Environ. Sci. Technol. 1997, 31, 2218-2222
Elution of Pyrene from Activated Carbon into an Aqueous System Containing Humic Acid MASAMI FUKUSHIMA,† KIYOSHI OBA, SHUNITZ TANAKA,* KEN NAKAYASU, HIROSHI NAKAMURA, AND KIYOSHI HASEBE Division of Materials Science, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, Japan
The elution of pyrene (Pyr) into an aqueous solution containing humic acids (HAs) was investigated using activated carbon (AC) as an adsorbent of Pyr. The apparent water solubility of Pyr that adsorbed to the AC was increased by the presence of HAs. To quantitatively interpret the water solubility enhancement of Pyr in the presence of HAs, the partition coefficient of Pyr was evaluated using a micellelike partition model that took into account the partition of Pyr between water and the HA phase. The relationship between the apparent water solubility of Pyr and the concentration of HA was not linear, as expected by that model. To explain this result, we introduced a Langmuir isotherm that assumed the re-adsorption of Pyr on the AC in a micelle-like partition model. The experimental data set fitted well to the derived equation, and the partition and adsorption coefficients were evaluated based on this model. The elution of trace amounts of Pyr from the adsorbent as affected by the presence of dissolved organic carbon was simulated using the derived coefficients.
TABLE 1. Elemental Composition of Humic Acids HAs
origin
WHA
%C %H %N %O
commercial (Wako Pure Chemicals) SHA peaty soil (Shinshinotsu) BHA peaty soil (Bibai) SAHA peaty soil (Sarobetsu peatery)
63.0 2.8
1.4 32.8
52.2 5.4 57.3 5.7 58.4 4.7
2.1 40.3 2.9 34.1 3.6 33.3
NOS (11). Particulate matter in an aqueous environment can act as an adsorbent for DOCs, such as HAs, which have surface-active characteristics (12). Elution of NOS, such as PAH, from particulate matter might be complicated as a result of such an adsorption. Although numerous studies concerning the enhancement in water solubility of PHAs by the presence of HAs have been published (13-16), the data were largely interpreted based on a simple aqueous system without particulate matter. Moreover, only a few studies have been reported relative to the adsorption of PAHs to solids from an aqueous solution of DOCs (17, 18). The elution of PAH from the PAH-contaminated particulate matter has been insufficiently studied especially from aqueous systems that contain HAs. In the present work, the elution of PAH from activated carbon (AC) was investigated in the presence of HAs. Activated carbon was used for the adsorbent of PAH because of its ability to strongly bind PAHs as well as other organic matter (19-21). Pyrene is highly reactive and is one of the predominant PAHs present in the environment. For example, Pyr is observed as a metabolite in finfish that has been exposed to PAH originating from combustion processes (22). Pyrene, therefore, was used as an example of PAHs in the present work. The partition of Pyr between water and the AC and HA phases was quantitatively interpreted by taking into account the adsorption of HAs on the AC.
Experimental Section Introduction Polycyclic aromatic hydrocarbons (PAHs) are toxic and hazardous organic pollutants in the aqueous environment. These are released via a variety of pathways (1-5). Seepage from underwater petroleum is thought to constitute a major input for marine PAHs (1). PAHs also originate from refinery effluent (2), diesel fuel (3), and coal tar (4). Moreover, airborne PAHs, which are adsorbed on particulate matter, can be deposited into lakes, and these result in the pollution of the aqueous environment (5). The behavior and fate of various nonpolar organic solutes (NOS) including PAHs, which are adsorbed on particulate matter, are closely related to the distribution and transportation of pollutants between various compartments in aqueous systems. Dissolved organic carbon (DOC), such as humic acids (HAs) (6, 7), can facilitate the distribution and transportation of NOS. Chiou et al. (8-10) showed an enhancement in the water solubility of NOS, such as DDT, by the presence of HAs. This observation strongly suggests that DOC may play an important role in the diffusion of NOS from particulate matter into an aqueous environment. Particulate matter represents another compartment that may be involved in the distribution and transportation of * Corresponding author telephone: +81-11-706-2219; fax: +8111-716-6101; e-mail address:
[email protected]. † Present address: Department of Hydrospheric Environmental Protection, National Institute for Resources and Environment, 16-3, Onogawa, Tsukuba-shi, Ibaraki 305, Japan.
