Correspondence/Rebuttal Cite This: Environ. Sci. Technol. 2018, 52, 5514−5516
pubs.acs.org/est
Response to Comment on The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics
I
disruption of these donor−acceptor interactions.4 This hypothesis conflicts with high-pressure size exclusion chromatography measurements published recently10 for DOM treated with sodium borohydride. In addition, it is difficult to reconcile how an anionic reductant, such as borohydride, but not neutral solvent molecules, could access such complexes in a static, anion-encircled hydrophobic microenvironment. Another remaining question is how solvents of such different polarity could have no effect on DOM spectra, when pH so readily affects the optical properties of DOM, as reported by Blough and Del Vecchio. In the referenced pH experiments,11−13 the CT model holds that increased pH deprotonates phenolic donors, which leads to increased charge-transfer excitation. However, for donor−acceptor pairs to be pH-sensitive, the donors would have to be solvent accessible, contrary to the proposed CT model. These observations demonstrate that the evidence presented in support of a CT model (i.e., reactivity with borohydride and pH-sensitive spectra) also contradicts the provision that donor−acceptor complexes reside in a solventinaccessible microenvironment. A final argument against solvent-inaccessible chromophores and fluorophores is that fluorescence quantum yield was significantly changed by solvent polarity at all excitation wavelengths measured (Figure 3c in manuscript),1 extending well into the visible wavelength range. This change in fluorescence intensity with solvent polarity across the DOM absorption spectrum indicates that these fluorophores are solvent-accessible. Lastly, Blough and Del Vecchio assert that the formation of particulates for soil humic acids in tetrahydrofuran supports the hypothesis that donor−acceptor moieties are in a solvent inaccessible hydrophobic microenvironment. The data presented in Figure 4 of our study1 contradicts this view. A solvatochromic shift in fluorescence spectra of >20 nm was observed for soil humic acids in acetonitrile and a soil fulvic acid in both acetonitrile and tetrahydrofuran at a range of excitation wavelengths. We hypothesized that this solvatochromism in the fluorescence spectra could be due to exciplex formation, but not ground state donor−acceptor complexes due to the lack of solvatochromism in the corresponding absorbance spectra. This result gives us confidence that solvent polarity would affect the dynamics of donor−acceptor complexes present in the other isolates investigated, if they were present. Furthermore, in the apolar organic solvent systems investigated, carboxylates would be protonated,14 removing a speculated stabilizing force, and thus encouraging complex dissociation. Lastly, the view that the low solubility of soil humic acids in tetrahydrofuran is evidence for a hydrophobic microenvironment does not fit our observation that these materials dissolved f irst and then, after 24 h, showed evidence of precipitates (Supporting Information section 3.4).1 The hypothesis that the stabilization by charged moieties controls
n response to our recent study regarding the role of chargetransfer (CT) interactions in dissolved organic matter (DOM) photophysics,1 Blough and Del Vecchio2 raise concerns about the use of solvent polarity, viscosity, and temperature to test for the prevalence of CT interactions in DOM. This topic is of great significance to environmental chemistry and engineering due to the ubiquity and reactivity of DOM in these systems. Blough and Del Vecchio argue against the types of potential donor−acceptor complexes occurring in DOM proposed in Figure 1 of McKay et al. (2018),1 which includes independent, covalently tethered, and conjugated donor−acceptor moieties. The counterargument2 asserts that the types of donor− acceptor complexes we propose play a limited role, suggesting instead that the model for polydopamine, as proposed by Dreyer and co-workers,3 also applies to DOM. In this model, structural units composing DOM would form static complexes as a result of H-bonding, π-stacking, and charge-transfer interactions.3,4 It is hypothesized that such donor−acceptor complexes in DOM are hindered kinetically and thermodynamically from dissociating into their respective independent moieties. Blough and Del Vecchio state that