Adsorption of Lead (II) from Aqueous Solution using Activated Carbon Prepared from Raffia Palm (Raphia Hookeri) Fruit Epicarp

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1 IOSR Journal of Applid Chmistry (IOSR-JAC) -ISSN: Volum 9, Issu 7 Vr. I (July 2016), PP Adsorption of Lad (II) from Aquous Solution using Activatd Carbon Prpard from Raffia Palm (Raphia Hookri) Fruit Epicarp Ghogomu, J. N 1, Muluh, S. N 1, Ajifack, D. L 1, Alongamo, A.A.B 1, Noufam, D. T 1 1 (Dpartmnt of Chmistry,Faculty of Scinc, Univrsity of Dschang, P.O. Box 6, Dschang, Camroon) Abstract: Activatd carbons from raphia hookri fruit picarps obtaind by chmical activation using ZnCl 2 (CAZn), H 3 PO 4 (CAH) and KOH (CAK) wr utilizd for th rmoval of lad (II) ion from aquous solution. Sampls wr charactrizd by FTIR, Bohm mthod, phpzc, iodin numbr, bulk dnsity and surfac ara. Batch adsorption xprimnts wr prformd to study th ffcts of contact tim, solution ph, adsorbnt mass and initial adsorbat concntration at 27 o C. Optimal conditions from quilibrium studis wr as follows: contact tims of 80 minuts for CAZn and CAH and 100 minuts for CAK, ph = 6 for all sampls, adsorption capacitis of mg/g for CAZn, mg/g for CAH and 28.0 for CAK. Adsorption was modld using Langmuir, Frundlich, Tmkin and Dubinin-Radushkvich isothrm modls. Equilibrium data wr bst rprsntd by Langmuir isothrm modl with monolayr adsorption capacitis of mg/g, mg/g and mg/g for CAZn, CAH and CAK rspctivly. Adsorption kintics was via psudo-first ordr, psudo-scond ordr, Elovich and intra-particl diffusion modls. Kintic studis rvald that th adsorption procss followd psudo-scond ordr modl. Raphia hookri basd activatd carbons (CAZn and CAH) ar shown to b promising matrials for th adsorption of lad(ii) ions from aquous solutions. Kywords: Activatd carbon, Adsorption, Chmical activation, Lad (II) ions, Raphia hookri. I. Introduction Th problm of watr contamination is nithr nw nor limitd to a particular gographical ara. Ecotoxicity spcifically from wastwatr contaminatd with dys, hydrocarbons and havy mtals from various manufacturing procsss has bcom a srious nvironmntal issu in rcnt yars. Unlik organic pollutants, th majority of which ar suscptibl to biological dgradation, ths havy mtals do not dgrad into harmlss nd products and thir prsnc in strams and laks lads to bioaccumulation in living organisms causing halth problms in animals, plants, and human bings [1,2]. Lad (Pb), a havy mtal, has rcntly gaind incrasing attntion owing to its high toxicity to living organisms [3]. Lad ions ar takn into th body via inhalation, ingstion or skin absorption. Lad accumulats mainly in bons, brain, kidny and muscls and may caus many srious disordrs lik anamia, kidny disass tc. Lad is also known to caus mntal rtardation, rducs hamoglobin rproduction ncssary for O 2 transport and it intrfrs with normal cllular mtabolism. Lad qually has damaging ffcts on th body s rproductiv and cntral nrvous systm [4]. As a rsult of ths, nvironmntal rgulatory boards oftn spll out limitations as to th maximum concntration of lad in natural watr and for wastwatr discharg. Th currnt U.S. Environmntal Protction Agncy (EPA) and World Halth Organization (WHO) standard for lad in wastwatr and drinking watr is 0.5 and 0.05 mg/l, rspctivly [5]. Svral tratmnt mthods hav bn suggstd, dvlopd and usd to rmov havy mtals from wastwatr, som of which includ: chmical prcipitation, ion xchang, mmbran sparation, complxation, adsorption, solvnt xtraction and distillation [6] ar all gard towards gratr amlioration of th quality of watr/nvironmnt [7]. Amongst ths mthods, adsorption which is rportd in this study has shown high potntials and simplicity in th dpollution of industrial wastwatr, spcially in th limination of lad [8]. Nvrthlss, its fficincy dpnds largly on th adsorbnt (cost, availability, and its rgnration) put togthr. Th cost of prparing activatd carbon from agricultural wasts is ngligibl whn compard to th cost of commrcial activatd carbon. Prsnt day rsarchrs hav utilizd diffrnt low-cost matrials as adsorbnts which at tims ar lss fficint. [9] prpard militia frrugina plant lavs-basd activatd