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Materials. The HAs were extracted and purified from peat soils (Shinshinotsu, Bibai, and Sarobetsu in Hokkaido) and a commercial product (Wako Pure Chemicals) according to a protocol recommended by the International Humic Substances Society (IHSS) (23). These are referred to as SHA, BHA, SAHA, and WHA, respectively. These origins and elemental compositions are summarized in Table 1. The commercial HA (WHA) had a higher carbon content and a lower nitrogen content than the peat HAs. The AC powder was obtained from Wako Pure Chemicals (special reagent grade, produced from coal) and used without further purification. Preparation of Activated Carbon Adsorbing Pyr. A test adsorbent of Pyr used in the present work (AC-Pyr) was prepared by adsorbing Pyr on the AC. The mixture of 1 g of AC powder and 100 mL of 10 mM Pyr in ethanol solution was stirred for 24 h. Subsequently, this mixture was filtered through a silica-fiber filter (Advantec QR-80), in which 97% of 0.3-µm particles (dioctyl phthalate) could be collected. The powder of AC-Pyr on the filter was then collected. After being air-dried in the dark, the powder was transferred into a brown bottle and stored in a desiccator over silica gel. The amount of Pyr in the AC-Pyr was estimated by measuring Pyr in the filtrate, which indicated that the AC-Pyr contained 0.6 mmol of Pyr g-1 of AC. Moreover, the FTIR spectra of AC, Pyr, and AC-Pyr (Figure 1) were observed using a KBr disk containing 0.3% (w/w) of the samples. Analysis of Pyr Dissolved in the Aqueous Solution. A 0.01-g sample of the AC-Pyr was placed into a test tube, and
S0013-936X(96)00722-5 CCC: $14.00
1997 American Chemical Society
FIGURE 1. FTIR spectra of AC, Pyr, and AC-Pyr. then 10 mL of an aqueous solution of HA, adjusted to pH 6 with acetate buffer, was added. After shaking the test solution at 25 °C in the dark, it was filtered through a 1.2-µm membrane filter. The filtrate, containing water-soluble Pyr, was transferred into a separatory funnel, and the Pyr was twice extracted with 5 mL of hexane. The concentration of Pyr in the hexane solution was measured by spectrofluorimetry. The concentration of HA in the filtrate was measured by visible absorbance at 400 nm (24), and then HA adsorbed to the AC was calculated by subtracting the amount of HA in the filtrate from the total initially added. The concentration of HA in the aqueous solution was represented as the concentration of DOC, [DOC]aq, according to the previous work (25).
Results and Discussion Adsorption of Pyr on the AC. Figure 1 shows FTIR spectra of Pyr, AC-Pyr, and AC, respectively. It is known that the FTIR spectra of PAHs have specific strong bands corresponding to the C-H deformation of aromatic hydrocarbon at 900675 cm-1 (26). The strong peaks at 830, 730, and 700 cm-1 (solid arrows in Figure 1), which are assigned to those C-H deformations, were observed in the spectrum of Pyr. These peaks also appeared in the spectrum of AC-Pyr but not in the AC alone. This clearly demonstrates that Pyr is adsorbed to the AC. However, sharp peaks at 1800-900 cm-1 of Pyr were not observed in the spectrum of AC-Pyr. This appeared to be due to the small amount of Pyr adsorbed to the AC. The amount of Pyr adsorbed to the AC was evaluated by measuring Pyr in the filtrate (120 mg g-1 AC). This corresponds to 10% (w/w) of Pyr used in the measurement of the FTIR spectrum of Pyr itself. Hence, the peak absorbance of Pyr on the AcPyr was approximately 10 times smaller than that of Pyr. On the other hand, the broad peaks at 1650 and 1640 cm-1 were observed in both the AC-Pyr and the AC. These peaks can be attributed to aromatic CdC stretching (27). If the aromatic moieties in AC would contribute to the interaction of Pyr, these peaks would be expected to shift significantly. However, a shift in these peaks was not observed. Thus, the adsorption of Pyr on the AC was not due