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Sorption Selectivity in Natural Organic Matter Probed with Fully Deuterium-Exchanged and Carbonyl-13C‑Labeled Benzophenone and 1 H−13C NMR Spectroscopy Xiaoyan Cao,‡ Charisma Lattao,§ Joseph J. Pignatello,§ Jingdong Mao,*,†,‡ and Klaus Schmidt-Rohr*,∥,⊥ †
Department of Chemistry, College of Sciences, Nanjing Agricultural University, Nanjing 210095, Jiangsu, China Department of Chemistry and Biochemistry, Old Dominion University, 4541 Hampton Boulevard, Norfolk, Virginia 23529, United States § Department of Environmental Sciences, The Connecticut Agricultural Experiment Station, P.O. Box 1106, New Haven, Connecticut 06504, United States ∥ Department of Chemistry, Iowa State University, Hach Hall, Ames, Iowa 50011, United States ‡
S Supporting Information *
ABSTRACT: Specific functional-group or domain interactions of fully deuterium-exchanged, carbonyl-13C-labeled benzophenone and different types of natural organic matter (NOM) were investigated through two-dimensional 1H−13C heteronuclear correlation NMR spectroscopy. The sorbents included Beulah-Zap lignite, type II kerogen (IL-6), Pahokee peat, Amherst humic acid, and a polystyrenepoly(vinylmethyl ether) (PS-PVME) blend. PS-PVME consists of PS and PVME chains that are mixed on a scale of Beulah > PS-PVME > Pahokee peat > Amherst HA (Table S2). The Freundlich linearity index, N, follows the order IL-6 kerogen < Beulah < Pahokee peat < Amherst HA < PS-PVME. The KF and N values for 8647
dx.doi.org/10.1021/es501129f | Environ. Sci. Technol. 2014, 48, 8645−8652
Environmental Science & Technology
Article
analyzed most easily after extracting the 1H spectrum associated with benzophenone 13CO as a vertical slice from the 2D HETCOR spectrum and comparing them at different mixing times. The average spectrum of all protons in the sample, useful as a reference, can be obtained by summing the 1H spectra over the full range (0−220 ppm) of 13C chemical shifts, at tm,e = 10.25 ms, since it most closely approaches magnetization equilibrium. Lastly, the 1H spectra associated with mostly nonprotonated aromatic C (of the sorbents) are also displayed for comparison purposes. It is reasonable to hypothesize that benzophenone-(13CO)-d10 intercalates in the aromatic rings of the sorbents. If the hypothesis is valid, the proton spectra of benzophenone 13CO would be similar to those of nonprotonated aromatic C, indicating their similar chemical environments. On the other hand, a stronger alkyl proton peak associated with benzophenone 13CO relative to that associated with nonprotonated aromatic C would indicate some sorption of benzophenone near alkyl segments. Benzophenone-( 13 CO)-d 1 0 in Polystyrene-poly(vinylmethyl ether) (PS-PVME). The PS-PVME polymer blend contains contrasting aromatic−alkyl and polar−nonpolar functionalities (Figure 1(a)). The 2D HETCOR spectra (Figure 1(b-c)) indicate the proximity of aromatic and alkyl groups. For instance, as the mixing time is increased from 0.01 to ca. 0.3 ms, the PVME methoxyl and OCH C signals already develop cross peaks to PS aromatic protons (Figure 1(b-c), see PS and PVME structures in Figure 1(a)). These results confirm mixing of PS and PVME at the nanoscale. At mixing times of 0.01 ms and ca. 0.3 ms, the 1H spectra associated with the 13 CO of benzophenone at 13C chemical shifts of 195 ppm (Figure 1(d)) show major signals from aromatic protons near 7 ppm as well as weaker correlations with alkyl protons near 2 ppm. This shows that the 13CO of benzophenone is found predominantly near PS aromatic protons. The cross section of 13 CO (195 ppm) after 10 ms spin diffusion shows increasing contributions from O-alkyl protons (Figure 1(d)). This indicates transfer of magnetization from more distant O-alkyl and alkyl protons to 13 CO. The proton spectrum corresponding to nonprotonated aromatic C (146 ppm) (Figure 1(e)) shows a relatively weaker aromatic peak and equally strong alkyl peak. This shows that benzophenone 13C O is in closer proximity to the core of the aromatic regions than is the nonprotonated aromatic C, which is linked to the alkyl backbone of PS (Figure 1(a)). A possible reason for the preferential sorption of benzophenone to the aromatic “core” of polystyrene (Figure 1(f)) is a π−π EDA interaction with the aromatic rings in polystyrene: the benzophenone ring is moderately electron accepting due to the electron withdrawing ability of the Ar−CO substituent, whereas the aromatic ring in polystyrene is weakly electron donating due to the polarizability of the π electrons and induction of the alkyl substituent. Benzophenone-(13CO)-d10 in Beulah-Zap Lignite. Beulah-Zap lignite is a coal of the lowest rank and has an average polycyclic aromatic cluster size of nine carbons.30 Beulah-Zap lignite has mostly nonpolar alkyl and aromatic components but little polar alkyl carbon (Figure S1(b) and Table S1). Figure S8 shows the 2D HETCOR spectra, for three tm,e values of BeulahZap lignite sorbed with 3.2 wt % of benzophenone-(13CO)d10. These spectra and their cross sections, see Figure 2(d), show aromatic-alkyl cross peaks at all mixing times, indicating that many alkyl segments are directly linked to aromatic rings and do not form distinct domains. In addition to this mixed
