Coconut coir activated carbon: an adsorbent for removal of lead from aqueous solution

Transcription

1 Ravag of th Plant III 95 Coconut coir activatd carbon: an adsorbnt for rmoval of lad from aquous solution M. Chaudhuri & S. N. B. Saminal Dpartmnt of Civil Enginring, Univrsiti Tknologi PETRONAS, Malaysia Abstract Th purpos of this study was to prpar activatd carbon from an agricultural solid wast and assss its ffctivnss in adsorptiv rmoval of lad from aquous solution. Activatd carbon was prpard from coconut coir and its charactristics wr compard with that of a commrcial bituminous coal-basd activatd carbon. Th coconut coir activatd carbon possssd highr surfac and micropor aras, micropor volum and avrag por diamtr, and wll dvlopd mso- and micropors. Adsorption of lad from aquous solution by th coconut coir activatd carbon was xamind. Batch adsorption tst showd that xtnt of adsorption was dpndnt on lad concntration, contact tim, ph and activatd carbon dos. Adsorption was low at acidic ph and incrasd up to ph 5, with marginal incras up to ph 6. Equilibrium adsorption was attaind in 2.5 h. Lad adsorption followd psudo-scond ordr kintics. Equilibrium lad adsorption data for th coconut coir activatd carbon and commrcial activatd carbon wr dscribd by th Langmuir and Frundlich isothrm modls. Coconut coir activatd carbon showd highr lad adsorption capacity [7.75 (Langmuir) and 3.63 (Frundlich)] compard with th commrcial activatd carbon [7.55 (Langmuir) and.87 (Frundlich)]. Coconut coir activatd carbon is a suitabl substitut for commrcial activatd carbon in th adsorptiv rmoval of lad from watr. Kywords: adsorption, activatd carbon, coconut coir, havy mtal, lad. Introduction Toxic havy mtals hav bcom an cotoxicological hazard of prim intrst and incrasing significanc owing to thir tndncy to accumulat in living doi:.2495/rav

2 96 Ravag of th Plant III organisms []. Lad is found in wastwatr from industris such as battry, cramic and glass manufacturing, mtal plating and finishing, printing and tanning, and production of lad additivs from gasolin [2]. Th tchniqus to rmov lad from watr includ ion xchang, chmical prcipitation, rvrs osmosis, vaporation, mmbran filtration and adsorption [3]. Adsorption by activatd carbon is a promising tchniqu as it nabls th rmoval of trac amount of lad. Howvr, high cost of commrcial activatd carbon limits its us in dvloping countris and thr is a growing nd to prpar activatd carbon from locally availabl wast matrials. Coconut coir is a rsidu in th procssing of coconut and is an agricultural solid wast. It is rich in lignin (6-45%), hmicllulos (24-47%) and pctin (2%) contnt [4, 5]. Th carboxylat and phnolic groups of lignin, hmicllulos and pctin ar known as th main sits of mtal binding [5]. In this study, activatd carbon was prpard from coconut coir and its charactristics wr compard with that of a commrcial bituminous coal-basd activatd carbon. Adsorption of lad from aquous solution by th coconut coir activatd carbon was xamind and its lad adsorption capacity was compard with that of th commrcial activatd carbon. 2 Matrials and mthods 2. Activatd carbon Coconut coir activatd carbon (CCAC) was prpard by soaking washd and drid coconut coir ovrnight in % potassium hydroxid solution followd by washing with distilld watr to rmov fr potassium hydroxid and drying at 5±5 C for 24 h. It was thn subjctd to activation at 9 C for 3 min in an atmosphr of nitrogn. Th carbon obtaind was washd with distilld watr and thn with % hydrochloric acid. Th carbon was washd furthr with distilld watr to rmov th fr acid and drid at 5±5 C for 24 h. Th carbon was ground to a finr siz of 22-5 µm and usd in adsorption tst. A commrcial bituminous coal-basd activatd carbon was obtaind from th Calgon Carbon Corporation, Pittsburgh, PA. It was ground to a siz of 22-5 µm and usd in adsorption tst. 2.2 Adsorption tst Batch adsorption tst was carrid out by shaking 25 ml of lad nitrat [Pb(NO 3 ) 2 ] solution with. g of activatd carbon in a stopprd glass bottl placd on an orbital shakr at 5 rpm and room tmpratur (22 C). Aftr a prdtrmind contact tim, th bottl was rmovd from th shakr and th suprnatant was filtrd through Whatman No. filtr papr and analysd for lad concntration by Mthod 3 B of Standard Mthods [6]. Th ffct of ph (-6), contact tim (-8 min), lad concntration (2-6 mg Pb/L), and activatd carbon dos (2- g/l) on adsorption was valuatd. Adsorption isothrm was dtrmind by batch quilibrium tst at th optimum ph and

