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1 Chemicl Engineering Journl 8 () 76 8 Contents lists ville t SciVerse ScienceDirect Chemicl Engineering Journl jo u r n l hom epge: Adsorent ility of lignin-sed ctivted crons for the removl of p-nitrophenol from queous solutions Luis M. Cotoruelo,, Mrí D. Mrqués, Frncisco J. Díz, José Rodríguez-Mirsol, Jun J. Rodríguez, Tomás Cordero Deprtmento de Ingenierí Químic, Fcultd de Ciencis, Universidd de Málg, 97 Málg, Spin Ingenierí Químic, Fcultd de Ciencis, Universidd Autónom de Mdrid, 89 Mdrid, Spin r t i c l e i n f o Article history: Received 6 Octoer Received in revised form Decemer Accepted Jnury Keywords: p-nitrophenol Activted cron Adsorption Equilirium Thermodynmics Kinetics s t r c t This pper reports the p-nitrophenol (PNP) removl from queous solutions y dsorption onto ctive crons (ACs). Nine ACs were prepred from cid-precipitted euclyptus krft lignin following twostep process consisting in CO prtil gsifiction (75 85 C) fter croniztion (5 8 C) in N tmosphere. The mount dsored rnged from to. mmol/g, nd it is relted to the initil concentrtion of dsorte, temperture, ph, urn off of the ctivted crons, nd contct time. The equilirium results were fitted y the Temkin, Duinin-Rdushkevich, nd Guggenheim-Anderson-de Boer equtions. The min thermodynmic mgnitudes were estimted s well, nd their vlues indicted tht the dsorption processes were spontneous nd exothermic. The kinetic study showed tht the processes re of pprent second order relted to the concentrtions of the empty ctive sites on the ACs surfce. The vlues of the effective diffusivities hve een clculted nd they hve suggested tht the internl diffusion controls the net mss trnsfer. The results otined in the present work cn e for the enefit of the preprtion of new dsorents, s well s the primry design of the dsorption equipment with seprtion or environmentl purposes. Elsevier B.V. All rights reserved.. Introduction Progressive increses in the industril ctivity produce lrge mounts of toxic compounds. Among them, p-nitrophenol (PNP) cn enter the environment during the production of dyes, pesticides nd drugs. Nitrophenols re toxic to plnts nd nimls. Due to its hrmful effects, wstewters contining phenolic compounds must e treted efore to their dischrge to the receiving wter odies. Mny different types of dsorent my e evluted to remove sustituted phenols from wstewters. They include, mong others, XAD polymeric resins [] nd ctivted crons, which still re the dsorents widest used to this end [ ]. On the other hnd, s we hve reported in previous works [,], the preprtion of ctivted crons from krft lignin is n interesting possiility for the lignin enefit s n lterntive wy for the lck liquors from the cellulose pulp industry, s they re usully processed. So, the ojective of this work hs een to study the dsorption of PNP from queous solutions using ACs prepred in our lortory. The ACs derived from euclyptus krft lignin hve een chosen for their vlule properties sed on their well developed surfce re, Corresponding uthor. Tel.: + 957; fx: E-mil ddress: lcot@um.es (L.M. Cotoruelo). surfce chemicl nture, nd their thermo-chemicl stility. Also, the use of PNP dsorption from wter or orgnic (enzene, heptne, nd xylene) solutions ecme referenced procedure in the surfce study of vried dsorents, s lumin, crons, cement, metl powders, silic, titni, pigments, sugr, nd wool [5 7].. Mterils nd methods The experimentl procedures followed in the preprtion nd chrcteriztion studies of the ACs s dsorents (here nmed s: A, B, B, C5, D, E, E, E6, nd E) were previously descried in detil elsewhere [8]. The rw mteril ws euclyptus krft lignin supplied y the Empres Ncionl de Celuloss s otined y cid precipittion of krft lck liquors. This lignin ws cronized under N tmosphere nd then it ws ctivted y prtil gsifiction with CO during different times to otin ctivted crons with different urn off. The porous structures of the ACs were chrcterized y mens of dsorption desorption of N t 77 K nd CO t 7 K using Quntchrome pprtus (Autosor- model) nd mercury porosimetry using Crlo Er mercury porosimeter. Ultimte nlyses of the ACs were crried out in Perkin-Elmer (model CHN) nlyzer /$ see front mtter Elsevier B.V. All rights reserved. doi:.6/j.cej...6

