Comparative analysis of the efficiencies of two low cost adsorbents in the removal of Cr(VI) and Ni(II) from aqueous solution

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1 African Journal of Environmnal Scinc and Tchnology Vol. 3 (11), pp , Novmbr 2009 Availabl onlin a hp:// ISSN X 2009 Acadmic Journals Full Lngh Rsarch Papr Comparaiv analysis of h fficincis of wo low cos adsorbns in h rmoval of Cr(VI) and Ni(II) from aquous soluion O. Olayinka Khind*, T. Adund Oluwaoyin, and O. Oyyiola Adronk Dparmn of Chmisry, Univrsiy of Lagos Akoka, Yaba, Lagos, Nigria. Accpd 22 Spmbr, 2009 Havy mals prsn in was wars which ar rlasd ino h nvironmn by various indusris hav bcom a srious problm. This rsarch is focusd on h comparison of wo agriculural wass in h rmoval of Ni(II) and Cr(VI) ions from aquous soluion and h ffc of HCl modificaion. Th ffcs of adsorbn dos, iniial concnraion, conac im, ph and mpraur on adsorpion wr invsigad. Gnrally, h rsul showd an incras in adsorpion by Cr(VI) wih incras in mass of adsorbn wih h xcpion of coconu husk which consisnly had an adsorpion of 96 ± 3% irrspciv of h adsorbn loading. Th coconu husk adsorbns(unmodifid coconu husk 83 ± 3 and 81.4 ± 4.7%, HCl modifid coconu husk 96 ± 4 and 24.1 ± 4% wr found o b mor fficin for Cr(VI) and Ni(II) rspcivly han ak r bark adsorbns(unmodifid ak r 61.5 ± 3 and 80.9 ± 2.2%, HCl modifid ak r 81.9 ± 2.5 and 29.9 ± 2.6%). HCl modifid adsorbns wr found o br adsorbns for Cr(VI) ion whil h unmodifid adsorbns wr br adsorbn for Ni(II) ion for all h paramrs sudid. Th daa for h adsorpion of Cr (VI) and Ni(II) was sd wih Frundlich adsorpion isohrm modl and i was found o b suiabl for h adsorpion of Cr(VI) and Ni(II) for boh coconu husk and ak r bark. Th kinic sudy of h adsorpion procss showd ha adsorpion of Cr(VI) and Ni(II) or boh coconu husk and ak r bark was found o follow h scond-ordr. Ky words: Agriculural wass, adsorpion, havy mals, adsorbns, aquous soluion. INTRODUCTION Indusrial aciviis such as mining, lcroplaing, anning, mallurgical opraion and manufacuring hav ld o h rlas of havy mals ino h nvironmn (Samara, 2006). Unlik h organic polluans which ar bio-dgradabl, havy mal lik Ni(II) and Cr(VI) ar no bio-dgradabl (Rordamswag, 2006), hus, making hm a sourc of gra concrn. Havy mals bioaccumula in living organisms raching lvls ha caus oxicological ffcs (Samara, 2006). Human halh, agriculural dvlopmn and h cosysms ar all a risk unlss war and land sysms ar ffcivly managd (Rao al., 2002). In h nvironmn, chromium xiss in wo sabl forms which ar h + III and h + VI sas. Whil chromium (III) is known o b an ssnial rac lmn in plans and animal mabolism, h chro- *Corrsponding auhor. kolayi @yahoo.com. Tl: mium (VI) is oxic, carcinognic and muagnic (Igw al., 2005). Nickl oxiciy has bn rpord o caus pulmonary fibrosis and inhibi many nzymaic funcions (Liphadzi and Kirkham, 2005). As a rsul of h oxicological ffcs of hs mals, inrs in havy mal rmoval from was war has bn on h ris. Som mhods hav bn dvlopd o rmov havy mals from was war bfor discharg ino h war bodis. Ths mhods includ rducion, prcipiaion, ion xchang rvrs osmosis, dialysis and adsorpion by coad carbon. Mos of hs mhods ar xpnsiv so ar no affordabl for a dvloping counry lik Nigria. Thy also hav limid applicaion as hy canno rmov mals a low concnraion (Faur-Brasqu al., 2002). This has hrfor ld o h us of agriculural wass in rmoval of havy mals. Agriculural wass ar characrizd by rady availabiliy, affordabiliy, cofrindlinss and high upak capaciy for havy mals du o h prsnc of funcional groups which can bind mals o ffc h rmoval of havy mal from ffluns

