The Effect of Temperature on the Adsorption of 4-Nitrophenol onto Palm Shell based Activated Carbon

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1 Th Effct of Tmpratur on th Adsorption of 4-Nitrophnol onto Palm Shll basd Activatd Carbon Norzilah Abdul Halif, Wan Mohd Ashri Wan Daud, Iskandar Mohd Noor, Ch Rosmani Ch Hassan. Dpt. of Chmical Enginring, Faculty of Enginring, Univrsity of Malaya, Kuala Lumpur, Malaysia ABSTRACT: A sris of xprimnts was conductd to study th adsorption of 4-NP onto Palm Shll basd Activatd Carbon (PSAC) and th rsults obtaind wr compard to two commrcial products, on is Coal basd Activatd Carbon (CBAC) and anothr is Coconut Shll basd Activatd Carbon (CSAC). Th xprimnts wr carrid out in batch mod with th tmpraturs of th opration wr varid from 25 to 40 C and th 4-NP concntrations wr varid from 500 to 2000 mg/l. Th Langmuir and Frundlich isothrms wr found to rprsnt th masurd adsorption data wll ( R 2 > 0.90). Th rsults show that in all cass, incrasing th tmpratur from 25 to 40 C, th affinity dcrass and CSAC has th lowst (26.13%), followd by CBAC (46.13%) and thn PSAC (50.30%). Similar trnds also found in th adsorption intnsity, whr CSAC was th lowst (11.12%), followd by PSAC (21.66%) and CBAC th highst (27.94%). Th findings of this invstigation suggst that PSAC can b usd as an ffctiv adsorbnt for th rmoval of 4-NP from wastwatr. Kywords: Adsorption, 4-nitrophnol, palm shll basd activatd carbon, Frundlich isothrm, Langmuir isothrm. (1) INTRODUCTION Th prsnc of phnol and phnolic compounds in watr and wastwatr has bn of grat public concrn and it is on of th most frqunt contaminants at hazardous-wast sits. Nitrophnols ar usually formd du to th photo dgradation of psticids and ths phnols hav considrabl solubility in watr. Thr ar vry toxic to flora and fauna and thrfor, pos srious nvironmntal problms. In ordr to rduc this problm, various rgulatory agncis hav st a limit on th concntration of phnolic compounds in industrials fflunt bfor it can b safly dischargd to watr bodis and in Malaysia, th concntration should not xcd 10 µg/l [1]. Adsorption has bn found to b suprior and prfrrd choic than othr tchniqus for watr r-us bcaus of low initial cost, simplicity of dsign, as of opration and insnsitivity to toxic substancs. Activatd carbons of high porosity, high surfac ar frquntly usd in industry for purification and nvironmntal rmdiation. It can b producd from a numbr of prcursors such as coal, wood and agricultural wasts. Rcognizing th conomic drawback of activatd carbon, such as high cost, many invstigators hav studid th fasibility of using chap, yt commrcially availabl matrials as potntial adsorbnts such as th low rank coal [2], ash [3], and clay [4]. Anothr viabl option is by convrting agricultur wast to activatd carbon. Palm shll is an agricultural wast from palm oil industry and in South East Asia thr is an

