DOI: 10.57760/sciencedb.35176
title: Magnesium Ion-Induced Efficient Crystallization of Macallisterite: A Combined Strategy of Sonochemistry Assistance and pH Regulation
title: Magnesium Ion-Induced Efficient Crystallization of Macallisterite: A Combined Strategy of Sonochemistry Assistance and pH Regulation

description: The overall strategy includes optimization of macallisterite synthesis to improve the product yield, and additive-assisted crystallization for the purpose of morphology control and is schematically illustrated in Figure 1.Magnesium salts effects on the yield. A sodium borate solution was prepared by dissolving 7.7313 g boric acid and 1.8007 g of sodium hydroxide in 74 mL deionized water. Then, magnesium salts of magnesium chloride hexahydrate (MgCl2.6H2O, 40.6620 g), magnesium sulfate heptahydrate (MgSO4·7H2O, 49.3000 g) and magnesium nitrate hexahydrate (Mg(NO3)2·6H2O, 56.4600 g) were dissolved in the sodium borate solution, respectively, to obtain three macallisterite supersatuted solutions. Thereafter, these supersaturated solutions were placed in the parallel crystallizer (CrystalSCAN PB4, Technobis Crystallization Systems, Figure S1) and cooled to 15 ℃ for 20 hours under stirring rate of 400 rpm for the macallisterite crystallization. The solid product was filtrated and washed sequentially with deionized water (2 × 10 mL) and anhydrous ethanol (2 × 10 mL), and then dried in a vacuum oven at 45 ℃ for 12 hours for the powder X-ray diffraction (PXRD) characterization. The boron concentration in the mother solution was analyzed by titration method. The product yield was calculated by the following:                             (1)Y is the product yield, %; c1 and c2 are the boron mass fraction of the supersaturated solution and the mother liquid, respectively; m0 and m1 are the total mass of the supersaturated solution and the mother liquid, respectively, g.Solution pH effects on the yield. The impact of the initial solution pH on product yield was investigated by adjusting the mounts of sodium hydroxide following the Na/B molar ratio of 0.20, 0.26, 0.30, 0.36, 0.40 and 0.50. Specifically, sodium borate solutions were prepared by dissolving 7.7313 g boric acid and 1.0004 g, 1.3005 g, 1.5006 g, 1.8007 g, 2.0009 g and 2.5011g sodium hydroxide in 74 mL of deionized water. Then, magnesium chloride hexahydrate (40.6620 g) were dissolved in the above sodium borate solutions to obtain macallisterite supersaturated solutions with pH of 4.90, 5.35, 5.80, 6.05, 6.30 and 6.60, respectively. These supersaturated solutions were placed in the parallel crystallizers for the macallisterite crystallization. The yield was calculated and the product was characterized by the PXRD.Ultrasonic time and temperature effects on the yield. The macallisterite supersaturated solutions were prepared by dissolving 7.7313 g boric acid, 1.8007 g sodium hydroxide and 40.6620 g magnesium chloride hexahydrate in 74 mL deionized water. These solutions were placed in a polydimethylsiloxane (PDMS) oil bath and heated at 60, 80, or 100 ℃ with 50 kHz ultrasonic irradiation of 5, 10, 20, 30 or 40 minutes, respectively, using an SS-GEN-IND-35K1200W ultrasonic generator. Then the supersaturated solutions were moved to the parallel crystallizer for the macallisterite crystallization. Finally, the yield was calculated and the product was characterized by the PXRD, differential scanning calorimetry (DSC) and scanning electron microscopy (SEM).Raman spectroscopy investigation on the borate species in solution. Raman spectra were acquired using a laser micro-Raman spectrometer (Thermo Fisher Scientific, USA) with a 532 nm solid-state laser as the excitation source. The prepared solutions were transferred into quartz capillary tubes, ensuring the removal of air bubbles. The instrument was calibrated before sample collecting. The laser power used is 10 mW, the sample was collected with a exposure time of 40 s for 10 times at room temperature, covering a spectral range from 400 to 950 cm-1 with a resolution of approximately 1 cm-1.DFT calculation of the polymerization of borate species in solution. Density functional theory (DFT) calculations were conducted to quantify the thermodynamic driving forces and clarify the charge transfer mechanisms underlying the Mg2+-induced polymerization of borate species in the macallisterite supersaturated