Uptake Fluoride from Water by Starch Stabilized Layered Double Hydroxides

water

Article Uptake Fluoride from Water by Starch Stabilized Layered Double Hydroxides

Jiming Liu, Xiuping Yue *, Xinyu Lu and Yu Guo College of Environment Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China; [email protected] (J.L.); [email protected] (X.L.); [email protected] (Y.G.) * Correspondence: [email protected]; Tel.: +86-351-6010-243

Received: 14 May 2018; Accepted: 4 June 2018; Published: 7 June 2018

Abstract: A novel starch stabilized Mg/Al layered Double hydroxides (S-LDHs) was prepared in a facile approach and its fluoride ion removal performance was developed. Characterization of S-LDHs was employed by using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and particle size distribution. The adsorption property was studied through the assessment of the adsorption isotherms, kinetic models, thermal dynamics, and pH influence. The result shows that a low loading of starch of 10 mg onto layered double hydroxides (LDHs) could obviously improve the fluoride removal rate. The S-LDHs had three times higher the adsorption capacity to fluoride than that of Mg/Al LDHs to fluoride. The particle size was smaller and the particle size distribution was narrower for S-LDHs than that for Mg/Al LDHs. The Langmuir adsorption isotherm model and pseudo-second-order kinetic model fitted well with the experimental data. In thermodynamic parameters, the enthalpy (∆H0) value was 35.63 kJ·mol−1 and the entropy (∆S0) value was 0.0806 kJ·mol−1K−1. The values of ∆G0 were negative, implying the adsorption process is spontaneous. S-LDHs reveals stable adsorption property in a wide pH range from 3 to 9. The mechanism for fluoride adsorption on S-LDHs included surface adsorption and interaction ion exchange.

Keywords: layered double hydroxides; hydrotalcite; starch; ﬂuoride; adsorption property; thermodynamics

1. Introduction Many reports indicate high fluoride content exists in drinking water in many countries such as China, Russia, Pakistan, and South Africa [1–3]. Use of drinking water containing high concentrations of fluoride can cause dental fluorosis, nervous system diseases, and even cancer [4–6]. The world health organization (WHO) has set the guideline on fluoride in drinking water at 1.5 mg/L [7]. Plenty of methods have been attempted to lower high levels of fluoride, including electrodialysis [8], adsorption [9–12], precipitation [13], ion exchange [14] and membrane filtration [15]. Among them, adsorption is found to be a more appropriate method to remove the fluoride in drinking water due to the fact that it is relatively simple in design and convenient to operate. In recent years, Layered double hydroxides (LDHs) have attracted increasing attention in catalysis, adsorption, ion exchange, etc., and have been regarded as an effective adsorbent in reducing the contents of excessive anions, including fluoride [12,16–18]. LDHs are composed of alternating stacked 2+ 3+ cationic layers and interlayer compensation anions. The basic structural formula is: M 1−xM x n− 2+ 3+ (OH)2(A )x/n·mH2O, wherein M and M represent divalent and trivalent cations, respectively, n− 2− and A represents an n-valent anion. Typical LDHs are Mg6Al2(OH)16CO3 ·4H2O, which have 2+ 3+ 2− a structure similar to brucite Mg(OH)2, and consist of Mg and Al ions on both sides, with CO3 anion and water molecules in the middle, like sandwich bread. LDHs own a large interlayer

Water 2018, 10, 745; doi:10.3390/w10060745 www.mdpi.com/journal/water Water 2018, 10, 745 2 of 11 surface, which can capture all kinds of different exchangeable anionic species, provide it with a high 2− removal efficiency of anionic compounds. However, investigation shows that carbonate anion (CO3 ) is easily generated by CO2 in the air, enters the intermediate layer and its strong affinity prevents the substitution of other anions unless it is further calcined, thus leading to LDHs that cannot be regarded as exchangeable inorganic anionic materials [19]. It has been reported that the removal rate of fluoride ions by Mg/Al LDHs is only 29.8% [20]. To overcome these drawbacks, a kind of corn starch was applied to the preparation of synthesizing starch-stabilized LDHs, and its usage in fluoride removal was examined. The starch-stabilized nanoparticles have many merits and have received worldwide attention because of their good biodegradability, environmental friendliness, and low cost [21]. The starch-based materials exhibited much less agglomeration and greater reactivity than those without starch, thereby accelerating the 2− physical property of the compounds. Liang and Zhao studied the removal of ASO4 by a starch stabilized magnetite and the results showed that the adsorption capacity of the stabilized magnetite is 14% higher than that of original magnetite [22]. Therefore, in this present work, a novel starch stabilized Mg/Al LDHs material (S-LDHs) was developed to study its fluoride ion removal performance. Characterization of S-LDHs was employed by using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and particle size distribution. The adsorption mechanism was studied through the assessment of adsorption isotherms, kinetic models, thermal dynamics, and pH influence.

