Effects of Ionic Strength on the Chromophores of Dissolved Organic

Apr 21, 2015 - Address: 333 More Hall, Box 352700, University of Washington, Seattle, WA 98195-2700. E-mail: [email protected]. Phone (206) 543-0785...
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Effects of Ionic Strength on the Chromophores of Dissolved Organic Matter Yuan Gao,*,† Mingquan Yan,‡ and Gregory V. Korshin† †

Department of Civil and Environmental Engineering, University of Washington, Seattle, Washington 98195, United States Department of Environmental Engineering, College of Environmental Sciences and Engineering, Peking University, the Key Laboratory of Water and Sediment Sciences, Ministry of Education, Beijing 100871, China

‡

S Supporting Information *

ABSTRACT: This study examined effects of variations of the ionic strength (IS) on the absorbance of dissolved organic matter (DOM). The measurements performed for DOM of allochthonous (Suwannee River humic and fulvic acids, SRHA and SRFA) and autochthonous (Pony Lake fulvic acid, PLFA) origin showed that increases of IS (which was controlled by additions of sodium perchlorate) from 0.001 to 0.3 mol/L were accompanied by increases of the absorbance of DOM. The extent of the increase of DOM absorbance observed at increasing IS was consistently greater at higher pH values, and it followed the order of PLFA < SRFA < SRHA. The absolute values of the spectral slopes of the log-processed absorbance spectra of DOM calculated for a 350 to 400 nm wavelength range decreased proportionally to the logarithm of IS values. This result was hypothesized to be indicative of the deprotonation of the DOM chromophores at increasing IS values, which was supported by model calculations showing that values of the spectral slopes were nearly linearly correlated with the extent of IS-induced deprotonation of the operationally defined phenolic groups in DOM.

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INTRODUCTION Dissolved organic matter (DOM) is one of the most important components of soils, sediments and aquatic environments.1 One of the well-documented effects of DOM is its influence on concentrations and mobility of metals in the environment. Prior research has shown that metal concentrations tend to increase in the presence of DOM due to the formation of metal−DOM complexes and metal/DOM colloidal particles,2−6 although metal/DOM complexes tend to have lower toxicity compared to the free metal ions.7 The extent of DOM−metal binding and the nature of DOM functionalities engaged in interactions with metal cations are site-specific because DOM can be formed autochthonously from its precursors generated in the water body per se (e.g., algae) or it can be produced via allochthonous processes involving the degradation of terrestrial biota.8 The diversity of DOM sources and generation pathways result in a highly complex nature of DOM whose molecules have varying molecular weights, conformations, proton affinities and concentrations of carboxylic, phenolic and other metal cation-binding functional groups,9,10 and other properties that, for engineered systems, are also affected by specifics of water treatment processes.11 Prior studies have shown that the modeling of metal−DOM systems must take into account both specific interactions between Men+ ions and DOM functionalities, as well as the electrostatically controlled interactions affected by variations of © 2015 American Chemical Society

ionic strength (IS). Prior studies have also provided evidence that concentrations of background cations such as Na+ are important for such aspects of DOM chemistry due to the supramolecular properties of DOM molecules and their aggregation,9,12−15 DOM adsorption on the surfaces of mineral particles and their colloidal behavior.16−18 Because environmental concentrations of sodium and other background cations (e.g., K+, Ca2+, Mg2+) tend to be several orders of magnitude higher than those of trace level metals (e.g., Pb2+ and Cu2+), this could result in a strong competition between the background and trace-level cations for binding sites in DOM.19 Several theoretical approaches, for instance the Non-Ideal Competitive Adsorption (NICA) model coupled with the Donnan electrostatic submodel have been developed to describe the binding of many cations to DOM over a wide range of conditions.20−22 In the NICA−Donnan and related models, metal-binding DOM functionalities are separated into operationally defined carboxylic and phenolic moieties, each with a continuous Gaussian-type distribution of pK values.23,24 The Donnan model can be used to quantify concentrations of counterions (e.g., Na+) accumulating to balance the charge of Received: Revised: Accepted: Published: 5905

