Characterization of Bottom Ash as an Adsorbent of Lead from Aqueous Solutions

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1 Environ. Eng. Rs. 010 Dcmbr,15(4) : Rsarch Papr DOI: /r pissn ISSN X Charactrization of Bottom Ash as an Adsorbnt of Lad from Aquous Solutions Joan B. Gorm 1, Marla C. Maniquiz 1, Soon Sok Kim 1, Young Gyu Son 1, Yun-Ta Kim, L-Hyung Kim 1 1 Dpartmnt of Civil and Environmntal Enginring, Kongju National Univrsity, Chonan , Kora Dpartmnt of Ocan Enginring, Pukyong National Univrsity, Busan, , Kora Abstract This study invstigatd th potntial of using bottom ash to b usd as an adsorbnt for th rmoval of lad (Pb) from aquous solutions. Th physical and chmical charactristics of bottom ash wr dtrmind, with a sris of laching and adsorption xprimnts prformd to valuat th suitability of bottom ash as an adsorbnt matrial. Trac lmnts wr prsnt, such as silicon and aluminum, indicating that th matrial had a good adsorption capacity. All havy mtals lachd during th Kora standard laching tst (KSLT) passd th rgulatory limits for saf disposal, whil batch adsorption xprimnts showd that bottom ash was capabl of adsorbing Pb (xprimntal q = 0.05 mg/g), whrin th adsorption rat incrasd with dcrasing particl siz. Th adsorption data wr thn fittd to kintic modls, including Lagrgrn first-ordr and Psudo-scond ordr, as wll as th Elovich quation, and isothrm modls, including th Langmuir, Frundlich and Dubinin-Radushkvich isothrms. Th rsults showd that psudo-scond ordr kintics was th most suitabl modl for dscribing th kintic adsorption, whil th Frundlich isothrm bst rprsntd th quilibrium sorption onto bottom ash. Th maximum sorption capacity and nrgy of adsorption of bottom ash wr mg/g and 7.01 KJ/mol, rspctivly. Kywords: Adsorption, Bottom ash, Equilibrium isothrm, Kintic constant, Laching, Lad 1. Introduction Th prsnc of havy mtals in watr systms is a srious nvironmntal problm du to thir toxic, cumulativ and nonbiodgradabl charactristics. Lad (Pb) is considrd on of th most toxic havy mtals, which has bn incrasingly introducd into watr bodis. According to th Ministry of Environmnt in Kora, th prmissibl limit for Pb in drinking watr is 0.05 mg/l [1]. Human xposur to Pb from diffrnt sourcs, such as storag battris, lad manufacturing, tir war and mining, causs svral illnsss, and unlik most organic pollutants, it is gnrally rfractory and difficult to biologically dtoxify []. Various tratmnt procsss, such as chmical prcipitation, ultra-filtration and lctrochmical dposition, hav bn dvlopd for th rmoval of havy mtals [3]. Howvr, th application of such mthods is limitd du to tchnical or conomical constraints, which has lad to th sarch for chapr and mor asily obtainabl matrials for adsorption of havy mtals. Numrous studis hav assssd th us of rcycld matrials, such as sawdust [], ric hull [4], coconut hull [5], fly ash [6] and maiz husk [7], for th ffctivnss in th rmoval of Pb from wastwatr. Th rising gnration of coal-burning powr plants in Kora has rsultd in incrasd disposal of th associatd by-products, such as boilr slag, fly ash and bottom ash. Ths coal-fird powr plants hav gnratd about 5 million tons of coal bottom ash [8]. This hug amount of ash rquirs larg aras for its disposal, which may vntually bcom a sourc of pollution. Thrfor, altrnats for th utilization of ths by-products hav to b dvlopd. Bottom ash, which accounts for 5-15% of th by-products producd during coal powr gnration, has alrady bn usd as structural fill, and construction and road bas matrials. Rcntly, thr has bn growing intrst in th utilization of bottom ash as a sorbnt for various pollutants, spcially in watr. Svral studis hav invstigatd th possibl utilization of bottom ash as an adsorbnt for th rmoval of various havy mtals from wastwatr [9-11]. Th particl siz, inhrnt larg surfac ara and high porosity of bottom ash mak it