Document Type : Original Research Paper
1 Nuclear Fuel Cycle Research School, Nuclear Science and Technology Research Institute,
2 Nuclear Science and Technology Research Institute, Tehran, Iran
3 Amirkabir University of Technology Tehran
Small anionic radius, high affinity to act as a ligand, and the formation of a great number of various organic and inorganic compounds are characteristics of fluoride (F−) ions. The fluoride (F−) ions are present in various inorganic and organic materials in soil, rocks, air, plants, and animal bodies.
The high solubility (in water) of the fluoride compounds would lead to the production of dissociated fluoride ions in groundwater and surface [1-4]. Two main sources of fluoride (F−) ions in groundwater and on the surface as anionic contaminants are geochemical reactions and disposal of industrial wastewaters [1-4].
The industrial activities which lead to producing fluoride contaminated wastewater originate form are glass and ceramic productions, electroplating, semiconductor manufacturing, coal-fired power stations, beryllium extraction, brick and ironworks, and aluminum smelters . Human and other creatures’ health can be affected by the pollution of groundwater with fluoride ions.
An excess intake of fluoride is harmful to our health; for instance, it can cause dental/skeletal fluorosis [6-8].
As an unwanted hazardous intake, the presence of fluoride ions over the threshold concentration of 1.5 mg/L in water can be harmful . The studies showed that drinking water is the primary source of daily intake of the fluoride ions and consumption of drinking water with high fluoride concentrations for a long time can induce various defects in birth, reproduction, immunological and anatomical problems including dental and skeletal fluorosis . Therefore the removal of fluoride (F−) ions from drinking water and wastewater are very vital and important. Different chemical and physical treatment methods have been used for the uptake of fluoride from wastewater. These methods are coagulation, precipitation, surface adsorption , ion exchange, and membrane processes (Nano-filtration, Reverse osmosis)  and electrolytic treatment [11, 15-18].Among these methods, adsorption is found to be the most commonly used and popular method arising out of its simple operation, low cost, and highly efficient technique [13-19]. Various natural and synthesized adsorbents have been used for the removal of fluoride ions from water. Natural materials such as zeolites, clays, and chitosans are inexpensive, green, and available adsorbents, but the low capacity has limited their usage in fluoride removal. The problem of natural adsorbents (low capacity) can be tackled by modification of natural adsorbents or utilization of synthesized materials. In recent years, several rare and transition metal oxides such as zirconium oxide, titanium oxide, cerium oxide, and iron(III) oxide have been used for the treatment of wastewater to eliminated fluoride ions treatment [21-22]. The high adsorption capacities and environmental friendliness are the attractive properties of these metal oxides as adsorbents for the uptake of fluoride ions from wastewater. Also, the results cleared that the multimetal oxides have higher fluoride uptake capacity, especially when the rare metals are present as multimetal oxides [21-22]. The main reasons for the high fluoride (F−) removal capacity of the metal/multimetal oxides can be described by the modification of the net charge of metal/multimetal oxide surface that facilitates the removal of fluoride through physisorption [21-22].
More recent investigations revealed that rare-earth elements, such as Lanthanum and Cerium when associated with other metals forming composite materials, such as Al-Fe-La , Fe-Mg-La , Fe-La, Fe-La-Ce, Ca-Al-La , Mg-Ce-La , Fe-Al-Ce , Zr-Al-La  develop a high affinity to fluoride. It is known that the synergistic interactions of metals promote an improvement in the adsorptive capacity of materials for fluoride. As compared to Cerium, Mn oxide is considered a low-cost material, and its composites also have good adsorption capacity for fluoride [11, 29], as well as aluminum oxide. So, mixing rare earth metals (La, Ce) with low-cost metals (Al, Mn) could be advantageous to achieve high adsorption capacities at a neutral pH solution.
Also, the adsorbent material’s fluoride removal performance can be associated with the synergistic effect of some properties such as structure, porosity, and Ce-Al-Mn interactions in the surface material.
