dactylon-based ACTIVATED CARBON

Transcription

1 GANAPATY ALAGUMUTHU VELLAISAMY VEERAPUTHIRAN RAMASWAMY VENKATARAMAN Chmistry Rsarch Cntr, Sri Paramakalyani Collg, Tamilnadu, India SCIENTIFIC PAPER UDC :58: DOI: /HEMIND A FLUORIDE SORPTION USING Cynodon dactylon-based ACTIVATED CARBON This study dals with th application of Cynodon dactylon-basd thrmally activatd carbon for fluorid toxicity. Th batch adsorption tchniqu was followd at nutral ph as a function of contact tim, adsorbnt dos, adsorbat concntration, tmpratur and th ffct of co-anions. Th data indicat that th prpard adsorbnt surfac sits ar htrognous in natur and that fits into a htrognous sit-binding modl. Th prsnt systm followd th Rdlich Ptrson isothrm as wll as th Langmuir adsorption isothrm modl. Lagrgrn psudo-first-ordr, psudo-scond-ordr, intra particl diffusion and Elovich kintics wr modld to dscrib th adsorption rat of fluorid, and dtrmind as this schm followd psudo-scond-ordr kintics. Th calculatd nthalpy chang, ΔH, and ntropy chang, ΔS, for th adsorption procss wr kj/mol and J/mol K, rspctivly, and showd ndothrmic xprinc. Instrumntal analysis of XRD, FTIR and SEM givs an ida about th fluorid binding ability of adsorbnt. Th high fluorid lvl in drinking watr has bcom a critical halth hazard of this cntury as it inducs intns impact on human halth including skltal and dntal fluorosis [1]. Though fluorid is an ssntial constitunt for both humans and animals, it can b ithr bnficial or dtrimntal to human halth dpnding on th lvl of fluorid in drinking watr [2]. In India, this problm is common in placs such as Andhra Pradsh, Tamilnadu, Karnataka, Krala, Rajasthan, Gujarat, Uttar Pradsh, Punjab, Orissa and Jammu and Kashmir [3]. Th fr fluorid lvl in drinking watr was idntifid at 3.02 mg/l in Kadayam block of Tamilnadu [4]. A fluorid survy in Nilakottai block of Tamilnadu showd a positiv corrlation btwn th prvalnc of dntal fluorosis in childrn and lvls of fluorid in portabl watr is 3.24 mg/l [5]. Adsorption is on of th significant tchniqus in which fluorid is adsorbd onto a mmbran, or a fixd bd packd with rsin or othr minral particls. Many natural and low cost matrials such as rd mud [6,7], zirconium imprgnatd coconut shll carbon [8], cashw nut shll carbon [9], ground nut shll carbon [10] and clays [11] hav bn usd as adsorbnts for fluorid rmoval from drinking watr. Rcntly, amorphous alumina supportd on carbon nanotubs [12], alignd carbon nanotubs [13], ion xchang polymric fibr [14], and an ion xchangr basd on a doubl hydrous oxid of Al and F (F 2 O 3 Al 2 O 3 H 2 O) [15] hav bn assayd for rmoving fluorid from drinking watr as wll as industrial wastwatr. Thus, it is important to dvlop or find chapr adsorbnts for fluorid rmoval from watr that hav gratr fluorid adsorption capacitis lik th abov said adsorbnts. This papr concntrats on invstigating low cost matrials for fluorid sorption which can ffctti- Corrsponding author: G. Alagumuthu, Chmistry Rsarch Cntr, Sri Paramakalyani Collg, Alwarkurichi , Tamilnadu, India. livivra@yahoo.co.in Papr rcivd: 12 July, 2010 Papr accptd: 16 Sptmbr, 2010 vly rmov fluorid from aquous solutions at a rlativly low lvl. Th novl adsorbnt obtaind by burning, carbonization and thrmal activation of th Cynodon dactylon, posssss an apprciabl dfluoridation fficincy. Th thrmally activatd carbon should hav a high surfac ara and strong sorption capacity towards various sorbats [16]. This adsorbnt is abundantly availabl in all dry and wt lands in hug amount. W rport hr th rsults of dfluoridation studis using Cynodon dactylon. This study lads to th assumption that fluorid dposition occurs by th forcs of adsorption ovr th surfac of th activatd carbon and this was charactrizd by th surfac morphological studis of th adsorbnt matrial. In addition, th dynamics and kintics of th adsorption procss ar discussd. EXPERIMENTAL