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Journal of Environmental Chemical Engineering

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Fluoride adsorption from aqueous solution using activated carbon obtainedfrom KOH-treated jamun (Syzygium cumini) seed

Ramya Araga, Shantanu Soni, Chandra S. Sharma⁎

Creative & Advanced Research Based On Nanomaterials (CARBON) Laboratory, Department of Chemical Engineering, Indian Institute of Technology, Hyderabad, Kandi-502285, Telangana, India

A R T I C L E I N F O

Keywords:Jamun seedKOH activationActivated carbonFluoride ionAdsorption

A B S T R A C T

In this study, for the first time, jamun seed derived activated carbon was used as an adsorbent for removal offluoride from water. Activated carbon was prepared by KOH activation of jamun seed and subsequent pyrolysisat 900 °C. The fluoride sorption experiments were carried out under batch mode to optimize the various in-fluencing parameters such as contact time (0–3 h), dosage of adsorbent (20–500 mg), initial fluoride con-centration (2–20 mg L−1), temperature (298–308–318 K), and pH (2.5–10). The contact time and pH for max-imum fluoride uptake were observed at 120 min and 2.5 respectively. Maximum adsorption capacity (3.65 mg g-1) of fluoride on activated carbon was found for 10 mg L−1 of initial fluoride concentration using 0.4 g L−1

adsorbent dosage. The equilibrium data were found to follow Dubinin-Radushkevich isotherm among the threeapplied isotherm models (Freundlich, Temkin, and Dubinin-Radushkevich) and pseudo-second-order kineticmechanism with the rate constant of 0.036 g mg−1 min−1. Thermodynamic analysis revealed that the adsorp-tion process was exothermic in nature. Performance of the prepared adsorbent was compared with other re-ported biomass derived activated carbons and it was observed that the proposed adsorbent is efficient in terms ofits adsorption capacity. In addition to synthetic samples, field water samples collected from fluoride affectedvillages were also tested for adsorption experiments.

1. Introduction

Consumption of contaminated water will have hazardous influenceson human health. Among the various aquatic pollutants, fluoride is oneof the most abundant elements present in environment and it con-taminates the groundwater by both natural and artificial causes [1].Fluoride concentration in drinking water is a real health concern be-cause ingestion of excessive fluoride (permissible value of fluoride indrinking water suggested by World Health Organization (WHO) is1.5 mg L−1) causes thyroid disorder, Alzheimer’s syndrome, osteo-sclerosis (brittle bones) and dental/skeletal fluorosis [2]. The excessconcentrations of fluoride in groundwater have been recognized inmany developed and developing nations including USA, Asia, andAfrica. In India, fluoride was firstly detected in 1937 at Nellore districtof Andhra Pradesh and later it has been found that 17 states of India areaffected with high fluoride content, especially Telangana, Rajasthan,Madhya Pradesh, Uttar Pradesh and Gujarat [3].

Many methods have been developed to reduce the fluoride contentfrom water, namely adsorption, coagulation and precipitation, ion ex-change and membrane process [3,4]. However, ion exchange and

membrane processes include high operational and maintenance cost,toxic sludge generation, and complicated procedure to treat thefluoridated water. Coagulation and precipitation is the traditionalmethod being used for the removal of fluoride from water and Nal-gonda technique is widely used to treat the fluoride contaminated waterin many countries but the resulting high residual aluminum con-centration (2–7 mg L−1) is the major disadvantage of this technique.Among all these techniques, adsorption is widely studied and adoptedbecause of its low cost, easy maintenance, simple operation and theefficient performance [4,5].