2 Khind al. 361 (Olayinka al., 2007). Panu hull (an agriculural was) was found o b 36 ims mor fficin compard o granular acivad carbon which is h widly usd adsorbn (Prisamy and Namasivayan, 1995). Ohr agriculural wass which hav bn invsigad for hir mal adsorpion propry includ sugar can bagass, soybans hulls, coon sd hull, ric bran, ric husk and sraw as fficin havy mal adsorbns (Nasim al., 2004). Was a, coconu shll, coconu husk and ak r bark which ar all agriculural wass wr also found o b fficin mal adsorbns. Adsorpion of Cr(VI) and Ni(II) wr ph dpndn, conac im dpndn and adsorbn loading dpndn (Olayinka al., 2005; Olayinka al., 2007). Acivad Amarind (Tamarindus indica) sds using sodium biocarbona and HCl acivad nm lavs wr found o b fficin adsorbn for Cr(VI) (Babu and Gupa, 2008, 2009). I was also found ha sodium hydroxid modificaion of oa and wha sraws improvd adsorpion of havy mal for oa sraw biomass bu did no improv ha of wha sraw biomass. Howvr boh h modifid and unmodifid wr good nough o rmdia havy mals (Rios al., 1999). Prvious rsarchs hav shown ak r back and coconu husk o b good adsorbn du o hir high lignin conn (Olayinka and Adu, 2005; Olayinka al., 2005). Th coconu husk, a was in h agriculural scor is a low cos adsorbn for havy mal bu has found us in h furniur indusry for making foo mas, in h radiional mdicin scor as mdicin for raing fvr in childrn and for making fir for cooking in local sings. Tak r bark a was obaind from h imbr indusry in Nigria is only usd in making fir for cooking. Hnc a comparison of hir adsorpiv propris if in favor of h ak r bark will lad o a consrvaion of h coconu husk and incras h us of ak r back as is conomic valu is lss. Th aim of his sudy was o compar h us of modifid coconu husk and ak r bark in h rmoval of Ni(II) and Cr(VI) from aquous soluion. Th rsarch involvd h modificaion of boh ak r bark and coconu husk wih HCl and invsigaion of som xprimnal condiions such as ph of soluion, conac im, adsorbn loading and mpraur as i rlas o adsorpion of h mal o h adsorbns. I also involvd h us of Frunlich adsorpion isohrm modl o fi in h daa. Kinic sudy using psudo firs ordr and scond ordr kinic modl wr also usd o invsiga h mchanism of adsorpion. MATERIALS AND METHODS Adsorbns Th adsorbns wr prpard as dscribd by Hanafiah al. (2006). Each of h adsorbns (coconu husk and ak r bark) wr cu ino small pics and blndd. Thy wr washd wih disilld war o rmov dir and colour and air- drid for 24 h o avoid hrmal dacivaion of h adsorbn surfac rspciv- ly. Thy wr sivd o pass hrough a 2 mm sainlss sl siv and a porion of ach of h adsorbn was sord in clan polyhyln conainrs (labld as unmodifid adsorbn) prior o analysis. For h modificaion of h adsorbns, abou 400 g of ach of h washd adsorbns wr mixd wih 600 ml of 0.1 mol dm -3 HCl. Th mixur was had a 120 C for 30 min wih occasional sirring. Each of h adsorbn was sparad from war using Bucknr funnl and a vacuum pump and washd wih disilld war unil h washings wr fr of color and h ph of wash soluion was abou 7. Th washd adsorbns wr airdrid for 24 h and labld as HCl - modifid coconu husk and HCl- modifid ak r bark rspcivly. Th concnraion of Cr(VI) and Ni(II) in ach of h