. History-dependent sorption in humic acids and a lignite in the context of a polymer model for natural organic matter. Environ Sci Technol 2004;38: 5853–62. Luthy RG, Aiken GR, Brusseau ML, Cunningham SD, Gschwend PM, Pignatello JJ, et al. Sequestration of hydrophobic organic contaminants by geosorbents. Environ Sci Technol 1997;31:2432–40. Northcott GL, Jones KC. Partitioning, extractability, and formation of nonextractable PAH residues in soil. 1. Compound differences in aging and sequestration. Environ Sci Technol 2001;35:1103–10. Piatt JJ, Backhus DA, Capel PD, Eisenreich SJ. Temperature-dependent sorption of naphthalene, phenanthrene, and pyrene to low organic carbon aquifer sediments. Environ Sci Technol 1996;30:751–60. Pignatello JJ. The measurement and interpretation of sorption and desorption rates for organic compounds in soil media. Adv Agron 1999;69:1-73. Pilorz K, Bjorklund E, Bowadt S, Mathiasson L, Hawthorne SB. Determining PCB sorption/desorption behavior on sediments using selective supercritical fluid extraction. 2. Describing PCB extraction with simple diffusion models. Environ Sci Technol 1999;33:2204–12.
Q. Zhao et al. / Science of the Total Environment 444 (2013) 311–319 Robertson BK, Alexander M. Sequestration of DDT and dieldrin in soil: disappearance of acute toxicity but not the compounds. Environ Toxicol Chem 1998;17:1034–8. Schwarzenbach RP, Westall J. Transport of nonpolar organic compounds from surface water to groundwater. Laboratory sorption studies. Environ Sci Technol 1981;15: 1360–7. Song YF, Wilke BM, Song XY, Gong P, Zhou QX, Yang GF. Polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and heavy metals (HMs) as well as their genotoxicity in soil after long-term wastewater irrigation. Chemosphere 2006;65: 1859–68. Steinberg SM, Pignatello JJ, Sawhney BL. Persistence of 1,2-dibromoethane in soils: entrapment in intraparticle micropores. Environ Sci Technol 1987;21:1201–8. Weber WJ, Huang WL. A distributed reactivity model for sorption by soils and sediments. 4. Intraparticle heterogeneity and phase-distribution relationships under nonequilibrium conditions. Environ Sci Technol 1996;30:881–8. Xing B, Pignatello JJ. Time-dependent isotherm shape or HOCs in soil organic matter: implications for sorption mechanism. Environ Toxicol Chem 1996;15:1282–8.
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Xing B, Pignatello JJ. Dual-mode sorption of low-polarity compounds in glassy poly(vinyl chloride) and soil organic matter. Environ Sci Technol 1997;31:792–9. Yang K, Zhu LZ, Lou BF, Chen BL. Correlations of nonlinear sorption of organic solutes with soil/sediment physicochemical properties. Chemosphere 2005;61:116–28. Zhang D, Pan B, Wu M, Zhang H, Peng H, Ning P, et al. Cosorption of organic chemicals with different properties: their shared and different sorption sites. Environ Pollut 2012;160:178–84. Zhao Q, Li P, Stagnitti F, Ye J, Dong DB, Zhang YQ, et al. Effect of aging and freeze– thawing on extractability of pyrene in soil. Chemosphere 2009;76:447–52. Zhao Q, Weise L, Li PJ, Yang K, Zhang YQ, Dong DB, et al. Ageing behavior of phenanthrene and pyrene in soils: a study using sodium dodecylbenzenesulfonate extraction. J Hazard Mater 2010;183:881–7. Zhu LZ, Feng SL. Synergistic solubilization of polycyclic aromatic hydrocarbons by mixed anionic–nonionic surfactants. Chemosphere 2003;53:459–67.