solutions. All computations were performed using the Gaussian 16 software package. The B3LYP functional was employed in conjunction with the D3BJ dispersion correction for all computations. Geometry optimizations and frequency calculations utilized the 6-311+G (d,p) basis set for all atoms. Geometry optimizations and frequency analyses were carried out for all atoms. To accurately model the aqueous synthesis environment, the Solvation Model based on Density (SMD) was employed, with water as the solvent, during both the optimization and frequency calculations.This computational protocol was applied to a series of key structures: the dominant borate species (B3O3(OH)4-, B4O5(OH)42-, B5O6(OH)4-, and B6O7(OH)62-), the Mg(H2O)62+ cation, and their resulting coordination complexes of the [Mg(H2O)6-n(B3O3(OH)4)]+, [Mg(H2O)6-n(B4O5(OH)4)], [Mg(H2O)6-n(B5O6(OH)4)]+ and [Mg(H2O)6-n(B6O7(OH)6)]. All optimized structures were confirmed to be true local minima on the potential energy surface, as evidenced by the absence of imaginary frequencies.The Gibbs free energy (ΔG) for the complexation process was calculated using the following expression:             (2)where GComplex, GBoron species, and GMg(H₂O)₆²⁺ represent the single-point energies of the optimized complex, the isolated boron species, and the isolated Mg(H2O)62+ cation, respectively. The single-point energies were corrected for basis set superposition error (BSSE) via the counterpoise method. The Gibbs free energy change (ΔG) for the polymerization reactions was computed to evaluate their thermodynamic spontaneity. Natural bond orbital (NBO) analysis at the same level of theory was performed on the optimized complexes to quantify the charge transfer between Mg2+ and the boron species. Finally, electrostatic potential (ESP) maps were generated and visualized using the VMD program. These maps were rendered based on the optimized structures that were confirmed to be true local minima (as evidenced by the absence of imaginary frequencies), providing visual insight into the charge distribution and reactive regions within the complexes.Preparation of the Macallisterite Spherulites via Additive-Assisted Batch Crystallization. The sodium dodecyl sulfonate (SDS) was used as an additive to prepare the uniform macallisterite spherulites for the purpose of the crystal agglomeration improvment.SDS Concentration Effects on Macallisterite Spherulites Preparation. The macallisterite supersaturated solutions were prepared by dissolving 7.7313 g boric acid, 1.8007 g sodium hydroxide and 40.6620 g magnesium chloride hexahydrate in 74 mL deionized water by adding 0.025, 0.05, 0.10, 0.20, 0.50, or 1.00 g SDS, respectively. These supersaturated solutions were placed in a polydimethylsiloxane oil bath and heated at 80 ℃ for 20 minutes of 50 kHz ultrasonic irradiation by using an SS-GEN-IND-35K1200W ultrasonic generator. Then they were moved to the parallel crystallizers for the macallisterite crystallization. The product was characterized by the PXRD and SEM.Solution pH Effects on Macallisterite Spherulites Preparation. The macallisterite supersaturated solutions with initial pH values of 4.90, 5.35, 5.80, 6.05, 6.30, and 6.60 were prepared by dissolving 7.7313 g boric acid and 1.0004 g, 1.3005 g, 1.5006 g, 1.8007 g, 2.0009 g or 2.5011g sodium hydroxide, 40.6620 g magnesium chloride hexahydrate in 74 mL of deionized water, respectively. 0.10 g SDS was then added in the above supersaturated solutions. The resulting solutions were placed in a polydimethylsiloxane oil bath of 80 ℃ for ultrasonic irradiation and then moved to the parallel crystallizer for the macallisterite crystallization. Finally, the products were characterized by the PXRD and SEM.Crystallization Time Effects on Macallisterite Spherulites Preparation. Nine macallisterite supersaturated solutions were prepared by dissolving 7.7313 g of boric acid, 1.8007 g of sodium hydroxide and 40.6620 g in 74 mL of deionized water with 0.10 g SDS addition, respectively. And then the resulting solutions were used for the macallisterite crystallization after ultrasonic irradiation in an oil bath (80 ℃, 20 min, 50 kHz). Finally, the products were characterized by the PXRD and SEM.Powder X-ray Diffraction (PXRD). The phase composition and crystal structure of the synthesized macallisterite samples were characterized using Powder X-ray