2 L.M. Cotoruelo et l. / Chemicl Engineering Journl 8 () Tle Preprtion conditions nd finl properties of the ACs. AC A B B C5 D E E E6 E A BET (m /g) A S (m /g) V microp. CO (cm /g) c V microp. N (cm /g) d V mesop. (cm /g) e V mcrop. (cm /g) f Croniztion temp. (K) Activtion temp. (K) Activtion time (h) 5 6 Burn off (%) g Yield (% weight) Ultimte nlyses (% dry sh free sis) C (%) O (%) H (%) Inorgnic mteril (%) Determined y dsorption of N t 77 K (BET eqution with A m =.6 nm ). Externl surfce determined from the N dsorption isotherm y S method. c Determined y dsorption of CO t 7 K (Duinin-Rdushkevich eqution). d Determined from the N dsorption isotherm y S method. e (From to 8 nm width) estimted from the N dsorption isotherm y tking volume dsored t P/P o =.95 s liquid nd (from 8 to 5 nm width) estimted y mercury porosimetry y tking cumultive pore volume t r = 5 nm. f Estimted y mercury porosimetry y tking cumultive pore volume. g Clculted s the smple weight loss reltive to completely devoltilized krft lignin. For the liquid phse dsorption, previous experimenttion included progrm for the influences of the initil concentrtion of PNP, s well s the dsorent dose. An dsorent dose of. g AC/L (dry sis), nd rnge from 5 to mg PNP/L were set s selected for the tests in this present work. So, the equilirium nd the kinetic experiments were crried out y putting the doses of ctivted cron weighted into glss flsks. A stock solution of PNP (Merck, 99.5% purity) ws prepred in distilled wter nd the desired concentrtions were otined y successive dilutions. ml of queous solutions of different initil concentrtions (C o :.6.57 mmol/l) were dded to the glss flsks to contct the ACs, nd mechniclly gitted t 8 rpm using n oritl incutor (Gllenkmp, model INR-5). The smples were kept t different test tempertures in the rnge 8 K K. The contct times vried etween nd dys (depending on the smples). The solutions from the kinetic tests were nlyzed for different contct times up to h. To nlyze the PNP concentrtions, clirtion plot ws prepred y mesuring the sornce of smples with different predetermined concentrtions (UV vis spectrophotometer Vrin, model Cry E, mx : 5 nm). To void the ph influence on the sornce of the smples, ll smples were previously cidified y dding. M HCl.. Results nd discussion.. Chrcteriztion of the ctivted crons Euclyptus krft lignin is not porous mteril (A BET < m /g) s supplied, nd it hs no dsorption cpcity s is. A first thermochemicl process is necessry to develop ny specific surfce re, nd it hs to e followed y physicl ctivtion. A high grde of porosity (microporosity nd certin grde of mesoporosity) is developed y the ctivtion process. The resulting ACs hve heterogeneous physicl nd chemicl structure. The former rises from the existence of pores with different sizes. The ltter is originted from vriety of functionl groups (minly in the form of cron oxygen) on the surfce. Tle summrizes some of the chrcteristics of the ACs, which hve een extensively discussed in previous work [8]. The surfce re nd the micropore volume sustntilly increse with urn off; widening of micropores, nd n incresing mesopores contriution re oserved nd, t high urn off levels, significnt development of mcroporosity