Adsorbnt prparation Th Cynodon dactylon sampl, common nam Brmuda grass, was collctd from narby Wstrn Ghats ara and washd thoroughly with doubl distilld watr. Th matrial was thn kpt asid for shadow dry and thn drid at K for 24 h. It was washd with doubly distilld watr in ordr to rmov th fr acid and drid at th sam tmpratur for 3 h. Latr, th drid adsorbnt was thrmally activatd in Muffl furnac at 1073 K (hr w avoid acid tratmnt for charring). Th rsulting product was coold to room tmpratur and sivd to th dsird particl sizs, namly, <53, , , and msh. Finally, th product was stord in vacuum dsiccators until rquird. Sorption xprimnts Th sorption isothrm and kintics xprimnts wr prformd by batch adsorption xprimnts and wr carrid out by mixing 1.25 g (obtaind by th study ffct of adsorbnt dos) of sorbnt with 100 ml of sodium fluorid containing 3 mg/l as initial fluorid 23

2 concntration. Th mixtur was agitatd in a thrmostatic shakr at a spd of 250 rpm at room tmpratur. Th dfluoridation studis wr conductd for th optimization of various xprimntal conditions lik contact tim, initial fluorid concntration, adsorbnt dos, particl siz and influnc of co-ions with fixd dosag. Th ragnts usd in this prsnt study ar of analytical grad. A fluorid ion stock solution (100 mg/l) was prpard and othr fluorid tst solutions wr prpard by subsqunt dilution of th stock solution. All th xprimnts wr carrid out at room tmpratur. Fluorid ion concntration was masurd with a spcific ion slctiv lctrod by us of total ionic strngth adjustmnt buffr II (TISAB II) solution to maintain ph and to liminat th intrfrnc ffct of complxing ions [9]. Th ph of th sampls was also masurd by Orion ion slctiv quipmnt. All othr watr quality paramtrs wr analyzd by using standard mthods [17]. Th kintic studis of th sorbnt wr carrid out in a tmpratur controlld mchanical shakr. Th ffct of diffrnt initial fluorid concntrations, viz., 2, 4, 6, 8 and 10 mg/l at four diffrnt tmpraturs, viz., 303, 313, 323 and 333 K on sorption rat wr studid by kping th mass of sorbnt as 1.25 g and volum of solution as 100 ml in nutral ph. Th fluorid concntration rtaind in th adsorbnt phas, q (mg/g), was calculatd according to [18]: q c c W 0 = (1) whr c 0 and c ar th initial and rsidual concntration at quilibrium (mg/l), rspctivly, of fluorid in solution; and W is th wight (g) of th adsorbnt. Charactrization of sorbnts Th physicochmical proprtis of th activatd carbon ar shown in Tabl 1. Th X-ray diffraction (XRD) pattrn of adsorbnt was obtaind using a Brukr AXS D8 Advanc, Inst ID: OCPL/ARD/ X-ray diffractomtr. Fourir transform infrard (FTIR) spctra wr rcordd using a Nicolt 6700, Thrmo Elctronic Corporation, USA, spctrophotomtr. Th scanning lctron microscopy (SEM) analysis prformd using a Philips XL-20 lctron microscop. Computations wr mad using a Microcal Origin (Vrsion 6.0) softwar. Th accuracis of fits ar discussd using rgrssion corrlation cofficint, r, and chi-squar analysis (SSE). Th chi-squar statistic tst is basically th sum of th squar of th diffrnc btwn th xprimntal data and data obtaind by calculating from th modls, with ach squard diffrnc dividd by th corrsponding data obtaind by calculating from th modls. Th quivalnt mathmatical statmnt [19] is: χ ( q q, m) 2 2 =Σ (2) q, m whr q,m is quilibrium capacity obtaind by calculating from th modl (mg/g), and q is xprimntal data of th quilibrium capacity (mg/g). Thory of isothrm and kintic modling Th abilitis of four widly usd isothrms, th thortical Langmuir, mpirical Frundlich, Tmkin and Rdlich Ptrson isothrms, to modl th adsorption quilibrium data wr xamind. To xprss th mchanism of fluorid adsorption onto th surfac of adsorbnt, th kintic modls psudo-first-ordr, psudo-scond-ordr, intra particl diffusion, Elovich modls and thrmodynamic paramtrs ar usd to analyz th prsnt adsorption data to dtrmin th rlatd kintic paramtrs. Th Langmuir