to chemical interaction with aromatic moieties, but rather due to a physical interaction or permeation into the pores as described in the literature (21). Effect of Shaking Time on the Elution of Pyr and the Adsorption of HA. Figure 2a shows the dependency of apparent water solubility of Pyr (Sw*) on shaking times of the test solution containing WHA and AC-Pyr. Ten milliliters of mixtures of AC-Pyr and WHA was prepared for each shaking time, and Pyr eluted by the aqueous solution was measured as described in the Experimental Section. In the absence of WHA, the apparent water solubility was 6 × 10-9 M. However,
FIGURE 2. Effect of shaking time on Sw* (a) and [DOC]aq (b); WHA, 100 mg L-1, pH 6 (0.02 M acetate buffer). the Sw* value increased in the presence of 100 mg L-1 WHA, and this value reached 5 × 10-7 M. This was 100 times larger than that without WHA. These results demonstrate that the presence of HA contributes to the enhancement of elution of Pyr from the AC-Pyr. On the other hand, it is generally known that AC is capable of adsorbing a variety of organic matter (19-21). Hence, it could be expected that large amount of HAs adsorbs to the AC. The WHA remaining in the aqueous solution decreased during the shaking time, reaching a plateau after 40 h (Figure 2b). In the presence of 100 mg L-1 WHA, approximately 30% of WHA was adsorbed to the AC. This shows that the adsorption of HA on AC may seriously influence the partition equilibria by Pyr between water and the HA phase. Although the Sw* values were uneven (Figure 2a), the elution of Pyr from the AC-Pyr reached equilibrium rapidly with WHA. However, the adsorption of the HA on the AC reached equilibrium after 40 h as shown in Figure 2b. Therefore, in view of this, the apparent water solubility of Pyr was measured after shaking the test solution for 48 h. Evaluation of Partition and Adsorption Coefficients. To quantitatively interpret the enhancement in water solubility of NOS by the presence of HAs, the partition coefficients of NOS were evaluated by assuming a micelle-like partition with water and the HA phase. In this model presented by Chiou and co-workers (8-10), the Sw* values increased with [DOC]aq as shown in
Sw* ) Sw0(1 + Kdoc[DOC]aq)
(1)
where Sw0 and Kdoc denote the water solubility of NOS without
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FIGURE 3. Relationships between [DOC]aq and Sw*. Dotted points, experimental data set; lines, curve-fitting to eq 8, pH 6 (0.02 M acetate buffer). DOC and the partition coefficient between water and HA phase, respectively. Equation 1 shows the linear relationships between Sw* and [DOC]aq, and this has also been used to evaluate the Kdoc values of PAHs (13-16). Figure 3 shows the relationship between Sw* and [DOC]aq in the presence of ACPyr. These were not linear relationships, as expected in eq 1 and quickly reached a plateau. In the absence of HA, Pyr was dissolved in aqueous solution only in amounts consistent with the dissolution equilibrium. In the presence of HA, Pyr in the aqueous phase would diffuse into the HA phase according to a micelle-like partition equilibrium, and the Sw* value would be larger than that without HA as expected by eq 1. However, if the adsorption equilibrium of the Pyr species partitioning into HA phase (HA-Pyr) on AC is also considered, the Sw* value with AC would be smaller than for the system without AC. Thus a substantial Sw* value in the presence of AC-Pyr is obtained by subtracting HA-Pyr adsorbing on AC ([Pyr]HAAC) from the Sw* value in the absence of AC-Pyr was shown in
Sw* ) Sw0(1 + Kdoc[DOC]aq) - [Pyr]HAAC
[DOC]ads/[DOC]max 1 - [DOC]ads/[DOC]max
[DOC]add
[DOC]max aHA )
1 + aHA [DOC]add
9
log Kdoc
log aHA-Pyr
log aHA
log e280
WHA SHA BHA SAHA
4.9 4.3 4.4 5.2
1.6 1.6 1.8 2.8
1.8 1.8 1.7 2.7
3.0 2.7 2.6 3.2
