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Impact of acid-base conditions on defluoridation by induced crystallization
G Model JIEC 4852 No. of Pages 11
Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
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Impact of acid-base conditions on defluoridation by induced crystallization Linyu Denga,b,c,* , Ying Wanga,b,c , Jianqi Zhoua,b,c , Tinglin Huanga,b,c, Xin Suna,b,c a
School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China Key Laboratory of Northwest Water Resources, Environment and Ecology, Ministry of Education, Xi’an University of Architecture and Technology, Xi’an 710055, China c Shaanxi Key Laboratory of Environmental Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 June 2019 Received in revised form 30 October 2019 Accepted 6 November 2019 Available online xxx
Defluoridation by induced crystallization under extreme pH and bicarbonate/carbonate alkalinity of an aquatic environment was researched. At the conditions of initial pH 4–10 and alkalinity of less than 9.92 mM, the F could be successfully removed by the induced crystallization of Ca5(PO4)3F on the PR surface within 1 h. With a prolonged time of 144 h, the F could be also removed by the formation of Ca5(PO4)3F at initial pH 2 and alkalinity 9.92 mM. However, in extreme base aquatic environments, such as pH 12 or alkalinity 17.12 mM, the F was partially removed by the induced crystallization of FCO3apatite (Ca9.316Na0.36Mg0.144(PO4)4.8(CO3)1.2F2.48) and the F could be completely removed by adjusting pH to a range of 4–10. Additionally, the safe disposal of spent crystal seed is successfully achieved as an extra benefit. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Defluoridation Induced crystallization Initial pH Initial alkalinity Removal mechanisms
Introduction Although F is a micronutrient needed in trace quantities to strengthen skeletal tissues and teeth, F concentrations above 1.5 mg L 1 were associated with greater severity of the incurable disease fluorosis [1]. In the United States, the U.S. Environmental Protection Agency (U.S. EPA) sets a maximum content level of F of 4.0 mg L 1, while California imposes a strict 2.0 mg L 1 restriction. The World Health Organization (WHO) drinking water standard suggests that the lowest desirable F concentration is 0.5 mg L 1, whereas the maximum permissible limit is 1.5 mg L 1 [2]. F concentrations in groundwater within western China's Shaanxi Province [3], the Wailapally granitic aquifer in India [4], Khan Younis City in Palestine [5], and the Thar Desert of southeastern Pakistan [6,7] all exceed the WHO drinking water guideline of 1.5 mg L 1. Thus, defluoridation had to be performed before the groundwater was used as a source of drinking water or the industrial wastewater was discharged into the natural aquatic environment. Major processes for defluoridation from water and wastewater include precipitation, adsorption, coagulation, ion exchange,
* Corresponding author at: School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China. E-mail address: [email protected] (L. Deng).
electrodialysis, electrocoagulation, and other various electrochemical and membrane processes [8–13]. Among these methods, adsorption is a feasible candidate if the spent adsorbent could be disposed of safely. Recently, some researchers reported that the precipitation of F as fluorapatite can be utilized for removal of F from water [14–16]. Consequently, the safe disposal of spent adsorbent was successfully achieved as an additional benefit. Larsen et al. [15,16] reported that the fluorapatite precipitation process with bone char or hydroxyapatite could reduce the F concentration of water. Dahi [17] showed that, when using bone char that was already saturated with respect to the F , the content of F in the raw water can be reduced from 8.2–13 mg L 1 to less than 1.0 mg L 1 under deliberately prolonged operation conditions. Our previous research also showed that when using fluorapatite and calcite as seed crystals, the induced crystallization process could reduce the F concentration from 9.5 to lower than 1.0 mg L 1 [18]. To use the technique of fluorapatite precipitation in a practical application, two key facts must be taken into account [15]. First, precipitation demands a solution that is supersaturated with respect to fluorapatite. Second, a mean of seeding the precipitation of F -containing apatite is required. The fluorapatite precipitation process can also be termed as induced crystallization. Supersaturation and precipitation with respect to fluorapatite in F -containing water were realized by introducing CaCl2 and NaH2PO4 to the solutions. During this process, the addition of
https://doi.org/10.1016/j.jiec.2019.11.010 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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H2PO4 to the solution would allow them to be hydrolyzed, so H2PO4 , HPO42 , and PO43 were all present in the solution. The distribution of these phosphates is strongly affected by the pH and alkalinity in the solution. It was found that when there was 7.39 mM of bicarbonate in the artificial water with a F concentration of 9.2 mg L 1, F could not be efficiently removed by fluorapatite induced crystallization, compared with same water without bicarbonate [15]. Therefore, bicarbonate and carbonate alkalinity and pH affect the induced fluorapatite crystallization obviously, but the mechanisms are not clear. The defluoridation kinetics and mechanisms with different pH and alkalinity should be investigated further. Phosphate rock (PR) is an abundant ore and has been utilized as the raw material for the phosphatic fertilizer industry [19–21]. It is composed mainly of fluorapatite and impurities of silica (Si), iron (Fe), and aluminum (Al) oxides, carbonates, and clay minerals [21– 23]. Recently, apatite has been focused in the field of wastewater treatment in the reports of Drouiche's group [24,25]. Given that fluorapatite is the major component of PR, it is possible that PR can be used as seed crystals to remove F from the aqueous solution. Moreover, the necessity to dispose of spent adsorbent was overcome because new generated solid fluorapatite could take part in the F complex as a seed nucleus sustainably. In this study, we investigated the possibility of using PR as the seed crystal in the induced fluorapatite crystallization process and assessed the impact of initial pH and alkalinity on defluoridation efficiencies and kinetics characteristics. The Visual MINTEQ thermodynamic equilibrium model was applied to calculate dissolved and precipitate species during the induced crystallization process. Scanning electron microscopy combined with energy-dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), N2 Brunauer– Emmett–Teller (BET) specific surface areas, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR) were performed to investigate the mechanisms of defluoridation from an aqueous solution during induced crystallization using PR as the seed crystal. Materials and methods Materials and characterization PR powder was obtained from Taizhou Changpu Chemical Reagent Co. (Jiangsu, China), was sieved to a particle size of <75 mm in diameter and washed with ultrapure water to remove soluble impurities. The specific surface area of raw PR was 1.309 m2/g. It was then dried at 105 C for 12 h. All chemicals used were analytical grade and were purchased from the SinoPharm Chemical Reagent Co., Ltd. (Shanghai, China), or Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China), and used without further purification. All solutions were prepared with ultrapure water. The fluoride stock solution was prepared with NaF to a concentration of 0.05 M. F working solutions were freshly diluted from stock solution to a concentration of 9.5 mg L 1 (0.5 mM). The carbonate and bicarbonate stock solutions were prepared with Na2CO3 and NaHCO3 to a concentration of 1.0 M, respectively. The Ca2+ stock solution was prepared with CaCl2 to a concentration of 2.0 M. The PO43 stock solution was prepared with NaH2PO4 to a concentration of 1.0 M. Both Ca2+ and PO43 solutions were used as precipitants. The Ca2+ and PO43 concentrations were controlled by addition of stock solutions of each ion into the beaker containing the F working solution so as not to dilute the reaction system. Concentrations of F , PO43 , and Ca2+ were determined using anion and cation chromatography (IC; ICS-1100, Thermo Scientific, USA). Aqueous samples were filtered through a 0.22-mm membrane filter before IC tests. The pH was measured using a