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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Effect of freeze–thawing cycles on soil aging behavior of individually spiked phenanthrene and pyrene at different concentrations Qing Zhao a, b, Baoshan Xing b, Peidong Tai a, Hong Li c, Lei Song b, Lizhu Zhang b, d, Peijun Li a,⁎ a
Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA 01003, USA Wenzhou Vocational College of Science and Technology, Wenzhou, Zhejiang 325006, China d School of Science, Harerbin Institute of Technology, Harerbin, Heilong Jiang 150001, China b c
H I G H L I G H T S ► ► ► ►
Concentration effect on PAHs' extraction was studied. Changes in extraction efficiency rate caused by slow sorption were analyzed. Conceptual model of the interactions between SOM and chemicals was proposed. A new way on studying the aging behavior of HOCs was suggested.
a r t i c l e
i n f o
Article history: Received 24 January 2012 Received in revised form 16 November 2012 Accepted 17 November 2012 Available online 29 December 2012 Keywords: Aging Freeze–thawing Soil Pyrene Phenanthrene Concentration effect
a b s t r a c t This work was initiated to study the concentration effects on phenanthrene and pyrene extraction during process of aging in phaeozem and burozem with or without freeze–thawing cycles. 1, 10, and 100 μg g−1 phenanthrene and pyrene contaminated soils were extracted by 10,000 mg L−1 surfactant SDBS at various times. The extraction amount decreased with increasing contact time. Aging process could be divided into two stages: an initially rapid and then a slow sorption period. The amount extraction of 1 μg g−1 pyrene in phaeozem was higher at the initial sorption stage. The time for 100 μg g−1 phenanthrene and pyrene extraction efficiency to reach equilibrium in both soils were shorter than 1 and 10 μg g −1. Changes in extraction efficiency rate caused by slow sorption were affected by contact time, physicochemical properties of soils and chemicals and the applied concentration of chemicals. For 1 and 10 μg g−1 phenanthrene and pyrene contaminated soils, freeze–thawing cycle increased the extraction efficiency at the initial 1st and 8th days. The extraction efficiency with no freeze–thawing cycle was higher than that with freeze–thawing cycles at 30 and 120 days. A conceptual model was proposed to account for this process. © 2012 Elsevier B.V. All rights reserved.
1. Introduction With an increasing amount of hydrophobic organic compounds (HOCs) entering soil during their production, application, transport, handling and disposal, their environmental risks in soil have gained broad concerns. Aging of HOCs in soil, which results from the movement of compounds from accessible soil compartments into less or inaccessible compartments and exhibits reduced extractability to solvents and bioavailability to microorganisms, is one of the most important behaviors that must be taken into consideration in risk estimation (Northcott and Jones, 2001; Chung and Alexander, 2002; Abu and Smith, 2005; Song et al., 2006; Louchart and Voltz, 2007; Zhao et al., 2009). There is a virtually universal accord in the scientific community that HOCs' aging in soil plays an important role both in pollutant sequestration and removal. For ⁎ Corresponding author. Tel.: +86 24 83970367; fax: +86 24 83970368. E-mail address: [email protected] (P. Li). 0048-9697/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.scitotenv.2012.11.062
pollutant sequestration, we aim for accelerating the aging process in order to decrease the activity and toxicity of HOCs. For pollutant removal, we aim for diminishing aging process to increase HOCs' mobility, bioavailability and extractability and to maximize the remediation efficiency. Freeze–thawing cycle has great influence on HOCs' sorption and desorption in both aged and unaged soils (Zhao et al., 2009). Freeze–thawing cycles may lead to either breakdown or formation of soil aggregates (Lehrsch et al., 1991; Lehrsch, 1998). However, the mechanism of this process is still unclear. In addition, study of relevant variables, for example concentration effects, in the process is limited. As in the light of actual conditions, HOCs distribute in a wide range of concentrations in soil. Previous studies showed that aging behavior occurred at both low and high level concentrations (Robertson and Alexander, 1998; Chung and Alexander, 1999). Regarding sorption of HOCs as a mass transfer process, aging phenomenon can be explained to mass transfer resistance — the resistance of the matrix to molecular
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previous studies (Zhao et al., 2009, 2010). A certain amount of soils were added into stock solutions (0.15 g phenanthrene/pyrene in 0.8 L hexane) with continuous mixing. The slurry was then mixed with 0.02% w/w NaN3. The mixture was then stirred every 15 min to allow the solvent evaporate slowly and to make sure thorough mixing. The prepared soil samples were then moved into a bottle and kept in darkness in 25 °C thermostated containers. The extraction of phenanthrene and pyrene was determined at the following times 1, 4, 8, 13, 16, 30, and 120 days. For samples (2 g) that went through different frequencies of freeze–thawing cycles (1, 4 and 6 times), the freeze–thawing cycles were conducted to determine the day (30th and 120th days).
diffusion. In this process, concentration gradient is the diffusion driving force making HOC molecules migrate to achieve maximum entropy (Pignatello, 1999). The frequent observations of nonlinear sorption isotherms for HOCs reflect that as the concentration increases, the percentage of aged HOCs in soil will decrease. What's more, concentration affects not only the ratio of sorbed to solution-phase HOCs in short time periods but also the onset of slow kinetics of sorption and the extent of short-term desorption (Chung and Alexander, 1999; Divincenzo and Sparks, 1997). Thus, the specific objective of this study was to evaluate concentration effects on the extractability of phenanthrene and pyrene during the process of aging in two different soils with or without freeze–thawing.