HA-Pyr on AC is probably similar to that of an HA molecule. In order to confirm this, the absorbance at 400 nm of HA was compared with that of HA-Pyr. The absorbance for 50 mg L-1 HA was not different from that for the filtrate after shaking for 48 h with Pyr. This shows that the binding of Pyr with HA does not affect the adsorption of HA on AC. The adsorption of HA-Pyr was treated as the adsorption of HA by using the Langmuir isotherm. The coverage of HA to the AC surface, θHA ()[DOC]ads/[DOC]max), corresponds to that of HA-Pyr (θHA-Pyr), and eq 3 can be replaced by on for the HA species, HA-Pyr, to give
aHA-Pyr[DOC]add )
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 8, 1997
θHA-Pyr 1 - θHA-Pyr
(5)
Since the term (1 - θHA-Pyr)[DOC]add corresponds to [DOC]aq, the following relation could be derived from the rearrangement of eq 5:
aHA-Pyr[DOC]aq ) θHA-Pyr
(6)
Moreover, the total amount of Pyr partitioning into HA in the absence of AC corresponds to Sw* - Sw0, and the following equation can be written by combining with eq 6:
[Pyr]HAAC ) (Sw* - Sw0)aHA-Pyr[DOC]aq
(7)
Equation 2 can be rearranged by using eq 7 in the following form:
(4)
Equation 4 denotes the relationship between [DOC]add and ([DOC]add - [DOC]aq)/[DOC]add. The adsorption coefficient of HA on the AC (aHA) can then be evaluated by a nonlinear least square regression analysis of the relationships between [DOC]add and ([DOC]add - [DOC]aq)/[DOC]add (Figure 4). These results are summarized in Table 2. On the other hand, HA and HA-Pyr species would adsorb on the AC. However, since the Pyr molecule is a much smaller molecule than an HA molecule, the adsorption behavior of
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HAs
(3)
where aHA, [DOC]ads and [DOC]max represent the experimental adsorption coefficient of HA, the concentration of DOC adsorbing on AC, and the maximum adsorption concentration of DOC, respectively. The [DOC]ads value can be calculated by subtracting [DOC]aq remaining in the aqueous solution from that added initially ([DOC]add). Therefore, the Langmuir isotherm for eq 3 was rearranged into
[DOC]add - [DOC]aq
TABLE 2. Evaluation of Various Parameters for HAs
(2)
The adsorption of organic matter on AC has usually been interpreted by a Langmuir isotherm (21), and the adsorption of HA on AC can be represented by
aHA[DOC]add )
FIGURE 4. Langmuir plots of the humic acids. Dotted points, experimental data set; lines, curve-fitting to eq 4, pH 6 (0.02 M acetate buffer).
Sw* )
Sw0(1 + Kdoc[DOC]aq + aHA-Pyr[DOC]aq) 1 + aHA-Pyr[DOC]aq
(8)
Equation 8 shows the relationships between Sw* and [DOC]aq, and Kdoc and aHA-Pyr could be evaluated by a nonlinear least square regression analysis of the experimental data set. The solid lines in Figure 3 show the calculated curves by using eq 8, and these curves were fitted excellently to the experimental data points. The evaluated Kdoc and aHA-Pyr values are summarized in Table 2. The errors for twice analyses were in the range of
FIGURE 5. Theoretical [DOC]aq vs Sw* curves calculated using eqs 1 and 8. Parameters used for calculation: log Kdoc, 5.2; log aHA-Pyr, 2.8 for SAHA. Dotted line, without adsorbent (calculated by eq 1); solid line, with adsorbent (calculated by eq 8). 0.1-0.3. The log Kdoc values of SAHA and WHA were larger than those of BHA and SHA. From the elemental composition in Table 1, it is clear that the carbon to hydrogen ratio (C/H) of WHA is the largest among four HAs. This suggests that WHA contains large amounts of unsaturated carbons, since the C/H ratio is a simple index of degree of unsaturation (28). Moreover, Gauthier et al. (29) showed that the partition coefficient of Pyr for various HAs increased with the contents of their aromatic moieties. The absorption at 280 nm (e280) is one index of the aromatic contents of HAs (30). From the log e280 values in Table 2, the aromatic contents of SAHA and WHA were larger than those of SHA and BHA. The larger log Kdoc values for WHA and SAHA also correspond to larger amounts of aromatic moieties. Moreover, the log aHA-Pyr