PREPARATION AND ADSORPTION STUDIES OF PHOSPHORYLATED CELLULOSE MICROSPHERES

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1 CELLULOSE CHEMISTRY AND TECHNOLOGY PREPARATION AND ADSORPTION STUDIES OF PHOSPHORYLATED CELLULOSE MICROSPHERES DANA M. SUFLET, IRINA POPESCU and IRINA M. PELIN Ptru Poni Institut of Macromolcular Chmistry, 41A, Grigor Ghica Voda Ally, , Iasi, Romania Corrsponding author: D. M. Suflt, Rcivd Novmbr 1, 2015 Nw anionic microsphrs basd on monobasic cllulos phosphat wr obtaind by chmical cross-linking with pichlorohydrin, using th watr-in-oil invrs mulsion tchniqu. SEM and FTIR spctroscopy wr usd to analys th morphology and chmical structur of th microsphrs. Th main charactristics, such as th wrack dnsity, swlling dgr, and th xchang capacity, wr invstigatd. Ths nw anionic microsphrs could b usd as potntial adsorption matrials. In this rspct, th adsorption capacity of mthyln blu and rhodamin 6G takn as modls was studid. Th Langmuir modl fittd th adsorption data bttr than th Frundlich modl. Morovr, th kintic adsorption data wr in clos agrmnt with th psudo-scond ordr kintic modl, which supportd th assumption that dy adsorption occurs by physical intractions. Intra-particl and Boyd s modls suggstd that th intra-particl diffusion was not only a rat-controlling procss, but also th film diffusion procss playd som rol in th adsorption on th monobasic cllulos phosphat microsphrs. Kywords: cllulos phosphat, microsphrs, adsorption kintics INTRODUCTION Dys ar widly usd in industris, such as txtils, lathr, papr, printing, plastics, pharmacutical, cosmtic, food, tc. Som dys and thir dgradation products may b toxic, allrgnic, mutagnic, and carcinognic. 1 Consquntly, thy ar important sourcs of watr pollution and thir rmoval bcoms a major problm for nvironmntal managmnt. Amongst numrous tchniqus of dy rmoval, adsorption is th most usd and th most fficint. 2 Th us of adsorbnts basd on polysaccharids and thir drivativs is a big advantag du to thir rnwability, biodgradability and non-toxicity. 3-5 Amongst polysaccharids, cllulos is th most abundant natural polymr and thus on of th most potntial rsourcs to rplac th absorbnts basd on synthtic polymrs. In this rspct, cllulos and its drivativs can b usd to obtaind cllulosicbasd matrials, which could b mployd as chap dy adsorbnts To our knowldg, th obtaining of adsorbnt matrials basd on monobasic cllulos phosphat (PCll) has not bn rportd in th litratur. Compard to othr cllulos drivativs, cllulos phosphats possss th advantags of low cost, asy prparation and low toxicity. 12,13 From this point of viw, in this papr, nw anionic microparticls basd on PCll wr obtaind by chmical cross-linking with pichlorohydrin using th watr-in-oil invrs mulsion tchniqu. Th morphology and siz distribution of microparticls wr analysd by scanning lctron microscopy and th chmical structur was confirmd by FTIR spctroscopy. Th main charactristics of ths nw microsphrs, such as th swlling dgr in various ph solutions and th xchang capacity, wr dtrmind. In ordr to us th microsphrs as adsorption matrials, th adsorption capacity of cationic dys (mthyln blu and rhodamin 6G wr slctd as modl compounds) was studid using th batch-adsorption tchniqu. Th influnc of th microparticls dosag, ph, and initial dy concntration on th microparticls adsorption capacity was followd at quilibrium. Exprimntal data wr xamind using svral isothrm modls in ordr to stablish th most appropriat modl to dsign th adsorption of Cllulos Chm. Tchnol., 51 (1-2), 23-34(2017)