adsorbn was drmind by soaking 1 g of h adsorbn in 10 ml d-ionizd war for on hour wih coninuous agiaion, afr which i was cnrifugd a 2000 rpm wih a Rmi R-8C laboraory cnrifug. Th suprnaan was carfully dcand ino acid cland polyhyln conainrs and analyzd using a Flam Aomic Absorpion Spcrophoomr (FAAS) Prkin Elmr A Analys 200. Th ph of h adsorbns usd (modifid and unmodifid) was drmind according o Briish Sandard ISO (2005), using a Mlr Toldo ph mr. Th caion xchang capaciy of h adsorbn was drmind by sauraing h adsorbing si of 50 g adsorbn wih 33 ml of 1 M sodium aca. Th xchangabl sodium ions wr rmovd by washing h saurad adsorbn wih 95% hanol. 33 ml of 1 M soluion of sodium aca was usd o wash h adsorbn hr ims and h suprnaans of ach wash wr kp in a volumric flask and mad up o h mark wih disilld war. Th sodium conn was drmind using a flam aomic absorpion spcroscopy wih csium chlorid usd as an ionizaion supprssan. Th surfac ara of h coconu husk and ak r bark adsorbns wr drmind by iodin adsorpion mhod (Akporhonor al., 2007). Th amoun of iodin absorbd from aquous soluion was simad by iraing a blank wih sandard hiosulpha soluion and compard wih iraing agains iodin conaining h sampl coconu husk and ak r bark rspcivly. Indicaor usd was sarch. Th surfac ara was calculad as follows. Surfac ara (mg/g) = V blank V blank x V M x V blank x W Whr; V blank = Volum of blank usd. V sampl= Volum conaining adsorbn. M = Molariy of iodin. W = Wigh of adsorbn usd. Adsorbas Th soluions of Cr(VI) and Ni(II) mal ions wr prpard from analyical grad K 2Cr 2O 7 and NiCl 2 rspcivly. For h prparaion of 1000 mgl -1 soluions of ach of Cr(VI) and Ni(II), g of K 2Cr 2O 7 and g of NiCl 2 rspcivly wr dissolvd in dionizd disilld war and mad up o 1000 ml in sandard flasks. Th prpard aquous soluions (1, 000 mg/l) (sock soluions) wr dilud wih d-ionizd war o obain h working sandard soluions. Th main characrisics of hs ions ar summarizd in Tabl 1. Adsorpion xprimns For h adsorpion xprimn, h ffc of adsorbn loading on h adsorpion of h ions was invsigad. Bwn g of h adsorbn was wighd rspcivly ino conical flasks. 20 ml of 10 mg/l soluion of ach of h mal ion soluion was addd and

3 362 Afr. J. Environ. Sci. Tchnol. Tabl 1. Main characrisics of mal ions sudid. Propry Cr(VI) Ni(II) Aomic wigh (gmol -1 ) Formula K 2Cr 2O 7 NiCl 2.6H 2O 20 C (gcm -3 ) Ionic radius Pauling lcrongaiviy h mixur shakn wih an IKA HS 260 basic rciprocaing shakr a 150 rpm for 30 min. Th mixurs wr cnrifugd a 2, 000 rpm for 30 min, h suprnaan dcand and h mal ion conn was drmind using a Prkin Elmr AA Analys 200 Flam AAS. Th prcnag adsorpion was drmind by; C o C o C a x100 Whr; C o = Iniial concnraion of soluion, C a = Concnraion of h soluion afr adsorpion. In ordr o invsiga h ffc of concnraion on h adsorpion of mal ions, 0.4 g of h adsorbn was addd ino 20 ml ach of varying concnraions (bwn mg/l) of h mal ion soluions. Th mixurs wr shakn, cnrifugd and h concnraion of h mal ions adsorbd was drmind. Th ffc of conac im was also invsigad by adding 0.4 g of h adsorbn o 20 ml of 10 mg/l and shakn using varying conac ims (bwn min) and h prcnag of adsorbd ions drmind. Th ffc of ph on adsorpion of h ions was invsigad using 0.4 g of h adsorbn and 20 ml of 10 mg/l and h ph of h soluion was adjusd bwn 2 and 12. Th mixur was shakn for 90 min, cnrifugd and h amoun of ion adsorbd was drmind. Th adsorbns usd wr unmodifid ak r