Data for the synthesis, characterization, and use of xerogels as adsorbents for the removal of fluoride and bromide in aqueous phase

Groundwater with high fluoride concentrations has been recognized as one of the serious concerns worldwide. Besides, the fluoride released into the groundwater by slow dissolution of fluoride-containing rocks, various industries also contribute to fluoride pollution [1]. Excess intake of fluoride leads to various health problems such as dental and skeletal fluorosis, cancer, infertility, brain damage, thyroid diseases, etc. [2]. On the other hand, bromide is naturally present in surface and groundwater sources. However, during the chlorination process, bromide can be oxidized to HOBr, which can react with natural organic matter in water to form brominated organic disinfection byproducts, which are very harmful to human health [3]. Among various methods for water treatment, the adsorption process has been widely used and seems to be an efficient and attractive method for the removal of many contaminants in water, such as anions, in terms of cost, simplicity of design, and operation [4], [5]. In the past years, xerogels and carbon xerogels, a new type of adsorbents, which are synthesized by the sol-gel polycondensation of resorcinol and formaldehyde, have gained attention due to their moldable texture and chemical properties [6]. Moreover, melamine addition in resorcinol and formaldehyde xerogels adds basic groups on its surface, favouring Lewis acid-base interactions between xerogels and other components by adsorption [7]. In this data article, the synthesis of three resorcinol-formaldehyde (R/F) xerogels with an increasing amount of melamine (M) was carried out by colloidal polymerization (molar ratios of M/R = 0.5, M/R = 1.0, and M/R = 2.0). Additionally, samples of M/R = 0.5 xerogel were carbonized at 400, 450, and 550 °C under an inert atmosphere to increase their specific area. Organic and carbon xerogels obtained were characterized by FTIR, TGA, SEM, Physisorption of N2, and the pH at the point of zero charge (pHPZC). All organic xerogels were also tested as adsorbents on the removal of fluoride and bromide ions from aqueous phase. The Freundlich, Langmuir, and Radke-Prausnitz isotherm models were applied to interpret the experimental data from adsorption equilibrium. Additionally, the data of the mass of the xerogel needed to remove fluoride and bromide from groundwater and fulfill the maximum concentration levels are also included.

