Sorption of Eu(III) on Humic Acid or Fulvic Acid Bound to Hydrous

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Environ. Sci. Technol. 2008, 42, 6532–6537

Sorption of Eu(III) on Humic Acid or Fulvic Acid Bound to Hydrous Alumina Studied by SEM-EDS, XPS, TRLFS, and Batch Techniques X . L . T A N , † X . K . W A N G , * ,† H . G E C K E I S , ‡ AND TH. RABUNG‡ Institute of Plasma Physics, Chinese Academy of Sciences, P.O.Box 1126, 230031, Hefei, P.R. China, and Forschungszentrum Karlsruhe, Institut fu ¨ r Nukleare Entsorgung, P.O.Box 3640, D-76021 Karlsruhe, Germany

Received March 10, 2008. Revised manuscript received June 16, 2008. Accepted June 17, 2008.

To identify the effect of humic acid (HA) and fulvic acid (FA) on the sorption mechanism of Eu(III) on organic-inorganic colloids in the environment at a molecular level, surface adsorbed/ complexed Eu(III) on hydrous alumina, HA-, and FA-hydrous alumina hybrids were characterized by using X-ray photoelectron spectroscopy (XPS) and time-resolved laser fluorescence spectroscopy (TRLFS). The experiments were performed in 0.1 mol/L KNO3 or 0.1 mol/L NaClO4 under ambient conditions. The pH values were varied between 2 and 11 at a fixed Eu(III) concentration of 6.0 × 10-7 mol/L and 4.3 × 10-5 mol/L. The different Eu(III)/FA(HA)/hydrous alumina complexes were characterized by their fluorescence emission spectra ((5D0f7F1)/ (5D0f7F2)) and binding energy of Eu(III). Inner-sphere surface complexation may contribute mainly to Eu(III) sorption on hydrous alumina, and a ternary surface complex is formed at the HA/ FA-hydrous alumina hybrid surfaces. The sorption and species of Eu(III) in ternary Eu-HA/FA-hydrous alumina systems are not dominated by either HA/FA or hydrous alumina, but are dominated by both HA/FA and hydrous alumina. The results are important for understanding the sorption mechanisms and the nature of surface adsorbed Eu(III) species and trivalent chemical homologues of Eu(III) in the natural environment.

Introduction The environmental behavior of lanthanide and actinide ions has attracted intense interest as they are constituents of the long-lived radioactive waste. Obviously, it is the knowledge of the sorption mechanism which is of importance to assess the radionuclide behavior in natural environments. Potentially released radionuclides such as Np, Pu, Am, and Cm may be adsorbed onto natural mineral surfaces, and this retention enhances the retardation of radionuclides in the natural environment (1). Eu(III) is a trivalent lanthanide and a chemical homologue of trivalent actinides as both trivalent lanthanides and actinides exhibit similar sorption properties (2). Thereby, the interaction of Eu(III) with natural minerals and oxides has been studied extensively by using batch techniques (3–12), chelating resins (13–15), spectroscopic * Corresponding author phone: +86-551-5592788; fax: +86-5515591310; e-mail: [email protected] (X.K.Wang). † Chinese Academy of Sciences. ‡ Forschungszentrum Karlsruhe. 6532

