Removal of fluoride from water using a novel sorbent lanthanum-impregnated bauxite
© The Author(s) 2016
Received: 2 October 2015
Accepted: 19 August 2016
Published: 26 August 2016
A novel sorbent, Lanthanum-Impregnated Bauxite (LIB), was prepared to remove fluoride from water. To understand the surface chemical composition and morphology, LIB was characterized using X-ray diffraction and scanning electron microscopy techniques. Experiments were performed to evaluate the sorption potential, dose of sorbent, kinetics, equilibrium sorption capacity, pH and influence of anions for defluoridation by LIB. Equilibrium isothermal studies were conducted to model the sorption and regeneration studies were carried out to evaluate the reusability of LIB. The results showed that LIB, at a dose of 2 g/L could remove 99 % of fluoride from an initial concentration of 20 mgF/L. Kinetic studies revealed the best fit of pseudo second order model. The sorption followed Langmuir isotherm model and the maximum sorption capacity of LIB for removal of fluoride was found to be 18.18 mg/g. Naturally occurring pH of water was found to be favorable for sorption. Usually occurring anions in water except nitrates influenced sorption of fluoride by LIB.
Excessive fluoride in drinking water causes serious health problems such as brittleness of bones, dwarfishness, fluorosis and cancers (Chinoy 1991). The maximum contaminant level (MCL) of fluoride in drinking water is 1.5 mg/L, according to the World Health Organization (2004). Groundwater with fluoride concentration >1.5 mg/L is prevalent in several regions of the world, warranting treatment (Yeşilnacar et al. 2016; Atasoy et al. 2013; Vijaya Kumar et al. 1991; Gaciri and Davies 1993; Czarnowski et al. 1996). Several technologies such as adsorption (Vivek Vardhan and Karthikeyan 2011), coagulation and flocculation (Emamjomeh and Sivakumar 2006), electrodialysis (Adhikary et al. 1989), electrocoagulation (Khatibikamala et al. 2010) and reverse osmosis (Simons 1993) have been tried to remove fluoride from water with varying degrees of success. Chemical precipitation of fluoride using alum and lime, known as Nalgonda Technique (Nawlakhe et al. 1978) can be used for fluoride removal. However, it poses some problems such as generation of large volumes of sludge, which is difficult to deal with. Adsorption is considered to be a feasible technique especially for household applications or for small communities (Srimurali et al. 1998). Various sorbents such as activated alumina (Boruff 1934; Fink and Lindsay 1936; Swope and Hess 1937), bone char (Nemade et al. 2002), bauxite (Sujana and Anand 2011), magnesium amended activated alumina (Maliyekkal et al. 2008) and rice husk (Vivek Vardhan and Karthikeyan 2011) have been tried (Bhatnagar et al. 2011; Ayoob et al. 2008). Among various adsorbents used activated alumina is deemed to be the selective sorbent for removal of fluoride from water (Boruff 1934; Fink and Lindsay 1936; Swope and Hess 1937). However, due to some drawbacks such as optimum removal at a low pH value of 5.5, its practical scope of applicability is limited.
Recently various rare earth materials such as lanthanum (Na and Park 2010), lanthanum modified activated alumina (Cheng et al. 2014), lanthanum oxide (Nagendra Rao and Karthikeyan 2012), lanthanum impregnated green sand (Vivek Vardhan and Srimurali 2016), cerium (Xu et al. 2001) and yttrium (Raichur and Basu 2001) have been used as sorbents for removal of fluoride from water. Though lanthanum has got good affinity for fluoride, there are some difficulties related to its use as an adsorbent. Compounds of lanthanum are present in fine powder form. Application of lanthanum compounds in powder form for adsorption is associated with practical limitations such as difficulty in separation from liquid, impeded hydraulic flow and leachate of metal with treated water (Maliyekkal et al. 2008). To overcome these problems, lanthanum had to be fixed onto a suitable substrate. Bauxite is an ore of aluminum and is abundantly available at low cost. In the present investigation, an attempt has been made to impregnate lanthanum onto bauxite, in order to develop a low-cost adsorbent and also to study the synergetic effect of lanthanum and bauxite on fluoride removal as well as to overcome the drawbacks associated with the use of lanthanum powder. Lanthanum Impregnated Bauxite (LIB) was prepared using La2CO3. La2CO3 is the base material for synthesis of other forms of Lanthanum and is available at low-cost. Also the quantity of La2CO3 that goes into impregnation for synthesis of LIB is very less. So, when used on a massive scale, LIB turns out to be a very low-cost adsorbent. However, the exact cost analysis will be done in future studies. LIB was characterized using X-ray diffraction (XRD) studies and Scanning Election Microscopy (SEM). Adsorption experiments were conducted in batch mode. Experiments involving Kinetics, isothermal equilibrium, pH and regeneration studies were carried out to evaluate the practical feasibility of application of LIB as an adsorbent for removal of fluoride from water.