bark, unmodifid coconu husk. In ordr o invsiga h ffc of modifying h adsorbn, ach of h xprimns wr rpad using, HCl - modifid ak r bark and HCl-modifid coconu husk. For qualiy conrol, h xprimns wr prformd in riplicas. Adsorpion isohrm modls Th Frundlich adsorpion isohrm modls wr usd. Th Frundlich isohrm (Fundlich, 1906) is xprssd as N = K f C 1/n Whr; N = x/m (amoun of mal ions pr uni mass of adsorbn) whn h quilibrium concnraion is C. To simplify h quaion, i was linarizd hus: Log N = Log K f + 1/nLog C. Th Frundlich Isohrm Cofficins wr drmind by ploing LogN agains Log C. Th Frundlich quaion is an mpirical quaion basd on adsorpion on a hrognous surfac. Th consans indica h adsorpion capaciy and h adsorpion innsiy. Adsorpion kinics Th mchanism of h adsorpion of Cr(VI) and Ni(II) was sd using psudo firs-ordr and scond-ordr kinic modls. Psudo firs-ordr kinics Th non-linar form of psudo firs-ordr quaion is givn by: dn d = k ( N N ) (Ho and Mckay, 1999) ad Whr N and N ar h amouns of ach of Cr(VI) and Ni(II) adsorbd a quilibrium im and any insan of im () rspcivly. K ad is h ra consan. Th ingrad ra law hn bcoms: log( N N ) = log N kad Plo of log(n-n) vs. givs a sraigh lin and h adsorpion ra consan k ad can b compud. Scond-ordr kinics Applicabiliy of h scond ordr kinics is sd wih h ra quaion: dn d = k ( N N ) 2 2 Whr; k 2 is h scond ordr ra consan. From h boundary condiions = 0 o = and N = 0 o N = N, h ingrad ra law bcoms: ( N N ) N = + h N 1 = N + k 2 and his can b rwrin as: Whr; h = is h iniial sorpion ra as 0. Th plo of /N = givs a linar rlaionship and N, k 2 and h. (Ho and Mckay, 1999, Gupa and Babu, 2009) RESULTS AND DISCUSSION Physicochmical propris of Adsorbns Slcd characrisics of h adsorbns wr invsigad and ar prsnd in Tabl 2. Th adsorbns wr slighly acidic for HCl modifid adsorbns and nural for unmodifid adsorbn. For mos applicaions, ph of 6-8 is hough o b accpabl (Akporhonor and Egwaikhid, 2007). Larg surfac ara is a gnral rquirmn for good adsorbns. Surfac ara is a singl mos imporan characrisic of acivad carbon dsignd

4 Khind al. 363 Tabl 2. Physicochmical characrisics of h adsorbns. Paramrs Tak r bark Coconu husk ph HCl Modifid adsorbn 6 6 ph Unmodifid adsorbn 7 7 Caion xchang capaciy(cec)(mq/100g) Havy mal conn (Cr(VI)) No dcd No dcd Havy mal Conn (Ni) No dcd No dcd Surfac ara (m 2 /g) Figur 1a. Effc of conac im on adsorpion of chromium ion. for adsorpion of compounds from liquid mdia (Akporhonor and Egwaikhid, 2007). Th surfac ara of coconu husk was found o b largr (20.31 mg 2 /g) compard o ha of ak r bark (17.45 m 2 /g). Ni(II) and Cr(VI) conn wr no dcd in h wo adsorbns which man ha hs adsorbns had lil or no Ni(II) and Cr(VI) conaind in hm o xaggra h rsuls. Th caion xchang capaciy was found o b mq/100 g. Effc of conac im on h adsorpion of mal ions Th rsuls for h ffc of conac im on adsorpion of Cr(VI) and Ni(II) ar shown in Figurs 1a and b. A ph of 3 was usd for Cr(VI) soluion whil ph 7 was usd for Ni(II) soluion. Th coconu husk adsorbns gav br prcn adsorpion han h ak r bark adsorbns as h conac im incrasd. I was obsrvd ha, h HClmodifid coconu husk consisnly had a consan prcnag adsorpion (95%) for chromium all hrough h conac ims usd which man quilibrium was rapidly rachd and as quickly as 30 min h adsorpion sis whr alrady saurad o maximum upak capaciy. This maks i h mos fficin adsorbn of h adsorbns usd in his rsarch for Cr(VI) adsorpion. For