ALKALI EXTRACTION OF HEMICELLULOSE FROM DEPITHED CORN STOVER AND EFFECTS ON SODA-AQ PULPING

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1 ALKALI EXTRACTION OF HEMICELLULOSE FROM DEPITHED CORN STOVER AND EFFECTS ON SODA-AQ PULPING Hli Chng, a Huaiyu Zhan, a, * Shiyu Fu, a and Lucian A. Lucia b A biorfinry using th procss of hmicllulos pr-xtraction and subsqunt pulping provids a promising way for th utilization of straw biomass and rsolution of problms rlatd to silicon. In this work, hmicllulos was xtractd from dpithd corn stovr with sodium hydroxid solution bfor soda-aq pulping. Componnts of th xtracts wr quantifid by ion chromatography. Th paramtrs (alkali concntration and tmpratur) affcting hmicllulos pr-xtraction wr optimizd. Th main constitunt of hmicllulos in corn stovr was xylan, which accountd for 18.1% of th dpithd raw matrial. Mor than 90% of th xylan can b xtractd undr th optimal conditions: NaOH concntration of 10%, tmpratur of 75ºC, and tim of 2h. Solid fractions aftr xtraction wr subjctd to soda-aq pulping. In comparison with control pulp obtaind without xtraction, it was found that alkali pr-xtraction could improv th brightnss and dcras kappa numbr of th subsqunt pulp, causing a slight loss of yild, viscosity, dnsity, and burst strngth, but an obvious improvmnt of tar strngth. Morovr, th silicon contnt was dcrasd by 79.8% whn th xtraction conditions wr st at 75 ºC with alkali concntration of 8%, suggsting that pr-xtraction of hmicllulos is a potntial way to solv silicon problms associatd with alkalin pulping of stovr. Kywords: Corn stovr; Alkali pr-xtraction; Hmicllulos; Silica rmoval; Soda-AQ pulping Contact information: a Stat Ky Laboratory of Pulp and Papr Enginring, South China Univrsity of Tchnology, Guangzhou, China; b Dpartmnt of Forst Biomatrials Scinc and Enginring, North Carolina Stat Univrsity, Raligh, NC , Unitd Stats; * Corrsponding author. Tl: ; pphyzhan@scut.du.cn INTRODUCTION Lignocllulosic biomass has bn gaining incrasing attntion rcnt yars owing to its grat potntial as sustainabl fdstock for th production of biofuls and chmicals, a systm for which th trm biorfinry has bn applid (Kamm 2004). Sinc thr is a grat risk of comptition for raw matrials with th pulping industry, th concpt of an Intgratd Forst Products Biorfinry (IFBR) was dvlopd, suggsting a pr-xtraction of hmicllulos prior to pulping (van Hiningn 2006). During kraft pulping, much of th hmicllulos, along with most of th lignin, is dissolvd into th pulping liquor, and th mixtur is finally burnd in an alkali rcovry systm to produc stam and lctricity. Considring that th hat valu of hmicllulos is much lowr than that of lignin (van Hiningn 2006), burning is not a good way to mak full us of hmicllulos. Undrgoing an IFBR procss, th hmicllulos polysaccharids can b pr-xtractd to produc thanol or modifid to manufactur advancd matrials. Such Chng t al. (2010). Alkali xtraction of stovr, BioRsourcs 11(1),

2 approachs hav significant potntial to provid additional incom for pulp or papr mills. Svral studis hav focusd on pr-xtraction of hmicllulos using diffrnt mthods of wood tratmnt, in combination with pulping. Mnds t al. (2009) prformd an auto or acid-catalysd pr-hydrolysis tratmnt of ucalyptus bfor kraft pulping. Th xtractd hmicllulos polysaccharids could b furthr frmntd to produc thanol; manwhil, th blaching rquirmnts and brightnss rvrsion of pulp wr found to b significantly dcrasd. Aldajani and Tschirnr (2008) rportd NaOH xtraction of populus wood bfor kraft