2.3. Freeze–thawing technique 2. Materials and methods Soil samples were frozen in a refrigerator at −15 °C. After freezing, the samples were then kept in 25 °C incubator for thawing. A freeze– thawing cycle included 8 hour freezing and 8 hour thawing.
2.1. Sorbents and chemicals Two different types of uncontaminated soils, burozem and phaeozem, were used in this study. All soils were collected from the 0–10 cm upper layer, air-dried, sieved through a 2 mm sieve and did not contain detectable levels of pyrene and phenanthrene. The physical and chemical properties of burozem and phaeozem are described elsewhere in detail (Zhao et al., 2009, 2010). Phaeozem had 5.21 ±0.32% organic carbon, 39.8 ±2.8% sand, 24.7 ± 2.5% silt, and 34.5 ± 2.2% clay. Burozem had 3.28± 0.10% organic carbon, 45.2 ± 3.0% sand, 43.0 ± 2.2% silt, and 11.8± 1.5% clay. The organic matter aromaticity of phaeozem and burozem determined by NMR was 18.8% and 22.3% respectively. The surface area of phaeozem and burozem determined by nitrogen adsorption–desorption isotherms at 77 K was approximately 13.7 m 2 g−1 and 24.8 m2 g−1 respectively. Phenanthrene and pyrene were purchased from Sigma Chemical Co. Both of them have a purity >98%. The logKow of phenanthrene was 4.57 (Zhu and Feng, 2003), and logKow of pyrene was 4.88 (Fan et al., 2008).
2.4. Sample extraction Extraction experiments of phenanthrene and pyrene were performed using a batch equilibration technique at 25±1 °C, as described in our previous work (Zhao et al., 2010). Briefly, 2 g of contaminated soil and 20 mL of 10,000 mg L−1 sodium dodecylbenzenesulfonate (SDBS) solution were added to 25-mL screw cap vials. The vials were sealed with aluminum foil-lined Teflon screw caps and then placed on a shaker for 24 h at 150 rpm. After centrifugation (4000 rpm for 15 min), 1 mL of the supernatant was removed and analyzed by HPLC (Agilent 1200) with a flow rate of 0.7 mL min−1 and a mobile phase of 90% methanol and 10% water. HPLC analyses were performed with a C18 reverse-phase column (ODS2, 4.6 mm×250 mm), a VWD detector (G1314B) and a FLD detector (G1321A). The sorbed phenanthrene and pyrene by sorbents were then calculated by mass difference. Experimental uncertainties were evaluated in vials without soil, which showed that total uncertainty was less than 4% of the initial concentrations. Previous study (Zhao et al., 2010) showed that the extraction efficiency (ratio of extracted amount via total amount) of phenanthrene (range from 1 μg g−1 to 100 μg g−1) in phaeozem and burozem was 81.1%–91.8% and 86.1%– 94.5% respectively. With respect to pyrene, it was 73.4%–85.2% and
2.2. Aging of chemicals 1, 10 and 100 μg g −1 pyrene and phenanthrene contaminated soils were prepared and stored, following the approach described in our 1.0
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71.3%–92.2% in phaeozem and burozem, respectively. All samples were performed with duplications. Data were statistically analyzed using SPSS 13.0 for windows (SPSS Inc., Chicago, Illinois). 3. Results 3.1. Aging process of phenanthrene and pyrene in soils The extraction amount of phenanthrene and pyrene in phaeozem and burozem by SDBS decreased with increasing contact time and could be divided into two stages: initially rapid and then slow sorption periods (Fig. 1). For example, the extraction amount decreased from 0.860 and 0.904 μg g−1 on the 1st day to 0.782 and 0.801 μg g−1 on the 30th day, which was 9.1% and 11.4% reduction in 1 μg g−1 phenanthrene contaminated phaeozem and burozem, respectively. On the 120th day, the extraction amount in phaeozem and burozem decreased to 0.620 and 0.759 μg g−1, which was 27.9% and 16.0% reduction with respect to the 1st day, respectively. The reduction in extraction amount indicates the increase in the amount of less extractable, more desorb-resistant fraction of compound which might probably be attributed to the aging behavior of compounds in soils. Generally speaking, the extraction amount of phenanthrene and pyrene in phaeozem was lower than that in burozem, however, the amount of extraction of 1 μg g−1 pyrene in phaeozem was higher than that in burozem. Soil organic carbon content is one of the most important factors that affect organic compounds' sorption in soils. Many studies had pointed out that sorption amount of compounds especially nonpolar ones in soil