Thus the mixed valance of metals in multimetal oxide have an additional net charge on their surface compared with the single metal oxides and the additional surface charge induces a stronger van der Waals attraction force for the adsorption of ions resulting in higher ion uptake capacity. In the current work Ce–Mn–Al tri-metal composite oxide was synthesized by the electrochemical methods and its potentials as an adsorbent for adsorption of fluoride and treatment of fluoride contaminated water were assessed thorough investigation of the effect of influencing factors such as time, initial concentration and pH. The adsorbents were characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and the Fourier transform infrared spectroscopy (FTIR).
Chemicals and reagents
The materials; Mn(NO3)24H2O, Al(NO3)39H2O, NaNO3, Ce(NO3)36H2O HNO3, NaOH, and TIZAB (Trans-1,2-Diaminocyclohexane-N,N,N′,N′-tetraacetic acid monohydrate) purchased from Merck (Germany), were utilized without further purification. The solutions were prepared from distilled water. The working electrode (cathode) was made up of stainless steel plates (316 L, 5×12×0.5 mm), placed between the two-rod graphite electrodes acting as anodes. The current densities of 5, 15, and 35 mA cm-2 were applied for deposition at different periods.
Preparation of adsorbent (Ce-Mn-Al tri-metal) and fluoride measurement.
The Al-Mn-Ce nano oxide was deposited at current densities of 5, 15, and 35 mAcm-2. The main reactions at work electrode (cathodic reactions) for metal deposition were OH- electro-generation reactions or dissolved oxygen, water, and nitrate ions reductions reactions as follows [30-34].
These reactions led to the production of OH¯ ions, increasing the local pH and deposition of Mn-Ce-Al hydroxide at the cathode surface as:
Just after the completion of the reaction, the Mn-Ce-Al-(OH)x powder was stripped from the electrode surface. The mixed hydroxide was then converted to an oxide by heat treatment:
The F- ions adsorption experiments were conducted in the batch mode and the effects of influencing parameters such as initial concentration, pH, and time were investigated. The stock fluoride (F-) solution was prepared by dissolving an appropriate amount of NaF in distilled water. 0.05 mg of the Mn-Ce-Al-(OH)x powder was poured into a specified volume of F- aqueous solution, and stirred in a temperature-controlled shaker. The stirring speed of 400 rpm was chosen for all the experiments
The products were characterized by X-ray diffraction pattern STOE STADI MP diffractometer equipped with monochromatized CuKα radiation (k = 0.154 nm, 40 kV], Scanning Electron Microscopy (SEM, model EM-3200 China). The fluoride concentrations were determined by a Metteler Toledo Seven Compact S220 pH/Ion meter (Switzerland) equipped with an ISTEK fluoride ion-selective electrode.
RESULTS AND DISCUSSIONS
Fig. 2 shows the SEM images of the Mn-Ce-Al composites prepared at different current densities of 5, 15, and 35 mAcm-2. Based on the figure, the surface morphologies show significant changes when the current density varied from 5 to 35 mAcm-2. At a high current density of 35 mAcm-2, the one-dimensional structure (nanowire with an average diameter of 50nm) is produced, whereas the decrease in the current density leads to the disappearance of the nanowire and appearance of the nanosheet structures. Moreover, the particle sizes are decreased with increasing the current density and turning to a smooth wire shape structure. Energy dispersive X-ray (EDX) spectroscopy results of the synthesized samples at different current densities are shown in Fig.1d. As seen, the Al, Mn, and Ce elements are not the same in all samples. The Mn content increases by increasing the current density.
Based on Ksp values as:
OH- generation at electrode surface leads to the formation of Mn(OH)2, Ce(OH)3, and Al(OH)3, respectively. Also, it is well known that the MnO2 structures are one-dimensional (rod, wire). In other words, the crystal growth mechanism of MnO2 is anisotropic growth. The chemical potential of the MnO2 in solution and inherent structure of the MnO2 material tend to anisotropic growth mechanism in particles formation. In the first step, due to the high surface energies of the initial synthesis nuclei, the accumulation takes place and spherical nanoparticles form (Ostwald Ripening). Then, the spherical nanoparticles would gradually transform into a one-dimensional structure (wire or rod). The gradual conversion of the nanoparticles into the one-dimensional structure (wire or rod) can be attributed to their anisotropic nature and the preferential one-dimensional growth of Mn components. In other words, from the thermodynamic and kinetic points, the crystal growth of Mn components in one axis is prevailed entailing one-dimensional structures. Thanks to literature, we can easily find that the common structures of manganese hydroxide/oxide are one-dimensional [32,34].