diffraction (PXRD) on a D/max 2500 VL/PC diffractometer (Rigaku SmartLab, Japan). The measurements were performed using Cu Kα radiation (λ=1.5418 Å) generated at 45 kV and 200 mA. The diffraction patterns were recorded in the 2θ range of 2° to 40° with a scanning speed of 5° per minute and a step size of 0.01°.pH Measurement. The pH of all solutions was measured using a calibrated Seven Excellence pH meter (Mettler Toledo, Switzerland) equipped with an Inlab expert Pro-ISM pH electrode. The instrument was calibrated daily prior to measurements using standard buffer solutions (pH 4.01, 7.00, and 10.01 at 25 ℃, Merck KGaA, Germany). All readings were taken at an ambient temperature of 25±1 ℃ with automatic temperature compensation enabled, ensuring an accuracy of ±0.02 pH units.Scanning Electron Microscopy (SEM). The microstructure and crystal morphology of the macallisterite products were examined using an SU8010 field emission scanning electron microscope (Hitachi High-Technologies Corporation, Japan). Samples were fixed on conductive adhesive tape and sputter-coated with a 5 nm thick Pt layer to enhance conductivity. Observations were conducted under high vacuum conditions at accelerating voltages of 5.0 or 10.0 kV. Images were captured using the HITACHI PC-SEM software at a working distance of approximately 8 mm.Thermal Analysis (DSC-TG). The thermal behavior, including dehydration, decomposition, and phase transitions, was investigated using a synchronous thermal analyzer (Setaram, France). 10-20 mg Samples were placed in alumina crucibles and heated from room temperature to 800 ℃ (or 1200 ℃) at a constant rate of 5 ℃•min-1 under a dynamic nitrogen atmosphere (flow rate of 50 mL•min-1). Both differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) signals were simultaneously recorded to monitor changes in heat flow and mass loss, respectively.Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). The elemental composition of the macallisterite samples was quantified using an inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 720ES, USA). Samples were digested in appropriate acids prior to analysis. Calibration curves were constructed using certified standard solutions for each element of interest. All measurements were performed in triplicate, and the results are reported as average concentrations.
description: The overall strategy includes optimization of macallisterite synthesis to improve the product yield, and additive-assisted crystallization for the purpose of morphology control and is schematically illustrated in Figure 1.Magnesium salts effects on the yield. A sodium borate solution was prepared by dissolving 7.7313 g boric acid and 1.8007 g of sodium hydroxide in 74 mL deionized water. Then, magnesium salts of magnesium chloride hexahydrate (MgCl2.6H2O, 40.6620 g), magnesium sulfate heptahydrate (MgSO4·7H2O, 49.3000 g) and magnesium nitrate hexahydrate (Mg(NO3)2·6H2O, 56.4600 g) were dissolved in the sodium borate solution, respectively, to obtain three macallisterite supersatuted solutions. Thereafter, these supersaturated solutions were placed in the parallel crystallizer (CrystalSCAN PB4, Technobis Crystallization Systems, Figure S1) and cooled to 15 ℃ for 20 hours under stirring rate of 400 rpm for the macallisterite crystallization. The solid product was filtrated and washed sequentially with deionized water (2 × 10 mL) and anhydrous ethanol (2 × 10 mL), and then dried in a vacuum oven at 45 ℃ for 12 hours for the powder X-ray diffraction (PXRD) characterization. The boron concentration in the mother solution was analyzed by titration method. The product yield was calculated by the following:                             (1)Y is the product yield, %; c1 and c2 are the boron mass fraction of the supersaturated solution and the mother liquid, respectively; m0 and m1 are the total mass of the supersaturated solution and the mother liquid, respectively, g.Solution pH effects on the yield. The impact of the initial solution pH on product yield was investigated by adjusting the mounts of sodium hydroxide following the Na/B molar ratio of 0.20, 0.26, 0.30, 0.36, 0.40 and 0.50. Specifically, sodium borate solutions were prepared by dissolving 7.7313 g boric acid and 1.0004 g, 1.3005 g, 1.5006 g, 1.8007 g, 2.0009 g and 2.5011g sodium hydroxide in 74 mL of deionized water. Then, magnesium chloride