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had a positive relationships with soil organic carbon content (Feng et al., 2000; Yang et al., 2005). As phaeozem had higher organic carbon content (5.21± 0.32%), it had a higher adsorption capacity than burozem, and hence in most cases the extraction amount of phenanthrene and pyrene on phaeozem was lower than that on burozem. However, when the initial compound concentration was low, as there are enough sites for organic compounds' sorption, other effects such as the component of soil organic matter and sorption energy of sorption sites must be taken into consideration. Chemical molecule prefers to sorb on high energetic sites at low concentration (Zhang et al., 2012). Those sites may include absorption into condensed or “hard” organic polymeric matter or combustion residue and adsorption into microvoids or microporous minerals with porous surfaces (Luthy et al., 1997). The Freundlich exponential coefficient n can be regarded as an index of site energy distribution, the smaller the n, the greater contribution of high energetic sorption sites make (Xing and Pignatello, 1997). From Table S1, we can see that n value of pyrene sorption on burozem is smaller than phaeozem, indicating that there were more high energetic sites in burozem than in phaeozem. Hence, the amount extraction of 1 μg g−1 pyrene in phaeozem was higher. In order to compare the concentration effects of the samples, we selected extraction efficiency instead of extraction amount as ordinate (Fig. 2). The time for 100 μg g−1 phenanthrene and pyrene extraction efficiency to reach equilibrium in both soils were shorter than 1 and 10 μg g−1. With our experimental mixing time, 1 μg g−1 of phenanthrene and pyrene in both soils had not reached equilibrium yet. The extraction efficiency of 1 μg g−1 phenanthrene in both soils was higher
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than other concentration levels, while for pyrene, the extraction efficiency at 100 μg g−1 was the highest which was consistent with Chung and Alexander (1999). Also it is noteworthy that the change in extraction efficiency of phenanthrene and pyrene in both soils between 1 day and 120 days followed the order: 1 μg g−1 >10 μg g−1 >100 μg g−1, which might possibly arise from the onset of the slow kinetics of sorption. According to Xing and Pignatello (1996) and our recent research (Zhao et al., 2010), we assumed establishment of equilibrium for the fast state at 1 d. Then the extraction efficiency change rate caused by slow sorption, Rt (% d −1), can be estimated by the following equation: Rt ¼
Et −E1 t−1
where Et is the extraction efficiency on the t day (%), E1 is the extraction efficiency on the 1st day and represents as the fast sorption fraction. This equation can well reflect the slow sorption process because it shields the effects caused by fast sorption. The results were summarized in Fig. 3. Rt value of 1 μg g −1 phenanthrene and pyrene in both soils was higher than other concentration levels indicating that the proportion of extraction efficiency caused by slow sorption decrease with the increasing concentration. Rt decreased as the time increased. However, at the beginning of 4 or 8 days, Rt increased with increasing time in some cases under low concentration levels. This is because at the initial stage, there may be sufficient accessible sorption sites in glassy phase. The number of sites will decrease as the time increases,