2 DANA M. SUFLET t al. ionic microsphrs and th adsorption kintics wr fittd in ordr to obtain a bttr undrstanding of th dy adsorption mchanism onto this novl adsorbnt. EXPERIMENTAL Matrials Th monobasic cllulos phosphat (PCll, with a dgr of substitution of about 0.6) was synthsisd in our laboratory. 14 Epichlorohydrin (ECH), 1,2 dichlorothan, cllulos actat butyrat (CAB), sodium hydroxid, acton, and thylic thr (Sigma Aldrich, Grmany) wr usd as rcivd. Th mthyln blu (MB) and rhodamin 6G (R6G) wr purchasd from Sigma Aldrich, Grmany, and takn as modl dys. All xprimnts wr prformd using distilld watr, with 5 µs/cm conductivity. Synthsis of microsphrs Monobasic cllulos phosphat microsphrs (PCllMS) wr prpard by chmical cross-linking with ECH using th watr-in-oil (w/o) invrs (mini)mulsion. Brifly, 2 g of monobasic cllulos phosphat was dissolvd in 0.5 M NaOH aquous solution (40 ml). Th solution was pourd in 100 ml of disprsion mdium (1,2 dichlorothan) in which 3 g of CAB (as disprsion agnt) was dissolvd. Th w/o mulsion was stirrd for 1 h at 700 rpm for obtaining a stabl disprsion, thn 10 ml of ECH was addd and th cross-linking raction was carrid out at 55 C for 48 h. Th cross-linkd microparticls wr rcovrd by filtration and thn washd for rmoving rsiduals in th following ordr: 1,2 dichlorothan, acton, twic-distilld watr, acton, and thylic thr. Finally, th microsphrs wr compltly drid by ovrnight xposur to 60 C, undr vacuum. Analysis and charactrization of cllulos phosphat microsphrs Th microsphrs morphology and particl siz wr valuatd by scanning lctron microscopy (SEM, Quanta 200, FEI). Th wrack dnsity was dtrmind by wighing a volum of 1 ml of drid microsphrs placd in a graduatd cylindr (i.d. = 11 mm). Th swlling dgr was valuatd using th cntrifugation mthod 15 as follows: th dry microsphrs (20 mg) wr placd into stainlss stl baskts that hav a siv and a filtr papr at th bottom, and th baskts wr immrsd into watr with a crtain ph (adjustd with NaOH or HCl solutions). Aftr th swlling of th microsphrs (24 hours), th baskts wr cntrifugd at 1000 rpm for 15 minuts, whn only th swlld microparticls rmaind, without xcss solvnt. Th watr rtaind into th microparticls, masurd by wighing, was rportd to th amount of dry microsphrs. Th xchang capacity of th microsphrs (EC) was dtrmind by th titration mthod, using a Mtrohm all purposs 716 DMS Titrino apparatus quippd with a combind lctrod. Firstly, 100 mg of ionic microsphrs wr swolln in 20 ml watr for 12 h. Aftr that, an xcss of 0.1N HCl solution (5 ml) was addd and maintaind for 5 h, undr gntl stirring. Th suprnatant solution was collctd and titratd with a 0.1N NaOH solution. Th xchang capacity (EC) was xprssd as mq. phosphoric groups/g drid microsphrs. Dy adsorption Adsorption studis wr prformd at room tmpratur using th batch tchniqu. Th dy concntration was dtrmind spctrophotomtrically using a UV-Vis spctrophotomtr (Thrmo Fishr Scintific Evolution 201, USA) by using a linar calibration plot stablishd for MB and R6G at 663 nm and 526 nm, rspctivly. Effct of ph and initial dy concntration Th ffct of th initial ph on th adsorption capacity of PCllMS was invstigatd as follows: 20 mg of PCllMS wr addd to ach dy solution (20 ml; 100 mg/l) at ambint tmpratur and shakn at 200 rpm for 24 h. Th initial solution ph (btwn 2 to 11) was adjustd by