All reagents used in the present investigation were of analytical grade and procured from E. Merck Ltd, India. Water used in all batch sorption studies was laboratory distilled water prepared with a glass distillation unit (pH 6.7 ± 0.1 and specific conductivity 2.0 to 4.3 µS/cm). Stock solution of fluoride of 100 mg/L was prepared with distilled water using sodium fluoride. Aqueous fluoride solution was prepared by adding appropriate quantity of stock fluoride solution into distilled water and used in all adsorption experiments unless otherwise specified. LIB was prepared by thermal impregnation method as described below in adsorbent preparation. Raw bauxite was collected from mines at Mahboobabad, India. Lanthanum carbonate was purchased from Indian Rare Earths Limited, Aluva, Kerala, India.
Raw bauxite was crushed and sieved to get <75 micron particle size. Bauxite so obtained was heated in a muffle furnace at 400 °C for 4 h. This heated bauxite was cooled to room temperature in a desiccator and is called calcined bauxite. Calcined bauxite was stored in an air tight plastic container for further use. In a separate conical flask, La2(CO3)3 of 0.5 g was mixed with 50 mL of distilled water and dilute HCl was added to it drop wise till the La2(CO3)3 got completely dissolved. To this solution 20 g of prepared calcined bauxite was added and mixed using a magnetic stirrer for 3 h. The liquids were strained off and the solid material obtained was washed with distilled water. It was dried in a water bath at 110 °C for 6 h and subsequently heated in a muffle furnace at 950 °C for 4 h and then cooled. The material thus obtained was called LIB and used as an adsorbent in all further investigations. LIB was processed at high temperatures for lanthanum impregnation, whereas bauxite was calcined to improve its surface properties. So, this paper has the limitation of not studying bauxite and LIB, subject to same thermal treatment to bring out the exact differences due to lanthanum impregnation. Calcined bauxite is hereafter referred to as simply bauxite in this paper.
Characterization of adsorbent
LIB samples were analyzed by X-ray powder diffraction (XRD) technique before and after adsorption for studying its mineralogy. XRD analysis was carried out using a X-ray diffractometer, Philips: PW1830 with CuKα radiation. To study the surface morphology, scanning electron microscope (SEM) was used. SEM and EDAX images were obtained from a Carl Zeiss, EVO MA15 instrument. Particle size distribution was analyzed using Ankersmid particle size analyzer. Pore size analysis of bauxite and LIB were done by using a micropore analyzer (ASAP 2020, Micromeritics, USA) by Nitrogen chemisorption isotherm technique (Carabineiro et al. 2011).
