Exposure to fluoride beyond the recommended level for longer duration causes both dental and skeletal fluorosis. Thus, the development of cost-effective, locally available, and environmentally benign adsorbents for fluoride removal from contaminated water sources is absolutely required. In the present study, diatomaceous earth (diatomite) locally available in Ethiopia, modified by treating it with an aluminum hydroxide solution, was used as an adsorbent for fluoride removal from aqueous solutions. Adsorption experiments were carried out by using batch contact method. The adsorbent was characterized using FT-IR spectroscopy. Effects of different parameters affecting efficiency of fluoride removal such as adsorbent dose, contact time, initial fluoride concentration, and pH were investigated and optimized. The optimum adsorbent dose, contact time, initial fluoride concentration, and pH values were 25 g/L, 180 min, 10 mg/L, and 6.7, respectively. The performance of the adsorbent was also tested under optimum conditions using groundwater samples taken from Hawassa and Ziway. Langmuir and Freundlich isotherm models were applied to describe the equilibrium data. Compared to Langmuir isotherm (
Water is a source of life, a fundamental requirement for health and main need for industrialization. It is essential for all forms of growth and development: humans, animals, and plants. However, the suitability of water for a specific purpose can be affected by other substances dissolved or suspended in it. Contamination of drinking water by fluoride is one such example. In addition to arsenic and nitrate, which cause large-scale health problems, fluoride is classified as one of the contaminants of water for human consumption by the World Health Organization (WHO) [
Fluoride contamination of groundwater by natural as well as anthropogenic sources is a major problem worldwide, imposing a serious threat to human health. Water contamination by fluoride from industrial activities includes effluent discharge, fertilizers and pesticides, fluorosilicone and fluorocarbon polymer synthesis, coke manufacturing, glass and ceramic manufacturing, electronics manufacturing, electroplating operations, steel and aluminum manufacturing, metal etching (with hydrofluoric acid), and wood preservatives [
Fluoride enters into the human body through a variety of sources like water, food, air, medicine, and cosmetics. Among these, drinking water is the most common source which makes fluoride available to human beings [
Dental fluorosis is the most common manifestation of chronic use of high-fluoride water and is characterized by discoloured, blackened, mottled, or chalky-white teeth. Skeletal fluorosis occurs over long-time consumption of drinking water with >4 mg/L fluoride during adolescence which may disrupt the mineralization of bones leading to severe and permanent bone and joint deformations [
Fluoride contamination of groundwater and related health hazards are a worldwide problem, including various countries in Africa, Asia, and Europe as well as the USA and Australia. China and India are among the most affected, and other countries such as Ethiopia, Kenya, Ghana, and Tanzania have serious problems related to fluoride contamination [
Fluorosis is a disease for which no medical treatment exists and considered as crippling disease, and prevention is the only solution for this menacing problem. Fluoride poisoning (fluorosis) can be prevented or minimized by using alternative water sources (like surface water, low-fluoride groundwater, and rain water), increasing the nutritional status of the population at risk (adequate intake of calcium reduces the risk of dental fluorosis during the childhood), and by removing excessive fluoride from drinking water. Defluoridation of drinking water appears to be a simpler practical solution to prevent the adverse effects of fluoride. Hence, the development of defluoridation technologies, preferably low-cost and environmentally friendly, capable of reducing the fluoride concentration below the limit established by the WHO is of paramount importance [
Different technologies have been used for the removal of fluoride from drinking water including precipitation/coagulation, membrane-based processes, ion-exchange methods, and adsorption methods. Lime and alum are used to form insoluble precipitates with fluoride in precipitation and flocculation process. The fluoride concentration of the treated solution still remains about 8.0 mg/L due to the solubility of the formed precipitate. Chemical precipitation may also produce large amount of sludge [
Adsorption is the preferred technique for defluoridation at community and household levels in rural areas because of its low cost and ease of operation, high efficiency, easy accessibility, environmental benignity, and needless of operational skill and electric power to run, and since adsorbents can in principle be reused and recycled making it ideal for use in less-developed rural areas. It has the added advantage that it can be applied to a decentralized water supply system. The availability of different adsorbents in large amounts and low costs make them potential candidates for the defluoridation in remote areas [
Diatomites (diatomaceous earth) are siliceous sedimentary rocks composed mainly of amorphous hydrated or opaline silica (SiO2·
Despite the unique combination of physical and chemical properties of diatomite and its abundant availability in Ethiopia, its application for fluoride removal from drinking water in Ethiopia has not been investigated properly. The effectiveness of adsorption techniques is greatly dependent on the physicochemical properties of the adsorbent. Raw diatomaceous earth (DE) has a low fluoride removal potential. The highest percent fluoride removal at optimum adsorption conditions is between 23.4% and 25.6% for 8 mg/L fluoride at pH 2, contact time of 30 min, solid-liquid ratio of 0.4 g/50 mL, and shaking speed of 200 rpm [
A 1000 mg/L stock solution of sodium fluoride (NaF) was prepared by dissolving 2.21 g of anhydrous sodium fluoride (99.0% NaF, BDH Chemicals Ltd, England) in distilled water in a 1 L volumetric flask and diluting to the mark. Other standard fluoride solutions of the required concentrations for calibrating the fluoride ion-selective electrode (FISE) were prepared by serial dilution of the stock solution with distilled water.
