Fabrication of Manganese-Supported Activated Alumina Adsorbent for Defluoridation of Water

water

Article Fabrication of Manganese-Supported Activated Alumina Adsorbent for Deﬂuoridation of Water: A Kinetics and Thermodynamics Study

Kun You 1, Peijie Li 2, Jinxiang Fu 1, Ning Kang 1, Yujia Gao 1, Xiaoxiang Cheng 2,*, Yuehong Yang 3 and Furui Yu 3

1 School of Municipal and Environmental Engineering, Shenyang Jianzhu University, Shenyang 110168, China; [email protected] (K.Y.); [email protected] (J.F.); [email protected] (N.K.); [email protected] (Y.G.) 2 School of Municipal and Environmental Engineering, Shandong Jianzhu University, Jinan 250101, China; [email protected] 3 Shandong Urban Construction Vocational College, Jinan 250103, China; [email protected] (Y.Y.); [email protected] (F.Y.) * Correspondence: [email protected]

Abstract: Fluoride pollution frequently occurs in many underground drinking water sources due to discrepancies in the geological environment. To address this problem, a manganese-supported activated alumina (MnOOH-supported AA) adsorbent was proposed in the present study. The adsorbent was prepared with an impregnation method, then the morphology and microstructure were systematically characterized. Further, the adsorption kinetics and thermodynamics were systematically explored through static experiments to confirm the adsorption mechanism. The results showed that MnOOH was successfully loaded on the activated alumina (AA), and irregular and convex spinous structures were formed on the surface of particles. Compared with the AA, Citation: You, K.; Li, P.; Fu, J.; Kang, MnOOH-supported AA exhibited a significantly higher defluoridation rate, which has been doubled. N.; Gao, Y.; Cheng, X.; Yang, Y.; Yu, F. The kinetic behavior of fluoride adsorption on MnOOH-supported AA was governed by the quasi- Fabrication of Manganese-Supported Activated Alumina Adsorbent for second-order kinetics model with regression coefficients of 0.9862, 0.9978 and 0.9956, respectively. Defluoridation of Water: A Kinetics The adsorption rate was mainly ascribed to the intra-particle diffusion. Additionally, the Freundlich and Thermodynamics Study. Water isotherm equation fitted the adsorption thermodynamic process reasonably well compared with the 2021, 13, 1219. https://doi.org/ Langmuir adsorption model. Specifically, the correlation coefficients were 0.9614, 0.9383 and 0.9852 ◦ ◦ ◦ 10.3390/w13091219 at 25 C, 35 C and 45 C, respectively. The adsorption–desorption isotherm plot was similar to the Type V isotherm. The whole fluoride adsorption was a spontaneous endothermic reaction, and Academic Editor: An Ding controlled by chemical adsorption. These results demonstrated that MnOOH-supported AA as an alternative to the conventional AA showed promising potential for defluoridation in drinking water Received: 20 March 2021 treatment. Accepted: 24 April 2021 Published: 28 April 2021 Keywords: manganese-supported activated alumina (MnOOH-supported AA); fluoride; kinetics; thermodynamics Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction China has abundant groundwater resources, accounting for 30% of the total water resources. Due to stable water temperature, simple water-supply facilities, reliable water- supply capacity, favorable water quality and extensive distribution etc., the groundwater is Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. being developed on a large scale and has been a vital source of drinking water. However, This article is an open access article the water quality and safety cannot be guaranteed for nearly half of the groundwater. The distributed under the terms and water could be polluted by fluoride because of human production activities and natural conditions of the Creative Commons geochemical processes. Long-term use of high-fluoride water will do harm to human health, Attribution (CC BY) license (https:// causing endemic fluorosis such as dental fluorosis and skeletal fluorosis. The intelligence creativecommons.org/licenses/by/ quotient of children could be significantly affected if excessive fluoride is ingested [1,2]. 4.0/). In China (Figure S1), high-fluoride shallow groundwater is mainly distributed in Heilong

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Jiang, Jilin, Liaoning, Shanxi, Jiangsu, Ningxia and Gansu Provinces, etc. The fluoride content in groundwater is 2~5 mg/L, and sometimes even as high as 10 mg/L. Meanwhile, the phenomenon of high iron ions generally occurs in Heilong Jiang, Jilin, Liaoning, Shandong, Hubei and Hebei Provinces and the Nei Monggol Autonomous Region, etc. It is noteworthy that groundwater is a significant drinking water source in northern rural areas of China. In general, the occurrence of high-fluoride and high-iron are noticed depending on the geological structure. However, due to the local geological structure and economic environment conditions, there is still a lack of effective defluoridation measures in many rural areas. Therefore, there is an urgent technical demand by livelihood problems to relieve fluoride pollution. At present, there are five major types of commonly used fluoride removal methods: precipitation [3–5], ion exchange [6,7], membrane filtration [8,9], electrodialysis [10] and adsorption [11,12], all of which have their own advantages and shortcomings, as shown in Table1. Among these methods, adsorption is widely applied for low operating costs and simple operation process [13], and the most frequently used adsorbents include activated alumina (AA), zeolite and chitosan [14,15]; among these, AA is the most commonly used. Table2 shows the related research on adsorbents and adsorption effects abroad. Although the common adsorbents exhibit preferable adsorption capacity, there are still problems occurring in defluoridation, including low saturation adsorption capacity and increasing amounts of adsorbent dosage [16]. Recently, fabricating adsorbents coated with Ca, Zr, La, Ce, Mn and other elements have attracted increasing attention for fluoride removal, and the modified adsorbents have exhibited excellent removal. Calcium hydroxide used for fluoride removal could produce sludge with a high water content and an unstable fluoride concentration in the effluent. It is reported that manganese oxide (MnOOH) with a large surface area and microporous structure [17–19] is prevalent in epigenetic geochemical environments, which manifests excellent adsorption performance for anions [20]. At present, there are few studies related to AA loaded with MnOOH for fluoride removal in groundwater. In the present study, a manganese-supported AA (MnOOH-supported AA) adsorbent was evaluated for defluoridation from aqueous solutions. To be specific, AA with a size of 3~5 mm was selected, and MnOOH was loaded on AA using an impregnation method. The morphology and microstructure of MnOOH-supported AA were analyzed using various characteristic means. Moreover, the adsorption kinetics and adsorption thermodynamics were determined by a quasi-first-order model, a quasi-second-order model, the Weber and Morris model, the Freundlich isotherm equation and the Langmuir isotherm equation. The adsorption mechanism was then discussed on the strength of the experimental results. Water 2021, 13, 1219 3 of 19

Table 1. Comparative analysis of ﬂuoride removal methods.

Technique Category Materials Advantages Limitations Chemical Low cost, High amount of retained water (sludge dewatering is Calcium salt, etc. precipitation simple operation required prior to disposal) Can be expensive, efficiency depends on pH and the Precipitation presence of co-existing ions in water, adjustment and Coagulant Iron salt, Low cost, readjustment of pH is required, elevated residual sedimentation aluminum salt, etc. simple operation aluminum concentration, formation of sludge with a high amount of toxic aluminum fluoride complex Ion exchange Ion exchange Ion exchange resin High efficiency High cost of installation and regeneration Expensive, vulnerable to interfering ions (sulfate, Ion exchange Electrodialysis High efficiency, no need to dosage agents phosphate, chloride, bicarbonate, etc.), high operation cost, membrane toxic concentrate generated Separation membrane Reverse High efficiency, suitable for treating High cost of installation and maintenance, replacement of Reverse osmosis membrane osmosis high-fluoride water media after multiple regenerations Ultrafiltration Ultrafiltration membrane No risk of secondary contamination, High cost, pre-processing complexity, strict technical Nanofiltration Nanofiltration membrane suitable for treating high-fluoride water requirements Greater accessibility, low cost, simple Common ions interfere with fluoride adsorption, Activated operation, availability of wide range of Adsorption Activated alumina regeneration difficult, low adsorption efficiency under alumina adsorbents, produce high-quality water, high fluoride concentration environmentally friendly Water 2021, 13, 1219 4 of 19

Table 2. A summary of various adsorbents for ﬂuoride removal from water.