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techniques (3, 4, 16–18), and capillary methods (19). Alumina is used as model mineral of clay minerals, and clays are one of potential host rocks for final nuclear waste disposal. The results indicate that Eu(III) sorption to oxides is generally strongly pH dependent and, to a lesser extent, influenced by ionic strength. Sorption of Eu(III) onto clay minerals, however, depends clearly on ionic strength at low pH where ion exchange and outer-sphere sorption dominate. At high pH and ionic strength inner-sphere surface complexation dominates (9–12). Adsorption of Eu(III) on alumina has previously been studied by conventional batch titration techniques (5–16, 19). A mechanistic understanding of the underlying physicochemical process is necessary for the evaluation of longterm performance assessment of a nuclear waste repository. However, the mechanisms governing Eu(III) immobilization and sorption are still not completely understood. Humic substances (HSs) are known to have strong influence on the sorption and speciation of lanthanides and actinides in natural aqueous systems (20–29). Macroscopic investigations of Eu(III) sorption on oxides in the presence of HSs have been carried out in the last decades (9–15, 19). Generally the presence of HS increases sorption of cations at low pH but reduces the sorption at high pH values (10, 11, 26–28). The increase of sorption is interpreted by the adsorption of HS on mineral surface followed by the interaction of metal ions with surface adsorbed HS, whereas the decrease of sorption is explained by the formation of soluble M-HS complexes in aqueous solution (26). Identification of functional groups involved in M-HS bonding is important for assessment of the complexation of actinides with HSs in groundwater. Recent studies indicated that proton exchanging groups such as carboxylic acid and phenolic groups are the primarily responsible binding sites for the complexation of metal ions with HSs. Such chemical state information can be deduced from the chemical shifts of elemental lines in X-ray photoelectron spectroscopy (XPS) (30, 31). Time resolved laser fluorescence spectroscopy (TRLFS) has been proven a versatile tool to study both solution chemistry and surface complexation of HSs in relation to Eu(III) sorption (2–4, 32). Herein, we studied the sorption of Eu(III) on hydrous alumina as a function of pH in the presence and absence of HA or FA under ambient conditions. Two techniques (XPS and TRLFS) were applied to study the sorption mechanism at a molecular level and thereby to identify the formed species. X-ray diffraction was devoted to identify the crystal nature of the minerals in the samples. SEM-EDS experiments were performed to observe the oxide morphology before and after sorption.

Experimental Section Materials. Eu stock solution was prepared from Eu2O3 by dissolution, evaporation and redissolution in 10-3 mol/L perchloric acid. The hydrous alumina particles used in this work (Degussa, Aluminum Oxide C) have been previously used in sorption studies (2, 26, 33). The hydrous alumina was first washed with 0.1 mol/L HNO3, then with 0.1 mol/L NaOH and finally rinsed with Milli-Q water until the conductivity of the washing solution reached a stable value around 1.4-1.6 µS. The purified alumina was then stored as a suspension (66 g/L) in Milli-Q water, and has been aged for 4 years. The XRD analysis (Supporting Information Figure SI-1) indicated that the hydrous alumina is a mixing of bayerite, boehmite, and diaspore. The specific surface area determined by the N2-BET method is 105 m2/g (2, 33). The particle size is about 100 nm, and the point of zero charge 10.1021/es8007062 CCC: $40.75

 2008 American Chemical Society

Published on Web 08/01/2008

TABLE 1. 13C NMR Characteristics (Chemical Shift ppm) % of HA and FA a

HA FA a

0-50

51-105

106-160

161-200

aromaticity

15 16

21 28

47 19

17 39

57 30

Refs 35 and 36.