pulping. Whn th sam yild and high quality of pulp wr maintaind, about 20%-25% of th hmicllulos can b rcovrd. Yoon t al. (2008) dvlopd a hot-watr pr-xtraction tchniqu with subsqunt kraft pulping for IFBR of softwood, and ovr 40 kg hmicllulos pr ton of aspn chips was xtractd at 90 o C using 1.67mol/L NaOH, whil yilding th sam amount of pulp compard with th control cook. Usually, hmicllulos xhibits highr solubility in alkalin solutions than in acidic ons with lss dgradation to furfural substancs. Also, alkali pr-xtraction gnrally rquirs much mildr conditions of tmpratur, prssur, and chmical dosag than acidic conditions. Furthrmor, no ph adjustmnt or watr-washing is ndd for subsqunt alkalin (soda or kraft) pulping. Whn applid to non-wood biomass, an additional potntial advantag is that th silicon problm, which rfrs to th incras in viscosity of pulping liquor du to dissolvd silicon and formation of Na 2 O nsio 2, can b ovrcom. Th IFBR procss of woody biomass, such as ucalyptus, aspn, loblolly pin, tc., has bn rportd (Mnds t al. 2009; Aldajani and Tschirnr 2008; Yoon t al. 2008). Howvr, up to now, non-wood biomass matrials with high contnt of hmicllulos such as what straw (Sun t al. 2000), corn stovr (Gáspár t al. 2007), and sugarcan bagass (Brinzo t al. 2009; Sun t al. 2004), hav only bn studid for hmicllulos xtraction. To th knowldg of th authors, littl work has focusd on th subsqunt pulping procss using th pr-xtractd biomass. Corn stovr is abundant in China, th US, and many othr countris of th world, but this potntial sourc of fibrs has not bn fully utilizd, spcially as paprmaking raw matrials, bcaus of th significant amount of pith in it. Th pith is mainly composd of carbohydrat rsrvs (oligosaccharids and polysaccharids containing arabinos, xylos, mannos, galactos, and glucos) which tnd to absorb pulping chmicals and thus can b dissolvd, lading to high chmical consumption and low yilds; furthrmor, th fin natur of rsidual pith aftr pulping significantly rducs th drainag rat of th rsulting pulps, making washing and dwatring difficult. Finally, it has bn notd that pith clls, if lft in th final pulp, can rsult in rducd sht opacity (Byrd 2005). Givn ths disadvantags, dpithd corn stovr was usd in this study for hmicllulos xtraction and soda-aq (anthraquinon) pulping, noting that th pith from th corn stovr can b usd as animal fd. Th yild and composition of hmicllulos polysaccharids wr masurd to invstigat th ffct of xtraction tmpratur and chmical charg. Th pulp proprtis of soda-aq pulp with or without hydrogn proxid blaching as wll as silicon contnt of pulping liquor wr also xamind to study th fasibility of an Intgratd Forst Products Biorfinry for corn stovr. Chng t al. (2010). Alkali xtraction of stovr, BioRsourcs 11(1),

3 EXPERIMENTAL Matrials Dpithd corn stovr was obtaind from Shandong provinc, China, and was cut into pics of 2 cm lngth. Bfor xtraction, th matrial was washd with tap watr to rmov contaminants, and thn air-drid (moistur contnt around 8%) and stord at room tmpratur. Pr-Extraction of Hmicllulos Extraction of hmicllulos from corn stovr with alkali (NaOH) was prformd in a 15-litr lctrically hatd and thrmostatically controlld rotary digstr (ZQS 1, Shaanxi Univrsity of Tchnology Machinry Factory, China). Th solids loading of th fdstock was 300g (ovn-drid wight), and th mass ratio of liquid to solid was 5:1. Th alkali concntration and xtraction tmpratur wr varid within a rang of 2% to 10% and 65 C to 95 C, rspctivly. Th tim to tmpratur and tim at