S. No. Adsorbent Adsorption Capacity Concentration Range Contact Time pH 1 Fe-impregnated chitosan (Fe-CTS) 1.97 mg/g 10 mg/L 6 h - Magnetic iron oxide fabricated hydrotalcite/chitosan 2 5.03 mg/g 10 mg/L 20 min 5 (Fe3O4HTCS) 3 Hydrous zirconium oxide-impregnated chitosan beads 22.1 mg/g 9.7–369.2 mg/L 160 h 5 4 Sn(IV) chloride impregnated chitosan La3+ modiﬁed 17.63 mg/g 5–100 mg/L 30 min 6 Lanthanum-aluminum loaded hydrothermal palygorskite 5 1.30 mg/g 4.89 mg/L 540 min 7.5 (La-Al-HP) 6 Fe3+-modiﬁed bentonite clay 2.91 mg/g 10 ppm 30 min 2~10 7 MnO2 coated Na-bentonite 2.4 mg/g 5 mg/L 30 min 8 8 Hydroxyapatite nanorods 1.49 mg/g 10 mg/L 3 h 7 Sulfate-doped hydroxyapatite hierarchical hollow 9 28.3 mg/g 2–100 mg/L 2 h 3.0~10.0 microspheres Hydroxyapatite decorated with carbon nanotube composite 10 11.05 mg/g 15 mg/L 300 min 6 (CNT-HAP) 11 Hydroxyapatite montmorillonite (HAP-MMT) 16.7 mg/g 30 mg/L 30 min 5 12 Zirconium impregnated activated carbon (ZrAC) 5.4 mg/g 2.5–20 mg/L 180 min 4 13 Mg-Mn-Zr impregnated activated carbon (ACMg-Mn-Zr) 26.27 mg/g 5–30 mg/L 3 h 4 14 Alumina impregnated activated carbon 2.86 mg/g 10 mg/L 3 h 6.1 15 Activated alumina - 2–20 mg/L 24 h 6~8 16 Acid activated alumina 69.52 mg/g 10–60 mg/L 3 h 6.5 17 Nitric acid activated alumina 45.75 mg/g 40 mg/L 3 h 3.5 Water 2021, 13, 1219 5 of 19

2. Materials and Methods 2.1. Chemicals

In this study, AA, sodium ﬂuoride (NaF), ammonia (NH3·H2O), ammonium ferrous sulfate ((NH4)2Fe(SO4)2·6H2O) and hydroxylamine hydrochloride (H4ClNO) were purchased from Sinopharm Chemical Reagent Co., Ltd. (CHN). Aluminum sulfate (Al2(SO4)3), manganese sulfate (MnSO4), hydrochloric acid (HCl), phenanthroline (C12H8N2), ammonium acetate (CH3COONH4) and glacial acetic acid (CH3COOH) were provided by Tianjin Kemiou Chemical Reagent Co., Ltd. (CHN). Hydrogen peroxide (H2O2) and SPADNS reagent were purchased from Tianjin Damao Chemical Reagent Factory (CHN) and Amer- ica HACH company (USA), respectively. As shown in Table3, a ﬂuoride solution was prepared by dissolving a certain amount of NaF and (NH4)2Fe(SO4)2·6H2O in deionized water (DI water). Note that the actual initial concentration of the pollutant solution is subject to the actual measured value on that day. All chemicals were all of analytical grade and DI water (resistivity of 18.2 MΩ·cm) was used in all experiments.

Table 3. Reference sample preparation.

F− Total Fe 2 ± 0.5 2 ± 0.5 Concentration 5 ± 0.5 2 ± 0.5 (mg/L) 10 ± 0.5 2 ± 0.5

2.2. Fabrication of MnOOH-Supported AA 2.2.1. Modiﬁcation of AA

As shown in Figure1a, 100 g of AA particles was added in 200 mL of an Al 2(SO4)3 solution with a concentration of 4%. Subsequently, the solutions were mixed for 24 h at a speed of 120 r/min. After that, the obtained solutions were washed with DI water several times until the pH of the efﬂuent was stable, then dried in an oven for 2 h at 103 ◦C. The 2− modiﬁed AA was used after cooling to room temperature. It was noteworthy that SO4 in the solution could combine with AA by electrostatic or chemical adsorption, forming Water 2021, 13, x FOR PEER REVIEW 5 of 18 sulfate-type AA. The chemical reaction is expressed as follows: 2− − (Al2O3)n·2H2O + SO4 → (Al2O3)n·H2SO4 + 2OH (1)

FigureFigure 1. Schematic 1. Schematic diagram diagram for for the the preparation preparation process process of of MnOOH-supported MnOOH-supported AA. AA.

2.3. Adsorption Experiment The adsorption kinetic experiment was carried out as follows: 200 mL of a fluoride solution was treated with the addition of a certain amount of MnOOH-supported AA at pH 4 and a temperature of 25 °C. The mixture was then oscillated at a moderate speed of 120 r/min using a thermostatic oscillator. To reduce variations, measurements were conducted in three parallel experiments, and the results presented were the average values. The adsorption kinetic model on MnOOH-supported AA was established on the basis of the experimental data. With respect to the adsorption thermodynamic experiment, 7 g/L of MnOOH-supported AA was added into the fluoride solution at pH 4 and oscillated for 12 h to attain adsorption/desorption equilibrium. The initial concentrations of the fluoride solution were 2, 5, 10, 15 and 20 mg/L. In addition, the iron ion content was 2 mg/L. To study the effect of temperature on adsorption, the temperatures were controlled at 25 °C, 35 °C and 45 °C. The adsorption thermodynamic model was established, and the thermodynamic parameters of the adsorption process were also calculated. The mechanism of fluoride adsorption on MnOOH-supported AA was further discussed.

2.4. Mathematical Model and Basic Parameter Expression 2.4.1. Adsorption Kinetic Model To get insight into the adsorption properties and rate-limiting step during the adsorption process, the adsorption kinetics of fluoride were investigated with a quasi-first- order model, a quasi-second-order model [21] and the Weber and Morris model [22,23]. The mathematical expressions are shown as Equations (2)–(4).

( − ) = − (2) 1 = + (3) = / + (4)

where qe and qt (mg/g) represent the adsorption quantity at equilibrium and time, respectively; k1 and k2 are the rate constant of the quasi-first-order reaction and the quasi-second- order reaction, respectively; kp (mg/(g·min1/2)) represents the particle diffusion rate constant; t (min) is the adsorption time and C is a constant which characterizes the extent of diffusion.

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2.2.2. AA Coated with MnOOH Figure1b shows that MnOOH-supported AA was fabricated using an impregnation method. Firstly, 50 g of the modified AA particles was dispersed in 250 mL of a MnSO4 solution (0.06 mol/L). Thereafter, the mixture was continuously stirred, injected with ◦ 5.1 mL of H2O2 with a concentration of 30%, and heated to 95 C. Next, 75 mL of 0.2 mol/L NH3·H2O was poured into the above solution. The brown precipitates generated on the surface of AA were MnOOH. To increase the contact area, the as-obtained mixture was continuously stirred at 165 r/min for 6 h. After that, the samples were firstly dried at 100 ◦C for 2 h, then washed thoroughly 8 times with 70–80 ◦C hot DI water, and finally dried for 4 h at 100 ◦C. As presented in Figure1c, the MnOOH-supported AA was cooled in an indoor environment naturally for further use.

2.3. Adsorption Experiment The adsorption kinetic experiment was carried out as follows: 200 mL of a fluoride solution was treated with the addition of a certain amount of MnOOH-supported AA at pH 4 and a temperature of 25 ◦C. The mixture was then oscillated at a moderate speed of 120 r/min using a thermostatic oscillator. To reduce variations, measurements were conducted in three parallel experiments, and the results presented were the average values. The adsorption kinetic model on MnOOH-supported AA was established on the basis of the experimental data. With respect to the adsorption thermodynamic experiment, 7 g/L of MnOOH-supported AA was added into the fluoride solution at pH 4 and oscillated for 12 h to attain adsorption/desorption equilibrium. The initial concentrations of the fluoride solution were 2, 5, 10, 15 and 20 mg/L. In addition, the iron ion content was 2 mg/L. To study the effect of temperature on adsorption, the temperatures were controlled at 25 ◦C, 35 ◦C and 45 ◦C. The adsorption thermodynamic model was established, and the thermodynamic parameters of the adsorption process were also calculated. The mechanism of fluoride adsorption on MnOOH-supported AA was further discussed.

2.4. Mathematical Model and Basic Parameter Expression 2.4.1. Adsorption Kinetic Model To get insight into the adsorption properties and rate-limiting step during the adsorption process, the adsorption kinetics of ﬂuoride were investigated with a quasi-ﬁrst-order model, a quasi-second-order model [21] and the Weber and Morris model [22,23]. The mathematical expressions are shown as Equations (2)–(4).

ln(qe − qt) = lnqe − k1t (2)

t 1 t = + 2 (3) qt k2qe qe 1/2 qt = kpt + C (4)

where qe and qt (mg/g) represent the adsorption quantity at equilibrium and time, respectively; k1 and k2 are the rate constant of the quasi-ﬁrst-order reaction and the quasi- 1/2 second-order reaction, respectively; kp (mg/(g·min )) represents the particle diffusion rate constant; t (min) is the adsorption time and C is a constant which characterizes the extent of diffusion.

2.4.2. Adsorption Thermodynamic Model The Langmuir isotherm model and Freundlich isotherm model were applied to ﬁt the equilibrium adsorption data. The Langmuir and Freundlich models are shown as Equations (5) and (6): C C e = e 1 (5) qe qmax KLqmax Water 2021, 13, 1219 7 of 19

1 lnq = lnK + lnC (6) e F n e

where qmax (mg/g) represents the maximum adsorption capacity, Ce (mg/L) is the concentration of adsorbate at equilibrium, KL is the Langmuir constant, KF is the Freundlich constant, n refers to a measure of adsorption intensity (1/n = 0.5~1, the adsorption process is conducted easily; 1/n > 2, the adsorption process is arduous) [24].