Other than the conventional adsorbents that are being used fordefluoridation purpose such as activated alumina and alumina modifiedadsorbents, various unconventional biomass waste materials have beentested and reported for the fluoride removal in recent years e.g., acti-vated coconut fiber dust [6], sugarcane bagasse [7], activated carbonsderived from coconut shell [8], coconut fiber [9], rice straw [10], teaash [11], banana peel and coffee husk [12], bael shell [13], and Con-ocarpus erectus [14] because of their low cost, easy availability andefficient fluoride removal. Different functional groups present on thesurface of biomass-based adsorbents such as hydroxyl, carboxyl,

http://dx.doi.org/10.1016/j.jece.2017.10.023Received 16 June 2017; Received in revised form 2 October 2017; Accepted 12 October 2017

⁎ Corresponding author.E-mail address: cssharma@iith.ac.in (C.S. Sharma).

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Available online 18 October 20172213-3437/ © 2017 Elsevier Ltd. All rights reserved.

T

carbonyl, amide and amine may involve in the physiochemical inter-actions of fluoride ions [3].

The major objective of this study is to evaluate the fluoride removalefficiency of a novel and low-cost adsorbent which is jamun seed de-rived activated carbon from aqueous solution by carrying out the batchadsorption experiments. Among various chemical reagents (KOH,H3PO4, ZnCl2 etc.), KOH was found as efficient activating agent becauseof its well-defined micropore size distribution and high surface area ofresultant carbon material [15]. The jamun seed powder was im-pregnated with KOH solution to increase the surface area, consequentlyadsorption capacity. Various influencing parameters of defluoridationprocess viz., contact time, adsorbent dosage, initial fluoride con-centration, temperature, and pH were optimized and further desorptionstudies were also carried out to check the reusability of adsorbent. Thesuitability of prepared adsorbent was tested by field water samplescollected from the nearby fluoride affected villages.

2. Materials and methods

2.1. Materials

Jamun seed powder was purchased from Patanjali Ayurved Limited,India. Potassium hydroxide (KOH) was purchased from Merck, India.Sodium fluoride (NaF, 99%) and Sodium hydroxide pellets (NaOH,98%) were purchased from Alfa Aesar. Hydrochloric acid (HCl, 37%)was purchased from Sigma-Aldrich, India. All the materials were usedas received without any further purification. Deionized distilled water(resistivity of 18.2 MΩ-cm, Millipore) was used for all of the experi-ments.

2.2. Preparation of adsorbent

Jamun seed powder was mixed with the aqueous solution of KOH,with the precursor to KOH ratio of 1:0.1 by weight, and stirred at roomtemperature for 2 days as reported elsewhere [15]. Later the treatedsample was dried at 110 °C for 12 h to remove the water content. As-obtained dried sample was pyrolyzed in a tubular high-temperaturefurnace (Nabertherm GMBH) under high purity nitrogen (N2) (99.99%)atmosphere. N2 flow rate was maintained at 1.25 l min−1 after initialpurging. The pyrolysis process consisted of following steps: (a) fromroom temperature to 350 °C with the heating ramp of 2 °C min−1 (b) adwell at 350 °C for 30 min (c) from 350 °C to 900 °C with the heatingramp of 5 °C min−1 (d) dwell at 900 °C for 1 h and finally (e) coolingfrom 900 °C to room temperature. After cooling, the sample was takenout from the furnace tube and washed several times with hydrochloricacid solution (0.1 M) and distilled water to remove the residual KOH.The activated carbon sample was then dried at 110 °C overnight andcrushed into a fine powder prior to adsorption studies, which was la-beled as activated jamun seed (AJS). Another set of sample which is nottreated with KOH but pyrolyzed is also prepared (labeled as pyrolyzedjamun seed, PJS) for comparison of adsorption capacities.

2.3. Characterization

The morphology of as-prepared jamun seed derived inactivated andactivated carbon was examined using field emission scanning electronmicroscopy (FESEM) (Zeiss). The structural characterizations of carbonsamples were investigated by using powder X-ray diffractometer (XRD)(PANalytical) with Cu Kα radiation (0.154 nm) and Raman spectro-meter (Senterra, Bruker) with an excitation wavelength of 532 nm. Thepresence of functional groups in the prepared samples was investigatedby Fourier transform infrared spectroscopy (FTIR) (Tensor 37, Bruker).The Brunauer-Emmett-Teller (BET) surface area was calculated by theadsorption/desorption isotherms of N2 conducted at 77 K using BETsurface analyzer (Quantachrome autosorb). The isoelectric point ofKOH treated and pyrolyzed jamun seed was measured by zeta potential

analyzer (Delsa nano, Beckman Coulter).

