Fixed bed column performance of Tinospora cordifolia for defluoridation of water

A continuous adsorption study in a fixed-bed column was carried out by using Tinospora cordifolia as an adsorbent for the removal of fluoride from aqueous solution. The effect of flow rate, influent fluoride concentration and bed depth on the adsorption characteristics of adsorbent was investigated at pH 7. The dependencies of breakthrough curves on these parameters were confirmed from the data obtained. Modeling of data was done. Thomas, Yoon–Nelson and Adams–Bohart models were applied to experimental data to predict the breakthrough curves. These kinetic models were helpful to determine the characteristic parameters of column designing for defluoridation on a large scale. Thomas and Yoon-Nelson models were found to be more suitable for the description of the breakthrough curve than the Adams–Bohart model in the present study. It was concluded that the Tinospora cordifolia-packed column can be used for effective defluoridation of water.


GRAPHICAL ABSTRACT INTRODUCTION
Fluoride, part of numerous minerals and rocks, has become a significant pollutant for groundwater assets everywhere in the present reality. Little convergence of fluoride (up to 0.5 mg/l) is a basic need for dental enamel and bones; yet, when an individual is exposed to high accumulation of fluoride (more than 1. and so on; however, every single technique has its own impediment: some require enormous amounts of energy and synthetics; some influence the climate (Murugan & Subramanian ). Adsorption is one of the critical means of defluoridation. Removal of fluoride through a fixed bed section with appropriate adsorbent is a powerful methodology and has more extensive appropriateness (Gupta et al. 

Preparation of adsorbent
Tinospora cordifolia is the member of Meninspermaceae family. It is easily available in various locations of Durg district in Chhattisgarh. The bioadsorbent was first washed with distilled water to remove impurities. Then it is dried in the sun; after that, it was further dried in a hot air oven at 60 C till dryness. It was then powdered with the help of a grinder and used for removal of fluoride.

Column studies
A fixed-bed column study for fluoride removal from water by bioadsorbent was performed using a column of 4.5 cm diameter and 59 cm length. The column was packed with bioadsorbent between two supporting layers of glass beads.
The bulk density of adsorbent packed in the column was 0.078 gm/cm 3 . The study was conducted at a temperature of 29 ± 2 C and the pH of the fluoride solution was 7.0.
The column was treated with fluoride solutions of different concentrations under different experimental conditions to study the effect of flow rate (13, 21 ml/min), initial concentration of influent (3, 5, 7 mg/l) and bed depth (30, 36, 40 cm) on column adsorption.

Modeling of column adsorption
The behavior of the adsorption column was described by a breakthrough curve, which is a typical plot of the ratio of outlet solute concentration to inlet solute concentration in the fluid as a function of time from the start of flow or volume of effluent at a particular bed depth.
Data collected during practical work in the laboratory can be used to design full-scale adsorption columns. Successful design of a column adsorption process requires prediction of the concentration-time profile or breakthrough curve for the effluent. In order to describe the fixed-bed column behaviour and to scale up it for industrial applications, many mathematical models have been developed. In the present research work Thomas, Yoon-Nelson and Adams-Bohart models were used to obtain the kinetic behaviour of the column (Table 1).

Error analysis
In the linear regressive analysis, different formulas were used to calculate the value of correlation coefficient R 2 , but this may affect the accuracy of results significantly.
Therefore nonlinear regressive analysis can be chosen as a better way to minimise such errors of calculation (Ho where (C/C 0 ) c and (C/C 0 ) e are the ratio of effluent and influent fluoride concentrations obtained from models, and from experiment, respectively; N is the number of the experimental point. In order to verify the best fit model for the adsorption, it is necessary to analyze the data using SS, combined with the values of the determined coefficient (R 2 ).

RESULT AND DISCUSSION
Column study The most important parameters for the study of the breakthrough curve are flow rate, bed depth and initial inlet concentration. The effects of these parameters on the shape of the breakthrough curve and column performance were investigated at pH 7.

Effect of flow rate on breakthrough curves
The breakthrough curves at various flow rates (

Effect of bed depth on breakthrough curves
The breakthrough curves at different bed depths (30, 36, 40 cm) at constant flow rate of 13 ml/min and initial fluoride concentration of 5 mg/l at pH 7 are shown in Figure 2.
As we increase the adsorbent mass in the column, bed height increases, thus the surface area of the adsorbent increases and fluoride had more binding sites for adsorption (Zulfadhly et al. ). Thus adsorption capacity of the column q exp increases with increase in bed height ( Table 2).