carbon for th sorption of Pb(II) ions in aquous solution with a maximum adsorption capacity of about 3.3 mg/g of Pb(II) from a modl basd on th Frundlich adsorption isothrm. According to [10] maximum adsorption of Pb(II) ions by phosphoric acid activatd carbon prpard from flutd pumpkin sd shll was of th ordr of mg/g of adsorbnt. In anothr study, [11], invstigatd on th adsorption prformanc of H 3 PO 4 tratd Nipa Palm Nut (NPN) basd activatd carbon for th uptak of Pb(II) and rportd a monolayr adsorption capacity of 125 mg/g via th Langmuir modl. On thir own part, [12] showd that th maximum monolayr adsorption capacity of Pb(II) ions from aquous solution using palm shll activatd carbon was 13.4 mg/g. All ths rsults show that DOI: / Pag

2 th adsorbnts act as substituts to commrcial activatd carbon in wast-watr tratmnt. In th sam vin, this study has bn to valuat th ability of chmically activatd carbon prpard from Raphia hookri fruit picarps by zinc chlorid, phosphoric acid and potassium hydroxid for th adsorption of Pb(II) from aquous solutions. II. Matrials and Mthods 2.1 Sourc and prparation of adsorbnts All th ragnts and chmicals usd wr of analytical grad. Th working solutions wr prpard in distilld watr. Raffia palm (Raphia Hookri) fruit bunchs wr collctd from frsh watr swamp bushs of Pinyin Villag in Santa Sub-Division, Mzam Division of th North Wst Rgion of Camroon. Th fruits wr rmovd from th bunch, put in a plastic bag and lft for a wk. Thraftr, th picarps wr sparatd from th msocarp and ndocarp, washd and sun drid. Th drid picarps wr thn first crushd into smallr fragmnts with a mortar and pstl and latr furthr ground into finr particls using an lctric blndr. 2.2 Chmical Activation and Pyrolysis 10 g sampl of th prcursor matrial was imprgnatd using ithr solutions of H 3 PO 4, ZnCl 2 or KOH in a 1:1 (wight basis) imprgnations ratio. Th imprgnation was carrid out at 80 o C in a hot air ovn for 24h to achiv wll pntration of chmicals into th intrior of th prcursor matrial. Th imprgnatd sampls wr thn drid at 110 o C in an ovn for 12h. Th drid imprgnatd sampls wr transfrrd into porclain crucibls with lids. Thn th crucibls wr placd insid an lctric Muffl furnac and carbonizd at 400 o C for 1h, in th absnc of air. Th carbonizd sampls wr thn coold to room tmpratur and washd with 0.1 M hydrochloric acid (HCl) to rmov th rsidual chmical agnts followd by distilld watr until th ph valu of th rinsd watr was nutral. Th washd activatd carbon was thn drid in th ovn at 110 o C for 5 hours. Th sampls wr pulvrizd using a porclain mortar and pstl and stord in an airtight bottl for latr us. Th adsorbnts prpard by H 3 PO 4, ZnCl 2 and KOH activations wr dnotd as CAH, CAZn and CAK rspctivly and th prcursor matrial (biomass) dnotd B throughout this study. III. Charactrization of Adsorbnts 3.1 Analysis by Fourir-Transform-IR Spctrophotomtr (FTIR) In ordr to dtrmin th surfac functional groups xisting on th sampls, FTIR wr carrid out on sampls using infrard spctrophotomtr (Brükr alpha p Spctromtr) with thanol as solvnt with a rsolution of 4cm -1 within th intrval cm Surfac Function Dtrmination by th Bohm Mthod Titration of th total surfac acidity and basicity of th activatd carbons was carrid out via th Bohm mthod [13]. In th dtrmination of acidic surfac functions (carboxylic, lactonic, phnolic), 40 ml ach of dcimolar solutions of NaHCO 3, Na 2 CO 3, NaOH and HCl, wr introducd into diffrnt ractors and ach put in contact with 0.1 g of activatd carbon sampl. Titration of xcss bas was don by HCl solution. In th dtrmination of th basic surfac functions, 0.1 g of activatd carbon was put in contact with 40 ml of a dcimolar solution of HCl. Aftr stirring for 48 hours, th xcss acid was titratd with NaOH. 3.3 Iodin Numbr (IN) Th iodin numbr was masurd according to th procdur stablishd by th Amrican Socity for Tsting and Matrials (ASTM D ), Iodin numbr is a rlativ indicator of porosity in an activatd carbon. Th iodin numbr is dfind as th numbr of milligrams of iodin adsorbd by 1.0 g of carbon whn th iodin concntration of th filtrat is 0.02 N. Th iodin numbr is accptd as th most fundamntal paramtr usd to charactriz activatd carbon prformanc. Iodin numbr was mployd in this study as a tst for microporosity via volumtric analysis. This fundamntal tst for th potntials of th prpard activatd carbon dtrmins its micro-porosity up to valus as small as 2 nm. Th iodin numbr is obtaind from th following xprssion as dscribd by [13]: 25.4 x 30 - Vn (1) Iodin numbr = mac Whr m AC is th mass of activatd carbon (g), V n is th Volum of thiosulfat solution at quivalnc point (ml). 