Kinetic and thermodynamic studies for fluoride removal using a novel bio-adsorbent from possotia (Vitex negundo) leaf
© The Author(s). 2017
Received: 1 October 2017
Accepted: 27 November 2017
Published: 12 December 2017
Possotia leaf powder (PLP), a novel biosorbent obtained from a locally available wild plant with environment friendly and low cost nature, has found to be effective to remove fluoride from fluoride contaminated drinking water.
The kinetic and thermodynamic parameters for the adsorption have been studied. Its fluorideabsorption efficiency is found to be nearly 75% at natural pH range and is affected by the parameters likecontact time, absorbent dose, solution temperature and initial fluoride concentration.
The optimum contact time and absorbent dose were found to be 120 min and 3gm/L, respectively.Langmuir, Freundlich and Temkin isotherm are studied through this experimental data and found to be well followed. The adsorption kinetics data were fitted to pseudo first order, pseudo second order, and intraparticle diffusion models and was best fitted for pseudo second order model.
The thermodynamic study points out the efficiency of this bio-adsorbent at low temperatureindicating the physical nature of adsorption with weak adsorbent-adsorbate force of attraction. By studyingenthalpy, entropy and Gibb’s free energy via van’t Hoff plot, it was observed that the adsorption process wasfavorable and exothermic in nature.
Water is life, but we are still unable to make healthy drinking water available and affordable to all. Waste or contaminated water from any source must ultimately return to the natural water reservoir. A large number of people depend heavily on underground water due to lack of other water resources. People in the world are suffering as 80% of the diseases come from ill or deteriorated water, which is threatening for human life (Mamba et al., n.d.). Fluoride is one such life-threatening water contaminant. Its gaseous form is a very powerful oxidizing agent. Naturally, fluoride exists as a very reactive fluoride ion, and its natural abundance by weight in the Earth’s crust ranges from 0.065–0.09% (Mondal et al., n.d.-a). The World Health Organization (WHO) announced the acceptable range of fluoride in drinking water as 1–1.5 mg/L (Swain et al., n.d.; Mohan et al., n.d.).
Intake of fluoride from drinking water at the level of 1 mg/L enhances bone development and prevents dental carries. Exceeding the limit of fluoride intake may lead to dental and skeletal fluorosis. The main constituent of teeth and bone is hydroxyl apatite (Ca10(PO4)6(OH)2) and fluoride can substitute the hydroxide ion within the hydroxyl apatite crystal structure to form fluorapatite (Ca5(PO4)3F), which makes teeth and bones denser, harder, and more brittle (Yu et al., n.d.). Young children are the most susceptible as dental enamel and skeletal formation is most active during early childhood (Levin et al., n.d.). Moreover, excessive fluoride intact may interfere in DNA synthesis (Bhatnagar et al., n.d.). Therefore, researchers are trying to focus on various defluoridation processes. The most commonly used methods for defluoridation of drinking water are reverse osmosis, precipitation, membrane filtration, ion-exchange, etc. (Yadav et al., n.d.). Among these, adsorption is considered as the best method for its low maintenance cost and effectiveness towards fluoride removal even at low concentrations (Gandhi et al., n.d.). Hence, researchers have focused on locally available bio-adsorbents like Moringa oleifera (drum stick), tulsi, tamarind seed, tea ash (Mondal et al., n.d.-b), babool bark (Mamilwar et al., n.d.), banana peel (Bhaumik & Mandal, 2016), neem leaf, kikar leaf (Kumar et al., n.d.), papal leaf (Ramanjaneyulu et al., n.d.), red mud, fly ash, rice husk ash, and maize ash (Waghmare & Arfin, n.d.).
Vitex negundo, popularly known as possotia trees in the North Eastern Region of India, belong to the family Lamiaceae which are easily available in Assam, India, and are known for their medicinal properties. Studies have shown that chemicals isolated from the plant have potential anti-inflammatory, antibacterial, antifungal, and analgesic activities (Dharamasiri et al., n.d.; Gupta & Tandon, n.d.). Moreover, local people use it as vegetable leaf for treatment of rheumatism and deworming. Use of its bark for defluoridation was already reported (Suneetha et al., n.d.). But as it is a herb-like plant, many plants will be required for the preparation of the bio-adsorbent. Therefore, it is imperative to study if this particular leaf could be used for the wastewater treatment. With this background, this study was undertaken and a detailed kinetic and thermodynamic interpretation of the process involved has been presented in this manuscript.