2. Materials and Methods

2.1. Reagents and Synthesis of Adsorbents The reagents utilized in this study were of analytical grade (A.R.). The aqueous solutions were employed with deionized water. The Mg/Al layer double hydroxides were synthesized using the co-precipitation method [23]. Starch stabilized layered double hydroxides (S-LDHs) were synthesized by the modiﬁed co-precipitation method. Firstly, 10 mg of starch was dispersed in 1000 mL aqueous solution to form a starch solution. Secondly, a combined solution of 0.50 mol of Al(NO3)3·9H2O and 1.50 mol of Mg(NO3)2·6H2O and a combined solution of 0.5 mol/L Na2CO3 and 2.00 mol/L NaOH was mixed with starch solution slowly accompanied with vigorous stirring to maintain the pH at around 10. Subsequently, the suspension was aged, separated, and dried. Lastly, the resulting material passed through a 60 mesh screen and was collected for later use.

2.2. Characterization Methods The material’s surface features were obtained by FTIR spectrums on a Bruker TENSOR27 spectrophotometer (Bruker International AG, Rheinstetten, Baden-Württemberg, Germany) within the limit of 400–4000 cm−1. The materials elements were analyzed by XRD patterns in a Bruker D8 X (Bruker International AG, Rheinstetten, Baden-Württemberg, Germany). The scanning scope is 5~80◦ every 0.02◦ (2θ) with a step speed of 8◦·min−1. The particle size distribution was determined by Zetasizer NANO ZS90 (Enigma Business Park, Worcestershire, United Kingdom) with the maximum particle size range of 0.3 nm–10 µm, a concentration range of 0.1 ppm–40% w/v, and a detection angle of 175◦, 12.8◦.

2.3. Experimental Procedure Adsorption experiments were investigated in a lab-scale application. Adsorption isotherms were gained by mixing 0.2 g S-LDHs with 50 mL of water with fluoride a concentration of 10 mg/L, 20 mg/L, 50 mg/L, 80 mg/L, and 100 mg/L and the fluoride concentrations under equilibrium conditions were tested after 4 h of reaction. The kinetics experiment of fluoride adsorption was obtained with the dosage of S-LDHs 0.2 g combined with an initial fluoride concentration of 10 mg/L in a conical flask with 50 mL of water. The residual fluoride concentration was measured after a different contact time. Water 2018, 10, 745 3 of 11

TheWater experiments 2018, 10, x FOR were PEER done REVIEW in a thermostat with continuous oscillation at 180 rpm, 20 ± 1 ◦C. After3 of the 10 reaction the adsorbent was separated by ﬁltration. TheThe adsorptionadsorption valuevalue ofof ﬂuoridefluoride onon S-LDHsS‐LDHs couldcould bebe calculatedcalculated as as follows follows [ [24]:24]:

()CCV0  e (C0 − Ce)V qqe= (1)(1) e WW wherewhereq qee isis thethe amount ofof fluoridefluoride adsorbed adsorbed on on the the S-LDHs S‐LDHs at at equilibrium equilibrium time; time;C C0 0(mg/L) (mg/L) isis the initialinitial fluoridefluoride concentration,concentration, andand CCee (mg/L)(mg/L) is is the equilibrium fluoridefluoride concentration; V is thethe volumevolume ofof solutionsolution (L);(L); andand WW isis thethe weightweight ofof adsorbentadsorbent (g).(g).