tmpratur wr always st as 10 min and 120 min, rspctivly. Aftr xtraction, th rsidual solids and liquor wr sparatd using a 200 msh Nylon filtr. Th spnt liquor was collctd and stord at 4 C for furthr analysis, whil th solid fractions wr thoroughly washd with tap watr and thn air-drid for subsqunt pulping. Soda-AQ Pulping Th soda-aq pulping was carrid out in th sam lab-scal batch digstr for pr-xtraction as dscribd abov. Th cooking conditions wr: 12% ffctiv alkali (as NaOH), 0.05% AQ charg, liquor to solids ratio 4:1, tim to maximum tmpratur 150 C, 120 min, and tim at 150 C, 60 min. Aftr cooking, th pulp and liquor wr sparatd by filtring through a 200 msh Nylon filtr. Th pulp was xtnsivly washd with dionizd watr to rmov rsidual chmicals and othr contaminants. All pulp sampls wr stord in moist form at 4 ºC bfor analyzing or blaching. Th pulping liquor was also stord at 4 C for furthr analysis. Proxid Blaching In our prvious work it was found that corn stovr pulp was asy to blach with hydrogn proxid to achiv high brightnss. Th blaching conditions in this work wr as follows: Chlating tratmnt bfor hydrogn proxid blaching (Q): 10% (w/v) pulp consistncy, 0.3% (w/w) EDTA, tmpratur 60 C, tim 60min; alkalin proxid blaching (P): 10% pulp consistncy, chmical dosag of 3% H 2 O 2, 2% NaOH, and 0.15% MgSO 4. Th pulp was mixd with blaching agnts and thn maintaind at a tmpratur of 90 C for 4h. Chmical Composition of Corn Stovr Ground particls in th rang btwn 40 and 60 msh wr usd for th dtrmination of corn stovr chmical composition. Moistur contnt was dtrmind by drying a rprsntativ sampl at 105 o C in an ovn ovrnight. Ash contnt and bnzn/thanol xtractivs wr dtrmind according to TAPPI standard mthods T211 Chng t al. (2010). Alkali xtraction of stovr, BioRsourcs 11(1),

4 om-02 and T6 wd-73. Klason lignin, was dtrmind according to a mthod by Effland (1977), whil th acid solubl lignin contnt was dtrmind by TAPPI mthod 250. Bfor analysis of th monosaccharid contnt in th raw matrial, a two-stp hydrolysis with 72% and 4% sulfuric acid, rspctivly, was don by th standard procdur rcommndd by NREL (Davis 1998). Thn th hydrolysat was dilutd and filtrd through a 0.22μm syring filtr and analyzd by using an ion chromatography 3000 (Dionx Company, USA) with an lctrochmical dtctor and a PA20 column. Analysis of Polysaccharids in th Extract Th dtrmination of th saccharids in th alkalin xtraction liquor was also don with th IC3000 systm. Th liquor was adjustd to ph 5~6 using 6 mol/l HCl, and thn hydrolyzd by 4% sulfuric acid at 121 C for 1h. Th filtring and dilution stps wr also don bfor injction into th IC systm (Tunc and van Hiningn 2008a). As can b xpctd for most non-wood hmicllulos, th main componnt of corn stovr hmicllulos is xylan (Bastawd 1992), so th yild of xylan was usd to stimat alkali xtraction fficincy of hmicllulos in th xprimnt, -3 c 10 n v 132 Y= 100% m 18.1% 150 (1) whr Y is th xtraction yild of xylan (%); c is th concntration of xylos masurd by IC (mg/l); n is dilution tims; v is total volum of liquid in th xtraction systm (L); m is th o.d. wight of th corn stovr (g), 132/150 is th stoichiomtric factor btwn xylos and xylan, and 18.1% is th xylan contnt of th raw matrial. Dtrmination of Silicon Contnt Th silicon contnt dissolvd in th xtraction spnt liquor was masurd by silicomolybdat blu spctrophotomtry (Tong t al. 2005). Silicic acid mixd with ammonium molybdat can form th silicomolybdat yllow complx compound undr acidic conditions; rducing agnt was usd to rduc silicomolybdat