HCl-modifid ak r hr was an iniial incras in prcnag adsorpion bu afr 45 min conac im h prcnag adsorpion rmaind consan (54%). This migh b du o sauraion of h adsorpion sis afr 45 min afr which lil or no incras in prcnag adsorpion was obsrvd as conac im incrasd. For unmodifid coconu husk, h prcnag of chromium adsorbd incrasd ill 90 min afr which hr was a significan dcras in prcnag adsorpion. This could b as a rsul of dsorpion of h mal ions, whil h unmodifid ak r had h lows prcnag adsorpion of all h adsorbn usd for Cr(VI) adsorpion vn hough h prcnag adsorpion incrasd as conac im incrasd. In h cas of Ni(II), h prcnag adsorpion was consan for h coconu husk adsorbns whil hr wr sligh variaion for h ak r bark adsorbns. Th unmodifid and HCl modifid coconu husk adsorbns consisnly gav 96 and 50% adsorpion rspcivly. I could b said ha h mal upak was rapid and h adsorbns wr capabl of adsorbing mals rapidly wihin 30 min, whil for h ak r bark

5 364 Afr. J. Environ. Sci. Tchnol. Figur 1b. Effc of conac im on adsorpion of nickl ion. Figur 2a. Effc of ph on adsorpion of mal ion. hir prcnag adsorpion incrasd wih im. For h HCl-modifid ak bark, h adsorpion wihin h firs 45 min was rapid and afr which subsqun incras was slow. A similar rnd was obsrvd wih h unmodifid ak r bark as i rapidly adsorbd h Ni(II) wihin h 30 min conac im. Effc of ph on h adsorpion of mal ion Th adsorpion capaciy of h adsorbns as a funcion of hydrogn ion concnraion (ph) was drmind. Th rsuls for h adsorpion of Cr(VI) and Ni(II) ar shown in Figurs 2a and b. Th upak of havy mal is ph dpndn as sn in h Figurs 2a and b. This could b bcaus ph affcs h solubiliy of mals in soluion and h ions on h funcional groups of h adsorbn (Malik and Rams, 2007). Chromium adsorpion was high for all h adsorbns a low ph valus of 2, 3, 4 and 5.Th adsorpion capaciy of all h adsorbns for Cr(VI) dcrasd wih incrasing ph valus, bu for h HClmodifid adsorbn h dcras in prcnag adsorpion was sligh. Th dcras was obvious in h ak r bark adsorbns. This is probably du o h lowr surfac ara of ak r bark as sn in Tabl 1, hnc a lowr numbr of adsorpion sis as compard o h coconu husk. Gnrally, Cr(VI) is br adsorbd a low ph valus. This could b bcaus a low ph Cr(VI) xis as Cr 3+ and can hrfor b br adsorbd by h C = O funcional group on h adsorbn sinc oxygn is lcrongaiv and can arac ions ha ar posiivly chargd (Sharma and Forsr, 1994). Also according o Samana al. (2002), a low ph, hr is prsnc of a larg numbr of H + ions, which in urn nuralizs h

6 Khind al. 365 Figur 2b. Effc of ph on adsorpion of Ni(II) ion. ngaivly chargd adsorbn surfac by rducing h hin-dranc o h diffusion of dichroma ions. Th dcras in prcnag adsorpion of chromium a highr ph could b du o h high numbr of hydroxyl which causs hindranc o h diffusion of chroma ions (Olayinka al., 2007). Th following racion mchanism for adsorpion of Cr(VI) a diffrn ph is proposd by Baya (2002). 