Fluoride ion adsorption from wastewater using magnesium(II), aluminum(III) and titanium(IV) modi ﬁ ed natural zeolite: kinetics, thermodynamics, and mechanistic aspects of adsorption

Natural zeolite was modi ﬁ ed using metal ions, including magnesium(II), aluminum(III) and titanium(IV). The modi ﬁ ed zeolite was then used as an adsorbent for the investigation of the adsorption kinetics, isotherms, and thermodynamic parameters of ﬂ uoride ions in wastewater at various pHs and temperatures. The kinetics and thermodynamics for the removal of the ﬂ uoride ions onto the modi ﬁ ed zeolite have also been investigated. The ﬂ uoride ion adsorption capacity of the three types of modi ﬁ ed zeolites exhibited an increase, then decrease, with rising pH. The ﬂ uoride adsorption capacity of the modi ﬁ ed zeolites decreased with an increase in temperature. The pseudo-second-order model is more suitablefor describing theadsorption kineticdatathanthe pseudo- ﬁ rst-order model formodi ﬁ edzeolite and the adsorption process of the ﬂ uoride ions reveals pseudo-second-order kinetic behavior, respectively. It was found that the adsorption equilibrium data ﬁ t the Freundlich isothermal equation better than that of the Langmuir isothermal and Dubinin – Radushkevich (D – R) isothermal equations. Thermodynamic analysis suggests that the negative values of Δ G 0 and Δ H 0 further indicate that the ﬂ uoride adsorption process is both spontaneous and exothermic. The results of competitive adsorption tests suggest that the modi ﬁ ed metal zeolite materials adsorb ﬂ uoride ions with high selectivity.


INTRODUCTION
As a trace element, fluoride is essential to prevent dental cavities, but an excessive intake of the element can be detrimental to human health. Indeed, excess ingestion of fluoride can cause dental/skeleton fluorisis (Mahramanlioglu et al. ). It not only affects the teeth and skeleton, but its accumulation over a long period of time can lead to cancer, osteosclerosis (brittle bones and calcified ligaments), as well as neurological impairment in human beings (Harrison ). According to World Health Organization (WHO) norms, the acceptable fluoride concentration in drinking water is generally 0.5-1.5 mg/L (World Health Organization ). In China, the industrial standards of fluoride concentration in wastewater discharge are under 10 mg/L, and 1.0 mg/L in drinking water. The need to effectively reduce the fluoride concentration in industrial wastewater has become a vital task in contemporary society.
Fluoride water pollution mainly occurs via two ways, both of which are related to natural and human activities.
Fluoride is often present in minerals and it can be leached out due to erosion by rainwater, leading to contamination of ground and surface waters. Conversely, fluoride contamination occurs in a wide range of industrial wastewater generated by aluminum and steel production, metal finishing and electroplating, glass and semiconductor manufacturing, ore beneficiation, and fertilizer operation (Paulson ). Fuxin. There are rich natural zeolite mineral resources in this region, so the modified zeolite adsorption method not only reduces the cost, but also makes good use of the resources at hand. Zeolite is an aqueous frame structure made up of aluminum silicate minerals, the chemical formula for which is (Na, K) x (Mg, Ca, Sr or Ba) y (Al (xþ2y) Si (n-(xþ2y))-O 2n ·mH 2 O). The lattice is characterized by many pores and channels; therefore, it possesses a large specific surface area and exhibits good adsorption performance. Making the adsorbent with natural zeolite treated with the appropriate modification methods has always been a hot area of research both at home and abroad. Naturally occurring low-cost zeolites used as an ion adsorbent offer great potential for removing fluoride from industrial wastewater.
In recent years, considerable attention has been devoted to developing new adsorbents loaded with metal ions (Zhang et al. ). These metal ions, such as sodium, magnesium, aluminum, titanium, zirconium, and lanthanum, exhibit promising results for the removal of fluoride when they are loaded on zeolite. Rahmani found that the adsorption capacity of Al(III)-loaded zeolite for removing fluoride is larger than that of Fe(III)-loaded zeolite; this adsorbent performs desorption and regeneration processes well (Rahmani et al. ). In another investigation, Zhao et al. studied the combined removal of fluoride and arsenic by Fe(III)-loaded ligand exchange cotton cellulose and found that the adsorbent could simultaneously remove As(V) and F À efficiently from the aqueous solution (Han et al. ). Moreover, research on rare earth metal-loaded zeolite has also made progress, but the operating costs are typically high. Therefore, the long-term interests, low adsorption capacity, and high costs are problems restricting their wide application. In this paper, three types of modified zeolite were prepared by loading magnesium, aluminum, and titanium, respectively. The aim was to investigate and characterize the effect of different metal ion valences loaded on zeolite for the removal of fluoride ions. Other factors including adsorption equilibrium, adsorption kinetics, effect of various pHs, initial fluoride concentration, solution temperature, reaction time, and coexisting anions on fluoride removal were also investigated to evaluate the adsorption performance of the modified zeolites.

