Data on the removal of fluoride from aqueous solutions using synthesized P/γ-Fe2O3 nanoparticles: A novel adsorbent

Graphical abstract

How data was acquired All adsorption experiments were done in batch mode. After the adsorption process, the residual fluoride concentrations were estimated. The initial and residual fluoride concentrations in the solutions were analyzed using a UV-visible recording spectrophotometer (Shimadzu Model, CE-1021-UK) at 570 nm. Fourier-transform infrared spectroscopy (FT-IR) was done on a JASCO 640 plus machine (in the range of 400-4000 cm À1 ) to determine the functional groups present in the adsorbent before and after fluoride adsorption. The pH of the solution was measured using a MIT65 pH meter.

Data format Raw and analyzed
Experimental factors The influence of pH, contact time, initial fluoride concentration and P/g-Fe 2 O 3 nanoparticles dosage on the adsorption process. Kinetic and isotherm parameters were also presented.

Experimental features
Fluoride removal from aqueous solution using P/g-Fe 2 O 3 nanoparticles. P/g-Fe 2 O 3 nanoparticle characterization data obtained from FTIR. Kinetic and isotherm modeling of the removal process.

Trial registration Not applicable
Ethics Not applicable

Protocol data
The presented data established that P/g-Fe 2 O 3 nanoparticles can be applied for the removal of fluoride with great efficiency. Data on the isotherm, kinetics, and effect of process variables were provided, which can be further explored for the design of a treatment plant for the treatment of fluoride-containing industrial effluents where a continuous removal is needed on a large scale. FTIR data for P/g-Fe 2 O 3 nanoparticles were also provided. The dataset will also serve as a reference material to any researcher in this field.

Data
High concentration of fluoride is toxic and causes digestive disorders, fluorosis, endocrine, thyroid and liver damages, and also decreases the growth hormone [1,2]. In addition, it influences the metabolism of some elements such as calcium and potassium [3]. Fluoride must be properly reduced before its discharge to the water bodies. Adsorption can be considered as an effective method for the removal of fluoride [4,5]. The applicability of P/g-Fe 2 O 3 nanoparticles for fluoride removal was reported. Fourier transform infrared (FTIR) on the P/g-Fe 2 O 3 nanoparticles is given in Fig. 1. Fig. 2 shows the schematic illustration for the synthesis of P/g-Fe 2 O 3 nanoparticles. The functional groups present in the P/g-Fe 2 O 3 nanoparticles before and after fluoride adsorption are given in Table 1. The estimated adsorption isotherm and kinetic parameters are presented in Table 2.

Adsorption experiments
The adsorption experiment was conducted at batch mode using the one-factor-at-a-time (OFAT) method, that is, keeping a factor constant and varying the other factors to get the optimum condition of each variable. At first, for the purpose of this study, a stock solution of fluoride was prepared with distilled water from which other fluoride concentrations were prepared. The stock solution of fluoride (concentration of 1000 mg/L) was made by dissolving 2.21 g NaF in 1000 mL distilled water. A known mass of adsorbent (P/g-Fe 2 O 3 nanoparticles) was added to 1 L of the water samples containing different concentrations of fluoride. The pH of the water sample was adjusted by adding 0.1 N HCl or NaOH solutions. The removal efficiency was determined by varying the different adsorption process parameters such as pH (2)(3)(4)(5)(6)(7)(8)(9)(10)(11), contact time (15-120 min), initial fluoride concentration (10-50 mg/L) and P/g-Fe 2 O 3 nanoparticles dosage (0.01-0.1 g/L). To create optimal conditions, the solutions were agitated using orbital shaker at a predetermined rate (150 rpm). After each experimental run, the solution was filtered and the filtrate was analyzed for the residual fluoride concentration. The initial and residual fluoride concentrations in the solutions were analyzed by a UV-vis recording spectrophotometer (Shimadzu Model: CE-1021-UK) at a wavelength of absorbance (l max ): 570 nm [5].

Data analysis
The removal efficiency, R (%) and amount of fluoride adsorbed on P/g-Fe 2 O 3 nanoparticles, q e (mg/ g) of the studied parameters were estimated based on the following formulas [6][7][8]: Where C 0 and C f are the initial and residual fluoride concentrations (mg/g), respectively.
Where C 0 and C e are the initial and final equilibrium liquid phase concentration of fluoride (mg/g), respectively. M is the weight of the nano adsorbent (g) and V is the volume of the solution (L).