2.4. Batch adsorption experiments

All the adsorption experiments were carried out with 50 ml ofknown concentration of fluoride solution. After adding a known weightof adsorbent, the solution contained conical flasks were shaken in ashaking incubator (RIS-24 Plus, REMI India) at the speed of 150 rpm.Once the equilibrium is attained, the solution was taken out from theflask and filtered through the vacuum filtration system (Millipore). Theresidual fluoride concentration was determined with ion chromato-graphy (Basic IC Plus, Metrohm AG).

The amount of fluoride adsorbed by the carbon sample (qe, mg g−1)and percentage adsorption (%) were calculated using the followingequations respectively.

=−q V C C

W( )

ei e

(1)

=−

×C C

CPercentage adsorption (%) ( ) 100i e

i (2)

Where Ci and Ce are the initial and equilibrium concentrations of F− insolution (mg L−1); V is the volume of solution (L), and W is the weightof adsorbent (g) respectively.

2.5. Desorption experiments

In order to examine the regeneration ability of AJS, desorptionstudies were carried out with the fluoride adsorbed activated carbon.Initially, fluoride was adsorbed onto activated carbon by using10 mg L−1 of fluoride solution onto 300 mg of adsorbent. Once theequilibrium was reached, solution was filtered and the residue (fluorideadsorbed activated carbon) was further subjected to desorption.Desorption studies were carried at different pH values (4–10) by theaddition of 0.1 M HCl or 0.1 M NaOH and the suspensions weremaintained for 30 min in orbital shaker at the speed of 100 rpm. Laterthe residue was filtered, the filtrate fluoride concentration was mea-sured and the procedure was repeated at least for five times using thesame activated carbon.

3. Results and discussion

3.1. Characterization of adsorbent

3.1.1. MorphologyThe surface morphology of the untreated jamun seed powder, PJS

and AJS were examined by FESEM (as shown in Fig. 1). The particlesize distribution of untreated jamun seed powder (Fig. 1a and b) israndom and the particles were found to be agglomerated during theKOH treatment and pyrolysis process. Those agglomerated particleswere grinded using a mortar to obtain a fine powder and used for SEMimaging and later for fluoride adsorption studies. SEM micrographs ofthe activated sample (Fig. 1e and f) illustrate that the activated carbonparticles are porous in nature and such type of porosity is not seen inthe inactivated carbon sample (Fig. 1c and d).

3.1.2. Structural characterizationsThe powder X-ray diffraction patterns were recorded to study the

crystalline structure of the prepared charcoal samples and are shown inFig. 2(a). XRD patterns of both inactivated and activated carbon showthe broad noncrystalline peaks at 2θ of 25° and 43° corresponds to re-flections of (0 0 2) and (1 0 0) planes. The absence of distinct peaks forthe crystalline forms of carbon is evident for the disordered nature ofsamples [16].

Fig. 2(b) shows the deconvoluted Raman spectra of synthesizedinactivated and activated charcoal samples and it displays two broad

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peaks centered at about 1350 and 1580 cm−1, which are associatedwith characteristic D band (1350 cm−1) and G band (1580 cm−1). Theintensity ratio of D to G (ID/IG) is calculated to be 0.93 for inactivatedcarbon and 1.06 for activated carbon, which indicates both the carbonsare disordered carbons [17].

Disordered carbons (hard carbons) are known to have good ad-sorption properties due to the availability of high surface area and more

active sites on the surface because of randomly arranged crystallitesand graphene layers [16,18]. Consequently, jamun seed derived hardcarbon was tested for removal of fluoride.

