The presence of fluoride ions in water poses a significant danger to human health. In Tanzania, where the Rift Valley passes, some people are impaired due to elevated levels of fluoride in water. The purpose of this study was to prepare thermally activated
Water is one of life’s fundamental needs on earth. Natural sources or anthropogenic activities such as mining, farming, and construction may contaminate it. Fluoride is one of the noxious and detrimental pollutants. The presence of high-level fluoride ions in water bodies, including several regions of Africa, Asia, and China, is a global concern. More than 30 million people are seriously affected by fluorosis, and another 100 million are exposed to it in China, according to recent reports [
Fluoride is a naturally occurring product present in rocks, geochemical deposits, and natural water systems and enters the food chain via drinking water or plants and cereals [
To tackle the issue of fluoride in water, many remediation strategies have been identified. Precipitation, ion exchange, reverse osmosis, electrodialysis, distillation, chemical reduction, nanofiltration, and electrocoagulation are all used in these techniques. These approaches face challenges in terms of planning and operating costs and requirements for qualified engineers and manpower, despite the high remediation performance. A certain degree of fluoride remediation capability has been shown by these methods, but some of them can only be used in a small pH range (5-6), and some of them are too costly to consider for full-scale water treatment [
The study of fluoride removal using natural, synthetic, and biomass materials such as activated alumina, fly ash, alum sludge, chitosan beads, red mud, zeolite, calcite, hydrated cement, attapulgite, and acid-treated spent bleaching earth has received considerable attention in recent years [
For the preparation of thermally activated biosorbents, the precursor,
In order to eliminate dust and other unwanted materials, small pieces of pericarp were washed and then treated with hot distilled water to remove color and dried at 100°C using an oven. To extract moisture and other volatile impurities, dried materials were kept in the furnace at 150°C for 24 hours. After that, 100 g of the materials was treated with 50 mL of concentrated H2SO4 and was carbonized at various temperatures of 450, 500, 550, and 600°C using an electric furnace [
The initial weight of the sample (5 g) was measured and taken in a petri dish. It was then heated in an oven at a temperature of 105°C for 2 hours. After heating, it was cooled in a desiccator. The final weight of the dried sample has now been measured. Using equation (
The sample’s initial weight (5 g) was weighed and taken in a crucible closed with a lid and combusted in a muffle furnace for 7 minutes at a temperature of 925°C. The crucible was then cooled in a desiccator, and the final weight of the sample being combusted was weighed. By using equation (
The 1 g biosorbent sample was transferred into a 10 mL measuring cylinder, and the particle volume was recorded. This sample was put in a 20 mL distilled water beaker and boiled for 5 minutes to replace the air in the sample. The content was superficially dried and weighed. The pore volume for the sample was provided by the increase in the weight of the sample divided by the water density. By dividing the pore volume of the particle by the total volume of the particle [
The surface area and pore size of the biosorbents were determined using an automated gas sorption analyzer, Quanta Chrome Nova Win version 11.03, by measuring nitrogen adsorption-desorption at 77.35 K. The area of the surface was measured according to the method of Brunauer-Emmet-Teller (BET).