cn e detected. E series includes ACs with low, intermedite, nd high ctivtion degrees. The chemicl nture of the ACs surfce hs een lso studied nd reported in previous work [9]. XPS nd TPD nlyses suggested tht the surfce oxygen groups formed during gsifiction were the following: hydroxyl, ether, cronyl, croxylic cid, lctones, phenols, quinone, nd nhydrides of croxylic cids... Adsorption equilirium isotherms... Equilirium results The dsorption isotherm is n expression for the vrition of the dsorent dsorption cpcity (q e, mmol/g) which is in equilirium with the concentrtion of dsorte (, mmol/l) in the ulk of the solution t constnt temperture. Grphicl presenttions of the PNP dsorption isotherms on ll ACs t 9 K re depicted in Fig.. Fig. displys the E series isotherms for the ACs prepred when the ctivtion time vried. The shpe of the isotherms indictes high dsorent ffinity to the PNP t low concentrtions. The isotherms re concve (fvorle) with respect to the -xis for the entire concentrtion rnge studied. The highest dsorption cpcities corresponded to D nd E, in greement with their highest totl surfce development. The dsored mount progressed with the increse of the AC urn off, s consequence of the greter surfce re nd mesoporosity development. The isotherm of E reveled high dsorption cpcity even t low equilirium concentrtions. In contrst, the E isotherm provided wek enlrge on its cpcity vlues towrd the highest PNP concentrtions in the solution. To hve n ide out the expected yield from the dsorption system, it is importnt t first, to estlish the most pproprite correltions for the equilirium stte curves. Vrious isotherm eqution types like those from Guggenheim-Anderson-de Boer (GAB), Duinin-Rdushkevich (D-R) nd Temkin, hve een tested to descrie the equilirium dt.

3 78 L.M. Cotoruelo et l. / Chemicl Engineering Journl 8 () q e (mmol/g) 5 E E6 E E A B B C5 D Ln q e (q e in mmol/g) A B B C5 D..5. (mmol/l) ε (J /mmol ) 6 E E6 E E 8 Fig.. Experimentl dt (dots) nd GAB fits (curves) for the dsorption isotherms of PNP on ACs t 9 K: () A, B, B, C5, nd D; () E series. Fig.. Experimentl dt (dots) nd D-R fits (liner) for the dsorption isotherms of PNP on ACs t 9 K: () A, B, B, C5, nd D; () E series.... Guggenheim-Anderson-de Boer model The GAB model [] my e expressed s: K M GAB q e = () ( K )( + (K K ) ) where K nd K (L/mmol) re the isotherm constnts nd M GAB (mmol/g) is the equilirium concentrtion of dsorte on the solid phse corresponding to complete coverge of the dsorption sites. This eqution cn ecome Lngmuir isotherm for K =. Fig. nd Tle show the results of the fits to the GAB model (included the vlues of : verge qudrtic devition).... Duinin-Rdushkevich model The D-R eqution [] is expressed s: q e = M D-R exp( ˇε ) () Eq. () cn e linerized s: Ln q e = Ln M D-R ˇε () where M D-R (mmol/g) represents the theoreticl sturtion cpcity; ˇ (mmol /J ) is constnt relted to the dsorption energy; nd ε (J/mmol) is the Polnyi potentil, which is relted to the equilirium concentrtion s follows: [ ( )] ε = RT Ln + () where R is the gs constnt (.8 J/mmol K) nd T is the solute temperture (K). The Polnyi sorption potentil (ε) is the work required to remove molecule from its loction in the sorption spce to infinity, independent of the temperture. This model ssumes the heterogeneity of sorption energies within this spce. The vlues of M D-R nd ˇ my e estimted y plotting Ln q e vs. ε ; i.e., from the intercept nd the slope of the stright lines depicted y Eq. (). Tle shows the results of the fits to the D-R eqution nd the vlues of r (determintion coefficient). Experimentl dt nd their D-R fits re plotted in Fig.. Tle GAB, D-R nd Temkin isotherm constnts t 9 K nd coverge fctors. AC A B B C5 D E E E6 E GAB M GAB (mmol/g) K (L/mmol) K (L/mmol) E-.9E-.7E-.E- 5.E-.E-.E- 6.E- 9.9E- D-R M D-R (mmol/g) ˇ (mmol /J ) r E (J/mmol) Temkin M T (mmol/g) K T (L/mmol) E E (J g/mmol ) r Equivlent re occupied nd coverge fctors for C o:.57 mmol/l A PNP (m /g AC ) C.F. (A PNP/A BET)

