Hisanori Iwai*a,
Rodrigo Mundob and
Seiya Nagaoa
aLow-Level Radioactivity Laboratory, Institute of Nature and Environmental Technology, Kanazawa University, Nomi 923-1224, Japan. E-mail: h-iwai@se.kanazawa-u.ac.jp
bDivision of Material Chemistry, Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan
First published on 25th August 2021
The use of a glass fiber filter coated with polyethyleneimine (PcGF) for partitioning dissolved polycyclic aromatic hydrocarbons (PAHs) that are associated with humic substances (HSs) is reported. The PAHs pass through the PcGF, while HS-associated PAHs are trapped by electrostatic interaction between the HSs and the PcGF. Based on this strategy, free- and associated-PAHs can be separated by simple filtration. Approximately 60–90% of the deuterated benzo[a]pyrene (BaP-d12) that was added to the sample solutions was in the associated form with soil type HAs, while the percentages were lower in the case of aquatic HA (ca. 25%) and FAs (ca. 10–15%). Strong correlations (R2 = 0.84–0.90) were observed between the %-association for deuterated pyrene (Pyr-d10) or BaP and the degree of HS's aromaticity (logE280), regardless of the HS fractions or their origins. The separation technique was used to evaluate the association coefficient (logKassoc) and the capacity (Cassoc) for soil type HAs based on a Langmuir adsorption model. The logKassoc values were not highly dependent on the origin of the HA (ca. 3.5–4.5). The BaP-d12 and Pyr-d10-Cassoc values for the HA derived from compost were more than one order larger than the corresponding values for peat. The findings indicate that Cassoc values vary with the origin of the HA and affect the environmental behavior of PAHs. The present study reports on the development of a simple partitioning technique that does not require any special training and equipment.
Polycyclic aromatic hydrocarbons (PAHs), which are mainly released by the incomplete combustion of fossil fuels and organic matter, are well-known examples of hydrophobic organic pollutants.6–8 They have mutagenic and carcinogenic characteristics,6,7 thus making them important in regards to human health as well as other organisms.8 Because of this, the U.S. Environmental Protection Agency (USEPA) has listed 16 such components as priority pollutants.9 It was reported that 331–818 Gg of 16 PAHs was globally released into the atmosphere in 2007.9 After atmospheric release, the PAHs are deposited on terrestrial and aquatic environments where the HSs then become associated with aromatic species.4,10,11 Therefore, a knowledge of the affinity of HSs to be associated with PAHs would allow us to better understand their behavior and fate. The traditional methodology for estimating the association coefficient of HSs–PAHs involves fluorescence-quenching methods,10,12,13 the equivalent-dialysis methods,14 solubility enhancement methods,15 and reversed-phase separation.16,17 A tandem-cartridge technique was also recently proposed.11 All of the above methods have experimental advantages and disadvantages. For example, while fluorescence quenching represents an easily and commonly used method, it frequently overestimates associability values due to quenching, such as by collision, dissolved oxygen, and inner filtration. Because of these drawbacks, it is necessary to select methods that take sample properties, experiment situations, equipment, and cost into consideration. In addition, the development of new methodologies should also have the potential for use in future studies.
As described above, HSs contain acidic functional groups, such as carboxylic acid and phenolic-OH groups, and are typically negatively charged except under strongly acid conditions. As a result, they can be adsorbed on positively charged surfaces by the electrostatic interactions. This phenomenon is a common occurrence in the natural environment. For example, metal ions are adsorbed on clay minerals and dissolved organic matter in soil and aquatic environments.18,19 We previously reported on the functionalization of a glass fiber filter by coating it with a cationic polymer. The resulting filter was used to trap HSs on its surface by electrostatic interactions,20 while uncharged dissolved organic matter, such as PAHs, pass through these filters. In cases where HSs are associated with the PAH fraction, they could be trapped on the filter along with HSs. It would therefore be expected that free-PAHs could be separated from HSs by filtration using a filter coated with a cationic substance. Based on this, we examined the use of a glass fiber filter as a possible separation method. Separating free-PAHs and associated-PAHs was examined by a simple filtration through a filter coated with polyethyleneimine, a cation polymer. In addition, to investigate the influence of HSs on the solubility of PAHs, three types of humic (HA) and fulvic acids (FA) from different origins (bark-compost, peat soil, and groundwater) were employed in this work. The association coefficient and capacity of HAs to associate with PAHs were examined using this electrostatic separation technique.