yllow to silicomolybdat blu. A crtain volum of 1g/L standard silicon solution was takn to mix with 10 ml 25%(v/v) nitric acid in th plastic bakr. Thn 10mL of 10%(w/v) ammonium molybdat solution was addd to th mixtur. Aftr uniform mixing of th solutions, th bakr was maintaind in th watr bath for 12 min undr 30 C, thn 40 ml 5%(w/v) oxalic acid was put into th bakr, 40 ml 4.5%(w/v) ammonium frrous sulphat was addd into th bakr succssivly, th mixtur in th bakr was thn put into a 250mL volumtric flask and mad to constant volum, and th silica contnt was masurd with an Alignt 8453 UV Spctrophotomtr by using watr as blank ragnt. Charactrization of Pulps Th NaOH-AQ pulps with or without pr-xtraction wr charactrizd. Th kappa numbr, brightnss, viscosity, yild, and rsidual alkali wr masurd according to th standard tsting mthods. Rfining of th pulps was don on a PFI dvic. Handshts wr prpard from th rfind pulp according to TAPPI standard mthod T205. Physical proprtis, including dnsity, braking lngth, burst indx, and tar indx, Chng t al. (2010). Alkali xtraction of stovr, BioRsourcs 11(1),

5 of blachd pulps wr charactrizd using TAPPI standard mthods T494 cm-01, T403 om-02, and T414 om-04, rspctivly. All data rportd wr th man valu of at last 4 indpndnt masurmnts. RESULTS AND DISCUSSION Effcts of Tmpratur and Alkali Charg on th Yild of Hmicllulos Th chmical compositions of raw corn stovr biomass ar listd in Tabl 1. Th main composition of th hmicllulos polysaccharids was xylan, which accountd for 18.1% of th o.d. wight of th raw matrials. Th contnt of arabinan was much lowr (1.3%), whil galactan, mannan, and galacturonic acid wr undtctabl. Thrfor, hmicllulos rcovry was calculatd on th basis of xylan. Tabl 1. Chmical Composition of Corn Stovr Biomass (% on original matrial) Componnt Glucan Xylan Arabinan Klason Acid solubl Bnzn/thanol Ash lignin lignin xtractivs Wight prcnt Th ffcts of initial alkali concntration (varid from 2% to 10%) on th dissolution of polysaccharids ar shown in Fig. 1a. Th data showd that th dissolution prcntag of glucan was lss than 1.3%, indicating that narly all th loss of total carbohydrat (15%-30%) was du to th dissolution of hmicllulos. Th dissolution of xylos was incrasd significantly with th incras of alkali concntration. Whn 10% of NaOH was applid, ovr 90% of th xylos could b xtractd from solid substrats. As rportd by Sun t al. (2001) 89.3% of th original hmicllulos can b xtractd from partially dlignifid ground fast-growing poplar wood using 8.5% NaOH at 20 C for 16h. Ths rsults suggstd that a considrabl amount of hmicllulos could b xtractd at low tmpratur dspit th diffrnc of raw matrials. Th yild of hmicllulos from alkali xtraction at various tmpraturs was also studid. Th rsults (Fig. 1b) indicatd that th yild of hmicllulos was improvd by only 15%, maning that it incrasd from 68% to 78% whn th tmpratur was raisd from 65 C to 95 C. Manwhil, th dissolution of glucan was lowr than 1.5%. Figur 1b also suggsts that a considrabl amount of hmicllulos could b obtaind at vn low tmpratur (<65 C). Howvr, th objctiv of alkali pr-xtraction is not only to yild hmicllulos but also to disrupt th cll-wall for improving pulping prformanc by dissolving lignin and silicon, hydrolysing uronic acid and actyl groups, and swlling th cllulos. For an ffctiv IFBR procss, conditions with highr alkali concntration and tmpratur ar mor frquntly usd in ordr to minimiz th tim rquird for a high yild of hmicllulos. Effct of Alkali Pr-Extraction on Soda-AQ Pulping and QP Blaching Pr-xtraction of hmicllulos from corn stovr could caus a significant dissolution of carbohydrat, which may dcras th viscosity of subsqunt NaOH-AQ pulp. Figur 2a shows that viscosity dcrasd linarly with incrasing dissolution of Chng t al. (2010). Alkali xtraction of stovr, BioRsourcs 11(1),