3.1.3. Functional group characterizationThe FIIR spectra of prepared carbon samples were presented in

Fig. 3. No significant changes have been observed in the characteristic

Fig. 1. FESEM images of (a-b) untreated jamun seedpowder, (c-d) PJS and (e-f) AJS.

Fig. 2. (a) XRD and (b) Raman spectra of PJS and AJS.

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absorption peaks of the inactivated and activated carbon samples. Eventhough numerous functional groups were present on the surface ofcarbon, the peaks centered at 803 cm−1 (]CeH bend), 1028 cm−1 and1092 cm−1 (CeO stretch), 1260 cm−1 (CeO Stretch), 1630 cm−1

(C]C Stretch) and 3437 cm−1 (OeH stretch) are prominent amongthem [19]. Among these functional groups, the fluoride ions can bindeffectively to eOH and eCO functional groups [6].

3.1.4. Surface area characterizationThe surface area of product inactivated and activated carbons were

determined by degassing 100 mg of sample mass at 300 °C for 4 h toremove any adsorbed species before N2 adsorption experiment.Nitrogen adsorption isotherms were measured at 77 K adsorptiontemperature. The surface area calculated from BET plots (shown in Figs.4a and 4b ) is as follows: 13.67 m2 g−1 for PJS and 747.45 m2 g−1 forAJS whereas total pore volume is determined as 0.019 cm3 g−1 for PJSand 0.41 cm3 g−1 for AJS. The BET surface area and pore volume of theactivated carbon are increased prominently and it is evident of the greatinfluence of porogen addition to the biomass.

3.1.5. Point of zero charge determinationThe pH at which surface charge of the adsorbent takes a zero value

is defined as point of zero charge (pHPZC). At pHPZC, the charge ofpositive surface sites is equal to charge of negative surface sites. If thesolution pH is higher than pHPZC, the sorbent surface is negatively

charged and it could interact with positive species in the solution. If thesolution pH is lower than pHPZC, the sorbent surface is positivelycharged and it could interact with negative species in the solution [13].

The zeta potential of activated carbon suspension at different pHvalues was determined by zeta potential analyzer. For this purpose, thepH of carbon suspensions was adjusted to initial values between 2 and11, by adding either HCl or NaOH. Then zeta potential was measuredand plotted against initial pH, as illustrated in Fig. 5. The pH at whichzero-crossing occurs is taken as pHPZC and the value of pHPZC for theprepared activated carbon sample is determined as 4.9. To attract thefluoride ions, the adsorbent surface must be positively charged andpositive carbon surface can be achieved when the pH of the solution islower than pHPZC which is 4.9.

3.2. Adsorption experiments

3.2.1. Effect of agitation timeInitially, the adsorption of fluoride on prepared activated carbon

was studied as a function of time in order to find out the equilibriumtime for maximum adsorption. The time profile of fluoride adsorptiononto activated carbon is presented in Fig. 6. As-prepared activatedcarbon of 50 mg was used for 50 ml of 10 mg L−1

fluoride solution. Theamount of fluoride adsorption (mg g−1) increased with increase inagitation time initially, but gradually it became sluggish and ap-proached to nearly a constant value at the time of 120 min, beyondwhich there is no further adsorption. The active sites which are avail-able in the initial stage of adsorption become limited due to the bindingof fluoride ions. The occupation of remaining vacant active sites by the

Fig. 3. FTIR spectra of jamun seed derived carbon before and after activation.

Fig. 4. BET surface area analysis of jamun seed derivedcarbon (a) before and (b) after activation.

Fig. 5. Effect of pH on zeta potential of jamun seed activated carbon in aqueous sus-pension.

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fluoride ions is difficult because of the formation of repulsive forcesbetween adsorbate on the solid surface and the liquid surface [6].Further the data obtained were successfully used to evaluate the ad-sorption kinetics which were discussed in section 3.3.2.

