Indian Journal of Chemical Technology

Vol . l O. March 2003. pp. 2 1 7-222

A rticles

Cashewnut sheath carbon: A new sorbent for defluoridation of water

R Si vabalan", S Rcngara/, B an umathi Arabindoo" & Y M urugesan*

"Department or Chemistry, Anna University. Chennai 600 025. I ndiJ hDcpartlllcnt of Environmental Science and Engineering. Kwangju Institute of Science and Technology. Kwangju.

South Korea

Received 26 A ugusl 200 J; revised received 25 October 2002; {{('('{'pled 8 }mlllarv 2003

Cashcwnut sheath, an agricultural waste discarded largely in India is identified for the preparation of an effective sorb­ent for fluoride removal. Experiments were conducted for the sorption of fluoride from aqueous solution using activated carbon from cashewnut sheath (CSC) in two phases, namely, batch studies and column studies. The i nfluence of pH. ad­sorbent dose, particle s ize and contact t ime was investigated in batch studies. The adsorption process fol lows Freundlich ad­sorption isotherm. Continuous flow experiments in fixed bed col umns packed w ith activated carbon were carried out in or­der to assess the feasibi l i ty of this for field applications. The carbon bed after exhaustion was regenerated with 0. 1 N hydro­chloric acid. The influence of i n it ial concentration of fluoride ion, flow rate, particle size and concentration of the regener­ant on the performance of the column was studied. Comparative study was conducted w ith commercial l y available carbon impregnated with 2% aluminium sulphate sol ution. The resul ts revealed that CSC is found to be active and effic ient for

fluoride removal.

The social and physiological i mpact on chi ldren wi th unsightly discolouration of the teeth due to mottl i ng i s wel l -documented ' . I t i s wel l-known that fluoride i n drink ing water below 1 .0 mg/L reduces the preva­lence of dental caries by 50-60%. The chronic and tox ic effects of excessive in take of fluoride are usu­ally observed as skeletal abnormal i ties or damage

2.

The effects range from stiffness and rheumatism to a permanent cri pp l ing skeletal r igidity3. According to World Health Organisation (WHO) the maximum acceptable concentration4 i n drink ing water is 1 .5 mg/L 4• Hence there is a need for defluoridation of drink ing water i n order to prevent the occurrence of fluorosis .

Defluoridation has long been practiced in water supply and a variety of methods for fluoride removal are already known5.6. The removal methods i nclude chemical prec ipi tation7 , adsorption on activated alu-

. 8 .

h C) I d' I ' d mtna , Ion exc ange , e ectro ta YSls an reverse os-mosis . The high cost of acti vated carbon has st imula­ted i nterest i n examin ing the feasibi l i ty of using cheaper raw materials. M any reports have appeared on the development of acti vated carbon from cheaper and readi ly avai l able materials from various

, o·p I I . I sources -. n recent years some ow cost matena s

* For correspondence (E-Mai l : v_murugu @hotmai l .com; Fax: +9 1 -44-2200660)

such as tree bark 1 3 , fi sh bone charcoal '4 , almond shells ' 5, o l ive stones ' 6, o i I palm shel ls 1 7, coconut shel l ' 8, etc. have been used for the preparation of act i ­

vated carbon. I n the search for new and low cost agri­cul tural wastes as source material for activated car­bon, attempts have been made to prepare act ivated carbon from cashewnut sheath (esC) by chloride pro­cess " . Extensive characterisation studies have been performed to assess the suitab i li ty of the act ivated carbon. The defluoridation studies were conducted i n batch a s wel l a s column type conti nuous tlow meth­ods.

Experimental Procedure

Preparation and characterisation of activated

carbon

Acti vated carbon from cashewnut sheath (A/lacar­

diulIl occidentale L) was prepared by chloride proc­ess. I ni t ia l ly, the raw material was washed thoroughly wi th water to remove earthy matter and dried at

I 1 0°e. The material was broken i n to smal l pieces and soaked in 1 0% calc ium chloride solution for 24 h . The

soaked material was dried at I I Ooe and then sub­jected to pyrolysis fol lowed by thermal activation at

8S0-900oe for 30 min . After activation the carbon was washed w i th 1 0% hydrochloric acid to remove residual i norganic i mpurities and washed with water

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repeatedly and finally dried at 1 1 O°e. The carbon was

ground and sieved to particle s ize in the range of 300-800 micron for further characterisat ion.