pH meter (STARTER 3100/3C pro, OHAUS, USA). All tests were performed in triplicate and the mean (SD) reported. The morphology of the surface of the PR was determined by means of SEM (Quanta 600FEG, FEI, USA). The elemental components were semi-quantitatively determined using EDS (INCA Energy 350, Oxford Instruments, UK). The dimensional and morphological features of raw and spent PR were demonstrated by the TEM (JEM-2100Plus, JEOL, Japan). The compositions of the raw and spent PR were determined by XRD (Empyrean, PANalytical, The Netherlands) using Cu Kα radiation over an angular range of 5 –90 . The specific surface areas of raw and spent PR were measured with a surface area analyzer (ASAP2460, MICROMERITICS, USA), using N2 BET sorption methods. TGA was investigated by a TG-DSC Simultaneous Thermal Analyzer (TGA/ DSC3+, METTLER TOLEDO, Switzerland) from room temperature to 1100 C to analyze the thermal stability of the raw and spent PR. FTIR of the raw and spent PR also were recorded in the range 400– 4000 cm 1 on a FTIR analyzer (Nicolet iS50, Thermo Scientific, USA). FTIR spectroscopy was used to reliably quantitatively estimate the percentage of carbonate in carbonated apatite. The ratio of the extinction of the IR carbonate band at about 1415 cm 1 to the extinction of the PO43 band at approximately 575 cm 1 is linearly related to the carbonate content of the carbonated apatite [26]. This method facilitates carbonate estimation to an accuracy of better than 10% in the range 1%–12% wt./wt. The XPS analyses were determined with an X-ray photoelectron spectrometer (Thermo ESCALAB 250XI) with an Al Kα monochromatic source. The wide spectra energy was 100 eV. The high-resolution photoelectron spectroscopy of Ca2p, P2p, O1s, and C1s was obtained with the energy of 30.0 eV and step size of 0.1 eV. The C1s peak of 284.8 eV was used as a calibration for a binding energy of elements. Batch experiments 10 g of PR was added to polymethyl methacrylate beakers containing a 1-L F working solution exhibiting an initial F concentration of 9.5 mg L 1 (0.5 mM). The solution and PR were mixed at 400 rpm for 2 min with a mechanical stirring paddle. The PO43 stock solution was then added and mixed at 400 rpm for 5 min. The Ca2+ solution was subsequently added and mixed at 400 rpm for 5 min. This final solution was continuously stirred at 100 rpm for 60 min. The experiments were conducted at room temperature. The influence of the initial pH of the F solution was determined by changing the initial pH to between 2 and 12 with 0.1-M HCl and 0.1-M NaOH. The influence of alkalinity was determined by adding carbonate or bicarbonate to the F solution, adjusting the total alkalinity of the solution from 0.99 to 17.12 mM. The induced crystallization process was repeatedly conducted using the same PR materials. The spent PR was washed with deionized water and dried at 105 C for 12 h prior to its reuse as seed crystals. The induced crystallization, extraction, and reuse processes were repeated three times. The final solution of all the above experiments were filtered through a 0.45-mm membrane filter for subsequent testing. The residual F , PO43 , and Ca2+ concentrations and the final pH were then measured. These waterquality parameters were tested three times and the error bar in each data figure represents the standard deviation of three replicates. Chemical modeling The solute-solid equilibrium after induced crystallization was modeled based on Visual MINTEQ (ver. 3.1, Jon Petter Gustafsson, 2013). Visual MINTEQ can calculate the speciation of inorganic ions and complexes in waters and evaluate the effect of dissolving or
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precipitating solid phases on water chemistry [27]. To formulate the chemical model, initial pH, ionic strength, temperature, components (molar concentration of every ion) of the solution, finite solid (solid with a given amount), and possible solid should be first inputted. Then, Visual MINTEQ will repeat calculations until the solution is in equilibrium with respect to the solid. Finally, the equilibrium concentrations of various ion species, the yield of possible precipitate, and gas species will be outputted. Although SiO2 was one of the components of raw PR, we did not input SiO2 to the MINTEQ model because SiO2 is a chemically stable material, and is not nearly as involved in the chemical reactions in the experimental conditions used in this study.
the C-O bond at 880 and 728 cm 1 conforms to the presence of dolomite in the solid [32,33], in accordance with the XRD data. Fig. 1(c) and (d) show that the surface of PR is smooth and the main elements on the surface are carbon (C), oxygen (O), fluorine (F), phosphorus (P), and calcium (Ca). The surface also contained small quantities of iron (Fe), magnesium (Mg), aluminum (Al), and silicon (Si). The content of the main chemical composition of PR is given in Table S1. Furthermore, the precipitation-dissolution equilibrium of PR in aqueous phase showed the raw PR was stable at initial pH = 4–10 (Fig. S1).
Results and discussion
The effect of contact time on residual F , PO43 , and Ca2+ concentrations were shown in Fig. 2. Under the conditions of initial F concentration of 9.5 mg L 1, contact time 30 min, and the Ca/F molar ratios 5, 8, and 10, the residual F concentrations were 1.9, 1.3, and 1.1 mg L 1, respectively. It should be emphasized that a Ca/P/F molar ratio of 5:3:1 is the stoichiometric ratio required to precipitate fluorapatite from the solution. The observed differences in defluoridation (Fig. 2) with Ca/F ratio may result from the fact that a higher Ca/F molar ratio can provide a higher level of supersaturation of fluorapatite condition, which will accelerate the induced crystallization process. The kinetic fittings of residual F with different Ca/P/F ratios were also presented in Fig. 2. It should be stressed that, in the seeded crystal-growth method, the spontaneous nucleation of amorphous precursor phase is unavoidable and crystal growth on seed crystals occurs [34]. Therefore, the rate of fluorapatite precipitation denotes the crystal-growth rate of fluorapatite.
Characterization The XRD pattern, FTIR spectra, SEM image, and EDS spectrum are shown in Fig. 1. It can be seen from Fig. 1(a) that PR is composed of fluorapatite, dolomite, small quantities of calcite, and quartz. The FTIR result for PR is shown in Fig. 1(b). The peaks at 573 and 604 cm 1 were assigned to the P-O mode of the PO43 [28]. The peak at 964 cm 1 was attributed to the v1 vibration modes of the PO43 [29]. The peaks at 1049 and 1095 cm 1 were attributed to the v3 vibration modes of the PO43 [30]. The peak at 880 cm 1 was attributed to the v2 vibration modes of the C-O bond of the CO32 ion, and the peaks at 1432 and 1454 cm l were likely the v3 vibration modes of the C-O bond of the CO32 ion. These values agree with those usually reported in the literature for B-type carbonate fluorapatites [31]. The presence of absorption bands of
Effect of contact time and initial pH
Fig. 1. Characterization of raw PR: (a) XRD pattern, (b) FTIR spectra, (c) SEM image, and (d) EDS spectrum.
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Fig. 2. Effect of contact time on residual F (a) and kinetic fitting (pseudo-first- and -second-order kinetic) with Ca/P/F molar ratio (b) Ca/P/F = 5:3:1; (c) Ca/P/F = 8:3:1; (d) Ca/ P/F = 10:3:1). Initial F concentration, 9.5 mg L 1. C0 is the initial F concentration (mg L 1) and C is the F concentration (mg L 1) at time t.
During the calculation of the rate of fluorapatite precipitation, the period from 0 to 2–3 min was not used in the rate calculations because this period represented a rapid surface adsorption equilibration process not characteristic of crystal growth [34]. Therefore, in Fig. 2, the point of “time 0” was removed. It can be seen from Fig. 2 that the removal of F agrees well with the pseudo-second-order kinetic model with all Ca/P/F molar ratios, and the rate constant k2 was 0.00802, 0.00907, and 0.0107 mg L 1 min 1 with Ca/P/F molar ratios of 5:3:1, 8:3:1, and 10:3:1, respectively. According to Fig. 2, it is suggested that a reaction time of 60 min is enough for defluoridation. PO43 was added
following by stoichiometric ratio (P/F = 3) of fluorapatite, whereas excessive addition of Ca2+ (Ca/F = 8 or 10) was conducted to accelerate the reaction. Considering that Ca2+ was a desirable element in the field of aquatic environments compared to PO43 , the protocol of overdosage of Ca2+ was acceptable in here. Experimental results of residual F , PO43 , and Ca2+ concentrations and final pH under different initial pH values are shown in Fig. 3. When the initial pH increased from 4 to 10, the residual concentrations of F , PO43 , and Ca2+ were 0.90–0.99, 0.17–5.37, and 103.74–126.44 mg L 1, respectively. The initial pH (4–10) represented less effects on F , PO43 , and Ca2+. The reason was that
Fig. 3. Experimental results of residual F (a), PO43 (b), and Ca2+ (c) concentrations and final pH (d) under different initial pH values. Initial F concentration, 9.5 mg L 1. Ca:P: F molar ratio, 8:3:1.