August 28, 2014 April 17, 2015 April 21, 2015 April 21, 2015 DOI: 10.1021/acs.est.5b00601 Environ. Sci. Technol. 2015, 49, 5905−5912

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DOM molecules.25 The Donnan approach assumes that DOM molecules behave as a gel with a uniform distribution of electric charge and potential within that gel.7,22 Because in environmentally relevant conditions DOM molecules tend to have a net negative charge, it is neutralized by the nonspecific binding of counterions such as Na+ or hardness cations that accumulate in the Donnan volume (VD). The Donnan volume, an important parameter in the NICA−Donnan model, is deemed to include a humic molecule along with the diffuse part of the electric double layer surrounding it. Logarithms of VD values have been hypothesized to decrease as a linear function of the logarithm of the IS values of the solution7 The fundamental concepts and formal apparatus incorporated in the advanced models of DOM−metal cation interactions have been developed based on the data of diverse experimental techniques, notably potentiometric methods that use ion-selective electrodes, ISE,26,27 X-ray absorption fine structure spectroscopy (EXAFS),28−31 infrared (IR) spectroscopy and nuclear magnetic resonance (NMR). These methods allow determining important details of the behavior of free metals concentrations at varying conditions and many aspects of structural geometry of DOM−metal complexes. However, they virtually always require preconcentration of DOM or DOM−metal complexes, which, beside constituting considerable practical difficulties, has a potential to alter the characteristics of DOM and its complexes.32 Another major complication is that the extent of nonspecific binding of metal cations such as Na+ and effects of varying IS values on the intrinsic properties of DOM (e.g., their conformation, protonation of carboxylic and phenolic moieties) are difficult, if at all possible, to characterize directly by any of these methods. Such information can be obtained using optical spectroscopy methods that can be applied for systems having environmentally relevant DOM concentrations. These methods help discern changes of DOM chromophores and fluorophores that are indicative of the properties of the entire DOM ensemble.42,45,49,50,52,54 The sensitivity to such changes can be enhanced using a differential approach that quantifies changes of DOM absorbance or fluorescence as a function of reaction coordinates, for instance, proton or metal cation concentrations and it also greatly amplifies the sensitivity of the optical methods to such changes.33 Other parameters derived based on the analysis of DOM fluorescence or absorbance, for instance, differential logarithms of DOM absorbance have also been found to correlate strongly with concentration of DOM-bound metal cations.34,35 Prior studies38 have examined in some detail the effects of pH and IS variations on the absorbance of purified Aldrich humic acid (PAHA) deemed to be representative of humic species in soils, but the behavior of humic species originating from aquatic environments has not been studied in the context of ascertaining and separating effects of pH and IS on DOM chromophores. In this study, we used the method of differential spectroscopy to probe effects of IS variations on the chromophores of both autochthonous and allochthonous aquatic DOM. The understanding of effects of varying concentrations of background electrolytes on the optical properties of DOM may be useful for generating a direct experimental proof of the applicability of major concepts of the existing models of metal/DOM interactions as well as for separating contributions of specific and nonspecific binding of metals to DOM typical for aquatic environments.

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MATERIALS AND METHODS Experiments were carried out with samples of Suwannee River fulvic acid (SRFA) (standard number 1S101F), Suwannee

Figure 1. Differential absorbance spectra of SRFA at varying ionic strength (in mol/L) at pH 5.0, 7.0 and 9.0.

