ELIMINATION OF Cr(VI) USING ACTIVATED CARBON PREPARED FROM MUSHROOM MEDIUM

Transcription

1 CELLULOSE CHEMISTRY AND TECHNOLOGY ELIMINATION OF Cr(VI) USING ACTIVATED CARBON PREPARED FROM MUSHROOM MEDIUM LAN-SHU XU, FU-LI FAN, XIAO-XU CAI and XIAO-JUAN JIN MOE Ky Laboratory of Woodn Matrial Scinc and Application, Bijing Ky Laboratory of Lignocllulosic Chmistry, MOE Enginring Rsarch Cntr of Forstry Biomass Matrials and Bionrgy, Bijing Forstry Univrsity, 35, Qinghua East Road, Haidian, 83, Bijing, China Corrsponding author: Xiao-Juan Jin, Rcivd April, 5 Mushroom mdium, which is mainly composd of wood flour, is a kind of wast largly obtaind from th mushroom cultivation industry. Th typical mthod of rusing it is by burning it to gnrat hat. In th prsnt papr, mushroom mdium-basd activatd carbon was prpard and th rmoval for Cr(VI) through it was xplord. Th combind ffct of fiv procss paramtrs, i.. contact tim, ph, adsorbnt dosag, initial concntration and tmpratur, on Cr(VI) adsorption wr studid. Th maximum adsorption capacity of activatd carbon towards Cr(VI) was found undr th following conditions: ph, contact tim of 3 h, tmpratur of 6 C and adsorbnt dosag of. g. Th adsorbd amount of Cr(VI) dcrasd with an incras in th initial concntration. In addition, th kintics of Cr(VI) adsorption followd th psudo-scond-ordr rat xprssion. As rgards th adsorption isothrms, th data wr fittd bttr by th Langmuir modl than by th Frundlich modl. Kywords: activatd carbon, mushroom mdium, adsorption, Cr(VI) INTRODUCTION Th incrasing contamination of urban and industrial wastwatr by toxic mtal ions causs important nvironmntal pollution. Chromium (Cr) is on of th most toxic mtals that finds wid sprad us in various industris, including lctroplating, mtal finishing, lathr tanning and pigmnts. Chromium xists almost xclusivly in th Cr(III) oxidation stat or in th Cr(VI) oxidation stat. Cr(III) is sparingly solubl and lss toxic in natur. Howvr, Cr(VI) compounds ar vry toxic chmicals and gnotoxic carcinogns. Thus, th prsnc of Cr(VI) in th nvironmnt is posing srious problms and causing grat public concrn. Currntly, th drinking watr guidlin of th Environmntal Protction Agncy (EPA) in th US has stablishd th maximum contaminant lvl for chromium of. mg/l. To solv this srious problm for nvironmntal and human halth, an fficint limination of Cr(VI) from pollutd watr is ncssary. Svral mthods hav bn usd for trating Cr(VI) containing wastwatr, including prcipitation, xtraction, ion xchang, and adsorption. 3-4 Among all ths mthods, activatd carbon (AC) adsorption is th most conomically attractiv and fasibl, du to its asy opration and th dvlopmnt of various adsorbnts. 5-8 Howvr, commrcially availabl AC may b xpnsiv, for this rason, th production of low-cost, disposabl sorbnts for chromium rmoval is worth considring. Thr is a scond largst mushroom cultivation bas in China, which is locatd in Pingquan of Hbi Provinc. Th mushroom cultur mdium, composd mainly of wood flour, is gnrally abandond in landfills, amounting to a dozn tons pr yar. Most of th wast mushroom mdium is burnt, Cllulos Chm. Tchnol., 5 (-), 59-65(7)

2 LAN-SHU XU t al. which rlass toxic gass and polluts th nvironmnt. Thrfor, th wast mushroom mdium could b usd to prpar AC as a potntial adsorbnt for th rmoval of toxic pollutants from watr, which will nlarg its application and hlp to dal with th mrgnt wastwatr tratmnt challng in China. Th objctiv of this papr is to prpar AC from mushroom mdium and valuat its capacity to adsorb Cr(VI) from aquous solutions. Svral important factors, such as contact tim, adsorbnt dos, initial concntration, ph and tmpratur, wr invstigatd. Furthrmor, th adsorption of Cr(VI) was charactrizd by kintic modls and