3.2.2. Effect of adsorbent dosageFig. 7 shows the amount of fluoride removed as the function of

carbon dosages at 10 mg L−1 of initial fluoride concentration. Theamount of dosage was varied from 20 to 500 mg per 50 ml of fluoridesolution under the optimized condition of agitation time. The resultshows that by increasing the amount of adsorbent, the percentage re-moval also increases but decreases the adsorption capacity. Percentageadsorption has attained the equilibrium at the adsorbent dosage of300 mg (optimum weight of activated carbon) beyond which there is nosubstantial removal of fluoride.

Various factors are reported which can cause the effect of adsorbentdosage. Those includes: (a) with an increase in the adsorbent dosage,the unsaturation of adsorption sites may lead to drop in adsorptioncapacity (b) agglomeration of adsorbent particles at higher doses maylead to reduce in the surface area and increase the diffusional pathlength [10].

3.2.3. Effect of initial fluoride concentrationThe effect of initial fluoride solution concentration on adsorption

process was studied under optimum conditions of shaking time andcarbon dose as illustrated in Fig. 8. The adsorption capacity was variedfrom 0.1 mg g−1 to 1.25 mg g−1 with an increase of fluoride

concentration from 2.7 mg L−1 to 20.04 mg L−1, whereas the fluorideremoval efficiency is initially high for low F− concentrations because ofthe more interaction of F− ions with the adsorption sites. Further, itreduced at higher concentrations due to the saturation of availableactive sites [13].

3.2.4. Effect of solution pHIt can be seen from Fig. 9 that the adsorption is strongly dependent

on pH. Fluoride ion removal increases with a decrease in pH and theoptimum pH was observed at the value of 3. Jamun seed derived ac-tivated carbon acquires a positive charge at low pH (as discussed in thesection 3.1.5). Thus, high percentage of adsorption in the acidicmedium can be explained by the attractive forces of surface positivecharges towards the F− ions and low percentage adsorption in alkalinemedium is attributed to the repulsion of F− ions by the gradual increaseof negative surface charges. A similar effect of pH on fluoride adsorp-tion has been reported by other authors as well [10,11]. In case ofpH<3, no significant improvement was observed in the adsorptioncapacity of the prepared adsorbent material. Therefore, all the batchsorption experiments were carried out at the pH value of 3.

3.2.4.1. Possible adsorption mechanism:. The pH of solution was foundto be one of the most important factors that control the adsorption ofF− ions onto surface of activated carbon. The fluoride adsorption ontoprepared charcoal can be explained by the following two-step ligandexchange reaction as reported in the literature [10,20,21]:

Fig. 6. Effect of agitation time on F− adsorption of jamun seed derived activated carbon.

Fig. 7. Effect of carbon dosage on% adsorption and adsorption capacity of fluoride ionsonto activated jamun seed derived carbon.

Fig. 8. Effect of initial fluoride concentration on% adsorption and adsorption capacity offluoride ions onto as-prepared jamun seed derived activated charcoal.

Fig. 9. Effect of pH on adsorption of fluoride ions onto jamun seed derived activatedcarbon.

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-COH + H+ ↔ COH2+ (3)

-COH2+ + F− ↔ CF + H2O (4)

Hence, the combined reaction is

-COH + H+ +F− ↔ CF + H2O (5)

Where −COH represents the surface hydroxyl group of activatedcarbon. Another mechanism is proposed by Y. Wang et al. [22] as fol-lows:

COH + F− ↔ CF + OH− (6)

However, the difficulty with this mechanism is at higher pH valuesthere would be competition between the OH− and F− ions for the ac-tive sites of adsorbent.

3.2.5. Effect of temperature on fluoride removalThe rate of diffusion of adsorbate molecules through the boundary

layer and internal pores of adsorbent will increase with respect to in-crease in temperature due to decrease in the thickness of boundarylayer [23]. So, the effect of temperature on the fluoride adsorption rateonto prepared activated carbon was investigated at three differenttemperatures, i.e. 298, 308 and 318 K and presented in Fig. 10.