adding 0.1N NaOH or 0.1 N HCl. Th dy solutions wr collctd aftr 24 h and assayd by UV-Vis spctroscopy. Th ffct of dy concntration on th adsorption capacity of microsphrs was also invstigatd as follows: 20 mg microsphrs wr addd to ach dy solution (20 ml; mg/l) and allowd to intract for 24 h undr gntil shaking (200 rpm) at room tmpratur. Th final dy concntrations wr dtrmind by UV-Vis spctroscopy. Th amount of dy adsorbd by th microsphrs at quilibrium (q ) was calculatd from th diffrnc btwn th initial concntration of dy and th dy concntration in th suprnatant aftr adsorption (Eq. (1)): V ( C0 C f ) q = (1) m whr C 0 and C f ar th initial and final dy concntration (mg/l), V is th volum of th dy solution (L), and m is th mass of th adsorbnt usd (g). Th amount of absorbd dy onto microsphrs was also calculatd as prcntag of rmoval (R, %) according to Eq. (2): 100( C0 C f ) R(%) = (2) C 0 Equilibrium adsorption For quilibrium dy adsorption xprimnts, fixd amounts of th PCllMS (20 mg) wr quilibratd in 20 ml of aquous dy solutions of varying concntrations. Th rsidual quilibrium dy concntration in th solutions was calculatd from UV- 24

3 Cllulos Vis data. Th Langmuir isothrm modl dscribd by Eq. (3) was linarizd Eq. (4) as: K LC q = (3) 1 + a C L C al = 1 + C (4) q K L K L whr q th solid phas adsorbat concntration at quilibrium (mg/g), C th aquous phas adsorbat concntration at quilibrium (mg/l), and K L (L/g) and a L (L/mg) ar th Langmuir isothrm constants. Th plot of C /q vs. C givs a straight lin with th slop a L /K L and intrcpt 1/K L. Th maximum adsorption capacity of th adsorbnt (q m, mg/g) corrsponds to K L /a L. Hrby, a dimnsionlss constant commonly known as sparation factor (R L ), usd to prdict th adsorption bhaviour of dys, can b rprsntd as: 1 RL = (5) 1+ alc0 whr K L (L/mg) rfrs to th Langmuir constant rlatd to th nrgy of adsorption and C 0 rprsnts th initial dy concntration (mg/l). In this contxt, th lowr R L valu mans that adsorption is mor favourabl. In a mor complx xplanation, th R L valu indicats th adsorption natur to b ithr favourabl (0< R L <1), irrvrsibl (R L =0), linar (R L =1), or unfavourabl (R L >1). 16 Th Frundlich isothrm is an mpirical modl dscribing th non-idal and rvrsibl adsorption, not rstrictd to th formation of monolayr. This modl is widly applid in htrognous systms spcially for organic compounds or highly intractiv spcis, and it is dscribd by Eq. (6): q n = K C F (6) F 1 whr q th solid phas adsorbat concntration at quilibrium (mg/g), C th aquous phas adsorbat concntration at quilibrium (mg/l), K F th Frundlich constant (L/g) bing of th xtnt of adsorption, and 1/n F th htrognity factor bing an indicator of adsorption ffctivnss and th dviation from linarity of th adsorption procss. Th 1/n F valu rangs btwn 0 and 1 is a masur of adsorption intnsity or surfac htrognity, bcoming mor htrognous as its valu gts closr to zro. Th Frundlich isothrm in Eq. (6) can b linarizd, as shown in Eq. (7): 1 ln q = ln K F + ln C (7) nf Th Dubinin-Radushkvich isothrm is an mpirical modl initially concivd for th adsorption of subcritical vapours onto micropor solids, following a por filling mchanism. 