adsorption isothrm [20], prhaps th bst known of all isothrms, is oftn applid in solid/liquid systms to dscrib th saturatd monolayr adsorption. It can b rprsntd as: q q K c m a = (3) 1 + Kac whr q m is q for a complt monolayr (mg/g); K a is th adsorption quilibrium constant (L/mg). To valuat th adsorption capacity for a particular rang of adsorbat concntration, th aformntiond quation (Eq. (3)) can b usd in th following linar form: c 1 1 = c + (4) q q K q m a m Th constants q m and K a can b dtrmind by Eq. (4) from th slop of th linar plot of c /q vrsus c. Tabl 1. Txtur proprtis of th Cynodon dactylon basd activatd carbon No. Physical paramtr Valu 1 Surfac ara, m 2 /g Dnsity, g/cm Particl siz, µm >53 3 Spcific volum, dm 3 /kg Moistur, mass % Ash contnt, mass %

3 Frundlich adsorption isothrm [21], basd on adsorption on htrognous surfac is th arlist known rlationship dscribing th adsorption quilibrium and is givn by: q 1 KFc n = (5) whr K F and 1/n ar mpirical constants, indicating th adsorption capacity and adsorption intnsity, rspctivly. Equation (5) may b convrtd to a linar form by taking logarithms: 1 log q = log KF + log c (6) n Th plot of log q vrsus log c of Eq. (6) should rsult in a straight lin. From th slop and intrcpt of th plot, th valus for n and K F can b obtaind. Th Tmkin isothrm [22], th simpl form of an adsorption isothrm modl, has bn dvlopd considring th chmisorption of an adsorbat onto th adsorbnt, is rprsntd as: q = a+ blog c (7) whr q and c hav th sam maning as notd prviously and th othr paramtrs ar calld th Tmkin constants. Th plot of q vrsus log c will gnrat a straight lin. Th Tmkin constants a and b can b calculatd from th slop and intrcpt of th linar plot. Th Rdlich Ptrson isothrm [23] contains thr paramtrs and incorporats th faturs of th Langmuir and Frundlich isothrms. It can b dscribd as follows: Ac q = (8) + g 1 Bc Equation (8) can b convrtd to a linar form by taking natural logarithms: c A g c B q = + (9) ln( 1) ln ln Th thr isothrm constants, A, B, and g (0 < g < < 1), can b valuatd from th linar plot rprsntd by Eq. (9) using a trial and rror optimization mthod [24]. Th psudo-first-ordr kintic modl, th Lagrgrn rat quation, has bn th most widly usd rat quation for assigning th adsorption of an adsorbat from a liquid phas sinc 1898 [25]. Th linar form of th psudo-first-ordr quation is rprsntd as: kl log( q qt ) = log q t (10) whr q t is th adsorption capacity (mg/g) at tim t; k L (min 1 ) is th rat constant of th psudo-first-ordr adsorption raction. Th plot of log (q q t ) vrsus t should giv a straight lin from which k L and q can b calculatd from th slop and intrcpt of th plot, rspctivly. Th psudo-scond-ordr modl was dvlopd by Ho and McKay [26] to dscrib th adsorption of som mtal ion onto th adsorbnt. Th linar form of th psudo-scond-ordr adsorption kintic rat quation is xprssd as: t 1 t = + (11) q k q q 2 t 2 whr q and q t hav th sam maning as mntiond prviously, and k 2 is th rat constant for th psudoscond-ordr adsorption raction (g/mg min 1 ). Th valu of q and th psudo-scond-ordr rat constant, k 2, can b calculatd from th slop and intrcpt of th straight lin obtaind from th plot of t/q t vrsus t. Th initial adsorption rat can b obtaind as q t /t whn t approachs zro: h = k q (12) whr h 0 is th initial adsorption rat (mg/g min 1 ). In intraparticl diffusion modl [27], th adsorbat movs from th solution phas to th surfac of th adsorbnt particls in svral stps. Th ovrall adsorption procss may b controlld by on or mor stps (.g., film or xtrnal diffusion, por diffusion, surfac diffusion and adsorption on th por surfac, or a combination of mor than on stp). In a rapidly stirrd batch procss of adsorption, th diffusiv mass transfr can b rlatd by an obvious diffusion cofficint, which will fit xprimntal adsorption rat data. Normally, a procss is diffusion-controlld if its