The characterisation of act ivated carbon was car­ried out by adopti ng the standard procedures

1 9-2 1 . The moisture content of the carbon was determi ned by heat ing a known weight of the sample in an air oven

mai ntained at 1 1 0°C for about 4 h. The resi due was

ign i ted in a muffle furnace at 1 000°C for about 3 h to determine the ash content. Iron content in the ash was

determined by atomic absorption spectrophotometer (Perkin-Elmer 2380). A known quant i ty of carbon

was digested w i th freshly boi led water and the pH was measured wi th Orion p H electrode. Decolouris ing power of the carbon was determi ned us ing methylene blue solution. The amount of carbon required for 90% removal of phenol was taken as the phenol number. Surface area measurement was carried out us ing M i­

cromeritics pulse chemisorb 2700 equipment. The characteristics of the CSC are presented in Table I .

Batch study

Batch type adsorption experi ments were conducted with 1 00 mL of aqueous solution contai n ing 1 0 mg/L fluoride, adj usted to different pH values. The solution was taken i n leak proof reaction bottles. The solutions

were equ i l ibrated for 24 h i n a mechanical shaker at

27± 1 °e. After the equ i l ibration period, the carbon

was fil tered and fluoride in the fi l trate was est imated using pH/ISE meter (Model 7 1 0A , Orion Research Inc. , USA). Experi ments were performed to under­stand the effect of contact t i me, partic le s ize and car­bon dosage. Desorption studies were conducted us ing d i lute hydrochloric acid . The removal effic iency of CSC was compared wi th alumin ium sulphate impreg­nated commercial act ivated carbon .

Continuolls flow column experiment Generally, continuous flow adsorption process i m­

parts advantages over batch operation. Unless the se­

lect iv i ty of a particu lar ion to be removed is very large, batch operations lead to an i neffic ient u t i l i sa­tion of the adsorption capac i ty of the sorbent owing to the decrease of solute concentration as adsorption continues. In cont inuous operation, the adsorbent is permanently i n contact wi th solut ion of same concen­tration. Glass column of 25 mm diameter and 600 m m length was connected t o a reservoir o f 5 L capaci ty contai n ing fluoride solut ion, and a peristal t ic pump

was connected at the bottom of the col u mn to control the flow rate. The column was operated continuously

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I ndian J. Chern. Techno!. . M arch 2003

Table I -Characteristics of activated carbon

51 No. Control test

I Bu lk density (g/cc)

2 Moisture content (%)

3 Ash content (%)

4 Fixed carbon content (%)

5 Matter soluble i n water (%)

6 Matter soluble in acid (%)

7 pH

S Decolouris ing power ( mg/g)

9 Phenol number (mg)

1 0 Ion exchange capacity (meq/g)

I I Surface area (rn2/g)

Grade [ (294-7 1 0 Il) Grade [ [ (70-294 Il)

1 2 Iron content (%)

CSC

OA5

1 1 .75

6.09

93.9 1

2.70

5 .00

3 .6

49.5

0 .33

0.007

393. 8 1 000.0

N i l

CAC

O.OS 1 2.57

2.9 1

97.09

1 .5 5

4.58

9.2

7:1.5

5 . 1 5

N i l

296

N i l

and water height was mai ntai ned constant throughout the operation to mai ntain the flow accurately. A known quantity of carbon was weighed, sieved, washed wi th dis t i l led water to remove carbon fines and then packed closely in the column by tapping to m in im i se air voids in the carbon bed.

Results and Discussion

The close examination of the resul ts of characteri­sation s tudy shows that the CSC possesses less bulk dens i ty than commercial act ivated carbon, CAe. The h igh moisture content is due to the porous surface of the carbon. However, no attempt was made to deter­

mine the porosity of the carbons. Surface area of CSC i s more than CAe. The surface area values of the two varieties of CSC differ in part ic le s ize which clearly i ndicates that fine particles wi th greater surface area are more su i table for adsorption process. A pre l imi­nary screen i ng s tudy was performed to assess the sui tab i l i ty of CSC for the removal of tluoride from aqueous solut ion.

Effect of pH

I n order to opt imise the p H for effic ient fluoride removal, batch experiments were conducted wi th 1 00 mL of 1 0 mg/L of fluoride contain i ng known quanti ty of CSC and alumin ium su lphate i mpregnated CAC i n the p H range o f 2- 1 3 and the results are presented i n Fig. I . I t i s evident that percentage removal o f fluo­

ride i s h igher and remai ns constant i n a wide pH range of 3- 1 0 and above pH 10 i t rapidly decreases.