Dataset link: Data for the synthesis, characterization, and use of xerogels as adsorbents for the removal of fluoride and bromide in aqueous phase (Original data) a b s t r a c t Groundwater with high fluoride concentrations has been recognized as one of the serious concerns worldwide. Besides, the fluoride released into the groundwater by slow dissolution of fluoride-containing rocks, various industries also contribute to fluoride pollution [1] . Excess intake of fluoride leads to various health problems such as dental and skeletal fluorosis, cancer, infertility, brain damage, thyroid diseases, etc. [2] . On the other hand, bromide is naturally present in surface and groundwater sources. However, during the chlorination process, bromide can be oxidized to HOBr, which can react with natural Keywords: Xerogels Melamine Colloidal polymerization Fluoride and bromide ions Adsorption organic matter in water to form brominated organic disinfection byproducts, which are very harmful to human health [3] . Among various methods for water treatment, the adsorption process has been widely used and seems to be an efficient and attractive method for the removal of many contaminants in water, such as anions, in terms of cost, simplicity of design, and operation [4,5] . In the past years, xerogels and carbon xerogels, a new type of adsorbents, which are synthesized by the sol-gel polycondensation of resorcinol and formaldehyde, have gained attention due to their moldable texture and chemical properties [6] . Moreover, melamine addition in resorcinol and formaldehyde xerogels adds basic groups on its surface, favouring Lewis acid-base interactions between xerogels and other components by adsorption [7] . In this data article, the synthesis of three resorcinolformaldehyde (R/F) xerogels with an increasing amount of melamine (M) was carried out by colloidal polymerization (molar ratios of M/R = 0.5, M/R = 1.0, and M/R = 2.0). Additionally, samples of M/R = 0.5 xerogel were carbonized at 400, 450, and 550 °C under an inert atmosphere to increase their specific area. Organic and carbon xerogels obtained were characterized by FTIR, TGA, SEM, Physisorption of N 2 , and the pH at the point of zero charge (pH PZC ). All organic xerogels were also tested as adsorbents on the removal of fluoride and bromide ions from aqueous phase. The Freundlich, Langmuir, and Radke-Prausnitz isotherm models were applied to interpret the experimental data from adsorption equilibrium. Additionally, the data of the mass of the xerogel needed to remove fluoride and bromide from groundwater and fulfill the maximum concentration levels are also included.
© The textural properties such as specific area, pore volume, and average pore diameter were determined using adsorption/desorption isotherms of N 2 at 77 K (Micromeritics, ASAP 2020); specific area was obtained by using Brunauer-Emmett-Teller method (BET), and pH PZC . The adsorption equilibrium experiments were carried out in batch mode at pH = 5 and room temperature (25 ± 1 °C). The pH was monitored with a digital pH meter and adjusted by adding drops of 0.01 or 0.1 N NaOH or HNO 3 solutions.
( continued on next page ) The experimental adsorption equilibrium data for fluoride and bromide were analyzed using the adsorption isotherm models of Freundlich, Langmuir, and Radke-Prausnitz. Data format Raw Analyzed Parameters for data collection Xerogels were synthesized using resorcinol (R)-formaldehyde (F) and melamine (M) by colloidal polymerization with the following molar ratios of M/R = 0.5, M/R = 1.0, and M/R = 2.0. Also, samples of M/R = 0.5 xerogel were carbonized at 400, 450, and 550 °C under an inert atmosphere. Organic and carbon xerogels obtained were characterized by FTIR, TGA, SEM, Physisorption of N 2 , and the pH at the point of zero charge (pH PZC ). All xerogels were also tested as adsorbents on the removal of fluoride and bromide ions from aqueous phase. The mass of the xerogel needed to remove fluoride and bromide from groundwater and fulfill the maximum concentration levels was estimated. Description of data collection Xerogels were synthesized using the methodology proposed by Muehlemann et al. [9] . The xerogels M/R = 0.5 were carbonized in a horizontal tubular furnace Carbolite model 12/65/550 under a N 2 flow at 400, 450, and 550 °C for 2 h. The equilibrium adsorption experiments were carried out in a batch adsorber using 0.02 g of carbonized xerogel, 50 mL of fluoride, or bromide solution with different initial concentrations ranging from 10 to 100 mg L −1 .
The experiments were carried out at constant pH. The mass of fluoride or bromide adsorbed per gram of adsorbent (q), was obtained using a mass balance equation. The adsorption equilibrium data were fitted by Langmuir, Freundlich and Prausnitz-Radke isotherm models.

Value of the Data
• The data provided is valuable to develop xerogels of Resorcinol/Formaldehyde/Melamine by a simple method of colloidal polymerization. • Melamine addition increases the basic groups content, which improves fluoride and bromide ions adsorption onto xerogels. • These data show that fluoride and bromide adsorption on xerogels was not mainly dependent on the textural properties of adsorbents.  Table 1 exhibits the textural properties (specific area, total pore volume, and mean pore diameter) and pH PZC of xerogels and carbon xerogels samples.          a q = K f C 1 / n e K f is the Freundlich constant, n is the Freundlich constant (mg/g(L/mg) 1/n ); q is the amount of bromide or fluoride adsorbed per gram of adsorbent (mg/g), C e is the equilibrium bromide or fluoride concentration (mg/L), q m is the maximum adsorption capacity (mg/g), K L is the Langmuir constant (L/mg), A is the constant of the Radke-Prausnitz (L/g); B is the constant of the Radke-Prausnitz (L β /mg β ); β is the constant of the Radke-Prausnitz.%D is the average absolute percentage deviation (%), N is the number of experimental data points, q iexp is the mass of the fluoride or bromide adsorbed at equilibrium, and q ipred is the mass of the fluoride or bromide adsorbed at equilibrium predicted. b q = q m K L C e 1+ K L C e .  Tables 2 and 3 , respectively. Fig. 12 shows the adsorption isotherm of bromide from an aqueous solution on xerogels at pH = 5.0 and    Table 4 . All raw data associated with the figures in this work can be found in the supplementary material. All related primary data are deposited on Mendeley Data ( https://data.mendeley.com/ datasets/gvjd33sw57/2 ). The raw data associated with the Figs. 4 , 6-13 , 15 and 16 can be consulted at Dataset 1 to Dataset 11, respectively.