Batch adsorption experiments
Sorption experiments were conducted in batch mode using 250 mL Teflon flasks with a 100 mL of 20 mg/L of aqueous fluoride solution. A known quantity of adsorbent was added to the prepared fluoride solution in Teflon flasks. It was agitated using a rotary shaker of make Kaizen Imperial at 160 rpm and at room temperature for specific contact periods ranging from 0 to 360 ± 1 min. The solutions contained in the flasks were then withdrawn at specified contact periods, filtered with 42 Whatman filter paper of pore size 2.5 µm and analysed for residual fluoride using SPADNS method (APHA) (APHA et al. 1996) at 570 Nm. A spectrophotometer, Evolution 201, of Thermo Scientific make was used to analyze fluoride. The contact period, at which there was no further reduction of fluoride, is considered the equilibrium contact time. Similarly the optimum usage of adsorbent was studied by varying the sorbent dose ranging from 0 to 8 ± 0.01 g/L for a constant equilibrium contact time. To understand the influence of pH, sorption experiments were conducted at different pH values ranging from 2 to 12. Starting pH adjustments were made using diluted NaOH and H2SO4. pH was measured using a Hanna make, pH analyzer. Optimum values obtained during preliminary investigations for various parameters were used in all further detailed experimentation. Fluoride ion concentrations varying from 5 to 70 mg/L were used in sorption equilibrium investigations, to arrive at the best fitting isothermal model. The reporting fluoride concentration range by SPADNS method is from 0 to 1.4 ± 0.1 mg/L. Appropriate dilutions of samples were made when fluoride exceeded the above mentioned concentration range. Concentrations of lanthanum and aluminum were measured using atomic absorption spectrometer with a graphite furnace (AAS, GBC 932 Plus).
Kinetics of sorption
In the present investigation pseudo first order, pseudo second order and intraparticle diffusion models were studied to understand the kinetics of adsorption of fluoride using bauxite and LIB.
Pseudo first order equation and pseudo second order equation
Intraparticle diffusion analysis
Effect of competing ions
The influence of anions on the efficiency of fluoride sorption by LIB was investigated. In order to find this, various individual ions of Cl−, SO4 2−, PO4 3−, HCO3 − and NO3 − of concentrations up to 100 mg/L were added each separately into 20 mg/L of aqueous fluoride solution and adsorption experiments were carried out using 2 g/L of LIB as well as 6 g/L of bauxite. The liquid samples were then withdrawn after reaching equilibrium time and analyzed for residual fluoride concentrations. Wherever the influence of phosphates was analyzed, for every 16 mg/L of PO4 3−, an error correction of −0.1 mg/L was made to rectify its interference with SPADNS method (Hach company 1989–2014).
Determination of pH zero point charge
pH of zero point charge (pHzpc) was found by a batch equilibrium method (Rivera-Utrilla et al. 2001). Typically, NaCl of 0.01 M concentration and 50 mL in quantity was taken into six conical flasks. pH of these solutions were varied between 2 and 12 using H2SO4 or NaOH. One gram of LIB was added to each flask and a plot was drawn between pH before addition of LIB and pH after addition of LIB. This plot yielded a straight line. The flasks were then agitated at room temperature for 48 h and then the pH values were noted. Now a plot was drawn between pH value of solution before agitation and pH value after 48 h of agitation, which eventually yielded a curve. The intersection point of the straight line and the curve is the value of pHzpc of LIB. Similar experiments were conducted with bauxite. Results obtained by this method are in close agreement with the results obtained by Carabineiro et al. (2011).
After the agitated batch sorption experiments were conducted under optimal conditions with 20 mg/L aqueous fluoride solution, the liquids were strained off and the sorbent which got loaded to capacity was air dried for 48 h. Further it was desorbed by agitation with various eluents such as distilled water, NaOH and HCl, for a period of 180 min. The best desorbent was considered as the regeneration agent.
Cycles of regeneration
Regular batch adsorption experiments were conducted with a 20 mg/L of aqueous fluoride solution using adsorbent under optimal experimental conditions. After sorption, the spent sorbent, which got loaded to capacity was separated by filtration using a 42 Whatman filter paper and air dried for 48 h. Subsequently it was desorbed using the most appropriate eluent found through experimentation. Such regenerated sorbent was again separated and air dried to be used as a fresh sorbent for removal of fluoride from a 20 mg/L of aqueous fluoride solution. After agitation, the residual fluoride in solution was measured. This process was repeated several times until the residual fluoride exceeded the permissible limits. The number of cycles until fluoride in solution reached the permissible limit was considered the optimum cycles of reusability of sorbent.