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the interaction of H2PO43 with solid carbonates (CaCO3, CaMg (CO3)2) would generated bicarbonate in the solution. The buffering capacities of the bicarbonates and PO43 would keep the system pH at comparative neutral level, consisted with the results of the final pH (Fig. 3(d)). However, when the initial pH values of the system were 2 and 12, which present in an extreme aquatic environment, the residual F concentrations were 20.48 and 8.08 mg L 1 (Fig. 3(a)) respectively. The reduction in the effect of defluoridation may be due to insufficient buffer system capacity or a change in reaction mechanism. Defluoridation mechanism It is clear that the new substances formed on the surface of spent PR (Fig. 4(a)) comparing to raw PR (Fig. 1(c)). The dimensional and morphological features of the raw and spent PR were demonstrated by TEM and HRTEM and the results are presented in Fig. 5. The surface of raw PR was smooth (Fig. 5(a)), while the surface of spent PR was rough and covered with some pinky crystals (Fig. 5(c)). The BET surface area of raw and spent PR was 1.3090 and 2.1406 m2/g, respectively. The pinky crystals on the surface of spent PR contributed to the expansion of the surface area. Furthermore, the measured d spacings of lattice fringes were 0.29 and 0.23 nm, which were assigned to the (001) and (110) crystal planes of raw PR, respectively (Fig. 5(b)). Interestingly, the d spacings of lattice fringes of pinky coating were also 0.29 and 0.23 nm. The XRD pattern and FTIR analysis of spent PR (Figs. S2 and S3) were consistent with raw PR (Fig. 1(a)). These suggested that the newly formed pinky substances on the spent PR particles surface were the same as on raw PR. The lattice fringes images of the raw and spent PR were closely similar to that of Ca10(PO4)6(OH) F reported by Xiang Ge [35]. TGA was carried out from room temperature to 1100 C for raw and spent PR (Fig. S4). The results showed one mass loss stage (from 557 C to 783 C) for both raw and spent PR. This mass loss stage was attributed to the decomposition of calcite and dolomite, which were the main compositions of PR as shown in Fig. 1 [36,37]. The wide scan XPS spectra of raw and spent PR are presented in Fig. 6(a). This demonstrates that the surface of both of raw and spent PR consisted of F, O, Ca, C, and P. The F1s, P2p, and Ca2p XPS spectra of spent PR after defluoridation were similar with that of raw PR and the peak location was consistent with the literature [38,39]. The same composition of raw PR and newly formed pinky coating on spent PR, which were confirmed by TEM, BET, XRD, TGA, and XPS, suggested that the F was removed mainly through the formation of Ca5(PO4)3F.
5
Impact of extreme solution pH As described in “Effect of contact time and initial pH” section, a reduction of defluoridation under pH = 2 and 12 conditions can be observed obviously. Given that the extreme conditions of pH and alkalinity, such as acidic discharge from mining industries and strong basic condition in traditional defluoridation using lime, pH = 2 and 12 were selected to investigate the reliability of induced fluorapatite crystallization on PR and the probable mechanism. The equilibrium concentrations of ion species at different initial pH modeled by Visual MINTEQ are shown in Fig. S5. Surprisingly, the residual concentrations of F and PO43 calculated by the model were all near zero even when the initial pH was at 2 and 12 and the final pH between experimental results and the model output exhibited a distinguishing difference. Given that the model output results of F were similar to the experimental results (Fig. 3) at initial pH 4–10, the reliability of the MINTEQ simulation is good, as expected. The input parameters of MINTEQ were same as those used in the laboratory experiments. However, the contact time of the laboratory experiments was 1.0 h, whereas the Visual MINTEQ model provided the chemistries results based on fully aging. Therefore, 1 h did not seem enough for the system to reach equilibrium under an extreme acid-base condition. The experiments at initial pH values of 2 and 12 were performed at a prolonged range of contact times from 1 to 144 h, and the results are shown in Fig. 7(a). The residual F concentration was 0.11 mg L 1 and 3.47 mg L 1 at initial pH 2 and 12, respectively, when the contact time was prolonged to 144 h. The behaviors of kinetic models at initial pH 2 and 12 are shown in Fig. 7(b) and (c), respectively, which demonstrate that the regression correlations (R2) and rate constants k2 fitted with a pseudo-secondorder kinetic model were 0.9816 and 0.0001 mg L 1 min 1 for initial pH 2, and 0.9904 and 0.00002 mg L 1 min 1 for initial pH 12, respectively. Clearly, the rate constants k2 of pH 2 and 12 were less than that of pH 4–10. At initial pH 2, the residual F concentration was 0.11 mg L 1 after 144 h (Fig. 7). The F , PO43 , and Ca2+ dissolved obviously from PR in the first hour, and then the F , PO43 , and Ca2+ were supersaturated with respect to fluorapatite (Ca5(PO4)3F). Thereafter, fluorapatite precipitate was formed gradually when the reaction increased gradually to 144 h. The XRD result (Fig. S6) shows that the spent PR at initial pH 2 is composed of fluorapatite, dolomite, small quantities of calcite, and quartz. The FTIR result (Fig. S7) of the spent PR at initial pH 2 was similar to the raw PR (Fig. 1(b)). Therefore, prolonging the reaction time could cover the negative effect of extreme acid condition.
Fig. 4. SEM image and EDS spectrum of spent PR.
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Fig. 5. TEM analysis of raw and spent PR. (a) TEM image of raw PR; (b) HRTEM lattice fringe image and d spacings of raw PR; (c) TEM image of spent PR; (d) HRTEM lattice fringe image and d spacings of spent PR.
At initial pH 12, the residual F concentration was 3.47 mg L 1 at 144 h (Fig. 7). The XRD (Fig. S8) and FTIR (Fig. S9) results of the spent PR at pH 12 are similar to those of the raw PR. Precipitate fitting by MINTEQ in Table 1 shows that the concentrations of FCO3-apatite (Ca9.316Na0.36Mg0.144(PO4)4.8(CO3)1.2F2.48) of spent PR at initial pH 12 was 2706 mg L 1, which was higher than that of raw PR 2200 mg L 1. Furthermore, the percentage of CO32 in FCO3apatite of spent PR at initial pH 12 detected by FTIR was 6.6%, which was greater than that in the raw PR (6.4%). The FTIR and MINTEQ results confirmed that the F was partially removed by the formation of FCO3-apatite. However, the residual concentration of F of the model output was nearly zero (Fig. S4). The model output and experimental results were inconsistent because OH competed with F for Ca2+ and PO43 to form precipitate. Effect of alkalinity Experimental results of residual F , PO43 , and Ca2+ concentrations and final pH under different initial alkalinities are shown in Fig. 8. At initial alkalinity of 0, 0.99, and 1.95 mM, the residual F concentrations of the experimental results were 1.04, 1.21, and 2.09 mg L 1, respectively (Fig. 8). Therefore, the residual F increased with increasing initial total alkalinity. At higher initial alkalinities of 9.92 and 17.12 mM, the residual F concentrations of the experimental results were 7.34 and 7.97 mg L 1, respectively (Fig. 8). This suggested that the high total alkalinities of solution can inhibit the defluoridation process in this system. Nevertheless, the residual F concentrations of the MINTEQ model outputs were all near zero at the initial alkalinities of 9.92 and
17.12 mM (Fig. S10). One main reason for the differences between the experimental results and the model outputs was that the contact time of 1 h was not enough to allow the solution to reach equilibrium, as described in “Impact of extreme solution pH” section. Hence, the experiments at initial alkalinities of 9.92 and 17.12 mM were performed under a prolonged contact time of 144 h (Fig. 9(a)). When the contact time increased up to 144 h, the residual F concentration at an initial alkalinity of 9.92 mM was 0.70 mg L 1 (Fig. 9(a)). Therefore, high alkalinity increased the required reaction time of induced fluorapatite crystallization to achieve equilibrium. Under an initial alkalinity condition of 17.12 mM, the residual F concentration decreased from 7.97 to 6.26 mg L 1 when the reaction time increased from 1 to 144 h (Fig. 9(a)). The improvement of defluoridation efficiency was negligible compared with that of lower-alkalinity conditions. The downturn of defluoridation under high-alkalinity conditions was attributed to the competition between OH and F with Ca2+ and PO43 to form precipitate, similar to the effect of pH 12, as discussed in “Impact of extreme solution pH” section. The XRD and FTIR results of the spent PR at initial alkalinities of 9.92 and 17.12 mM (Figs. S11–S14) support the above statement. The regression correlation (R2) and rate constant k2 fitted with a pseudo-second-order kinetic model were 0.9702 and 0.0001 mg L 1 min 1 for initial total alkalinity 9.92 mM, 0.9033 and 0.000001 mg L 1 min 1 for 17.12 mM, respectively (Fig. 9(b) and (c)). The decrease of rate constants with alkalinity was confirmed. A schematic expressing the induced crystallization process on the surface of PR at different initial pH values and alkalinities is
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Fig. 6. High resolution XPS spectra of raw PR and spent PR: (a) wide scan spectra; (b) F1s spectra; (c) P2p spectra; (d) Ca2p spectra.