17 It is gnrally applid to xprss th adsorption mchanism with a Gaussian nrgy distribution onto htrognous surfac. q 2 k DR ε = qs (8) 1 ε = RT ln(1 + ) (9) C E = 1 (10) 1 2 (2k DR ) whr q is th solid phas adsorbat concntration at quilibrium (mg/g), q s is th maximum adsorption capacity of dy ion (mg/g), k DR is dnotd as th D-R isothrm constant (mol 2 /kj 2 ), and th paramtr ε can b corrlatd as in Eq. 9, whr R, T and C rprsnt th gas constant (8.314 J/mol K), absolut tmpratur (K), and adsorbat quilibrium concntration (mg/l), rspctivly. Th fr nrgy of adsorption, E (kj/mol), is dfind as th fr nrgy whn on mol of ion is transfrrd to th surfac of a solid from infinity in solution (Eq. 10). Th valu of E is usd to stimat th adsorption typ. Th magnitud of E, lss than 8 kj/mol, indicats that th adsorption procss is physical in natur, whil th rang of valu 8-16 kj/mol indicats that th procss of adsorption procds by ion xchang. Chmisorption is charactrizd by E valus highr than 40 kj/mol. Th Tmkin isothrm quation assums that th adsorption is charactrizd by a uniform distribution of binding nrgy, up to som maximum binding nrgy, and that th hat of adsorption of all th molculs in th layr would dcras linarly with convrg du to adsorbnt-adsorbat intractions. Th Tmkin isothrm (11) can b linarizd as shown in Eq. (12): RT q = ln( at C ) (11) bt RT RT q = ln at + lnc (12) bt bt whr b T is th Tmkin constant rlatd to th hat of adsorption (kj/mol) and a T is th quilibrium binding constant corrsponding to th maximum binding nrgy (L/mg), R is th gas constant (8.31 J/mol K), and T (K) is th absolut tmpratur. Adsorption kintics In ordr to valuat th ffct of control tim on th dy adsorption capacity of PCllMS, sampls of 20 mg microparticls wr immrsd in 20 ml of dy solution ( mg/l) for diffrnt contact durations (up to 26 h) and shakn at 200 rpm. Th dy concntration in th suprnatant was dtrmind by UV-Vis spctroscopy. Th xprimntal data wr analysd by svral kintics modls: Lagrgrn rat known as psudo-first-ordr kintics modl (Eq. 13), th psudo-scond-ordr (Eq. 14), intra-particl diffusion, Eq. 15, and Boyd s (Eq. 16) modls wr applid in ordr to invstigat th diffusion 18, 19 mchanism. 25

4 DANA M. SUFLET t al. k1 log( q qt ) = log q t (13) t 1 1 = + t 2 q k q q (14) q t t = kit + I (15) qt Bt = ln(1 ) (16) q whr q t (mg/g) and q (mg/g) indicat th dy adsorbd amount at tim t (min) and quilibrium, rspctivly, k 1, k 2, and k i ar th psudo-first ordr rat constant (min -1 ), th psudo-scond ordr rat constant (g/mg min), and th intra-particl diffusion rat constant (mg/g min 1/2 ) for th adsorption procss rspctivly; t is tim (min), and I is th intrcpt of th y axis, which givs an ida about th thicknss of th boundary layr. Th B t is a mathmatical function that rprsnts th fraction of adsorbat adsorbd at diffrnt tims and it is usd to idntify whthr xtrnal transport or intra-particl transport controls th rat of sorption. Th Boyd s modl assums that th bound layr surrounding th adsorbnt particl is th main rsistanc to diffusion. RESULTS AND DISCUSSION Prparation and charactrization of microsphrs Th microsphrs basd on PCll wr synthsisd by chmical cross-linking with ECH using th w/o invrs mulsion tchniqu (Schm 1). Th chmical structur of PCllMS was confirmd by FTIR spctromtry (Fig. 1). In th FTIR spctrum of PCllMS, th following charactristic bands wr obsrvd: a larg band at 3349 cm -1 corrsponding to th OH groups; a band at 2899 cm -1 attributd to th CH 2 groups; th band at 1653 cm -1 owing to th associatd watr; th bands btwn 1432 and 1282 cm -1 corrsponding to CH 2 and C OH from th glucosidic units; a larg band at cm -1 attributd to C O C from th glucosidic units, and a band at 