4 L.M. Cotoruelo et l. / Chemicl Engineering Journl 8 () q e (mmol/g) A B B C5 D E E6 E E q e (mmol/g) 5 E series q e = V pore r =.9 E..8 E C5 B D B E6..6 V pore (cm /g). E A Fig.. Amount of PNP dsored s function of the ACs totl pore volume Ln ( in mmol/l) Fig.. Experimentl dt (dots) nd Temkin fits (liner) for the dsorption isotherms of PNP on ACs t 9 K: () A, B, B, C5, nd D; () E series. The constnt ˇ gives the men dsorption free energy (E) when the dsorte is trnsferred from the infinity in the solution to the surfce of the solid. It cn e clculted s: E = (5) ˇ The mgnitude of E (J/mmol) is useful for estimting the mechnism of the dsorption step (Tle ). In the present cse, with E < 8 kj/mol, it is ssumed tht the physicl forces re the min ffecting the dsorption mechnism.... Temkin model The Temkin isotherm [] descries the ehviour of the dsorption systems on heterogeneous surfces. This model ssumes tht: () the dsorption het of ll the molecules in the lyer decreses linerly with the coverge due to the dsorte dsorte interctions nd () the dsorption is chrcterized y homogeneous distriution of inding energies, up to some mximum inding energy. The Temkin eqution my e expressed s: q e = ( RT ) Ln(K T ) (6) If RT/ = M T ; then Eq. (6) cn e linerized s: q e = M T Ln K T + M T Ln (7) where K T (L/mmol) is the inding equilirium constnt corresponding to the mximum inding energy, nd M T (mmol/g) is relted to the het of dsorption. A plot of q e vs. Ln mkes possile the determintion of M T nd K T from the slope nd intercept, respectively. Tle nd Fig. show the results of the fits to the Temkin eqution nd the vlues of r. The vlues of the equivlent re occupied (A PNP ), s well s the corresponding coverge fctor (C.F.: A PNP /A BET ) reched t the mximum initil concentrtion used (C o :.57 mmol/l) hve een clculted. According to Eq. (8), ny q e vlue cn e trnsformed to PNP equivlent surfce re (A PNP ) which would cover g of AC ssuming tht the PNP molecules tke prllel orienttion (flt.5. dsorption) on the AC surfce nd the surfce re covered y ech molecule dsored is 5.5 Å [5]: ( ) ( ) m PNP m ( PNP mmolpnp ) A PNP = 6. q e (8) g AC mmol PNP g AC Tle shows the vlues of the coverge fctors for the PNP dsorption. Those re in the rnge from to 8% of the BET specific surfce res. When compring the different ACs (except E with E), we hve oserved tht the coverge fctors increse s urn offs increse. This is the consequence of the ctivtion grde influence, s comintion of the micro nd mesoporosity developed. Fig. shows the mximum dsorption cpcity (C o :.57 mmol/l) of PNP vs. the totl pore volume for ll the studied ACs. A liner-like trend hs een otined for E series; it confirms the fvorle effect of the ctivtion step. E hs very low cpcity (.7 mmol/g) while E shows very high cpcity (. mmol/g). This is in greement with their internl surfce re development, nd more especilly with the volume of mesopores... Effect of the ph on the dsorption The dsorption of PNP t different ph (from to ) is depicted in Fig. 5. The removl of PNP ws the mximum t cid ph, ut it ws slightly decresing, nd when the ph of the solution incresed to vlues higher thn 7, especilly up to lkline, the dsorption of PNP decresed shrply. PNP ( wter solule solid) is modertely cid in wter (pk = 7.5). When the solution ph is cid; for exmple ph = ; then: [p-nitrophenol]/[p-nitrophenolte] =. 5, lmost ll the q e (mmol/g) PNP on E C o :.79 mmol/l T: 9 K w:. g/l 6 8 ph Fig. 5. ph effect on the dsorption equilirium of PNP.