rat is dpndnt on th rat at which componnts diffus toward ach othr. Th possibility of intraparticl diffusion was xplord using th intraparticl diffusion modl: q = k t + C (13) t 1/2 id whr k id is th intraparticl (por) diffusion rat constant (mg/g min 0.5 ) and C is th intrcpt that givs an ida about th thicknss of th boundary layr. Th largr th valu of C, th gratr th boundary-layr ffct. Elovich modl [28], which is basd on chmisorption phnomna, is xprssd as in a linar quation [29] as: qt 1 = β lnαβ + ln t (14) β whr α is th initial sorption rat (mg/g min 1 ) and β is th dsorption constant (g/mg) during th xprimnt. From th intrcpt and slop of th straight lin obtaind from th plot of q t vrsus ln t, th valus for α and β wr calculatd. 25

4 Thrmodynamic paramtrs. Th thrmodynamic paramtrs for th adsorption procss in solution hav bn calculatd using th following standard thrmodynamic rlations: ΔG = RTln K 0 (15) whr ΔG is th standard fr nrgy chang (kj/mol), T is th tmpratur (K) and R is univrsal constant (8.314 J mol 1 K 1 ). Th sorption distribution cofficint, K 0, for th sorption raction was dtrmind from th slop of th plot ln (q /c ) against c at diffrnt tmpraturs and xtrapolating to zro c [10]. Th sorption distribution cofficint may b xprssd in trms of ΔH and ΔS as a function of tmpratur: ln K 0 = ΔH /RT + ΔS /R (16) whr ΔH is th standard nthalpy chang (kj/mol) and ΔS is th standard ntropy chang (kj mol 1 K 1 ). Th valus of ΔH and ΔS can b obtaind from th slop and intrcpt of a plot of ln K 0 against 1/T [30]. RESULTS AND DISCUSSION Effct of contact tim and initial fluorid concntration Contact tim plays a vry important rol in adsorption dynamics. Th ffct of contact tim on adsorption of fluorid onto Cynodon dactylon is shown in Figur 1. Batch adsorption studis using th concntrations 2.0, 3.0, 4.0, 6.0, 8.0 and 10.0 mg/l of fluorid solution and with 1.25 g of th adsorbnt wr carrid out at 303 K as a function of tim to valuat th dfluoridation and adsorption rat constants. Th adsorption of fluorid incrass with tim and gradually attains quilibrium aftr 105 min. From Figur 1, th tim to rach quilibrium conditions appars to b indpndnt of initial fluorid concntrations. Thrfor 105 min was fixd as minimum contact tim for th maximum dfluoridation of th sorbnt. Th adsorption of fluorid dcrasd from 84 to 51% by incrasing fluorid concntration from 2.0 to 10.0 mg/l. Furthr, it was obsrvd that th rmoval curvs ar smooth and continuous, indicating th possibility of th formation of monolayr covrag of th fluorid ion at th intrfac of adsorbnt. Effct of particl siz Th dfluoridation xprimnts wr conductd using Cynodon dactylon with fiv diffrnt particl sizs, viz. >53, , , and µm. As th adsorption procss is a surfac phnomnon, th dfluoridation fficincy of th sampl with 53 µm rgistrd high dfluoridation fficincy du to largr surfac ara. Th prcntags of fluorid rmoval by th sampl with diffrnt particl sizs ar studid. Hnc, th matrial with particl siz of 53 µm has bn chosn for furthr xprimnts. Highr prcntag of adsorption by Cynodon dactylon with smallr particl siz is du to th availability of mor spcific surfac ara on th adsorbnt surfac. Figur 1. Prcntag of fluorid adsorbd on Cynodon dactylon vrsus tim for diffrnt initial fluorid concntrations. Influnc of adsorbnt dos Th influnc of varying concntrations of adsorbnt on th adsorption of fluorid at nutral ph is shown in Figur 2. Whil incrasing th adsorbnt dos proportional rmoval obsrvd for fluorid until som xtnt. Aftr that, th curv laps as flat indicating th highr fluorid adsorption occurs at 1.25 g and th followings rmains constant. A distribution cofficint, K D, rflcts th binding ability of th surfac for an lmnt. Th K D valus of a systm mainly dpnd on ph and typ of surfac. Th distribution cofficint valus for fluorid and Cynodon dactylon at nutral ph wr calculatd [31] by mans of: Figur 2. Variation of fluorid rmoval for diffrnt adsorbnt dosags at constant tmpratur. 26