adsorption isothrms. EXPERIMENTAL Matrials Th mushroom mdium was obtaind from th Pingquan Mushroom Cultivation Company in Hbi Provinc (Hbi, China). Th mushroom mdium mainly consistd of wood flour. Th chmical composition was th following: ash (GB/T 74-8).7%, lignin (GB/T 337-8) 8.39% and holocllulos (GB/T ) 6.4%. Othr chmicals of analytical grad wr purchasd from Bijing Lanyi Chmical Ragnt (Bijing, China). Doubl distilld watr was usd for prparation of all rquird solutions. Prparation of activatd carbon Th procss of prparing AC with KOH includs two stps. In th carbonization stp, th mushroom mdium was ground and thn was hatd to a carbonization tmpratur of 5 C at a hating rat of C/h and was hld at this tmpratur for h. Th carbonizd sampls wr scrnd out with sivs to hold th 4-6 msh fractions for furthr applications. In th activation stp, 6 g of sampl was soakd in a 5% KOH solution for 4 h at th imprgnation ratio of 3:. Th soakd sampls wr thn put in an lctric furnac and hatd up to about 85 C at a hating rat of C/min and wr hld at this tmpratur for 5 min. Thn, th AC was washd squntially with a 5% HCl solution and with distilld watr until th solution was nutral. Finally, th sampls wr drid at 5 C in an ovn for 6 h and kpt as an adsorbnt in a vacuum sald bakr for th nxt xprimntal stps. Charactrization of activatd carbon Iodin numbr was dtrmind at 33 K basd on th Standard Tst Mthod ASTM Dsignation: D Th nitrogn adsorption-dsorption data wr rcordd at a liquid nitrogn tmpratur of 77 K. Th nitrogn adsorption isothrm was masurd ovr a rlativ prssur (P/P ) rang, from approximatly 6 to. BET surfac ara was calculatd from N adsorption isothrms using th Brunaur-Emmtt-Tllr (BET) quation. Por siz distribution in th micropor rang was obtaind by th Barrtt Joynr Halnda (BJH) mthod. Cr(VI) adsorption xprimnts Th Cr(VI) solution was prpard by dissolving.566 g of potassium dichromat (K Cr O 7 ) in ml distilld watr. Cr(VI) solutions wr prpard by dissolving K Cr O 7 in distilld watr. Th Cr(VI) solution with a Cr(VI) concntration of mg/l was prpard in th nxt work. At th nd of th adsorption procss, th suprnatant liquids wr filtrd with.45-µm filtr mmbrans. Th concntration of Cr(VI) was dtrmind by a UV/vis spctrophotomtr with,5-diphnyl-carbazid in acid mdium at a wavlngth of 54 nm. Th amount of adsorption at quilibrium, q (mg/g), was calculatd by: ( co c ) v q = m () whr C and C ar th initial and th quilibrium concntrations of Cr(VI) in solution (mol/l), rspctivly; v is th volum of th solution (L), and m is th mass of AC usd (g). RESULTS AND DISCUSSION Charactrization of th adsorbnt Th optimum iodin numbr of th solution is of 68 mg/g for Cr(VI) adsorption. Th nitrogn adsorption dsorption isothrm of AC85 (Fig. (a)) was usd to dtrmin th surfac ara and por-siz distribution of th AC85. Th isothrm of th AC sampl rsmbls a combination of typ-i and typ-ii isothrms, which is in accordanc with th Intrnational Union of Pur and Applid Chmistry (IUPAC) classification. This adsorption bhavior xhibits a combination of microporous-msoporous structur. Manwhil, th isothrm shows an almost flat platau at highr rlativ prssur, indicating th prsnc of micropors. 9 Figur (b) shows th por siz distribution of th AC. As w know, micropors ar lss than nm wid, msopors ar -5 nm wid, and macropors ar mor than 5 nm wid. As can b sn from Figur (b), th pors of th prpard AC includd micropors and msopors. Th txtural paramtrs of th AC at 85 C show strictly msopor surfac ara and significant micropor surfac ara, whil th BET surfac ara could rach m /g. 6