3 Ravag of th Plant III 97 contact tim for adsorption with 25 ml of - mg Pb/L lad solution and. g activatd carbon. 3 Rsults and discussion 3. Activatd carbon charactristics Th coconut coir activatd carbon (CCAC) and commrcial activatd carbon (CAC) wr charactrisd for various physical and chmical paramtrs, as shown in tabl. Th CCAC possssd highr surfac and micropor aras, micropor volum and avrag por diamtr than thos of CAC. According to scanning lctron micrograph, as shown in fig., th CCAC containd wlldvlopd mso- and micropors compard to CAC. Tabl : Charactristics of CCAC and CAC. Paramtr Surfac Ara (m2/g) Micropor Ara (m2/g) Micropor Volum (ml/g) Avrag Por Diamtr (Å) Ash Contnt (%) Bulk Dnsity (g/ml) ph CCAC Figur : CCAC CAC CAC Scanning lctron micrograph of CCAC and CAC.

4 98 Ravag of th Plant III 3.2 Effct of ph on adsorption Effct of ph on lad adsorption by CCAC from a 2 mg Pb/L lad solution in 24 h is shown in fig. 2. It is obsrvd that adsorption was low at low acidic ph and incrasd (39.6%) up to ph 5, with marginal incras up to ph 6. Abov ph 6, lad hydroxyl complxs and lad hydroxid form and hnc was not considrd. For adsorption of lad by CAC, maximum adsorption (33.2%) also occurrd at ph 5 (data not shown). At low acidic ph, lad adsorption was hindrd possibly as a rsult of comptition btwn hydrogn and lad ions for adsorption sits, with an apparnt prpondranc of hydrogn ions. As ph incrasd, ngativ charg dnsity on th activatd carbon surfac incrasd du to dprotonation of adsorption sits, thus lad adsorption incrasd. Similar obsrvations hav bn rportd for adsorption of lad by activatd carbon prpard from palm shll [7], Militia frrugina plant lav [8], sugar can bagass and ucalyptus sawdust [9] and coconut shll and palm sd hull [], and activatd biochar prpard from pinwood and ric husk []. All subsqunt adsorption tsts wr conductd at ph 5. Adsorption, % ph Figur 2: Effct of ph on lad adsorption by CCAC. 3.3 Effct of contact tim and lad concntration on adsorption Effct of contact tim and lad concntration on lad adsorption by CCAC ar shown in fig. 3. Extnt of adsorption incrasd with dcras in lad concntration and incras in contact tim. Equilibrium adsorption was attaind in 5 min (2.5 h). For lad adsorption by CAC, quilibrium adsorption was also attaind in 2.5 h (data not shown). A contact tim of 2.5 h was usd in all subsqunt adsorption tsts.

5 Ravag of th Plant III 99 4 Adsorption, % mg Pb/L 4 mg Pb/L 6 mg Pb/L Tim, min Figur 3: Effct of contact tim and lad concntration on lad adsorption by CCAC. 3.4 Effct of activatd carbon dos on adsorption Effct of activatd carbon dos on lad adsorption from a 2 mg Pb/L lad solution by CCAC is shown in fig. 4. Adsorption incrasd with activatd carbon dos and attaind maximum adsorption (ca %) at 8 g/l activatd carbon dos. Adsorption, % Activatd carbon dos, g/l Figur 4: Effct of activatd carbon dos on lad adsorption by CCAC.