a good choic for us as a low-cost adsorbnt. Howvr, appropriat assssmnts of th matrial to valuat its toxicity and th possibility of its rcycling and rus hav to b prformd. Th objctiv of this rsarch was to dtrmin th suitability This is an Opn Accss articl distributd undr th trms of th Crativ Commons Attribution Non-Commrcial Licns ( org/licnss/by-nc/3.0/) which prmits unrstrictd non-commrcial us, distribution, and rproduction in any mdium, providd th original work is proprly citd. Rcivd Octobr 04, 010 Accptd Novmbr 17, 010 Corrsponding Author lhyung@kongju.ac.kr Tl: Fax: Copyright Th Koran Socity of Environmntal Enginrs 07

2 Joan B. Gorm, Marla C. Maniquiz, Soon Sok Kim, Young Gyu Son, Yun-Ta Kim, L-Hyung Kim of bottom ash to b usd as an altrnativ mdium for th rmoval of havy mtals in wastwatr. In this study, th physical, chmical and minralogical compositions of bottom ash wr analyzd. Morovr, th total havy mtal contnt was xamind and th laching charactristics of bottom ash wr invstigatd to valuat th capability of this matrial to rlas toxic havy mtals xcding th Koran rgulatory limits. Batch adsorption xprimnts wr prformd to assss th potntial of bottom ash for th adsorption of Pb. Hraftr, th kintic and quilibrium constants of adsorption wr stablishd and modld.. Mthods.1. Exprimntal Matrial Th bottom ash sampls wr obtaind from a coal incinrator in Kora. It has a combination of various sizs dpnding on th incinration procss. Rprsntativ wightd sampls wr sivd, with th sampls groupd as follows: <1.19 mm, mm and mm siz rangs for th analysis of its physical, chmical, laching and adsorption charactristics. Th ph, loss on ignition (LOI) and organic mattr contnt of th sampls wr dtrmind. Th spcific surfac ara of th bottom ash sampls wr masurd using an ASAP 000 analyzr (Edwards High Vacuum Intrnational, Crawly, W.Sussx, UK) with N adsorption... Elmntal and Morphological Analysis Th scanning lctron microscopy-nrgy disprsiv spctroscop (SEM-EDS) was usd to dtrmin th lmntal and morphological compositions of th bottom ash sampls. Th SEM-EDS was opratd at an acclrating voltag of 30 kv for th minral analyss of rprsntativ sampls in ordr to provid information on th physical proprtis of th matrial. Th EDS dtctor is abl to dtct lmnts with atomic numbr qual to or gratr than six..3. Minralogical Analysis Th bottom ash sampls, in powdr form, wr analyzd using an X-ray diffractomtr (XRD) for th qualitativ valuation of th common and prdominant phass within th ash. Th diffractomtr was opratd at 40 kv and 40 ma, ovr th rang of θ from 0 to 70, with a dtctor spd of 1 /min. Th unknown substancs wr idntifid by comparing th pattrns of thir diffraction data against a databas archivd in th Powdr Diffraction Fil of th Intrnational Cntr for Diffraction Data..4. Total Contnt and Laching Tst To dtrmin th total havy mtal contnt of th sampls, 1.0 g of bottom ash was digstd using a microwav ovn with Nitric/Hydrochloric acid solution, and analyzd using inductivly coupld plasma atomic mission spctroscopy (ICP-AES). Th lachability of th havy mtals from th bottom ash was assssd according to th Kora standard laching tst (KSLT). Fifty grams of bottom ash sampls wr mixd with an xtraction fluid (W:V ratio=1:10) composd of 500 ml distilld watr and HCl (ph= ). Th sampls wr vigorously shakn (185 rpm) in a watr bath shakr at a constant tmpratur of 0 o C for 6.5 hrs. Th lachat was vacuum filtrd through a 1.0-μm filtr papr, and th concntrations of Pb, Coppr (Cu), Arsnic (As), Chromium (Cr) and Cadmium (Cd) in th suprnatant liquid wr thn analyzd using ICP-AES..5. Batch Adsorption Tst Batch adsorption xprimnts wr carrid out by using 100 g of bottom ash for kintic adsorption, and 10, 0, 50 and 100 g of bottom ash for quilibrium adsorption, in 500 ml of solution, containing 10 mg/l of Pb. Th mixturs wr shakn (100 rpm) in a watr bath shakr at a constant