896 cm -1 for th β-d-glucosidic bonds. In addition, th bands of th phosphat groups could b obsrvd in th spctrum of PCllMS: cm -1 (P H bond), 1210 cm -1 (P=O bond), and 845 cm -1 (P O C bond). Th particl siz distribution and th morphology of PCllMS wr invstigatd by SEM. Th sphrical structur of th cllulos phosphat microparticls and th siz distribution btwn µm ar shown in Figur 2. Th main charactristics of PCllMS, such as th wrack dnsity, xchang capacity (EC), and particl sizs, ar prsntd in Tabl 1, and th ffct of initial ph solution on th watr rtntion of microsphrs is prsntd in Figur 3. It can b obsrvd that th initial ph of mdium dos not influnc th watr rtntion rat of PCllMS, th amount of ight grams pr gram microsphrs rmaining constant through th ph domain. This bhaviour can b xplaind by th high dgr of cross-linking and by th low valu of pk a of monobasic phosphat groups (pk a =2.4). 20 Dy adsorption Influnc of initial ph and sorbnt dosag on dy adsorption Th ph is on of th most important procss variabls whn considring dy adsorption. Th ffct of th solution ph on th amount of dy adsorbd on PCllMS at quilibrium was invstigatd varying th initial solution ph in th rang of , at 25 C (Fig. 4a), th othr paramtrs bing kpt constant. Schm 1: Synthsis of PCllMS by chmical cross-linking with pichlorohydrin 26

5 Cllulos Figur 1: FTIR spctra of parnt cllulos, monobasic cllulos phosphat and microparticls basd on phosphorylatd cllulos Figur 2: ESEM photographs of PCllMS. Gnral viw (a), surfac dtail (b), and th corrsponding siz distribution (c) Tabl 1 Main charactristics of phosphorylatd polysaccharid microsphrs a Sampl cod Wrack dnsity, g/ml EC, mqui./g Particl siz, µm PCllMS 0.53 ± ± a Th valus ar th man of thr indpndnt masurmnts ± SD 27

6 DANA M. SUFLET t al. Figur 3: Effct of ph on watr rtntion of PCllMS Figur 4: Effct of ph (a) and absorbnt dos (b) on adsorption of dys, at 25 C (adsorbnt dos = 20 mg, [dy] = 100 mg/l, contact tim = 48 h) At ph = 2, th monobasic phosphat groups from th cllulos drivativ ar in th protonatd form and th adsorption of cationic dys is clos to zro, but with th incras of th ph, th anionic groups bgin to dissociat and th dy adsorption incrass until th ph is around 6. Ovr this valu, th amount of dy adsorption into th microparticls rmaind almost constant. In ordr to study th ffct of th adsorbnt dosag on dy adsorption, various amounts of PCllMS (2-80 mg) wr placd in th dy solutions with a concntration of 100 mg/l. Th natural ph of R6G and MB in watr was 5.3 and 5.5, rspctivly. Th dy adsorption fficincy incrass with th incras of th adsorbnt dos (Fig. 4b). In th prsnc of 80 mg PCllMS, almost complt adsorption (90%) of MB was achivd, whil th rmoval prcntag for R6G was of only 70%. This diffrnc can b attributd to a mor voluminous structur of R6G compard with MB. Incrasing th adsorbnt dos, additional adsorbnt sits bcom availabl and hnc incrasing amounts of th dy ar rmovd from th solution. Equilibrium adsorption Th dy adsorption procss is govrnd by mass transfr from solution to adsorbnt, which can b xplaind mathmatically by quilibrium isothrm and kintic modls Th adsorption proprtis and quilibrium data illustrat how adsorbat molculs intract with th adsorbnt and ar thrfor vry important to optimiz th us of cllulos phosphat microsphrs. Exprimntal data wr xamind using svral isothrm modls (Fig. 5a and b) in ordr to stablish th most appropriat modl to dsign th adsorption of ionic microsphrs. Th corrsponding paramtrs ar listd in Tabl 2. Th shap of th curvs indicats isothrms of L typ, suggsting a progrssiv saturation of th microsphrs. 22 Th linarizd form of th Langmuir isothrm (Fig. 5c) fits th data masurd for both dys, 28