Fig. 7. (a) Defluoridation at initial pH 2 and 12, (b) kinetic plot of defluoridation at initial pH 2, and (c) kinetic plot of defluoridation at initial pH 12. Initial F concentration = 9.5 mg L 1. Ca:P:F molar ratio = 8:3:1.
summarized in Fig. 10. At initial pH 2, PR was partially dissolved in the first 1 h, and F , PO43 , and Ca2+ in the solution formed Ca5(PO4)5F on the surface of PR in the subsequent time to 144 h. At an initial pH 4–10, the induced crystallization of Ca5(PO4)3F occurred on the PR surface within 1 h, which was same to that at an initial alkalinity of 9.92 Mm within an extended time of 144 h. At an initial alkalinity of 17.12Mm, FCO3-apatite and Ca5(PO4)3OH were
formed on the PR surface in 144 h. At initial pH 12, FCO3-apatite, Ca5(PO4)3OH, and Mg(OH)2 were formed on the PR surface in 144 h. Stability for recycle runs The residual concentrations of F , PO43 , and Ca2+ as well as final pH after the cycle utilization of PR as seed crystals are shown
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Table 1 Solids modeled by MINTEQ and CO32 contenta in apatiteb in PR detected by FTIR. Condition
Raw PRd pH = 2.00 pH (4–10) pH = 12.00 Alky = 9.92 mM Alky = 17.12 mM
MINTEQ model output (mg L
1
CO32 (%) detected by FTIR
) c
Ca5(PO4)3F
FCO3-apatite
Ca5(PO4)3(OH)
CaMg(CO3)2
CaCO3
Mg(OH)2
400 650 650 – 650 –
2200 2207 2207 2706 2207 2706
– – – 232 – 231
4300 3404 3974 3846 4085 4287
1300 1280 1390 1630 1410 1390
– – – 137 – –
6.4 4.5 5.4 6.6 6.0 6.8
This CO32 content denotes CO32 content in apatite and does not include the CO32 content in CaMg(CO3)2 and CaCO3. Apatite includes Ca10(PO4)6F, FCO3-apatite and Ca10(PO4)6(OH)2. c The molecular formula of FCO3-apatite is Ca9.316Na0.36Mg0.144(PO4)4.8(CO3)1.2F2.48. d Contents of Ca10(PO4)6F2, FCO3-apatite, Ca10(PO4)6(OH)2, CaMg(CO3)2, CaCO3, and Mg(OH)2 in this row were calculated from XRD and FTIR data, as were the MINTEQ model input conditions. a
b
Fig. 8. Experimental results of residual F , PO43 , and Ca2+ concentrations and final pH under different initial alkalinities. Initial F concentration, 9.5 mg L 1. Ca:P:F molar ratio, 8:3:1.
in Fig. 11. When the Ca:P:F molar ratio was 5:3:1, the residual F after the third cycle was 1.6 mg L 1, which exceeds the WHO drinking water guideline of 1.5 mg L 1. When the Ca:P:F molar ratios were 8:3:1 and 10:3:1, the residual F after three cycles were all below 1.5 mg L 1. These results indicate that when the Ca:P:F molar ratio was 5:3:1, the PR could be reused twice, but when the Ca:P:F molar ratio was 8:3:1 or 10:3:1, the PR could be reused at least three times. It should be recognized that the residual PO43 increased with the cycle number, but the residual Ca2+ concentration and final pH did not vary significantly. This is a curious phenomenon. The F , PO43 , and Ca2+ ions in the solution theoretically formed fluorapatite crystals on the PR surface. Thus, residual F , PO43 , and Ca2+ concentrations should all be very low. It is possible that the addition of NaH2PO4 to the solution leads to CO32 dissolution, generating Ca2+ ions, because calcite and dolomite are both constitutes of the PR (Fig. 1(a)). Therefore, the F , PO43 , CO32 , and Ca2+ ions formed both FCO3-apatite and fluorapatite on the surface of the PR, and the ratio of the newly formed FCO3-apatite/
fluorapatite increased with cycle number. This process would allow the residual PO43 to increase with cycle number while maintaining consistent residual F and Ca2+ concentrations. Comparison of techniques for defluoridation Table 2 summarizes techniques for defluoridation. Adsorption is the main technique used to remove F–. The technique used in this work can effectively remove F–. Considering the extremely low solubility and the continued use as a seed crystal of Ca5(PO4)3F, the necessity for spent material dispose can be negligible compared with other defluoridation techniques, i.e., traditional adsorption, electrocoagulation, etc. Reliability under extreme aquatic environment conditions This research puts forward a possible technology for the defluoridation for most of the aquatic environment. The F could be removed to a satisfactory concentration by forming Ca5(PO4)5F.
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Fig. 9. (a) Defluoridation at initial alkalinities of 9.92 and 17.12 mM, (b) kinetic plot of defluoridation at initial alkalinity 9.92 mM, and (c) kinetic plot of defluoridation at initial alkalinity 17.12 mM. Initial F concentration, 9.5 mg L 1. Ca:P:F molar ratio, 8:3:1.
Fig. 10. Schematic of defluoridation mechanism by induced crystallization on PR at different pH values and alkalinities.
However, in some other extreme aquatic environments, such as pH 2 or 12, the treatment of objective water could be achieved through prolongating the reaction time or adjusting pH to a range of 4–10; this strategy was also effectual under higher-alkalinity conditions. Conclusions The impact of acid-base conditions on defluoridation by induced fluorapatite crystallization on PR was investigated in this work. The residual F could decrease from 9.5 mg L 1 to below the U.S. EPA limit within 1 h when the initial pH is in the range 4–10 as
well as when the initial alkalinity is less than 9.92 mM. At the initial pH 2 and initial alkalinity 9.92 mM, the residual F could satisfy the drinking water guideline as the reaction time was prolonged up to 144 h. However, at the extreme base aquatic environment (initial pH 12 and initial alkalinity 17.12 mM), the objective of defluoridation could be achieved by adjusting the initial pH to 4–10. The MINTEQ modeling, BET, XRD, FTIR, SEM-EDS, TEM, TGA, and XPS analysis revealed the objective of defluoridation was achieved by the formation of Ca5(PO4)3F at an initial pH less than 10 and alkalinity less than 9.92 mM. In particular, F was removed successfully by the induced crystallization of FCO3-apatite (Ca9.316Na0.36Mg0.144(PO4)4.8(CO3)1.2F2.48) under an extreme base
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Fig. 11. Residual F , PO43 , and Ca2+ concentrations and final pH in cycle use of PR as seed crystals.
Table 2 Comparison of techniques for removal of F–. Techniques a
Adsorption Adsorptionb Adsorptionc Adsorptiond Adsorptione Electrocoagulation Induced crystallization
Initial concentration (mg L 10 10 100 2 100 8.6 9.5
1
)
Residual concentration (mg L 2.5 1.5 40 0.3 40 1.4 0.9
1
)
Removal efficiencies (%)
Reference
75 85 60 84 60 85 90
[40] [41] [42] [43] [44] [45] This work
Adsorbents: a Fe3O4@La-Ce. b La/modified alumina. c MnCO3. d Microwave-assisted carbonized Azadirachta indica bark. e Powdered-drinking-water treatment residuals.
aquatic environment (the initial pH 12 and alkalinity 17.12 mM). This research has potential to guide the treatment of the common aquatic environment; in addition, extreme conditions could be pretreated to achieve defluoridation. Conflicts of interest None declared.
References [1] [2] [3] [4] [5] [6] [7]
Acknowledgments This work was supported by the Key Laboratory Program of Education Bureau of Shaanxi Province, China (Grant No. 19JS038), the National Natural Science Foundation of China (Grant No. 51308436), the National Natural Science Foundation of China (Grant No. 51278404), and the Shaanxi Science & Technology Coordination & Innovation Project (Grant No. 2015KTCL-03-15). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jiec.2019.11.010.