3 Mushroom Volum Adsorbd(cm3/g) (a) Rlativ prssur (P/P) Por Volum(%) (b) Por siz(nm) Figur : (a) AC85 nitrogn adsorption-dsorption; (b) AC85 por siz distribution curvs Rmoval fficincy (%) Tim (min) Figur : Effct of contact tim on adsorption fficincy (adsorbnt dosag =. g/l, volum of Cr(VI) solution = ml, c = mg/l, T = 4 C, ph ) Effct of contact tim Th ffct of contact tim was invstigatd to idntify th rat of its rmoval by th proposd adsorbnt. Ths xprimnts wr carrid out at an initial ph in th prsnc of mg/l K Cr O 7. Th rsults ar shown in Figur, whrfrom it is clar that th adsorption of Cr(VI) incrasd rapidly within th first h and thn lvlld off. Thrfor, th amount of Cr(VI) rmovd rachd a maximum aftr h. Th prcntag of adsorption of AC is up to 96.%. Th rapid adsorption at th initial contact tim is rlatd to th availability of a highr numbr of activ sits nar th surfac, which improvs th flux of Cr(VI) diffusion to th adsorbnt surfac. Th slowr adsorption rat at th final stag of th adsorption was apparntly du to th gradual saturation of th activ sits on th lss accssibl intrnal por surfacs of th AC. This rsult could b attributd to th instantanous utilization of th availabl adsorbing sits in th AC lattic. Bsids, th fast adsorption was du to th fast transfr of Cr(VI) ions to th surfac of th AC, whil th following slow adsorption was on account of th low concntration of th ions in th intra-particl pors of th AC. It was rportd that th rmoval of Cr(VI) using a cllulos microsphr-basd adsorbnt and activatd wd Slavonia cucullata rquird h and h to obtain adsorption quilibrium, rspctivly. - Effct of initial Cr(VI) concntration Th assssmnt of th ffct of initial concntration on th adsorption of Cr(VI) on AC was carrid out by varying th adsorptiv concntrations from 5 mg/l to 8 mg/l for 8 min, and th rsults ar shown in Figur 3. It was found that th adsorption of Cr(VI) was strongly dpndnt on th initial mtal ion concntration. As shown in Figur 3, th quilibrium adsorption capacitis incrasd whn initial concntration of Cr(VI) ions incrasd. Mor Cr(VI) ions stayd in th solution phas at highr concntrations of mtal bcaus of th saturation of th binding sits of AC. At low initial concntration, th ratio of th surfac activ sits to th AC in th solution was high, and hnc all AC may intract with th activ functional groups on th surfac of th carbon and b rmovd from th solution. In addition, with incrasd initial concntration, th numbr of activ adsorption sits is nough to accommodat th AC. 6

4 LAN-SHU XU t al. Rmoval fficincy(%) Rmoval fficincy(%) Initial concntration(mg/l) Figur 3: Effct of initial concntration on adsorption fficincy (adsorbnt dosag =. g/l, volum of Cr(VI) solution = ml, t = 8 min, T = 3 C, ph ) Dosag of AC (g) Figur 4: Effct of AC dosag on adsorption (xprimntal conditions: ph.; initial chromium concntration = mg/l; volum of solution = ml; T = 4 C) Rmoval fficincy (%) ph Figur 5: Effct of ph valu on adsorption fficincy (carbon dosag =.g; t = 3 h; Cr(VI) concntration = mg/l; volum of solution = ml; T = 4 C) Effct of adsorbnt dosag Th ffcts of AC dosag on th rmoval of chromium ar shown in Figur 4. In this study, xprimnts wr carrid out to xamin th influnc of adsorbnt dosag (.,.,.5,.,.5 and. g) at an initial concntration of mg/l and optimal ph valu of th adsorbnt for 8 min. Th rsult was dtrmind as follows: with incrasing adsorbnt dosag, highr phas contact ara, adsorption surfac ara and adsorption sits wr providd. As shown in Figur 4, whn th adsorbnt dosag was as high as. g, th rmoval ratio of Cr(VI) was of narly 9.9%. Howvr, th fficincy did not incras linarly with th incras in th adsorbnt dosag. Th incras in th prcntag of Cr(VI) adsorption whn incrasing th adsorbnt dosag was du to th incras in th numbr of adsorbnt sits availabl. Similar phnomna wr also obsrvd by othr rsarchrs. Parinda and Paitip indicatd that th optimum Cr(VI) concntration was 3 g/l (or.3% w/v), whn 99.99% of Cr(VI) adsorption was achivd. Effct of ph Th ph is on of th most important factors affcting many adsorption procsss. 