7 Cllulos ovr th whol concntration rang. Th valus of corrlation cofficints (R 2 ) wr of and for MB and R6G, rspctivly (s Tabl 2). Th Langmuir paramtr (a L ), known as binding constant, is rlatd to th nrgy of adsorption, and indicats th affinity or th binding strngth of th dy to th adsorbnt. Th high valus of a L indicat a strong adsorption of th dy on th adsorbnt. In our cas, th highr valu of a L for MB than a L of R6G implis a comparativly highr affinity of PCllMS for MB. Th good agrmnt btwn th xprimntal and th calculatd valus of q according to th Langmuir modl (Tabl 2) indicats that th dy forms a monolayr covrag onto th microsphrs surfac. It was also obsrvd that th valu of th adsorption capacity of PCllMS for R6G was highr than for th MB, though th volum of th R6G molcul is highr than th volum of th MB molcul. This bhavior could b attributd to th diffrnt numbr of mols adsorbd for ach dy into PCllMS: 134 mmol R6G and mmol MB pr gram of microsphrs. 24 Figur 5: Equilibrium adsorption isothrm of dys MB (a) and R6G (b) onto PCllMS. Langmuir (c) and Frundlich (d) modls dscribing th adsorption Figur 6: Sparation factor (R L ) as a function of initial dy concntration Figur 7: Tmkin plot for adsorption of dys MB and R6G onto PCllMS 29

8 DANA M. SUFLET t al. Tabl 2 Langmuir, Frudlich, Tmkin and Dubinin-Radushkvich adsorption paramtrs of PCllMS adsorbnt at 25 C by linar rgrssion modls Isothrm Langmuir Frundlich Tmkin Dubinin- Radushkvich Paramtrs Sampl MB R6G a L, L/mg K L, L/g q m, mg/g q,xp, mg/g R L R Low concntration K F, L/mg n F R High concntration K F, L/mg n F R a T, L/mg b T, J/mol R q DR, mg/g k DR, mol 2 /kj E, kj/mol R Although th xchang capacity of PCllMS was at a low lvl (EC = 0.24 mqui./g), th q valus wr rlativly high du to rtntion of dys not only by lctrostatic intractions, but also by hydrogn bonds. Th adsorption capacity of PCllMS was lowr, compard with that of som adsorbnts basd on anionic cllulos 8,25 drivativs, but it was still found to b highr than that of othr ionic or non-ionic cllulos matrials. 6,9,26 Th valu of R L for th adsorption of both dys was found to dcras with th incras in dy solution concntration (Fig. 6), and th calculatd valus for both dys btwn for MB and for R6G (Tabl 2) indicatd a favourabl dy adsorption procss. By plotting th xprimntal data using th linar Frundlich quation (7) (Fig. 5d), dviations from linarity wr obsrvd, obtaining low valus of R 2. Howvr, whn th concntration rang was dividd into two rgions (low and high concntrations) a good fit of rsults (R 2 >0.966) was obsrvd. Th valus of Frundlich isothrm paramtrs ar also includd in Tabl 2. This multi-linarity of th isothrm is normally attributd to irrgular nrgy distributions, du to diffrnt groups on th surfac, with diffrnt lvls of activation nrgis for th rang of sorption. 8,27 By plotting q vs. ln C (Fig. 7) using Tmkin modl (Eq. 12), th Tmkin constants a T and b T wr dtrmind as intrcpt and slop (Tabl 2). Th low valus of th Tmkin adsorption potntial, a T, could indicat a low potntial of th PCllMS microsphrs, and th valus of b T ( kj/ml) suggstd a wak intraction btwn adsorbat (dy) and adsorbnt (PCllMS). 