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Journal of Industrial and Engineering Chemistry xxx (2019) xxx–xxx
Contents lists available at ScienceDirect
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Impact of acid-base conditions on defluoridation by induced crystallization Linyu Denga,b,c,* , Ying Wanga,b,c , Jianqi Zhoua,b,c , Tinglin Huanga,b,c, Xin Suna,b,c a
School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China Key Laboratory of Northwest Water Resources, Environment and Ecology, Ministry of Education, Xi’an University of Architecture and Technology, Xi’an 710055, China c Shaanxi Key Laboratory of Environmental Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 June 2019 Received in revised form 30 October 2019 Accepted 6 November 2019 Available online xxx
Defluoridation by induced crystallization under extreme pH and bicarbonate/carbonate alkalinity of an aquatic environment was researched. At the conditions of initial pH 4–10 and alkalinity of less than 9.92 mM, the F could be successfully removed by the induced crystallization of Ca5(PO4)3F on the PR surface within 1 h. With a prolonged time of 144 h, the F could be also removed by the formation of Ca5(PO4)3F at initial pH 2 and alkalinity 9.92 mM. However, in extreme base aquatic environments, such as pH 12 or alkalinity 17.12 mM, the F was partially removed by the induced crystallization of FCO3apatite (Ca9.316Na0.36Mg0.144(PO4)4.8(CO3)1.2F2.48) and the F could be completely removed by adjusting pH to a range of 4–10. Additionally, the safe disposal of spent crystal seed is successfully achieved as an extra benefit. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Defluoridation Induced crystallization Initial pH Initial alkalinity Removal mechanisms
Introduction Although F is a micronutrient needed in trace quantities to strengthen skeletal tissues and teeth, F concentrations above 1.5 mg L 1 were associated with greater severity of the incurable disease fluorosis [1]. In the United States, the U.S. Environmental Protection Agency (U.S. EPA) sets a maximum content level of F of 4.0 mg L 1, while California imposes a strict 2.0 mg L 1 restriction. The World Health Organization (WHO) drinking water standard suggests that the lowest desirable F concentration is 0.5 mg L 1, whereas the maximum permissible limit is 1.5 mg L 1 [2]. F concentrations in groundwater within western China's Shaanxi Province [3], the Wailapally granitic aquifer in India [4], Khan Younis City in Palestine [5], and the Thar Desert of southeastern Pakistan [6,7] all exceed the WHO drinking water guideline of 1.5 mg L 1. Thus, defluoridation had to be performed before the groundwater was used as a source of drinking water or the industrial wastewater was discharged into the natural aquatic environment. Major processes for defluoridation from water and wastewater include precipitation, adsorption, coagulation, ion exchange,
* Corresponding author at: School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China. E-mail address: [email protected] (L. Deng).
electrodialysis, electrocoagulation, and other various electrochemical and membrane processes [8–13]. Among these methods, adsorption is a feasible candidate if the spent adsorbent could be disposed of safely. Recently, some researchers reported that the precipitation of F as fluorapatite can be utilized for removal of F from water [14–16]. Consequently, the safe disposal of spent adsorbent was successfully achieved as an additional benefit. Larsen et al. [15,16] reported that the fluorapatite precipitation process with bone char or hydroxyapatite could reduce the F concentration of water. Dahi [17] showed that, when using bone char that was already saturated with respect to the F , the content of F in the raw water can be reduced from 8.2–13 mg L 1 to less than 1.0 mg L 1 under deliberately prolonged operation conditions. Our previous research also showed that when using fluorapatite and calcite as seed crystals, the induced crystallization process could reduce the F concentration from 9.5 to lower than 1.0 mg L 1 [18]. To use the technique of fluorapatite precipitation in a practical application, two key facts must be taken into account [15]. First, precipitation demands a solution that is supersaturated with respect to fluorapatite. Second, a mean of seeding the precipitation of F -containing apatite is required. The fluorapatite precipitation process can also be termed as induced crystallization. Supersaturation and precipitation with respect to fluorapatite in F -containing water were realized by introducing CaCl2 and NaH2PO4 to the solutions. During this process, the addition of
https://doi.org/10.1016/j.jiec.2019.11.010 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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H2PO4 to the solution would allow them to be hydrolyzed, so H2PO4 , HPO42 , and PO43 were all present in the solution. The distribution of these phosphates is strongly affected by the pH and alkalinity in the solution. It was found that when there was 7.39 mM of bicarbonate in the artificial water with a F concentration of 9.2 mg L 1, F could not be efficiently removed by fluorapatite induced crystallization, compared with same water without bicarbonate [15]. Therefore, bicarbonate and carbonate alkalinity and pH affect the induced fluorapatite crystallization obviously, but the mechanisms are not clear. The defluoridation kinetics and mechanisms with different pH and alkalinity should be investigated further. Phosphate rock (PR) is an abundant ore and has been utilized as the raw material for the phosphatic fertilizer industry [19–21]. It is composed mainly of fluorapatite and impurities of silica (Si), iron (Fe), and aluminum (Al) oxides, carbonates, and clay minerals [21– 23]. Recently, apatite has been focused in the field of wastewater treatment in the reports of Drouiche's group [24,25]. Given that fluorapatite is the major component of PR, it is possible that PR can be used as seed crystals to remove F from the aqueous solution. Moreover, the necessity to dispose of spent adsorbent was overcome because new generated solid fluorapatite could take part in the F complex as a seed nucleus sustainably. In this study, we investigated the possibility of using PR as the seed crystal in the induced fluorapatite crystallization process and assessed the impact of initial pH and alkalinity on defluoridation efficiencies and kinetics characteristics. The Visual MINTEQ thermodynamic equilibrium model was applied to calculate dissolved and precipitate species during the induced crystallization process. Scanning electron microscopy combined with energy-dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), N2 Brunauer– Emmett–Teller (BET) specific surface areas, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR) were performed to investigate the mechanisms of defluoridation from an aqueous solution during induced crystallization using PR as the seed crystal. Materials and methods Materials and characterization PR powder was obtained from Taizhou Changpu Chemical Reagent Co. (Jiangsu, China), was sieved to a particle size of <75 mm in diameter and washed with ultrapure water to remove soluble impurities. The specific surface area of raw PR was 1.309 m2/g. It was then dried at 105 C for 12 h. All chemicals used were analytical grade and were purchased from the SinoPharm Chemical Reagent Co., Ltd. (Shanghai, China), or Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China), and used without further purification. All solutions were prepared with ultrapure water. The fluoride stock solution was prepared with NaF to a concentration of 0.05 M. F working solutions were freshly diluted from stock solution to a concentration of 9.5 mg L 1 (0.5 mM). The carbonate and bicarbonate stock solutions were prepared with Na2CO3 and NaHCO3 to a concentration of 1.0 M, respectively. The Ca2+ stock solution was prepared with CaCl2 to a concentration of 2.0 M. The PO43 stock solution was prepared with NaH2PO4 to a concentration of 1.0 M. Both Ca2+ and PO43 solutions were used as precipitants. The Ca2+ and PO43 concentrations were controlled by addition of stock solutions of each ion into the beaker containing the F working solution so as not to dilute the reaction system. Concentrations of F , PO43 , and Ca2+ were determined using anion and cation chromatography (IC; ICS-1100, Thermo Scientific, USA). Aqueous samples were filtered through a 0.22-mm membrane filter before IC tests. The pH was measured using a