values of the HAs were similar to each other, except for SAHA. If the hypothesis in eq 5 is valid, the log aHA-Pyr values would be in good agreement with the log aHA values evaluated by the Langmuir plot using eq 4. As shown in Table 2, these values are, in fact, in good agreement. This supports the conclusion that log aHA-Pyr values can be evaluated by the presented, model which takes into consideration the re-adsorption of HA-Pyr. On the other hand, for the case of the large value of log Kdoc, the value for WHA, the Sw* increased steeply with [DOC]aq. Although the log Kdoc value of SAHA was larger than that of WHA, the Sw* values of SAHA were smaller than those of the WHA. However, the log aHA-Pyr value for the SAHA was much larger than that of the WHA. Therefore, the smaller value of Sw* for the SAHA is attributed to large amounts of HA-Pyr species being adsorbed on the AC-Pyr. Moreover, a comparison of log aHA-Pyr and log aHA values shows that the adsorption behavior of HA-Pyr on AC is similar to the HA molecule. These results led to the conclusion that the elution of Pyr from the AC-Pyr in the presence of HAs was explained by both interactions of the partition of Pyr into the HA phase and re-adsorption of HA-Pyr species on the AC-Pyr. Simulation of Sw* vs [DOC]aq Curves. Distribution and transportation of PAHs from particulate matter would be complicated by various matrixes in an aqueous environment. Moreover, since the concentrations of Pyr in the particulate matter (several tens of µg g-1 order) are much smaller than that in the AC-Pyr (31), the elution behavior of Pyr will be difficult to observe in aqueous systems. Therefore, we attempted to simulate the elution behavior of trace amounts of Pyr adsorbing on the particulate matter by using the simple model as described in eq 8. First, the amount of Pyr eluted from the adsorbent in the absence of DOC needs to be calculated. The distribution coefficient of Pyr between AC and water (KwAC) could be calculated by using the Sw0 value and the amount of Pyr in
the AC-Pyr (1 × 105). Assuming that Pyr was eluted from the adsorbent containing 50 µg g-1 Pyr into water, the Sw0 value could be calculated by using the KwAC value (2.5 × 10-12 M ≈ 0.5 ng L-1). In the presence of DOC, the water solubility enhancement of Pyr could be simulated by using eq 1 without adsorbent and eq 8 with adsorbent. The Sw* vs [DOC]aq curves were simulated by using log Kdoc and log aHA-Pyr values for the various HAs in Table 2. Moreover, since it was reported that concentrations of DOC in natural waters were in the range of 0.1-25 mg L-1 (32, 33), the simulations were performed in this range. One of the examples for the simulation curves is shown in Figure 5. In the lower [DOC]aq region (10 mg L-1), the Sw* value calculated using eq 1 was 10-50% larger than that calculated by eq 8, which took into account the re-adsorption of HA-Pyr on the adsorbent. The simulation curves showed that the effect of the re-adsorption on the Sw* values was substantial in the higher [DOC]aq region. In the case of trace Pyr in natural adsorbent, the amount of Pyr eluted into the aqueous systems would be too small to detect. Therefore, the derived equation that considered the re-adsorption of HA-Pyr was useful for estimating enhancement of water solubility by Pyr from natural adsorbent by DOCs such as HAs.
Acknowledgments The authors would like to acknowledge the Japanese Ministry of Education, Science and Culture for their support of this work under a Grant-in-Aid for scientific research (07558205). We would also like to thank Dr. K. Sasaki in the Faculty of Technology, Hokkaido University, for her guidance of recording FTIR spectra and useful discussion.
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Received for review August 20, 1996. Revised manuscript received March 28, 1997. Accepted April 2, 1997.X ES960722J X
Abstract published in Advance ACS Abstracts, June 1, 1997.