5 8 L.M. Cotoruelo et l. / Chemicl Engineering Journl 8 () 76 8 q t (mmol/g) t (min) 8 ph :. ph : 6.6 ph : 7.9 ph :. ph : 8.9 Fig. 6. Kinetic of PNP dsorption on E6 t different ph. C o:.79 mmol/l; T: 9 K; w:. g/l. q e (mmol/g) molecules of PNP in solution re in their moleculr form. At low ph, there is nerly no electrosttic repulsion; so, the dsored mount is the mximum. The lrgest mount dsored t low ph my e the result of the strong chemicl ond etween the lonely pir of electrons on the hydroxyl group in PNP nd the surfce of AC, s well s the existing forces from the dispersive interctions etween the romtic ring of the PNP nd specific sites on the ctivted cron surfce. At high ph vlues, the PNP molecules re more ionized to p-nitrophenolte nions; the numer of negtively chrged sites increses nd the numer of positively chrged sites decreses. The surfce groups on the AC ecome nionic (negtive chrge), s croxilte ( COO ) nd phenolte (Ph O ) ions, nd they re not fvorle for the dsorption of nions. So, the shrp decrese in the dsorption cpcity t sic ph my e ttriuted to the electrosttic repulsion etween the p-nitrophenolte molecules nd the negtively chrged dsorent surfce. Moreover, the low dsorption t high ph is proly due to the presence of OH ions competing with the phenoltes for the residul positively chrged dsorption sites on the ACs surfce. As complementry study, the kinetic dsorption dt for PNP on E6 t different ph hve een otined (Fig. 6). It cn e oserved tht t low ph, the dsorption cpcity nd the rte increse s consequence of the occurring strong ttrction forces. Nevertheless, it my not e prcticl to mintin extreme ph vlues ecuse it would need ddition of chemicls to lower the ph nd the effluent would need finl neutrliztion step efore its dischrge... Effect of the temperture on dsorption The temperture influences the dsorption equilirium nd its vritions produce displcement from or towrd the phse dsored. Also, n increse in temperture generlly improves the soluility of the molecules (if in liquid phse) nd their diffusion within the pores of the dsorent mterils. Fig. 7 shows the dsorption cpcity of B nd E t different tempertures. The isotherms show similr ehviours, with strong ffinities of the PNP molecules for the surfces, s evidenced y the initil slopes. The mounts dsored of PNP rose with the decrese of the temperture, indicting the pprent exothermic nture of the entire process. Similr effect hs een reported y other uthors [6, ]. In generl, physicl dsorption processes re typiclly exothermic; n increse in temperture cuses rection in the opposite direction, i.e., this produces desorption of molecules from the less energetic ctive centres, resulting displcement in the equilirium towrd the fluid phse. The thermodynmic nlysis provides informtion out the process ility to tke plce nd the stility of the dsored phse s well. The chnges on the free energy ( G) hve een estimted t 9 K for ll the ACs, nd t (mmol/l) Fig. 7. Experimentl dt (dots) nd GAB fits (curves) for the dsorption isotherms of PNP t different tempertures: () B; () E. M GAB in (mmol/g), K, K in (L/mmol). four tempertures for B nd E. In ddition, for these ltter ACs, the enthlpy ( H) nd the entropy ( S) chnges hve een lso clculted. The free energy of dsorption hs een clculted from the expression derived from the Gis dsorption eqution [5]. If we refer the free energy chnge per mol of solute dsored, the following finl expression cn e otined: G = RT q e d (9) q e where q e is relted to y the dsorption isotherm. In the present work, s criterion, the fits of the GAB eqution hve een used for the clcultion of the equilirium