Information about the degree of microporosity is given by the iodine number. It is a measure of the capacity of activated carbon to adsorb compounds of low molecular weight. A typical solution of iodine was prepared and stored in a light-resistant container by dissolving 2.7 g of I2 and 4.1 g of KI in 1 liter of deionized water. The standard solution of iodine was treated with the biosorbents under analysis, and the number of iodines was calculated according to the Das et al. [
The methylene blue number is described as the maximum quantity of dye adsorbed by 1.0 g of adsorbent. It is intended for the determination of the degree of adsorbent mesoporosity. The degree of mesoporosity means that medium-sized molecules such as methylene blue dye may be adsorbed by the adsorbent. At room temperature, 10.0 g of activated carbon was mixed with 10.0 mL of methylene blue solution at varying concentrations (10, 25, 50, 100, 250, 500, and 1000 mg/L) for 24 hours. Using a UV/visible spectrophotometer, the remaining concentration of methylene blue was analyzed. Using equation (
The morphology, pore size, area, and volume of TAADFPs were visualized using a scanning electron microscope (SEM) (SSX-550, Shimadzu, Japan). By scanning through an electron beam, SEM enables the visualization of the surface features of a solid sample by images [
To assess the cation and anion exchange power of sorbents, Boehm titration was used [
The point of zero charge, pHpzc, quantifies the acidic or basic character of the carbon. The carbon surface is positively charged at
The Fourier transform infrared (FT-IR) spectrophotometer was used, as defined by Adewuyi and Pereira [
The fluoride ion-selective electrode system (Hach, USA) was calibrated using five standards of fluoride solutions diluted from a stock solution in the range of 1 to 50 mg/L to ensure good performance of the instrument and create a linear equation for the determination of unknown concentrations. The logarithm graph of fluoride ion concentrations (mg/L) against potential (mV) showed a straight line with slopes ranging from -54.94 to -55.73 mV, within the theoretical range of -50 to -60 mV at 25°C [
Response surface methodology (RSM), which is an effective statistical technique for multiparameter optimization, was used in this study to obtain a series of experiments designed to obtain an optimal response [
According to the experimental conditions set by the program, batch experiments were carried out where 100 mL of simulated water was mixed with each adsorbent and agitated at room temperature at 200 rpm on a mechanical shaker. Then, a Whatman No. 42 filter paper was used to filter the mixture. The fluoride concentrations were measured using the fluoride ion-selective electrode potentiometric method. The concentration of fluoride ion adsorbed on the adsorbent was measured as the difference between the initial solution concentration and the postsorption solution concentration. The adsorption efficiency (%) and the adsorption capacity (mg adsorbate adsorbed/g of adsorbent) for each experiment were determined using equations (
The isotherms of Langmuir and Freundlich adsorption were studied at a pH of
Kinetic studies were carried out at different time intervals of 10-50 minutes by adjusting the pH at
To regenerate the adsorbent, desorption experiments were carried out using NaCl (0.1 M) in the desorbing media-distilled water. After the adsorption process, the adsorbent was separated and washed several times with deionized water to remove any unadsorbed pollutant ions, and then, desorption studies were performed. The pollutant-loaded biosorbents were transferred and agitated with 1 L of eluant solution, NaCl in the water shaker bath for 1 hour. It was again filtered, and then, the concentrations of pollutant ions desorbed in the filtrate were determined. The eluted biosorbents were washed several times with distilled water to remove excess salt and acid. The biosorbents regenerated were then used in the next biosorption process. Three cycles of biosorption-desorption were conducted, and the experiments were operated under the same conditions during the entire process. For each cycle, the adsorption efficiency of the regenerated adsorbent was determined.
The moisture contents of TAADFPs activated at 450, 500, 550, and 600°C were found as 3.5, 3.3, 3.1, and 2.8%, respectively. It decreased as the activation temperature increased. This is because the dehydration rate of the activated carbon increased with an increase in activation temperature. Any porous material tends to absorb moisture. The moisture content of all the adsorbents was near normal and comparable with the values reported elsewhere [