River humic acid (SRHA) (standard number 2S101H) and Pony Lake fulvic acid (PLFA) (standard number 1R109F) obtained from the International Humic Substances Society (IHSS). In the case of SRHA, 0.01 M NaOH was used to prepare a stock solution of this DOM sample. SRFA and PLFA 5906

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⎛ Aj ⎞ D ln Ai ,nm = ln⎜ ⎟ ⎝ A ref ⎠

dissolved readily in pure water. The dissolved organic carbon (DOC) concentrations in the DOM stock solutions were in the range of 100−140 mg/L, as determined by a Shimadzu TOCVcsh carbon analyzer. Concentrations of trace-level elements present in DOM solutions diluted to a 5 mg/L concentration as DOC were measured by a PerkinElmer ELAN ICP-MS instrument (PerkinElmer Instruments, Shelton CT). Results of these measurements (Table S1 in the Supporting Information) demonstrate that interferences from the background metals on the measurements of DOM absorbance were negligible, except possibly for those from iron. However, because this study used the differential approach, effects of any background component affecting the system were compensated by subtracting the reference spectrum recorded at the same concentrations of the trace-level species present in the system. Ionic strength of DOM solutions was controlled by adding required amounts of analytical grade sodium perchlorate NaClO4 (Aldrich Chemical Co., Milwaukee, WI). In these experiments, requisite volumes of NaClO4 and DOM stock solutions were added in 200 mL containers to reach a 5 mg/L DOC concentration whereas the concentration of NaClO4 was varied from 0.001 to 0.3 mol/L. pH of the solutions was controlled by adding small amounts of HClO4 or NaOH. After each addition of NaClO4 stock solution and a 30 min equilibration time, aliquots were taken to record their absorbance spectra acquired with a 5 cm quartz cuvette by a Shimadzu UV-2700 spectrophotometer. The technical specifications of Shimadzu UV-2700 quote the photometric accuracy of that instrument at ±0.002, ±0.003 and ±0.006 absorbance units (a.u.) for 0.5, 1.0 and 2.0 absorbance levels, respectively.36 The repeatability of that instrument is quoted at ±0.001, ±0.001 and ±0.003 a.u., respectively for the same conditions. Additional measurements carried out in our laboratory demonstrated that for conditions used in this study, the repeatability error averaged over the wavelength range of interest (e.g., 230−400 nm) was 99.5% purity, BioXtra grade purchased from Sigma-Aldrich) was added. This buffer does not affect the ionic strength of DOM solutions and does not absorb light at wavelengths >ca. 230 nm.37 Calculations of Absorbance-based Parameters. Differential spectra associated with IS variations were calculated as described in prior research:32,34 DASi ,nm =

(2)

As defined above, calculations of differentials of the logtransformed spectra allow ascertaining effects of variations of system parameters on DOM chromophores that per se may have relatively low intensity but are sensitive to changes of the chemical state of DOM molecules. As a result, this approach allows examining changes of the absorbance spectra of DOM in a much wider range of wavelengths compared with their conventional linear representation.

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RESULTS AND DISCUSSIONS Ionic Strength Effects on Differential Spectra of DOM. As observed in prior studies, the unprocessed (zero-order)

Aj − A ref length × DOC

(1)

Figure 2. Differential absorbance spectra (DAS) of SRHA for varying ionic strength (in mol/L) at pH 5.0 and 9.0.

In the above formula, DASi,nm is the differential absorbance at any given wavelength, Aj and Aref are absorbance intensities measured respectively at a selected NaClO4 concentration and a relevant reference concentration (e.g., 0.001 mol/L in this study). DOC is the concentration of organic carbon (mg/L), and length is the cell length (in cm). Spectral slopes in the range of wavelengths 350 to 400 nm (S350−400) were determined as the slope of the linear correlation (for this range, the R2 was over 0.999) that fits the log-transformed DOM absorbance spectra in the range between 350 and 400 nm for any specific pH and sodium perchlorate concentration. Differentials of the log-transformed spectra of DOM were also calculated as defined below:

spectra of these DOM samples did not have any specific features (Figure S1a of the Supporting Information) and their changes caused by IS variations were subtle in some conditions (e.g., at pH 5.0 and for PLFA solution). Nonetheless, when quantified using the differential approach and also via calculations of log-processed spectra, these changes were found to be consistent and interpretable. Important aspects of this trend can be discerned via the comparison of differential absorbance spectra (DAS) at a constant pH but increasing IS values (Figures 1−3). Figure 1 demonstrates that in the case of SRFA, IS-differential spectra (calculated using the absorbance of SRFA in the presence of 5907