2H + 2H + 2H + 2H + + 2HCrO 4-2H 2 CrO 4 2H 2 O+Cr 2 O 7-2 2CrO 3 + H 2 O A lowr ph, dominan form of Cr(VI) is HCrO 4- whil h surfac of adsorbn is chargd posiivly. Th H 2 CrO 4 and CrO 3 xis as poly-nuclar spcis a high chromium concnraion and a low ph valu. In h cas of Ni(II), all h adsorbns had high adsorpion of bwn 65 o 99% a phs of 8-9. I is apparn ha Ni(II) is srongly adsorbd a highr ph valus. Th opimum ph for h adsorbns could b said o b ph 7 and 8. This could b du o an incrasing ngaiv charg dnsiy on h adsorbn surfac. A ph grar han 8, Ni(II) rmoval was mosly du o prcipiaion and no by sorpion. A similar rsul was obaind whn sawdus and ohr wass wr usd as sn in Tabl 3. Effc of adsorbn loading on h adsorpion of Cr(VI) and Ni(II) Th rsul for h adsorpion of Cr(VI) and Ni(II) is shown in Figurs 3a and b rspcivly. For Cr(VI), HCl-modifid coconu husk consisnly rmovd almos 100% of h Cr(VI) from h synhic fflun rgardlss of h adsor- bn loading. This could b bcaus h HCl- modifid coconu husk has posiivly chargd sis du o h acid and h Cr(VI) in soluion xis as chroma ion which is ngaiv. Th prsnc of a highr numbr of adsorpion sis on a small mass of h adsorbn probably accouns for h rason why variaion in adsorbn loading had no ffc sinc 0.2 g of adsorbn alrady rmovd abou 100%. Th highr numbr of adsorpion sis on HClmodifid coconu husk compard o HCl-modifid ak r bark could probably b as a rsul of h highr surfac ara of h coconu husk. I was also obsrvd ha as adsorbn loading of h HCl-modifid ak r bark incrasd, adsorpion incrasd. Thr was found o b a ris in adsorpion from 15.9 o 81.9% as h mass of adsorbn was varid from 0.2 o 1 g. This may b aribud o incrasd surfac ara and availabiliy of mor adsorpion sis. This is in agrmn wih h findings of Priasivayam al. (1995). Th prcnag adsorpion of Cr(VI) by h unmodifid coconu husk and unmodifid ak r bark incrasd wih incrasing adsorbn dosag. A similar rnd was obsrvd wih unmodifid ak bark. This is also aribud o incrasd surfac ara and h availabiliy of mor adsorpion sis. In h cas of Ni(II), h unmodifid adsorbns rmovd mor Ni(II) compard o h HCl-modifid adsorbns. Th prcnag of Ni(II) rmovd incrasd o 99.77% as h adsorbn loading of h unmodifid coconu husk incrasd from 0.2 g o 0.6 g whil a sharp dcras in adsorpion was noicd a 0.8 g. This may b du o aggrgaion of adsorpion sis rsuling in dcras in oal adsorbn surfac ara of paricls availabl o Ni(II) and an incras in diffusion pah lngh. Th lowr prcnag of adsorpion of boh HCl-modifid coconu husk and HCl-modifid ak r bark as

7 366 Afr. J. Environ. Sci. Tchnol. Tabl 3. Maximum adsorpion capaciy of various low cos adsorbns and agriculural was (Gupa and Babu, 2009). S/NO Adsorbn Maximum adsorbn capaciy qm (mg g-1) Opimum ph 1. Acivad nm lavs Acivad Carbon (Filrasorb-400) Sawdus Coconu husk fibr Ta facory was Laf Mould Pin ndls Sugar b pulp Palm prssd-fibrs Maiz cob Sugar can bagass Acivad charcoal Acivad amarind sds Almond Polymr grafd sawdus Mapl sawdus Acivad alumina Cacus Biomass rsidual slurry Was a Walnu shll Ric husks Soya cak CH; coconu husk, TB; ak r bark. Figur 3a. Effc of adsorbn loading on Cr(VI) ion. compard o h unmodifid coconu husk and unmodifid ak r bark for Ni(II) could b as rsul of rpulsion bwn h xcss posiiv binding sis and Ni(II) ion which ar posiiv in soluion. Th unmodifid coconu husk gav a highr prcnag adsorpion whn compard o h unmodifid ak r bark. This could b as