2.4.3. Adsorption Thermodynamic Parameters To better expound the adsorption mechanism, the adsorption thermodynamic parameters, i.e., adsorption enthalpy (∆H), adsorption free energy (∆G) and adsorption entropy (∆S), were assessed. ∆H could be calculated through the Van’t Hoff equation, as shown in Equation (7):

1 ∆H ln = − + lnK0 (7) Ce RT

where ∆H (kJ/mol) is the isosteric enthalpy variation of adsorption, Ce (mol/L) is the equilibrium concentration of the solution at the absolute temperature T (K), R is the gas constant (8.314 J/(mol·K)) and K0 is the equilibrium adsorption distribution coefﬁcient. ∆G can be obtained with the Gibbs free energy equation. If the adsorption isotherm can be described using the Freundlich equation model, the ∆G obtained is independent of q when the mass concentration of the adsorbate is low, expressed as follows:

∆G = −nRT (8)

where ∆G (kJ/mol) is the change of adsorption free energy and n is the Freundlich exponent. ∆S could be obtained using the Gibbs–Helmholtz equation, as shown in Equation (9):

∆H − ∆G ∆S = (9) T

2.5. Analytical Methods The water quality indexes were detected by the Standard Test Method for Drinking Water (GB/T 57750-2006). The detection and analysis method, and standard limits for drinking water are displayed in Table4. The surface morphology of MnOOH-supported AA was observed by scanning electron microscopy coupled with energy dispersive X-ray spectrometry (SEM- EDS) (S-4800, Hitachi, Japan), and the photomicrographs were recorded at an acceleration voltage of 5 kV and a constant temperature. The specific surface was measured by N2 air- suction desorption on an automatic static physical adsorption instrument (Autosorb-IQ2-MP, Quantachrome, America). The total specific surface was calculated based on the multi-point Brunauer, Emmentt and Teller equation (P/P0 = 0.005–0.3) with point range of 30 points for adsorption and 30 points of desorption. X-ray photoelectron spectra (XPS) were measured utilizing ESCALAB 250Xi (Thermo Fisher Scientific, China) with an Al (hv = 1486 eV) X-ray source at the standard mode. Prior to measurement, conductive tape was stuck on the tinfoil, and the sample powder was evenly spread on the surface of conductive tape. The sample surface was then covered with flat tinfoil, which was placed on the table press at a pressure of 1.5 T for 1 min. Lastly, the sample sheet was placed on the sample table to determine the XPS spectra. Fourier-transform infrared spectrometry (FTIR) (NICOLET iS50, Thermo Nicolet Corporation, America) was applied to analyze the infrared adsorption spectroscopy in transmission mode. Specifically, 10 mg of the sample was mixed evenly with 500 mg of potassium bromide powder using a sample mixer and compressed into a thin slice for examination. The infrared range was set to 4000–400 cm−1 and each scan was checked 32 times. Water 2021, 13, x FOR PEER REVIEW 7 of 18

evenly spread on the surface of conductive tape. The sample surface was then covered with flat tinfoil, which was placed on the table press at a pressure of 1.5 T for 1 min. Lastly, the sample sheet was placed on the sample table to determine the XPS spectra. Fourier- transform infrared spectrometry (FTIR) (NICOLET iS50, Thermo Nicolet Corporation, America) was applied to analyze the infrared adsorption spectroscopy in transmission mode. Specifically, 10 mg of the sample was mixed evenly with 500 mg of potassium bromide powder using a sample mixer and compressed into a thin slice for examination. The infrared range was set to 4000–400 cm−1 and each scan was checked 32 times.

Table 4. Experiment detection method.

Standard Limits for Drinking Water Test Item Detection and Analysis Method (mg/L) Fluorometric F- 1.0 spectrophotometry Phenanthroline Fe 0.3 Water 2021, 13, 1219 3. Resultsspectrophotometry 8 of 19 T 3.1. CharacterizationDirect-readin ofg methodMnOOH-Supported AA - 3.1.1. Surface Morphology and Phase Composition Table 4. Experiment detection method. It is well known that the adsorption reaction on the particle surfaces is directly Standard Limits for Test Item Detection and Analysis Method affected by the surface morphology. Hence, Drinkingthe surface Water (mg/L) morphologies of three kinds of Fluorometric AAF− were directly observed by SEM, as depicted in1.0 Figure 2. It was discovered that the spectrophotometry surface appearance Phenanthrolineof modified AA was markedly different from ordinary AA. To be Fe 0.3 specific, obvious bulksspectrophotometry were heaped up on the ordinary AA surface, and the porous T Direct-reading method - structure was also inconspicuous. By contrast, the pre-treated AA had a certain pore 3. Resultsstructure, which contributed to fluoride adsorption. This phenomenon could be explained 3.1.by Characterization the fact that of MnOOH-Supported the impurities AA present in AA were removed by the modification, leading 3.1.1.to Surface an increase Morphology of and porosity. Phase Composition It can be seen in Figure 2b that the surface of MnOOH- supportedIt is well known AA that formed the adsorption an irregular reaction on theand particle convex surfaces spinous is directly structure. affected It was noticed that a by the surface morphology. Hence, the surface morphologies of three kinds of AA were directlywell-developed observed by SEM, pore as depicted structure in Figure was2. achieved, It was discovered which that was the surfaceconsistent with the finding of appearanceWang ofet modified al. [25]. AA Meanwhile, was markedly differentthe EDS from analysis ordinary AA. in ToFigure be specific, 3 indicates that MnOOH- obvious bulks were heaped up on the ordinary AA surface, and the porous structure wassupported also inconspicuous. AA clearly By contrast, had more the pre-treated surface AA oxygen had a certaincontent pore than structure, the ordinary AA, and a small whichamount contributed of Mn to fluoride element adsorption. was also This observed. phenomenon To could this be end, explained it can by thebe concluded that MnOOH factwas that the successfully impurities present loaded in AA on were the removed AA surface. by the modification, Apparently, leading better to an dispersion of MnOOH increase of porosity. It can be seen in Figure2b that the surface of MnOOH-supported AA formedcould an irregular be achieved, and convex and spinous the surface structure. of It was MnOOH-supported noticed that a well-developed AA was rougher than that of porethe structure original was achieved, AA, resulting which was consistentin excellent with the ad findingsorption of Wang ability. et al. [25 ].As compared with the Meanwhile,unmodified the EDS AA, analysis the in specific Figure3 indicates surface that of MnOOH-supported MnOOH-supported AA clearly AA had increased from 2.684 m2/g more surface oxygen content than the ordinary AA, and a small amount of Mn element was 2 alsoto observed. 3.311 Tom this/g, end,and it MnOOH-supported can be concluded that MnOOH AA was contained successfully a loaded range on of the pores with different sizes, AAexhibiting surface. Apparently, better better porosity dispersion properties. of MnOOH could Th bee achieved,above andresults the surface showed that the MnOOH- of MnOOH-supportedsupported AA AA with was a rougher large thanamount that of theof originaladsorpti AA,on resulting sites enhanced in excellent the fluoride adsorption adsorption ability. As compared with the unmodified AA, the specific surface of MnOOH- supportedcapacity. AA increased from 2.684 m2/g to 3.311 m2/g, and MnOOH-supported AA contained a range of pores with different sizes, exhibiting better porosity properties. The above results showed that the MnOOH-supported AA with a large amount of adsorption sites enhanced the fluoride adsorption capacity.

a b

Figure 2. SEM images of fresh AA (a) and MnOOH-supported AA (b). Water 2021, 13, x FOR PEER REVIEW 8 of 18

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Figure 3. EDS results of AA (a) and MnOOH-supported AA particles (b).

3.1.2. XPS AnalysisFigure 3. EDS results of AA (a) and MnOOH-supported AA particles (b). Figure 3. EDS resultsFigure of 3.AAEDS (a) results and MnOO of AA (aH) and-supported MnOOH-supported AA particles AA particles (b). (b). The XPS spectra3.1.2. of XPS AA Analysis before and after modification are presented in Figure 4. It can be clearly observed3.1.2. that XPS the Analysis elements O, Al and Mn were the main constituent elements 3.1.2. XPS Analysis The XPS spectra of AA before and after modification are presented in Figure 4. It can of MnOOH-supportedThe AA XPS, consistent spectra of AA with before the and results after modiﬁcation of EDS. areIn addition, presented in the Figure XPS4. It peak can - The XPS spectrabe of clearly AA before observed and that after the modification elements O, are Al presentedand Mn were in Figure the main 4. It constituent can elements fitting analysis forbe the clearly Mn2p observed peak that of the MnOOH elements O,-supported Al and Mn were AA the is mainshown constituent in Figure elements 5. Ap- of be clearly observedMnOOH-supportedof that MnOOH the elements-supported AA, O, consistent AlAA and, consistent with Mn the were results with the ofthe main EDS. results In constituent addition, of EDS. the In XPSelements addition, peak-ﬁtting the XPS peak- ofparently, MnOOH the-supported Mn2panalysisfitting bandAA analysis for, consistent in the the Mn2p for MnOOH the peakwith Mn2p of the MnOOH-supported-supported resultspeak of of MnOOH EDS.AA AA wasIn- supported isaddition, shownmainly in the FigureAcomposedA XPS is5 .shown Apparently,peak of- in two Figure 5. Ap- fittingpinnacles analysis: the for MnOOH theparently,the Mn2p Mn2p peak band the peak inMn2pat the 641.5of MnOOH-supported MnOOH band eV inand -thesupported the MnOOH AAMnO was A -peaksupportedA mainly is shown at composed 653.4 AA in eV,wasFigure of two accountingmainly pinnacles: 5. Ap- composed the for of two MnOOH peak at 641.5 eV and the MnO peak at 653.4 eV, accounting for 59.2% and 40.7% parently,59.2% and the 40.7% Mn2p pinnaclesrespectively. band in: thethe MnOOHThese results -peaksupported revealedat 641.5 AA eV that was and the mainlythe surface MnO composed peak of AA at 653.4was of two success-eV, accounting for respectively.59.2% and 40.7% These resultsrespectively. revealed These that the results surface revealed of AA was that successfully the surface loaded of AA with was success- pinnaclesfully loaded: the withMnOOHMnOOH. MnOOH. peak at 641.5 eV and the MnO peak at 653.4 eV, accounting for 59.2% and 40.7% respectively.fully loaded These with MnOOH. results revealed that the surface of AA was successfully loaded with MnOOH.