3.4 Standardization of Iodin Solution 10 ml of 0.02 N iodin solutions wr pipttd into a conical flask. 2-3 drops of starch solution wr addd. Th pal yllow colour of iodin solution turnd blu and was titratd with N sodium thiosulphat till it bcam colourlss. DOI: / Pag

3 3.5 Dtrmination of ph at point of zro charg (ph pzc ) ph at point of zro charg, phpzc, corrsponds to a ph at which th surfac charg is nul. phzpc of th activatd carbon sampls wr stimatd according to standard procdur [13] ml of a dcimolar solution of NaCl was introducd into ractors ach containing 0.1 g of activatd carbon to b analyzd. Th ph of ach solution was adjustd by addition of dcimolar solutions of NaOH or HCl (by varying valus of ph btwn 2 and 12). ph zpc was dtrmind by th intrsction of th rprsntativ curvs ph (final) = f[ph (initial)] and th first bisctor curv. Stirring was maintaind at 27 C for 48 hours with th aid of a multi-agitator systm (Edmund Bühlr GmbH) shaking at 150 rpm. Th contnts of th ractors wr thn filtrd with Whatman N 1 filtr papr aftr which th ph of th final solutions was masurd. 3.6 Bulk Dnsity (g/cm 3 ) A masuring cylindr was wighd and thn filld with th prpard activatd carbon and gntly tampd until no chang in th lvl of th sampl was obsrvd. Th volum occupid by th packd sampl was rcordd (Vs). If Wc is th wight of th mpty cylindr and W th wight of th cylindr and dry sampl, thn wight of th dry sampl Ws was obtaind by [14]: W S = W - W C (2) Th bulk dnsity (B d ) was calculatd using [14]: wight of dry Sampl (g) B (3) 3 (g/cm ) = d Volum of packd dry sampl W (g) B (4) 3 S (g/cm ) = d V 3 S (cm ) Whr Vs is th volum occupid by th packd sampls. 3.7 Surfac Ara Th diamtr (assuming sphrical shap) of th activatd carbon was obtaind by passing th crushd activatd carbon through siv siz of 100 m and th xtrnal surfac ara was calculatd by th rlation [14]: (cm /g) Surfac ara (m /g) = B 3 d (g/cm ).P d whr B d is th bulk dnsity and P d is th particl siz (particl diamtr). IV. Bacth Adsorption Exprimnts 4.1 Prparation of Pb(II) Stock solution Th mothr solution was prpard by dissolving g of Pb(NO 3 ) 2 insid a 1L volumtric flask with frshly prpard distilld watr. Solutions of various concntrations usd in th adsorption studis wr obtaind by diluting portions of th stock solution with distilld watr. 4.2 Equilibrium Studis Using Batch Mthod Equilibrium and kintic adsorption xprimnts wr carrid out in a multi-agitator systm (Edmund Bühlr GmbH SM 30) maintaind at 27 o C. All th xprimnts xcpt th study of th ffct of ph wr carrid out at a ph of 6. For ach xprimnt, 0.5 g of adsorbnt was masurd and put into a 250 ml scrw cap conical flask containing 20 ml of Pb(II) ions at th dsird initial concntration which rang from mg/l in this study. Aftr agitating th solutions for a prdtrmind tim intrval ( minuts), sampls wr filtrd using filtr papr (Whatman N o 1). Th filtrd solution was put into tst tubs containing dithizon and ammoniacal buffr solutions to form a complx. Th concntration of th rsidual Pb(II) ions was dtrmind spctrophotomtrically using a UV-Vis Spctrophotomtr (JENWAY) at λ max = 490 nm. Th quantity of Pb(II) ions adsorbd by a unit mass of adsorbnt [Q (mg/g)] and th adsorption prcntag (% Adsorption) at an instant was calculatd from th diffrnc btwn th concntrations of lad (II) bfor and aftr adsorption using th quations [15]: (5) Q = C - C o W V C - C C o % Adsorption = x 100 o (6) (7) DOI: / Pag