(pHpzc) is about 8.9 (2, 33). The site concentration calculated from the titration data in 0.1 mol/L NaClO4 amounts to 1.96 × 10-4 equiv/g corresponding to a site density of 1 site/nm2 (2, 34). Soil humic acid (HA) and fulvic acid (FA) were extracted from the soil of Hua-Jia county (Gansu province, China), and were characterized in detail (35, 36). Cross-polarization magic angle spinning (CPMAS) 13C NMR spectra of HA and FA were divided into four chemical shift regions, 0-50 ppm, 51-105 ppm, 106-160 ppm, and 161-200 ppm. These regions were referred to as aliphatic, carbohydrate, aromatic, and carboxyl regions. The percentage of total intensity for each region is estimated by integrating the CPMAS 13C NMR spectra with each region, and the fraction of aromatic groups calculated by expressing aromatic C as percentage of the sum of aliphatic C (0-105 ppm) + aromatic C (106-160 ppm) is listed in Table 1. The results of potentiometric titrations of FA and HA are presented in Supporting Information Figures SI-2 and SI-3, and in Table SI-1. Sorption Experiments. The experiments were carried out under ambient conditions at T ) 20 ( 1 °C in the presence of 0.1 mol/L KNO3 or 0.1 mol/L NaClO4 by using batch technique. All solutions were prepared using Milli-Q water, and all experiments were conducted in polyethylene tubes. The aqueous suspension was mixed with a solution containing the background electrolyte KNO3 or NaClO4, HA or FA, hydrous alumina and Milli-Q water. The radiotracer 152 + 154Eu(III) was used in the batch sorption experiments. Humic acid or FA was first equilibrated with the aqueous alumina suspension for 3 days, and then Eu(III) solution was added into the HA/ FA-hydrous alumina suspension to study the sorption of Eu(III) on HA/FA-hydrous alumina hybrids. The pH value of the solution was adjusted with negligible amounts of 0.1 or 0.01 mol/L HNO3 or KOH. The blank experiments (i.e., no hydrous alumina) indicated that sorption of Eu(III) on the tube walls was negligible. Our pre-experiments of kinetic sorption indicated that sorption equilibrium could be achieved in several hours. The polyethylene tubes containing oxide and aqueous solution were shaken for 3 days to attain sorption equilibration, and then the solid was separated from supernatant solution by centrifugation at 18 000 rpm for 30 min. The Eu(III) concentration in the supernatant was analyzed by liquid scintillation counting (Packard 3100 TR/ AB liquid scintillation analyzer, PerkinElmer) with ULTIMA GOLD AB (Packard) scintillation cocktail. X-Ray Photoelectron Spectroscopy (XPS). For spectroscopic analysis, the solid was separated by filtration and subsequently washed with 0.1 mol/L KNO3 solution to remove non adsorbed Eu(III), and then the solid phases were dried at 50 °C. Only part of free water on solid phases was removed in this treatment, and this treatment did not result in any surface species modification. X-ray photoelectron spectroscopy data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al KR radiation. The pressure in the analysis chamber was maintained below 3 × 10-9 mbar. The XPS photoelectron binding energies (BE) of the adventitious carbon species, i.e., the C 1s line at 284.8 eV was used to correct the observed binding energies for surface charging (37). Curve fitting and decomposition were achieved as-

FIGURE 1. Effect of pH on the sorption of Eu(III) on bare, HA coated and FA coated hydrous alumina particles. C(hydrous alumina) ) 4.4 g/L, C(FA/HA)(initial) ) 10 mg/L, T ) 20 ( 1 °C, C(NaClO4) ) 0.1 mol/L, C(Eu(III))(initial) ) 6.0 × 10-7 mol/L (A); C(KNO3) ) 0.1 mol/L, C(Eu(III))(initial) ) 4.3 × 10-5 mol/L (B). suming Gaussian-Lorentzian fitting following Shirley background subtraction. Time Resolved Laser Fluorescence Spectroscopy (TRLFS). TRLFS measurements were performed using a pulsed Nd:YAG pumped dye laser system (Continuum, Powerlite 9030, ND 6000). A laser pulse energy of at most 3 mJ was measured by a photodiode. The fluorescence emission was detected by an optical multichannel analyzer consisting of a polychromator (Chromex 250) with a 300 lines/mm grating. The emission spectra were recorded at room temperature in the 480-750 nm range, within a constant time window of 1 ms at the constant excitation wavelength of 394 nm (2). For measuring the time dependence of fluorescence emission decay, the delay time between laser pulse and camera gating was scanned with time intervals of 10 and 30 µs.