At high current density (of 35 mAcm-2), because of the high OH- concentration generation in the initial step of the electrosynthesis and based on KSP (Mn(OH)2, the Mn value in the deposit is high (as it was approved by EDS results). So, Mn(OH)2 has a primary and significant role in the structure formation, and the structure of the product is conducted by the anisotropic nature of Mn; consequently, the nanowire is produced. As current density decreases, the amount of the Mn hydroxide in the deposit is decreased (see the EDS result) and the amounts of Ce and Al which tend to be irregular and three-dimensional are increased in the deposit thus, the crystal growth does not follow the anisotropic process. As a result, the crystal growth diverts from the one-dimensional to the three-dimensional mechanism. In the following items some of the major nanostructure mechanisms formations will be discussed:
Growth control by surfactant and additives
The crystal shape and morphology of the metal oxide result from the different crystal growth rates in various directions because the surface energy levels are not the same in different planes of the crystal. The high-index planes usually have higher surface energy. However, the exact mechanism for the formation of these architectures has not been fully understood .
Thus, faster growth along the special direction can lead to the phenomenon of special morphology (particles, wire, tube, rod, etc.). To get various morphologies necessitates the control of the growth rate of different facets of the nuclei [36-37]. For this purpose, surfactant, templates, or other specific assets are applied. For example, the NH3 molecules in solution can form an ordinate bond with the M ions on the surface; so, they can cancel the crystal growth. The density of the adsorbed NH3 depends on the density of the metal ions on the crystal plane. The densities of the metal ions in the crystal planes are not the same. For example, in the CuO crystal the order of Cu ions in the crystal is as: (001) > (100) > (010) (Li et al, 2008). In the solution with the initial concentration of Cu2+ and high NH3 concentration, all primary CuO nuclei planes are coordinated, resulting in a weak driving force for further aggregation and 1D, 2D, and 3D nanoplates formation. With an increase in Cu2+ concentration, the growth in 010 would be dominant because NH3 is bonded to Cu ions in 001 and 100 planes, slowing down the crystal growth in these directions, while the growth in the 010 direction would be faster resulting in the nanorods or nanowires (Fig.2a) (Li et al, 2008). The non-identical density of Mn2+ ions in the crystal planes of as-prepared Mn(OH)2 is responsible for faster growth of particles in a direction and 1D nanostructure formation.
Oriented attachment 
Oriented attachment is a crystal growth mechanism in which the attachment of the small crystallites on suitable planes or facets along the same directions occurs. In the oriented attachment, large particles could be formed in an irreversible and highly oriented manner. Commonly, the particle or sphere and or any other irregular shape structures (the result of crystal growth in three dimensions), would be resulted. However, the control of the crystal growth in the special direction by a surfactant, capping agent, etc. is possible (Fig.2b).
Scrolling of sheets
In as-prepared M(OH)2, the dangling OH groups in the layer edges under certain conditions, link together (hydrogen bond interactions). As a consequence, the edges can be connected and the sheets roll to form M(OH)2 tubular form  (Fig.2c).
The XRD patterns of as-prepared and temperature annealed (300 and 600 ˚C) samples are shown in Fig.3. As can be seen, an as-prepared sample has an amorphous structure, since no signs of crystalline phase can be seen in the pattern. While the heat-treated samples show clear and sharp peaks related to Al-Mn-Ce oxide, indicating an improvement most in the crystal quality of the nanostructures. Also, the results show that the intensities of the peaks were improved with the temperature increasing.
Adsorption experiments were carried out using a batch method. In order to determine the F-adsorption capacity of Al-Mn-Ce oxide samples synthesized at different current densities, 0.05 g of the adsorbents was added to 20 ml of NaF solutions with concentrations of 25mgL-1 (Fig.4). The samples were shaken at 400 rpm and 25oC for 24 hours. After shaking, the sorbent from the solution was separated by filtration and the concentration of fluoride ions was measured by an F- selective electrode. The sorption capacity of the samples was evaluated according to the following expression [40-41]:
where qe (mg g-1) is the amount of F- ions removed per unit mass of an adsorbent, Ci and Ce (mg L−1) are F- ions concentrations in the liquid phase at t=0 and t=te respectively, V (L) is the volume of suspensions and w (g) is the mass of the adsorbent. The results show that the samples have approximately the same F- adsorption capacity and sample 1 (synthesized at 35 mA cm-2) has a slightly higher capacity. Therefore, sample 1 with nanowire morphology was selected for further studies.