3-4 From Figur 5, it can b sn that th Cr(VI) adsorption onto AC is dramatically dcrasd with an incras in ph from. to 8.. On of th rasons for th bttr adsorption capacity obsrvd at low ph (ph < 3.) may b attributd to th lctrostatic attraction btwn positivly chargd - groups of th biomatrial surfac and th HCrO 4 anion. Morovr, th dcras in adsorption at highr ph valus may b du to th comptitivnss of Cr(VI) spcis (Cr O - 7, CrO - 4 ) and OH - ions in th bulk matrial. 5-6 Th rsult is similar to that obtaind for Cr(VI) rmoval from aquous solution by adsorption on tratd sawdust and activatd cow dung carbon. 7 6

5 Mushroom Rmoval fficincy (%) T( C) Figur 6: Effct of tmpratur on adsorption fficincy (adsorbnt dosag =. g/l, volum of Cr(VI) solution = ml, c = mg/l, t = 8 min, ph ).5 4 y =.6x (a) (b) 3.5 R = y = -.43x R =.9499 (q-q) ln t/min t/q t/min Figur 7: (a) First-ordr adsorption rat; (b) Scond-ordr adsorption rat of Cr(VI) on AC Effct of tmpratur It is wll known that tmpratur plays an important rol in th adsorption procss. Th tmpratur was varid from 5 to 7 C, at a contact tim of 8 min and Cr(VI) ion concntration of mg/l. It can b obsrvd in Figur 6 that th adsorption of Cr(VI) ion at various tmpratur incrass with th incras of tmpratur. As th tmpratur was incrasd from 5 to 6 C, Cr(VI) ion rmoval sharply incrasd. As th tmpratur was furthr incrasd from 6 to 7 C, Cr(VI) ion rmoval kpt at a constant lvl. Whn th tmpratur was 6 C, th rmoval ratio of Cr(VI) was narly 95.9%. Rsults indicat that maximum adsorption for Cr(VI) was achivd at 6 C. This could b attributd to th formation of bttr structurd adsorbd Cr(VI) ions on th carbon surfac. Manwhil, th adsorption of Cr(VI) ion on AC particls is an xothrmal procss, implying a physical adsorption procss whn th tmpratur is highr than 6 C. A similar phnomnon was found in th adsorption of Cr(VI) on sulphuric acid-tratd what bran. 8 Adsorption kintic studis Th kintics of adsorption of Cr(VI) from aquous solution has bn discussd in viw of th psudo-first-ordr quation and th psudo-scond-ordr quation. 9- Ths kintic rat quations can b writtn as follows: In( q q) = Inq k t () t q t (3) = + k q q whr q (mg/g) is th amount of Cr(VI) adsorbd at tim t (min); q (mg/g) is th amount of Cr(VI) adsorbd at quilibrium, k l (/min) is th first-ordr adsorption rat constant; k (g mg/min) is th rat constant of scond-ordr adsorption. In Figur 7(a) and (b), th fitting plots using th psudo-first-ordr quation and th psudo-scond-ordr quation ar shown. A comparison of th data obtaind by th two modls is givn in Tabl. Th cofficint of dtrmination (R ) for th formula showd that th psudo-scond-ordr quation was mor 63

6 LAN-SHU XU t al. suitabl for th xprimntal data. Th data showd that a good agrmnt btwn th xprimntal and th calculatd q valus was obsrvd by using th psudo-scond-ordr quation. Th obtaind rsults suggstd that th psudo-scond-ordr modl provids th bst corrlation of th data, basd on th assumption that th rat-limiting stp may b causd by chmical sorption or chmisorptions involving valncy forcs through sharing or xchang of lctrons btwn adsorbnt and adsorbat. Adsorption isothrms Th Langmuir and Frundlich modls wr th most common modls to dscrib th adsorption isothrms. -7 Th Langmuir adsorption isothrm is givn by th following quation: 8 c = q q K m L c + q m (4) whr q m (mg/g) is th amount of adsorption corrsponding to complt monolayr covrag and b (/mg) is th Langmuir constant rlatd to th nrgy or nt nthalpy of adsorption. Th linar plots of C /q vrsus C ar shown in Figur 8(a). R L shows th adsorption capacity (R L =/(+K L q