FigureFigure 4. XPS 4. patternXPS pattern of AA (ofa) andAA MnOOH-supported(a) and MnOOH- AAsupported particles AA (b). particles (b). Figure 4. XPS pattern of AA (a) and MnOOH-supported AA particles (b). Figure 4. XPS pattern of AA (a) and MnOOH-supported AA particles (b).

Figure 5. 5.XPS XPS peak-ﬁtting peak-fitting analysis analysis for the for Mn2p the peakMn2p of peak MnOOH-supported of MnOOH-supported AA. AA.

Figure 5. XPS peak-3.1.3.fitting FTIR analysis for the Mn2p peak of MnOOH-supported AA. Figure 5. XPS peak-fitting analysis for the Mn2p peak of MnOOH-supported AA. The typical FTIR spectra of AA and MnOOH-supported AA are shown in Figure 6. 3.1.3. FTIR The -OH stretching vibration at 3455.84 cm−1, H2O bending vibration at 1617 cm−1, and 3.1.3. FTIR The typical FTIRCO32- spectrasymmetric of stretchingAA and MnOOH vibration- supportedat 1384.44 cm AA−1 are are clearly shown observed. in Figure The 6. reasonable The typical FTIR spectra of AA and MnOOH-supported AA are shown in Figure 6. The -OH stretchingexplanation vibration for at this 3455.84 phenomenon cm−1, H was2O thatbending AA with vibration a large atsurface 1617 area cm −1and, and hydrophilic- The -OH stretching vibration at 3455.84 cm−1, H2O bending vibration at 1617 cm−1, and CO32- symmetric stretching vibration at 1384.44 cm−1 are clearly observed. The reasonable CO32- symmetric stretching vibration at 1384.44 cm−1 are clearly observed. The reasonable explanation for this phenomenon was that AA with a large surface area and hydrophilic- explanation for this phenomenon was that AA with a large surface area and hydrophilic-

WaterWater 20212021, 13, 13, x, 1219FOR PEER REVIEW 10 of 19 9 of 18

Water 2021, 13, x FOR PEER REVIEW 9 of 18 3.1.3. FTIR ity inevitably absorbed H2O and CO2 in the air. Note that the low peak values corresponding Theto H typical2O and FTIR CO2 spectra vibration of AA were and observed, MnOOH-supported implying AAlow are contents shown inof Figure H2O 6and. CO2 in −1 −1 Thethe -OHadsorbent. stretching The vibration typical atpeak 3455.84 at 605.64 cm ,H cm2O−1 bending was attributed vibration to at 1617Al-O cm bond, and vibration in ityCO inevitably2− symmetric absorbed stretching H2 vibrationO and CO at21384.44 in the air. cm− Note1 are clearlythat the observed. low peak The values reasonable correspond- unmodified3 AA, while a higher peak at 586.84 cm−1 was noticed in MnOOH-supported ingexplanation to H2O forand this CO phenomenon2 vibration waswere that observed, AA with aimplying large surface low area contents and hydrophilicity of H2O and CO2 in AA due to the combination of Al-O and Mo-O bonds. The above results revealed that theinevitably adsorbent. absorbed The H typical2O and COpeak2 in at the 605.64 air. Note cm that−1 was the at lowtributed peak values to Al corresponding-O bond vibration in there was little variation in the functional groups of MnOOH-supported AA as compared unmodifiedto H2O and COAA,2 vibration while a werehigher observed, peak at implying586.84 cm low−1 was contents noticed of H in2O MnOOH and CO2-supportedin −1 AAthewith adsorbent.due the to control the The combination; typical however, peak of anat 605.64Al increased-O and cm Mo -wasOH-O attributed stretchingbonds. The to Al-O vibration above bond results vibrationpeak revealed was in obtained. that −1 thereunmodiﬁedHence, was the little AA, content variation while ofa higher -inOH the peakwas functional atenhanced, 586.84 groups cm whichwas of MnO noticed providedOH in-supported MnOOH-supported more adsorption AA as compared sites for AA due to the combination of Al-O and Mo-O bonds. The above results revealed that there withdefluoridation. the control ; however, an increased -OH stretching vibration peak was obtained. was little variation in the functional groups of MnOOH-supported AA as compared with Hence,the control; the however, content an of increased -OH was -OH enhanced, stretching vibration which provided peak was obtained. more adsorption Hence, the sites for defluoridation.content of -OH was enhanced, which provided more adsorption sites for deﬂuoridation.

Figure 6. FTIR of AA before and after modification.

Figure3.2. Analysis 6.6.FTIR FTIR of of AA InfluentialAA before before and andFactors after after modification. and modific Regenerationation. Effects 3.2.3.2.1. Analysis Effect of of Influential MnOOH Factors-Supported and Regeneration AA on Fluoride Effects Removal 3.2.3.2.1. Analysis EffectFigure of of7 MnOOH-Supported depictsInfluential the Factors effects AAand of AA onRegeneration Fluoride before and Removal Effects after modification on fluoride removal. 3.2.1.For Figureboth Effect MnOOH7 depictsof MnOOH- thesupported effects-Supported of AA AA and beforeAA theon and Fluorideoriginal after modification AA,Removal the performance on fluoride removal.of defluoridation Forwas both Figureimproved MnOOH-supported 7 depicts with anthe increase effect AAs and of in AA thedosages. originalbefore Specifically, and AA, theafter performance modification with 1 g/L of defluoridation ofon the fluori adsorbentde remov treat-al. wasForment, both improved the MnOOH removal with-supported rates an increase of AA AA inand dosages.and MnOOH the original Specifically,-supported AA, with the AA performance 1 were g/L of8.41% the adsorbentofand defluoridation 15.4%, respec- treatment, the removal rates of AA and MnOOH-supported AA were 8.41% and 15.4%, wastively. improved When 15 with g/L an of increase AA and in MnOOH dosages.-supported Specifically, AA with was 1 addedg/L of ,the respectively, adsorbent treat-the re- respectively. When 15 g/L of AA and MnOOH-supported AA was added, respectively, the duction rates were 31.75% and 73.33%. As compared with AA, it appeared that MnOOH- ment,reduction the ratesremoval were rates 31.75% of and AA 73.33%. and MnOOH As compared-supported with AA, AA it were appeared 8.41% that and MnOOH- 15.4%, respectively.supportedsupported When AA AA 15 displayed displayed g/L of superiorAA superior and fluorideMnOOH fluori removal-desupported removal performance, AAperformance was with added the, with defluoridation, respectively, the defluoridation the re- ductionraterate doubled. doubled. rates were 31.75% and 73.33%. As compared with AA, it appeared that MnOOH- supported AA displayed superior fluoride removal performance, with the defluoridation 10 80 MnOOH-supported AA MnOOH-supported AA rate doubled. Control Control

108 80 MnOOH-supported AA MnOOH-supported AA 60 Control Control

86 60 40

64 40 20 42 Fluoride removal (%) Fluoride concentration (mg/L) 20 20 0 Fluoride removal (%) 1 3 5 7 10 15 Fluoride concentration (mg/L) Adsorbent dosage (g/L) 0 0 Figure 7.1Effect of3 the MnOOH-supported5 7 10 AA15 and AA on deﬂuoridation. Figure 7. EffectAdsorbent of the MnOOH dosage (g/L)-supported AA and AA on defluoridation.