tmpratur of 0 C. Small sampls of th solution (0.0 ml) wr takn out at prdtrmind tim intrvals to masur th volution of th adsorbat concntration. Th sampls wr filtrd through a 0.45-µm mmbran, and th rsidual mtal concntrations of suprnatant liquid wr dtrmind using ICP-AES. 3. Rsults and Discussion 3.1. Physical Proprtis Th physical proprtis of coal bottom ash vary dpnding on th typ, sourc and finnss of th parnt ful, as wll as th oprating conditions of th powr plant [1]. Tabl 1 shows th physical proprtis of th bottom ash groupd by particl siz rang. It was apparnt that ph, LOI and surfac ara incrasd with dcrasing particl siz. Th ph of th bottom ash, whn mixd with watr (initial ph=7.0), changs to High ph is th major factor controlling th vntual mtal laching of th matrial. Th ph has an impact on th adsorption sits, as ths ar known to b ph dpndnt. As ph incrass, th lctrostatic attraction of th adsorbnt for mtals is nhancd. Of th thr groups, <1.19 mm had th highst surfac ara. Sinc finr particls hav a gratr spcific surfac ara, this will provid an incrasd ffctiv contact ara for th adsorbnt and th adsorbat. 3.. Chmical and Minralogical Proprtis Th morphology of th bottom ash sampls was affctd by th combustion tmpratur and cooling rat during th incinration procss. Fig. 1 shows th SEM photographs takn at 10- µm magnification. At this rang, hollow cnosphrs wr dtctd along th surfac of th bottom ash sampls. Th marking points in th figurs rprsnt th spot points during th EDS analyss. Th rsults (Tabl ) show that th prdominant lmnts in th bottom ash sampls wr oxygn (O), silicon (Si), aluminum (Al) and carbon (C). Th diffractogram in Fig. shows th XRD pattrn of th bottom ash sampls. It was found that th matrial was amorphous in natur, as indicatd by th prsnc Tabl 1. Physical charactristics of bottom ash Proprty <1.19 mm mm mm ph Loss on ignition (%) Surfac ara (m /g) DOI: /r

3 Charactrization of Bottom Ash as an Adsorbnt of Lad from Aquous Solutions a b Fig.. X-ray diffraction pattrn of bottom ash. of quartz (SiO ) and mullit (Al 6 Si O 13 ). Significant amounts of silicon and aluminum wr found in th bottom ash sampls, which wr similar to thos found in othr matrials, such as zolits, which ar known to b good adsorbnts for th rmoval of havy mtals Havy Mtal Contnt and Laching Charactristics c Fig. 1. Scanning lctron microscopy micrographs of bottom ash for particl siz rangs: (a) <1.19 mm; (b) mm; (c) mm. A good adsorbnt matrial is considrd saf and fficint whn it can ffctivly rmov havy mtals, without rlasing potntially hazardous substancs. Th highst attntion is paid towards th solubl havy mtals that may migrat into soil and groundwatr. Th total and laching concntrations of havy mtals in th bottom ash ar shown in Tabl 3. Th rsults show that th total concntrations of havy mtals in bottom ash wr in th ordr: As > Cr > Pb > Cu > Cd. Bottom ash contains rlativly small amounts of havy mtals, particularly volatil mtals, such as Cd and Pb [10]. As shown in Tabl 3, arsnic constitutd th largst proportion of th havy mtals prsnt in th bottom ash. Unitd Stats Gological Survy (USGS) studis indicat that, during coal combustion in modrn coal-fird utilitis, % of th arsnic is capturd in coal combustion byproducts, which ar incorporatd into th silicat minrals constituting th bulk of th solid ash [13]. Th rsults from th laching tst showd havy mtal concntrations from th thr particl siz groups of lss than 0.3 mg/l, passing th Koran rgulatory limit. Th rgulatory limit for Pb and Cd is 3.0 mg/l, 1.5 mg/l for As and Cr, and 0.3 mg/l for Cd. Th amounts of havy mtals lachd from th bottom ash wr in th following ordr: As > Cu > Pb > Cr > Cd. It was also obsrvd that th amounts of Pb, Cr, Cu and As lachd from th bottom ash incrasd with dcrasing particl siz. Tabl. Elmntal composition of bottom ash (%) Particl siz O Si Al C Mg P F K Na Ca Ti <1.19 mm mm mm