In order to evaluate the adsorption kinetic, 0.05 g of sample 1 was suspended in20 ml of 25 mg g-1 NaF solution at 25 °C and 400 rpm for various intervals time ranging from 15 minutes to 24 h (Fig. 5).
Fig. 5 depicts the variation of qt (mg g−1) versus the contact time (h) of the F- adsorption by Al-Mn-Ce Oxide. The results indicated that the uptake of F- ions was quite rapid and the requested time (min) for achieving the highest capacity was about 200 min. In other words, more than 80 % of F- ions were uptake from the solution during the three hours initial contact times. The fitting of experimental data with two important kinetic models of pseudo-second-order and pseudo-first-order models shows that the pseudo-second-order is more in agreement with the obtained data compared with the pseudo-first-order model. The pseudo-first-order and pseudo-second-order kinetic models are given in Equations 11 and 12, respectively [40-41].
Where, qe and qt are the sorption capacity (mg/g) in equilibrium state and time t, respectively. The KI and KII are kinetic rate constants. In the pseudo-second-order model, the availability of the active sites on the sorbent for ions (sorbates) is regarded as the overall sorption rate-limiting factor. In other words, the uptake capacity of the sorbent is proportional to its occupied active sites number [42-43].
To find the effect of the initial concentration on the fluoride removal, 0.05 g of adsorbent was added to the 20 ml of NaF solution with the concentration of 25, 125, 250, and 500 mg g-1. The suspensions were shaken at 400 rpm for 24 h at 25 to 60 oC (Fig. 6.). The results indicated that the uptake of fluoride by the composite is first increased and then become constant with increasing the F- concentration. The maximum sorption capacity of 48 mg/g was found at 500 mg L-1 initial concentration which reflects the high ability of the synthesized composite for F-ions adsorption compared with the reported materials (Table 2).
Adsorption Mechanism and effect of solution pH
The adsorption mechanism of F- by the metal oxides is complicated. Based on the chemical composition, the functional groups of Al-Mn-Ce Oxide, and reports  the adsorption mechanism can be proposed in order to determine the potential application of the obtained adsorbent.
The net surface charge, due to the defects of lattice, and the surface hydroxyl groups  on Al-Mn-Ce Oxide are the active adsorption agents for the uptake (adsorption) of F- in solution, by the electrostatic interaction and ion-exchange (ligand or ion-exchange with the fluoride ions) following the equations. Due to the similarity of OH− and F− ionic radii and the higher coordination ability of F− compared with OH−, the coordination of Al-Ce-Mn oxide with F−is better than that of OH−(strong chemical bond with metal), therefore, the ligand-exchange would happen . Based on the EDX results the sample with a greater Ce value (the sample obtained at 35 mA cm-2) showed higher adsorption capacity, hence it seems that the Ce–OH was the preferential adsorption site for the fluorine (F-) adsorption by the ligand exchange mechanism. However, the role of Al3-OH and Mn3-OH with higher positive charges in fluorine (F-) adsorption by electrostatic attraction cannot be ignored. In the same condition, the pure oxide of MnO2, Al2O3, and CeO3 was synthesized and their fluorine (F-) adsorption capacity was evaluated. The results showed that the adsorption capacities of MnO2, Al2O3, and CeO3 are less than 15 mg g-1. This result cleared that the synergistic interaction between Al, Ce, and Mn in the Al-Ce-Mn composite makes the special and favorite structure a desirable surface with a high active adsorption site for the fluoride adsorption. In other words, the multivalent cations such as La3+, Ce3+, and Zr4+ dramatically change the surface properties of the composites and their affinity for fluoride. Since these metals are expensive, preferably, some cheaper metals such as Al, Fe, Mn, etc. are mixed with these metals to prepare the hybrid adsorbents.