m )). Whn <R L <, it has a fin adsorption. Th Frundlich adsorption isothrm is givn by th following quation: 9 q = K f C / n (5) whr K f and n ar Frundlich constants rlatd to adsorption capacity and adsorption intnsity in Figur 8(b). Usually, for a good adsorbnt, < n<. Th abov rlation is prsntd in Tabl. As can b sn from Figur 8(a), th valu of th cofficint of dtrmination (R ) is.988, and it was highr for th Langmuir isothrm than for th Frundlich isothrm, which is Th rsult shows that th adsorption isothrm of Cr(VI) xhibitd Langmuir bhavior. Tabl Paramtrs of adsorption rat of Cr(VI) AC-KOH Psudo-first-ordr quation Psudo-scond-ordr quation q /mg/g K /min R C/q (a) y =.84x +.68 R = C Inq y =.7x R = InC Figur 8: (a) Langmuir adsorption isothrm; (b) Frundlich adsorption isothrm (b) Tabl Adsorption isothrm constants for Cr(VI) by AC AC-KOH Langmuir Frundlich q m K L R L R /n K F R

7 Mushroom CONCLUSION Th study indicats that mushroom mdium can b dvlopd into an ffctiv adsorbnt for th rmoval of Cr(VI) from aquous solutions. Th xprimntal rsults show that a dosag of AC of. g, raction tim of 3 h, tmpratur of 6 C and ph ar th optimum conditions for Cr(VI) rmoval. With an incras of th initial concntration of th chromium solution, th Cr(VI) rmoval rat rducd. In addition, th adsorption bhaviour of Cr(VI) by AC is mor in lin with th Langmuir modl. Considring th dynamics quation of Lagrgrn primary and scondary data on th rat of adsorption, th adsorption bhavior of AC for Cr(VI) followd th law of scondary dynamics. ACKNOWLEDGMENT: This study was fundd by th Stat Forstry Administration, projct 487: Th study on th tchnology and mchanism of th activatd carbon lctrod from wast hard board. REFERENCES P. Baroni, R. S. Viira, E. Mnghtti, S. M. Da and M. M. Bppu, J. Hazard. Matr., 5, 55 (8). J. K. Yang and S. M. L, Chmosphr, 63, 677 (6). 3 J. Y. Qiu, Z. Y. Wang, H. B. Li, L. Xu, J. Png t al., J. Hazard. Matr., 66, 7 (9). 4 F. J. Alguacil, C. Caravaca and M. I. Martin, J. Chm. Tchnol. Biotchnol., 78, 48 (3). 5 N. Ptrov, T. Budinova, M. Razvigorova, E. Ekinci, F. Yardim t al., Carbon, 38, 69 (). 6 A. B. Garcia, A. M. Alonso, C. A. Lon and J. Tascon, Ful, 77, 63 (998). 7 B. Saha, M. H. Tai and M. Strat, Procss. Saf. Environ., 79, (). 8 X. J. Jin, Z. M. Yu and Y. Wu, Cllulos Chm. Tchnol., 46, 79 (). 9 Y. S. H, Z. Li, H. X. Xi, J. G. Guo and Q. B. Xia, Ion Exchang Adsorpt.,, 376 (4). C. Li, Y. Zhang, J. Png, H. Wu, J. Q. Li t al., Radiat. Phys. Chm., 8, 967 (). S. S. Baral, S. N. Das, G. R. Chaudhury, Y. V. Swamy and P. Rath, J. Chm. Eng., 39, 45 (8). P. Suksaby and P. Thiravtyan, J. Environ. Manag.,, (). 3 G. Arslan and E. Phlivan, Biorsour. Tchnol., 98, 836 (7). 4 Y. Wang, X. J. Wang, M. Liu, X. Wang and Z. Wu, Ind. Crop. Prod., 39, 8 (). 5 S. P. Duby and K. Gopal, J. Hazard. Matr., 45, 465 (7). 6 D. D. Das, R. Mahapata, S. N. Das and R. S. Thakur, J. Colloid. Intrf. Sci., 3, 35 (). 7 S. S. Baral, S. N. Das and P. Rath, J. Biochm. Eng., 3, 6 (6). 8 A. Ozr and D. Ozr, Environ. Tchnol., 5, 689 (4). 9 S. Lagrgrn, J. Vtnsk. Handl., 4, (898). Y. S. Ho, J. C. Y. Ng and G. M. Kay, Sp. Purif. Mthods, 9, 89 (). T. Altun and E. Phlivan, Food. Chm., 3, 693 (). N. Tazrouti and M. Amrani, Biorsourcs, 4, 74 (9). 3 T. Chn, T. Wang, D. J. Wang, H. R. Xu and J. Q. Zhao., Matr. Rs. Bull., 46, 44 (). 4 E. Lornc-Grabowska and G. Gryglwicz, Dys Pigmnts, 75, 36 (7). 5 Y. S. Ho and G. McKay, Procss Biochm., 34, 45 (999). 6 G. Rojas, J. Silva, J. A. Flors, A. Rodriguz, M. Ly t al., Sp. Purif. Tchnol., 44, 3 (5). 7 V. O. Arif, K. Trilstari, J. Sunarso, N. Indraswati and S. Ismadji, CLEAN - Soil, Air, Watr, 36, 937 (8). 8 9 I. Langmuir, J. Am. Chm. Soc., 43, 36 (98). H. Frundlich, J. Phys. Chm., 6, (96). 65