4 Joan B. Gorm, Marla C. Maniquiz, Soon Sok Kim, Young Gyu Son, Yun-Ta Kim, L-Hyung Kim Tabl 3. Contnt and laching concntration of havy mtals in bottom ash Havy Contnt (mg/kg) Laching concntration (mg/l) mtal <1.19 mm mm mm <1.19 mm mm mm Pb Cu As Cr Cd (min), q (mg/g) th adsorption capacity at quilibrium and K (min 1 ) th rat constant for th psudo-first-ordr modl. Th psudo-scond-ordr modl Eq. () can b xprssd as follows [15]: t 1 t = + () q K q q t s whr K s is th rat constant for th psudo-scond-ordr modl (g/mg min). Basd on th scond ordr modl, th initial adsorption rat (h 0 ) and half adsorption tim (τ 1/ ) can b stimatd according to Eqns. (3) and (4): Fig. 3. Adsorption capacity of adsorbnt Adsorption Charactristics Th rsults obtaind from th tim-dpndnt xprimnts for th rmoval of Pb using bottom ash ar shown in Fig. 3. Th trnd shows that th amount of mtal ions rmovd incrasd with incrasing contact tim. Of th thr particl siz rangs, <1.19 mm dmonstratd th highst and fastst adsorption of Pb. As shown in Fig. 3, th absorption capacity for Pb incrasd with dcrasing particl siz of th bottom ash. Sinc adsorption is a surfac phnomnon, th highr adsorption rat was attributd to th grat accssibility to pors and th largr surfac ara. Th adsorption of Pb attaind quilibrium btwn 1000 and 4500 min of contact tim Kintic Modls To furthr xplain th adsorption mchanism, kintic modls, such as th psudo-first and psudo-scond-ordr modls, as wll as th Elovich quation wr tstd to fit th xprimntal data obtaind from th Pb rmoval xprimnts. Ths quations wr linarizd, and th cofficint of rgrssion (R ) was usd to dtrmin th adquatnss of th diffrnt modls to fit th adsorption procss. Assuming first ordr kintics, th Lagrgrn Eq. (1) was usd to dtrmin th psudo-first ordr [14]. K log(q qt ) = log(q ) t (1).303 whr q t (mg/g) is th amount of adsorbat absorbd at tim t h = K s q (3) 0 / τ 1 = 1 / K q (4) Th Elovich modl is givn by Eq. (5) [16]: s 1 1 q = t ln( αβ ) lnt β + β (5) whr α and β ar th paramtrs of th Elovich rat quation obtaind from th linar rgrssion analysis of th qt = F(t) function. Th fitting of th adsorption xprimntal data to thr thortical modls ar shown in Figs From th slop and intrcpt of th straight lin of log(q -q t ) vrsus t, t/q vrsus t, and t q t vrsus ln t, th corrsponding constants wr obtaind. Tabl 4 shows th kintic paramtrs and corrlation cofficints xtractd from th thr quations. A trnd in th rat constants showd that K, K s and α incrasd with dcrasing particl siz, whil β incrasd with particl siz. This was attributd to th availabl adsorbing sits on th surfac of th adsorbnt. Among th thr quations, th psudo-scond ordr quation gav th most consistnt dtrmination cofficints (R = ) and satisfactory fitings (P<0.0001), indicating that th adsorption raction can b approximatd to th psudo-scond ordr modl. Morovr, th amount adsorbd (q ), as prdictd by th psudo-scond ordr modl, agrd bttr with th xprimntal data, as shown in Tabl Equilibrium Isothrm An adsorption isothrm, which is usually th ratio of th quantity adsorbd to that rmaining in th solution at a fixd tmpratur, bst dscribs th quilibrium rlationships btwn th adsorbnt and adsorbat. Th adsorption data wr fittd to thr sorption isothrms; namly, th Langmuir, Frun- DOI: /r

5 Charactrization of Bottom Ash as an Adsorbnt of Lad from Aquous Solutions Tabl 4. Paramtrs of th kintic constants for Pb adsorption Particl siz rang Psudo-first Psudo-scond Elovich quation K R K s h 0 x 10 4 τ 1/ R ß α R <1.19 mm mm mm Tabl 5. Comparison of th xprimntal q (mg/g) with thos prdictd by th thortical modls Particl siz rang Exprimntal Psudofirst ordr Psudoscond ordr <1.19 mm mm mm Fig. 4. Psudo-first ordr sorption plots. dlich and Dubinin-Radushkvich Isothrms. Langmuir thory is basd on th assumption that sorption taks plac at homognous monolayr sits, with no furthr