FigureOn 7. Effecthe onet of the hand, MnOOH an irregular-supported and AA convex and AA spiny on defluoridation. structure was seen on the surface of MnOOH-supported AA particles. The special structure provided more active sites for ad- sorptionOn the reaction one hand,s [26] an, hence irregular increasing and convex the reduction spiny structure efficienc wasy of seen fluori onde the. On surface the other of MnOOHhand, when-supported MnOOH AA-supported particles. AAThe was special dispersed structure in theprovided solution, more the active ions located sites for o nad- the sorption reactions [26], hence increasing the reduction efficiency of fluoride. On the other hand, when MnOOH-supported AA was dispersed in the solution, the ions located on the

Water 2021, 13, x FOR PEER REVIEW 10 of 18

adsorbent surface were stressed unevenly due to the internal and surface forces of the particles, causing ions to have interface energy [27]. To reduce the interface energy, Water 2021, 13, 1219 11 of 19 MnOOH-supported AA underwent surface hydroxylation by absorbing water molecules [19] and MnOOH reverted to Mn(OH)2. Simultaneously, the surface of MnOOH-supported AA was covered by a certain amount of hydroxide radicals [28], thereby increasing On the one hand, an irregular and convex spiny structure was seen on the surface the active sites for removing fluoride [29]. Since OH- and F- with a similar hydrate ionic of MnOOH-supported AA particles. The special structure provided more active sites for adsorptionradius can reactions occur ion [26ic], hencecoordination increasing reaction the reduction in an efficiency acidic solution, of fluoride. fluori On thede otherwas removed hand,through when the MnOOH-supported exchange of hydroxyl AA was groups dispersed on the in theadsorbent solution, surface the ions located[30]. The on chemical the re- adsorbentaction equations surface wereare shown stressed as unevenly follows: due to the internal and surface forces of the particles, causing ions to have interface energy [27]. To reduce the interface energy, MnOOH- (AlO) ⋅ MnOOH + HO → (AlO) ⋅ Mn(OH) + OH (10) supported AA underwent surface hydroxylation by absorbing water molecules [19] and MnOOH revertedR to− Mn(OH)(AlO)2.⋅ Simultaneously,Mn(OH) + 2F the→ surfaceR − (Al ofO MnOOH-supported) ⋅ MnF + 2OH AA (11) was covered by a certain amount of hydroxide radicals [28], thereby increasing the active In addition, an underground hot spring water in Liaoning Province was used to in- sites for removing fluoride [29]. Since OH− and F− with a similar hydrate ionic radius canvestigate occur ionicthe fluoride coordination removal reaction performance. in an acidic solution,As can be fluoride seen in was Table removed S1, under through the condi- thetion exchange of 5.6 mg/L of hydroxyl of F- concentration groups on the and adsorbent 7 g/L of surface the adsorbent, [30]. The chemicalthe removal reaction rates of AA equationsand MnOOH are shown-supported as follows: AA were 37.72% and 67.75%, respectively. It was concluded that MnOOH-supported AA had a better fluoride removal effect than AA, and the fluoride ( ) · + → ( ) · ( ) + − removal rate wasAl2O increased3 n MnOOH by moreH2O than Al70%.2O3 n Mn OH 2 OH (10) − − R − (Al2O3) · Mn(OH) + 2F → R − (Al2O3) · MnF2 + 2OH (11) 3.2.2. Effect of pH on Fluoriden Adsorption2 n In addition, an underground hot spring water in Liaoning Province was used to investigateThe effect the fluoride of pH on removal fluoride performance. removal was As investigated, can be seen in and Table the S1, results under are the shown in conditionFigure 8. ofObviously, 5.6 mg/L of the F− fluorideconcentration removal and 7effect g/L ofdecreased the adsorbent, with the the removal increase rates in pH of values. AASpecifically, and MnOOH-supported with the F- concentration AA were 37.72% incre andasing 67.75%, from respectively. 2 to 5 mg/L, It was the concluded F- adsorption ca- thatpacity MnOOH-supported increased from AA 0.16 had to a better 0.41 fluoride mg/g, removaland the effect removal than AA, rate and was the fluoridepromoted from removal77.09~80.3% rate was to 80.85 increased~83.27%. by more Based than on 70%. these results, a pH of 4 was determined for the thermodynamic and kinetic studies. 3.2.2. Effect of pH on Fluoride Adsorption Meanwhile, the fluoride removal effects of MnOOH-supported AA with a particle The effect of pH on fluoride removal was investigated, and the results are shown in size of 0.5–1.5 mm under different pH conditions were compared and analyzed. As shown Figure8. Obviously, the fluoride removal effect decreased with the increase in pH values. Specifically,in Figure S2 with, the thefluoride F− concentration removal rate increasing and adsorption from 2 to amount 5 mg/L, of the the F− adsorbentadsorption gradually capacitydecrease increasedd with the from increase 0.16 toin 0.41pH. mg/g,Under andacidic the pH removal condition rates was, the promoted fluoride removal from effect 77.09~80.3%of the system to was 80.85~83.27%. better. It can Based also on be these seen results, in these a pHresults of 4 that was determinedthe fluoride for removal the effect thermodynamicof small particles and was kinetic significantly studies. better than that of large particles.

0.6 100 2 mg/L 2 mg/L 5 mg/L 5 mg/L 90

70 0.3 60

50 Fluoride removal (%) removal Fluoride

40 Fluoride adsorption capacity (mg/g) capacity adsorption Fluoride 0.0 30 3 4 5 6 7 8 9 pH value Figure 8. 8.Effect Effect of of pH pH on on ﬂuoride fluoride removal removal by MnOOH-supported by MnOOH-supported AA. AA.

3.2.3.Meanwhile, Effect of Coexisting the fluoride Anions removal on effects Fluoride of MnOOH-supported Adsorption AA with a particle size of 0.5–1.5 mm under different pH conditions were compared and analyzed. As shown in FigureFigure S2, the 9 describes fluoride removal the effect rates and of adsorptioncoexisting amount anions of on the theadsorbent fluoride gradually adsorption of decreasedMnOOH- withsupported the increase AA inand pH. AA Under. As acidic shown pH in conditions, Figure 9a the, there fluoride were removal various effect effects of ofcoexisting the system anions was better. on F- It removal can also beby seen MnOOH in these-supported results that AA. the fluoride The F- removal effect performance ofwas small greatly particles affected was significantly by carbonate, better while than thata slight of large impact particles. was produced by chloride and nitrate. Specifically, the removal rates were reduced from 22.89% to 0.5% due to carbonate,

Water 2021, 13, 1219 12 of 19

3.2.3. Effect of Coexisting Anions on Fluoride Adsorption Figure9 describes the effects of coexisting anions on the fluoride adsorption of MnOOH- Water 2021, 13, x FOR PEER REVIEW 11 of 18 supported AA and AA. As shown in Figure9a, there were various effects of coexisting anions on F− removal by MnOOH-supported AA. The F− removal performance was greatly affected by carbonate, while a slight impact was produced by chloride and nitrate. Specifically, theand removal bicarbonate rates resulted were reduced in a decrease from 22.89% in the toremoval 0.5% due rate to from carbonate, 24.88% and to 18.41%. bicarbonate This resultedwas mainly in a owing decrease to inthe the competitive removal rate adsorption from 24.88% between to 18.41%. interfering This anions was mainly and F owing-, lead- toing the to competitivea reduction adsorptionin the adsor betweenption effect. interfering It can be anions seen and from F− Figure, leading 9b tothat a reductionthe removal in therate adsorption decreased effect.with an It increasing can be seen concentration from Figure 9ofb thatinterfering the removal anions. rate The decreased effect of withbicar- anbonate increasing and sulfate concentration on fluoride of interfering removal anions. decreased The from effect 16.92% of bicarbonate to 1% and sulfate29.35% on to fluoride15.42%, removalrespectively. decreased from 16.92% to 1% and 29.35% to 15.42%, respectively.

a b

FigureFigure 9.9. EffectEffect ofof coexistingcoexisting anionsanions onon FF−- adsorptionadsorption by by MnOOH MnOOH-supported-supported AA AA ( (aa)) and and AA AA ( (bb))..

3.2.4.3.2.4. RegenerationRegeneration ofof MnOOH-SupportedMnOOH-Supported AAAA TheThe MnOOH-supportedMnOOH-supported AA AA was was regenerated regenerated using using the the acid-base acid-base regeneration regeneration process, pro- and the results are shown in Tables5 and6. Here, 2.5% Na CO was selected as the cess, and the results are shown in Tables 5 and 6. Here, 2.5% Na2 2CO3 3 was selected as the regeneratingregenerating agent to to obtain obtain the the greatest greatest effect. effect. Firstly, Firstly, a certain a certain amount amount of MnOOH of MnOOH--sup- supported AA was regenerated, followed by 0.2 mol/L H2SO4 pickling for 0.5 h; finally, it ported AA was regenerated, followed by 0.2 mol/L H2SO4 pickling for 0.5 h; finally, it was was dried after washing with distilled water for 0.5 h. It was concluded that the effluent dried after washing with distilled water for 0.5 h. It was concluded that the effluent mass mass concentration of F− of MnOOH-supported AA was 0.96 mg/L after regeneration concentration of F- of MnOOH-supported AA was 0.96 mg/L after regeneration for 2 h for 2 h and the removal rate reached 80.8%. Compared with the first removal effects, the and the removal rate reached 80.8%. Compared with the first removal effects, the removal removal rate of fluoride ions decreased by 0.2%. rate of fluoride ions decreased by 0.2%.