6 Ravag of th Plant III 3.5 Adsorption kintics To idntify th kintics of lad adsorption by CCAC, two commonly usd kintic modls i.. psudo first-ordr and psudo scond-ordr [2] wr mployd. dq k( q q) () dt dq dt k (2) 2 2 ( q q) whr, q is th amount of solut adsorbd at quilibrium pr unit wight of adsorbnt (mg/g), q is th amount of solut adsorbd at tim t pr unit wight of adsorbnt (mg/g), and k and k 2 ar raction rat constants. Th following linarisd tim dpndnt functions wr obtaind by intgrating and rarranging qns. () and (2) for th boundary conditions t = to > and q = to >. log( q k ) ln t q q (3) 2.33 t q t 2 k2q q (4) For psudo first-ordr and psudo scond-ordr kintic modls, th raction rat constants calculatd from qns. (3) and (4) ar shown in tabl 2 and th kintic plots ar shown in fig. 5 and 6. R 2 valus as wll as figs. 5 and 6 indicat that kintics of lad adsorption by CCAC is bttr xprssd by th psudo scond-ordr kintic modl. Complianc to th psudo scond-ordr kintic modl strongly suggsts chmical adsorption or chmisorption btwn lad and CCAC. Tabl 2: Psudo first-ordr and psudo scond-ordr raction rat constants for lad adsorption by CCAC. Lad concntration (mg Pb/L) Psudo first-ordr Psudo scond-ordr k (h - ) R 2 k 2 (g/mg h) R

7 Ravag of th Plant III.5.5 log (q-q) y = -.43x R² =.956 y = -.4x R² =.937 y = -.2x R² = mg Pb/L 4 mg Pb/L 6 mg Pb/L t, min Figur 5: Psudo first-ordr kintic plot for lad adsorption by CCAC y =.2x R² =.9978 t/q 5 y =.7x R² =.9984 y =.6x R² = mg Pb/L 4 mg Pb/L 6 mg Pb/L t, min Figur 6: Psudo scond-ordr kintic plot for lad adsorption by CCAC.

8 2 Ravag of th Plant III 3.6 Adsorption isothrm In adsorption in a solid-liquid systm, th distribution ratio of th solut btwn th liquid and th solid phas is a masur of th position of quilibrium. Th prfrrd form of dpicting this distribution is to xprss th quantity q as a function of C at a fixd tmpratur, th quantity q bing th amount of solut adsorbd pr unit wight of th solid adsorbnt, and C th concntration of solut rmaining in th solution at quilibrium. An xprssion of this typ is trmd an adsorption isothrm [3]. Th Langmuir adsorption isothrm is q Q bc bc o (5) whr, Q is th numbr of mols of solut adsorbd pr unit wight of adsorbnt in forming a monolayr on th surfac (monolayr limiting adsorption capacity) and b is a constant rlatd to th nrgy of adsorption. Th Frundlich adsorption isothrm is q K C f / n (6) whr, K f is th Frundlich constant (adsorption capacity) and /n rprsnts th adsorption intnsity. Adsorption isothrm for lad adsorption by CCAC and CAC wr dtrmind by batch quilibrium tst using optimum contact tim and ph (2.5 h and ph 5) for lad adsorption. Th isothrms wr fittd to th linar form of th Langmuir quation [C /q = /(bq o ) + C /Q o ], fig. 7 and Frundlich quation [ log q log K f (/ n) log C ], fig. 8. Th valus of Langmuir constants Q and b, and Frundlich constants K f and /n ar shown in tabl 3. Valus of Q and K f indicatd highr lad adsorption capacity of CCAC compard with CAC. C/q y =.323x R² =.99 y =.29x R² =.9948 CCAC C Figur 7: Langmuir adsorption isothrm for lad adsorption.

9 Ravag of th Plant III 3.8 y =.239x R² =.9958 log q y =.3566x R² =.99 CCAC CAC log C Figur 8: Frundlich adsorption isothrm for lad adsorption. Tabl 3: Langmuir and Frundlich constants for lad adsorption. Activatd carbon 4 Conclusions Langmuir constant Frundlich constant Q o b K f /n CCAC CAC Th coconut coir activatd carbon (CCAC) possssd highr surfac and micropor aras, micropor volum and avrag por diamtr, and wlldvlopd mso- and micropors compard to thos of a commrcial bituminous coal-basd activatd carbon (CAC). Maximum lad adsorption occurrd at ph 5 and quilibrium adsorption was attaind in 2.5 h. Lad adsorption by CCAC followd psudo scond-ordr kintics. Th CCAC showd highr lad adsorption capacity compard with CAC. Th CCAC is a suitabl substitut for commrcial activatd carbon in th adsorptiv rmoval of lad from watr. Acknowldgmnts Th authors ar thankful to th managmnt and authoritis of th Univrsiti Tknologi PETRONAS (UTP) and th Civil Enginring Dpartmnt, UTP for providing facilitis for this rsarch.

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