Isotherm and kinetic modeling
An important physiochemical subject in terms of the evaluation of adsorption processes is the adsorption isotherm, which provides a relationship between the amount of fluoride adsorbed on the  solid phase and the concentration of fluoride in the solution when both phases are in equilibrium [9].
To analyze the experimental data and describe the equilibrium status of the adsorption between solid and liquid phases, the Langmuir, Freundlich, and Temkin isotherm models were used to fit the adsorption isotherm data. Several kinetic models have been applied to examine the controlling mechanisms of adsorption processes such as chemical reaction, diffusion control, and mass transfer [10]. Three kinetics models, namely pseudo-first-order, pseudo-second-order, and intraparticle diffusion models were used in this study to investigate the adsorption of fluoride on P/g-Fe 2 O 3 nanoparticles.

Langmuir isotherm
For the Langmuir model, it is assumed that adsorbates attach to certain and similar sites on the adsorbent's surface and the adsorption process occurs on the monolayer surface. The Langmuir equation can be rearranged to linear form for the convenience of plotting and determining the  isotherm constants, K L and q m by drawing a curve of l/q e versus 1/C e [11,12]: Where q e (mg/g) is the amount of fluoride adsorbed per specific amount of adsorbent, C e is the equilibrium concentration of the fluoride solution (mg/L), K L (L/mg) is Langmuir constant, and q m (mg/g) is the maximum amount of fluoride required to form a monolayer.

Freundlich isotherm
The Freundlich model is an empirical relationship between the parameters, q e, and C e . It is obtained by assuming a heterogeneous surface with nonuniform distribution of the adsorption sites on the adsorbent surface, and can be expressed by the following equation [13,14]: Where K f and 1/n are the Freundlich constants related to adsorption capacity and adsorption intensity, respectively. The Freundlich constants can be obtained by plotting a graph of Log q e versus Log C e based on the experimental data by applying the linear form of the Freundlich isotherm Eq. (4): Temkin isotherm In Temkin model, the surface adsorption theory was corrected considering possible reactions between the adsorbent and adsorbate. This model can be expressed as the following equation [15]: Where A T and B 1 are the Temkin constants. B 1 is related to the heat of adsorption and A T is the equilibrium binding constant.

Lagergren kinetic model
Adsorption kinetic models are used to examine the rate of adsorption process and the potential rate controlling step. The Lagergren (pseudo-first-order) rate equation is expressed as Eq. (7) [16,17]: Log q e À q t ð Þ ¼ Log q e ð Þ À k 1 2:303 t ð7Þ

Ho kinetic model
The Ho (pseudo-second-order) rate equation is given as [12,18]: Where q e (mg g À1 ) and q t (mg g À1 ) are the amounts of fluoride adsorbed at equilibrium and at time t, respectively, K 1 (min À1 ) is the pseudo-first-order rate constant of adsorption, and K 2 (g mg À1 min À1 ) is the pseudo-second-order rate constant.

Intraparticle diffusion
For the intraparticle diffusion model (Eq. (9)), c is the intercept (mg/g) and K pi is the slope. If intraparticle diffusion is involved in the adsorption process, then the plot of t 0.5 versus q t would result in a linear relationship, and the intraparticle diffusion would be the controlling step if this line passed through the origin (C = 0). When the plots do not pass through the origin (C 6 ¼ 0), this is indicative of some degree of boundary layer control and this further shows that the intraparticle diffusion is not the only rate controlling step, but also other processes may control the rate of adsorption [19,20].
Where q t (mg/g) is the amount of fluoride adsorbed at time t (min) and K pi (mg/g min) is the intraparticle diffusion model rate constant. The estimated adsorption isotherm and kinetic parameter are presented in Table 2. Fig. 6 shows the adsorption kinetic (Ho) plot for fluoride removal on P/g-Fe 2 O 3 nanoparticles. The removal of fluoride on P/g-Fe 2 O 3 nanoparticles followed the Ho kinetic model with a correlation coefficient (R 2 ) of 0.999 at 25 mg/L, suggesting that the rate-limiting step is a chemical adsorption process [21]. The isotherm data fitted into the Freundlich, Langmuir and Temkin isotherms but fitted more to the Langmuir isotherm, which indicates a monolayer adsorption on a homogeneous surface [14].

Funding sources
This paper is the result of the approved project at Zabol University of Medical Sciences.