Results and discussion
EDAX of LIB
Error (1 sigma) (Wt%)
Influence of sorbent dose
Influence of contact time
Comparison of parameters of kinetic models for adsorption of fluoride onto LIB and bauxite
qe (exp) = 9.8 (mg/g)
qe (cal) = 5.462 (mg/g)
K1 = 0.0152 (min−1)
R2 = 0.8921
qe (cal) = 10.75 (mg/g)
K2 = .0037 (min−1)
R2 = 0.9965
Kid = 0.438 (g mg−1 min−1)
C = 2.9041
R2 = 0.6986
qe (exp) = 3.1666 (mg/g)
qe (cal) = 0.9156 (mg/g)
K1 = 0.0154 (min−1)
R2 = 0.8956
qe (cal) = 1.781(mg/g)
K2 = 0.0225 (min−1)
R2 = 0.9964
Kid = 0.073 (g mg−1 min−1)
C = 0.4837
R2 = 0.699
Pore size characteristics of bauxite, LIB and activated alumina
BET surface area
BJH adsorption cumulative volume of pores between 17.000 and 3000.000 Å diameter
BJH Desorption cumulative volume of pores between 17.000 and 3000.000 Å diameter
Adsorption average pore width (4 V/A by BET)
BJH Adsorption average pore diameter (4 V/A)
BJH Desorption average pore diameter (4 V/A)
Equilibrium isothermal studies
Langmuir and freundlich isotherm models
Comparison of isothermal constants for adsorption of fluoride onto LIB and bauxite
qmax = 18.18 mg/g
b = 0.541
R2 = 0.997
Kf = 5.794 mg/g
n = 2.753
R2 = 0.805
qmax = 7.722 mg/g
b = 0.379
R2 = 0.992
Kf = 1.902 mg/g
n = 2.085
R2 = 0.861
Comparison of isothermal constants for adsorption of fluoride onto LIB and bauxite
Sujana and Anand (2011)
Titanium rich bauxite
Das et al. (2005)
Na and Park (2010)
Lanthanum modified activated alumina
Cheng et al. (2014)
Lanthanum impregnated chitosan flakes
Jagtap et al. (2011)
Lanthanum incorporated chitosan beads
Bansiwal et al. (2009)
Alumina metallurgical grade
Magnesia amended activated alumina
Maliyekkal et al. (2008)
Mixed rare earth oxides
Raichur and Basu (2001)
Lanthanum impregnated bauxite
Influence of pH
Influence of anions
LIB was prepared by thermal impregnation method, and it reduced fluoride from distilled water from 20 to 0.7 mg/L.
LIB at a dose of 2 g/L removed fluoride up to 99 % from an initial concentration of 20 mg/L of aqueous fluoride solution, whereas bauxite at a dose of 6 g/L removed up to 94 % of fluoride from an initial concentration of 20 mg/L of aqueous fluoride solution.
The time taken for defluoridation by LIB was 120 min and it followed pseudo second order reaction, which indicates that the mechanism involved could be chemisorption. Pore diffusion seems to be the rate limiting step. The time taken for defluoridation by bauxite was 150 min.
The sorption process by LIB conformed to Langmuir isotherm model. The maximum sorption capacity according to this model was 18.18 mg/g, which was close to observed experimental values. Bauxite followed a similar trend with a best fit to Langmuir isotherm model. The maximum sorption capacity of bauxite was found to be 7.722 mg/g.
A pH range of 6.5–8.5 was found to be optimum for LIB, for removal of fluoride from water, which is a naturally occurring pH for waters. Bauxite exhibited optimum fluoride removal from pH 5.0 to 6.5.
Addition of NO3− to aqueous fluoride solution water brought the residual fluoride concentration after sorption to 1.4 mg/L, whereas other individual ions added such as Cl−, SO4 2−, PO4 3− and HCO3 − caused the final fluoride concentration after sorption to be more than 1.5 mg/L, probably due to competition of ions.
4 % NaOH regenerated LIB by 95 % and the effective number of cycles after regenerations were found to be 3 for removal of fluoride up to permissible limit.
VVCM and SM conceived and designed this study. VVCM mainly and SM partly performed the experiments. Both authors read and approved the final manuscript.
The authors profusely thank and acknowledge the financial assistance in the form of waiver of Article Processing Charges, offered by Springer Open Waivers and Biomedcentral Waivers, towards publishing this paper.
The authors declare that they have no competing interests.
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