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Aggregation, and Dissociation of Aqueous Al Induced by Fluoride Substitution Xiao Wang, Gong Zhang, Xiaoning Fu, Chengzhi Hu, Ruiping Liu, Huijuan Liu, Xiufang Xu, and Jiuhui Qu Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 02 May 2017 Downloaded from http://pubs.acs.org on May 2, 2017
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Aggregation, and Dissociation of Aqueous Al13 Induced by
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Fluoride Substitution
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Xiao Wang1,4, Gong Zhang1,4, Xiaoning Fu3, Chengzhi Hu1,4, Ruiping Liu2,4, Huijuan
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Liu*1,4, Xiufang Xu*3, and Jiuhui Qu2,4
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1
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Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China
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2
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Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China
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3
State Key Laboratory of Environmental Aquatic Chemistry, Research Center for
Key Laboratory of Drinking Water Science and Technology, Research Center for
Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry
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(Ministry of Education), Nankai University, Tianjin, 300071, China
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4
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*Corresponding author:
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Huijuan Liu, Tel: +86-10-6284-9128, E-mail:
[email protected] 14
Xiufang Xu, E-mail:
[email protected] University of Chinese Academy of Sciences, Beijing, 100049, China
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Abstract
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The ɛ-Keggin ion AlO4Al12(OH)24(H2O)127+ (ɛ-K Al137+) is a double-edged sword,
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because it commonly acts as a toxic component toward aquatic organisms, but also is
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considered to be an effective coagulant. Gaining deeper insight into the
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transformation of ɛ-K Al137+ in the presence of coexisting ligands would have
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significant implications for water environmental science, as well as for practical water
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purification. The aggregation and dissociation of aqueous Al137+ induced by Fluoride
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(F-) substitution were herein investigated using Nuclear Magnetic Resonance,
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Electrospray Ionization- Mass Spectrometry and theoretical calculations. The F-
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substitution on η-OH2 sites was extremely fast, reducing the charge of ɛ-K Al137+ so
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that the repulsive force between fluorinated Al13 species was immediately reduced.
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Consequently, fluorinated Al13 aggregated, with the formula [Al13F5]2+, which was
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demonstrated by calculating the Gibbs free energy changes (∆rG) of the substitution
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reactions involved. Moreover, the replacement of η-OH2 with F- weakened the
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strength of Al- OHa/b bonds and thus prompted the replacement of µ-OHa/b with F-. In
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addition, fluorination prompted [Al13F5]2+ to dissociate to oligomers.
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Keywords:
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Aluminum polymer • Al13 •Fluoride ligand • watersheds • coagulant
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Introduction
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Aluminum (Al) is the most abundant metal in Earth's geological environment, and
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research on Al chemistry plays an important part in the fields of geoscience, water and 2
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soil environments, and ecology.1-3 A variety of Al species exist, including solute
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molecules (Al monomers, Al polymers) and hydroxide precipitates (e.g. gibbsite),4-6
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among which the ε-isomer Keggin AlO4Al12(OH)24(H2O)127+ (ε-K Al137+) polymer has
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been demonstrated thermodynamically to be a crucial intermediate species in the
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transformation of aqueous Al to Al hydroxide precipitates.7,8 With multiple
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coordinated hydroxyl groups and high charges, the ε-K Al137+ structure includes 12
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edge-shared AlO6 octahedra in four sets of triple linked trimers around a central AlO4
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tetrahedron.9, 10 The ε-K Al137+ polymer has been detected in watersheds polluted by
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mine drainage and acid rain,11, 12 and has a toxicity that is rather general, and more
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virulent to plants and fishes than Al monomers.13-15 Previous studies have reported
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that ε-K Al137+ is a crucial component for mineral paragenesis and dissolution, as well
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as for sorption reactions at the solid-water interface,16,
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transformation of ions in the geochemical environment.