5 K c s D = (15) cw whr c s is th concntration of fluorid in th solid particls (mg/kg) and c w is th concntration in watr (mg/l). It is sn that th distribution cofficint incrass with an incras in adsorbnt concntration, indicating th htrognous surfac of th adsorbnt [18]. It is obsrvd in Figur 3 that K D incrass with an incras in adsorbnt concntration at constant ph. If th surfac is homognous, th K D valus at a givn ph should not chang with adsorbnt concntration. All th forthcoming xprimnts wr carrid out using a constant adsorbnt dos of 1.25 g. Figur 4. Intrfrnc of co-ions on dfluoridation studis using Cynodon dactylon adsorbnt. Figur 3. Th rlationship btwn distribution cofficint K D with diffrnt adsorbnt dosags. Effct of intrfring co-ions Th ffcts of coxisting anions such as sulfat, nitrat, chlorid, and bicarbonat on fluorid adsorption by th Cynodon dactylon adsorbnt wr xamind and th rsults ar givn in Figur 4. Chlorid and nitrat ions did not noticably intrfr with fluorid rmoval vn at a concntration of 500 mg/l, whil sulfat ions bgan to show som advrs ffcts whn th SO 4 2- concntration incrasd. Howvr, bicarbonat showd grat comptitiv adsorption with fluorid. Th fluorid adsorption amount dcrasd quickly from 83.7 to 51.5% with th incras of bicarbonat concntration mg/l, and thn dcrasd slightly with furthr incras of bicarbonat concntration. This may b attributd to th comptition of bicarbonat ions with th fluorid ions at th activ sit, on th surfac of th sorbnts. Th slctiv natur of th fluorid by th sorbnt dpnds on siz, charg, polarizability, lctrongativity diffrnc, tc. Th ordr of intrfrnc for fluorid rmoval obsrvd as in th following ordr, HCO 3 >SO 4 2 >Cl NO 3 for th adsorbnt Cynodon dactylon. A similar trnd was rportd whil studying zirconium imprgnatd cashw nut shll carbon as a sorbnt for fluorid rmoval [9]. Adsorption isothrms Th quilibrium data isothrm analysis for fluorid adsorption onto th Cynodon dactylon at ph 7.0 (±0.2) and at tmpraturs of 303, 313, 323 and 333 K ar shown in Figurs 5 8. Rsults indicat that th adsorbnt has a high affinity for fluorid adsorption undr ths conditions. Th quilibrium data has bn analyzd by linar rgrssion of isothrm modl quations, viz. Langmuir (Figur 5), Frundlich (Figur 6), Tmkin (Figur 7), and Rdlich Ptrson (Figur 8). Th rlatd paramtrs obtaind by calculation from th valus of slops and intrcpts of th rspctiv linar plots ar shown in Tabl 2. Th prsnt data fit th Langmuir and Rdlich Ptrson modls (Figs. 5 and 8) wll (r 2 > 0.99). Th avrag monolayr adsorption capacity, q m, obtaind for Cynodon dactylon is mg/g. Th valu Figur 5. Langmuir isothrms obtaind by using linar mthod for th adsorption of fluorid using activatd adsorbnt at various tmpraturs. 27

6 for th Rdlich Ptrson constant A that is obtaind as high ( L/g) for th maximum optimizd valu for g ( ), which indicats th high affinity of Cynodon dactylon for fluorid. Ths high g-valus of th Rdlich Ptrson modl rquird to dscrib th bst fit of th prsnt data indicatd that th adsorption of fluorid is du to th Langmuir monolayr surfac adsorption. Howvr, th Frundlich isothrm modl, basd on multilayr adsorption, dscrib th data fairly wll (r in th rang). Th Frundlich adsorption constants, K F, obtaind from th linar plot ar btwn 3.0 and 3.4. Th Frundlich cofficint, n, which should hav valus ranging from 1 to 10, is high ( ), and that supports th favorabl adsorption of fluorid onto th adsorbnt. Th linar plot for Tmkin adsorption isothrm, which contains th faturs of chmisorption, rlativly dscribd th prsnt isothrm adsorption data (r is in th rang). This indicatd that th adsorption of fluorid onto th adsorbnt might b happnd by chmisorptions with physical forcs (i.., physisorption followd by chmisorption). Hnc, th ordr of isothrm quations obyd by th