d-PAHs | Abbreviations | Concentrations | Water solubilityb at 25 °C (ng L−1) | |
---|---|---|---|---|
Mix solution (ng μL−1) | Samplea (ng L−1) | |||
a The case of a 100 μL of d-PAHs mix was added to 200 mL of HS solution.b From Tobiszewski and Namieśnik (2012).21 | ||||
Naphthalene-d8 | Nap-d8 | 3.46 | 1730 | 3.1 × 107 |
Acenaphthene-d10 | Ace-d10 | 1.24 | 620 | 1.6 × 107 |
Phenanthrene-d10 | Phe-d10 | 2.32 | 1160 | 1.1 × 106 |
Pyrene-d10 | Pyr-d10 | 0.13 | 65 | 1.3 × 105 |
Benzo[a]pyrene-d12 | BaP-d12 | 0.135 | 67.5 | 1.5× 103 |
Fluorescence spectra of 10 mg L−1 solutions of HcHA and ApHA, 1 mg L−1 solution of MgHA, and 0.5 mg L−1 solutions of FAs diluted in 0.05 M Tris–HCl buffer (pH 7.00) were measured using a fluorescence spectrometer F-7100 (Hitachi, Japan). The fluorescence properties were estimated based on the humification index (HIX) and biological index (BIX). HIX is the ratio of the integral fluorescence intensities within the emission wavelength from 400–480 nm to 330–345 nm at an excitation wavelength of 254 nm.26 BIX is the ratio of the fluorescence intensity at 380 nm to 430 nm of the emission wavelengths which were excited at 320 nm.27
The PAHs were analyzed by HPLC, as described in previous reports.28,29 The system was comprised of an LC-20AT, CTO-20A column oven, and RF-20Axs fluorescence detector (Shimadzu, Japan). An inertsil ODS-P column (diameter 4.6 mm, length 250 mm, 5 μm, 30 °C) was employed for the separation. The wavelength pares of excitation/emission (nm) to detect PAHs were follows: 280/340 for Nap-d8, Ace-d10 and Phe-d10; 331/392 for Pyr-d10; 264/407 for BaP-d12. The mobile phase was a mixture of acetonitrile and ultrapure water operated under a gradient elution from 55–99% of acetonitrile in ultrapure water at a flow rate of 1 mL min−1.
Fig. 2 shows the HS absorption behavior on a PcGF, which was investigated using aqueous HcHA (pH 7.00 ± 0.02 adjusted with 0.01 M Tris–HCl). More than 95% of the HcHA was adsorbed on the PcGF from 50–200 mL of a 2 mg L−1 solution of HcHA, and the ratio of adsorption (%) then decreased gradually with filtration volume (ca. 60% after 500 mL). In the case of 5 mg L−1 and 10 mg L−1 HcHA solution, the adsorption ratio exceeded 90% when their filtration volumes were under 100 and 50 mL, respectively. However, after filtering 500 mL of these solutions, the values decreased to approximately 38% and 20% for 5 mg L−1 and 10 mg L−1 aqueous solutions of HcHA, respectively (Fig. 2). Consequently, a reasonable filtration volume for adsorbing dissolved humic substances with a PcGF was determined to be 200 mL, 100 mL, and 50 mL filtration volumes for 2 mg L−1, 5 mg L−1, and 10 mg L−1 of humic substances, respectively.