28 Th adsorption data wr also fittd with th linarizd form of th D-R isothrm modl (Eq. 8), and th obtaind paramtrs ar prsntd in Tabl 2. Th valus of th fr nrgy of adsorption (E), ar limitd to and kj/mol for MB and R6D, rspctivly. Basd on ths data, it can b concludd that th physical adsorption playd a dominating rol in th adsorption procss of dy adsorption onto th PCllMS. From th abov discussion, in trms of R 2 valus, th applicability of th abov modls for th prsnt xprimntal data followd approximatly th ordr: i) for MB: Lamgmuir > Frundlich > Dubinin-Radushkvich > Tmkin; ii) for R6G: Lamgmuir > Tmkin > Frundlich > Dubinin-Radushkvich. It is clar that for th 30

9 Cllulos whol concntration rang, th quilibrium adsorption data wr bttr xplaind by th Lamgmuir isothrm, for both dys. Adsorption kintics Adsorption kintics is a vital valuation lmnt for an adsorption procss that dpnds on th adsorbat-adsorbnt intraction and systm conditions. Th ffct of th contact tim and th concntration of dy solution on th adsorption kintics ar prsntd in Figur 8. As xpctd, th ovrall trnd was th incras of th adsorption in tim, bcoming constant aftr 24 hours. Th dy adsorption incrasd with th incrasing of initial dy concntration from 100 to 500 mg/l. In ordr to valuat th kintic mchanism, th xprimntal data wr analysd by svral kintic modls: psudo-first ordr (Eq. 13), psudo-scond (Eq. 14), intra-particl diffusion (Eq. 15), and Boyd s modl (Eq. 16), as shown in Figurs 9 and 10. Th kintic paramtrs stimatd from ths plots (prsntd in Tabl 3) show that th R 2 valus ar highr (>0.98) for th psudo-scond ordr kintic modl (for both dys) than for th psudo-first ordr on. This suggstd that th adsorption kintics of dys on PCllMS microsphrs follows th psudo-scond ordr kintic modl and supports th assumption that th dy adsorption occurs by physic-sorption. Th intra-particl diffusion modl was mployd to find th probabl dy-adsorption mchanism. It has bn rportd that if th linar plot of q t vs t 1/2 passs through th origin, th intra-particl diffusion mchanism is considrd as a rat-limiting stp for th adsorption procss; othrwis, th film diffusion procss is th prdominant mchanism of th adsorption procss. Plotting th xprimntal data according to th intra-particl modl (Fig. 10 a and b), th multi-linarity in thr rgions was obsrvd. Th first rgion is attributd to th diffusion of dys through th solution to th xtrnal surfac of microsphrs or to th boundary layr diffusion of solut molculs (film diffusion or xtrnal mass transfr) with an instantanous blocking of th availabl sits on th xtrnal surfac of microsphrs. Th scond rgion rprsnts th intra-particl diffusion procss, whn th dy molculs ntr th porous structur of microparticls. Th last rgion, which is th slowst stag, can b attributd to th quilibrium phas, whr th adsorption sit on th microsphrs and th concntration of th dy solution ar limitd. 8,29 As shown in Tabl 3, non of th rgions has I valus qual to zro, which indicats that ths lins do not pass through th origins. This mans that th adsorption procss occurrd by both xtrnal diffusion and intraparticl diffusion. 30 Boyd s modl (Eq. 16) also confirmd ths obsrvations, whn th Boyd plots did not pass through th origin (Fig. 10 c and d). This suggsts, for th prsnt systm, that th film diffusion plays a crtain rol in th ratlimiting procss, and th intra-particl diffusion is not th only rat-controlling procss in th adsorption of dys on th PCllMS. Figur 8: Adsorption kintics of MB (a) and R6G (b) onto PCllMS microsphrs 31