prsnt data is Rdlich Ptrson > Langmuir > Frundlich > Tmkin isothrm. Th ffct of isothrm shap can b usd to prdict whthr an adsorption systm is favourabl or unfavourabl. Tan t al. [32] usd th ssntial faturs of th Langmuir isothrm which can b xprssd in trms of a dimnsionlss constant sparation factor or quilibrium paramtr R L, dfind by th following rlationship: R L 1 = (16) 1 + K c a 0 Figur 6. Plot of th Frundlich isothrm for fluorid adsorption on Cynodon dactylon. whr R L is a dimnsionlss sparation factor, c 0 th initial fluorid concntration (mg/l) and K a th Langmuir constant (L/mg). Th paramtr R L indicats th isothrm shap accordingly: R L >1 1 btwn 0 and 1 0 Typ of Isothrm Unfavorabl Linar Favorabl Irrvrsibl Tabl 2. Isothrm paramtrs obtaind using th linar mthod for th adsorption of fluorid onto cynodon dactylon at diffrnt tmpraturs Isothrm Paramtr Tmpratur, K Langmuir isothrm q m / mg g K a / dm 3 mg r SSE Frundlich isothrm K F / mg g 1 (dm 3 mg 1 ) 1/n /n r SSE Rdlich-Ptrson isothrm g B / (dm 3 mg 1 ) g A / dm 3 g r SSE Tmkin isothrm a / dm 3 g b r SSE

7 th conditions usd for th xprimnts. Figur 7. Adsorbnt rspons to Tmkin isothrm for fluorid rmoval at diffrnt tmpraturs. Figur 9. Sparation factor R L valus vrss initial fluorid concntration for various tmpraturs drivd by Langmuir constants. Figur 8. Rdlich Ptrson isothrms for th sorption of fluorid ions by using Cynodon dactylon at various tmpraturs. A figur with a rlationship btwn R L and c 0 is prsntd in Figur 9 to show th ssntial faturs of th Langmuir isothrm. Tabl 3 shows th valus of R L for Cynodon dactylon at diffrnt xprimntal tmpraturs. In this work, th R L valus calculatd in th studid rang of fluorid concntration ar dtrmind to b in th rang of , which suggsts favorabl adsorption of fluorid onto th studid adsorbnt, undr Adsorption kintics Th rsults of th rat of fluorid adsorption on th Cynodon dactylon surfac, as a function of th initial fluorid concntration, ar shown in Figur 1. Th fluorid adsorption was fast, up to 105 min, and thn it bcam slow. Th initial rapid adsorption was prsumably du to activ surfac of th adsorbnt. Th slow adsorption in th latr stag rprsnts a gradual uptak of fluorid at th innr surfac. Ths data wr analyzd using kintic quations, viz. psudo-first-ordr (Figur 10), psudo-scond-ordr (Figur 11), intraparticl diffusion (Figur 12) and Elovich (Figur 13) modls. Th various rlatd kintic paramtrs, obtaind by calculation from th slops and intrcpts of th plots ar shown in Tabl 4. Th Lagrgrn psudo-first-ordr kintic quation dscribs th kintic data somwhat bttr (r is in th rang) for fluorid concntrations of mg/l. Th psudo-scond-ordr kintic quation dscribs th prsnt data bst fit (r is in th rang) in th concntration rang of fluorid usd for th adsorption study (Figur 11). Th quilibrium adsorption capacity, q, valuatd from th psudo-scond-ordr plot was dtrmind to incras from 1.70 to Tabl 3. R L valus at diffrnt tmpraturs, which wr calculatd using Langmuir constants No. Fluorid conctration Tmpratur, K mg/l

8 incrasing concntration indicatd a highr por sorption possibility of fluorid onto th adsorbnt at room tmpratur. From th Elovich modl, th r valus wr found to b btwn and and fairly support to th formation of chmisorption followd by th physisorption. Th prsnt obsrvation is consistnt with th rportd works [9]. Figur 10. Psudo-first-ordr kintic fit for fluorid adsorption onto Cynodon dactylon at room tmpratur for diffrnt initial fluorid concntrations. Figur 12. Plot for constant intra-particl diffusion at diffrnt tmpraturs. Figur 11. Psudo-scond-ordr kintic fit for fluorid adsorption onto Cynodon dactylon at room tmpratur for diffrnt initial fluorid concntrations mg/g as th fluorid concntration incrasd from 2.0 to 10.0 mg/l, which suggsts that th studid adsorbnt should b a good adsorbnt for scavnging fluorid from th contaminatd watr. Th