Table 5. Inﬂuence of regenerant concentration on regeneration effects (Na2CO3). Table 5. Influence of regenerant concentration on regeneration effects (Na2CO3). Regenerant Concentration F− Concentration Removal Rate Regenerant Concentration F- Concentration Removal Rate (mg/L) (mg/L) (%) (mg/L) (mg/L) (%) 1 2.48 50.40 1 22.48 1.3250.40 73.60 2 2.51.32 173.60 80.16 2.5 51 0.9880.16 80.56 5 7.50.98 0.9780.56 80.75 10 0.96 80.95 7.5 0.97 80.75 10 0.96 80.95 Table 6. Inﬂuence of regenerant time on regeneration effects (Na2CO3). Table 6. Influence of regenerant time on regeneration effects (Na2CO3). Regenerant Time F− Concentration Removal Rate Regenerant(h) Time F- Concentration(mg/L) Removal(%) Rate (h)1 (mg/L) 1.17 (%) 76.79 11.5 1.17 1.13 76.79 77.58 1.52 1.13 0.96 77.58 80.8 2.5 1.17 76.79 23 0.96 1.45 80.8 71.23 2.5 1.17 76.79 3 1.45 71.23

Water 2021, 13, x FOR PEER REVIEW 12 of 18

3.3. Kinetic Study of Fluoride Adsorption Water 2021, 13, 1219 3.3.1. Quasi-First-Order Model and Quasi-Second-Order Model 13 of 19 The adsorption dynamic curves under different initial mass concentrations of fluo-

3.3.ride Kinetic are displayed Study of Fluoride in Figure Adsorption 10. It is worth noting that a similar variation tendency of the 3.3.1.adsorption Quasi-First-Order rate cu Modelrve was and observedQuasi-Second-Order while treating Model different raw water. The adsorbed amountThe adsorption rapidly dynamicincreased curves in under600 min different of adsorption initial mass concentrations time, irrespective of fluoride of the dosage. This arecould displayed be ascribed in Figure to 10 the. It fact is worth that notingMnOOH that- asupported similar variation AA possessed tendency of rich the active sites, lead- adsorptioning to a rapid rate curve transformation was observed whileof F- treatingfrom the different solution raw water.to the The surface adsorbed. It was clear that the amount rapidly increased in 600 min of adsorption time, irrespective of the dosage. This couldadsorption be ascribed of fluori to thede fact was that mainly MnOOH-supported external diffusion AA possessed in the rich initial active adsorption sites, process. Sub- leadingsequently, to a rapid the transformationadsorption sites of F− fromwere the gradually solution to thecovered surface. by It wasfluori clearde that with an increase in theadsorption adsorption time of fluoride, and was the mainly solvent external diffusion diffusion rate in theslowed initial adsorptiondown accordingly. process. After that, the Subsequently, the adsorption sites were gradually covered by fluoride with an increase defluoridation primarily occurred in the internal pores of particles. F- diffused from the in adsorption time, and the solvent diffusion rate slowed down accordingly. After that, theouter defluoridation surface of primarily particles occurred to the in inner the internal surface pores of of micropore particles. F− whendiffused the from adsorption time in- thecreased outer surface from of600 particles min to to the1500 inner min. surface With of microporethe extension when the of adsorptionadsorption time time, the adsorbed increasedamount fromslowly 600 minincreased to 1500 min. until With it thetended extension to be of adsorptionstable. In time, an adsorption the adsorbed time of over 1500 amount slowly increased until it tended to be stable. In an adsorption time of over 1500 min, themin, adsorption the adsorption sites in the sites surface in andthe innersurface pores and were inner all occupied. pores were At this all moment, occupied. the At this moment, adsorptionthe adsorption of fluoride of onfluori MnOOH-supportedde on MnOOH AA-supported came to an equilibrium AA came state. to an equilibrium state.

1.4

1.2

1.0

0.8

0.6

0.4 10 mg/L 5 mg/L Adsorbed amount (g/mg) 0.2 2 mg/L

0.0 0 1000 2000 3000 4000 5000 Time (min) FigureFigure 10. 10.Adsorption Adsorption dynamic dynamic curve of curve fluoride of on fluori MnOOH-supportedde on MnOOH AA. -supported AA. Figure 11 presents the kinetic curves of fluoride with initial concentrations of 2, 5 and 10Figure mg/L for 11 adsorption presents on the MnOOH-supported kinetic curves of AA. fluori The correlationde with initial coefficients concentrations (R2) of 2, 5 and were10 mg/L 0.8060, for 0.9764 adsorption and 0.9632 on as MnOOH evaluated by-supported the quasi-first-order AA. The model correlation (Figure 11 coefficientsa). (R2) were R2 According0.8060, 0.9764 to the quasi-second-orderand 0.9632 as evaluated model, the by valuesthe quasi were-first 0.9862,-order 0.9978 model and 0.9956, (Figure 11a). Accord- respectively (Figure 11b). It could be discovered that the experimental data perfectly fitted withing to the the quasi-second-order quasi-second- equation.order model, The results the R demonstrated2 values were that 0.9862, the quasi-second- 0.9978 and 0.9956, respec- ordertively model (Figure could 11 describeb). It could the entire be discovered defluoridation that process, the whichexperimental was in accordance data perfectly fitted with withthe previousquasi-second studies-order [20,27,31 equation.]. During theThe adsorption results process,demonstrated some fluoride that passedthe quasi-second-order through the orifice and diffused into the pores of MnOOH-supported AA, which was describedmodel could as physical describe adsorption. the Someentire other defluoridation fluoride was adsorbed process, owing whic to theh fluoride–was in accordance with hydroxylprevious exchange studies reaction, [20,27,31] which. During was chemical the adsorption adsorption. Therefore,process, thesome adsorption fluoride passed through processthe orifice of fluoride and diffused was composed into ofthe physical pores adsorption of MnOOH and-supported chemical adsorption, AA, which and was described as chemicalphysical adsorption adsorption. played Some the dominant other role. fluori In general,de was the adsorbed entire defluoridation owing to kinetics the fluoride–hydroxyl of MnOOH-supported AA were better fitted by the quasi-second-order model [32,33], includingexchange external reaction, diffusion, which inner was diffusion, chemical and surface adsorption. adsorption, Therefore, etc. [34]. the adsorption process of fluoride was composed of physical adsorption and chemical adsorption, and chemical adsorption played the dominant role. In general, the entire defluoridation kinetics of MnOOH-supported AA were better fitted by the quasi-second-order model [32,33], including external diffusion, inner diffusion, and surface adsorption, etc. [34].

Water 2021, 13, x FOR PEER REVIEW 13 of 18 Water 2021, 13, 1219 14 of 19

Time(min) (a) 0 1000 2000 3000 4000 5000 0 (b) 7000 2 mg/L 2 mg/L 5 mg/L y = 1.2883x+616.9346 y10 = -0.00080x-0.0583 5 mg/L 6000 2 10 mg/L 2 -1 R2 = 0.9632 10 mg/L R = 0.9862 5000 y = 0.8060x+295.6528 -2 5 4000 R2 = 0.9978

y = -0.00084x-0.5989 2 t/qt R2 = 0.8060 3000

ln(qe-qt) -3

2000 y10 = 0.7373x+246.2219 y5 = -0.00101x-0.0855 -4 R2 = 0.9956 R2 = 0.9764 1000

-5 0 0 1000 2000 3000 4000 5000 Time(min) FigureFigure 11. 11.Fitting Fitting of the of fluoride the fluori adsorptionde adsorption process with theprocess quasi-first-order with the kinetics quasi model-first (-aorder) and the kinetics quasi-second-order model (a) andkinetics the model quasi (b-).second-order kinetics model (b). 3.3.2. Weber and Morris Model 3.3.2. Weber and MorrisThough Model the quasi-second-order model could describe the entire adsorption process well, the intra-particle diffusion was not well assessed. Thus, the Weber and Morris model was Though the quasi-second-order model could describe the entire adsorption process used to further analyze the inner diffusion effect [22]. Figure 12 exhibits the plots of qt versus well, the intra-particlet for diffusion initial concentrations was not ofwell 2, 5 andassessed. 10 mg/L. Th Sainius, etthe al. Weber [35] reported and that Morris the main model rate- 1/2 was used to further limitinganalyze reaction the inner was intra-particle diffusion diffusion effect when[22]. the Figure curve of12qt exhibitsversus t thewent plots through of the origin and presented favorable linear relationships. In Figure 12, irrespective of the initial t q versus t for initialconcentration, concentrations the fit linesof 2, are 5 straight and 10 and mg/L. distinct Saini inflection et pointsal. [35] appear reported at 23 min 1/2that. What the main rate-limiting reactionis noteworthy was is thatintr alla-particle fit lines never diffusion pass through when the origin.the curve The results of qt revealedversus that t1/2 went through the originthe adsorption and presented rate was controlled favorable not only linear by intra-particle relationships diffusion. In butFigure also by 1 external2, irre- 1/2 1/2 spective of the initialdiffusion. concentration, Specifically, the the fit first lines (0~23 are min straight) and the and second distinct (23~67 inflection min ) adsorption points processes were assigned to external diffusion and intra-particle diffusion, respectively. the appear at 23 min1/2. parametersWhat is noteworthy of the Weber and is that Morris all equation fit lines are never listed inpass Table through7. As seen, the the origin. linear The results revealedcorrelation that the ofadsorption MnOOH-supported rate was AA controlled on fluoride was not relatively only by preferable intra-particle in the entire dif- 1/2 first adsorption process (0~23 min ). This could imply that the1/2 reaction was mainly fusion but also by external diffusion. Specifically, the first (0~23 min ) and the1/2 second controlled by external diffusion. For the second adsorption process (23~67 min ), qt and 1/2 (23~67 min ) adsorptiont1/2 also process showedes a relatively were assigned good linear to relationship, external diffusion though R2 was and lower intra than-particle that of diffusion, respectively.the first the adsorption parameters process. of the It can Weber be concluded and Morris from the equationC and k values are that listed diffusion in Ta- in ble 7. As seen, the linearthis stage correlation was chiefly of dependent MnOOH on the-supported internal diffusion AA on rate. fluori The abovede was results relatively indicated that defluoridation using MnOOH-supported AA was the common result of external and 1/2 preferable in the entireintra-particle first adsorption diffusion. process (0~23 min ). This could imply that the reaction was mainly controlled by external diffusion. For the second adsorption process Table 7. Parameters of the Weber and Morris model. (23~67 min1/2), qt and t1/2 also showed a relatively good linear relationship, though R2 was 1/2 1/2 lower than that of the firstC0 adsorption process.0~23 min It can be concluded from23~67 the min C and k values that diffusion in this stage(mg/L) was chieflyR2 dependeknt on the C internalR 2diffusionk rate. The Cabove results indicated that defluoridation2 0.9112 using 0.0212 MnOOH−-0.0494supported 0.8294 AA was 0.0067 the common 0.2738 re- 5 0.9789 0.0347 −0.0524 0.9658 0.0097 0.5357 sult of external and intra10-particle 0.9494 diffusion. 0.0318 −0.0548 0.9648 0.0116 0.5372