pH meter (STARTER 3100/3C pro, OHAUS, USA). All tests were performed in triplicate and the mean (SD) reported. The morphology of the surface of the PR was determined by means of SEM (Quanta 600FEG, FEI, USA). The elemental components were semi-quantitatively determined using EDS (INCA Energy 350, Oxford Instruments, UK). The dimensional and morphological features of raw and spent PR were demonstrated by the TEM (JEM-2100Plus, JEOL, Japan). The compositions of the raw and spent PR were determined by XRD (Empyrean, PANalytical, The Netherlands) using Cu Kα radiation over an angular range of 5 –90 . The specific surface areas of raw and spent PR were measured with a surface area analyzer (ASAP2460, MICROMERITICS, USA), using N2 BET sorption methods. TGA was investigated by a TG-DSC Simultaneous Thermal Analyzer (TGA/ DSC3+, METTLER TOLEDO, Switzerland) from room temperature to 1100 C to analyze the thermal stability of the raw and spent PR. FTIR of the raw and spent PR also were recorded in the range 400– 4000 cm 1 on a FTIR analyzer (Nicolet iS50, Thermo Scientific, USA). FTIR spectroscopy was used to reliably quantitatively estimate the percentage of carbonate in carbonated apatite. The ratio of the extinction of the IR carbonate band at about 1415 cm 1 to the extinction of the PO43 band at approximately 575 cm 1 is linearly related to the carbonate content of the carbonated apatite [26]. This method facilitates carbonate estimation to an accuracy of better than 10% in the range 1%–12% wt./wt. The XPS analyses were determined with an X-ray photoelectron spectrometer (Thermo ESCALAB 250XI) with an Al Kα monochromatic source. The wide spectra energy was 100 eV. The high-resolution photoelectron spectroscopy of Ca2p, P2p, O1s, and C1s was obtained with the energy of 30.0 eV and step size of 0.1 eV. The C1s peak of 284.8 eV was used as a calibration for a binding energy of elements. Batch experiments 10 g of PR was added to polymethyl methacrylate beakers containing a 1-L F working solution exhibiting an initial F concentration of 9.5 mg L 1 (0.5 mM). The solution and PR were mixed at 400 rpm for 2 min with a mechanical stirring paddle. The PO43 stock solution was then added and mixed at 400 rpm for 5 min. The Ca2+ solution was subsequently added and mixed at 400 rpm for 5 min. This final solution was continuously stirred at 100 rpm for 60 min. The experiments were conducted at room temperature. The influence of the initial pH of the F solution was determined by changing the initial pH to between 2 and 12 with 0.1-M HCl and 0.1-M NaOH. The influence of alkalinity was determined by adding carbonate or bicarbonate to the F solution, adjusting the total alkalinity of the solution from 0.99 to 17.12 mM. The induced crystallization process was repeatedly conducted using the same PR materials. The spent PR was washed with deionized water and dried at 105 C for 12 h prior to its reuse as seed crystals. The induced crystallization, extraction, and reuse processes were repeated three times. The final solution of all the above experiments were filtered through a 0.45-mm membrane filter for subsequent testing. The residual F , PO43 , and Ca2+ concentrations and the final pH were then measured. These waterquality parameters were tested three times and the error bar in each data figure represents the standard deviation of three replicates. Chemical modeling The solute-solid equilibrium after induced crystallization was modeled based on Visual MINTEQ (ver. 3.1, Jon Petter Gustafsson, 2013). Visual MINTEQ can calculate the speciation of inorganic ions and complexes in waters and evaluate the effect of dissolving or
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precipitating solid phases on water chemistry [27]. To formulate the chemical model, initial pH, ionic strength, temperature, components (molar concentration of every ion) of the solution, finite solid (solid with a given amount), and possible solid should be first inputted. Then, Visual MINTEQ will repeat calculations until the solution is in equilibrium with respect to the solid. Finally, the equilibrium concentrations of various ion species, the yield of possible precipitate, and gas species will be outputted. Although SiO2 was one of the components of raw PR, we did not input SiO2 to the MINTEQ model because SiO2 is a chemically stable material, and is not nearly as involved in the chemical reactions in the experimental conditions used in this study.
the C-O bond at 880 and 728 cm 1 conforms to the presence of dolomite in the solid [32,33], in accordance with the XRD data. Fig. 1(c) and (d) show that the surface of PR is smooth and the main elements on the surface are carbon (C), oxygen (O), fluorine (F), phosphorus (P), and calcium (Ca). The surface also contained small quantities of iron (Fe), magnesium (Mg), aluminum (Al), and silicon (Si). The content of the main chemical composition of PR is given in Table S1. Furthermore, the precipitation-dissolution equilibrium of PR in aqueous phase showed the raw PR was stable at initial pH = 4–10 (Fig. S1).
Results and discussion
The effect of contact time on residual F , PO43 , and Ca2+ concentrations were shown in Fig. 2. Under the conditions of initial F concentration of 9.5 mg L 1, contact time 30 min, and the Ca/F molar ratios 5, 8, and 10, the residual F concentrations were 1.9, 1.3, and 1.1 mg L 1, respectively. It should be emphasized that a Ca/P/F molar ratio of 5:3:1 is the stoichiometric ratio required to precipitate fluorapatite from the solution. The observed differences in defluoridation (Fig. 2) with Ca/F ratio may result from the fact that a higher Ca/F molar ratio can provide a higher level of supersaturation of fluorapatite condition, which will accelerate the induced crystallization process. The kinetic fittings of residual F with different Ca/P/F ratios were also presented in Fig. 2. It should be stressed that, in the seeded crystal-growth method, the spontaneous nucleation of amorphous precursor phase is unavoidable and crystal growth on seed crystals occurs [34]. Therefore, the rate of fluorapatite precipitation denotes the crystal-growth rate of fluorapatite.
Characterization The XRD pattern, FTIR spectra, SEM image, and EDS spectrum are shown in Fig. 1. It can be seen from Fig. 1(a) that PR is composed of fluorapatite, dolomite, small quantities of calcite, and quartz. The FTIR result for PR is shown in Fig. 1(b). The peaks at 573 and 604 cm 1 were assigned to the P-O mode of the PO43 [28]. The peak at 964 cm 1 was attributed to the v1 vibration modes of the PO43 [29]. The peaks at 1049 and 1095 cm 1 were attributed to the v3 vibration modes of the PO43 [30]. The peak at 880 cm 1 was attributed to the v2 vibration modes of the C-O bond of the CO32 ion, and the peaks at 1432 and 1454 cm l were likely the v3 vibration modes of the C-O bond of the CO32 ion. These values agree with those usually reported in the literature for B-type carbonate fluorapatites [31]. The presence of absorption bands of
Effect of contact time and initial pH
Fig. 1. Characterization of raw PR: (a) XRD pattern, (b) FTIR spectra, (c) SEM image, and (d) EDS spectrum.
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Fig. 2. Effect of contact time on residual F (a) and kinetic fitting (pseudo-first- and -second-order kinetic) with Ca/P/F molar ratio (b) Ca/P/F = 5:3:1; (c) Ca/P/F = 8:3:1; (d) Ca/ P/F = 10:3:1). Initial F concentration, 9.5 mg L 1. C0 is the initial F concentration (mg L 1) and C is the F concentration (mg L 1) at time t.
During the calculation of the rate of fluorapatite precipitation, the period from 0 to 2–3 min was not used in the rate calculations because this period represented a rapid surface adsorption equilibration process not characteristic of crystal growth [34]. Therefore, in Fig. 2, the point of “time 0” was removed. It can be seen from Fig. 2 that the removal of F agrees well with the pseudo-second-order kinetic model with all Ca/P/F molar ratios, and the rate constant k2 was 0.00802, 0.00907, and 0.0107 mg L 1 min 1 with Ca/P/F molar ratios of 5:3:1, 8:3:1, and 10:3:1, respectively. According to Fig. 2, it is suggested that a reaction time of 60 min is enough for defluoridation. PO43 was added
following by stoichiometric ratio (P/F = 3) of fluorapatite, whereas excessive addition of Ca2+ (Ca/F = 8 or 10) was conducted to accelerate the reaction. Considering that Ca2+ was a desirable element in the field of aquatic environments compared to PO43 , the protocol of overdosage of Ca2+ was acceptable in here. Experimental results of residual F , PO43 , and Ca2+ concentrations and final pH under different initial pH values are shown in Fig. 3. When the initial pH increased from 4 to 10, the residual concentrations of F , PO43 , and Ca2+ were 0.90–0.99, 0.17–5.37, and 103.74–126.44 mg L 1, respectively. The initial pH (4–10) represented less effects on F , PO43 , and Ca2+. The reason was that
Fig. 3. Experimental results of residual F (a), PO43 (b), and Ca2+ (c) concentrations and final pH (d) under different initial pH values. Initial F concentration, 9.5 mg L 1. Ca:P: F molar ratio, 8:3:1.