hexahydrate (40.6620 g) were dissolved in the above sodium borate solutions to obtain macallisterite supersaturated solutions with pH of 4.90, 5.35, 5.80, 6.05, 6.30 and 6.60, respectively. These supersaturated solutions were placed in the parallel crystallizers for the macallisterite crystallization. The yield was calculated and the product was characterized by the PXRD.Ultrasonic time and temperature effects on the yield. The macallisterite supersaturated solutions were prepared by dissolving 7.7313 g boric acid, 1.8007 g sodium hydroxide and 40.6620 g magnesium chloride hexahydrate in 74 mL deionized water. These solutions were placed in a polydimethylsiloxane (PDMS) oil bath and heated at 60, 80, or 100 ℃ with 50 kHz ultrasonic irradiation of 5, 10, 20, 30 or 40 minutes, respectively, using an SS-GEN-IND-35K1200W ultrasonic generator. Then the supersaturated solutions were moved to the parallel crystallizer for the macallisterite crystallization. Finally, the yield was calculated and the product was characterized by the PXRD, differential scanning calorimetry (DSC) and scanning electron microscopy (SEM).Raman spectroscopy investigation on the borate species in solution. Raman spectra were acquired using a laser micro-Raman spectrometer (Thermo Fisher Scientific, USA) with a 532 nm solid-state laser as the excitation source. The prepared solutions were transferred into quartz capillary tubes, ensuring the removal of air bubbles. The instrument was calibrated before sample collecting. The laser power used is 10 mW, the sample was collected with a exposure time of 40 s for 10 times at room temperature, covering a spectral range from 400 to 950 cm-1 with a resolution of approximately 1 cm-1.DFT calculation of the polymerization of borate species in solution. Density functional theory (DFT) calculations were conducted to quantify the thermodynamic driving forces and clarify the charge transfer mechanisms underlying the Mg2+-induced polymerization of borate species in the macallisterite supersaturated solutions. All computations were performed using the Gaussian 16 software package. The B3LYP functional was employed in conjunction with the D3BJ dispersion correction for all computations. Geometry optimizations and frequency calculations utilized the 6-311+G (d,p) basis set for all atoms. Geometry optimizations and frequency analyses were carried out for all atoms. To accurately model the aqueous synthesis environment, the Solvation Model based on Density (SMD) was employed, with water as the solvent, during both the optimization and frequency calculations.This computational protocol was applied to a series of key structures: the dominant borate species (B3O3(OH)4-, B4O5(OH)42-, B5O6(OH)4-, and B6O7(OH)62-), the Mg(H2O)62+ cation, and their resulting coordination complexes of the [Mg(H2O)6-n(B3O3(OH)4)]+, [Mg(H2O)6-n(B4O5(OH)4)], [Mg(H2O)6-n(B5O6(OH)4)]+ and [Mg(H2O)6-n(B6O7(OH)6)]. All optimized structures were confirmed to be true local minima on the potential energy surface, as evidenced by the absence of imaginary frequencies.The Gibbs free energy (ΔG) for the complexation process was calculated using the following expression:             (2)where GComplex, GBoron species, and GMg(H₂O)₆²⁺ represent the single-point energies of the optimized complex, the isolated boron species, and the isolated Mg(H2O)62+ cation, respectively. The single-point energies were corrected for basis set superposition error (BSSE) via the counterpoise method. The Gibbs free energy change (ΔG) for the polymerization reactions was computed to evaluate their thermodynamic spontaneity. Natural bond orbital (NBO) analysis at the same level of theory was performed on the optimized complexes to quantify the charge transfer between Mg2+ and the boron species. Finally, electrostatic potential (ESP) maps were generated and visualized using the VMD program. These maps were rendered based on the optimized structures that were confirmed to be true local minima (as evidenced by the absence of imaginary frequencies), providing visual insight into the charge distribution and reactive regions within the complexes.Preparation of the Macallisterite Spherulites via Additive-Assisted Batch Crystallization. The sodium dodecyl sulfonate (SDS) was used as an additive to prepare the uniform macallisterite spherulites for the purpose of the crystal