Sivabalan el at. : Cashewnut sheath carbon: A new sorbent for deOuoridation of water A rticles

1 00

80

� ;> 0 60 E

� (1) :2 0 40

.3 u.. ;;R 0 20

0

0 2 4 6 8 1 0 1 2 1 4

p H

Fig. I -Fluoride removal versus p H

60% removal is reached a t p H 2 and shows zero effi ­ciency at pH 1 3 . H ence pH of the solution was mai n­tai ned at 6.9 for further studies. In the case of CAC, the maximum removal effici ency of 55% was ob­served in a pH range 5-8. Thus the CSC is efficient i n the removal of fluoride over a wide range of pH val­ues.

Effect of carbOll dose and particle size

The results of variation i n the removal of fluoride for different doses (0. 1 to 2 g) of the two types of CSC and CAC at a contact period of 24 h are g iven i n Fig. 2 . An increase i n the fluoride removal i s observed as the dose of the adsorbent i ncreases and also the overal l percentage removal is more for the finer

adsorbent (grade " 70-294 ).1) than the coarser

adsorbent (grade I 294-7 1 0 ).1) . This is because the fine particles provide more surface area for adsorption. The increase in the rate of defluoridation is observed up to 8 and 5 giL for grade I and " carbons respectively. Further i ncrease i n the dosage does not show any s ignificant removal efficiency. This i s due to the overlapping of the act ive s i tes at higher concentrations and thus reduce the net surface area22

• The resul ts clearly i ndicate that the opt imum carbon dosage of 7 and 5 giL of grade I and I I carbons respectively are required for 80.0% removal of fluoride. B u t in the case of CAC a h igher dose of 1 4 giL shows a lesser efficiency (56%).

Effect of contact time

Fig. 3 exhibits the vanatlOn of fluoride removal from aqueous solution CSC and CAC with respect to contact t ime. The removal of fluoride increases with time unti l an equi l ibrium fluoride concentration in the

-; ;> 0 E � " � 0 :::l w:: � 0

-; ;> 0 E � ., :2 0 :::l i:i: � 0

1 00

80

60

40 -- Grade I __ Grade II

20 __ CAC

0

0 0.4 0.8 1 .2 1 .6 Caroon dosage (g)

Fig. 2--Fluoride removal versus carboll dosage

1 00 ·

80

60

40

20

0

0 4 8

-+- Grade 1 I

1 2 Contact time (h)

-- Grade I -- CAC

1 6 20

Fig. 3-Fluoride removal versus contact t ime

2

24

effluent is attained. It is observed that 30.8% removal takes p lace within 30 min of contact t ime with 8 giL of grade I carbon and 50.4% removal of fluoride is observed wi th i n 2 h whi le a total of 9 1 .6% removal i s achieved only i n the next 22 h . I n the case of grade I I carbon a dose of 5 giL removes 40.5% fl uoride with in 30 min and 55 .6% within 2 h whi le a total of 80.9% removal i s achieved in the next 22 h. The increase in the rate of fluoride removal is not very s ignifi cant after 1 6 and 1 2 h of contact t ime for grade I and I I carbons respectively. I n the case o f CAC, 56% re­moval takes place within 4 h of contact t i me. This data c learly indicates that CSC is more effective than CAC for the removal of fluoride. The resul ts of batch study experiments for the removal of tluoride are gi ven in Table 2.

Desorption of fluoride

Di lu te hydrochloric acid has been found to be the

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Tnblc 2-0ptimum condi t ions for mnximum adsorption of tluoride on CSC and CAC

SI No. Conditions CSC CAC

OJ) E " -e � '\) " v .." 'L: 0 ::J C

2 3

4

1 0

R

6

4

2

0

0

Adsorben t dosage (gIL)

Grade 1 (294 -7 1 0 �l) 8 1 4

Grade " (70-294 �l) 5

pH 6.9 6.9 Contact t ime (hI')

Grade I (294 - 7 1 O �) 1 6 6

Grndc " (70-294 �) 1 2

Perccntage removal (%) Grade I (294 -7 1 0 �l) 87.6 56

Gmde " (70-294 �l) 75 . 1

-+- CSC

--- CAC

2 3 4 5 Concentration o f regcnerant (N & % )

Fig. 4-Desorption of fluoride

sui table reagent for the desorption of fluoride. Vari­ous concentrations viz, 0.02, 0.04, 0.06, 0.08, 0. 1 , 0.2, 0.4, 0.5 and 1 .0 N HCI were attempted for the de­sorption study. From the resu l ts (Fig. 4) it is observed that approxi mately 0. 1 N HCI is requ i red for quant i ­tat ive desorption of fluoride from cashewnut sheath carbon . Above 0. 1 N, there is no s ign ificant change i n desorption capaci ty . Hence 0 . 1 N HCI has been cho­sen for regeneration. In the case of CAC 2% solut ion of al umin ium sulphate showed good regeneration .