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the interaction of H2PO43 with solid carbonates (CaCO3, CaMg (CO3)2) would generated bicarbonate in the solution. The buffering capacities of the bicarbonates and PO43 would keep the system pH at comparative neutral level, consisted with the results of the final pH (Fig. 3(d)). However, when the initial pH values of the system were 2 and 12, which present in an extreme aquatic environment, the residual F concentrations were 20.48 and 8.08 mg L 1 (Fig. 3(a)) respectively. The reduction in the effect of defluoridation may be due to insufficient buffer system capacity or a change in reaction mechanism. Defluoridation mechanism It is clear that the new substances formed on the surface of spent PR (Fig. 4(a)) comparing to raw PR (Fig. 1(c)). The dimensional and morphological features of the raw and spent PR were demonstrated by TEM and HRTEM and the results are presented in Fig. 5. The surface of raw PR was smooth (Fig. 5(a)), while the surface of spent PR was rough and covered with some pinky crystals (Fig. 5(c)). The BET surface area of raw and spent PR was 1.3090 and 2.1406 m2/g, respectively. The pinky crystals on the surface of spent PR contributed to the expansion of the surface area. Furthermore, the measured d spacings of lattice fringes were 0.29 and 0.23 nm, which were assigned to the (001) and (110) crystal planes of raw PR, respectively (Fig. 5(b)). Interestingly, the d spacings of lattice fringes of pinky coating were also 0.29 and 0.23 nm. The XRD pattern and FTIR analysis of spent PR (Figs. S2 and S3) were consistent with raw PR (Fig. 1(a)). These suggested that the newly formed pinky substances on the spent PR particles surface were the same as on raw PR. The lattice fringes images of the raw and spent PR were closely similar to that of Ca10(PO4)6(OH) F reported by Xiang Ge [35]. TGA was carried out from room temperature to 1100 C for raw and spent PR (Fig. S4). The results showed one mass loss stage (from 557 C to 783 C) for both raw and spent PR. This mass loss stage was attributed to the decomposition of calcite and dolomite, which were the main compositions of PR as shown in Fig. 1 [36,37]. The wide scan XPS spectra of raw and spent PR are presented in Fig. 6(a). This demonstrates that the surface of both of raw and spent PR consisted of F, O, Ca, C, and P. The F1s, P2p, and Ca2p XPS spectra of spent PR after defluoridation were similar with that of raw PR and the peak location was consistent with the literature [38,39]. The same composition of raw PR and newly formed pinky coating on spent PR, which were confirmed by TEM, BET, XRD, TGA, and XPS, suggested that the F was removed mainly through the formation of Ca5(PO4)3F.
5
Impact of extreme solution pH As described in “Effect of contact time and initial pH” section, a reduction of defluoridation under pH = 2 and 12 conditions can be observed obviously. Given that the extreme conditions of pH and alkalinity, such as acidic discharge from mining industries and strong basic condition in traditional defluoridation using lime, pH = 2 and 12 were selected to investigate the reliability of induced fluorapatite crystallization on PR and the probable mechanism. The equilibrium concentrations of ion species at different initial pH modeled by Visual MINTEQ are shown in Fig. S5. Surprisingly, the residual concentrations of F and PO43 calculated by the model were all near zero even when the initial pH was at 2 and 12 and the final pH between experimental results and the model output exhibited a distinguishing difference. Given that the model output results of F were similar to the experimental results (Fig. 3) at initial pH 4–10, the reliability of the MINTEQ simulation is good, as expected. The input parameters of MINTEQ were same as those used in the laboratory experiments. However, the contact time of the laboratory experiments was 1.0 h, whereas the Visual MINTEQ model provided the chemistries results based on fully aging. Therefore, 1 h did not seem enough for the system to reach equilibrium under an extreme acid-base condition. The experiments at initial pH values of 2 and 12 were performed at a prolonged range of contact times from 1 to 144 h, and the results are shown in Fig. 7(a). The residual F concentration was 0.11 mg L 1 and 3.47 mg L 1 at initial pH 2 and 12, respectively, when the contact time was prolonged to 144 h. The behaviors of kinetic models at initial pH 2 and 12 are shown in Fig. 7(b) and (c), respectively, which demonstrate that the regression correlations (R2) and rate constants k2 fitted with a pseudo-secondorder kinetic model were 0.9816 and 0.0001 mg L 1 min 1 for initial pH 2, and 0.9904 and 0.00002 mg L 1 min 1 for initial pH 12, respectively. Clearly, the rate constants k2 of pH 2 and 12 were less than that of pH 4–10. At initial pH 2, the residual F concentration was 0.11 mg L 1 after 144 h (Fig. 7). The F , PO43 , and Ca2+ dissolved obviously from PR in the first hour, and then the F , PO43 , and Ca2+ were supersaturated with respect to fluorapatite (Ca5(PO4)3F). Thereafter, fluorapatite precipitate was formed gradually when the reaction increased gradually to 144 h. The XRD result (Fig. S6) shows that the spent PR at initial pH 2 is composed of fluorapatite, dolomite, small quantities of calcite, and quartz. The FTIR result (Fig. S7) of the spent PR at initial pH 2 was similar to the raw PR (Fig. 1(b)). Therefore, prolonging the reaction time could cover the negative effect of extreme acid condition.
Fig. 4. SEM image and EDS spectrum of spent PR.
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Fig. 5. TEM analysis of raw and spent PR. (a) TEM image of raw PR; (b) HRTEM lattice fringe image and d spacings of raw PR; (c) TEM image of spent PR; (d) HRTEM lattice fringe image and d spacings of spent PR.
At initial pH 12, the residual F concentration was 3.47 mg L 1 at 144 h (Fig. 7). The XRD (Fig. S8) and FTIR (Fig. S9) results of the spent PR at pH 12 are similar to those of the raw PR. Precipitate fitting by MINTEQ in Table 1 shows that the concentrations of FCO3-apatite (Ca9.316Na0.36Mg0.144(PO4)4.8(CO3)1.2F2.48) of spent PR at initial pH 12 was 2706 mg L 1, which was higher than that of raw PR 2200 mg L 1. Furthermore, the percentage of CO32 in FCO3apatite of spent PR at initial pH 12 detected by FTIR was 6.6%, which was greater than that in the raw PR (6.4%). The FTIR and MINTEQ results confirmed that the F was partially removed by the formation of FCO3-apatite. However, the residual concentration of F of the model output was nearly zero (Fig. S4). The model output and experimental results were inconsistent because OH competed with F for Ca2+ and PO43 to form precipitate. Effect of alkalinity Experimental results of residual F , PO43 , and Ca2+ concentrations and final pH under different initial alkalinities are shown in Fig. 8. At initial alkalinity of 0, 0.99, and 1.95 mM, the residual F concentrations of the experimental results were 1.04, 1.21, and 2.09 mg L 1, respectively (Fig. 8). Therefore, the residual F increased with increasing initial total alkalinity. At higher initial alkalinities of 9.92 and 17.12 mM, the residual F concentrations of the experimental results were 7.34 and 7.97 mg L 1, respectively (Fig. 8). This suggested that the high total alkalinities of solution can inhibit the defluoridation process in this system. Nevertheless, the residual F concentrations of the MINTEQ model outputs were all near zero at the initial alkalinities of 9.92 and
17.12 mM (Fig. S10). One main reason for the differences between the experimental results and the model outputs was that the contact time of 1 h was not enough to allow the solution to reach equilibrium, as described in “Impact of extreme solution pH” section. Hence, the experiments at initial alkalinities of 9.92 and 17.12 mM were performed under a prolonged contact time of 144 h (Fig. 9(a)). When the contact time increased up to 144 h, the residual F concentration at an initial alkalinity of 9.92 mM was 0.70 mg L 1 (Fig. 9(a)). Therefore, high alkalinity increased the required reaction time of induced fluorapatite crystallization to achieve equilibrium. Under an initial alkalinity condition of 17.12 mM, the residual F concentration decreased from 7.97 to 6.26 mg L 1 when the reaction time increased from 1 to 144 h (Fig. 9(a)). The improvement of defluoridation efficiency was negligible compared with that of lower-alkalinity conditions. The downturn of defluoridation under high-alkalinity conditions was attributed to the competition between OH and F with Ca2+ and PO43 to form precipitate, similar to the effect of pH 12, as discussed in “Impact of extreme solution pH” section. The XRD and FTIR results of the spent PR at initial alkalinities of 9.92 and 17.12 mM (Figs. S11–S14) support the above statement. The regression correlation (R2) and rate constant k2 fitted with a pseudo-second-order kinetic model were 0.9702 and 0.0001 mg L 1 min 1 for initial total alkalinity 9.92 mM, 0.9033 and 0.000001 mg L 1 min 1 for 17.12 mM, respectively (Fig. 9(b) and (c)). The decrease of rate constants with alkalinity was confirmed. A schematic expressing the induced crystallization process on the surface of PR at different initial pH values and alkalinities is
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Fig. 6. High resolution XPS spectra of raw PR and spent PR: (a) wide scan spectra; (b) F1s spectra; (c) P2p spectra; (d) Ca2p spectra.