Volatile matter is due to the presence of organic compounds present in the raw material. This gives information on the suitability of the precursor for carbon preparation. The volatile matter content of TAADFPs activated at 450, 500, 550, and 600°C was found as 11.2, 10.3, 9.2, and 8.5%, respectively. The volatile matter content of activated carbon decreased with an increase in activation temperature. When activation temperature was increasing, devolatilization was increased, and hence, the prepared activated carbon had a low volatile matter content. This implies that most of the volatile organic matter escaped during the preparation of the materials, and it is an indication that the activated carbons were well devolatilized. Research by Veena Devi et al. [
The pore volumes of TAADFPs activated at 450, 500, 550, and 600°C were found as 3.0, 3.8, 4.0, and 4.8 cm3/g, respectively. The pore volumes increase with an increase in activation temperature. This is due to the increase in the rate of evaporation and devolatilization of small molecules leaving behind pores. The recommended pore volume ranged from 2 to 5 cm3/g [
Porosity is the ratio of a material’s pore volume to the bulk volume (adsorbent). The number of pores present in the adsorbents is defined by porosity. Therefore, porosity increases the adsorption power of the adsorbent [
The surface areas of TAADFPs activated at 450, 500, 550, and 600°C were 385.44, 399.27, 445.71, and 447.70 m2/g, respectively. All the adsorbents had a considerably large surface area and thus had a high adsorption potential on the surface. Adsorbents with a very high surface area of 200-300 m2/g have a significant number of active sites including functional groups to promote the adsorption of different anions and cations from water [
The pore diameters of TAADFPs activated at 450, 500, 550, and 600°C were 0.3055, 3.0341, 3.0375, and 3.0471 nm, respectively. A pore diameter of less than 2 nm is known to have microporosity, and greater than 2 nm is mesoporous, according to the IUPAC classification [
It is a measure of the adsorbent’s ability to adsorb low-molecular-weight compounds as the iodine molecule is relatively small and so provides a measure of the surface area available to small adsorbates such as fluoride. The iodine number gives information about the degree of microporosity. Generally, the higher the iodine number, the greater the sorption capacity. The iodine numbers of TAADFPs activated at 450, 500, 550, and 600°C were found as 515, 590, 620, and 650 mg/g, respectively. It increased from low activation temperature to high activation temperature. This is due to an increase in dehydration and devolatilization rate as the result of an increase in processing temperature. The values obtained in this study are within the reported values (440–700 m2/g) by Veena Devi et al. [
Large methylene blue number values suggest that mesopores are possessed by the materials. The degree of mesoporosity is an indicator that medium-sized molecules such as methylene blue dye in mesopores can be adsorbed by the adsorbent [
The morphology of TAADFPs activated at 450, 500, 550, and 600°C can be seen in micrographs obtained from a scanning electron microscope (SEM) (Figure
Scanning electron micrographs of TAADFPs activated at (a) 450°C, (b) 500°C, (c) 550°C, and (d) 600°C.
The reduction of OH- ions in the solution was very significant in this study. The anion exchange capacity of TAADFPs activated at 450, 500, 550, and 600°C was 91, 94, 96, and 99%, respectively. All the adsorbents can efficiently adsorb anions. The reduction of OH- ions in the solution was increased as the processing temperature increased. This is because exposure of functional groups and the development of pores increased as temperature increased due to complete combustion, dehydration, and devolatilization. Also, adsorption of OH- was mainly attributed to ion exchange and electrostatic attraction due to the presence of exchangeable chloride ions of adsorbent and positively charged adsorbent surface, respectively [
pH at point of zero charge was studied in the pH range of 2 to 12 with an interval of 2. The pH at which the line intersects the axis corresponding to
pH at point of zero charge for TAADFPs activated at (a) 450°C, (b) 500°C, (c) 550°C, and (d) 600°C.
The functional groups and the effect of the activation temperature on the surface functional groups of TAADFPs were analyzed using the FT-IR spectrophotometer. The FT-IR spectral analysis showed that many functional groups for the adsorption process are available on the surface of adsorbents.
All TAADFP spectra showed similar peaks (Figure
The FT-IR spectra for raw and TAADFPs activated at different temperatures.
To get optimal conditions for optimum adsorption, the response surface methodology (RSM) with Design-Expert software suggested 21 experiments with different sets of operational parameters (Table
Different experiments suggested by Design-Expert software for the optimization of conditions of pH, initial concentration (IC), contact time (CT), and adsorbent dose (AD) for defluoridation using TAADFPs.