The percentage of associated Pyr-d10 and BaP-d12 were calculated from their relative areas to the corresponding values for controls, and the results are shown in Fig. 4. In all HS variants, the PAHs that were associated with HA were stronger than those for FAs. The %-associated fraction of BaP-d12 varied with the origin of HAs (ca. 25–92%). Although HcHA showed the highest level of association with BaP-d12 (ca. 92%), the value for the %-associatioin for HcFA was significantly lower (ca. 12%). A similar trend was observed in the case of the other HS series. Interestingly, approximately 10–15% of the added BaP-d12 was associated with FAs regardless of their origins. The %-associated BaP-d12 for MgHA was quite small compared to those of HcHA and ApHA (ca. 60%), suggesting that MgHA has an FA-like character rather than that of an HA. Small amounts of associated Pyr-d10 were also observed but only in the cases of HcHA (ca. 8%) and ApHA (ca. 2%). These findings indicate that the behavior of PAH, in particular PAH molecules with larger numbers of rings, would be influenced by the presence of HA rather than the FA fraction.
Hardwood bark compost | Aberdeenshire peat | Mobara groundwater | |||||
---|---|---|---|---|---|---|---|
HcHA | HcFA | ApHA | ApFA | MgHA | MgFA | ||
UV-vis indexes | logE400/E600 | 0.85 | 1.38 | 0.96 | 1.28 | 1.22 | 1.34 |
logE600 | 0.776 | 0.468 | 0.577 | 0.387 | 0.563 | 0.495 | |
logE280 | 1.81 | 1.50 | 1.67 | 1.46 | 1.59 | 1.31 | |
Fluorescence indexes | HIX | 21.4 | 10.4 | 18.7 | 6.62 | 5.01 | 3.68 |
BIX | 0.416 | 0.511 | 0.359 | 0.582 | 1.00 | 0.900 |
To better understand the relationship between PAH associability (%) and the characteristic of the HSs, the %-associations of BaP-d12 and Pyr-d10 for each HS sample were plotted, as shown in Fig. 5. The percentage values of associated BaP-d12 (% BaP-assoc) and Pyr-d10 (% Pyr-assoc) increased with decreasing logE400/E600, increasing logE600 and logE280, and these relations were well-correlated exponentially (R2 = ca. 0.85–0.93). A good relation was found for HIX to % PAH-assoc (R2 = ca. 0.75–0.86). However, the plots were more spread out in the case where the X-axis was BIX (R2 < 0.35), indicating that very low levels of bacterial by-products contributed to the PAH that was associated with humic fractions. Aromatic components in the HS structure could serve as a high-affinity association site for PAHs, as evidenced by the finding that a larger aromatic content contributed to an increase in % PAH-assoc. These findings are consistent with previously reported findings.10,15,36 Chin et al. (1997) suggested that a larger molecular size could provide spaces for association with planer-shaped PAH molecules to occur.15 HSs are supermolecular structures made up of the humification of low molecular-sized precursors; the molecular size of HSs thus increases with the degree of humification.3,37 In addition, the aromaticity of HSs tends to increase with the humification process.37,38 These mechanisms provide a plausible reason for the strong correlation between the % PAH-assoc vs. the humification indexes (logE600, logE400/E600, and HIX). The degree of humification and aromaticity appear to be key determinants in the association of PAHs with HSs (Fig. 5).