3.3. Results and Discussion

3.1.3.1. Characteristics ofof LDHsLDHs RepresentativeRepresentative XRD XRD patterns patterns of theof Mg-Althe Mg LDHs‐Al LDHs and starch-based and starch LDHs‐based adsorbents LDHs adsorbents are displayed are indisplayed Figure1 .in It Figure can be 1. seen It can that be Mg-Al seen that LDHs Mg materials‐Al LDHs displays materials sharp displays peaks sharp in (003), peaks (006), in (003), and (013), (006), showingand (013), that showing the materials that the are materials the characteristic are the characteristic reﬂections reflections of the double of the layer double hydroxide layer hydroxide structures (Figurestructures1a). (Figure As for the1a). samples As for the with samples starch with stabilized, starch the stabilized, characteristics the characteristics of the original of the diffraction original peakdiffraction still exist, peak but still the exist, peak but value the ispeak weakened value is and weakened the width and of the the width characteristic of the characteristic peak is broadened, peak is indicatingbroadened, that indicating the material that the has material changed has (Figure changed1b). The(Figure XRD 1b). patterns The XRD after patterns the ﬂuoride after adsorptionthe fluoride areadsorption shown inare Figure shown1c. in The Figure peaks 1c. inThe (003), peaks (006), in (003), and (009)(006), becomeand (009) wider become and wider the peak and strength the peak becomesstrength weaker,becomes which weaker, shows which that shows the material that the crystallinity material crystallinity reduced after reduced the adsorption. after the adsorption.

Figure 1. X‐ray Diffraction (XRD) pattern of (a) Mg/Al LDHs, (b) S‐LDHs, (c) Samples after fluoride Figure 1. X-ray Diffraction (XRD) pattern of (a) Mg/Al LDHs, (b) S-LDHs, (c) Samples after ﬂuoride removal on S‐LDHs. removal on S-LDHs.

The FTIR study of Mg‐Al LDHs, S‐LDHs and S‐LDHs before and after adsorption were exhibited to detectThe FTIRthe presence study of of Mg-Al characteristic LDHs, S-LDHs functional and S-LDHsgroups. beforeAs can and be seen after from adsorption Figure were 2a in exhibited the FTIR to detect the presence of characteristic functional groups. As can be seen from Figure2a in the FTIR spectrum, the region between 3400 and 3700 cm−1 has broad bands for the ‐OH stretching vibrations spectrum, the region between 3400 and 3700 cm−1 has broad bands for the -OH stretching vibrations of the hydroxyl groups [25]. The band at 1653 cm−1 is for the adsorbed inter‐layer water. The peak at of the hydroxyl groups [25]. The band at 1653 cm−1 is for the adsorbed inter-layer water. The peak at 1372 cm−1 is for the vibration of CO32− and NO3−. The band below 1000 cm−1 is for the metal bands. −1 2− − −1 1372While cm for theis for FTIR the spectrum vibration ofof COS‐LDHs3 and (Figure NO3 2b),. The the band weak below band 1000 at ~1315 cm cmis for−1 and the ~1030 metal bands.cm−1 is While for the FTIR spectrum of S-LDHs (Figure2b), the weak band at ~1315 cm −1 and ~1030 cm−1 is assigned to CH2–O–CH2 [26], indicating that starch and LDHs effectively bonded together. The FTIR assignedspectra of to the CH S2‐LDHs–O–CH after2 [26 fluoride], indicating adsorption that starch are shown and LDHs in Figure effectively 2c. The bonded band at together. 3503 cm The−1 shifted FTIR −1 spectrato 3468 ofcm the−1, showing S-LDHs afterthat the ﬂuoride anions adsorption and the hydroxyl are shown groups in Figure interacted2c. The bandon the at adsorbent. 3503 cm shifted to 3468 cm−1, showing that the anions and the hydroxyl groups interacted on the adsorbent.

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FigureFigure 2. FourierFourier-transform 2. Fourier‐transform‐transform infrared infrared infrared spectroscopy spectroscopy spectroscopy (FTIR) (FTIR) (FTIR) pattern pattern pattern of of (a ) (of aMg/Al) ( Mg/Ala) Mg/Al LDHs, LDHs, LDHs, (b) (Sb ‐()LDHs,b S-LDHs,) S‐LDHs, (c) (c) (Samplesc) SamplesSamples after after afterfluoride ﬂuoride fluoride removal removal removal on onS ‐onLDHs. S-LDHs. S‐LDHs.