Figure 2. 1H spectra extracted at the 13C chemical shift of 13CO (200 ppm) at different mixing times for Beulah-Zap lignite sorbed with 3.2 wt % benzophenone-( 13 CO)-d 10 (a) and 0.5 wt % benzophenone-(13CO)-d10 (b). (c) Proton projection along 0− 220 ppm at tm,e = 10.25 ms. (d) 1H spectra extracted at the 13C chemical shift of nonprotonated aromatic C (143 ppm) at different mixing times.
matrix, Beulah-Zap lignite contains polymethylene domains of >5 nm radius that do not equilibrate with the aromatic-rich matrix within the 10 ms mixing time (Figure S8).8 At tm,e = 0.01 ms, the 1H spectra extracted at benzophenone 13CO (Figure 2(a)) from 0.5 and 1 ms LG-CP HETCOR spectra show a slightly greater contribution from aromatic than alkyl protons. With increasing tm,e the alkyl proton contribution increases more than the aromatic proton contribution. At tm,e = 0.3 ms, the proton slice associated with 13CO shows nearly equal contribution from aromatic and alkyl protons. At tm,e = 10.25 ms, the intensity of the alkyl proton signal continues to grow, indicating transfer of magnetization from the more distant alkyl protons to the immediate environment of 13CO. Furthermore, the signal growth in the alkyl region occurs most prominently in the 2−3 ppm region, which is associated with alkyl protons two bonds away from aromatic C, and not with those of the polymethylene protons (near 1.5 ppm, indicated by the vertical dashed line). The lignite sorbed with 0.5 wt % of benzophenone-(13CO)-d10 showed essentially the same results (Figure 2(b)). Hence, benzophenone-(13CO) is not associated with polymethylene domains but preferentially sorbed to the mixed aromatic-alkyl matrix. If benzophenone(13CO) were associated predominantly with polymethylene protons, the proton spectrum at tm,e = 0.01 ms would show a pattern indicating a major contribution from alkyl protons near 1.5 ppm, which is not the case. If benzophenone-(13CO) were uniformly distributed, its proton spectrum at tm,e = 0.01 ms would show a pattern similar to the sum over 0−220 ppm at 10.25 ms (Figure 2(c)), which is also not the case. It is further instructive to compare the proton spectra associated with 13CO (Figure 2(b)) and aromatic C (Figure 2(d)). That they are similar, exhibiting both aromatic and alkyl proton peaks, shows that benzophenone is located near the edges of the aromatic rings that link to alkyl segments. The slightly greater contribution from alkyl protons associated with 13 CO suggests that, additionally, some benzophenone is sorbed in alkyl-rich regions. Benzophenone-(13CO)-d10 in Kerogen IL-6. IL-6 kerogen is a type II kerogen with aromatic clusters averaging 7−9 8648
dx.doi.org/10.1021/es501129f | Environ. Sci. Technol. 2014, 48, 8645−8652
Environmental Science & Technology
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rings.28 In IL-6 kerogen, aromatic C accounts for 59% of total C, and about 2/3 of the aromatic C atoms are not protonated.28 The fraction of H in alkyl segments is 77%, while the fraction of H in aromatic segments is only 18%.28 The short-range 1H−13C correlation NMR spectrum28 documents the abundant linkages between aromatic and alkyl units. The separation of the polymethylene domain8 from the aromatic-rich matrix is demonstrated by the 13C-detected 1H inversion−recovery spectra shown in Figure 3(a). The aromatic C signals are
Benzophenone-(13CO)-d10 in Pahokee Peat. The presence of spatially distinct nonpolar alkyl and carbohydrate domains in the mixed aromatic−alkyl matrix within Pahokee peat is demonstrated by the 2D HETCOR spectra (Figure 4 (a-
Figure 3. (a) 13C-NMR detected 1H inversion recovery of kerogen IL6 sorbed with benzophenone-(13CO)-d10, after the indicated recovery delays. (b) The 1H spectra extracted at the 13C chemical shift of 13CO (197 ppm) at different mixing times. (c) Proton projection along 20−220 ppm at tm,e = 10.25 ms. (d) The 1H spectra extracted at the 13C chemical shift of nonprotonated aromatic C (138 ppm) at different mixing times.