In the adsorption, process pH is considered as an important factor that affects the adsorbent surface and sorbate ions as well as the competitor’s behavior in solution. The pH effect on F- adsorption was studied in 25 mg g-1 NaF solution at 25 °C by adjusting the solution pH with the NaOH or HNO3. As can be seen, the uptake capacity has little change at pH below 6 and it has a sharp decline with increasing solution pH. Based on the ligand exchange mechanism (Fig.6) in an acidic solution as a result of the protonation of hydroxyl groups convert them into good leaving groups for substitution viz. exchange of the F- ions. Also, in a low solution pH, the surface positive net charge of Al-Mn-Ce Oxide increases. Based on the electrostatic attraction mechanism, it leads to a stronger attraction of the F- ions and an increase in the adsorption capacity, in the long run. The F- uptake capacity is decreased with further increasing the pH values (˃6) which can be attributed to the competition between hydroxyl and fluoride ions for the active adsorption sites and decreasing of positive net charge of Al-Mn-Ce surface at a high pH .
The F- adsorption was described by Langmuir, Freundlich, and Temkin, isotherm models (Fig. 8).
Linear expression of Langmuir adsorption isotherm is as follows (Eq.13
Ce/qe =1/bK + Ce/b (13)
where qe is the uptake capacity (mg g-1), Ce is the F equilibrium concentration (mg L-1),
and K (L mg˗1) and b are the Langmuir constants related to the uptake capacity and adsorption energy respectively .
In the Freundlich isotherm model, the sorption mechanism is based on the multilayer location of the species on a heterogeneous surface with variable binding energy . The Freundlich expression is as follows:
where qe(mg g-1) is the uptake capacity, Ce(mg L-1) is the F- equilibrium concentration, KF (mg/g) and 1/n are Freundlich describing the quantity and quality of the adsorption, respectively.
The Temkin isotherm is based on the interactions between the sorbent and sorbate and linear decrease of the sorption heat of ions in the layer (than logarithmically) [64-66]. The model is expressed as:
qe= RT/b Ln(ATCe) (15)
qe= BLnAT + BLn Ce…….
where AT (L/g) is the equilibrium binding constant, bT is the Temkin isotherm constant, R(8.314J/mol/K) is the gas constant, T(K) is the temperature, B (J/mol) is a constant related to the sorption heat.
The results of the modeling with linearized Langmuir, Freundlich, Temkin models and their correlation coefficients are shown in Fig. 7. The correlation coefficient results show that the Langmuir model fitted the experimental data better than the others. The isotherm parameters for the Langmuir plot are given in Table 3. The maximum sorption capacity (qmax) of the Al–Mn-Ce estimated from the Langmuir model was 49.53 mg g−1 at 25 °C which was compatible with the experimental result (48 mg g-1).
Al-Mn-Ce oxide was synthesized for the first time through the electrochemical base generation method at three different current densities(5, 15, and 35 mAcm-2). The samples were characterized by XRD, SEM, and EDX techniques. The results indicated the chemical composition, uptake capacity, and morphology changes of samples with varying current densities. Three nanostructure mechanisms of the formation such as Growth control by surfactant and additives, Oriented attachment, and Scrolling of sheets were discussed. The adsorption of fluoride ions from an aqueous solution by the synthetic Al-Mn-Ce oxide was studied under different experimental conditions such as time, pH, and initial concentration. In the same condition, the pure oxide of MnO2, Al2O3, and CeO3 was synthesized and their fluorine (F-) adsorption capacity was evaluated. The results showed that the adsorption capacities of MnO2, Al2O3, and CeO3 are less than 15 mg g-1. The synthesized Al-Mn-Ce Oxide nanowire (synthesized at 35 mA cm-2) exhibited a high defluoridation capacity of 48 mg g−1. The adsorption mechanisms were conducted through the electrostatic attraction, as well as the ionic exchange/ligand exchange process. The hydroxyl groups and positive net charge due to the defect in the crystal structure of the Al-Mn-Ce oxide surface play important roles in the adsorption process. The F- adsorption kinetics and isotherms follow Langmuir and pseudo-second-order models, respectively. The findings revealed that Al-Mn-Ce oxide nanowire is a promising material for the removal of fluoride ions in relatively short contact time and a wide pH range.
CONFLICTS OF INTEREST
There are no conflicts to declare.