4 V. Rsults and Discusions Fourir Transform Infrard spctroscopy (FTIR) In viw of idntifying th diffrnt functional groups and chromophors rsponsibl for th various signals, th rsults obtaind by Fourir Transform Infra-Rd spctra analysis ar prsntd in Figs. 1 and on Tabl 1. Th FTIR spctra of th activatd carbons pr-tratd with diffrnt imprgnation solutions ar shown in Fig. 1. For th spctra of CAZn, CAH and CAK th rduction in th intnsitis of th broad band cntrd at cm -1 and th pak at cm -1 can b attributd to th disapparanc of H 2 O molculs aftr activation. It is qually obsrvd that thr is th disapparanc of th pak at cm -1 aftr activation and carbonization du to th dcomposition of som of th surfac functions during pyrolysis. Tabl 1: Intrprtation of th IR spctrum of B Absorption bands (cm -1 ) Chromophors and functional groups OH of carboxylic, phnols, alcohols, watr 2928 CH of aromatic mthoxyl groups 1698 C=O of ktons, aldhyds, lactons or of th carboxylic groups carboxylic acid drivativs and amino acids C=C bonds in th aromatic rings CH 2 and/or O-H of carboxylic and phnol groups C-O is charactristic of phosphorus and phosphoruscarbonatd compounds of th phosphoric acid activatd carbon C-O of a scondary alcohol Para substitution of th bnzn ring. 506 C-X bonds of halogno alkans Figur 1: suprposd FT-IR spctra of (a) biomass (B), (b) CAZn, (c) CAK and (d) CAH 5.2 Dtrmination of oxygn containing functional groups Th rsults of th quantification of th surfac acidic and basic functions of th activatd carbon sampls (CAZn, CAH and CAK) obtaind via th Bohm mthod is prsntd on Tabl 2. Chmical analysis of th surfac rvald that th sampls contain mor acidic (carboxylic and phnolic groups) than basic functions. This obsrvation is cohrnt with th phpzc valus obtaind for ach of th activatd carbon sampls. Tabl 2: Quantification of surfac oxygn-containing groups by th Bohm mthod Sampls Carboxylic (mq/g) Lactonic (mq/g) Phnolic (mq/g) Total acid (mq/g) Total basic (mq/g) Total (mq/g) CAZn CAH CAK Iodin Numbr (IN) Iodin numbr is a fundamntal paramtr usd to dtrmin th activity of activatd carbons. It givs a bttr stimat of th amount of activ surfac prsnt on th activatd carbon. It is a masur of th micropor contnt of th activatd carbon. Th micropors ar rsponsibl for th larg surfac ara of activatd carbon particls and ar cratd during th activation procss [16]. Th rsults of th Iodin Numbr obtaind togthr with th bulk dnsity and surfac ara valus for th diffrnt adsorbnts ar prsntd in Tabl 3. From Tabl 3, th rsults suggst that CAZn with th largst iodin numbr has mor micropors and thus a largr surfac ara than CAH and CAK. DOI: / Pag

5 Final ph Adsorption of Lad (II) from Aquous Solution using Activatd Carbon Prpard from Raffia Palm Tabl 3: Iodin numbr, bulk dnsity and surfac ara valus for th diffrnt adsorbnts Adsorbnt Imprgnation ratio Bulk dnsity (g/cm 3 ) IN (mg/g) Surfac ara(m 2 /g) CAZn 1: CAH 1: CAK 1: ph at point of zro charg (phpzc) Th phpzc obtaind for ach activatd carbon (Fig. 2) is cohrnt with th quantification of th surfac functional groups obtaind via th Bohm mthod. Ths phpzc valus prmit th dtrmination of th acidic or basic charactr of th activatd carbon and to know, according to th ph of th solution, th nt surfac charg. According to Fig. 2, it can b noticd that phpzc is 5.6, 5.0 and 5.2 rspctivly for CAZn, CAH and CAK. Hnc, in xprimnts whrin phpzc > ph sampls will hav thir surfacs positivly chargd whil for thos in which phpzc < ph will hav thir surfacs ngativly chargd CAZn CAH CAK Initial ph Figur 2: Graph for th phpzc for CAZn, CAH, CAK and B Effct of ph ph is on of th most important variabls affcting th amount of havy mtal adsorbd by an adsorbnt, bcaus ph influncs surfac charg, dgr of ionisation (protonation) of th functional groups on th adsorbnt. Th xprimnts wr don undr diffrnt ph valus (btwn 2 and 8) kping othr paramtrs constant. Th rsults ar prsntd in Fig. 3. By incrasing ph, incras in adsorption capacity of Pb(II) was obsrvd and th maximum adsorption capacity was obtaind at ph 6 corrsponding to mg/g, mg/g, mg/g and mg/g (CAZn, CAH, CAK and B) rspctivly. Pb(II) adsorption dcras as ph ros byond th optimum ph 6 bcaus Pb 2+ starts prcipitating as Pb(OH) 2. At vry low ph (ph <3), low valus of adsorbd Pb(II) ions wr obsrvd. This can b attributd to th facts that H + ions concntration in solution is high, thus th functional groups on th adsorbnt surfac ar protonatd. Hnc Pb(II) ion adsorption is low du to comptition btwn th H + and Pb 2+ ions for fr adsorption sits. Th optimum ph of 6 was maintaind in this study in subsqunt paramtr dtrminations. Figur 3: ffct of ph on lad (II) ion adsorption onto CAZn, CAK, CAH and B. (C o = 100 ppm, adsorbnt dos = 0.5 g, ph = 6, Contact tim = 80 min, agitation spd = 150 rpm and T = 27 o C.) DOI: / Pag