The particle size distribution of Mg/Al LDHs and S‐LDHs is shown in Figure 3. It can be seen The particleThe particle size distributionsize distribution of Mg/Al of Mg/Al LDHs LDHs and S-LDHs and S‐LDHs is shown is shown in Figure in3 Figure. It can be3. It seen can that be seen that the average diameter is 1358 nm and the particle size distribution is scattered. The average the averagethat the diameter average isdiameter 1358 nm is and 1358 the particlenm and size the distribution particle size is distribution scattered. The is average scattered. particle The sizeaverage particle size of S‐LDHs was 421 nm, and the particle size distribution was relatively concentrated, of S-LDHsparticle was size 421 of nm,S‐LDHs and thewas particle 421 nm, size and distribution the particle was size relatively distribution concentrated, was relatively which concentrated, showed which showed that the particle size distribution of the stabilized starch was more uniform. thatwhich the particle showed size that distribution the particle of thesize stabilized distribution starch of the was stabilized more uniform. starch was more uniform.

(a) (a)

(b) (b)

Figure 3. Size distribution of Mg/Al LDHs (a) and S‐LDHs (b). FigureFigure 3. Size 3. distributionSize distribution of Mg/Al of Mg/Al LDHs LDHs (a) and (a) S-LDHsand S‐LDHs (b). (b). 3.2. Adsorption Study 3.2. Adsorption3.2. Adsorption Study Study 3.2.1. Dosage Effect of Starch Content 3.2.1.3.2.1. Dosage Dosage Effect Effect of Starch of Starch Content Content The dosage effect of the starch on the fluoride removal is displayed in Figure 4. It can be seen TheThe dosage dosage effect effect of the of starchthe starch on the on fluoridethe fluoride removal removal is displayed is displayed in Figure in Figure4. It can4. It be can seen be seen that the starch content has an obvious effect on the fluoride removal. The removal rate increased thatthat the starchthe starch content content has anhas obvious an obvious effect effect on the on fluoridethe fluoride removal. removal. The The removal removal rate rate increased increased greatly with the increment of the starch dosage. When the dosage amount of the starch is about 10 greatlygreatly with with the increment the increment of the of starch the starch dosage. dosage. When When the dosage the dosage amount amount of the starch of the is starch about is 10 about mg, 10 mg, the removal percentage of the fluoride reached 98%, which is three times higher than the removal the removalmg, the removal percentage percentage of the fluoride of the reachedfluoride 98%,reached which 98%, is which three times is three higher times than higher the than removal the removal rate rate of the Mg/Al LDHs. The first reason is that the starch is in an anion state under alkaline condition, of therate Mg/Al of the LDHs.Mg/Al LDHs. The first The reason first reason is that is the that starch the starch is in an is in anion an anion state state under under alkaline alkaline condition, condition, inhibiting the entry of carbon dioxide and increasing the removal rate of the fluoride. The second inhibitinginhibiting the entrythe entry of carbon of carbon dioxide dioxide and and increasing increasing the removalthe removal rate rate of the of fluoride.the fluoride. The The second second reason is that the starch promotes the higher dispersion of LDHs, thus increasing the surface area of reasonreason is that is that the starchthe starch promotes promotes the higher the higher dispersion dispersion of LDHs, of LDHs, thus thus increasing increasing the surfacethe surface area area of of S‐LDHs and making the adsorption point more abundant. S-LDHsS‐LDHs and and making making the adsorption the adsorption point point more more abundant. abundant.

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100

0 0 5 10 15 20 starch content(mg)

FigureFigure 4. 4.Effect Effect of of starch starch content content on on ﬂuoride fluoride removal. removal.

3.2.2. Adsorption Isotherm 3.2.2. Adsorption Isotherm The experimental adsorption data of the fluoride on S‐LDHs is determined by the isotherm The experimental adsorption data of the fluoride on S-LDHs is determined by the isotherm models models of Langmuir and Freundlich [27]. The Langmuir isotherm model supports monolayer of Langmuir and Freundlich [27]. The Langmuir isotherm model supports monolayer adsorption, adsorption, and the Freundlich isotherm is related to the multi‐layer, non‐linear adsorption process. and the Freundlich isotherm is related to the multi-layer, non-linear adsorption process. The correlation The correlation coefficient (R2) and the root mean square error (RSME) are compared and evaluated, coefficient (R2) and the root mean square error (RSME) are compared and evaluated, which is the best which is the best fit of the two isotherms. fit of the two isotherms. The linearized expression of the Langmuir isotherm is: The linearized expression of the Langmuir isotherm is: 1 C C 1 (2) e= e + (2) qe qm KLqm The linearized expression of the Freundlich isotherm is: The linearized expression of the Freundlich isotherm1 is: ln ln ln (3) 1 ln qe = ln KF + ln Ce (3) where qm is the calculated adsorption amount (mg/g);n KL is the Langmuir constant (L/mg). KF is the Freundlich constant; 1/n is the heterogeneity factor. where q is the calculated adsorption amount (mg/g); K is the Langmuir constant (L/mg). K is the RMSEm was calculated for evaluating the straight lineL fit and the equation is as follows [28]:F Freundlich constant; 1/n is the heterogeneity factor. ∗ . RMSE was calculated for evaluatingRSME the straight line / fit and the equation is as follows [28]: (4)