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and the rest of chemical molecules have to diffuse into interior area or pores. Hence after 8 days, Rt decreased with the increasing time. For example, Rt increased from 0.193% d −1 on the 4th day to 0.412% d −1 in 10 μg g −1 pyrene contaminated phaeozem. The Rt value of phenanthrene and pyrene in burozem was higher than that in phaeozem. For example, Rt of 10 μg g −1 pyrene in burozem on the 16th day was 0.412% d −1, while in phaeozem, it was 0.208% d −1. The Rt value of pyrene in both soils was a bit higher than that of phenanthrene. For example, Rt of 1 μg g −1 pyrene in phaeozem on the 13th day was 0.962% d −1 compared to 0.646% d −1 of 1 μg g −1 phenanthrene in phaeozem on the 13th day. We also assumed establishment of equilibrium for the fast state at 8 d (Table S2). The result was consistent with 1 d assessment. Hence we can safely draw the conclusion that slow sorption processes of organic compounds in soil were affected by contact time, physicochemical properties of soils and chemicals, and the concentration of chemicals. 3.2. Effects of freeze–thawing cycles on the extraction efficiency The extraction efficiencies by 10,000 mg L−1 SDBS of 1, 10, and 100 μg g−1 phenanthrene and pyrene with or without freeze–thawing cycles in phaeozem and burozem during various time periods are shown in Figs. 4 and 5, respectively. For 1 and 10 μg g−1 phenanthrene contaminated soil samples, freeze–thawing cycles increased extraction efficiency at the initial 1st and 8th days, However, for pyrene in burozem, an increase in extraction efficiency following a freeze–thawing cycle was
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burozem, no freeze-thawing burozem, freeze-thawing
1
10
100%
100
burozem, no freeze-thawing burozem, freeze-thawing
90%
a
b
50%
1
10
100%
100
burozem, no freeze-thawing burozem, freeze-thawing
90%
a
b
a
80%
b
b
60%
50%
a
a
70%
a
60%
50%
100
b
b
a
1
120 day 100%
a
70%
50%
315
a b
80%
a
a
a 70%
70%
70%
60%
60%
60%
a
a
b
80%
a
a
a
a
70%
b 50%
1
10
100
50%
60%
50% 1
10
50%
100
1
Concentration (µg
10
100
1
10
100
g-1)
Fig. 4. Extraction efficiency by 10,000 mg L−1 SDBS of 1, 10, and 100 μg g−1 phenanthrene with or without freeze–thawing cycles in phaeozem and burozem during various time periods. Values are the means of the duplicates. Error bars represent the maximum and minimum. Values among different freeze–thawing treatments followed by the same letter (a–b) were not significantly different at p b 0.05.
observed only after day 1. While for pyrene in phaeozem, a decrease was found from the 1st day. According to their Freundlich exponential coefficient n (Table S1), the values of phenanthrene in phaeozem and burozem
1 day
90%
phaeozem, no freeze-thawing phaeozem, freeze-thawing
80% 70%
a b
60%
Extraction efficiency
b
a
30 day
phaeozem, no freeze-thawing phaeozem, freeze-thawing
60%
a
a
10
1
100% burozem, no freeze-thawing burozem, freeze-thawing
b
90%
a
80%
b a
10
100%
burozem, no freeze-thawing burozem, freeze-thawing
90% 80%
50%
b
100
b
40%
a
a
b
b
b
50%
1
10
100%
100
burozem, no freeze-thawing burozem, freeze-thawing
80%
a
a
60%
a 50%
50%
10
60%
b
a
70%
b
40%
1
10
100%
100
burozem, no freeze-thawing burozem, freeze-thawing
90%
a
80%
b
70%
a
60%
a
a
a b
phaeozem, no freeze-thawing phaeozem, freeze-thawing
70%
a
60%
b
90%
70%
1
100
a
b
a
50%
100
90% 80%
a
40% 1
120 day
phaeozem, no freeze-thawing phaeozem, freeze-thawing
70% 60%
a
50%
40%
90% 80%
a b
70%
a
50%
70%
8 day
90% 80%
a
were both smaller than the values of pyrene, indicating that more phenanthrene was sorbed on rubbery domain (partitioning), which will then cause the later start of the slow kinetics. The extraction efficiency then
a
b
50%
a
10
100 1 Concentration(µg g-1)
b a
b
40%
40%
1
a
60%
30%
10
100
1
10
100
Fig. 5. Extraction efficiency by 10,000 mg L−1 SDBS of 1, 10, and 100 μg g−1 pyrene with or without freeze–thawing cycles in phaeozem and burozem during various time periods. Values are the means of the duplicates. Error bars represent the maximum and minimum. Values among different freeze–thawing treatments followed by the same letter (a–b) were not significantly different at p b 0.05.