6 5.5.1 Effct of Adsorbnt dosag Th ffct of adsorbnt dos was dtrmind at an initial mtal ion concntration of 100 ppm and at ph 6. Th rsults summarizd in Fig. 4 indicat that, with an incras in mass of adsorbnt in th rang 0.1g to 1 g, adsorption capacity rapidly dcrass. This ffct can b du to som adsorption sits rmaining unsaturatd during th adsorption procss. Low Pb(II) rmoval as adsorbnt dos incrasd may b du to th fact that th adsorption of Pb(II) is limitd by monolayr adsorption. This is corroboratd by th valu of th corrlation cofficint, R 2, from Langmuir isothrm. Figur. 4: Effct of adsorbnt dos on th uptak of lad (II) ions onto CAZn, CAK, CAH and B. (C o = 100 ppm, adsorbnt dos = 0.5 g, ph = 6, Contact tim = 80 min, agitation spd = 150 rpm and T = 27 o C Effcts of Contact Tim Th rsult of prcntag Pb 2+ adsorbd is shown in Figur 5. It was found that th amount of Pb 2+ ions adsorbd incrasd with incrasing contact tim. It can b obsrvd from th figur that a high rat of Pb 2+ ions adsorption occurrd in th first 20 to 60 mins for all th adsorbnts and thraftr th rat of adsorption of th adsorbat spcis into th adsorbnt was found to b slow. Th tim rquird to rach quilibrium on Pb 2+ adsorption was 80 minuts for CAZn and CAH and 100 minuts for CAK. Th initial rapid adsorption is du to th availability of th ngativly chargd surfac of th adsorbnts for adsorption of cationic Pb 2+ spcis prsnt in th solution. Th latr slow adsorption is probably du to th lctrostatic hindrancs btwn adsorbd positivly chargd adsorbat spcis into th surfac of adsorbnts and th availabl cationic adsorbat spcis in solution, and th slow por diffusion of th solut ion into th bulk of th adsorbnt [17]. Figur 5: Effct of contact tim on th adsorption of lad (II) onto CAZn, CAK, CAH and B. (C o = 100 ppm, adsorbnt dos = 0.5 g, ph = 6, Contact tim = 80 min, agitation spd = 150 rpm and T = 27 o C 5.5 Effct of initial concntration of lad Th ffct of initial Pb 2+ ion concntration on th adsorption procss was carrid out with initial concntrations ranging from 100 to 700 ppm. Th xprimntal rsults on th adsorption capacity ar prsntd in Fig. 6. Th rsult obtaind show that an incras in th initial concntration lads to an incras in th quantity of Pb2 + ion adsorbd. Howvr, th prcntag adsorption dcrasd with incras in initial ion concntration. DOI: / Pag

7 Th adsorption capacity of Pb 2+ incras bcaus incrasing th initial concntration lads to an incras in th amount of Pb 2+ ion in th solution rsulting in an incras in collision btwn th molculs of th mtals and th adsorption sits. With this incras in collision, th adsorption sits bcom mor quickly saturatd. Thus, at highr Pb 2+ concntrations, th availabl adsorption sits bcom fwr and hnc th adsorption of Pb 2+ bcoms mor or lss constant. Figur 6. : ffct of initial concntration on lad (II) adsorption on CAK, CAZn, CAH and B. (C o = 100 ppm, adsorbnt dos = 0.5 g, ph = 6, Contact tim = 80 min, agitation spd = 150 rpm and T = 27 o C Adsorption isothrm modls Adsorption isothrms ar modls usd to charactriz th rlationship btwn th adsorbnt (solid) and th adsorbat (solut), and knowldg of th adsorption capacity of th adsorbnt can b dscribd by svral isothrm modls such as Langmuir, Frundlich, Tmkin and Dubinin-Radushkvich (D-R) modls [18] Langmuir adsorption isothrm Th Langmuir modl assums that th uptak of mtal ions occurs on a homognous surfac by monolayr adsorption without any intraction btwn adsorbd ions, that uniform nrgis of adsorption ar involvd, and that thr is no transmigration of adsorbat in th plan of th surfac [19]. Th linarizd Langmuir isothrm quation can b xprssd as: C KL C = + Q Q Q m m (8) Whr, K L is th Langmuir adsorption constant (L/mg ) which is rlatd to th maximum sorption capacity and nrgy of adsorption, C is th quilibrium concntration (mg/l), Q is th amount adsorbd pr amount of adsorbnt at quilibrium (mg/g) and Q m (mg/g) is an indicator of monolayr adsorption capacity. Th fficincy of th Langmuir adsorption procss is furthr assssd by (9): 1 R L = 1 + K L C o Whr R L is a dimnsionlss constant rfrrd to as sparation factor, K L is th Langmuir constant rlatd to th nrgy of adsorption and C o is th initial solut concntration (mg/l). If R L > 1 adsorption is said to b unfavorabl, R L = 1 for linar adsorption, 0 < R L < 1 for favorabl adsorption and R L = 0 for irrvrsibl adsorption. 