0.5 where qt is the adsorbed (mg/g) at time t gained from∗ 2 the lab experiments, and qt* is the fluoride RSME = [∑ (qt − qt ) /(n − m)] (4) capacity adsorbed (mg/g) forecasted by the two models. The samples number is expressed by the n wherevalue qandt is thefitted adsorbed parameters (mg/g) number at time is expressedt gained by from the the m value. lab experiments, and qt* is the fluoride capacityFigure adsorbed 5 and (mg/g) Table 1 forecasted show the byresults the two of the models. isotherm The samplescurves fitting number with is expressedthe Freundlich by the andn valueLangmuir and fitted equations. parameters The results number showed is expressed that the by theLangmuirm value. isotherm model had a higher R2 and lowerFigure RMSE5 and value, Table which1 show indicated the results that the of thereaction isotherm process curves might fitting be a withmonolayer the Freundlich adsorption and with Langmuirchemisorption equations. on the Theadsorbent results surface. showed It thatcan be the seen Langmuir that the isotherm maximum model adsorption had a capacity higher R is2 21.72and lowermg/g, RMSE which value, is three which times indicated higher than that thethe reaction value of process previously might investigated be a monolayer for Mg/Al adsorption LDHs. with The chemisorptionvalue of KL was on greater the adsorbent than zero, surface. indicating It can that be the seen adsorption that the system maximum was adsorptionfavorable. capacity is 21.72 mg/g, which is three times higher than the value of previously investigated for Mg/Al LDHs. The value of KL was greaterTable than 1. Isotherm zero, indicating adsorption that models the of adsorption fluoride onto system S‐LDHs. was favorable. Langmuir Isotherm Freundlich Isotherm Table 1. Isotherm adsorption models of fluoride onto S-LDHs. KL qm (mg/g) R2 RMSE KF n R2 RMSE 0.123 21.72 0.9987 0.0102 2.719 1.7393 0.9884 0.0804 Langmuir Isotherm Freundlich Isotherm 2 2 KL qm (mg/g) R RMSE KF n R RMSE 0.123 21.72 0.9987 0.0102 2.719 1.7393 0.9884 0.0804

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FigureFigure 5. 5.Curve Curve ﬁt fit of of Langmuir Langmuir and and Freundlich Freundlich isotherm. isotherm.