2 24 N Abdul Halif, WMA Wan-Daud, I Mohd-Noor, CR Ch-Hassan. abundanc of this matrial and currntly it has no significant tchnical application. In Malaysia alon approximatly 10 million tons of palm shll was gnratd annually and it crats larg disposal problm. Svral studis concludd that a good quality activatd carbon could b producd from palm shll [5,6]. Currntly, thr is no maningful study has bn conductd on th possibility of using PSAC in liquid phas applications such as watr tratmnt. Svral studis hav bn carrid out to dtrmin th quilibrium rlationship on adsorption of phnol on activatd carbon using isothrm modl such as Frundlich and Langmuir [7,8]. Thr ar many factors that affct th adsorption capacity such as particl siz [9], initial concntration [10], ph [11], and tmpratur [12]. Studis on adsorption quilibrium bhavior with tmpratur variation from 10 to 80 C hav bn carrid out [13,14]. In this study 4-NP is usd as th modl adsorbat, sinc it has bn rcommndd as rprsntativ of aquous organic pollutants [2]. Th aim of this work is to study th suitability of PSAC to rmov 4-NP from aquous solutions and th ffcts of tmpratur (25, 30, 35, and 40 C) on th quilibrium isothrm. Th rsults obtaind will b compard to th rsults obtaind using two commrcial activatd carbons, CSAC and CBAC. (2) EXPERIMENTAL 2.1 Matrials Adsorbat 4-NP with purity gratr than 99.5% supplid by Scharlau Chmi S.A. was with distilld watr to prpar a dsird concntration of simulatd wastwatr. dilutd Adsorbnts Th palm shll basd activatd carbon was supplid by local company, whil commrcial coal and coconut shll basd activatd carbons wr obtaind from a multinational company. 2.2 Analyss Nitrogn Adsorption/Dsorption Isothms Th sampl porosity is dtrmind using nitrogn adsorption/dsorption isothrm at 77 K wr dtrmind volumtrically using a Micromritics ASAP 2010 Porosimtr. Bfor 4 th xprimnt bgan, th adsorbnts wr dgassd ( 10 mmhg ) at 393 K. Th surfac aras of th sampls wr masurd basd on Brunaur-Emmt-Tllr (BET) mthod and th t- plot mthod was applid to calculat th micropor volum. 2.3 Exprimntal Procdurs Prparation of Adsorbnt PSAC, CBAC and CSAC wr grindd and sivd into a fixd particl siz rang ( µm). Aftr that th sampls wr drid in ovn at 110ºC ovrnight and thn coold down in a dsiccator.

3 Th Effct of Tmpratur on th Adsorption of 4-Nitrophnol onto Palm Shll basd Activatd Carbon Dtrmination of Equilibrium Tim For ach kintics xprimnt, 0.2 g of activatd carbon was contactd with 100 ml of 4-NP solution of 500 mg/l in 250 ml Ermnlayr flasks and was shakn at room tmpratur at 200 rotations pr minut (rpm) for svral durations (5, 10, 15, 20, 25, 30, 35, 40 h). Th contnts of ach flask wr thn filtrd and th concntrations wr masurd using a spctrophotomtr. Ths xprimnts wr conductd to stablish an quilibrium tim to b usd as contact tim in th subsqunt isothrm xprimnts Equilibrium studis A typical xprimnt bgan by changing 0.2 g of activatd carbon into a 250 ml Ermnlayr flask containing 100 ml of 4-NP solution having concntrations 500, 800, 1000, 1500, 1800 and 2000 mg/l for a contact tim of 35h. Th xprimnt was rpatd at diffrnt tmpraturs of 30, 35, and 40 C. Thn, suspnsions wr filtrd through Whatman No. 542 filtr papr and th rsidual adsorbat concntrations wr dtrmind spctrophotomtrically. Th isothrms wr thn analysd using Frundlich and Langmuir modls. Th Frundlich quation modl can b xprssd as: Q f 1 n = K C (2) Whr K F and n ar mpirical constants dpndnt on svral nvironmnt factors such as such tmpratur. In a linar form; lnq ln 1 = K f + lnc n (3) Th Langmuir modl is basd on a monolayr of th adsorbat adsorbd onto th carbon surfac and th modl can b xprssd as follows: Q X mkc L = (4) 1 + KC Whr; Q = amount of solut adsorbd on th adsorbnt (mg of adsorbat pr g of adsorbnt) C = concntration of adsobat in solution (mg/l) X m =maximum adsorption capacity corrsponding to complt monolayr covrag (mg of solut adsorbd pr g of adsorbnt) K L = Langmuir constant rlatd to nrgy of adsorption Eq. (3) can b rarrangd in a linar form; C 1 C = + (5) Q X K X m L m