Calibration and determination were carried out by addition of total ionic strength adjustment buffer (TISAB). The TISAB was prepared following a recommended procedure by dissolving 57 mL glacial acetic acid (100%, Sigma-Aldrich Laborchemikalien, Germany), 58 g sodium chloride (Oxford Laboratory, Mumbai, India), 7 g of sodium citrate (BDH Chemicals, England), and 2 g of EDTA (Scharlau Chemie S. A., Barcelona, Spain) in 500 mL distilled water; and then the pH was adjusted to 5.3 with 5 M sodium hydroxide (Scharlau Chemie S. A., Barcelona, Spain) and then made up to 1000 mL in a volumetric flask with distilled water [
The DE used for the study was collected in polyethylene plastic bags from Bedele Brewery, Oromia Regional State, Ethiopia. The material was washed with distilled water to remove dirt and dried under sun. The coating of aluminum hydroxide onto diatomite was carried out according to the method used by Wang and Peng [
An electronic balance (Adam Equipment, Model WL 3000, UK), with precision of 0.0001 g was used for weighing adsorbents and chemicals for solution preparation. An oven (Digit heat, J. P. Selecta, Spain) was used for drying the adsorbent and glass wares during analysis. A pH meter (HANNA instrument, HI 9025, Singapore) equipped with a pH glass electrode was used to measure the pH values of sample solutions. A pH/ISE meter (Orion model, EA 940 Expandable Ion Analyzer, USA) equipped with a combination fluoride ion-selective electrode (Orion Model 96-09, USA) was employed for the determination of fluoride in the samples and standards solutions. Spectrum 65 FT-IR (Spectrum 65 FT-IR spectrometer (PerkinElmer, USA) was used to record IR spectra.
To evaluate the likely changes in the functional groups of the material in contact with fluoride solution, the FT-IR spectroscopic analyses of both raw DE and aluminum hydroxide-treated DE were done using a Spectrum 65 FT-IR spectrometer (PerkinElmer, USA) in the KBr pellet in the range 4000–400 cm−1.
The bath adsorption and defluoridation studies were conducted in order to optimize various experimental parameters like contact time, initial fluoride concentration, adsorbent dose, and pH which can affect the adsorption efficiency of fluoride onto DE. Batch mode adsorption studies were carried out by agitating 25 g/L of the adsorbent in 50 mL of 10 mg/L fluoride solutions at pH 6.7 for 180 min taken into 250 mL plastic bottles. The fluoride solutions were agitated by a magnetic stirrer with a hot plate at room temperature. The pH was adjusted to the desired level either with 0.1 M NaOH or 0.1 M HCl. The total ionic strength adjustment buffer (TISAB) solution was added to both samples and standards in the ratio 1 : 1 in order to regulate the ionic strength of the samples and standard solutions. It was also used to avoid interferences from polyvalent cations such as Al(III), Fe(III), and Si(IV), which are able to form complex and precipitate with fluoride and reduce the free fluoride concentration in the solution. In order to determine the slope and intercept of the electrode, the fluoride ion-selective electrode was calibrated prior to each experiment. The pH meter was also calibrated at every measurement by using pH calibration buffers. All experiments were carried out at a temperature of 23 ± 2°C.