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Figure 3. Differential absorbance spectra (DAS) of PLFA for varying ionic strength concentrations (in mol/L) at pH 9.0.

Figure 5. Correlations between logarithms of ionic strength and spectral slopes S350−400 measured for SRHA, SRFA and PLFA.

which was almost 1 order of magnitude higher than that at pH 5. The differential spectra of SRFA prior to their normalization by the cell length and DOC concentration are shown in Figure S1b,c of the Supporting Information. These data demonstrate that changes of the absorbance of SRFA caused by IS variation exceed by 1 order of magnitude or more than the precision limit expected for the instrument used in our experiments. Results for SRHA (Figure 2) demonstrate that, as was the case with SRFA, the intensity of the differential spectra of SRHA increased with the pH. For instance, at pH 5, there were no prominent peaks and the intensity of the differential spectra at all wavelength was 370 nm. The intensity of the IS-differential spectra of SRFA rose with the pH. For example, when the solution pH was 5.0 and the IS was increased from 0.001 to 0.2 mol/L, the intensity of the peak at 330 nm was 0.00024 (L·mg−1·cm−1) at the highest IS concentration. The same change of the IS at pH 9.0 resulted an intensity of this peak (0.0017 L·mg−1 ·cm−1), 5908

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Figure 6. Comparison of differential spectra for SRFA, SRHA and PLFA at varying ionic strengths (vs 0.001 mol/L reference ionic strength) with a constant pH 8.1, and varying pH (vs 8.1 reference pH) with a constant ionic strength 0.04 mol/L.

level. This was 4.5 times higher than the intensity of that band at pH 5. Figure 3 shows the differential spectra for PLFA. In contrast with allochthonously generated SRFA and SRHA, PLFA is produced autochthonously via the degradation of microbial biomass, is rich in nitrogen and has a low hydrophobicity and aromaticity40−43 In accord with the lower aromaticity of PLFA,44−46 the response of its chromophores to IS changes was nearly 1 order of magnitude lower than that observed for SRFA and SRHA. Contributions of specific features in the differential absorbance of PLFA were different from those of SRFA and SRHA. Although the band with the maximum at 330 nm was

observed for PLFA as well as for SRFA and SRHA, the band at wavelengths >370 nm that was highly prominent for SRFA and especially SRHA had a low intensity, or it was nearly absent in the case of PLFA. The low intensity of this characteristic band (>370 nm) in the case of PLFA is likely to be a manifestation of a lower susceptibility of PLFA aromatic chromophores, which are less abundant in PLFA compared with SRFA or SRHA, to changes induced by IS variations. Effects of Ionic Strength on Log-Transformed Spectra of DOM. Prior research has shown that the absorbance of DOM at wavelength greater than ∼300 nm decreases exponentially with the observation wavelength.47−52 Accordingly, calculations of log-transformed DOM absorbance spectra 5909

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Figure 7. Correlations between spectral slopes and NICA−Donnan model calculations of charges associated with the phenolic groups in DOM: (a) correlations between the absolute values of spectral slopes measured for SRFA and SRHA at varying ionic strengths and fixed pH values; (b) correlations between differentials of the spectral slopes and phenolic charges for pH variations at a constant ionic strength.