Fig. 7. (a) Defluoridation at initial pH 2 and 12, (b) kinetic plot of defluoridation at initial pH 2, and (c) kinetic plot of defluoridation at initial pH 12. Initial F concentration = 9.5 mg L 1. Ca:P:F molar ratio = 8:3:1.
summarized in Fig. 10. At initial pH 2, PR was partially dissolved in the first 1 h, and F , PO43 , and Ca2+ in the solution formed Ca5(PO4)5F on the surface of PR in the subsequent time to 144 h. At an initial pH 4–10, the induced crystallization of Ca5(PO4)3F occurred on the PR surface within 1 h, which was same to that at an initial alkalinity of 9.92 Mm within an extended time of 144 h. At an initial alkalinity of 17.12Mm, FCO3-apatite and Ca5(PO4)3OH were
formed on the PR surface in 144 h. At initial pH 12, FCO3-apatite, Ca5(PO4)3OH, and Mg(OH)2 were formed on the PR surface in 144 h. Stability for recycle runs The residual concentrations of F , PO43 , and Ca2+ as well as final pH after the cycle utilization of PR as seed crystals are shown
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Table 1 Solids modeled by MINTEQ and CO32 contenta in apatiteb in PR detected by FTIR. Condition
Raw PRd pH = 2.00 pH (4–10) pH = 12.00 Alky = 9.92 mM Alky = 17.12 mM
MINTEQ model output (mg L
1
CO32 (%) detected by FTIR
) c
Ca5(PO4)3F
FCO3-apatite
Ca5(PO4)3(OH)
CaMg(CO3)2
CaCO3
Mg(OH)2
400 650 650 – 650 –
2200 2207 2207 2706 2207 2706
– – – 232 – 231
4300 3404 3974 3846 4085 4287
1300 1280 1390 1630 1410 1390
– – – 137 – –
6.4 4.5 5.4 6.6 6.0 6.8
This CO32 content denotes CO32 content in apatite and does not include the CO32 content in CaMg(CO3)2 and CaCO3. Apatite includes Ca10(PO4)6F, FCO3-apatite and Ca10(PO4)6(OH)2. c The molecular formula of FCO3-apatite is Ca9.316Na0.36Mg0.144(PO4)4.8(CO3)1.2F2.48. d Contents of Ca10(PO4)6F2, FCO3-apatite, Ca10(PO4)6(OH)2, CaMg(CO3)2, CaCO3, and Mg(OH)2 in this row were calculated from XRD and FTIR data, as were the MINTEQ model input conditions. a
b
Fig. 8. Experimental results of residual F , PO43 , and Ca2+ concentrations and final pH under different initial alkalinities. Initial F concentration, 9.5 mg L 1. Ca:P:F molar ratio, 8:3:1.
in Fig. 11. When the Ca:P:F molar ratio was 5:3:1, the residual F after the third cycle was 1.6 mg L 1, which exceeds the WHO drinking water guideline of 1.5 mg L 1. When the Ca:P:F molar ratios were 8:3:1 and 10:3:1, the residual F after three cycles were all below 1.5 mg L 1. These results indicate that when the Ca:P:F molar ratio was 5:3:1, the PR could be reused twice, but when the Ca:P:F molar ratio was 8:3:1 or 10:3:1, the PR could be reused at least three times. It should be recognized that the residual PO43 increased with the cycle number, but the residual Ca2+ concentration and final pH did not vary significantly. This is a curious phenomenon. The F , PO43 , and Ca2+ ions in the solution theoretically formed fluorapatite crystals on the PR surface. Thus, residual F , PO43 , and Ca2+ concentrations should all be very low. It is possible that the addition of NaH2PO4 to the solution leads to CO32 dissolution, generating Ca2+ ions, because calcite and dolomite are both constitutes of the PR (Fig. 1(a)). Therefore, the F , PO43 , CO32 , and Ca2+ ions formed both FCO3-apatite and fluorapatite on the surface of the PR, and the ratio of the newly formed FCO3-apatite/
fluorapatite increased with cycle number. This process would allow the residual PO43 to increase with cycle number while maintaining consistent residual F and Ca2+ concentrations. Comparison of techniques for defluoridation Table 2 summarizes techniques for defluoridation. Adsorption is the main technique used to remove F–. The technique used in this work can effectively remove F–. Considering the extremely low solubility and the continued use as a seed crystal of Ca5(PO4)3F, the necessity for spent material dispose can be negligible compared with other defluoridation techniques, i.e., traditional adsorption, electrocoagulation, etc. Reliability under extreme aquatic environment conditions This research puts forward a possible technology for the defluoridation for most of the aquatic environment. The F could be removed to a satisfactory concentration by forming Ca5(PO4)5F.
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Fig. 9. (a) Defluoridation at initial alkalinities of 9.92 and 17.12 mM, (b) kinetic plot of defluoridation at initial alkalinity 9.92 mM, and (c) kinetic plot of defluoridation at initial alkalinity 17.12 mM. Initial F concentration, 9.5 mg L 1. Ca:P:F molar ratio, 8:3:1.
Fig. 10. Schematic of defluoridation mechanism by induced crystallization on PR at different pH values and alkalinities.
However, in some other extreme aquatic environments, such as pH 2 or 12, the treatment of objective water could be achieved through prolongating the reaction time or adjusting pH to a range of 4–10; this strategy was also effectual under higher-alkalinity conditions. Conclusions The impact of acid-base conditions on defluoridation by induced fluorapatite crystallization on PR was investigated in this work. The residual F could decrease from 9.5 mg L 1 to below the U.S. EPA limit within 1 h when the initial pH is in the range 4–10 as
well as when the initial alkalinity is less than 9.92 mM. At the initial pH 2 and initial alkalinity 9.92 mM, the residual F could satisfy the drinking water guideline as the reaction time was prolonged up to 144 h. However, at the extreme base aquatic environment (initial pH 12 and initial alkalinity 17.12 mM), the objective of defluoridation could be achieved by adjusting the initial pH to 4–10. The MINTEQ modeling, BET, XRD, FTIR, SEM-EDS, TEM, TGA, and XPS analysis revealed the objective of defluoridation was achieved by the formation of Ca5(PO4)3F at an initial pH less than 10 and alkalinity less than 9.92 mM. In particular, F was removed successfully by the induced crystallization of FCO3-apatite (Ca9.316Na0.36Mg0.144(PO4)4.8(CO3)1.2F2.48) under an extreme base
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Fig. 11. Residual F , PO43 , and Ca2+ concentrations and final pH in cycle use of PR as seed crystals.
Table 2 Comparison of techniques for removal of F–. Techniques a
Adsorption Adsorptionb Adsorptionc Adsorptiond Adsorptione Electrocoagulation Induced crystallization
Initial concentration (mg L 10 10 100 2 100 8.6 9.5
1
)
Residual concentration (mg L 2.5 1.5 40 0.3 40 1.4 0.9
1
)
Removal efficiencies (%)
Reference
75 85 60 84 60 85 90
[40] [41] [42] [43] [44] [45] This work
Adsorbents: a Fe3O4@La-Ce. b La/modified alumina. c MnCO3. d Microwave-assisted carbonized Azadirachta indica bark. e Powdered-drinking-water treatment residuals.
aquatic environment (the initial pH 12 and alkalinity 17.12 mM). This research has potential to guide the treatment of the common aquatic environment; in addition, extreme conditions could be pretreated to achieve defluoridation. Conflicts of interest None declared.
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Acknowledgments This work was supported by the Key Laboratory Program of Education Bureau of Shaanxi Province, China (Grant No. 19JS038), the National Natural Science Foundation of China (Grant No. 51308436), the National Natural Science Foundation of China (Grant No. 51278404), and the Shaanxi Science & Technology Coordination & Innovation Project (Grant No. 2015KTCL-03-15). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jiec.2019.11.010.
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