10 DANA M. SUFLET t al. Figur 9: Psudo-first (a and b) and scond ordr (c and d) plots for th adsorption of MB and R6G on PCllMS microsphrs Figur 10: Intra-particl diffusion plot (a and b) and Boyd s plot (c and d) for MB and R6G on PCllMS microsphrs 32

11 Cllulos Tabl 3 Psudo-first ordr, psudo-scond, and intra-particl diffusion kintics for adsorption of acid dys on PCllMS microparticls Dy Mthyln Blu Rhodamin 6G C 0, mg/l k 1, min Psudo-first-ordr Psudo-scondordr Intra-particl diffusion Rgion 1 Rgion 2 Rgion 3 q, mg/g R k 2, g/mg.min q, mg/g R k i1, g/g.min -1/ I 1, mg/g R k i2, g/g.min -1/ I 2, mg/g R k i3, g/g.min -1/ I 3, mg/g R CONCLUSION Nw monobasic cllulos phosphat microsphrs wr synthsisd by chmical crosslinking with ECH using th w/o invrs mulsion tchniqu. Scanning lctron microscopy highlightd th sphrical shap and µm siz of th microparticls. Th fficincy of th monobasic cllulos phosphat microsphrs as adsorbnt for th rtntion of cationic dys from aquous solution has bn invstigatd. Th adsorption capacity, vry low in acidic mdia, incrass with th solution ph, attaining high valus at ph = 6, whn all th phosphoric groups of th sorbnt ar dissociatd. Th Langmuir modl fittd to th adsorption data bttr than th Frundlich modl. Th shap of th curvs indicats isothrms of L typ, suggsting a progrssiv saturation of th microsphrs. Th kintic adsorption data wr in clos agrmnt with th psudo-scond ordr kintic modl and supports th assumption that th dy adsorption occurs by physic-sorption. Intra-particl and Boyd s modls suggstd that th intra-particl diffusion was not th only rat-controlling procss, and that th film diffusion procss also plays som rol in th adsorption of dys on th monobasic cllulos phosphat microsphrs. ACKNOWLEDGEMENTS: This work was supportd by a grant of th Romanian National Authority for Scintific Rsarch and Innovation, CNCS UEFISCDI, projct numbr PN-II-RU- TE REFERENCES 1 M. T. Yagub, T. K. Sn, S. Afroz and H. M. Ang, Adv. Colloid Intrfac Sci., 209, 172 (2014). 2 G. Crini, Biorsour. Tchnol., 97, 1061 (2006). 3 G. Crini, Prog. Polym. Sci., 30, 38 (2005). 4 C. Pohontu, M. Popa, J. Dsbrirs and L. Vrstiuc, Cllulos Chm. Tchnol., 50, 609 (2016). 5 L. Ochiuz, M. Hortolomi, I. Stolriu and M. Brca, Cllulos Chm. Tchnol., 50, 569 (2016). 6 L. S. Silva, L. C. B. Lima, F. C. Silva, J. M. E. Matos, M. R. M. C. Santos t al., Chm. Eng. J., 218, 89 (2013). 7 F. C. Silva, L. C. B. Lima, R. D. S. Bzrra, J. A. Osajima and E. C. S. Filho, in Cllulos Fundamntal Aspcts and Currnt Trnds, ditd by M. Poltto and H. L. Ornaghi Jr., InTch., 2015, Chaptr 5. 8 N. K. Gol, V. Kumar, N. Misra and L. Varshny, Carbohyd. Polym., 132, 444 (2015). 9 I. A. Udotok, R. M. Dimmick, L. D. Wilson and J. V. Hadly, Carbohyd. Polym., 136, 329 (2016). 10 S. Hokkann, A. Bhatnagar and M. Sillanpaa, Watr Rs., 91, 156 (2016). 11 N. A. El-Klsh and G. A. Mahmoud, Cllulos Chm. Tchnol., 49, 881 (2015). 12 P. L. Granja, L. Pouysgu, M. Ptraud, D Jso, C. Baquy t al., J. Appl. Polym. Sci., 82, 3341 (2001). 13 T. Groth and W. Wagnkncht, Biomatrials, 22, 2719 (2001). 14 D. M. Suflt, G. C. Chitanu and V. I. Popa, Ract. Funct. Polym., 66, 1240 (2006). 15 K. W. Pppr, D. Richnbrg and D. K. Hal, J. Am. Chm. Soc., 15, 3129 (1952). 16 K. Y. Foo and B. H. Hamd, Chm. Eng. J., 156, 2 (2010). 33

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