Activated tamarind seed coat: a green biosorbent to remove fluoride from aqueous solutions

A novel, inexpensive, easily available and eco-friendly biosorbent, Tamarindus indica activated seed coat, has been evaluated for its capability to remove fluoride from water. Batch experiments were carried out to study the effect of various parameters affecting the biosorption such as pH (2–10), biosorbent dose (0.05 g/L to 0.35 g/L), contact time (10–80 min) and initial fluoride concentration (0.001–0.006 g/L) for the biosorption of fluoride at room temperature. The maximum removal of fluoride was found at pH 6, biosorbent dose 0.3 g and contact time 60 min. Physicochemical characterization studies revealed the suitable morphology and chemical functional groups present on the biosorbent. Isothermal data agreed well with the Langmuir isotherm adsorption model with R value 0.976 and KL 0.1. The biosorption interface of fluoride onto Tamarindus indica activated seed coat obeyed the pseudo-second-order rate equation with R 0.976. The present study demonstrates that Tamarindus indica activated seed coat can effectively remediate fluoridecontaminated water.


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From a literature review, it is learnt that Tamarind seed is a household material and is left as waste after removing Tamarind pulp for food preparation. Since this material is familiar to all kinds of people, an attempt has been made in the present study to use activated seed coat biomass for the removal of fluoride. Using both laboratory aqueous solution and groundwater field samples, the study aims to devise a simple, cheap and viable defluoridation method that could be adopted easily by village communities and urban dwellers.

MATERIALS USED AND METHODOLOGY Fluoride stock preparation
A stock of 2.21 g anhydrous sodium fluoride was weighed and immediately dissolved in 100 mL of distilled water.
Then the volume was made up to 1,000 mL to make the final fluoride stock solution. Various concentrations from 0.05 to 0.3 g/L fluoride solutions were prepared freshly from the stock whenever required.

Preparation of biosorbent
The tamarind seed used in this study was collected from the kitchen as a waste material. It was soaked in tap water for an hour to remove the adhering pulp, washed well with the same tap water and then with double distilled water until reaching pH 7, dried in an air oven at 150 C for six hours, its coat was manually separated and it was crushed with pestle and mortar and then sieved with a 100-micron-size mesh to get uniform size (as shown in Figure 1) particles. The obtained fine powder was stored for further experiments in an airtight container to protect it from humidity (Tang & Zhang ).

Analytical instruments
To adjust the pH of buffers, prior to starting the experiment the pH meter was calibrated with standard pH buffer solutions (Hanna Instruments) and adjusted.
For the measurement of fluoride ion concentration, a sophisticated fluoride meter (Extech FL700, USA) was used.
Before proceeding to the preparation of different fluoride concentration solutions, we ensured the machine was calibrated with Total Ionic Strength Adjustment Buffer (TISAB) (Tang & Zhang ). The potentiometer method was employed to measure the fluoride ion concentration in different solutions.

Batch biosorption experiment
Bisorption experiments were carried out by varying pH, initial concentrations (mg/L), biosorbent dose (g/0.1 L), contact time (minute) and initial fluoride concentration.
All experiments were carried out at room temperature of 21-25 C. The influence of pH (2.0-10.0), biosorbent dose (0.05-0.35 g/50 mL), contact time (10-60 min) and initial fluoride concentration (0.001-0.006) was evaluated during the present study in a 100 mL Erlenmeyer flask, and 50 mL of fluoride solution of known concentration was added. The contents (biosorbent/50 mL solution) were kept in constant shaking for a fixed time in an orbital shaker, and then the solids were separated through filtration using Whatman 42 filter paper and the volume was finally adjusted to 25 mL. Then solutions were collected for analysis, and residual fluoride concentration was determined using the fluoride ion selective meter (Extech FL700, USA). Each experiment was conducted three times, and average values are reported. Control experiments, performed without addition of adsorbent, confirmed that the sorption of fluoride on the walls of the Erlenmeyer flasks was negligible. The percentage removal of fluoride was obtained using the equation: where, Q e ¼ fluoride ion concentration sorbed at equilibrium Percentage fluoride ion removal (%) was considered by using the following equation: Fluoride ion removal efficiency (%) ¼

Langmuir isotherm model
The Langmuir isotherm model follows the hypothesis below: • each energetic site always interacts with only one adsorbate, • all the sorbate molecules are adsorbed on to specific restricted sites, • none of the adsorbed molecules will interact with neighboring ones, and • surface adsorption sites are always equally active.
The Langmuir isotherm equation: where, Q e ¼ biosorbent equilibrium fluoride ion concentration (mg/g), C e ¼ equilibrium fluoride ion concentration in the solution (mg/g), Q max ¼ biosorbent maximum adsorption capacity (mg/g) A linear form of Equation (3) is given below: The Langmuir constant values, q max and K L , were calculated from the slope and intercept of the linear plot of C e /q e versus C e . Important features of the Langmuir isotherm model were expressed by the equilibrium parameter separation factor, i.e. R L , represented by the equation given below: The resultant values of the R L equation signify the biosorption isotherm type: if the R L value is equal to 1 then the biosorption isotherm is considered as linear, if 0 is less than R L and R L is less than 1 this indicates biosorption is a favourable condition, if R L is greater than 1 then the biosorption isotherm is in an unfavourable condition and if R L is equal to 0 then the biosorption isotherm indicates an irreversible reaction. log

Freundlich isotherm model
The Freundlich isotherm constants 1/n and K f are considered from the intercepts and slopes of the linear plot of logQ e versus logC e .