data wr analyzd by th intraparticl diffusion kintic quation and Figur 12 shows a plot of th mass of fluorid adsorbd pr unit mass of adsorbnt (q t / mg g 1 ) vrsus th squar root of contact tim (t 0.5 / min 0.5 ). It is indicatd that th xtrapolation of th first linar portion of th plots should not pass through th origin, so th adsorption rat of fluorid onto th adsorbnt is not solly controlld by por diffusion. Thus, th adsorption data indicatd that th rmoval of fluorid from th aquous phas onto th studid adsorbnt was a rathr complx procss, mayb involving both boundary-layr diffusion and intraparticl diffusion. Th incras of k id (which dnots th avrag por diffusion rat constant, mg/g min 0.5 ) with Thrmodynamic paramtrs Th ffct of tmpratur is a major influnc in th sorption procss. Hnc, th sorption of Cynodon dactylon was monitord at four diffrnt tmpraturs 303, 313, 323 and 333 K undr th optimizd condition and thrmodynamic paramtrs, viz., standard fr nrgy chang, ΔG, standard nthalpy chang, ΔH, and standard ntropy chang, ΔS, wr calculatd using Eqs. (15) and (16) and prsntd in Tabl 5 (Figur 14). Th ngativ valus of ΔG indicatd th spontanity of th sorption raction. Th positiv valus of ΔH indicatd th ndothrmic natur of th sorption procss. Th positiv valu of ΔS showd th incrasing randomnss at th solid/liquid intrfac during sorption of fluorid. Th rsults showd an incras in adsorption capacity of fluorid with incrasing tmpratur, which is prsumably du to th control of th adsorption procss by diffusion phnomnon. Thus, th rsults indicat th ndothrmic natur of th diffusion controlld adsorption procss. Thy also indicat incrasd disordr in th systm with changs in th hydration of adsorbing fluorid ions [33]. Instrumntal analysis Th surfac, morphology, and siz distribution of th Cynodon dactylon adsorbnt particls wr obsrvd by mans of SEM, XRD and FTIR spctral analysis. Th morphology of th fluorid tratd adsorbnt is shown 30

9 Tabl 4. Kintic paramtrs for sorption of fluorid on cynodon dactylon for various fluorid concntrations at room tmpratur Kintic Modl Paramtrs Initial fluorid concntration, mg/dm Psudo-first-ordr k L / min q / mg g r SSE Psudo-scond-ordr k 2 / g mg 1 min h / mg g 1 min r SSE Intra-particl diffusion k p / mg g 1 min C r SSE Elovich modl α / mg g 1 min β / g mg r SSE Figur 13. Linar plot for Elovich kintic modl for th tmpraturs 303, 313, 323 and 333 K. in Figur 15, and it can b obsrvd that th particls ar prsntd as surfac txtur, flock and diffrnt lvls of porous surfac. Th forms and sizs of th particls ar vry irrgular aftr fluorid tratmnt. Th XRD pattrns of raw and fluorid tratd matrial ar givn in Figur 16. Th XRD data of th fluorid tratd adsorbnt providd vidnc of considrabl modification ovr th crystal clavags by indicating som pak apparanc at 2θ valus of 29.38, and and intnsity diffrncs at and A similar trnd was obsrvd by Shihabudn t al. [34] in thir study of fluorid rmoval from aquous solutions. This shows th strong adsorption of fluorid on th surfac of th adsorbnt. Th FTIR spctrum obtaind (Figur 17) for Figur 14. Plot of Gibbs fr nrgy chang ΔG, vrsus tmpratur T. th adsorbnt displayd th following major bands: cm 1 : O H strtch; cm 1 : C H strtch; cm 1 : C O strtch; cm 1 : C OH twist. It is rflcting th complx natur of adsorbnt and shows significant band shifting and intnsity changs du to fluorid sorption (Figurs 17 and 18). Fild trial Th dfluoridation fficincy of Cynodon dactylon in th fild lvl was xprincd with th sampl collctd from a nar by fluorid-ndmic villags. About 1.0 g of sorbnt was addd to 100 ml of fluorid watr sampl and th contnts wr shakn with constant tim at room tmpratur. Ths rsults ar prsntd in Tab- 31