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which regulate the
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Due to Lewis acid-base complexation and electrostatic interactions, coexisting
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ligands (fluoride, carboxylate and natural organics) strongly react with the Al137+
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polymer, leading to the morphological transformation of both.18-20 The ubiquitous F-
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can dissolve the ε-Keggin structure during water-rock interaction.21 The ε-K Al137+
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polymer has been reported to be an effective component for coagulation and
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complexation of contaminants in water purification and soil remediation,22-24 and
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exhibits especially superior aqueous fluoride (F-) removal efficiency compared to
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other aluminum species.25 Therefore, investigation on the interaction mechanism of 3
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ε-K Al137+ and F- would be helpful not only to understanding the transformation and
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migration of Al13 and F in the geological environment, but also to provide a basis for
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the application and regulation of Al polymers in water treatment, which has attracted
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wide attention from environmental researchers.
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According to previous research, the hydroxyl coordination on the Keggin structure
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is divided into three types with different activities according to the configuration:
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outer binding water (η-OH2), hydroxyl between two trimers (µ-OHa), and hydroxyl
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between two Al in one trimer (µ-OHb), which are the binding sites for ligands.26, 27 19F
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nuclear magnetic resonance (NMR) results confirmed that three types of hydroxyl
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coordination
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GaO4Al12(OH)24(H2O)127+.28 The substitution path involved the transfer of F to µ-Fa/b
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from η-F. In the meantime, based on the theoretical predictions, F-substitution of ε-K
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Al137+ at µ-OHa, µ-OHb, and η-OH sites decreased the reactivity at η-OH2, and the
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structure was readily dissociated in solutions.29 These Al137+-F theories provided us an
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ideal platform to gain deep insight into the processes of complexation, aggregation,
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and precipitation in the presence of Al in complex natural water system, which is
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beneficial to polluted watershed remediation and drinking water treatment.
can
be
substituted
by
F- at different
reaction
rates
on
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Combining experimental analysis and theoretical predictions, we herein focused on
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the study of the interaction mechanism between ε-K Al137+ and F-, to reveal the
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essential reactions involved in water environments and water purification. The
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transition of Al species and F species during the reaction of ε-K Al137+ and F-, as well 4
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as the rates of F- substitution on three different oxygen sites of ε-K Al137+, were
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quantitatively investigated using liquid and solid Magic-Angle-Spinning (MAS) NMR
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detection methods. In addition, the reaction products were characterized via
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Electrospray Ionization Quadrupole - Time of Flight - Mass Spectrometry
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(ESI-TOF-MS), and their formation mechanisms were elucidated by combining
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experimental results and theoretical calculations. Finally, the environmental
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significance of the results was discussed.
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Experimental Section
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Preparation of ε-Keggin Al137+ solution
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ε-K Al137+ solution was obtained by purifying a polyaluminum chloride solution
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prepared with an electrolysis process.30-32 The
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(NMR, Bruker AvanceIII 400, USA) spectrum of the prepared solution exhibits a peak
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at 63 ppm which is assigned to the Al(O)4 tetrahedral site of ɛ-K Al13, and a peak at 0
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ppm due to aluminum monomers (Figure S1). The percentages of ɛ-K Al13 and
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monomers were 98.25% and 0.56% respectively, which were quantified based on the
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areas of the signals.
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Reaction of ɛ-K Al137+ and F-
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27
Al Nuclear Magnetic Resonance
Fluoride stock solutions (NaF, pH 6.0) were added into ɛ-K Al13 solutions (pH 5.0)
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with two concentrations levels as total Al, according to the F-Al molar ratios (RF:Al) of
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1:15, 2:15, 3:15, 4:15, 5:15, and 6:15, with rapid stirring at 20 ºC for two hours. In
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either polluted watersheds or drinking water treatment plants, the aqueous pH is 5
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around 6.5-7.0. Due to the similar Al-F complex reaction in the presence of Al13, the
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coagulation performance of AlCl3 almost maintained the same level in pH range from
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5 to 7.25,33 In order to deep insight into Al13-F reaction mechanism, it was reasonable
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to slightly lower the pH value for the high-purity Al13 structure, so that confirms the
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absence of interference from other Al species. Thus, the pH was herein maintained at
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~5.0. In the experiments, 0.1~0.6 mL of 8 mM or 0.5~3.0 mL of 0.8 M NaF solutions
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were added in 20 mL of 0.6 mM or 0.3 M ɛ-K Al13 solution, respectively, and the pH
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values of the mixtures can be predicted to be 5.00~5.01 or 5.01~5.05. The actual pH
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values of 5.01~ 5.05 and 5.05~ 5.20 were a little higher than the predictions (Table
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S1), which was due to the release of hydroxyl ions during the reaction process.