3.2.3.3.2.3. Adsorption Adsorption Kinetic Kinetic Studies Studies TheThe adsorption adsorption kinetics kinetics can can identify identify the the process process of theof the adsorption adsorption efficiency efficiency and and help help to verify to verify the fluoridethe fluoride adsorption adsorption mechanism. mechanism. The adsorptionThe adsorption rate andrate fluoride and fluoride uptake uptake kinetics kinetics on S-LDHs on S‐LDHs were studiedwere studied based based on two on reaction two reaction based modelsbased models (pseudo (pseudo first and first pseudo and pseudo second) second) and a diffusion and a diffusion based modelbased (intraparticlemodel (intraparticle diffusion diffusion model). Themodel). pseudo-first The pseudo order‐first model order assumes model the assumes reaction the is reversible,reaction is thereversible, pseudo-second the pseudo order‐second model order regards model chemisorption regards chemisorption as the rate limiting as the step.rate limiting The intraparticle step. The diffusionintraparticle assumes diffusion that theassumes plots passthat throughthe plots origin. pass through The models origin. can The be expressedmodels can as be follows expressed [29]: as pseudo-firstfollows [29]: order model [30] pseudo‐first order model [30] ln (qe − qt) = ln (qe) − k1t (5) pseudo-second order model [31] ln ln (5) t 1 t pseudo‐second order model [31] = + 2 (6) qt k2qe qe 1 (6) intraparticle diffusion model [32] 1/2 qt = KFt + C (7) intraparticle diffusion model [32] −1 The common parameters, k1 represents the rate constant of adsorption (min ) obtained from the / (7) plot of ln(qe − qt) against t. k2 means the pseudo-second rate constant (g/(mg·min)) obtained from the 0.5 linearThe plot common of t/qt against parameters,t. KF is k1 the represents intraparticle the rate diffusion constant rate of constant adsorption (mg/(g (min·min−1) obtained)). from the plot Theof ln( resultsqe − qt) of against the fitting t. k2 means kinetics the models pseudo were‐second summarized rate constant in Figure (g/(mg6 ∙andmin)) Table obtained2. A pseudo- from the 2 secondlinear plot order of kinetic t/qt against was provedt. KF is the to be intraparticle fitted with diffusion the experiment rate constant data by (mg/(g the high∙min R0.5(>0.99))). and low RMSE,The which results implied of the chemisorption. fitting kinetics The models values were of k 2summarizedwere low, indicating in Figure that 6 and the adsorptionTable 2. A pseudo process‐ ratessecond were order fast. kinetic was proved to be fitted with the experiment data by the high R2 (>0.99) and low RMSE, which implied chemisorption. The values of k2 were low, indicating that the adsorption Table 2. Adsorption kinetic models for fluoride removal on S-LDHs and the calculated constants. process rates were fast. Kinetic Models Parameter Table 2. Adsorption kinetic models for fluoride removal on S‐LDHs and the calculated constants. −1 2 k (min ) qe (mg/g) R RMSE Pseudo-first-order 1 Kinetic Models Parameter 0.0771 2.036 0.9872 0.2401 k1 (min−1) qe (mg/g) R2 RMSE 2 Pseudo‐firstk ‐(g/(mgorder ·min)) qe (mg/g) R RMSE Pseudo-second-order 2 0.0771 2.036 0.9872 0.2401 0.1467k2 (g/(mg∙min)) 2.6107 qe (mg/g) 0.9995R2 RMSE 0.1194 Pseudo‐second‐order k (mg/(g·min0.5))0.1467 R2 2.6107 RMSE0.9995 0.1194 - intraparticle diffusion F kF (mg/(g∙min0.5)) R2 RMSE ‐ intraparticle diffusion2.4289 0.8471 0.7321 - 2.4289 0.8471 0.7321 ‐

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FigureFigure 6. 6.Kinetics Kinetics of of ﬂuoride fluoride adsorption adsorption on on the the S S−−LDHsLDHs fromfrom ((aa)) experiment,experiment, modelsmodels byby ( (bb)) pseudo pseudo ﬁrst,first, ( c(c)) pseudo pseudo second, second, and and ( d(d)) intraparticle intraparticle diffusion. diffusion.

3.2.4. Adsorption Thermodynamic Studies 3.2.4. Adsorption Thermodynamic Studies The variation of temperature can be used to analyze the thermodynamic properties of LDHs– The variation of temperature can be used to analyze the thermodynamic properties of fluoride interaction. In this study, temperature was controlled in a range of 20–50 °C. ΔG0, ΔS0 and LDHs–ﬂuoride interaction. In this study, temperature was controlled in a range of 20–50 ◦C. ∆G0, ∆S0 ΔH0 were calculated by the Van’t Hoff equation: and ∆H0 were calculated by the Van’t Hoff equation: Δ RTln, (8) ∆G0 = −RTlnb, (8) , (9) 0 0 0 ∆G = ∆H− T∆S , (9) ln , (10) ∆S0 ∆H0 ln b = −−1 −1 , (10)0 where, R is the universal gas constant (8.314 J∙molR K RT), b is adsorption isothermy constant, ΔG is the change in free energy, ΔH0 is the standard enthalpy and ΔS0 is the standard entropy, T is the where, R is the universal gas constant (8.314 J·mol−1 K−1), b is adsorption isothermy constant, temperature (K). Here, b value can be calculated from the Langmuir constant KL∙Qm. ΔH0 and ΔS0 ∆G0 is the change in free energy, ∆H0 is the standard enthalpy and ∆S0 is the standard entropy, parameters can be obtained, respectively, from the slope and intercept of the plot of lnb vs. 1/T. The T is the temperature (K). Here, b value can be calculated from the Langmuir constant K ·Q . ∆H0 and results are shown in Table 3. It can be seen that the values of ΔG0 are negative, indicatingL mthe sorption ∆S0 parameters can be obtained, respectively, from the slope and intercept of the plot of lnb vs. 1/T. process is spontaneous in nature. As the temperature rises, the reaction becomes relatively easy. The The results are shown in Table3. It can be seen that the values of ∆G0 are negative, indicating the positive value of ΔH0 confirms the sorption process is endothermic. The positive value of ΔS0 sorption process is spontaneous in nature. As the temperature rises, the reaction becomes relatively indicates that the reaction is spontaneous, and the solid‐liquid interface is random at the time of easy. The positive value of ∆H0 conﬁrms the sorption process is endothermic. The positive value of adsorption. ∆S0 indicates that the reaction is spontaneous, and the solid-liquid interface is random at the time of adsorption.Table 3. The value of thermodynamic parameter for the adsorption of fluoride by the S‐LDHs.