The amount of fluoride adsorbed (adsorption capacity),
The fluoride removal efficiency (percent fluoride removal) was calculated using the following equation [
Adsorption isotherms are useful for describing how the adsorbate will interact with the adsorbent (understanding the mechanism of the adsorption) and give an idea about the theoretical maximum adsorption capacity of the adsorbent. The equilibrium data were tested for fitness into the Langmuir and Freundlich isotherm models. Langmuir isotherm [
To understand the mechanism of fluoride-binding process on the adsorbent, the different functional groups found in the sorbent material are the key factors [
Fourier-transform infrared spectrum of raw diatomite (a) and aluminum hydroxide-treated diatomite after adsorption (b).
In Figure
The amount of contact surface between an adsorbent and adsorbate solution plays an important role in adsorption process. The effect of adsorbent mass on fluoride removal efficiency was studied by varying the mass of the adsorbent, viz., 2, 5, 10, 15, 20, and 25 g/L, while keeping other parameters constant at their respective optimum conditions (initial fluoride concentration 10 mg/L, pH = 6.7, contact time = 180 min, and stirring rate = 150 rpm). The percentage fluoride removal was determined, and the results are shown in Figure
Effect of adsorbent dose on the adsorption capacity of diatomite modified by treating it with aluminum hydroxide (initial fluoride concentration 10 mg/L, pH = 6.7, contact time = 180 min, and stirring rate = 150 rpm).
Figure
The effect of initial concentration on the extent of removal of the fluoride was studied by varying the concentrations from 5 to 70 mg/L, while keeping other parameters constant at their respective optimum values (pH = 6.7, contact times of 180 min, and adsorbent dose of 25 g/L). The results obtained were plotted as percentage removal of fluoride versus initial concentration of the fluoride ion in the solution as shown in Figure
Effect of initial fluoride concentration on the adsorption capacity of diatomite modified by treating it with aluminum hydroxide (adsorbent dose = 25 g/L, contact time = 180 min, pH = 6.7, and stirring rate 150 rpm).
As can be seen from Figure
The pH of the solution is an important factor in the adsorption process since it affects the adsorbent surface properties and ionic forms of fluoride in the solution [
Effect of pH on the adsorption capacity of diatomite modified by treating it with aluminum hydroxide (initial fluoride concentration 10 mg/L, adsorbent dose = 25 g/L, contact time 180 min, and stirring rate = 150 rpm).
Most adsorbents used in fluoride removal have narrow-working pH ranges and usually show optimum performance in acidic pH range. One study indicated that maximum fluoride removal efficiencies were obtained at pH 3 for fluoride adsorption by pumice from aqueous solutions [
The study of the effect of contact time on the fluoride removal efficiency was carried out by varying it from 15 to 1440 minutes, keeping other parameters constant at optimum values (pH = 6.7, dose of adsorbent 25 g/L, and initial concentration of fluoride of solution of 10 mg/L). Figure
Effect of contact time on the adsorption capacity of diatomite modified by treating it with aluminum hydroxide (initial fluoride concentration = 10 mg/L, adsorbent dose = 25 g/L, pH = 6.7, and stirring rate = 150 rpm).
As can be seen from Figure
Bark of babool [
An adsorption isotherm is the graphical representation of the amount of fluoride adsorbed per unit weight of the adsorbent as a function of its equilibrium concentration in the bulk solution at constant temperature. It gives general idea about the maximum amount of fluoride ions that could be removed and the effectiveness of the adsorbent in removing fluoride ions from water [
The linear form of Langmuir isotherm is most commonly used and is given as follows [
The values of Langmuir constants
The values of
The linear Langmuir isothermal plot and corresponding constants are given in Figure
Langmuir adsorption isotherm for fluoride removal by diatomite modified by treating it with aluminum hydroxide under optimum conditions.
Calculated Langmuir and Freundlich isotherm parameters.
Calculated Freundlich isotherm constants | Calculated Langmuir isotherm constants | |||||
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1/ |
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0.29 | 0.461 | 0.985 | 0.44 | 0.2 | 0.888 | 1.67 |
The plot of
Freundlich adsorption isotherm for fluoride removal by diatomite modified by treating it with aluminum hydroxide under optimum conditions.