Pyr-d10 | BaP-d12 | |||||
---|---|---|---|---|---|---|
logK | C (ng mg−1-HA) | r2 | logK | C (ng mg−1-HA) | r2 | |
HcHA | 3.52 | 33.9 | 0.982 | 4.20 | 503 | 0.950 |
ApHA | 4.21 | 3.10 | 0.948 | 4.46 | 134 | 0.927 |
HS + PAH ⇌ HS–PAH. | (1) |
Fig. 6 Langmuir plots of [PAH]free vs. [PAH]assoc. Upper and down are the case of Pyr-d10 and BaP-d12, squares and triangles represent the case of HcHA and ApHA, respectively. |
Eqn (1) is rewritten to the equilibrium below using the dimension of each concentration,
[HS] + [PAH]free ⇌ [PAH]assoc, | (2) |
From eqn (2), the association coefficient, Kassoc, is then expressed as shown below:
(3) |
Here, the total association capacity in HS, Cassoc, can be defined as the sum of the unassociated and associated sites in the HS, as described below:
Cassoc = [HS] + [PAH]assoc. | (4) |
The relationship between the concentrations of free and associated PAH with HS can then be expressed from eqn (2)–(4), as below:
(5) |
The values of Kassoc and Cassoc were estimated by fitting eqn (5) to the experimental plots using the least squared method. The fitting for eqn (5) is shown as solid and broken lines in Fig. 6. The fitting curves were fitted well to the plots (r2 = 0.923–0.982), suggesting that a Langmuir-type adsorption reaction is a suitable concept for explaining the association between the PAHs and HAs. Based on this concept, the results indicate that HAs contain a specific association site for PAHs, resulting in a monolayer molecular association. The evaluated associability values were summarized in Table 3. The values of logKassoc for ApHA (ca. 4.2 for Pyr-d10, 4.5 for BaP-d12) were slightly larger than those for HcHA (ca. 3.5 for Pyr-d10, 4.2 for BaP-d12). These values, however, were slightly lower compared with values obtained using the fluorescence quenching method: 4.3 or 4.96 for river HA-Pyr,13,15 the reverse phase method: 4.5–4.6 for compost soil-benzo(e)pyrene,17 and the equilibrium dialysis method: 4.9–6.3 for dissolved organic matter-BaP.36 The separation under flow conditions (3–6 mL s−1) permitted the labile species of PAH-assoc to be desorbed from the HSs that were trapped on the PcGF, and this is considered to be one of the causes of the underestimation of Kassoc compared to the other methods.13,15,17 In general, in field studies, PAHs are fractionated into particulate and dissolved-phase species, and the particulate phase is defined as the fraction trapped on a glass fiber filter (>0.5 μm).28,29 The particulate phase PAHs is regarded as the species adsorbed on the surface of the organic or inorganic particle cores. The experimental conditions used here are the same as that of field studies; the method proposed here was anticipated to be a useful approach to investigating the influence of the associability of the particulate surface on determining the distribution of PAHs in aquatic environment surveys. The Cassoc values varied depending on the sample; in both cases of Pyr and BaP, the HcHA contained a significantly larger Cassoc than that for ApHA. This result is in agreement with the highest %-PAHassoc for HcHA showed in Fig. 4. The high aromaticity of HcHA would permit it to have a large number of binding sites for PAHs. From these results, the PAH appears to be associated with a specific site in the HA molecules, and the logKassoc values were in the range of ca. 4–6, regardless of the origin of the HSs.13,15,17,18 However, the values for Cassoc varied significantly, depending on the origin of the HA. We, therefore, conclude that the adsorption capacity is the most important factor that significantly influences the behavior of PAHs in the environment rather than the logKassoc value.
In addition, the PcGF method was applied to estimate the associability of HSs with PAHs in aqueous. The Langmuir adsorption isotherm was fitted well to the relationship between the concentrations of free- and associated-PAHs (Pyr-d10 or BaP-d12), indicating that PAHs are associated with a specific site in the HSs. The values of logKassoc and Cassoc were then evaluated by fitting the isotherm. The logKassoc value was approximately 3.5–4.5, while Cassoc was significantly dependent on the origins of the HAs, suggesting that the PAHs behavior would be influenced by the magnitude of Cassoc value rather than logKassoc of HSs in the environment.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra04953d |
This journal is © The Royal Society of Chemistry 2021 |