Biosorbent characterization
Fourier transform infrared (FT-IR) spectrum The surface functional groups on the activated carbon of the biosorbent were examined using FTIR. The Fourier transform infrared (FT-IR) spectrum was recorded on a Thermo-Nicolet Nexus 670 FT-IR spectrophotometer (Ther-moFisher Scientific Inc., Madison, WI, USA) using KBr pellets containing 1% finely ground EPS and the spectrum was collected at a resolution of 4 cm À1 in the wavenumber region of 400-4,000 cm À1 .

Scanning electron microscope (SEM)
The SEM is one of the most versatile instruments available for the examination and analysis of microstructure morphology and chemical composition characterizations. The surface morphology analysis of the biosorbent was performed using a scanning electron microscope (SEM). The experimental solution was added to aluminium stubs and air dried. The sample was gold sputtered using an SC7620 Sputter Coater device and analyzed by scanning electronic microscopy.

Brunauer-Emmett-Teller (BET) analysis
The surface area of the biosorbent was determined by using a Brunauer-Emmett-Teller (BET) analyzer. The BET surface area was determined at 77 K by nitrogen adsorption using a Micromeritics ASAP 2020 V3.04 H surface area analyzer.

Fourier transform infrared spectroscopic (FTIR) analysis
The FTIR spectrum of Tamarindus indica activated seed coat biomass after biosorption (shown in Figure 3) was analyzed to understand the nature of the functional groups. The number of peaks displayed from 4,000 to 500 cm À1 in the FTIR spectra represents the complex nature of the biosorbent. The first peak was found in the range of the wavelength 3,300-3,600 cm À1 , i.e. due to the biosorption of fluoride onto the biosorbent surface.

Effect of pH
The pH of an aqueous solution is a significant monitoring

Effect of contact time (min)
The time required for Tamarindus indica activated seed coat biomass to equilibrate upon the adsorption of the fluoride ions was recorded at various time intervals. It was observed that the contact time required for adsorption was directly proportional to fluoride removal. As shown in Figure 4(c), a gradual increase was observed from ten minutes to 60 minutes and then a fall was seen in the peak after 60 minutes.
This indicates that the adsorption capacity of the activated seed coat was rapid and active in less than 60 minutes.
Another study proposed that the rapid nature of the activated biocarbon helps in quick adsorption of fluoride on to its surface. A fall in the biosorption after a certain time period could be due to limited surface area or previously occupied ions (Dong & Wang ).

Effect of initial fluoride ion concentration
At a constant pH of 6.0 and 60 minutes of contact time, an attempt was made to identify the initial fluoride ion con- (). These results obviously show that temperature is an unfavourable factor for fluoride sorption on the biosorbent.

Biosorption isotherm models
The process of a biosorption isotherm expresses the biosorbent and sorbate equilibrium relationship and also expresses the adsorbed ion distribution within the liquid and solid phases in the equilibrium state. In addition to this, equilibrium modeling also gives the information to study the mechanism involved in the Hence, the present Tamarindus indica activated seed coat n value (1.49) was between 0 and 10, which indicates a good biosorption capacity.

Biosorption kinetics
The kinetics explains the sorption mechanism of the acti-  results also correlated with the pseudo-first-order models.
The kinetics of the sorbate and biosorbent interface depends on many factors such as the characteristic features of sorbate and biosorbent, pH of the aqueous medium, temperature of the biosorption reaction, typical time of contact between sorbate and biosorbent and mass-transport course of action (Achak et al. ).
Pseudo-first-order and pseudo-second-order kinetic model plots are shown in Figure 5(c) and 5(d).

Thermodynamic parameters
In order to study the feasibility of the process using Tamar Gibbs free energy, ΔG ¼ À2:303ÃRT Ã log 10 K L Enthalpy, ΔH ¼ log 10 K L (T 2) À log 10 K L (T1) where, R is the ideal/universal gas constant (i.e. 8.314 J mol À1 K À1 ), T is the absolute temperature (K), and    Table 3.

Desorption studies
It is necessary to regenerate and reuse biosorbent to maintain the cost-effectiveness of the product and maximize its efficiency. Also, it is noteworthy that during desorption of the biosorbent it should not undergo any stressful damage that affects its biosorption capacity. In the present study, we regenerated the activated Tamarindus indica seed coat biomass with 10% sodium hydroxide (NaOH) solution and recovery efficiency was tested for up to three cycles of biosorption and desorption. The elution efficiency was increased to 90% with 10% NaOH in all three cycles. This high regeneration capacity of the biosorbent indicates that the overall biosorption system is efficient enough to remove fluoride from fluoride-contaminated water.

CONCLUSION
In the present study, preparation of activated carbon from tamarind seeds, characterization and fluoride removal