With the jamun seed derived activated carbon the adsorption ca-pacity was found to be higher at 25 °C up to the fluoride ion con-centration of 5.5 mg L−1, but there is no significant variation in theadsorbent performance at the temperatures of 25 and 35 °C. Later athigher concentrations, maximum capacity was attained at 45 °C and ifwe increase the temperature further to 55 °C, desorption occurred. Atmuch higher solution concentrations i.e., beyond 10 mg L−1, the lessertemperatures were found to be favorable for adsorption.

3.3. Adsorption isotherms, kinetics and thermodynamics study

3.3.1. Adsorption isotherms studyAdsorption isotherm data reveals the distribution of adsorbate mo-

lecules between the solid phase and liquid phase when the adsorptionprocess reaches the equilibrium. We can predict the adsorption capacityby analyzing the isotherm data and this is one of the key parametersrequired to design the adsorption system [24]. The experimental ad-sorption equilibrium data of F− ions onto jamun seed resultant acti-vated charcoal was fit into the linearized isotherm models of Freun-dlich, Temkin, and Dubinin-Radushkevich. The mathematicaldescriptions of these models are given by the following equations[24–26]:

Freundlich adsorption isotherm: = +q Kn

Clog log 1 loge f e (7)

Kf (mg g−1) and n are Freundlich isotherm constants related to ad-sorption capacity and adsorption intensity.

Temkin adsorption isotherm:

= +q B C B Aln lne e (8)

=B RTb (9)

A (L g−1) and b (J mol−1) are Temkin isotherm constants where b is theheat of adsorption.

Dubinin-Radushkevich (D-R) adsorption isotherm:

= −q q Kεln lne m2 (10)

= +ε RTC

ln(1 1 )e (11)

=EK

12 (12)

Where qm is the D-R theoretical maximum adsorption capacityFig. 10. Effect of temperature on adsorption of fluoride ions onto prepared activatedcarbon.

Table 1Summary of equilibrium isotherm parameters for the fluoride ion adsorption onto acti-vated jamun seed derived carbon at different temperatures.

Temperature (K) Freundlich constant

Kf (mg g−1) N R2

298 0.035 0.477 0.928308 0.013 0.377 0.959318 3.02 0.199 0.761

Temperature (K) Temkin constant

A (Lmg−1) b (J mol−1) R2

298 0.45 1783.7 0.927308 0.41 1686.9 0.904318 0.38 1336.6 0.968

Temperature (K) D-R constant

qm (mg g−1) K (mol2 kJ−2) E (kJmol−1) R2

298 2.27 3.44 0.38 0.986308 2.53 4.07 0.35 0.985318 8.42 8.18 0.25 0.939

Fig. 11. Dubinin-Radushkevich plots for fluoride adsorption onto activated jamun seedcarbon at different temperatures.

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(mg g−1), K is D-R isotherm constant related to adsorption energy(mol2 kJ−2), ε is Polanyi potential and E is the mean free energy ofadsorption (kJ mol−1).

qe is adsorption capacity of activated carbon at equilibrium(mg g−1), Ce is equilibrium concentration of adsorbate (mg L−1) and Ris the universal gas constant (8.314 J mol−1 K−1). The isotherm con-stants of the above-mentioned models obtained by least square methodare listed in Table 1. The fitting of these isotherms is judged based onthe value of regression coefficient (R2).

By comparing the results presented in Table 1, adsorption offluoride ions onto prepared activated carbon was found to be well fitted

with D-R isotherm because the R2 values (varied from 0.986 to 0.939)are higher for D-R isotherm than both Freundlich and Temkin iso-therms. The values of K and qm were calculated from the slope andintercept of ln qe verses ε2 plots as shown in Fig. 11.

The values of E were found as 0.38, 0.35 and 0.25 kJ mol−1 at ex-perimental temperatures, which confirms that the F− adsorption ontoprepared jamun seed derived activated carbon is physisorption innature (E < 8 KJ mol−1 indicates physical adsorption). The majoradvantage of physisorption is it provides better regeneration scope[27].