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However, the pH value was still within the range compatible with the ɛ-K Al13 species.
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The samples were filtered through ultra-filtration membranes (3000 Da) to separate
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the filtrates and precipitates. The reaction products in the filtrates of low concentration
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samples (AlT: 0.6 mM) were detected by ESI-TOF-MS (ACQUITY UPLC/Xevo G2
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Q TOF, Waters, USA). In addition, the filtrates of high concentration samples (AlT:
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0.3 M) were qualitatively and quantitatively analyzed using
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AvanceIII 500, USA), and the precipitates were qualitatively analyzed using
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MAS NMR. Inductively coupled plasma optical emission spectrometry (ICP-OES,
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Optima 2000 Perkin Elmer, USA) and a Dionex 2000 ion chromatography system
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(ICS, Dionex 2000, USA) were used to determine the total Al and F concentrations in
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filtrates. 6
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F/27Al NMR (Bruker 27
Al/19F
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Concentration changes of fluorinated sites The changes of F- substitution sites on the ɛ-K Al137+ polymer during the reaction 19
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were evaluated by real-time
F NMR. To meet concentration requirements for rapid
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detection by 19F NMR, the total Al concentration of ɛ-K Al13 solutions was 0.3 M, and
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the F concentration was 0.02 M. The first spectrum was acquired at 4.5 min after
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adding fluoride, and NMR measurement was continued for 1057 min.
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Zeta potential detection
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Zeta potentials and the size distribution in the suspensions after Al137+-F- reaction
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in 0.6 mM of AlT were measured using dynamic light scattering (DLS, Zetasizer Nano
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ZS90, Malvern, USA), with 0.3 ~100 µm measurable size range. To maintain the ionic
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strength, 10 mM NaCl was added into the suspensions. The zeta potential was
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obtained from the measurement of the electrophoretic mobility according to Henry’s
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equation (equation (1)).34 U/E is the electrophoretic mobility (m2 s-1 V-1), ζ is the zeta
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potential (V), ε is the solvent dielectric permittivity (kg m V-2 s-2), η is the viscosity
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(kg m-1 s-1), and F(ka) is Henry’s function (dimensionless). ε and η values were
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measured using a microwave network analyzer (Angilent N5224A, USA) and
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viscometer (BrookField DV2TLV, USA), respectively. F(ka) was set to 1.5.35
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U 2εζF (ka) = E 3η
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ESI-TOF-MS analyses
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(1)
The ESI-TOF-MS spectra were recorded on a micromass hybrid quadrupole
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time-of-flight mass spectrometer equipped with an electrospray ion source. Sodium
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formate solution was used to correct the mass spectra before the detection, and a
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leucine-enkephalin solution was used for real-time correction to ensure the accuracy
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of the mass spectrometry. All mass spectral data were performed in positive ion mode.
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The instrumental conditions were as follows: capillary voltage 3500.0 V, sample cone
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voltage 70 V (40, 60, and 150 V), extraction cone voltage 5 V, source temperature
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120 ℃, desolvation temperature 150 ℃, cone gas (N2) flow rate 300 L h-1, and mass
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range 50~1000.36, 37
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NMR analyses
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27
Al and
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F NMR were recorded on a Bruker AvanceIII 500 Spectrometer, with 27
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130 MHz and 470 MHz resonance frequencies respectively.
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were: repetition delay 2 s, frequency 130.3 MHz, scan width 39682.5 Hz, and number
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of scans 256. 19F NMR parameters were: repetition delay 2 s, frequency 470.6 MHz,
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scan width 71428.6 Hz, and number of scans 128. NaAlO2 (80 ppm) and NaSiF6
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(-129 ppm) solutions of known concentrations were employed as external standards
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for 27Al NMR and 19F NMR respectively, acting as the references for chemical shifts
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and relative peak areas. The concentrations of fluorinated sites and Al species were
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calculated according to the functional relationships between the concentrations of a
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series of NaF or AlCl3 (pH