the dynamics of these static complexes would not be affected by temperature and solvent polarity in the same way as dynamic donor− acceptor complexes, offering an alternative explanation for the lack of change in spectral shape presented in our study. Further, the absence of solvatochromism reported in our study could potentially be due to solvent-protected donor−acceptor complexes. This model hypothesizes that this hydrophobic core is further stabilized by charged outer groups. Although we welcome the criticism and further discussion, we disagree with the presented counterarguments. First, the model for the primary structure of polydopamine proposed by Dreyer et al. is controversial within the literature.3,5 Specifically, a subsequent study argued that polydopamine monomers are covalently linked, as opposed to being held together by noncovalent interactions (e.g., chargetransfer).5 The merits of these individual studies are beyond the scope of this response, but it is noteworthy that there is debate over the structure of a material that is synthesized from a known monomer unit (i.e., dopamine), whereas the structural units in DOM are much more heterogeneous and poorly characterized. In addition, the isolation of donor and acceptor moieties from solvent is inconsistent with other lines of evidence used to support a CT model. For example, putative acceptor and donor moieties in the CT model, aromatic ketones/aldehydes and phenols/polyphenols/alkoxy phenols, respectively, exhibit aqueous phase photochemistry.6−9 If such CT complexes are protected from the solvent, they would also be expected to be protected from participating in bimolecular reactions in the aqueous phase. Furthermore, according to the CT model, reduction of DOM with sodium borohydride would result in DOM with substantially lower molecular weight due to © 2018 American Chemical Society
Published: April 16, 2018 5514
DOI: 10.1021/acs.est.8b01807 Environ. Sci. Technol. 2018, 52, 5514−5516
Environmental Science & Technology
Correspondence/Rebuttal
Author Contributions
solubility in apolar solvents is further contradicted by the work of Aiken and Malcolm (1987), in which Suwannee River Fulvic Acid (SRFA) was dissolved in tetrahydrofuran at concentrations up to ∼1.6 g L−1 for vapor pressure osmometry measurements.15 If solubility of organic matter is limited by a hydrophobic core that requires stabilization by charged moieties, it questions the relative abundance and environmental importance of these structures in aquatic DOM, given that high concentrations of SRFA can be dissolved into an organic solvent.15 We disagree that our study “ignored” or relied on a “selfselected portion” of the available data. Our paper was certainly focused on temperature and solvent effects, but this tight focus was intentional. Solvent and temperature effects on absorbance and fluorescence are arguably the most important tests for CT complexes. Ultimately, the successful model of DOM optical properties must reconcile both previous experimental data and the data presented in our paper. We have made a start in bringing together the different observations from various studies in a table in the Supporting Information, which juxtaposes the common features of DOM absorbance and fluorescence spectra against the explanations presented by the CT and superposition models (Supporting Information Table S1).1 Finally, we welcome this comment, because it represents the constructive exchange and debate of scientific ideas. The CT model has been widely accepted by many members of the environmental chemistry community.16−19 However, a motivation for our study was the recognition that the classic tests known to affect organic donor−acceptor complexes (i.e., solvent and temperature perturbations) have thus far been absent in the development of the contemporary CT model. Therefore, we feel that our study represents an important contribution to this field and demonstrates that additional work is needed to present a unified photophysical model for DOM. We hope there will be continued efforts in this field to better understand the chemical and structural basis for DOM photophysical behaviors.
⊥
Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) McKay, G.; Korak, J. A.; Erickson, P. R.; Latch, D. E.; McNeill, K.; Rosario-Ortiz, F. L. The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics. Environ. Sci. Technol. 2018, 52 (2), 406−414. (2) Blough, N. V.; Del Vecchio, R. Comment on “The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics.. Environ. Sci. Technol. 2018, 45, 1−3. (3) Dreyer, D. R.; Miller, D. J.; Freeman, B. D.; Paul, D. R.; Bielawski, C. W. Elucidating the Structure of Poly(dopamine). Langmuir 2012, 28 (15), 6428−6435. (4) Del Vecchio, R.; Schendorf, T. M.; Blough, N. V. Contribution of Quinones and Ketones/Aldehydes to the Optical Properties of Humic Substances (HS) and Chromophoric Dissolved Organic Matter (CDOM). Environ. Sci. Technol. 2017, 51 (23), 13624−13632. (5) Liebscher, J.; Mrówczyński, R.; Scheidt, H. A.; Filip, C.; Hădade, N. D.; Turcu, R.; Bende, A.; Beck, S. Structure of Polydopamine: A Never-Ending Story? Langmuir 2013, 29 (33), 10539−10548. (6) Sun, L.; Qian, J.; Blough, N. V.; Mopper, K. Insights into the Photoproduction Sites of Hydroxyl Radicals by Dissolved Organic Matter in Natural Waters. Environ. Sci. Technol. Lett. 2015, 2 (12), 352−356. (7) Canonica, S.; Hellrung, B.; Wirz, J. Oxidation of Phenols by Triplet Aromatic Ketones in Aqueous Solution. J. Phys. Chem. A 2000, 104 (6), 1226−1232. (8) Canonica, S.; Freiburghaus, M. Electron-rich phenols for probing the photochemical reactivity of freshwaters. Environ. Sci. Technol. 2001, 35 (4), 690−695. (9) Packer, J. L.; Werner, J. J.; Latch, D. E.; McNeill, K.; Arnold, W. A. Photochemical fate of pharmaceuticals in the environment: Naproxen, diclofenac, clofibric acid, and ibuprofen. Aquat. Sci. 2003, 65 (4), 342−351. (10) McKay, G.; Couch, K. D.; Mezyk, S. P.; Rosario-Ortiz, F. L. Investigation of the Coupled Effects of Molecular Weight and ChargeTransfer Interactions on the Optical and Photochemical Properties of Dissolved Organic Matter. Environ. Sci. Technol. 2016, 50 (15), 8093− 8102. (11) Phillips, S. M.; Bellcross, A. D.; Smith, G. D. Light Absorption by Brown Carbon in the Southeastern United States is pH-dependent. Environ. Sci. Technol. 2017, 51 (12), 6782−6790. (12) Yan, M.; Korshin, G. V.; Claret, F.; Croué, J.-P.; Fabbricino, M.; Gallard, H.; Schäfer, T.; Benedetti, M. F. Effects of charging on the chromophores of dissolved organic matter from the Rio Negro basin. Water Res. 2014, 59, 154−164. (13) Dryer, D. J.; Korshin, G. V.; Fabbricino, M. In Situ Examination of the Protonation Behavior of Fulvic Acids Using Differential Absorbance Spectroscopy. Environ. Sci. Technol. 2008, 42 (17), 6644− 6649. (14) Cox, B. G. Acids, Bases, and Salts in Mixed-Aqueous Solvents. Org. Process Res. Dev. 2015, 19 (12), 1800−1808. (15) Aiken, G. R.; Malcolm, R. L. Molecular weight of aquatic fulvic acids by vapor pressure osmometry. Geochim. Cosmochim. Acta 1987, 51, 2177−2184. (16) Sharpless, C. M.; Blough, N. V. The importance of chargetransfer interactions in determining chromophoric dissolved organic matter (CDOM) optical and photochemical properties. Environ. Sci.: Processes Impacts 2014, 16 (4), 654−671. (17) Phillips, S. M.; Smith, G. D. Light Absorption by Charge Transfer Complexes in Brown Carbon Aerosols. Environ. Sci. Technol. Lett. 2014, 1 (10), 382−386. (18) Phillips, S. M.; Smith, G. D. Further Evidence for Charge Transfer Complexes in Brown Carbon Aerosols from Excitation−
Garrett McKay†,⊥ Julie A. Korak†,‡,⊥ Paul R. Erickson§ Douglas E. Latch∥ Kristopher McNeill*,§ Fernando L. Rosario-Ortiz*,† †
■
These authors contributed equally to this work.
Department of Civil, Environmental and Architectural Engineering, University of Colorado, Boulder, Colorado 80309, United States ‡ Bureau of Reclamation, Department of the Interior, PO Box 25007, Denver, Colorado 80225, United States § Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, 8092 Zurich, Switzerland ∥ Department of Chemistry, Seattle University, Seattle, Washington 98122, United States
AUTHOR INFORMATION
Corresponding Authors
*(K.M.) E-mail:
[email protected]. *(F.L.R-O.) E-mail:
[email protected]. ORCID
Kristopher McNeill: 0000-0002-2981-2227 Fernando L. Rosario-Ortiz: 0000-0002-3311-9089 5515
DOI: 10.1021/acs.est.8b01807 Environ. Sci. Technol. 2018, 52, 5514−5516
Environmental Science & Technology
Correspondence/Rebuttal
Emission Matrix Fluorescence Spectroscopy. J. Phys. Chem. A 2015, 119 (19), 4545−4551. (19) Maizel, A. C.; Remucal, C. K. Molecular Composition and Photochemical Reactivity of Size-Fractionated Dissolved Organic Matter. Environ. Sci. Technol. 2017, 51 (4), 2113−2123.
5516
DOI: 10.1021/acs.est.8b01807 Environ. Sci. Technol. 2018, 52, 5514−5516