8 Khind al. 367 Figur 3b. Effc of adsorbn loading on Ni(II) ion. Iniial concnraion adsorba in soluion (mg/l) Figur 4a. Effc of iniial concnraion on adsorpion of Ni(II) ion. as a rsul of h highr surfac ara and CEC of h coconu husk as agains h ak r. Th CEC of an adsorbn is a masur of h numbr of caions ha 100 g of adsorbn can rmov from soluion. Th CEC mchanism of rmoval of ions is basd on ion xchang. Easily xchangd ions ar displacd from hir sis and rplacd by biggr caions in soluion which adsorb o hs sis on h adsorbn. Ni(II) in soluion xiss as a caion and coconu husk wih highr valu of CEC xchangd mor of i as agains h ak r bark. Th ffc of concnraion on h adsorpion of h mal ions Th rsuls of h ffc of concnraion on adsorpion of Cr(VI) and Ni(II) o coconu husk and ak r back ar shown in Figur 4a and b. Th HCl-modifid coconu husk was found o hav h highs adsorpion of Cr(VI).Gnrally, h adsorpion of Cr(VI) by h adsorbns incrasd as h iniial concnraion of h Cr(VI) ion was incrasd. I was abl o adsorb a concnraion of 96 mg/l whn an iniial concnraion of 100 mg/l soluion of Cr(VI) ion was usd. Th high adsorpion of h Cr(VI) by h HCl-modifid coconu husk could b as a rsul of high surfac ara which mans high adsorpion numbr of adsorpion sis. Gnrally boh h HCl-modifid and unmodifid ak r bark had lowr adsorpion. This may probably b du o h owr surfac ara. In h cas of Ni(II), h unmodifid coconu husk had h highs concnraion of Ni(II) adsorbd followd by h unmodifid ak r bark. Th HCl had a lowr concnraion of Ni(II) rmovd. This is probably as a rsul of Ni(II) bing posi-

9 368 Afr. J. Environ. Sci. Tchnol. Figur 4b. Effc of iniial concnraion on adsorpion of Cr(VI) ion Tabl 4. Frundlich isohrm for h adsorbn on nickl and chromium adsorpions. Adsorbn/mal in soluion Equaion of graph K f 1/n N R 2 HCl modifid TB/Cr Y = x Unmodifid TB/Cr Y = x HCl modifid TB/Ni Y = x Unmodifid TB /Ni Y = x HCl modifid CH/Cr Y = 1.045x Unmodifid CH/Cr Y = x HCl modifid CH/Ni Y = x Unmodifid CH /Ni Y = x ivly chargd as compard o h ngaiv chroma. Th amoun of Ni(II) rmovd by som of h adsorbns was almos consan afr crain poins as h concnraion of h iniial soluion incrasd from mg/l. This probably indicad ha a a concnraion of 5, 8 and 12 mg/l of Ni(II) HCl modifid ak r bark, HCl modifid coconu husk and unmodifid ak r bark rspcivly, h availabl binding sis had bn saurad and h adsorbn could no longr bind furhr ions as h numbr of sis availabl on h adsorbn is consan. Furhrmor, h prsnc of comping ion (H 3 O + ) from h HCl modifid adsorbn could also hindr h Ni(II) adsorbing capabiliy of h adsorbn (Pandy al., 2007). Adsorpion isohrm Th Frundlich Isohrm Cofficins wr drmind by ploing LogN agains Log C and h rsuls ar shown in Tabl 4. Th Frundlich quaion is an mpirical quaion basd on adsorpion on a hrognous surfac. Th consans indica h adsorpion capaciy and h adsorpion innsiy. Th linariy of h plo showd h rprsnaiv naur of adsorpion on h adsorbns sd. Th corrlaion rgrssion co-fficin showd ha havy mal adsorpion was favorabl. Th valus obaind wr comparabl o valus obaind from h sudy of ohr adsorbns (Igw al., 2007). Th ak r bark had lowr R 2 valus and highr K f han ha of h coconu husk. This shows ha h coconu husk is a br adsorbn for h mal ions. Kinic adsorpion kinics Th kinic sudis of adsorpion of Cr(VI) and Ni(II) o ak r bark and coconu husk was carrid ou using h firs ordr and scond ordr modls on xprimnal daa and h valus obaind ar as sn in Tabl 5. Th rgrssion cofficins obaind from h psudo firs ordr kinic graph wr low. This implis non applicabiliy of h psudo firs ordr kinic modl o h xprimnal daa of h adsorpion of boh Cr(VI) and Ni(II) o ak r bark and coconu husk. Th scond ordr kinic modl kinics was applid by ploing /N vs. gav high valus of rgrssion corrlaion co-fficin as sn in Tabl 4. This implid ha h mchanism of adsorpion of boh ions on coconu husk and ak r bark followd scond ordr kinics.