10 Tabl 5. Thrmodynamic paramtrs of fluorid sorption on Cynodon dactylon No. Thrmodynamic Paramtr Tmpratur, K Valu 1 ΔG / kj mol ΔH / Kj mol ΔS / J mol 1 K Figur 15. Scanning lctron microscop viw of fluorid- -tratd thrmally activatd Cycnodon dactylon adsorbnt. Figur 16. XRD pattrn of pur and fluorid tratd adsorbnt. Figur 17. FTIR spctra of frshly prpard thrmally activatd adsorbnt. l 6. Thr is a significant rduction in th lvls of othr watr quality paramtrs in addition to fluorid. It is vidnt from th rsult that th sorbnt, Cynodon dactylon basd adsorbnt can b ffctivly mployd for rmoving th fluorid from watr. 32

11 Figur 18. FTIR spctra of fluorid loadd adsorbnt. Tabl 6. Physico-chmical paramtrs of dfluoridatd drinking watr from fild Watr quality paramtr Bfor tratmnt Aftr tratmnt Fluorid concntration, mg/l ph Elctrical conductivity, µs/cm Chlorid concntration, mg/l Total hardnss, mg/l Total alkalinity, mg/l Rgnration study Any adsorbnt is conomically viabl if th adsorbnt can b rgnratd and rusd in many cycls of opration. For chcking th dsorption capacity of th sorbnt, th matrial was subjctd to an adsorption at an initial fluorid concntration of 3 mg/l. Th xhaustd adsorbnt was rgnratd using 0 10% NaOH. At 2% NaOH concntration, Cynodon dactylon basd adsorbnt had dsorbd up to th lvl of 67.4% of fluorid. To tst th adsorption potntial of rgnratd adsorbnt, two mor cycls of adsorption dsorption studis wr carrid out by maintaining th initial conditions of th sam. In third cycl, th adsorbnt capacity has shown 19%. From th obsrvations this adsorbnt having somwhat rus potntial for fluorid rmoval. CONCLUSION Th activatd carbon was prpard succssfully from Cynodon dactylon and studid in batch mod. Fluorids hav bn rmovd by th lvl of 83.77% whil kping 3.0 mg/l fluorid concntration and 1.25 g dosag of adsorbnt at nutral ph. Howvr, th prsnc of bicarbonat ions intrfr th ffctiv rmoval of fluorid. Th sorption of fluorid using this adsorbnt followd Rdlich Ptrson isothrm as wll as Langmuir isothrms and was found to b spontanous and ndothrmic in natur. Th rat of sorption followd th psudo-scond-ordr kintic modl and occurrd through intraparticl diffusion. Th usd adsorbnts could b rgnratd by 67.4% using of 2% sodium hydroxid. Basd on th abov said dscriptions, Cynodon dactylon bioadsorbnt can b utilizd to rmov fluorid slctivly from watr. Acknowldgmnt Financial support for th projct by th Dpartmnt of Scinc and Tchnology (DST), Govrnmnt of India, Nw Dlhi, undr th major rsarch projct is gratfully acknowldgd. 33

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13 IZVOD SORPCIJA FLUORIDA NA AKTIVNOM UGLJENIKU DOBIJENOM IZ ZUBAČE (Cynadon dactylon) Ganapaty Alagumuthu, Vllaisamy Vraputhiran, Ramaswamy Vnkataraman Chmistry Rsarch Cntr, Sri Paramakalyani Collg, Tamilnadu, India (Naučni rad) U ovom radu proučavana j primna trmički aktiviranog ugljnika dobijnog iz biljk zubača (Cynadon dactylon) za uklanjanj fluorid iz vod za pić. Adsorpcija j rađna u šaržnim uslovima u nutralnoj srdini u zavisnosti od vrmna kontakta, adsorbovan doz, koncntracij adsorbata, tmpratur i prisutnih anjona. Dobijni rzultati ukazuju da su adsorbciona msta na površini adsorbnta htrognog karaktra i da s mogu opisati modlom htrognih vzujućih msta. Dati sistm s pokorava Rdlich Ptrson-ovoj kao i Langmuir-ovoj izotrmi. Brzina adsorpcij fluorida j modlovana Lagrgrn-ovim rakcijom psudo-prvim rdom, rakcijom psudo-drugim rdom, difuzijom unutar čstic, i Elovich- -ovom kintikim modlom. Nađno j da j kintika adsorpcij psudodrugog rda. Izračunat vrdnosti standardn promn ntalpij i ntropij za procs adsorpcij su 8,725 kj/mol i 0,033 J/mol K, rdom, koj ukazuju na to da j procs ndotrman. Adsorbat aktivni ugalj j ispitivan rndgnskom strukturnom analizom (XRD), infracrvnom spktroskopijom (FTIR) i skanirajućom lktronskom mikroskopijom (SEM) radi boljg razumvanja njgov sposobnosti za adsorbciju fluorida. Ključn rči: Fluoridi Cynodon dactylon Zubača Adsorpcija Izotrm Kintički modli Ky words: Fluorids Cynodon dactylon Adsorption Isothrms Kintic modls 35