Preparation of required solutions
Stock solution of the adsorbate NaF of concentration 100 mg/L is prepared by dissolving 0.22 g of NaF in 1.0 L of distilled water. SPADNS solution is prepared by dissolving 958 mg SPADNS [sodium 2-(parasulfo-phenylazo)-1,8 dihydroxy-3,6 naphthalene disulfonate] in 500 mL of distilled water. The zirconium-oxychloride solution is prepared by first dissolving 133 mg of zirconium-oxychloride octahydrate (ZrOCl2∙8H2O) in 25 mL of distilled water, then adding 350 mL of concentrated HCl and making the total volume of the solution up to 500 mL with distilled water. By mixing equal volume of SPADNS and zirconium-oxychloride solution, the SPADNS reagent is prepared which is used in the experiments.
A stock solution of 0.05 M copper (II) bisethylenediamine complex is prepared by mixing 50 mL of 0.1 M CuCl2 solution with 102 mL of 1.0 M ethylene diamine solution and is used to determine the cation exchange capacity of the bio-adsorbent.
Preparation of possotia leaf powder
The bio-adsorbent possotia leaf powder is prepared from the matured leaves of the possotia tree. The leaves are collected, cleaned thoroughly, and dried for 3–4 days. The dried leaves are crushed into fine powder by using a mechanical grinder. The resultant powder is sieved and fractionated using a series of sieves with different sizes. The powder is then again dried in an air oven at 60–70 °C for 12 h and washed with distilled water to remove all the color pigments so that it will not interfere in spectroscopic studies. Then the sample is kept at room temperature and used for further experiments.
where C o and C t are initial (t = 0) and final (t = t) fluoride concentrations in milligrams per liter when the adsorption was carried out for a time interval of t minutes, with an adsorbent amount of milligrams per liter.
Cation exchange capacity
Anion exchange capacity
Results and discussion
Identification of surface functional groups with IR spectroscopy
The effect of pH
The effect of contact time
The effect of adsorbent dose
The adsorption isotherm is nothing but a mathematical relationship between the amount adsorbed on the adsorbent surface and the pressure (in case of gaseous adsorbate) or the concentration (in case of liquid) of the adsorbate in equilibrium at a constant temperature. To describe the adsorbate-adsorbent interactions quantitatively and to study the effectiveness of the absorbent for the particular adsorbate, the following adsorption isotherms have been evaluated.
Equilibrium isotherm parameters of the defluoridation process
PLP dose (g/L)
A T (L/g)
In the present case, the 1/n values lies between 0 and 1 (Table 1), and hence, fluoride adsorption on a PLP surface at a low concentration is favorable. The same magnitude of “n” values suggests that the retention of fluoride from the solution takes place by ionic interactions.
Kinetic study on adsorption process is significant as it provides valuable information about the reaction pathways and the mechanism of sorption interactions. Besides, it describes the solute uptake rate which controls the residence time of the adsorbate at the solid–solution interface. Therefore, it is important to predict the rate at which a pollutant is removed from aqueous solutions for designing appropriate water treatment processes.
Kinetic parameters of the pseudo first- and second-order kinetics
Pseudo 1st-order parameters
Pseudo 2nd-order parameters
q e (mg/g)
q e (mg/g) from plot
k 1 (min−1)
q e (mg/g)
q e (mg/g)from plot
k 2 (L/mol/min)
The slope of linear portion of the plot gives the value of rate constant K. As could be observed from the figure, the linear portion of the plot does not pass through the origin indicating that the mechanism of fluoride adsorption is complex and both the surface adsorption and the intra-particle diffusion contribute to the rate-determining step (Bharali & Bhattacharyya, n.d.).
where ∆G is Gibb’s free energy change and T is the respective absolute temperature.
Values of thermodynamic parameters for fluoride adsorption on PLP
C e (mg/L)
Cation exchange capacity
The ability to react with positively charged molecules is called the cation exchange capacity (CEC) of that substance. Higher CEC indicates the higher negative charge of the substance and hence can bind more cations. The cation exchange capacity of PLP is estimated to be 0.0074 (meq/g).
Anion exchange capacity
The anion exchange capacity (AEC) of a substance indicates the degree to which it can adsorb and exchange anions. The anion exchange capacity of PLP is estimated to be 1.25 meq/g.
The PLP can remove more than 70% of fluoride from a 3-ppm aqueous solution on its own at a normal pH range, at a contact time of 120 min and 3 g/L of its dose.
Adsorption of fluoride by PLP follows a pseudo second-order mechanism, and the mechanism of fluoride removal is found to be complex. The surface adsorption as well as the intra-particle diffusion contributes to the rate-determining step.
Significant amount of fluoride can be removed in the pH range 7–8 by using PLP, which makes it suitable to utilize in defluoridation of drinking water especially in rural areas where sophisticated facilities are not available.
It is a cost affordable, easily available, and environment-friendly bio-adsorbent.
Further studies need to be carried out for evaluation of the bio-adsorbent for reusability.
PS conceived of the study and contributed in design and organization of the manuscript. HKB carried out the adsorption and kinetic experiments. RKB helped in analysing the data. All authors read and approved thefinal manuscript.
The authors declare that they have no competing interests.
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