3.3.2. Adsorption kinetics studyAdsorption kinetics describes the adsorption mechanism of ad-

sorbate on adsorbent and it depends on the nature of adsorbent,structural properties, adsorbate concentration and adsorbent-adsorbateinteractions. To understand the mechanism of F− ions onto jamun seedresulted activated carbon, the data were tested with pseudo-first-orderand pseudo-second-order kinetic models. The linear form of kineticequations is as follows [25]:

Pseudo-first-order model:

− = −q q q k tlog( ) log2.303e t e cal,exp ,

1(13)

Where qe,exp (mg g−1) is the adsorption uptake at equilibrium, qt(mg g−1) is the amount of fluoride adsorbed on the adsorbent at thetime of t, k1 (min−1) is the first-order rate constant, qe,cal and k1 werecalculated from the intercept and slope of log (qe–qt) versus t plot aspresented in Fig. 12(a).

Pseudo-second-order model:

Fig. 12. Kinetic modeling of F− adsorption onto AJS(a) Pseudo-first-order model (b) Pseudo-second-order model (initial F− concentration: 10 mg L−1,pH:3, Temperature: 298 K).

Table 2Kinetic constants for fluoride adsorption onto jamun seed derived activated carbon.

Initial Fluoride Concentration (mg L−1) Pseudo-first-order model Pseudo-second-order model

qe,exp (mg g−1) k1 (min−1) qe,cal (mg g−1) R2 qe (mg g−1) K2 (g mg−1 min−1) R2

10 2.5 0.02 1.094 0.941 2.675 0.036 0.997

Fig. 13. Van’t Hoff plot of fluoride ion adsorption onto jamun seed activated carbon.

Table 3Thermodynamic parameters of F− adsorption onto jamun seed activated carbon.

Temperature (K) ΔG° (kJ mol−1) ΔS° ( × 10−3 kJ mol−1 K−1) ΔH° (kJ mol−1)

298 2.8 −42.98 −9.95308 3.3318 3.7

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= +tq k q

tq

1

t e e22 (14)

Where k2 (g.mg−1 min−1) is the rate constant of second-order ad-sorption, qe and k2 were determined from the slope and intercept of t/qtversus t plot as shown in Fig. 12(b).

Constants determined for the kinetic models are presented inTable 2. The regression coefficient (R2 = 0.997) for the pseudo-second-order adsorption model is higher than the value obtained for pseudofirst-order kinetics (R2 = 0.941) and closer to unity. However, the R2

value of the pseudo first-order model is not satisfactory. In addition,adsorption capacity calculated (qe = 2.675 mg g−1) from the second-order kinetics is in good accordance with the experimental adsorptioncapacity of F− ions (qe,exp = 2.5 mg g−1). So, the F− adsorption ontojamun seed derived activated carbon is well fitted with pseudo-second-order reaction kinetic model.

3.3.3. Adsorption thermodynamics studyInvestigation of thermodynamic parameters (Gibbs energy change

(ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°)) plays an im-portant role in estimating the adsorption mechanism. The mentionedparameters were determined using the following equations [28]:

= −ΔG RT Klnoo (15)

= −K ΔSR

ΔHRT

ln oo o

(16)

Where R is the universal gas constant (8.314 J mol−1 K−1), T is theabsolute temperature (K) and Ko is the thermodynamic equilibriumconstant. The values of ΔH° and ΔS° were determined from the slopeand intercept of Van’t Hoff plot of ln Ko versus 1/T as shown in Fig. 13and are summarized in Table 3.

The positive values of ΔG° suggests that adsorption process is non-spontaneous and it requires some energy from an external source for theadsorption reaction. The negative value of ΔH° suggests that the ad-sorption phenomenon is exothermic while the negative entropy valueimplies a lesser degree of randomness at the solid-solution interface[28]. It causes the adsorbate molecules to escape from the solid phase toliquid phase, therefore the amount of adsorption will decrease [29].