Results and Discussion Macroscopic Sorption Experiments. Figure 1 shows the pH dependent sorption of Eu(III) on bare, HA coated and FA coated hydrous alumina particles. The initial concentrations of Eu(III) in Figure 1A and B were 6.0 × 10-7 mol/L and 4.3 × 10-5 mol/L, respectively. As found in earlier studies, sorption of Eu(III) is strongly dependent on pH, which suggests that surface complexation contributes to the sorption of Eu(III) (11, 13). The presence of HA/FA enhances the sorption of Eu(III) on HA/FA-hydrous alumina hybrids at pH < 7 in agreement with earlier investigations. At pH < 7, the negatively charged HA and FA are easily adsorbed on the positive charged hydrous alumina surface. Surface adsorbed VOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Relative proportion of species distribution of Eu(III) in solution in the presence of HA (A) and absence of HA (B). I ) 0.1 mol/L KNO3, T ) 20 ( 1 °C, C(HA) ) 10 mg/L, C(Eu(III)) ) 6.0 × 10-7 mol/L. HA and FA can form strong complexes with Eu(III) and thereby enhances the sorption of Eu(III) on hydrous alumina particles. At pH < 8, (96 ( 2) % HA/FA is adsorbed to hydrous alumina according to the analysis of HA/FA concentration in supernatant using UV-vis spectrophotometric analysis (see Supporting Information Figure SI-4). The sorption of HA/FA on hydrous alumina decreases with increasing pH at pH > 8, and about 80% HA/FA is adsorbed at pH ∼10. It is also interesting to note that the influence of FA on Eu(III) sorption to FA-hydrous alumina hybrids is higher than that of HA at low pH values. This may be attributed to the fact that FA has a higher carboxylic group content than HA and therefore binds more Eu(III) at the hydrous alumina surface. The proton exchanging site density of FA (2.71 × 10-2 mol/g) is higher than that of HA (6.46 × 10-3 mol/g) (Supporting Information Table SI-1), which explains the stronger sorption of Eu(III) on FA-hydrous alumina hybrids than on HAhydrous alumina hybrids at low pH values. It is well-known that the proportion of surface species (≡ SOH uncharged surface group; ≡SOH+ 2 positively charged group; and ≡SO- negatively charged group) of hydrous alumina change with increasing pH. The protonation reaction + + of ≡SOH forms ≡SOH+ 2 at low pH (i.e., ≡SOH+H f ≡SOH2 ) - at high and the deprotonation reaction of ≡SOH forms ≡SO pH values (i.e., ≡SOH f ≡SO-+H+). It was reported that the free Eu3+ ion was the predominant species at pH < 6.5 in the absence of hydrous alumina, HA or FA; the ≡SOHsurface group contributes mainly to the sorption of Eu3+ ion (9, 14). With increasing pH, functional groups are progressively 6534

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FIGURE 3. 3A: Eu 3d XPS spectra for Eu-hydrous alumina (a), Eu-HA-hydrous alumina (b), Eu-FA-hydrous alumina (c), and Eu-HA (d). C(hydrous alumina) ) 4.4 g/L, C(FA/HA)(initial) ) 10 mg/L, pH 6.3 ( 0.1, C(KNO3) ) 0.1 mol/L, C(Eu(III))(initial) ) 4.3 × 10-5 mol/L, the sorption of Eu(III) was carried out at T ) 20 ( 1 °C, and the solid phase was dried at T ) 50 ( 1 °C. 3B: The deconvolution of Eu 3d5/2 spectra for Eu-hydrous alumina, Eu-HA-hydrous alumina, and Eu-FA-hydrous alumina deprotonated, forming negative surface charge. The attractive force between the anionic surface sites and cationic metal ions easily results in the formation of metal-ligand hydrous alumina complexes. It is necessary to inspect the speciation of Eu(III) in solution to discuss the possible species of Eu(III) in the binary or ternary systems. The thermodynamic constants of Eu(III) are listed in Supporting Information Table SI-2. The presence of ternary carbonatohumate and hydroxohumate complexes and binary carbonate and hydroxyl complexes are shown in Figure 2A and B according to the calculation. The main species of Eu(III) are Eu3+ and Eu(NO3)2+ in binary Eu-hydrous alumina system and Eu3+(HA) in ternary Eu-HA-hydrous alumina system at pH < 6. At low pH values, the complex of Eu3+(HA) is easily adsorbed on hydrous alumina, and thereby enhances Eu(III) sorption on FA/HA-hydrous alumina hybrids. This also explains the enhancing sorption of Eu(III) in ternary system at pH < 6. The sorption of Eu(III) at pH > 8, where HA/FA is not any more completely bound to hydrous alumina, can be considered as a result of the