10 Khind al. 369 Tabl 5. Calculad kinic paramrs for scond ordr kinic modl for h adsorpion of Cr(VI) and Ni(II) using coconu and ak r bark. Adsorbn/mal in soluion Equaion of graph R 2 HCl modifid TB/Cr Y = 0.124x Unmodifid TB/Cr Y =0.1970x HCl modifid TB/Ni Y = x Unmodifid TB /Ni Y = x HCl modifid CH/Cr Y = x Unmodifid CH/Cr Y = x HCl modifid CH/Ni Y = x Unmodifid CH /Ni Y = 0.105x Conclusion A comparison of wo low cos adsorbns (Coconu husk and Tak r bark) showd ha coconu husk as an adsorbn was mor fficin in mal ion rmoval from soluion whn compard wih h ak r bark as an adsorbn. This can b aribud o h larg surfac ara and high CEC valu of coconu husk. Th coconu husk whn modifid wih HCl was found o b h bs adsorbn for Cr(VI) adsorpion as Cr(VI) ion in soluion xiss as chroma ion which is a ngaiv ion and asily xchangs for H + providd by h HCl whil h unmodifid coconu husk was h bs adsorbn for Ni(II) ion adsorpion. This was bcaus h concnraion of H + o comp wih Ni(II) for h adsorpion si wr lss compard o h HCl- modifid adsorbns. REFERENCE Akporhonor EE, Egwaikhid PA (2007). Rmoval of Slcd Mal ions from Aquous Soluion by Adsorpion ono Chmically Modifid Maiz Cobs. Acad. J., 4: Babu BV, Gupa S (2008). Adsorpion of Cr(VI) using acivad nm lavs as an adsorbn: Kinic sudis, Adsorpion. 14: Babu BV, Gupa S (2009). Rmoval of Cr(VI) from waswar using acivad amarind sds as an adsorbn, J. Environ. Eng. Sci., 7: Baya B (2002). Comparaiv sudy of adsorpion propris of Turkish fly ashs. II. Th cas of chromium (VI) and cadmium (II), J. Hazard Mar, 95: Gupa S, Babu BV (2009). Rmoval of Toxic Mal Cr(VI) from Aquous Soluions Using Sawdus as Adsorbn: Equilibrium, Kinics and Rgnraion Sudis. Chm. Eng. J., 150: Faur-Brasqu C, Kadirvlu K, L Cloirc P (2002). Rmoval of mal ions from aquous soluion by adsorpion ono acivad carbon clohs adsorpion complion wih organic mar. Carbon, 40: Hanafiah MAK, Ibrahim SC, Yahaya MZA (2006). Equilibrium adsorpion sudy of lad ions ono sodium hydroxid modifid Lalang (Impraa cylindica) laf powdr. J. Appl. Sci. Rs., 2: Ho YS, Mckay G (1999). A comparison of Chmisorpion Kinic Modls Applid o Polluan Rmoval on Various Sorbns, Trans. Chm. E., 76B: Igw JC, Ogunw DN, Abia AA (2005). Compiiv Adsorpion of Zn (II), Cd(II) and Pb(II) ions from Aquous and Non-aquous Soluion by Maiz Cob Husk. Afr. J. Biochnol., 4 (10): Liphadzi MS, Kirkham MB (2005). Phyo-rmdiaion of soil conaminad wih havy mals, a chnology for rhabiliaion of h nvironmn. S. Afr. J. Bo., 71: Malik UW, Rams B (2007). Spcrophoomrric Drminaion of Chromium III using Poassium hxacyanofrra II. Mirochimica Aca J., 3(63) 5-6: Nasim AK, Shaliza I, Piarapakaran S (2004). Eliminaion of Havy Mals from Was War using Agriculural Was as Adsorbns. Rv. Papr Malaysian J. Sci., 23: Olayinka KO, Alo BI, Adu T (2005). Sorpion of Havy Mals from Elcroplaing Efflun by Low-cos Adsorbns1: Us of Saw Dus and Tak Tr Bark. J. Nig. Environ. Soc., 2(3): Olayinka KO, Alo BI, Adu T (2007). Sorpion of Havy Mals from Elcroplaing Effluns by Low Cos Adsorbns II: Us of Was a, Coconu shll and Coconu husk. J. Appl. Sci., 7(16): Priasamy K, Namasivayam C (1995). Rmoval of Nickl (ii) from Aquous Soluion and Nickl Plaing Indusry Was War using an Agriculural Was: Panu hulls. Was Manag. J., 15(1): Pandy KP, Chouby S, Pandy M, Kalmal SSKK, Chandrashkhar K (2007). Biosorpiv Rmoval of Ni(II) from Was War and Indusrial Efflun. In. J. Environ. Rs. Publ. Halh, 4 (4): Rordamswg BV (2006). Havy Mals, Lnnch pp.1-4. Samara SA (2006). Toxiciy, Havy Mals. Mdicin pp.1-2. Samana AK, Basu JK (2000). Rmoval of Cr(VI) (VI) from aquous soluion by using low cos adsorbn. Indian J. Environ. Pro., 20: Sharma DC, Fosr CF (1994). Th ramn of chromium waswar using h sorpiv ponial of laf mould. Biors. Tchnol., 49: Rios JV, Bss-Obro RT, Timann KJ, Garda-Torrsdy JLO (1999). Invsigaion of mal ion binding by agriculural by-producs. Procdings of 1999 confrnc on hadzardous was rs.