sorption abl to tak plac at an occupid sit. Th rarrangd linarizd Langmuir isothrm is prsntd as follows Eq. (6) [4]: C q 1 k q = + (6) a m C q m whr q m is th amount of mtal sorbd for a complt monolayr (mg/g) and k a th sorption quilibrium constant. Th adsorption capacity and intnsity of th adsorbat towards th adsorbnt ar stimatd using th Frundlich isothrm [7]. Th linarizd Frundlich adsorption isothrm is givn as follows Eq. (7) [17]: logq = logk + 1/nlogC f (7) Fig. 5. Psudo-scond ordr sorption plots. whr K f and 1/n ar th Frundlich mpirical constants. Th third modl usd was th Dubinin-Radushkvich isothrm, which stimats th porosity of a matrial and th nrgy of adsorption. Th linar form of th quation and th nrgy of adsorption (E) ar prsntd as follows [7]: lnq = lnq B RT ln(1 + 1/ C ) (8) D D E = 1/ (9) B D Fig. 6. Elovich quation plots. whr q D is th Dubinin-Radushkvich constant and B D th fr nrgy of sorption pr mol of sorbat. Th constants wr obtaind from th slop and intrcpt of straight lin plots of C / q vrsus C, log q vrsus log C, and ln q vrsus RT ln (1+1/C ). According to Ahmad t al. [], th Frundlich isothrm is an indication of th surfac htrognity of th adsorbnt, whil th Langmuir isothrm only suggsts th surfac homognity of th adsorbnt. Basd on th quilibrium plots shown in Figs. 7-9 and th quilibrium isothrm constants prsntd in Tabl 6, th Frundlich isothrm gav th highst corrlation, making this th most suitabl modl for th sorption of Pb onto bottom 11

6 Joan B. Gorm, Marla C. Maniquiz, Soon Sok Kim, Young Gyu Son, Yun-Ta Kim, L-Hyung Kim ash. This lads to th conclusion that bottom ash is composd of htrognous and multi-layrd surfacs, which prformd a similar adsorption phnomnon. Th highr th valu of K f, th gratr th adsorption intnsity. Thrfor, sizs <1.19 mm hav th gratst adsorption capacitis, which was similar to th rsults obtaind from th kintic modls. Th maximum sorption capacity and nrgy of adsorption of bottom ash (<1.19 mm siz) drivd from th Dubinin-Radushkvich isothrm wr mg/g and KJ/mol, rspctivly. 4. Conclusions Fig. 7. Langmuir isothrm plots. Fig. 8. Frundlich isothrm plots. This study was conductd to assss th ffctivnss of th us of bottom ash as a mdium for th adsorption of Pb in wastwatr. Dtaild charactrizations of th physical, chmical, laching and adsorption capacitis of bottom ash wr prformd. Basd on th rsults, th following conclusions wr drawn: Th significant amounts of silica and aluminum dtctd on th surfac of th bottom ash should mak it a good absorbnt of havy mtals. Th KSLT prformd on th bottom ash sampls indicatd that th havy mtals concntrations in all th lachats wr incapabl of laching to xcd th Koran limits for saf disposal. Th adsorption and laching procsss wr strongly affctd by paramtrs such as th surfac ara, porosity and contact tim. Th adsorption capacity of bottom ash incrasd with dcrasing particl siz. Th most suitabl kintic modl for providing th bst corrlation of th adsorption kintics data was th psudo-scond ordr modl. Bottom ash is mad up of htrognous, multi-layrd surfacs, which ar availabl for adsorption, as dmonstratd by th Frundlich isothrm, th govrning quilibrium modl. Th maximum sorption capacity and nrgy of adsorption of bottom ash obtaind from th Dubinin-Radushkvich isothrm wr mg/g and KJ/mol, rspctivly. Acknowldgmnts This rsarch was supportd by th Basic Scinc Rsarch Program, through th National Rsarch Foundation of Kora (NFR), undr th projct nam of Dvlopmnt of Eco-frindly Rinforcmnt Matrial for Rcycling Bottom ash (Projct No.: ). Th authors ar gratful for thir support. Fig. 9. Dubinin-Radushkvich isothrm plots. Tabl 6. Paramtrs of th quilibrium constants for Pb adsorption Particl siz Langmuir isothrm Frundlich isothrm Dubinin-Radushkvich isothrm q m K a R K f 1/n R q D B D R <1.19 mm mm mm DOI: /r

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