Run | pH | IC (mg/L) | CT (min) | AD (g) | Adsorption efficiency (%) for TAADFPs activated at different temperatures | |||
---|---|---|---|---|---|---|---|---|
450°C | 500°C | 550°C | 600°C | |||||
1 | 12.00 | 5.00 | 30.00 | 10.00 | 57.12 | 78.72 | 70.55 | 72.73 |
2 | 2.00 | 50.00 | 120.00 | 10.00 | 91.72 | 93.97 | 89.70 | 93.17 |
3 | 7.00 | 27.50 | 75.00 | 5.50 | 92.99 | 94.47 | 84.43 | 89.16 |
4 | 3.00 | 50.00 | 25.00 | 5.50 | 71.24 | 78.79 | 70.37 | 78.80 |
5 | 6.00 | 5.00 | 30.00 | 1.00 | 68.63 | 67.68 | 79.19 | 91.94 |
6 | 4.00 | 27.50 | 75.00 | 5.50 | 92.37 | 93.15 | 91.27 | 91.57 |
7 | 2.00 | 27.50 | 75.00 | 5.50 | 67.61 | 92.57 | 90.52 | 98.36 |
8 | 5.00 | 5.00 | 120.00 | 10.00 | 63.85 | 91.22 | 91.70 | 89.60 |
9 | 10.00 | 50.00 | 120.00 | 1.00 | 68.26 | 78.71 | 53.85 | 58.91 |
10 | 5.00 | 27.50 | 30.00 | 5.50 | 92.68 | 93.96 | 77.42 | 96.36 |
11 | 8.00 | 27.50 | 30.00 | 5.50 | 91.01 | 93.70 | 77.39 | 88.70 |
12 | 4.00 | 27.50 | 25.00 | 5.50 | 88.91 | 93.17 | 86.22 | 87.16 |
13 | 6.00 | 50.00 | 25.00 | 1.00 | 79.35 | 78.48 | 71.95 | 76.94 |
14 | 3.00 | 27.50 | 120.00 | 10.00 | 95.55 | 96.50 | 96.88 | 94.67 |
15 | 6.00 | 27.50 | 75.00 | 5.50 | 94.29 | 56.62 | 91.94 | 93.44 |
16 | 2.00 | 50.00 | 30.00 | 10.00 | 88.41 | 91.92 | 80.05 | 64.94 |
17 | 12.00 | 5.00 | 30.00 | 5.50 | 53.98 | 50.94 | 75.45 | 59.49 |
18 | 10.00 | 27.50 | 75.00 | 5.50 | 87.46 | 93.15 | 85.05 | 93.15 |
19 | 6.00 | 27.50 | 75.00 | 1.00 | 46.34 | 75.87 | 63.85 | 71.99 |
20 | 4.00 | 27.50 | 120.00 | 5.50 | 93.26 | 93.17 | 97.65 | 94.67 |
21 | 8.00 | 5.00 | 120.00 | 1.00 | 63.02 | 75.92 | 62.96 | 71.54 |
To optimize the conditions for optimum adsorption performance, twenty-one experiments were performed with different sets of parameters obtained from response surface methodology (RSM). At 600°C, TAADFP demonstrated the highest efficacy of adsorption (98.36 percent) at pH 2, initial concentration of 27.5 mg/L, contact time of 75 minutes, and adsorbent dose of 5.5 g, respectively (Table
Figure
Mutual effect of pH and contact time on the adsorption efficiency of TAADFPs activated at (a) 450°C, (b) 500°C, (c) 550°C, and (d) 600°C.
The percentage of fluoride removal decreased with rising pH for all adsorbents. This is due to a rise in OH- concentration, which competed for adsorption with F-. The surfaces of the adsorbents were negatively charged above pHpzc (
In this study, the contact time ranged from 30 to 120 minutes. Initially, adsorption increased from 30 and 75 minutes between two phases, that is, adsorbent and solution containing adsorbate ions (Figure
The entire adsorption process is a function of the initial concentration of the pollutant ions, which makes it an important factor to be determined for effective sorption. The effect of initial concentration was investigated from 5 to 50 mg/L. The removal efficiency of adsorbents increased with increasing initial fluoride concentrations (Figure
Mutual effect of adsorbent dose and initial concentration on the adsorption efficiency of TAADFPs activated at (a) 450°C, (b) 500°C, (c) 550°C, and (d) 600°C.
The adsorption efficiency increased with an increase in the adsorbent dose (Figure
There was a consideration of adsorption isotherm studies to describe the nature of the biosorbent surface and the affinity of the adsorbent at a fixed operating condition. Both Langmuir and Freundlich adsorption isotherm models were studied.