Table 3. The value of thermodynamic parameterThermodynamic for the adsorption Paramete of ﬂuorider by the S-LDHs. Temperature (°C) b △G0 (KJ/mol) △H0 (KJ/mol) △S0 (J/(mol∙K)) 20 9.73 −12.44 Thermodynamic Parameter ◦ Temperature ( C) 30 b 4.25 −10.78 ∆G0 (KJ/mol) 35.63∆H 0 (KJ/mol) 0.0806∆ S0 (J/(mol·K)) 40 2.67 −9.93 − 2050 9.73 2.55 −10.1212.44 30 4.25 −10.78 35.63 0.0806 40 2.67 −9.93 Arrhenius50 equation representing 2.55 the variance−10.12 between the temperature and the activation energy of adsorption. The activation energy calculation can be obtained using the Formula (11).

A/, (11) where, k is the pseudo second‐order reaction rate constant, (g/(mg∙min)), Ea is activation energy, (kJ/mol). The calculated activation energy is 38.83 kJ/mol, greater than 20 KJ/mol, indicating the

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Arrhenius equation representing the variance between the temperature and the activation energy of adsorption. The activation energy calculation can be obtained using the Formula (11).

−E /RT Water 2018, 10, x FOR PEER REVIEW k = Ae a , (11)8 of 10 reactionwhere, isk correspondedis the pseudo second-orderto the slow defluorination reaction rate constant, stage, controlled (g/(mg·min)), by theEa chemicalis activation reaction energy, of the fluoride(kJ/mol). ion Theand calculatedS‐LDHs. activation energy is 38.83 kJ/mol, greater than 20 KJ/mol, indicating the reaction is corresponded to the slow defluorination stage, controlled by the chemical reaction of the 3.3.fluoride Mechanism ion and Analysis S-LDHs. 3.3.pH Mechanism value is Analysis an important parameter for reflecting the adsorption property. In this paper, the isoelectric point (pHPZC) and variance of the pH value was used to analyze the proposed adsorption pH value is an important parameter for reflecting the adsorption property. In this paper, mechanism. pHPZC plays an important role on the surface properties of the material, determining the the isoelectric point (pHPZC) and variance of the pH value was used to analyze the proposed adsorption degree of difficulty of adsorption. The pHPZC of adsorbent can be determined by the salt supplement mechanism. pHPZC plays an important role on the surface properties of the material, determining the method [33]. As shown in Figure 7, the pHPZC of the S‐LDHs adsorbents is 9.22. The results of the degree of difficulty of adsorption. The pHPZC of adsorbent can be determined by the salt supplement variancemethod of [33 the]. Asinitial shown pH invalue Figure to 7the, the fluoride pH PZC adsorptionof the S-LDHs on S adsorbents‐LDH at 20 is °C 9.22. were The also results analyzed. of the It canvariance be seen of that the initialwith the pH initial value topH the value fluoride in the adsorption solution onvarying S-LDH from at 20 3.0◦C to were 9.0, also the analyzed.adsorption capacityIt can be of seenthe fluoride that with onto the the initial material pH value remained in the stable, solution while varying when from the 3.0 pH to value 9.0, the was adsorption higher than 9.0,capacity the adsorption of the fluoride capacity onto of the the material fluoride remained decreased stable, gradually, while but when still the kept pH a value comparably was higher high than level, indicating9.0, the adsorption that a wide capacity range of of the initial fluoride pH decreased value has gradually, little effect but on still the kept removal a comparably ability high of fluoride. level, Whenindicating the pH that value a wide is less range than of the initial pHPZC pH, the value charge has little surface effect is onpositively the removal charged, ability while of fluoride. when pH valueWhen is thehigher pH valuethan pH is lessPZC, thanthe charge the pH PZCsurface, the chargeis negatively surface charged. is positively Thus charged, the possible while whenreason pH is as follows:value isunder higher acidic than pHconditions,PZC, the charge the protonation surface is negatively makes the charged. surface Thus of the the adsorbent possible reason positively is charged,as follows: and underfluoride acidic ions conditions, are more theeasily protonation adsorbed makes onto S the‐LDHs. surface Under of the alkaline adsorbent conditions, positively the − OHcharged,− concentration and fluoride in the ions solution are more increases, easily adsorbed and competes onto S-LDHs. with Under the fluoride alkaline ion, conditions, indicating the that OH the fluorideconcentration ions could in the not solution effectively increases, enter and the competes layer of the with S‐ theLDHs. fluoride The ion, reason indicating why the that removal the fluoride rate of theions fluoride could notions effectively is still high enter under the layer alkaline of the conditions S-LDHs. The is reasonthat the why surface the removal of the rate starch of the is fluoridepositively chargedions is stilland high the underfluoride alkaline ions conditionsare efficiently is that adsorbed. the surface In of summary, the starch is according positively to charged the adsorption and the fluoride ions are efficiently adsorbed. In summary, according to the adsorption experiments and the experiments and the characterization analysis, fluoride ion adsorption by S‐LDHs is presumed characterization analysis, fluoride ion adsorption by S-LDHs is presumed mainly by the interaction of mainly by the interaction of the LDHs and the surface adsorption of S‐LDHs. the LDHs and the surface adsorption of S-LDHs.