5.8 Frundlich adsorption Isothrm Frundlich modl assums that th uptak of mtal ions occurs on a htrognous surfac by monolayr adsorption [20]. Th linarizd rlation (10) is givn blow: 1 log Q = log K f + log C n Whr: Q is th quantity of solut adsorbd at quilibrium (adsorption dnsity: mg of adsorbat pr g of adsorbnt). C is th concntration of adsorbat at quilibrium, 1/n (mg/l) is th adsorption intnsity or th DOI: / Pag (9) (10)

8 htrognity factor and K f is th Frundlich constant rlatd to th adsorption nrgy (mol 2 /kj 2 ). Ths last two constants ar dpndnt of tmpratur and th natur of sorbnt and sorbat. 5.9 Tmkin adsorption isothrm. It is xprssd in linar form by th following rlationship btwn th amount adsorbd, Q, and concntration in solution at quilibrium, C, [19]: RT RT Q = In A T + In C b (11) T bt Whr, A T (L/g) is th Tmkin isothrm constant (quilibrium binding constant), B T (J/mol) is a constant rlatd to hat of sorption, R is th gas constant (8.314 JK -1. mol -1 ) and T is th absolut tmpratur (K). Similar to th Frundlich quation, th Tmkin modl taks into account th htrognity of th surfac [19] Dubinin-Radushkvich (D-R) Isothrm Th Dubinin-Radushkvich isothrm hlps to dtrmin th apparnt nrgy of adsorption, th charactristic porosity of adsorbnt towards th adsorbat and it dos not assum a homognous surfac or constant adsorption potntial. This isothrm is gnrally xprssd as follows [21]: Q = Q max (12) Whr, Q is th amount of adsorbat on th adsorbnt at quilibrium (mg/g); Q max is th thortical isothrm saturation capacity (mg/g); ε is th polanyi potntial givn as: β is a constant rlatd to th man fr nrgy of adsorption pr mol of th adsorbat adsorbd to th adsorbnt (mol 2 /KJ 2 ). Th nrgy can b obtaind using th following rlationship: Th linar form of D-R quation is xprssd as: 2 -βε 1 ε = RTIn 1 + C E = 1 2ε 2 In Q = In Q + βε max From th D-R isothrm, th magnitud of th man fr nrgy of adsorption shows if th adsorption procss follows physisorption, ion xchang or chmisorption. [22], rportd that th mans fr nrgy for physisorption < 40 KJ/mol and that for chmisorption > 40 KJ/mol. In ordr to find th most appropriat modl for th lad adsorption, th data wr fittd to ach isothrm modl. Th paramtrs of th diffrnt modls ar prsntd on Tabl 4 and only th Langmuir isothrm modl (R 2 = , , and for CAZn, CAH, CAK and B rspctivly) rprsntd on Figur 7 bst fittd th xprimntal data. Th homognous monolayr adsorption capacity of Pb(II) ions at 27 o C wr found to b qual to mg/g, mg/g, mg/g and mg/g for CAZn, CAH, CAK and B sampls rspctivly. Th shap of th Langmuir isothrm was invstigatd by th dimnsionlss constant sparation trm (R L ). In this invstigation th quilibrium paramtr was found to b in th rang 0 < R L < 1 as shown in Tabl 4. This indicatd that th sorption procss was vry favorabl and that th adsorbnt mployd xhibits good adsorption potntials. Tmkin isothrm modl prdicts a uniform distribution of binding nrgis ovr th population of surfac binding adsorption. Th valus of Tmkin constants A T and B T as wll as th corrlation cofficints ar listd in Tabl 4. th D-R isothrm prdict that th apparnt nrgy of adsorption (E) which stands at kj/mol, kj/mol, kj/mol and kj/mol rspctivly for CAZn, CAH, CAK and B wr all gratr than 40 kj/mol maning th mchanism of adsorption of Pb 2+ on all th adsorbnts was chmisorption in which strong chmical bonds ar involvd. (14) (15) (13) DOI: / Pag