Adsorptioll isotherms The Freundl ich model was appl ied for adsorption

equ i l ibrium for both CSC and CAe.

log (X/III) = log k + ( l /n) log Co

The resu l ts reveal that the adsorption of lluoride on CSC and CAC under opti mum condit ions at room

220

I ndian 1. Chclll. Tecll llol . . March 2003

1 .2

0.8

E 0.4 -

-X 0

• bIJ .£

-0.4 • CAC

-0.8

0 0.5 1 .5 2

log ee

Fig. 5-Freundl ich ndsorption equation for CSC and CAC sys­tems

1 0

� 8 --- 2.6 E - .. - 5 . 1 "Cl

6 c: ---6- 7.42 :l

<2 - 1 0. 1 " 4 :g 0 ::J 2 -w::

0 0 2 4 6 R 1 0

Volume treated i n l i tres

Fig. 6--Brenkthmugh curves of column runs with d i fferen t i n i t ia l fluoride concentration

temperature (27± 1 "C) obeys Freundl ich adsorption i sotherm. Freundl ich adsorption i sotherm represents the rel at ionsh i p between the amount of fluoride ad­sorbed by per un i t mass of the adsorbent (x/m) and the concentration of fluoride at equ i l i br ium (CJ. The constants k and 11 represent the adsorption capaci ty and i n tensi ty of adsorption respectively. The data ob­tai ned i n thi s study fi t wel l w i th Freundl ich adsorption i sotherm (Fig. 5). The plot of log (x/m) versus log Co for various i n i ti al concentration i s found to be l i near i ndicating the applicab i l i ty of Freundl ich adsorption i sotherm. The h igher values of k ( 1 .358 for CSC and 0.226 for CAC) i ndicates greater affi n i ty for fluoride and the Il value ( 1 .92 for CSC and 1 . 89 for CAC) shows good adsorption character of the carbon�:1 .

Continuolls flow column study

The performance of esc i n the removal of tl uoride has been studied i n column operation by vary ing the

Sivabalan ef al. : Cashewnut sheath carbon: A new sorbent for defluoridation of water A rticles

parameters viz . , flow rate, i n i tial fluoride concentra­

tion, particle size of the CSC and concentration of the

regenerant. Fig. 6 shows the breakthrough curves for different in i ti al fluoride concentrations. The saturated column is regenerated with 0. 1 N hydrochloric acid . I n order to fix the opti mum flow rate of the i nfluent for maximum fluoride removal , the fluoride solution i s passed through the column at various flow rates. Table 3 shows the effect of flow rate of the i nfluent on the defluoridation capacity. The defluoridation

capacity is maximum for a flow rate of 300 mUh. Though the defluoridation capaci ty of grade I I carbon

is higher (2522 mg/kg) than grade I carbon (2256 mg/kg), the flow rate decreases with decreas ing part i­cle s ize. Hence grade I carbon i s more sui table for tl uoride removal as the flow rate is h igh and the ad­sorption capacity i s also reasonable. CAC fluoride removal capacity is poor of 638 mg/kg.

An in teresting and noteworthy feature of the col­umn operation is that the total fluoride removal ca­paci ty of the bed appears to be h igher for water hav­

ing h igh fluoride content. Water contain ing 2.6 mg/L of fluoride shows low uptake capacity. However, the uptake capacity i ncreases wi th i ncrease in i n i tial fluo­ride concentration up to 7.6 mg/L. Beyond this con­centration, the adsorption capacity decreases consid­erably (Table 4). S i mi l ar studies conducted in the regeneration of the exhausted bed has shown that 0. 1 N HCl i s the su itable regenerant. The column opera-

Tahle 3-EfTect of flow rate of the influent on the defluoridation capacity

Flow rate Detluoridation capaci ty mLlh mg/kg

CSC CAC

300 2256 260

600 1 1 6 1 439

900 426

Table 4-lnfluence of in i t ia l fluoride concentration on defluori­dation capacity

In i tia l concentration of solu­tion

mg/L

2.6

5 . 1

7.6

1 0.0

Defl uoridation capacity mg/kg

727

1 088

23 1 0

1 8 5 1

t ion was stopped when the effluent fluoride concen­tration exceeds the permissible l im i t of 1 .5 mg/L to

prevent overexhaustion of the bed. Each col umn was recharged 6 t imes before rejecting the bed material .