agglomeration improvment.SDS Concentration Effects on Macallisterite Spherulites Preparation. The macallisterite supersaturated solutions were prepared by dissolving 7.7313 g boric acid, 1.8007 g sodium hydroxide and 40.6620 g magnesium chloride hexahydrate in 74 mL deionized water by adding 0.025, 0.05, 0.10, 0.20, 0.50, or 1.00 g SDS, respectively. These supersaturated solutions were placed in a polydimethylsiloxane oil bath and heated at 80 ℃ for 20 minutes of 50 kHz ultrasonic irradiation by using an SS-GEN-IND-35K1200W ultrasonic generator. Then they were moved to the parallel crystallizers for the macallisterite crystallization. The product was characterized by the PXRD and SEM.Solution pH Effects on Macallisterite Spherulites Preparation. The macallisterite supersaturated solutions with initial pH values of 4.90, 5.35, 5.80, 6.05, 6.30, and 6.60 were prepared by dissolving 7.7313 g boric acid and 1.0004 g, 1.3005 g, 1.5006 g, 1.8007 g, 2.0009 g or 2.5011g sodium hydroxide, 40.6620 g magnesium chloride hexahydrate in 74 mL of deionized water, respectively. 0.10 g SDS was then added in the above supersaturated solutions. The resulting solutions were placed in a polydimethylsiloxane oil bath of 80 ℃ for ultrasonic irradiation and then moved to the parallel crystallizer for the macallisterite crystallization. Finally, the products were characterized by the PXRD and SEM.Crystallization Time Effects on Macallisterite Spherulites Preparation. Nine macallisterite supersaturated solutions were prepared by dissolving 7.7313 g of boric acid, 1.8007 g of sodium hydroxide and 40.6620 g in 74 mL of deionized water with 0.10 g SDS addition, respectively. And then the resulting solutions were used for the macallisterite crystallization after ultrasonic irradiation in an oil bath (80 ℃, 20 min, 50 kHz). Finally, the products were characterized by the PXRD and SEM.Powder X-ray Diffraction (PXRD). The phase composition and crystal structure of the synthesized macallisterite samples were characterized using Powder X-ray diffraction (PXRD) on a D/max 2500 VL/PC diffractometer (Rigaku SmartLab, Japan). The measurements were performed using Cu Kα radiation (λ=1.5418 Å) generated at 45 kV and 200 mA. The diffraction patterns were recorded in the 2θ range of 2° to 40° with a scanning speed of 5° per minute and a step size of 0.01°.pH Measurement. The pH of all solutions was measured using a calibrated Seven Excellence pH meter (Mettler Toledo, Switzerland) equipped with an Inlab expert Pro-ISM pH electrode. The instrument was calibrated daily prior to measurements using standard buffer solutions (pH 4.01, 7.00, and 10.01 at 25 ℃, Merck KGaA, Germany). All readings were taken at an ambient temperature of 25±1 ℃ with automatic temperature compensation enabled, ensuring an accuracy of ±0.02 pH units.Scanning Electron Microscopy (SEM). The microstructure and crystal morphology of the macallisterite products were examined using an SU8010 field emission scanning electron microscope (Hitachi High-Technologies Corporation, Japan). Samples were fixed on conductive adhesive tape and sputter-coated with a 5 nm thick Pt layer to enhance conductivity. Observations were conducted under high vacuum conditions at accelerating voltages of 5.0 or 10.0 kV. Images were captured using the HITACHI PC-SEM software at a working distance of approximately 8 mm.Thermal Analysis (DSC-TG). The thermal behavior, including dehydration, decomposition, and phase transitions, was investigated using a synchronous thermal analyzer (Setaram, France). 10-20 mg Samples were placed in alumina crucibles and heated from room temperature to 800 ℃ (or 1200 ℃) at a constant rate of 5 ℃•min-1 under a dynamic nitrogen atmosphere (flow rate of 50 mL•min-1). Both differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) signals were simultaneously recorded to monitor changes in heat flow and mass loss, respectively.Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). The elemental composition of the macallisterite samples was quantified using an inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 720ES, USA). Samples were digested in appropriate acids prior to analysis. Calibration curves were constructed using certified standard solutions for each element of interest. All measurements were performed in triplicate, and the results are reported as average concentrations.