Materials
All chemicals used (Melamine, Formaldehyde (37%), Resorcinol, NaF, NaBr, NaOH, and HNO 3 ) were analytical grade and supplied from CTR Scientific. Stock solutions and a calibration curve of fluoride and bromide were prepared by dissolving appropriate quantities of NaF and NaBr, respectively, in deionized water.

Synthesis of xerogels
Xerogels were synthesized using the methodology proposed by Muehlemann et al. [9] . Xerogels were firstly prepared by dissolving 4 g of melamine (M) in 20 mL of deionized water (W) and 15 mL of formaldehyde (F) in a glass flask under constant stirring for 5 min at 55 °C. Then, 10 mL of 0.5 M NaOH solution was added to the M -F -W mixture and maintained under agitation until a uniform transparent solution was obtained, next 3 mL of 37% HCl solution and 7 g of resorcinol were added, agitation was maintained for 1 more min; finally, the mixture was poured into a Petri dish glass which was placed inside an oven for 48 h at 55 °C. The molar ratio was fixed to M/R = 0.5. Additionally, two more xerogels were prepared with the same procedure, but increasing the molar ratio to M/R = 1.0 and M/R = 2.0, respectively, by adjusting the amounts of melamine, resorcinol, HCl but maintaining the 15 mL of formaldehyde.
Lastly, xerogel M/R = 0.5 was carbonized in a horizontal tubular furnace Carbolite model 12/65/550 under a N 2 flow of 50 mL min −1 at 2 °C min-1 in the range of 55 to 115 °C, once the temperature of 115 °C was reached, it was kept for 30 min, then the temperature was increased to 400, 450 and 550 °C for 2 h on each temperature to finally obtain M/R = 0.5 400 °C, M/R = 0.5 450 °C, and M/R = 0.5 550 °C samples.

Adsorption experiments
The equilibrium adsorption experiments were carried out in a batch adsorber as describe elsewhere [10] with the following procedure: 0.02 g of carbonized xerogel sample was added to 50 mL of fluoride or bromide solution with different initial concentrations ranging from 10 to 100 mg L −1 . The adsorber was partially immersed in a thermostatic water bath. The carbonized xerogel sample and the solution were kept in contact until the equilibrium was reached; previous tests demonstrated that the equilibrium was reached in 7 days. The experiments were carried out at constant pH; therefore, the pH was monitored and adjusted by adding few drops of 0.01, 0.1N NaOH, and HNO 3 solutions. The mass of fluoride or bromide adsorbed per gram of adsorbent, (q), was obtained by using the following mathematical expression: Where: C 0 , C f , and C i represent the initial concentration, final concentration, and the concentration of sample i of fluoride or bromide solution (mg/L); m is the mass of the adsorbent, (g); N is the number of samples; q is the mass of fluoride or bromide adsorbed per gram of adsorbent, (mg/g); V 0 , V f , V i , and V a represent the initial volume, the final volume, the volume of sample i, and the total volume added of NaOH and HNO 3 solutions to adjust pH, (L).

Measurement concentrations
The bromide and fluoride concentration in aqueous solution was measured by a potentiometric method with a bromide or fluoride ion-selective electrode. The method required a calibration curve of seven standard solutions (concentrations ranging from 0.6 to 14 mg/L).

Declaration of Competing Interest
The authors declare that there is no conflict of interest.

Data Availability
Data for the synthesis, characterization, and use of xerogels as adsorbents for the removal of fluoride and bromide in aqueous phase (Original data) (Mendeley Data).