reltions. The G vlues (kj/mol) hve een clculted using Eq. (9) y numericl integrtion. In this cse, G depends on the temperture nd the grde of coverge (q e ). Tle shows the vlues of G t vrious levels of surfce coverge ( :.8,., nd. mmol/l). The negtive vlues of G indicte the fesiility of the processes nd the spontneous nture of the PNP dsorption on ll the ACs. Although the increse in induces higher mount dsored in the equilirium, we hve found tht, in some of cses, the solute vlues of G get lower. At high, the verge relesed energy descends when covering gret vriety of more nd less ctive sites. By contrst, for B, E nd E, with low urn off, A BET, nd porosity developed, G increses (verge molr vlues) little when increses. It seems tht it is consequence of their ctive sites distriution for dsorption, which hs to tke plce on the most ctive sites, minly locted the microporosity plces inside. On the other hnd, B nd E hve showed little or negligile increse in G when the temperture decresed. These complex phenomen re due to the dsorents surfce heterogeneity s well s the vriety of possile comined mechnisms to orgnize nd to drive the mss trnsfer steps..6.

6 L.M. Cotoruelo et l. / Chemicl Engineering Journl 8 () Tle Thermodynmics for dsorption t different equilirium concentrtions. (mmol/l), T (K), G nd H (kj/mol), S (J/mol K). AC T =.8 =. =. G H S T ( G=) G H S T ( G=) G H S T ( G=) A B B C D E E E E The enthlpy nd the entropy chnges for the dsorption of PNP on B nd E in rnge of temperture etween 8 K nd K hve een estimted y the Gis Helmholtz eqution: G = H T S () The vlues H nd S hve een determined from the intercept nd the slope, respectively, y plotting G vs. T (Fig. 8). The negtive vlues of H confirm the exothermic nture of the overll process (Tle ). Contrry to G, the solute vlues of H decresed little s the coverge level grew. This sitution is frequent in the dsorption on non-homogeneous surfces. It cn e explined ecuse the process is esier on the surfce sites t low coverge level, ttending to energy resons. The fits from Eq. () hve determined negtive vlues of S which result from lower grde of freedom of the molecules in the dsored phse thn in the fluid phse. In these cses ( H < ; S < ), the process cn only occur when T S < H. The process hs to ecome exothermic enough to overtke the disdvntge due to the negtive chnge ΔG (kj/mol) :.8 mmol/l r =.96 :. mmol/l r =.95 :. mmol/l r =.8 :.8 mmol/l r =.89 :. mmol/l r =.7 :. mmol/l r = T (K) Fig. 8. Thermodynmic estimtions for the dsorption of PNP on B nd E t different surfce coverge: () B; () E. 6 8 of entropy. If the temperture vlue is enough high, the term T S cn surpss the term H, nd the process does not tke plce. The highest vlues of T for the different cses nlyzed ccording to spontneous process hve een reported s T ( G=) in Tle. Nevertheless, the low ccurcy otined from the simplicity of Eq. () s used, with its liner regressions, llow thinking tht these results re numericl estimtions nd reltive trends for the thermodynmic mgnitudes nd their dependence on the vriles. One of these reltes to the porous structure of the dsorents, ecuse its ccessiility in energy terms vries s the dsorption tkes plce. Therefore, the vlues clculted for G, H nd S my represent the pprent ehviours ssocited with the dsorption processes s they hve een studied; t the intervl of tempertures nd the other experimentl conditions set..5. Kinetic study.5.. Empiricl model for the fluid phse The solid liquid dsorption processes on porous mterils my e descried s series of steps: externl mss trnsfer from the liquid phse to the solid surfce through the oundry