9 Figur 7: linar transformation of Langmuir isothrm Tabl 4: Rsults of adsorption paramtrs for th diffrnt modl isothrms Modls Paramtrs Adsorbnts CAZn CAH CAK B Langmuir Q o (mg/g) K L (L/mg) R Frundlich 1/n n K f (L/g) R Tmkin A T (L/g) b T (J/mg) B (kj/mol) R Dubinin- Radushkvich Q max (mg/g) β (mol 2 /J 2 ) 0.82x x x x10-4 E (kj/mol) R Adsorption Kintic Modls Svral kintic modls ar oftn usd in modling th adsorption mchanism of dissolvd soluts on adsorbnts. In this study, four kintic modls hav bn studid in dscribing th adsorption phnomnon of Pb (II) ions onto th two activatd carbon sampls studid hrin: psudo-first ordr, psudo-scond ordr, Elovich and intra-particl diffusion modls Psudo first-ordr modl In ordr to study th adsorption kintics of Pb(II) ions, th kintics paramtrs for th adsorption procss wr studid for contact tims ranging from 20 to 160 min by monitoring th quantity of Pb(II) adsorbd as a function of tim. Th data wr thn rgrssd against th Lagrgrn quation (12), which rprsnts a first ordr kintics quation [23]: K1 log Q - Q t = log Q + t whr, k 1 is th psudo-first-ordr rat constant (min -1 ); Q and Q t ar th adsorption capacitis at quilibrium and at a givn tim t xprssd in (mg/g) Psudo scond-ordr modl Rprsntd by (17) blow, th linarizd psudo-scond ordr chmisorption kintic modl [23] has bn frquntly usd in divrs xprimnts involving th adsorption of organics and havy mtals on activatd carbon: t 1 1 = + t Q K Q Q 2 t 2 whr, K 2 is th psudo-scond-ordr rat constant (mg.g 1.min -1 ). DOI: / Pag (16) (17)

10 5.14 Elovich modl Th linarizd Elovich quation is gnrally xprssd as: 1 1 Q t = In(αβ) + In t β β (18) whr, α is th initial sorption rat (mg g -1 min) and β is th dsorption rat constant (g mg -1 ) during any on xprimnt and by assuming that αβt >> 1[24] Intra-particl diffusion modl During th batch mod of opration, thr is a possibility of transport of sorbat spcis into th pors of th sorbnt, which is oftn th rat controlling stp. Th rat constants of intra particl diffusion (K id ) at diffrnt coppr ion concntrations wr dtrmind using th following linarizd quation [24]: 1 2 Q = K t + D (19) id Whr Q t is th amount of Pb(II) adsorbd at tim t and t 1/2 is th squar root of th tim. K id is th intraparticl diffusion constant (mg.g -1.min -1/2 ). Whn intra-particl diffusion plays a significant rol in controlling th kintics of th adsorption procss, straight lin plots through th origin ar obtaind and thir slops giv th rat constant, k id. Th slops and intrcpts of all th modl curvs wr usd to dtrmin valus of th constants as wll as th quilibrium capacity (Q ). Th rsults indicat that th adsorption procss follows only th scond-ordr kintic rat quation aftr fitting th adsorption data of Pb(II) ions onto th thr activatd carbon sampls and th biomass (Fig. 8) and with corrlation factors R 2 = , , and for CAZn, CAH, CAK and B rspctivly. This implis that adsorption of lad (II) ions onto th adsorbnts may occur through a chmical procss involving th valnc forcs of th shard or xchangd lctrons [25]. This mans that chmisorption raction bcoms mor prdominant in th rat-controlling stp for th lad systm. Th straightlin plot for psudo scond ordr rat quation has bn rprsntd on (Fig. 8) and th valus of all th modl paramtrs and th corrlation cofficints ar prsntd in Tabl 5. Th fact that linar plots for intra-particl diffusion modl for ach sampl did not pass through th origin is indicativ that th intra-particl diffusion was not a rat controlling stp. Figur 8: linarizd psudo- scond ordr plots for Co=100mg/L, V= 50 ml, ph= 6, adsorbnt dos = 0.5 g and T= 27 o C Tabl 5: Rsults of adsorption paramtrs for th diffrnt kintic modls Kintic Modls Paramtrs Adsorbnts CAZn CAH CAK B Psudo-first ordr Q (mg/g) K 1 (1/min) R Psudo-scond ordr Q (mg/g) K 1 (mg/g.min) R Elovich β (g/mg) DOI: / Pag