1.4 2 mg/L y = 0.0116x+0.5372 5 mg/L 102 1.2 2 10 mg/L R = 0.9648

1.0 y52 = 0.0097x+0.5357 R2 = 0.9658 y = 0.0318x+0.0548 0.8 101 R2 = 0.9494

(mg/g) y = 0.0347x-0.0524 t 0.6 51

q 2 R = 0.9789 y22 = 0.0067x+0.27381 0.4 R2 = 0.8294

0.2 y21 = 0.0212x-0.0494 R2 = 0.9112 0.0 0 10 20 30 40 50 60 70 2/1 2/1 t (min ) Figure 12. Fitting of the fluoride adsorption process with the Weber and Morris model.

Water 2021, 13, x FOR PEER REVIEW 13 of 18

y = -0.00084x-0.5989 2 t/qt R2 = 0.8060 3000

ln(qe-qt) -3

2000 y10 = 0.7373x+246.2219 y5 = -0.00101x-0.0855 -4 R2 = 0.9956 R2 = 0.9764 1000

-5 0 0 1000 2000 3000 4000 5000 Time(min) Figure 11. Fitting of the fluoride adsorption process with the quasi-first-order kinetics model (a) and the quasi-second-order kinetics model (b).

3.3.2. Weber and Morris Model Though the quasi-second-order model could describe the entire adsorption process well, the intra-particle diffusion was not well assessed. Thus, the Weber and Morris model was used to further analyze the inner diffusion effect [22]. Figure 12 exhibits the plots of qt versus t for initial concentrations of 2, 5 and 10 mg/L. Saini et al. [35] reported that the main rate-limiting reaction was intra-particle diffusion when the curve of qt versus t1/2 went through the origin and presented favorable linear relationships. In Figure 12, irrespective of the initial concentration, the fit lines are straight and distinct inflection points appear at 23 min1/2. What is noteworthy is that all fit lines never pass through the origin. The results revealed that the adsorption rate was controlled not only by intra-particle diffusion but also by external diffusion. Specifically, the first (0~23 min1/2) and the second (23~67 min1/2) adsorption processes were assigned to external diffusion and intra-particle diffusion, respectively. the parameters of the Weber and Morris equation are listed in Ta- ble 7. As seen, the linear correlation of MnOOH-supported AA on fluoride was relatively preferable in the entire first adsorption process (0~23 min1/2). This could imply that the reaction was mainly controlled by external diffusion. For the second adsorption process (23~67 min1/2), qt and t1/2 also showed a relatively good linear relationship, though R2 was lower than that of the first adsorption process. It can be concluded from the C and k values that diffusion in this stage was chiefly dependent on the internal diffusion rate. The above Water 2021, 13, 1219 results indicated that defluoridation using MnOOH-supported AA15 of 19was the common result of external and intra-particle diffusion.

Water 2021, 13, x FOR PEER REVIEW 1.4 14 of 18 2 mg/L y = 0.0116x+0.5372 5 mg/L 102 1.2 2 10 mg/L R = 0.9648

1.0 y = 0.0097x+0.5357 Table 7. Parameters of the Weber and52 Morris model. R2 = 0.9658 y = 0.0318x+0.0548 0.8 101 R2 =C 0.94940 0~23 min1/2 23~67 min1/2

(mg/g) y = 0.0347x-0.0524 t 0.6 51 2 2 q (mg/L) 2 R k C R k C R = 0.9789 y22 = 0.0067x+0.27381 0.4 2 0.9112 R2 = 0.82940.0212 -0.0494 0.8294 0.0067 0.2738 5 0.9789 0.0347 -0.0524 0.9658 0.0097 0.5357 0.2 y21 = 0.0212x-0.0494 10 R2 = 0.91120.9494 0.0318 -0.0548 0.9648 0.0116 0.5372 0.0 0 10 20 30 40 50 60 70 2/1 2/1 3.4. Kinetic Study of Fluorit (minde )Adsorption FigureFigure3.4.1. 12. Adsorption 12.Fitting Fitting of the of fluoride Isothermsthe fluori adsorptionde adsorption process with process the Weber with and Morris the Weber model. and Morris model. 3.4. KineticIn Figure Study of 1 Fluoride3, the Adsorptionvariation trend of three is was basically the same in 25 °C, 35 °C and 3.4.1.45 °C Adsorption. Significantly, Isotherms the equilibrium adsorption capacity gradually increased with an in- creasedIn Figure initial 13, theconcentration. variation trend ofAlthough three is was the basically slope the of samecurve in 25was◦C, reduced, 35 ◦C the unit adsorp- ◦ andtion 45 capacityC. Significantly, of MnOOH the equilibrium-supported adsorption AA improved capacity gradually constantly. increased This with was mainly attributed an increased initial concentration. Although the slope of curve was reduced, the unit adsorptionto the fact capacity that when of MnOOH-supported the initial concentration AA improved of constantly. fluoride This was was relatively mainly high, the solution attributedconcentration to the fact gradient that when increased the initial concentration on the inside of fluoride and was outside relatively of high, particles. the This was propi- solutiontious to concentration accelerate gradient the diffusion increased onrate the insideand enlarge and outside the of particles.equilibrium This was adsorption amount of propitious to accelerate the diffusion rate and enlarge the equilibrium adsorption amount offluori fluoridede [[22]22]. .

1.6

1.4

1.2

1.0

0.8

0.6 25℃ 0.4 35℃ 45℃ 0.2

Equilibrium adsorption capacity (mg/g) capacity adsorption Equilibrium 0 2 4 6 8 10 12 14 Fluoride equilibrium concentration (mg/L) FigureFigure 13. 13.Adsorption Adsorption isotherm isotherm curve of fluoride curve onof MnOOH-supportedfluoride on MnOOH AA. -supported AA. To get insight into the adsorption thermodynamic characteristics of MnOOH-supported AA, theTo Langmuir get insight adsorption into isotherm the adsorption was used to analyzethermodynamic the defluorination characteris properties;tics of MnOOH-sup- theported results AA are, shown the Langmuir in Figure 14. adsorption The related parameters isotherm of thewas Langmuir used to model analy andze the defluorination the Freundlich model at different temperatures are seen in Table8. It may be thought thatproperties the adsorption; the processresults of are MnOOH-supported shown in Figure AA on14 fluoride. The related was well parameters explained of the Langmuir usingmodel both and the the Langmuir Freundlich and Freundlich model isotherms. at different However, temperatures fluoride adsorption are seen was in Table 8. It may be morethought suitable that for the the Freundlichadsorption model, process with Rof2 values MnOOH of 0.9614,-supported 0.9383 and AA 0.9852 on atfluoride was well explained using both the Langmuir and Freundlich isotherms. However, fluoride adsorption was more suitable for the Freundlich model, with R2 values of 0.9614, 0.9383 and 0.9852 at 25 °C, 35 °C and 45 °C, respectively. In Table 8, the constant 1/n of the Freundlich equation is 0.5631, 0.6243 and 0.6241, which illustrated that adsorption was conducted easily in all cases. In addition, the KF values increased with an increase in adsorption temperature, suggesting that high temperatures contributed to the adsorption of fluoride on MnOOH-supported AA. The KL values were all positive under different temperatures, which accelerated the adsorption reaction. It was also observed that the KL values were enlarged when temperature increased, suggesting a chemical adsorption mechanism for the reaction system. Overall, the Freundlich isotherm was applicable to the adsorption data of all temperatures.