MINTEQ calculations were carried out to determine whether the observed changes of the spectral slopes induced by IS variations are correlated with the theoretically predicted changes of the deprotonation of the phenolic (or high-affinity site, HAS) and carboxylic (or low affinity site, LAS) sites in SRFA and SRHA in such conditions. The NICA−Donnan model parameters that were used in our calculations are compiled in Table S2 of the Supporting Information. Model calculations for PLFA were not performed because the intrinsic properties of this DOM are clearly different from those of SRFA and SRHA while the required protonation constants for PLFA are not included in the MINTEQ database. The model calculations showed that the S350−400 values determined for SRHA and SRFA for the entire range of pH and IS values used in the experiments were correlated with the charges of the phenolic sites in these DOM samples (Figure 7a). The correlation was nearly perfectly linear for SRHA, but for SRFA, it deviated from linearity for low charges of the phenolic groups for pH 5. In contrast, effects of IS variations on the charges of the carboxylic groups in SRFA and SRHA were not clearly correlated with the observed changes of the spectral slopes of the absorbance spectra. To examine further similarities in effects of variations of IS or pH values on DOM chromophores, changes of the spectral slopes vs pH at a constant ionic strength were also interpreted based on the NICA−Donnan model similarly to the approach presented in our recent study.58 Results of these calculations are presented in Figure S4 of the Supporting Information whereas Table S2 of the Supporting Information contains a compilation of the fitting parameters used in these calculations. The correlation between the differential spectral slopes and charges of DOM phenolic groups calculated for a constant IS and varying pHs are shown in Figure 7b. Comparison of the data presented in Figure 7a,b demonstrates that changes of the protonation of the phenolic groups in SRFA and SRHA induced by either variations of IS (Figure 7a) or pH (Figure 7b) result in similar trends. This observation provides additional support to the notion that the influence of IS variation on the chromophores in DOM is likely to be

and their differentials were carried out to account for contributions of DOM chromophore groups or relevant interchromophore interactions that per se have low intensities but can be sensitive to IS variations. Examination of the log-transformed spectra of DOM demonstrated that increases of IS values were accompanied by increases of the logarithms of DOM absorbance (Figure S2 of the Supporting Information), although similar to the zeroorder (unprocessed) spectra, the subtle changes and unclear patterns could not yield much useful information. However, the differentials of the log-transformed spectra (calculated as by eq 2) had a prominent increase at wavelengths >300 nm, as shown for SRFA in Figure 4. Similar data for SRHA and PLFA are presented in the Supporting Information (Figure S3). To quantify effects of IS variations on the DOM chromophores using a single index, slopes of the log-processed DOM absorbance spectra in the range of wavelengths 350−400 nm (denoted as S350−400) were determined as well. The S350−400 parameter was introduced based on the results of prior studies that have shown that for humic substances, spectral slope values tend to be correlated with apparent molecular weights of DOM found in a broad range of natural waters.53−55 The degradation of DOM by photolysis or chlorination causes the absolute values of the spectral slope to increase.56,57 In contrast, DOM deprotonation or DOM−metal complexation cause the absolute values of the spectral slopes to decrease.35 The dependence of S350−400 values vs IS logarithms is shown in Figure 5. This figure demonstrates that the absolute values of the slopes of the log-processed absorbance spectra underwent a consistent decrease at higher IS values. The notion that IS variations affect DOM chromophores in a way similar to that induced by their deprotonation caused by intentional increases of pH values was also suggested by the close resemblance of the shapes and intensities of the linear differential spectra of all examined DOM samples when either the IS values were varied at a constant pH, or the pH was varied at a constant IS (Figure 6). Because IS effect is similar to that expected to take place as a result of the deprotonation of DOM molecules, Visual 5910