11 Intra-particl diffusion α (mg/g.min) 5.23x x x R K id (mg/g.min) D R VI. Conclusion Th potntial of CAZn, CAH, CAK, and B as low-cost adsorbnts for th uptak of Pb 2+ ions from aquous solutions was stablishd. Th chmical analysis by th Bohm mthod and th rsults obtaind from FTIR analysis showd th prsnc of functional groups on th surfac of th activatd carbons. Th quilibrium adsorption of Pb(II) ion by CAZn, CAH, CAK and B was analyzd by th Langmuir, Frundlich, Tmkin and Dubinin-Radushkvich isothrm modls. Th Langmuir isothrm was th most appropriat modl with a monolayr adsorption capacity of Pb(II) ion of mg/g, mg/g and mg/g for CAZn, CAH and CAK rspctivly, working at ph = 6 at 27 C. Although kintic data for th Pb(II) adsorption wr fittd to four kintics modls, namly th psudo-first, psudo-scond-ordr, Elovich and intra-particl diffusion modl, only th psudo-scond-ordr modl, (R 2 = , , and for CAZn, CAH, CAK and B rspctivly) bst dscribd th adsorption procss for all th adsorbnts. From Tabl 6 blow, it is vidnt that our adsorbnt matrials ar mor fficint in th sorption of Pb(II) ions than othr litratur studis. Hnc, th prsnt study shows that th activatd carbons prpard from raphia hookri fruit picarps using zinc chlorid and phosphoric acid as activating agnt (CAZn and CAH) ar ffctiv adsorbnts for th adsorption of Pb(II) ion from aquous solution. Tabl 6: Comparison of adsorption capacity of lad (II) with othr adsorbnts Adsorbnts Q max (mg/g) ph Rfrnc militia frrugina plant lavs [9] flutd pumpkin sd shll [10] H 3PO 4 tratd Nipa Palm Nut (NPN) [11] Palm shll [12] CAZn Prsnt study CAH Prsnt study Acknowldgmnts Th authors gratfully acknowldg th support of this work by th Rsarch Laboratory of Noxious Chmistry and Environmntal Enginring of th Univrsity of Dschang. Rfrncs [1] U. Farooq, J. A. Farooq, M. A. Khan and M. Khan. Biosorption of havy mtal ions using what basd biosorbnts a rviw of th rcnt litratur, Biorsourc and Tchnology, 101, 2010, [2] M. Fomina and G. M. Gadd, Biosorption: currnt prspctivs on concpt, dfinition and application, Biorsourc Tchnology. 160, 2014, [3] A. Gundogdu,, D. Ozds, C. Duran, V. N. Bulut, M. Soylak, H. B. Snturk, Biosorption of Pb(II) ions from aquous solution by pin bark (Pinus brutia Tn.), Chmical Enginring Journal, 153, 2009, [4] F. J. Crino-Cordova, P. E. Díaz-Flor, R. B. García-Rys, E. Soto-Rgalado, R. Gómz-Gonzálz, and M. T. E. Bustamant- Alcántara Biosorption of Cu(II) and Pb(II) from aquous solutions by chmically modifid spnt coff grains. Intrnational Journal of Environmntal Scinc and Tchnology. 10, 2013, [5] P. Sivakumar, and P. N. Palanisamy Adsorption studis of basic Rd 29 by a non-convntional activatd carbon prpard from Euphorbia ssantiquorum L. Intrnational Journal of Chmical Tchnology and Rsarch, 03, 2009, [6] F. L. Fu, Q. Wang Rmoval of havy mtal ions from wastwatrs: a rviw, Journal of Environmntal Managmnt, 92, 2011, [7] D. M. Dragan, M. Milutin, A. Milosavljvić, D. Marinković, R.Z. ĐVljko, M. Jlna and L. B. Alksandar, Rmoval of coppr(ii) ion from aquous solution by high-porosity activatd carbon, NAME OF THE JOURNAL, 39, 2013, [8] K. Nashir, E. M. Yahaya, F. P. Muhamad, L. Mohamd, I. Abustan, S. B. Olugbnga, and A. A. Mohd, Adsorptiv Rmoval of Cu (II) Using Activatd Carbon Prpard From Ric Husk by ZnCl 2 Activation and Subsqunt Gasification with CO 2, Intrnational Journal of Enginring & Tchnology IJET IJENS, 01, 2011, [9] A. A. Mngisti, S. T. Silva, A. V. P. Rao and M. Singanan, Rmoval of lad (II) ions from aquous solution using activatd carbon from Militia Frrugina plant lavs. Bull. Chmical Socity of Ethiopia, 03, 2008, [10] A. I. Okoy, P. M. Ejikm and O. D. Onukwali, Lad rmoval from wastwatr using flutd pumpkin sd shll activatd carbon. Adsorption modlling and kintics, Intrnational Journal of Environmntal Scinc and Tchnology, 07, 2010, [11] S. N. M. Yusoff, A. Kamari, W. P. Putra, C. F. Ishak, A. Mohamd, N. Hashim, and M. Isa, Rmoval of Cu(II), Pb(II) and Zn(II) Ions from Aquous Solutions Using Slctd Agricultural Wasts: Adsorption and Charactrisation Studis, Journal of Environmntal Protction. 05, 2014, [12] Y. B. Onundi, A. A. Mamun, M. F. Al Khatib, and Y. M. Ahmd, Adsorption of coppr, nickl and lad ions from synthtic smiconductor industrial wastwatr by palm shll activatd carbon, Intrnational Journal of Enginring & Tchnology, 04, 2010, [13] D. L. Ajifack, J. N. Ghogomu, J. N. Ndi, & J. M. Ktcha, Dynamics and Equilibrium studis of th adsorption of Cu(II) from aquous solution by activatd Hibiscus Sabdariffa L. Stalk biomass, Intrnational Journal of Enginring Rsarch and Tchnology, 04, 2015, DOI: / Pag

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