Water 2021, 13, 1219 16 of 19

25 ◦C, 35 ◦C and 45 ◦C, respectively. In Table8, the constant 1/ n of the Freundlich equation is 0.5631, 0.6243 and 0.6241, which illustrated that adsorption was conducted easily in all cases. In addition, the KF values increased with an increase in adsorption temperature, suggesting that high temperatures contributed to the adsorption of ﬂuoride on MnOOH-supported AA. The KL values were all positive under different temperatures, which accelerated the adsorption reaction. It was also observed that the KL values were Water 2021, 13, x FOR PEER REVIEW enlarged when temperature increased, suggesting a chemical adsorption mechanism15 forof 18 the reaction system. Overall, the Freundlich isotherm was applicable to the adsorption data of all temperatures.

(a) 12 (b) 0.5 25C 25C 35C 35C 10 45C 0.0 45C

y35 = 0.5966x+3.1728 8 R2 = 0.8908 y = 0.6241x-1.0927 y = 0.6896x+3.1885 -0.5 45 25 R2 = 0.9852 y = 0.6243x-1.3024 2 35

R = 0.8968 2 R = 0.9383 6 LnQe Ce/Qe -1.0

y45 = 0.5111x+2.4568 4 y = 0.5631x-1.2932 R2 = 0.9522 25 -1.5 R2 = 0.9614 2 -2.0 0 2 4 6 8 10 12 14 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 Effulent equilibrium concentration (mg/L) LnCe FigureFigure 14. Fitting14. Fitting of the ﬂuorideof the fluori adsorptionde adsorption process with theprocess Langmuir with isotherm the Langmuir model (a) and isotherm the Freundlich model isotherm (a) and the modelFreundlich (b). isotherm model (b).

Table 8. Parameters of the adsorption isotherm models. Table 8. Parameters of the adsorption isotherm models. Temperature Langmuir Model Freundlich Model ◦ 2 2 Temperature C LangmuirKL Modelqe (mg/g) R KFreundlichF 1/n ModelR °C K25L q 0.216e (mg/g) 1.450R2 0.8968K 0.274F 0.56311/n 0.9614R2 35 0.188 1.676 0.8908 0.272 0.6243 0.9383 25 0.21645 0.2081.450 1.9570.8968 0.9522 0.274 0.335 0.5631 0.6241 0.98520.9614 35 0.188 1.676 0.8908 0.272 0.6243 0.9383 45 3.4.2. Isosteric0.208 Enthalpies1.957 for Adsorption0.9522 0.335 0.6241 0.9852 As different atoms and molecules were deposited on the surface, the surface energy of MnOOH-supported AA was unbalanced, thus generating Gibbs free energy. The deflu- 3.4.2. Isosteric Enthalpiesoridation for process Adsorption was that MnOOH-supported AA absorbed several fluorides from the As different atomssolution and to balance molecules its surface were free deposited energy. Along on with the the surface, adsorption the reaction, surface the energy Gibbs free energy slowly decreased and the surface energy came to a steady state [16,36]. As of MnOOH-supportedis well known,AA was the adsorption unbalanced, free energy thus is generating the embodiment Gibbs of the driving free energy. force. It can The defluoridation processbe seen was from that Table MnOOH9 that ∆G was-supported less than zero AA at absorbed 25 ◦C, 35 ◦C several and 45 ◦ C,fluori revealingdes thatfrom the solution to balancethe adsorption its surface reaction free was energy. spontaneous Along [37,38 ].with Meanwhile, the adsorption∆G gradually reaction, reduced with the increasing temperature, which meant that the higher temperature was conducive to the Gibbs free energy progressslowly ofdecreased the adsorption and reaction. the surface energy came to a steady state [16,36]. As is well known, theThrough adsorption the analysis free ofenergy adsorption is the isotherms, embodiment it was observed of the that driving improving force. the It can be seen from Tabletemperature 9 that was Δ beneficialG was less to the than adsorption zero at of fluoride25 °C, on35 MnOOH-supported °C and 45 °C, revealing AA. The that the adsorption∆ Hreaction was greater was than spontaneous zero, which showed [37,38 that]. Meanwhile, the whole adsorption ΔG gradually process was reduced an endothermic reaction. The variation of ∆H during the adsorption process principally resulted with increasing temperature,from chemical which reactions. meant The chemical that the reaction higher of fluoride temperature on an adsorbent was conducive surface was to the progress of themainly adsorption a displacement reaction. reaction with the activated hydroxyl group formed by the hydration Through the analysis of adsorption isotherms, it was observed that improving the temperature was beneficial to the adsorption of fluoride on MnOOH-supported AA. The ΔH was greater than zero, which showed that the whole adsorption process was an endothermic reaction. The variation of ΔH during the adsorption process principally resulted from chemical reactions. The chemical reaction of fluoride on an adsorbent surface was mainly a displacement reaction with the activated hydroxyl group formed by the hydration reaction. Generally, the increased temperature was not propitious to physisorption. This suggested that a chemisorption mechanism accorded with the defluoridation process onto MnOOH-supported AA. In addition, a positive ΔS higher than zero at three temperatures revealed an entropy production process for fluoride adsorption. During the adsorption process, the randomness of fluoride decreased and the confusion degree of the solid–liquid interface increased. This phenomenon could be explained by the fact that the exchange reaction between fluoride and water molecules on the adsorbent surface took place, leading to an improvement in the total ΔS in the entire system. We concluded that there was a preferable affinity between fluoride and MnOOH-supported AA. In the initial stage of the reaction, a great deal of heat was expended owing to the chemical reaction between fluoride and MnOOH-supported AA. As the coverage rate increased, active sites with high energy were gradually occupied and the amount of chemical reaction taking place was small. The

Water 2021, 13, 1219 17 of 19

reaction. Generally, the increased temperature was not propitious to physisorption. This suggested that a chemisorption mechanism accorded with the deﬂuoridation process onto MnOOH-supported AA.

Table 9. Thermodynamic parameters.

· C0 ∆H ∆G (J/mol) ∆S (J/(mol K)) (mg/L) (J/mol) 298 308 318 298 308 318 2 11,131.61 −11,973.36 −12,375.15 −12,776.94 77.53 41.54 43.30 5 3004.26 −1408.50 −1455.76 −1503.03 14.81 4.89 5.05 10 11,370.23 −7110.14 −7348.73 −7587.33 62.01 24.67 25.52 15 8057.76 −2433.97 −2515.64 −2597.32 35.21 8.45 8.72

In addition, a positive ∆S higher than zero at three temperatures revealed an entropy production process for fluoride adsorption. During the adsorption process, the randomness of fluoride decreased and the confusion degree of the solid–liquid interface increased. This phenomenon could be explained by the fact that the exchange reaction between fluoride and water molecules on the adsorbent surface took place, leading to an improvement in the total ∆S in the entire system. We concluded that there was a preferable affinity between fluoride and MnOOH-supported AA. In the initial stage of the reaction, a great deal of heat was expended owing to the chemical reaction between fluoride and MnOOH-supported AA. As the coverage rate increased, active sites with high energy were gradually occupied and the amount of chemical reaction taking place was small. The fluoride adsorbed later was attached to the adsorption sites with low energy, causing less heat production.

4. Conclusions (1) The surface of MnOOH-supported AA became rougher, exhibiting more adsorption sites and preferable porosity to enhance adsorption ability. The results obtained from XPS showed that the Mn2p peak of MnOOH-supported AA was composed of MnOOH at 641.5 eV and MnO at 653.4 eV, accounting for 59.2% and 40.7%, respectively, illustrating the ordinary AA was successfully loaded with MnOOH. (2) The defluoridation kinetics of MnOOH-supported AA fitted well with the quasi- second-order model, with R2 values of 0.9862, 0.9978 and 0.9956 in this present study. The adsorption process consisted of external diffusion, internal diffusion and chemical adsorption, among which, chemical adsorption was the primary adsorption mechanism. Additionally, the adsorption rate was governed by external diffusion and inter-particle diffusion conjointly, and the Weber and Morris diffusion was the main reason. (3) The adsorption of fluoride onto MnOOH-supported AA was better fitted by the Freundlich isotherm, showing chemical adsorption processes. The results showed that both higher and lower temperature were favorable to improving the adsorption effects. During the entire defluoridation process, it was observed that ∆S and ∆H were all positive and ∆G was negative, illustrating a spontaneous and an endothermic diffusion adsorption mechanism.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/w13091219/s1. Table S1. Comparative analysis of fluoride removal methods. Figure S1. Distribution of areas with excessive fluoride and iron in groundwater in China, fluoride exceeded area (a) and iron exceeded area (b). Figure S2. Effect of pH on fluoride removal efficiency (Φ 0.5–1.5 mm). Author Contributions: Data curation, K.Y. and N.K.; Formal analysis, N.K.; Funding acquisition, J.F.; Investigation, K.Y., N.K., Y.G., Y.Y. and F.Y.; Methodology, K.Y., X.C., Y.Y. and F.Y.; Project administration, J.F. and X.C.; Resources, J.F. and X.C.; Software, P.L. and Y.G.; Writing—original draft, K.Y.; Writing—review and editing, P.L. and X.C. All authors have read and agreed to the published version of the manuscript. Water 2021, 13, 1219 18 of 19

Funding: This research was funded by the Major Science and Technology Program for Water Pollution Control and Treatment, grant number “2018ZX070601001-3”. Data Availability Statement: Data are contained within the article. Acknowledgments: This research was supported by the Major Science and Technology Program for Water Pollution Control and Treatment (2018ZX070601001-3). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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