Langmuir isotherm is based on the assumption that a point of valance exists on the surface of the adsorbent and that each of these sites is capable of adsorbing one molecule. It is assumed that the adsorption sites have equal affinities for molecules of adsorbate and that the presence of adsorbed molecules at one site will not affect the adsorption of molecules at an adjacent site [
where
A plot of
Langmuir adsorption isotherms for fluoride adsorption by TAADFPs activated at (a) 450°C, (b) 500°C, (c) 550°C, and (d) 600°C.
The isotherm of Freundlich was interpreted as sorption to heterogeneous surfaces or surfaces that support varied affinity sites. Freundlich isotherm assumes unlimited sorption sites which correlate better with the heterogeneous surface of the adsorbent media. It is also believed that it first occupied the stronger binding sites and that with increasing degree of site occupation the binding strength decreases. The energy of fluoride ion binding to a site on an adsorbent in this model depends on whether the neighboring sites are already occupied or not. The expression for Freundlich adsorption isotherm is represented by the following linearized equation:
The plot of
Freundlich adsorption isotherms for fluoride adsorption by (a) TAADFP at 450°C, (b) TAADFP at 500°C, (c) TAADFP at 550°C, and (d) TAADFP at 600°C.
The adsorption capacities
To illustrate the mechanism of fluoride adsorption, kinetic experiments have been performed. To research the adsorption process, pseudo-first- and pseudo-second-order kinetics have been used.
The pseudo-first-order model is described by the linear equation:
Pseudo-first-order plots for fluoride adsorption by the adsorbents (a) TAADFP at 450°C, (b) TAADFP at 500°C, (c) TAADFP at 550°C, and (d) TAADFP at 600°C.
The pseudo-second-order model is described by the linear equation:
Pseudo-second-order plots for fluoride adsorption by the adsorbents (a) TAADFP at 450°C, (b) TAADFP at 500°C, (c) TAADFP at 550°C, and (d) TAADFP at 600°C.
The mean value of the correlation coefficient
Desorption experiments were conducted to evaluate the reusability of the spent adsorbents. According to Table
Desorption and resorption of fluoride by TAADFPs activated at 450, 500, 550, and 600°C using distilled water and 0.1 M NaCl (pH 5, initial fluoride concentration 5 mg/L, contact time 1 hour at room temperature).
TAADFPs activated at (°C) | First cycle | Second cycle | Third cycle | ||||||
---|---|---|---|---|---|---|---|---|---|
F- resorbed (mg/L) | F- desorbed (mg/L) | Percent removal (%) | F- resorbed (mg/L) | F- desorbed (mg/L) | Percent removal (%) | F- resorbed (mg/L) | F- desorbed (mg/L) | Percent removal (%) | |
450 | 4.31 | 3.92 | 86 | 4.25 | 3.80 | 85 | 3.52 | 3.10 | 70 |
500 | 4.34 | 3.94 | 87 | 4.32 | 3.55 | 86 | 4.14 | 3.45 | 82 |
550 | 4.46 | 4.21 | 89 | 4.39 | 3.85 | 88 | 4.25 | 3.64 | 85 |
600 | 4.58 | 4.32 | 92 | 4.35 | 4.13 | 87 | 4.26 | 3.95 | 85 |
The adsorbents exhibited good physical and chemical characteristics to act as biosorbents as depicted by SEM, BET, and IR analysis. The maximum removal efficiencies of TAADFPs activated at 450, 500, 550, and 600°C were 95.55, 96.50, 97.65, and 98.36%, respectively, which means that the adsorbents were effective for the removal of fluoride. All the TAADFPs researched in the present study followed the Freundlich isotherm model and second-order kinetics for the removal of fluoride. The TAADFPs examined in this study showed promising properties as a low-cost and effective adsorbent for the removal of fluoride from water.
All the data used to support the findings of this study is included within the article.
The authors declare that there is no conflict of interest with respect to the research, authorship, and/or publication of this article.