Figure 4. 2D HETCOR NMR spectra of the Pahokee peat sorbed with benzophenone-(13CO)-d10 with (a) 1 ms LG-CP (tm,e = 0.01 ms), (b) 1 ms HH-CP (tm,e = 0.3 ms), and (c) 1 ms HH-CP with 10 ms spin diffusion (tm,e = 10.25 ms). (d) 13C-NMR detected 1H inversion recovery of Pahokee peat sorbed with benzophenone-(13C O)-d10, after the indicated recovery delays. The 1H spectra extracted at the 13C chemical shifts of 13CO (197 ppm, panel e) and nonprotonated aromatic C (137 ppm, panel f) at different mixing times. (g) Proton projection along 0−220 ppm at tm,e = 10.25 ms.
nulled at a recovery interval of approximately 45 ms, while the polymethylene proton signals remain inverted at this interval and only pass through null at 60−70 ms. These differences in proton T1 values indicate incomplete 1H spin diffusion and therefore separation of the aromatic-rich matrix and polymethylene domains8 by more than ∼10 nm. Figure 3 (b) shows the proton slices of 13CO in benzophenone at different mixing times. All spectra contain a larger alkyl than aromatic 1H peak, which is not surprising considering the greater abundance of alkyl protons in the sample. The aromatic proton signal is not strong relative to the noise level. As mixing time increases, alkyl proton signals become predominant. When the high proportion of alkyl protons is taken into account (Figure 3(c)), however, preferential association of benzophenone with the polymethylene domain is not evident. The proton spectra associated with benzophenone CO (Figure 3(b)) and nonprotonated aromatic C (Figure 3(d)) both show comparable contributions from aromatic and alkyl protons, within experimental uncertainty due to noise. This again suggests that benzophenone is near the edges of the aromatic rings that link to alkyl segments.
c)) collected at different mixing times and the 13C-detected 1H inversion−recovery spectra13,31 (Figure 4(d)). The latter show 13 C NMR signals associated with rapidly relaxing protons are mostly aromatic and COO, consistent with previous observation.18 Their signals pass through null at a recovery interval of approximately 6 ms, while those of O-alkyl and nonpolar alkyl segments remain inverted until about 12 ms. The similarity in 1 H T1 values (∼12 ms) of the polymethylene and O-alkyl structures could arise from their spatial proximity and thus the averaging of relaxation parameters due to spin diffusion. However, the absence of pronounced polymethylene-carbohydrate cross peaks in the 2D spectra (Figure 4(a-c)) provides strong evidence for the presence of separate nonpolar alkyl and carbohydrate domains with radii larger than 5 nm that do not equilibrate within the 10 ms mixing time.8 By contrast, carbohydrate-like and lignin-like domains have been found to be intimately mixed in humic substances11 and whole soils.9 The 1 H spectrum associated with the 13 CO of benzophenone at a mixing time of 0.01 ms shows a major 8649
dx.doi.org/10.1021/es501129f | Environ. Sci. Technol. 2014, 48, 8645−8652
Environmental Science & Technology
Article
slice of 13CO at 0.01 ms and the overall protons spectrum obtained as the sum over 20−220 ppm at 10 ms mixing time (Figure 5(c)) suggests that the affinity of benzophenone for aromatic rings is greater than for carbohydrate and nonpolar alkyl components. Domain- and Functional Group-Based Sorption Selectivity of Benzophenone-(13CO)-d10. PS-PVME has units of PS and PVME that are efficiently mixed at the