4 26 N Abdul Halif, WMA Wan-Daud, I Mohd-Noor, CR Ch-Hassan. Whr; C = Initial concntration of adsorbat in solution (mg/l) o K = Langmuir constant rlatd to nrgy of adsorption L (3) RESULTS AND DISCUSSION 3.1 Por Structur Analysis Tabl 1: Structural paramtrs of CSAC, CBAC and PSAC. Adsorbnt δ t-plot BET 2 3 ( m g ) V mic ( cm g ) CSAC PSAC CBAC As shown in Tabl 1 CSAC has th highst BET surfac ara followd by PSAC and CBAC. It should b notd that BET surfac ara is apparnt surfac ara and do not rprsnt th tru surfac ara. Th valus wr providd to indicat a masur of comparativ variation in surfac ara as a function of typ of activatd carbons nitrophnol Analysis Th analysis was don using a Hitachi UV Spctrophotomtr and a good linarity is obtaind btwn th absorbncy vrsus 4-nitrophnol concntrations at maximum absorbncy wavlngth of nm. 3.3 Equilibrium Tim Th amount of 4-NP adsorbd incrass with tim bfor it gradually attains quilibrium in 35 hr. Th rmoval curvs ar smooth and continuous, indicating th formation of monolayr covrag onto surfac of th activatd carbon. Prvious studis shows that tim to achiv quilibrium wr varid from 24 hr [15] to 4 days [13]. 3.4 Equilibrium Studis Th adsorption isothrm rvals th distribution of adsorbat molcul in liquid and solid phas whn th adsorption procss has rachd an quilibrium stag. Th rsults indicat CSAC has th highst adsorptiv capacity and CBAC has th lowst. It is intrsting to not that although PSAC has th highst micropor volum, it has lowr 4-NP adsorption capacity as compard to CSAC. This could b du to th fact that not all th micropors availabl ar assssabl to 4-NP molculs. In this cas, BET surfac ara could b mor rflctiv to indicat 4-NP adsorption. Th isothrms wr analyzd using th Langmuir and Frundlich 2 modls and th rsults ar shown in Tabl 2 togthr with th rgrssion cofficint, r. At all tmpraturs, th sampls giv a bttr fit in th Frundlich and Langmuir modl.