3.4. Desorption study

Regeneration is an important step to develop a cost-effective ad-sorbent for fluoride removal. Desorption experiments were carried atdifferent pH values (4–11) with fluoride loaded adsorbents and deso-rption efficiency was found to be 96% beyond the pH of 6. It indicatesthat the recyclability of the jamun seed resulted activated carbon ispossible and henceforth recommended.

3.5. Field studies

The applicability of prepared jamun seed derived activated carbonwas tested with three different real water samples collected from thenearby fluoride affected villages (Nalgonda district, Telangana, India).

Fluoride concentration of the samples was measured as 2.65, 3.1 and3.2 mg L−1. The prepared adsorbent reduced the fluoride concentrationfrom above-mentioned values to the permissible level by using 15 g L−1

of adsorbent dose for 50 ml of sample, pH = 3, contact time = 120 minand temperature = 298 K. For field water sample the adsorbent dosagewas higher compared to the synthetic samples because of its less re-moval capacity at the lower concentrations as well as the presence ofco-anions viz., Cl− and NO3

− which competes for active sites of ad-sorbent.

3.6. Comparison with other biomass derived adsorbents

Table 4 shows the comparison that has been made between thejamun seed derived activated carbon and other biomass-based carbonadsorbents for the fluoride removal. Maximum adsorption capacity wasused as a parameter for comparison. The defluoridation capacity of AJSwas found to be superior over other adsorbents except for tea ash.

4. Conclusion

The activated carbon prepared from the jamun seed was successfullyused as an adsorbent for defluoridation of synthetic and field watersamples. Following conclusions can be drawn from the results of pre-sent study:

• High surface area carbon material was obtained by the pyrolysis ofjamun seed powder upon the addition of KOH as activating agent.

• Protonated surface hydroxyl groups of prepared adsorbent partici-pated in the defluoridation process.

• The equilibrium isotherm data are fitted well to Dubinin-Radushkevich model and adsorption kinetics could be well re-presented by pseudo-second-order kinetic model. Thermodynamicinvestigation reveals the non-spontaneity and exothermic nature ofadsorption.

• The minimum and maximum uptake of F− ions were found to be0.1 mg g−1 and 1.25 mg g−1 at the concentrations of 2.7 mg L−1

and 20.04 mg L−1 (dose: 6 g L−1, pH: 3, temperature: 25 °C, time:2 h) respectively.

• Recycling of activated carbon is feasible due to 96% of desorption.

• Maximum removal efficiency of PJS was measured as 3% (initial F−

concentration: 10 mg L−1, dose: 2 g L−1, pH: 3, time: 2 h), becauseof the availability of less surface area of adsorbent (as confirmed byBET analysis).

Moreover, the decreasing of fluoride concentration from realgroundwater samples to the permissible limits shows the scope ofprepared adsorbent on a real-time basis. Hence, it can be concluded thatthe as-prepared jamun seed derived activated carbon have the potentialto be effectively used for fluoride removal from water.

Contribution

CSS planned the work while RA and SS performed all the experi-ments. All authors analysed the data and prepared the manuscript.

Acknowledgement

RA acknowledges Dr. Manohar Kakunuri for his valuable discus-sions and suggestions, which significantly improved the quality of thismanuscript. CSS acknowledges Indian Institute of Technology,Hyderabad for providing necessary research facilities.

References

[1] WHO, Guidelines for Drinking Water Quality First Addendum to Third Addition,Vol.1, Recommendations, (2006).

Table 4Comparison of various biomass-derived carbons used for fluoride ion removal.

Adsorbent Adsorption capacity (mg g−1) Reference

Tea ash 3.75 [11]Banana peel 0.31 [12]Coffee husk 0.29 [12]Bael shell 2.45 [13]Coconut shell 0.96 [30]Morringa indica 0.23 [31]Jamun seed (activated) 3.65 Present workJamun seed (inactivated) 0.80 Present work

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