Materials
The natural zeolite used in this experiment was obtained from Fuxin, Liaoning Province, China. The zeolite was crushed and ground to provide ore with a particle size of   (1)).
and the fluoride removal rate expression (Equation (2)), where Q e is the adsorption capacity (mg/g) at equilibrium; S is the fluoride removal rate (

Effect of pH
A set of wastewater solutions with different pHs was used in the adsorption experiment. The fluoride removal process was studied over the pH range of 2 to 12, the results for which are shown in Figure 1. It was found that the pH of the wastewater plays a significant role in the adsorption process. The fluoride ion adsorption capacity of the three kinds of modified zeolite materials first showed an increase, then decrease, with rising pH. The Mg(II)-zeolite exhibited an adsorption capacity of 0.80 mg/g when the pH of the solution was 7, while that of the Al(III)-zeolite was 0.88 mg/g at neutral pH. The Ti(IV)-zeolite reached a maximum adsorption capacity of 1.64 mg/g at the optimum pH of 6. In contrast to this was the adsorption capacity of natural zeolite, which was 0.42 mg/g at pH ¼ 7. It can therefore be seen that the adsorption capacity of the modified zeolite is greater than that of the natural zeolite.
The pH of the wastewater can affect the amount of fluoride ions retrieved. At low pH, F À combines with H þ to form HF (K 1 ¼ 1.5 × 10 3 ) or HF À 2 (K 2 ¼ 3.9). This leads to low adsorption capacity, as F À cannot be adsorbed by both the natural and modified zeolites. There is also some debate over the nature of the zeolite structure. Because the zeolite surface has groups of -Al-OH and -Si-OH, the variable surface charges are generated by associating and desorbing these groups for H þ ions. At lower pHs, the con-  various zeolites decreased with an increase in wastewater temperature. The adsorption experiment can be adopted at room temperature (293 K) to reduce energy consumption.
The adsorption capacity of the zeolites exhibited large differences when their temperature was held at 293 K. While the adsorption capacity of Al(III)-zeolite is greater than that of Mg(II)-zeolite, Ti(IV)-zeolite's is largest. Despite this, the absorption capacity of the natural zeolite is smallest.

Effect of adsorbent dosage
The adsorbent dosage was varied in the following amounts: 0.5, 1, 2, 5, 10, and 20 g. It was then placed in the wastewater solution with 80 mg/L constant initial concentration, 100 mL solution volume, at 293 K temperature. After 300 min, the residual fluoride concentration was obtained, and the removal rate and adsorption capacity were calculated.

Adsorption kinetics
The adsorption data was applied to three different kinetic models: pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. In both the pseudo-first-order and pseudo-second-order models, the adsorption steps, including external diffusion, internal diffusion, and adsorption are lumped together. Linear forms of the pseudo-firstorder and pseudo-second-order equations are given in Equations (3) and (4), as well as intraparticle diffusion in Equation (5) where Q e and Q t are the amount of dye adsorbed (mg/g) on the adsorbents at equilibrium and at time t, respectively, k 1 is the rate constant of adsorption (1/h), k 2 is the rate constant of pseudo-second-order adsorption (g/mg h), k p is the rate constant for intraparticle diffusion (1/h), and C is the intercept. The linear plots of ln(Q e À Q t ) versus t for the first-  Table 3.
According to these correlation coefficient values, the second-order model is better suited to describe the  Langmuir model:

Adsorption isotherms
Freundlich model: lg Q e ¼ 1 n lg C e þ lg K f Dubinin-Radushkevich (D-R) model: In the Dubinin-Radushkevich isothermal equation, the energy of adsorption, E (kJ/mol), was E ¼ (2 K ) À1/2 . Q e is the amount absorbed at equilibrium (mg/g), C e is the    Table 4. The linear fitting figures of the adsorption isotherm are revealed in Figure 5 and the relevant data were obtained by using Equations (4)-(6). It is seen that the coefficient of determination (R 2 ) obtained from the Freundlich isotherm model is higher than that obtained using the Langmuir and D-R models for Mg(II)-zeolite and Al(III)-zeolite. The R 2 obtained from the Langmuir model is close to the R 2 obtained using the values of E in this study were found to be 9.8372, 9.5996, and 11.7053 kJ/mol at 293 K for the Mg(II)-zeolite, Al(III)zeolite and Ti(IV)-zeolite. These numbers suggest that the adsorption proceeds mainly by chemical adsorption.

Thermodynamic analyses
Thermodynamic parameters can provide in-depth information regarding the inherent energy changes associated with adsorption. Thermodynamic parameters, including the change in the Gibbs free energy (ΔG 0 ), enthalpy (ΔH 0 ), and entropy (ΔS 0 ) were determined to elucidate the adsorption process using the following equations and the information depicted in Table 5.
The thermodynamic parameters, including the acti- K ¼ Q e C e (9) where K is the equilibrium constant; Q e is the amount of dye adsorbed on the natural sepiolite from the solution at equilibrium (mg/g); C e is the equilibrium concentration of the dye solutions (mg/L); R is the gas constant (8.3144 J/mol·K); T is the wastewater temperature (K); and ΔG 0 can be calculated through R, T and lnK. ΔH 0 and ΔS 0 were calculated from the slope and intercept of van't Hoff plots of lnK against 1/T ( Figure 6).
According to the thermodynamic data, several conclusions can be drawn as follows: 1. The negative ΔH 0 indicates the adsorption process of the fluoride ions on the modified zeolites is exothermic.   2. The negative ΔG 0 shows that the adsorption process is spontaneous and irreversible. With the temperature increasing, the absolute value of ΔG 0 decreases. This phenomenon indicates higher temperature is not conductive to the adsorption reaction.
3. The value of ΔS 0 is negative. After the adsorption process was completed, the free space allowed for the movement of the fluoride ions before adsorption transforms into two-dimensional motion of the zeolites after adsorption.
The degrees of freedom of the fluoride ions reduces and the entropy of the whole system decreases.

Desorption and regeneration
Any adsorbent is economically viable if the adsorbent can be regenerated and reused in many cycles of operation.
The regeneration experiments were performed using 1.0 mol/L NaOH solution as the eluent. After desorption, three kinds of adsorbent were regenerated using 20 wt% MgSO 4 , 15 wt% Al 2 (SO 4 ) 3 and 10 wt% Ti(SO 4 ) 2 solution, respectively. These processes were described in the 'Materials and methods' section above. Adsorption tests on the regenerated samples indicate that three kinds of modified zeolites have no apparent reduction in adsorption capacity after five cycles; they reveal only 9%, 6%, and 5% reduction in adsorption capacity.

ADSORPTION MECHANISM
The surface of the natural zeolite was coated with a metal hydroxide after modification. While the modified metal (M) zeolite was in contact with the fluoride ions in solution, the fluoride ions can be adsorbed by exchanging with the active hydroxyl groups. The fluoride adsorption onto the neutral solid surface can be described by a ligand or ion exchange reaction mechanism when the pH of the medium remains neutral (∼7.00). This process can be expressed by Equations (12) and (13) as follows.
The mechanism of the fluoride removal process has been outlined in a similar manner to that discussed for metal oxides by Tor () and Sarkar et al. (). In these studies, the exchange between available hydroxide surface groups and fluoride ions occurs because both F À and OH À are isoelectronic and therefore have a comparable ionic radius. According to the adsorption effect of different modified metal zeolites, the surface of the Ti(IV)-zeolite may offer more active sites than that of the Mg(II) and Al ( The thermodynamic data indicated that the adsorption reaction was both spontaneous and exothermic. These adsorbents, which were acquired by loading metal ions such as Mg(II), Al(III), and Ti(IV) onto the natural zeolites can be regenerated using 1.0 mol/L NaOH solution as an eluent. The regenerated samples retain their good adsorptive performance, especially the Ti(IV)-loaded adsorbent.
The adsorbent is efficient with low chemical consumption and operating costs.