5 Th Effct of Tmpratur on th Adsorption of 4-Nitrophnol onto Palm Shll basd Activatd Carbon Frundlich Isothrm Th Frundlich modl has found wid accptanc bcaus of its accuracy and broad applicability. Th rsults of Frundlich analysis ar summarizd in Tabl 2. Of particular intrst ar th valus of th constant n and K F. Similar valus for th constant man similar adsorptiv proprtis for th carbons. This intnsity of adsorption procss, n indicats a favorabl adsorption whn 1<n<10, and it is mor favorabl as 1/n <1. From Tabl 2, in gnral CBAC has marginally th highr valu of n, followd by PSAC thn CSAC. Thus, it indicats that CSAC has slightly strongr forc that fixs th adsorbat, 4-NP onto its surfac as compard to PSAC and CBAC. K F is rlativ to th adsorption capacity and at low tmpratur of 25 and 30ºC, CBAC has marginally highr valu of K F, followd by CSAC thn PSAC. Howvr, at highr tmpratur, th diffrnc in th K F valus is significant Langmuir Isothrm Th valu of Langmuir adsorption isothrm for PSAC is givn in Tabl 2 and this indicats th monolayr covrag of 4-NP molculs onto th outr surfac of th adsorbnt. Th valus of Langmuir constant, X m and K L wr dtrmind from th slops and intrcpts of th plots rspctivly. Th adsorption of 4-NP onto activatd carbon is dirctly rlatd to th availabl surfac ara of th adsorbnt. Normally, larg surfac aras signify high adsorption of organic compounds. Hnc, surfac ara can b considrd as an important attribut in slctivity of an adsorbnt. Othr factors that influnc th adsorption ar th physical and chmical proprtis of th substrat, such as molcular dimnsion [3]. Th forcs of attraction btwn carbon and a molcul ar gratr whn th siz of th molcul is closr to th siz of th pors of th carbon. 4-NP has molcular dimnsion of nm whil most of activatd carbon tstd hr has por diamtr of 2-4 nm, with CSAC has th highst, followd by PSAC and CBAC. This could b partly contributd to th highr adsorption of 4-NP in CSAC. 3.5 Effct of Tmpratur Th tmpratur has significant ffcts on th adsorption procss. Incrasing th tmpratur is known to incras th rat of diffusion of th adsorbat molculs across th xtrnal boundary layr and in th intrnal pors of th adsorbnt particl, owing to th dcras in th viscosity of th solution. Howvr, changing th tmpratur will chang th quilibrium capacity of th adsorbnt for a particular adsorbat. In this part, a sris of xprimnts wr conductd at 25, 30, 35, and 40 C to study th ffct of tmpratur on th adsorption isothrm Frundlich Isothrm Tabl 2 dpicts th valus of paramtr K F and n at diffrnt tmpraturs. Th rsults show that both paramtrs dcras with th incras in tmpratur. For PSAC, th dcras in K F is 21.66, 27.94% for CBAC and 11.12% for CSAC. Th valu of n rprsnts th forc, which fixs th 4-NP molcul onto th adsorbnt. From Tabl 2, it is clar that for all th adsorbnts th strngth of th forcs for ach of adsorbnt was dcrasd with th incras in tmpratur. Th dcras could b attributd to th highr kintics nrgy possssd by th adsorbd molculs at highr tmpratur to scap from th surfac.

6 28 N Abdul Halif, WMA Wan-Daud, I Mohd-Noor, CR Ch-Hassan Langmuir Isothrm Th adsorption affinity of th thr carbons for 4-NP can b assssd by comparing th apparnt dissociation constant, K L, which is th masur of th stability of th complx formd btwn 4-NP and th functional groups on th carbon surfac undr spcifid. Tabl 2: Langmuir and Frundlich isothrms paramtrs T (ºC) Adsorbnt Langmuir Frundlich 2 X m K L r K F n 2 r 25 ( mg / g ) (l/mg) (l/g) CBAC PSAC CSAC CBAC PSAC CSAC CBAC PSAC CSAC CBAC PSAC CSAC xprimntal conditions. A small K L valu indicats that th carbon has a low affinity for 4-NP and vic vrsa. Whn th K L valu is high, mor surfacs ar covrd with adsorbat molcul du to strongr affinity of adsorbat molcul towards th surfac. Similarly, whn th hat of adsorption incrass, th amount adsorbd incrass du to th highr nrgy barrir that adsorbd molculs hav to ovrcom to b rlasd back to th liquid phas and mor bonds ar formd. In rlation to tmpratur, incrasing th tmpratur will dcras th amount adsorbd, X m at a givn prssur du to gratr nrgy acquird by th adsorbd molcul to scap from th surfac. From Tabl 2, th dcras in K L for ach of adsorbnts with th incras of tmpratur from 25 to 40 C ar 50.30% for PSAC, 46.31% for CBAC and 26.18% for CSAC, which mans that CSAC has a highst affinity and PSAC in th lowst. (4) CONCLUSIONS PSAC is rlativly a good adsorbnt for 4-NP adsorption. Th rsult obtaind show that it is slightly infrior as compard to CSAC and slightly bttr than CBAC. In all cass, whn tmpratur is dcrasd from 25 to 40 C, th adsorption affinity dcrass and th dcrasd was lowst for CSAC (26.13%), followd by CBAC (46.13%) and thn PSAC (50.30%). Similar trnds wr also found in th adsorption intnsity, with CSAC has th lowst rduction (11.12%), PSAC (21.66%) and CBAC th highst (27.94%).

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