Freundlich isotherm assumes that the uptake/adsorption of the adsorbate (fluoride) occurs on the heterogeneous surface by multilayer sorption. It is also assumed that the stronger binding sites are occupied first, and that the binding strength decreases with the increasing degree of site occupation [
The linear form of the Freundlich isotherm model is
In general, as the
The plot of log
Compared with the Langmuir isotherm (
Kinetic modeling gives information about adsorption mechanisms and possible rate-controlling steps such as mass transport or chemical reaction processes. The adsorption rate is an important factor for a better choice of material to be used as an adsorbent, where the adsorbent should have a large adsorption capacity and a fast adsorption rate. Pseudofirst-order and pseudosecond-order models were used to study the adsorption kinetics. For the pseudofirst-order model, the adsorption rate is expected to be proportional to the first power of concentration, where the adsorption was characterized by diffusion through a boundary [
The plot for pseudofirst-order is given in Figure
Pseudofirst-order plot for kinetic data for fluoride removal by diatomite modified by treating it with aluminum hydroxide under optimum conditions.
The pseudosecond-order model assumes that chemisorption may be the rate-controlling step in the adsorption processes. For the pseudosecond-order model under the initial and end boundary conditions
The equilibrium adsorption capacity (
The plot for pseudosecond-order is given in Figure
Pseudosecond-order plot for kinetic data for fluoride removal by diatomite modified by treating it with aluminum hydroxide under optimum conditions.
Table
Pseudofirst-order and pseudosecond-order kinetic constants and intraparticle diffusion model parameters.
Pseudofirst order | Pseudosecond order | Intraparticle diffusion model parameters | |||||
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0.0023 | 0.124 | 0.867 | 0.36 | 0.92 | 1 | 0.001 | 0.596 |
The value of correlation coefficient (
The probable mechanism controlling the sorption rate was evaluated using the intraparticle diffusion model. The McKay and Poots equation is expressed as [
According to the model, plot of uptake,
Intraparticle diffusion kinetic plot for fluoride removal using diatomite modified by treating it with aluminum hydroxide.
Thus, the probable mechanism controlling the rate of fluoride sorption onto the adsorbent is either the electrostatic attraction of fluoride ions to the positively charged sorbent surface or the ion-exchange at the surface [
The performance of aluminum hydroxide-treated DE was evaluated by applying it for defluoridation of real groundwater samples taken from Hawassa and Ziway, having 13.2 and 7.5 mg/L of initial fluoride concentrations, respectively. These are the areas located in the Rift Valley of Ethiopia having high fluoride concentrations in water. The defluoridations of real water samples were done at optimum experimental conditions, and the results obtained are presented in Figure
Application of fluoride removal from the Rift Valley water samples using diatomite modified by treating it with aluminum hydroxide (adsorbent dose 25 g/L; contact time 180 minutes).
As can be seen from Figure
Diatomite modified by treating it with aluminum hydroxide was found to be an effective adsorbent for the defluoridation of aqueous solution and natural groundwater. The maximum percent fluoride removal and adsorption capacity were 89% and 1.67 mg/g, respectively, for 10 mg/L fluoride-spiked water under optimum adsorption conditions (contact time: 180 min, adsorbent dosage: 25 g/L, pH 6.7, at room temperature, and shaking speed: 150 rpm). The adsorption data fitted well with Freundlich adsorption isotherm with a good correlation coefficient value which indicates multilayer sorption on the heterogeneous adsorbent surface. Sorption kinetics was studied by using pseudofirst-order and pseudosecond-order kinetic models. The data fitted better to pseudosecond-order kinetics which showed that the adsorption was by chemisorptions. Since intraparticle diffusion was not the rate-limiting mechanism, the adsorption rate-limiting step was most probably the process involving ion-exchange or attraction of F− to the sorbent surface. The results of the study showed that this low-cost adsorbent material, DE, can be employed for fluoride removal from groundwater and other water samples which contain excessive amount of fluoride which could be detrimental to human health. This adsorbent is cheaper, abundant, and easily available in huge amount in Ethiopia.
The data used to support the results of this study are included within the article, and any further information is available from the corresponding author upon request.
The authors declare that they have no conflicts of interest regarding the publication of this paper.
The authors would like to acknowledge Wollega University for covering different expenses associated with doing the research. The authors would like to thank the Department of Chemistry, Addis Ababa University, for allowing them to use their laboratory for doing the research. The authors are also grateful to Bedele Brewery for giving them the diatomite sample used as an adsorbent for defluoridation.