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(8) Thurman, E. M. Organic Geochemistry of Natural Waters; Springer: New York, 1985. (9) Leenheer, J. A. Systematic approaches to comprehensive analyses of natural organic matter. Ann. Environ. Sci. 2009, 3, 1−130. (10) Gao, Y.; Chen, D.; Weavers, L. K.; Walker, H. W. Ultrasonic control of UF membrane fouling by natural waters: Effects of calcium, pH, and fractionated natural organic matter. J. Membr. Sci. 2012, 401− 402 (0), 232−240. (11) Leenheer, J. A.; Croué, J.-P. Peer reviewed: Characterizing aquatic dissolved organic matter. Environ. Sci. Technol. 2003, 37 (1), 18A−26A. (12) Baalousha, M.; Motelica-Heino, M.; Coustumer, P. L. Conformation and size of humic substances: Effects of major cation concentration and type, pH, salinity, and residence time. Colloids Surf., A 2006, 272 (1), 48−55. (13) Iskrenova-Tchoukova, E.; Kalinichev, A. G.; Kirkpatrick, R. J. Metal cation complexation with natural organic matter in aqueous solutions: Molecular dynamics simulations and potentials of mean force. Langmuir 2010, 26 (20), 15909−15919. (14) Wall, N. A.; Choppin, G. R. Humic acids coagulation: Influence of divalent cations. Appl. Geochem. 2003, 18 (10), 1573−1582. (15) Yang, R.; van den Berg, C. M. Metal complexation by humic substances in seawater. Environ. Sci. Technol. 2009, 43 (19), 7192− 7197. (16) Feng, X.; Simpson, A. J.; Simpson, M. J. Chemical and mineralogical controls on humic acid sorption to clay mineral surfaces. Org. Geochem. 2005, 36 (11), 1553−1566. (17) Iskrenova-Tchoukova, E.; Kalinichev, A. G.; Kirkpatrick, R. J. Metal cation complexation with natural organic matter in aqueous solutions: Molecular dynamics simulations and potentials of mean force. Langmuir 2010, 26 (20), 15909−15919. (18) Majzik, A.; Tombácz, E. Interaction between humic acid and montmorillonite in the presence of calcium ions II. Colloidal interactions: Charge state, dispersing and/or aggregation of particles in suspension. Org. Geochem. 2007, 38 (8), 1330−1340. (19) Pinheiro, J. P.; Mota, A. M.; Benedetti, M. F. Lead and calcium binding to fulvic acids: Salt effect and competition. Environ. Sci. Technol. 1999, 33 (19), 3398−3404. (20) Avena, M. J.; Koopal, L. K.; van Riemsdijk, W. H. Proton binding to humic acids: Electrostatic and intrinsic interactions. J. Colloid Interface Sci. 1999, 217, 37−48. (21) Christl, I.; Kretzschmar, R. Relating ion binding by fulvic and humic acids to chemical composition and molecular size. 1. Proton binding. Environ. Sci. Technol. 2001, 35 (12), 2505−2511. (22) Milne, C. J.; Kinniburgh, D. G.; Van Riemsdijk, W. H.; Tipping, E. Generic NICA−Donnan model parameters for metal-ion binding by humic substances. Environ. Sci. Technol. 2003, 37 (5), 958−971. (23) Koopal, L. K.; Saito, T.; Pinheiro, J. P.; Van Riemsdijk, W. H. Ion binding to natural organic matter: General considerations and the NICA−Donnan model. Colloids Surf., A 2005, 265 (1), 40−54. (24) Montenegro, A. C.; Orsetti, S.; Molina, F. V. Modelling proton and metal binding to humic substances with the NICA−EPN model. Environ. Chem. 2014, 11 (3), 318−332. (25) Benedetti, M. F.; Van Riemsdijk, W. H.; Koopal, L. K. Humic substances considered as a heterogeneous Donnan gel phase. Environ. Sci. Technol. 1996, 30 (6), 1805−1813. (26) Christl, I.; Metzger, A.; Heidmann, I.; Kretzschmar, R. Effect of humic and fulvic acid concentrations and ionic strength on copper and lead binding. Environ. Sci. Technol. 2005, 39 (14), 5319−5326. (27) Christl, I. Ionic strength-and pH-dependence of calcium binding by terrestrial humic acids. Environ. Chem. 2012, 9 (1), 89−96. (28) Christl, I.; Kretzschmar, R. C-1s NEXAFS spectroscopy reveals chemical fractionation of humic acid by cation-induced coagulation. Environ. Sci. Technol. 2007, 41 (6), 1915−1920. (29) Frenkel, A. I.; Korshin, G. V.; Ankudinov, A. L. XANES study of Cu2+-binding sites in aquatic humic substances. Environ. Sci. Technol. 2000, 34 (11), 2138−2142.