Figure 7. pHpzcFigure of 7. SpHpzc‐LDHs of (a S-LDHs) and the (a effect) and theof initial effect ofpH initial (b) on pH fluoride (b) on ﬂuoride removal. removal.

4. 4.Conclusions Conclusions InIn this this work, work, starch starch stabilized stabilized Mg Mg-Al-LDHs‐Al‐LDHs werewere utilizedutilized as as adsorbents adsorbents and and the the properties properties of of Mg/AlMg/Al LDHs LDHs and and S‐LDHs S-LDHs were were compared. compared. The The fluoride ﬂuoride adsorption adsorption capacity capacity of the of the S‐LDHs S-LDHs was was three timesthree higher times higherthan that than of that the of Mg/Al the Mg/Al LDHs. LDHs. The The particle particle size size distribution distribution of thethe S-LDHs S‐LDHs is is more more uniform than that of the LDHs. The Langmuir adsorption isotherm model and pseudo‐second‐order kinetic model fit well with the experimental data. In the thermodynamic parameters, the enthalpy (ΔH0) value was 35.63 kJ mol−1 and the entropy (ΔS0) value was 0.0806 kJ mol−1K−1. The negative values of ΔG0 indicated the spontaneous nature of adsorption. The change in a wide range of initial pH values has little effect on the removal ability of the fluoride. The mechanism for fluoride removal on the S‐LDHs included surface adsorption and interaction ion exchange. Further investigations could focus on the reuse of the material and a dynamic monitor using real water.

Water 2018, 10, 745 9 of 11 uniform than that of the LDHs. The Langmuir adsorption isotherm model and pseudo-second-order kinetic model fit well with the experimental data. In the thermodynamic parameters, the enthalpy (∆H0) value was 35.63 kJ mol−1 and the entropy (∆S0) value was 0.0806 kJ mol−1K−1. The negative values of ∆G0 indicated the spontaneous nature of adsorption. The change in a wide range of initial pH values has little effect on the removal ability of the fluoride. The mechanism for fluoride removal on the S-LDHs included surface adsorption and interaction ion exchange. Further investigations could focus on the reuse of the material and a dynamic monitor using real water.

Author Contributions: Conceptualization, J.L. and X.Y.; Methodology, J.L.; Validation, J.L., and X.Y.; Formal Analysis, Y.G.; Investigation, Y.G.; Data Curation, X.L.; Writing-Original Draft Preparation, J.L.; Writing-Review & Editing, X.L.; Supervision, X.Y.; Project Administration, X.Y.; Funding Acquisition, J.L. Funding: This research was funded by Shanxi Provincial Natural Science Foundation, China, grant number 201601D102037. Acknowledgments: The authors are grateful for the financial support from Shanxi Provincial Natural Science Foundation, China (Grant No. 201601D102037). Conflicts of Interest: There are no conflict to declare.

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