Ratio of bed depth and column diameter is an important
parameter for column study. This ratio is related to the residence time (empty bed contact time, EBCT) of adsorbate on adsorbent. More the residence time in the column, more   EBCT is the relation between the bed length (Z) and the feed solution velocity (Q V ) (Abusafa & Yucel ) as given by: where A denotes the column cross sectional area (A ¼ ᴫr 2 where r is the radius of the column).
The values of EBCT obtained from different experimental conditions are shown in Table 2.
From the above table, it is clear that the maximum uptake (Q exp ¼ 3.40 mg/g) is obtained for higher bed depth (Z ¼ 40 cm) for the same influent concentration (C 0 ¼ 5 mg/L) and the same flow rate (Q ¼ 13 mL/min) because at higher bed depth, large surface area and more active sites of adsorbent is available for adsorption. It is also evident from the table that increase in influent concentration increases the uptake capacity due to increase in mobility of fluoride ions and increase in flow rate decreases the uptake capacity as higher flow rate of influent decreases the residence time of adsorbate on adsorbent.

Effect of influent fluoride concentration on breakthrough curves
The breakthrough curves were obtained at different concentrations (3, 5, 7 mg/l) at a constant bed depth of 36 cm and flow rate 13 ml/min. As the influent fluoride concentration increases, breakthrough time decreases (Figure 3). This may be due to relatively slower transport because of a decrease in the diffusion coefficient and decreased mass transfer coef-

Breakthrough curve models
Thomas model. The column data were fitted to the Thomas model. The Thomas rate constant k T and maximum adsorption capacity q were determined for different experimental conditions (Table 3). It is clear from Table 3 (Table 3).
Yoon-Nelson model. Column data were fitted to the Yoon Nelson model and the values of rate constant K YN and time required for 50% breakthrough τ were determined (Table 4).   (Table 4 and Figures 1-3).

It is clear from the table that the value of τ increases at slow
Adams-Bohart model. The Adams-Bohart adsorption model was applied to experimental data. The adsorption capacity (Na) and kinetic constant (k AB ) were calculated from the model (Table 5), which shows that the adsorption capacity of the column increases with increase in bed depth and initial concentration of the fluoride solution. Figures 1-3

Comparison of Thomas, Yoon-Nelson and Adams-Bohart models
In order to find out the fitness of Thomas, Yoon-Nelson and Adams-Bohart models with experimental data, values of correlation coefficients R 2 and value of error SS of these model were compared. For Thomas model, the values of R 2 range from 0.95 to 0.99 (Table 3) for the Yoon-Nelson model, R 2 ranges from 0.93 to 0.99 (Table 4), which is an indication of good agreement between experimental data with the data obtained from these models. The value of R 2 was found to be in the range of 0.80 to 0.95 (Table 5) for the Adams-Bohart model, which was slightly lower than that of the Thomas and Yoon-Nelson models under the same experimental conditions.
Comparing the values of SS from the Thomas, Yoon-Nelson and Adams-Bohart models (Tables 3-5

, 
) and the present study shows that it can also be used for removal of fluoride from water. Therefore Tinospora cordifolia as an adsorbent is very useful for removal of pollutants from water.
The FTIR analysis of fluoride-loaded biomass was done in our laboratory by Pandey et al. () and it was found that major changes were obtained in the region of 500-1,600 cm À1 . This range is assigned to amino group in bioadsorbent. Change in the frequency range of fresh and loaded biomass indicates that binding of fluoride in biomass occurs due to substitution of the amino group by fluoride.

Effectiveness of bioadsorbent used
The bioadsorbent Tinospora cordifolia used for removal of fluoride from water in the present study is easily available plant material. It is widely spread in Durg district in Chhattisgarh. Moreover, the plant possesses medicinal properties.
Bioadsorbent can be used for defluoridation at neutral pH.
Residue of this adsorbent after defluoridation process can be easily disposed of without changing the pH of the disposed field. Therefore, economically, its use is beneficial for removal of fluoride from water at large scale (Table 7).