6 hmicllulos. It could b sn that whn th dissolution of hmicllulos incrasd from 15% to 90%, th viscosity of th pulp dcrasd from 924mL/g to 720 ml/g. 100 ) (% 80 g ta 60 r c n p n40 tio lu 20 iso D 0 Xylos Total Carbohydrat Glucos Alkali concntration (%) (a) Dissolution prcntag (%) Xylos Total Carbohydrat Glucos Tmpratur ( C) (b) Figur 1. Effcts of alkali charg and xtraction tmpratur on th dissolution yild of carbohydrat from corn stovr: (a) Extraction tmpratur 75 C, tim 2h; (b) Alkali charg 6%, tim 2h ) /g L1000 (m sity 900 o isc V y = x R² = Dissolution of Hmicllulos (%) (a) Yild of NaOH-AQ Pulp(%) y = x R² = Dissolution of Hmicllulos(%) (b) Figur 2. Effcts of hmicllulos dissolution on viscosity (a) and yild (b) of subsqunt NaOH- AQ pulp. Th triangl symbol on th y-axis in Fig. 2 (a) rprsnts th viscosity of th control pulp, 1135mL/g; whil in Fig. 2 (b) it rprsnts th yild of th control pulp, 55.1%. Chng t al. (2010). Alkali xtraction of stovr, BioRsourcs 11(1),

7 A similar linar dcrasing trnd of pulping yild (calculatd basd on raw matrials) with th incras of dissolution of hmicllulos was also obsrvd (Fig. 2b). Th dissolution of hmicllulos involvd hydrolysis of str and thr bonds that link th hmicllulos to lignin (Sjöström 1993), so lignin is subsquntly much asir to dissolv during pulping of th pr-xtractd matrial. This was confirmd by th kappa numbr and brightnss of pulps (Tabl 2). Th kappa numbr dcrasd significantly by 63%, vn whn 15% of hmicllulos was dissolvd, compard with th control pulp. Dissolution of hmicllulos from 15% to 90% rsultd in a dcras in th kappa numbr of subsqunt NaOH-AQ pulp from 8.2 to 5.9. Tabl 2 also indicats that a mor considrabl amount of rsidual alkali was masurd in th pulping liquor from prxtractd lignocllulos than that from raw biomass. Th rason was probably du to partial rmoval of lignin, uronic acid, and actyl groups from corn stovr lignocllulos via hmicllulos pr-xtraction, thus rducing th dmand for alkali in subsqunt pulping. Th rsults also suggst that an IFBR procss combind hmicllulos prxtraction with NaOH-AQ pulping can b usd to substitut th procss of xtndd dlignification for a lowr kappa numbr of pulp. Tabl 2. Effct of Hmicllulos Pr-xtraction on NaOH-AQ Pulp Proprtis Pr-xtraction Alkali Concntration (%) Tmp. ( C) Kappa Numbr Rsidual alkali (g/l) Brightnss (%ISO) Dnsity* (g/cm 3 ) Braking Lngth* (km) Tar Indx* (mn m 2 /g) Burst Indx* (KPa m 2 /g) * Bating dgr 42 o SR. Physical proprtis of th NaOH-AQ pulp and thir corrsponding QP blachd pulp ar shown in Tabls 2 and 3. High hmicllulos contnt is hlpful for th bonding of fibrs and can improv th bonding strngth of th papr. From both Tabl 2 and Tabl 3 it could b found that as th xtraction condition intnsifid, th dnsity, burst indx, and braking lngth of both unblachd and blachd pulps wr all dcrasd, as compard with th control sampl. Th main composition of corn stovr hmicllulos is xylan, as confirmd by ICS. Xylan is a multi-hydroxyl compound. Th rfining procss can incras th fibrs swlling, hydration, and fibrillation dgr and improv its flxibility, so bonding strngth of th papr incrass and physical proprtis ar improvd (Wang 2005). Th rason for th dcras of th braking lngth, burst indx, Chng t al. (2010). Alkali xtraction of stovr, BioRsourcs 11(1),