FIGURE 4. TRLFS results for Eu(III) bound to HA, FA, hydrous alumina, HA-hydrous alumina, and FA-hydrous alumina hybrids. C(hydrous alumina) ) 4.4 g/L, C(Eu(III))(initial) ) 4.3 × 10-5 mol/L, C(FA/HA)(initial) ) 10 mg/L, C(KNO3) ) 0.1 mol/L, pH 6.3 ( 0.1, T ) 20 ( 1 °C. competition among Eu(III) with hydrous alumina, with surface adsorbed HA/FA, and with dissolved HA/FA. At pH > 8, the sorption of Eu(III) on HA/FA bound hydrous alumina does not reach levels achieved in systems free from HA/FA at high Eu(III) concentrations. This could be due to either residual Eu(CO3)(HA/FA) remaining in solution, thus reducing Eu(CO3)2 available for sorption, or residual HA/FA on hydrous alumina surface enhancing electrostatic repulsion of Eu(CO3)2 , or blocking potential sorption sites on hydrous alumina surface. Microscopic Spectroscopy Analysis. SEM Micrographs. Typical SEM images of hydrous alumina prior to and after Eu(III) sorption are shown in Supporting Information Figure SI-5. Hydrous alumina particles present as platelets with approximately 50∼100 nm thickness and variable shapes ∼500 nm in diameters. The presence of Eu(III) on several different zones of the hydrous alumina surface was confirmed by energy-dispersive spectroscopy (SEM-EDS) analysis (Supporting Information Figure SI-6). The figure shows that hydrous alumina is mainly composed of Al, O, and K; a small amount of Eu is also detected, which indicates the presence of surface adsorbed Eu(III) on hydrous alumina. XPS Investigations. In order to achieve molecular level information of Eu(III) sorbed onto hydrous alumina, XPS spectra were recorded to identify the Eu(III) species adsorbed on bare, HA and FA bound hydrous alumina particles. Changes of the peak shape or the energetic position of core