associated primarily with the changes of the protonation of the phenolic groups. The data presented above appear to strongly support the fundamental assumptions incorporated in the NICA-Donnan model. However, further work needs to be done with DOM of varying provenance to examine in more detail of effects of nonphenolic DOM moieties and determine whether the differences in the responses of SRFA, SRHA and PLFA to IS variations are defined by their different molecular weights, charges and/or conformations, or other mechanisms, for instance differences in the intrinsic chemistry of DOM chromophores.

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ASSOCIATED CONTENT

S Supporting Information *

Spectra of SRFA, log-transformed spectra of SRFA recorded at increasing ionic strengths, log-transformed differential absorbance spectra of PLFA and SRHA calculated for increasing ionic strengths, total metal concentration in DOM, effects of pH on the differential spectral slope and potentiometric and spectrophotometric NICA−Donnan parameters for protonation-active groups in SRHA and SRFA. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b00601.

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AUTHOR INFORMATION

Corresponding Author

*Y. Gao. Address: 333 More Hall, Box 352700, University of Washington, Seattle, WA 98195-2700. E-mail: gaoyuan@uw. edu. Phone (206) 543-0785. Fax (206) 685-9185. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the U.S. National Science Foundation (grant 0931676) and, partially, by U.S. National Geographic Air and Water Conservation Fund (grant GEFC1514). Gregory Korshin thanks the Foreign Experts Program of China for supporting his work at Peking University.

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REFERENCES

(1) Croue, J.-P.; Korshin, G. V.; Benjamin, M. M. Characterization of Natural Organic Matter in Drinking Water; American Water Works Association: Denver, CO, 2000; pp 5−8. (2) Dryer, D. J.; Korshin, G. V. Investigation of the reduction of lead dioxide by natural organic matter. Environ. Sci. Technol. 2007, 41 (15), 5510−5514. (3) Gao, Y.; Korshin, G. Effects of NOM properties on copper release from model solid phases. Water Res. 2013, 47, 4843−4852. (4) Liu, H.; Schonberger, K. D.; Korshin, G. V.; Ferguson, J. F.; Meyerhofer, P.; Desormeaux, E.; Luckenbach, H. Effects of blending of desalinated water with treated surface drinking water on copper and lead release. Water Res. 2010, 44 (14), 4057−4066. (5) Liu, H.; Korshin, G. V.; Ferguson, J. F. Interactions of Pb(II)/ Pb(IV) solid phases with chlorine and their effects on lead release. Environ. Sci. Technol. 2009, 43 (9), 3278−3284. (6) Peng, C.-Y.; Korshin, G. V. Speciation of trace inorganic contaminants in corrosion scales and deposits formed in drinking water distribution systems. Water Res. 2011, 45 (17), 5553−5563. (7) Kinniburgh, D. G.; van Riemsdijk, W. H.; Koopal, L. K.; Borkovec, M.; Benedetti, M. F.; Avena, M. J. Ion binding to natural organic matter: Competition, heterogeneity, stoichiometry and thermodynamic consistency. Colloids Surf., A 1999, 151 (1), 147−166. 5911

DOI: 10.1021/acs.est.5b00601 Environ. Sci. Technol. 2015, 49, 5905−5912

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DOI: 10.1021/acs.est.5b00601 Environ. Sci. Technol. 2015, 49, 5905−5912