8 and dnsity of th papr was also attributd to th rducd xylos contnt from hmicllulos xtractd matrial. Tabl 3. Effct of Hmicllulos Extraction on QP Blaching of th NaOH-AQ Pulp Pr-xtraction Braking Brightnss Dnsity* Burst Indx* Tar Indx* Alkali (%ISO) (g/cm 3 Lngth* Tmp. ) (KPa m 2 /g) (mn m 2 /g) Concntration (km) ( C) (%) * Bating dgr 42 o SR. Th ffct of hmicllulos xtraction on th tar indx was diffrnt from th othr strngth proprtis. With th strong xtraction conditions applid, tar indx of th QP blachd handsht incrasd first and rachd th maximum whn th alkali concntration was 8%. Tar indx of th unblachd pulp was also incrasd, but lowr than that of th blachd on. Tar indx of all th xtractd pulps was highr than that of th control. Th rsult was in accordanc with a prvious study by Wang (2005). It was found that aftr xtraction, th pulp was much asir to b blachd than th control. Undr th conditions usd, brightnss of th control sampl was 81.6% ISO, whil th brightnss of th blachd pulp with 10% NaOH xtraction bfor pulping rachd 85.6% ISO. Effct of Pr-xtraction on Silicon Contnt in Black Liquor Th ffcts of pr-xtraction conditions (alkali concntration and tmpratur) on silicon contnt in NaOH-AQ pulping black liquor ar shown in Fig. 3. Th rmoval prcntag of silicon was significantly incrasd with th alkali concntration. Whn prxtraction was prformd at 75 C with 10% alkali concntration, th silicon contnt in black liquor was rducd by ovr 80%. Th rason could b mainly ascribd to th ph chang bfor and aftr th xtraction. It has bn rportd that only if th ph valu rachs 14, th majority of SiO 2 in th black liquor is in th form of SiO 3 2-, which is much asir dissolvd in watr (Tong t al. 2005). So highr alkali concntration promotd th dissolution of silicon in watr during th xtraction procss. Chng t al. (2010). Alkali xtraction of stovr, BioRsourcs 11(1),

9 90 to 80 c t sp ) 70 r (% ith 60 w l p n o m 50 l sa silic f tro 40 o n l c o v 30 o m 20 R Alkali Concntration (%) (a) Rmoval of silicon with rspct to control sampl (%) Extraction tmpratur ( C) (b) Figur 3. Effcts of pr-xtraction of hmicllulos on th rmoval of silicon from subsqunt pulping liquor Th incras of xtraction tmpratur could also promot th rmoval of silicon. An avrag rat of dcras of silicon can b obtaind from th slop of th curv, indicating that th silicon contnt was dcrasd by 7.5% with a 10 C incras of tmpratur. CONCLUSIONS Sodium hydroxid was found to b an ffctiv agnt for th xtraction of hmicllulos from corn stovr. Whn xtraction tmpratur was 75 C and alkali concntration was 10%, mor than 90% of th original hmicllulos was dissolvd. Combind with a subsqunt NaOH-AQ pulping, alkali pr-xtraction can significantly rduc th kappa numbr of th subsqunt pulp. A loss of viscosity as wll as braking lngth and burst indx was obsrvd as a rsult of th dissolution of hmicllulos, whil tar indx was obviously improvd. Morovr, alkali xtraction of corn stovr hmicllulos had important ffct on silicon contnt in black liquor, achiving an ovr 80% rduction of th total silicon contnt in pulping liquor undr conditions usd. ACKNOWLEDGMENTS This work was supportd by National High Tchnology Program 863 (No.2007AA100704) and Major Stat Basic Rsarch Dvlopmnt Program (No.2010BC732206). W also acknowldg th SCUT-NCSU scholar xchang program that allowd portions of this work to b possibl. Chng t al. (2010). Alkali xtraction of stovr, BioRsourcs 11(1),

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