level spectra indicate chemical binding of Eu(III) to hydrous alumina. Figure 3A shows the XPS spectra of Eu 3d for Eu(III) adsorbed on the surfaces of hydrous alumina (a), HA-hydrous alumina (b), and FA-hydrous alumina hybrids (c). The binding energy value of 1135.3 eV found in the present study is close to the one reported by several authors for hydrolyzed Eu(III) species (38). The XPS spectra are characterized by two Eu 3d main lines, Eu 3d5/2 at 1135.3 eV and Eu 3d3/2 at 1165.1 eV in sample (a), with a peak separation of 29.8 eV between the two peaks. The binding energy of Eu 3d in samples (b) and (c) increases as compared to sample (a). It indicates that the Eu-HAhydrous alumina or Eu-FA-hydrous alumina interactions are stronger than Eu-hydrous alumina interaction (39, 40). The spectra are well-fitted with two components located at 1135.3 and 1137.4 eV associated to hydrous alumina and FA/HA bound hydrous alumina, respectively (Figure 3B). One surface species at 1135.3 eV was considered on hydrous alumina, whereas two components located at 1135.3 and 1137.4 eV were associated to hydrous alumina and FA/HA bound hydrous alumina. This XPS feature is perhaps associated with inner-sphere surface complexes of Eu(III). The XPS findings point to the existence of a ternary Eu3+ complex at the HA/ FA-hydrous alumina hybrid surfaces. The Eu-HA/FA-hydrous alumina bonds are formed on the surface of HA/FA bound hydrous alumina nanoparticles and thereby increase the sorption extent of Eu(III). VOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TRLFS Analysis. The position of the Eu(III) luminescence bands is independent of the speciation of the Eu(III) ion. The lifetime, intensity, and peak form of 5D0 f 7F1 (λ ) 592 nm) and 5D0 f 7F2 (λ ) 617 nm) transition change significantly when Eu(III) complexes/sorption are formed, which is a result of so-called “hypersensitive” transition (4). The peak forms and the ratio of the emission intensities obtained for the 5D0 f 7F1 (λ ) 592 nm) and 5D0 f 7F2 (λ ) 617 nm) transitions provide information on the Eu(III) speciation (32). Figure 4 shows the fluorescence emission spectra (Figure 4A and C) and time dependence of fluorescence emission decay (Figure 4B and D) of Eu(III) sorbed on HA, FA, hydrous alumina, HA-, and FA-hydrous alumina hybrids at pH 6.3 in 0.1 mol/L KNO3 solutions, respectively. The ratio of emission intensities of (5D0 f 7F1)/(5D0 f 7F2) of Eu(III) sorbed on hydrous alumina, HA-, and FA-hydrous alumina suspensions are 0.51 ( 0.01, 0.21 ( 0.01, and 0.20 ( 0.01, respectively (Supporting Information Table SI-3). It is interesting to note that there is little difference in the fluorescence spectra of Eu(III) in HA-hydrous alumina and FA-hydrous alumina suspensions, suggesting a similar binding state, which is significantly different to that of Eu(III)-hydrous alumina. At pH 6.3, the sorption extent is the same for Eu(III) interacted with HA-hydrous alumina and FA-hydrous alumina hybrids within the experimental uncertainties (see Figure 1). The fluorescence intensity of Eu(III)-hydrous alumina as a function of delay time is quite different to that of Eu(III)-HA/ FA-hydrous alumina. The shortening of fluorescence lifetime in the presence of HA/FA is well-known and attributed to energy transfer from the excited-state of Eu(III) to chromophoric groups of the FA/HA (41). It is quite sure that primarily the carboxylic groups are responsible for metal ion binding to HS. In case of ternary complexes, surface sorbed HA can bridge the cation to alumina surface, or the cation can be located between the surface and the HS. A third possibility is that the metal ion is sorbed to the mineral surface in the direct neighborhood of sorbed HS molecules (which could also be very attractive for the metal ion due to the negative charge given by the HS molecule). In all cases the metal ion fluorescence can be quenched by HS due to the small distances between metal ion and HS. Therefore, the fluorescence lifetimes should be shorter than in the system without HS, which is also found in our experiments. The fact that fluorescence quenching is observed shows however, in agreement with XPS, that Eu(III) is bound to FA/HA in the ternary system. A comparison of TRLFS spectra obtained with Eu(III)hydrous alumina, Eu(III)-FA, and Eu(III)-HA (Figure 4C and D) shows clear differences. The intensity ratio (5D0 f 7F )/(5D f 7F ) of Eu(III)-HA is different from that of 1 0 2 Eu(III)-FA and so is the fluorescence intensity as a function of delay time. Comparing to the spectra of Eu(III) in HAhydrous alumina and FA-hydrous alumina systems, one can see that the sorption and speciation of Eu(III) in HA-hydrous alumina and in FA-hydrous alumina are quite different to those of Eu(III) in hydrous alumina, in HA and in FA solutions, respectively. The result suggests that the sorption and species of Eu(III) in ternary Eu-HA-hydrous alumina or Eu-FAhydrous alumina systems are not dominated by either HA/ FA or hydrous alumina, but are dominated by both HA/FA and hydrous alumina. Fluorescence quenching is more pronounced for the Eu(III)-HA complex than for the Eu(III)-FA complex. Aromatic groups acting as chromophores are more abundant in HA than in FA (see Table 1), which explains the higher degree in quenching for Eu(III)-HA as compared to Eu(III)-FA. The differences make clear that the binding state in the Eu(III)-FA/HA-hydrous alumina is different to the two binary systems and a ternary complex is formed. This is again in agreement to XPS results and earlier studies on the behavior of Cm(III) (33). 6536

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The outcome of this study suggests that even low HA/FA concentrations, which may be present in natural groundwater, may influence mineral surface properties significantly and thus will have some impact on radionuclide sorption behavior. In this study, HA and FA have been extracted from the same soil and it is not surprising that they have similar functional groups to form complexes with Eu(III) at hydrous alumina surfaces. The batch results of Eu(III) sorption on HA/FA bound to hydrous alumina show no difference in sorption at pH 6.3, and the XPS and TRLFS results indicate that the same/similar structures and species of Eu(III) in the ternary systems are formed. The results are important to understand the influence of natural HSs on sorption and speciation of trivalent lanthanides and actinides in the natural environment.

Acknowledgments Financial support from National Natural Science Foundation of China (20677058; 20501019) and Centurial Project of CAS are acknowledged.

Supporting Information Available Acid-base titration and distribution of surface sites concentration of HA/FA; Sorption of HA/FA on hydrous alumina as a function of pH; The figure of SEM-EDS; The figure of XRD patterns. This material is available free of charge via the Internet at http://pubs.acs.org.

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