Conclusion Activated carbon prepared from cashewnut sheath,

a waste generated in the agricul tural sector, is capable of removi ng fluoride effecti vely from aqueous so lu­t ion. The removal of fluoride is more with fi ner vari­ety of carbon rather than the coarse. The study indi­

cates that removal of fluoride from aqueous solution depends on pH, contact ti me, particle s ize and dose of adsorbent . The adsorption of fluoride from aqueous sol u ti on with cashewnut sheath and commercial act i ­vated carbon conforms to a Freundlich equation based

on the formation of mono layer. Detluoridation ca­pacity is max imum at a flow rate of 300 mUh. As this flow rate can be conveniently mai ntai ned with grade I carbon, this carbon i s su i table for detluoridation. 0. 1 N HCl has been chosen for regeneration of the ex­hausted adsorbent. Each col umn i s recharged 6 ti mes before rejecting the carbon bed . An in teresting feature of the study is that tluoride uptake capacity i ncreases

with h igher in i ti al fluoride concentration.

Acknowledgement The authors gratefu l ly acknowledge the Raj i v Gan­

dhi National Dri nk ing Water Mission, Min i stry of

Rural Areas and Employment, New Delhi for finan­cial support to carryout this research work . Authors also wish to p lace on record the necessary fac i l i ties and congenial atmosphere provided by Anna Uni ver­s i ty , Chennai .

References M anahan S E, Toxicological Chemistry, 2nd edn. (Lewis Publ isher. M ichigan). 1 992.

2 Kempf G & McKay E, Mottled Enamel in a Segregated

Population (US Publ ic Health Repons), 45, 1 930. 2923. 3 Donnel T A O', The Chemistry I!i' Fluorine ( Pergamon Press.

Elmsford, New York). 1 973 . 4 World Health Organisation, Fluorille alld Fluorides. Ell vi­

TOllfIlental Health Criteria 36. Geneva. 1 984. 5 K i l ledar D J & B hargava D S. J Illst Pub Health Ellgineer.l'

(India), I ( 1 988) 6. 6 K i l l edar D 1 & Bhargava D S. J IllSt Pub Heallh Engineers

(India), 2 ( 1 988) 37 . 7 Nawlakhe W G, Kul karni D G, Pathak 13 N & l3ulusu K R .

Indian J Environ Hltl!. 1 7 ( I ) ( 1 975) 26.

8 Rubel, 1r F & Woosley Dale R. JA WWA , 7 1 ( I ) ( 1 979) 45.

9 Castle C, Schweizer M , S i monot M 0 & Sardin M. Chem

Eng Sci, 55 (2000) 334 1 .

22 1

A rticles

1 0 Deitz V R , Bibliography (�l Solid A dsorbellls, United States

Cane Sligar Re/ineries and B(lIIe Char Mmll(lactllres and

National Bllreall (�l S/{Indards (Washi ngton DC). 1 994. I I Rengaraj S, Banumathi Arabindoo & M urugesan V . Indian J

Chem Tecilllol, 6 ( 1 999) I . 1 2 Srin ivasan K. Bahvani Shankar Rao P. Periasamy K &

Ramadevi A. J Indian COllnc Chemists, 4 (2) ( 1 988) 6 1 . 1 3 Teles de Vascoreios L A & Gonzalez Beca C G, Eur Water

Pollllt Contml, 7(3) ( 1 997) 7 . 14 B hargava D S & Ki l ledar D J , Ill(lian J Eng Maler Sci. 2

( 1 995) 1 57 . 1 5 R i vera-Utri l la J & Ferro-Garcia M A , Carbon. 2 5 (5) ( 1 987)

645. 1 6 Lopez-Gonzalez J de D, M artinez-V i lchez F & Rodriguez­

Rei noso F. Carboll, 1 8 (6) ( 1 980) 4 1 3 .

222

I nd ian J . Chem. Techno! . , M arch 2003

1 7 Husse i n M Z. Tarmizi R S H. Zainal Z & I brah im R. Car­

bon. 34 ( I I ) ( 1 996) 1 447. 1 8 Laine J , Calafat A & Labady M . Carbon. 27 (2) ( 1 989) 1 0 1 . 1 9 lS I . A ctivated Carbon. Powdered and Gmnillar-Methods or

SOli/piing alld Tests ( Bureau of I nd ian Standards. New Delhi) . IS 877: 1 989.

20 APHA. Stalldard Methods Fir the Examination or Water {//ul

Wastewater, 1 7th edn. ( American Water Works Association. New York). 1 989.

2 1 Vogel A I , A Text book or Qllalllilatil'e Inorg(lI1ic /\ 1I(r/ysis,

3rd edn. (ELBS, London), 1 969. 22 K i l ledar D J & B hargava D S. J II/St Ellgineers (India), 70

( 1 990) 47. 23 M cKay G. B lair H S & Garden J R. J Appl PO/VII/ Sci, 1 27

( 1 982) 3034.