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decreased after freeze–thawing. For 100 μg g−1 phenanthrene and pyrene contaminated soils, the extraction efficiency decreased except in burozem at the initial 1st and 8th days. The different observations between the low concentration and high concentration might probably be attributed to the onset of the slow kinetics of sorption, which was caused by concentration effects (Divincenzo and Sparks, 1997). The lower the concentration is, the later the slow kinetics starts. The effects of the frequency of freeze–thawing cycles on the extraction efficiency of 1, 10 and 100 μg g−1 phenanthrene and pyrene in soils on the 30th and 120th days were investigated (Figs. 6 and 7) because it is common that soils experience several freeze–thawing cycles during winter. It is difficult to draw a general conclusion concerning the relationships with the extraction efficiency and the frequency of the freeze–thawing cycles because the extraction efficiency seems to fluctuate among different frequency cycles. However, in most cases, the extraction efficiency of no freeze–thawing cycle was higher than the other frequency cycles. Furthermore, similar to the extraction efficiency change, fluctuation extents of phenanthrene and pyrene in both soils also followed the order: 1 μg g−1 > 10 μg g−1 > 100 μg g−1.
contact time, physicochemical properties of soils and chemicals, the concentration of chemicals and the frequency of freeze–thawing cycles. As soil samples we used were air-dried and all contained soil organic matter (SOM) that was higher than 0.1 wt.%, we take SOM as the main sorption domain for phenanthrene and pyrene (Schwarzenbach and Westall, 1981; Weber and Huang, 1996). To further discuss the mechanisms of how freeze–thawing cycles affect the extractability of chemicals in soils and what roles contact time, physicochemical properties of soils and chemicals, the concentration of chemicals and the frequency of freeze–thawing cycles play in the whole sorption processes, we proposed a conceptual model (Fig. 8) of the mechanisms involved which is an expansion of the concept presented by Xing and Pignatello (1997), Lu and Pignatello (2004), Braida et al. (2003) and Lennartz and Louchart (2007). Adsorption to rubbery phase is a rapid, reversible process and the binding to the inner glassy section is a slower process that is limited by the diffusion controlled transport to the location of the reaction. Hence the extractability of phenanthrene and pyrene is mainly affected by the binding to the inner glassy sections. Freeze– thawing process mainly acts in the following ways. (1) The temperature will influence the diffusion process in SOM according to Arrhenius equation (Pilorz et al., 1999):
4. Discussion The following summarizes the findings here: (1) there were more high energetic sites in burozem than in phaeozem. (2) The sorption equilibrium time of high concentration chemical in soil was shorter than low concentration. (3) Slow sorption processes of organic compounds in soil were affected by contact time, physicochemical properties of soils and chemicals and the concentration of chemicals. (4) Freeze–thawing cycles affected extractability of chemicals in soils. The results may vary with
100%
where D is the diffusion coefficient, Ea is activation energy, R is the gaseous constant, T is the absolute temperature, and D0 is the diffusion coefficient obtained when extrapolating to very high temperatures. As freeze–thawing process contains a period that provides low temperature, hence it will decrease the
100%
30 days, Phe, Phaozem
freeze-thawing cycles=0 freeze-thawing cycles=1 freeze-thawing cycles=4 freeze-thawing cycles=6
90%
Extraction efficiency
D ¼ D0 exp ð−Ea =RTÞ
80%
70%
70%
60%
60%
100%
1
10
50%
100
100%
120 days, Phe, Phaozem
freeze-thawing cycles=0 freeze-thawing cycles=1 freeze-thawing cycles=4
90%
80%
70%
70%
60%
60%
1
10
1
10
100
120 days, Phe, Burozem
freeze-thawing cycles=0 freeze-thawing cycles=1 freeze-thawing cycles=4
90%
80%
50%
freeze-thawing cycles=0 freeze-thawing cycles=1 freeze-thawing cycles=4 freeze-thawing cycles=6
90%
80%
50%
30 days, Phe, Burozem
50% 100
1
10
100
Concentration (µg g-1) Fig. 6. Extraction efficiency by SDBS of 1 μg g−1, 10 μg g−1 and 100 μg g−1 phenanthrene in phaeozem and burozem under different frequencies of freeze–thawing cycle on the 30th and 120th days. Values are the means of the duplicates. Error bars represent the maximum and minimum.
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90%
90%
30 days, Pyr, Phaozem
freeze-thawing cycles=0 freeze-thawing cycles=1 freeze-thawing cycles=4 freeze-thawing cycles=6
80%
Extraction efficiency
317
120 days, Pyr, Phaozem
freeze-thawing cycles=0 freeze-thawing cycles=1 freeze-thawing cycles=4
80%
70%
70%
60%
60%
50%
50%
40%
40% 30%
30% 1
10
100
1
90%
10
90%
30 days, Pyr, Burozem
freeze-thawing cycles=0 freeze-thawing cycles=1 freeze-thawing cycles=4
80%
80%
70%
60%
60%
50%
50%
40%
40%
30%
freeze-thawing cycles=0 freeze-thawing cycles=1 freeze-thawing cycles=4
120 days, Pyr, Burozem
70%
100
30% 1
10
100
1
10
100
-1
Concentrations ( ug g ) Fig. 7. Conceptual diagram of the proposed interactions between SOM and organic chemical molecules with or without freeze–thawing at the process of short time sorption and long time sorption, respectively. At short time sorption stage, freeze–thawing mainly decreased the diffusion rate of the molecules. At long time sorption stage, freeze–thawing processes promote chemical molecules' sorption to those sites that require high activated energy in glassy phase. During freeze–thaw process, the glassy components can swell and thus more accessible to chemicals compared to those without freeze–thawing. The transient expansions in glassy domain of SOM, will inevitably slow down the desorption of organic chemicals in soil.
diffusion rate. Thus in this aspect, freeze–thawing process will increase the extraction efficiency. (2) Sorption is an exothermic reaction. Steinberg et al. (1987) found that the release of EDB increased as the temperature increased. Piatt et al. (1996) used batch and column techniques to investigate naphthalene, phenanthrene, and pyrene on low organic carbon sediments and discovered that the desorption rate constants decreased 1.2–2.6 times with temperature decreasing from 26 °C to 4 °C. Xing and Pignatello (1997) studied the sorption of 1,2-dichlorobenzene, 2,4-dichlorophenol and the herbicide metolachlor on poly(vinyl chloride) and soil, they suggest that sorption isotherm becomes increasingly linear from 6 to 90 °C. Hence, the temperature decrease during freeze– thawing process will promote chemical molecules' sorption to those sites that require high activated energy in glassy phase. (3) SOM, as a mixture of natural macromolecules consisting of rubbery (expanded, fluid-like) and glassy (condensed, relatively rigid) components, spans a range of glass transition temperatures, Tg (Leboeuf and Weber, 1997). During freeze–thaw process, the glassy components can swell and thus more accessible to chemicals compared to those without freeze–thawing. The transient expansions in glassy domain of SOM, will inevitably slow down the desorption of organic chemicals in soil.
In a word, freeze–thawing cycle is a complicated process that may either increase or decrease the extraction efficiency. According to the conceptual model (Fig. 8), depending on which mechanism takes a dominant role, we need to know the exact aging extent of the chemicals in soils. At short time sorption stage, most of the chemical molecules remain non-diffused (adsorbed in outer domain), freeze– thawing mainly decreased the diffusion rate of the molecules corresponding to the increased extraction efficiency. The concentration of chemicals will affect the onset of the slow sorption. The higher the concentration is, the earlier chemicals reached to the glassy domain. Hence, for 100 μg g −1 phenanthrene and pyrene contaminated soils, the extraction efficiency decreased except in burozem at the initial 1st and 8th days. At long time sorption stage, the diffusion rate is very low, freeze–thawing processes mainly promoted chemical molecules' sorption to those sites that require high activated energy in glassy phase. The expansion in glassy domain of SOM, will also increase the sequestration of organic chemicals in soil. Thus, the extraction efficiency of no freeze–thawing cycle was higher than the other frequency cycles at 30 and 120 days. An important implication of the freeze–thawing conceptual model is that much more attention should be taken into those newly contaminated soil sites that experience freezing and thawing, because the increased extractability of HOCs will give rise to the environmental risks
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extracted
glassy domainexpanded
extracted Rubbery Phase
Glassy Phase
Holes extracted
extracted
Fig. 8. Conceptual diagram of the proposed interactions between SOM and organic chemical molecules with or without freeze–thawing at the process of short time sorption and long time sorption, respectively. At short time sorption stage, freeze–thawing mainly decreased the diffusion rate of the molecules. At long time sorption stage, freeze–thawing processes promote chemical molecules' sorption to those sites that require high activated energy in glassy phase. During freeze–thaw process, the glassy components can swell and thus more accessible to chemicals compared to those without freeze–thawing. The transient expansions in glassy domain of SOM, will inevitably slow down the desorption of organic chemicals in soil.
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