lyer, internl diffusion within the porous prticle, nd dsorption on the surfce. For the present study, the results from D nd E were selected s exmples, on the sis of their surfce properties nd dsorptive ehviours. The mount q t (mmol/g) of PNP dsored on the AC ws clculted s the chnge in the queous phse concentrtion from the initil vlue y: q t = C o C t () w where C t (mmol/l) is the dsorte concentrtion t certin contct time t. At the sight of the shpe of the kinetic plots for D nd E, s showed in Fig. 9, n empiricl model ws used for the dsorte concentrtion decy curves in the queous phse. The experimentl results for the PNP dsorption were fitted to the following second order exponentil decy eqution: C t = y o + A exp( B t) + A exp( B t) () dq t dt = A B w exp( B t) + A B w exp( B t) () where y o, A, B, A, nd B re empiricl coefficients. Their vlues re shown in Fig. 9. The plot ttched to Fig. 9 shows the vlues derived from Eq. (). It is oserved shrp decrese in the dsorption rte during the first minutes of contct time. This step

7 8 L.M. Cotoruelo et l. / Chemicl Engineering Journl 8 () 76 8 C t (mmol/l) t (min) Fig. 9. Experimentl dt (dots) nd fits (curves) for kinetic of PNP dsorption on D nd E. C o:.79 mmol/l; T: 9 K; w:. g/l. y o, A, A in (mmol/l); B, B in (min ). corresponds to the surfce coverge in the widest pores nd the most ctive sites. A second step develops for 5 min pproximtely with n intermedite decresing rte. This step is the most interesting to the kinetic control for its reltion to the coverge into the nrrow mesopores nd micropores. The lst prt developed negligile dsorption rte which leds to the equilirium concentrtions, so extended s severl dys of contct time. The dsorption rte ws higher on D thn on E. It cn e explined ecuse of the first s higher surfce re nd pore volume (Tle )..5.. Kinetic model for the solid phse The kinetic dt hve een nlyzed y the Lgergren model [6] for order n; it is: dq t = k(q m q t ) n () dt where k is the dsorption rte constnt referred to the dsorte concentrtion on the solid, n is the pprent order of the process relted to the ville dsorent concentrtion (q m q t ), which is the driving force of the process, tht is, the vcnt nd ccessile ctive sites in the surfce of the dsorent, for ny time of contct. In this work, q m (mmol/g) represents the symptotic q t vlue in the function in the given experimentl conditions. However, the selected eqution (with its prmeters evluted for min of contct time) cnnot e used to predict the equilirium concentrtions (dt s fr s 8 dys). The vlues of q m my e clculted s: q m = C o y o { } D :.97 mmol/g E :.6 mmol/g (5) w In ddition, y integrtion of Eq. () for n =, we hve [7]: t = q t kq + t (6) q m m The vlues of k (g/mmol min) nd q m, cn e otined from the intercept nd the slope of the stright lines in the plots (t/q t vs. t; Fig. ). The vlues (q t, t) were those supplied y the experimentl fittings of the decy model. The liner regressions from Eq. (6) were cceptle, nd they provided q m vlues very close to those from Eq. (5), which verify the pplicility of this kinetic eqution for n pprent second order in the dsorption process of PNP. 8 t/q t (min g/mmol) D E q m :.98 k:. q m :.69 k:.6 liner fits r : t (min) Fig.. Kinetic order plots for PNP dsorption on D nd E. C o:.79 mmol/l; T: 9 K; w:. g/l; q m in (mmol/g); k in (g/mmol min). tht is to sy, single prmeter, effective or pprent tht we will denominte D i ; wht is representtive of ll the positions into the prticle. This pproch voids to differentite mong diffusion in the pore spces (pore diffusion), diffusion cross the surfces which cn occur susequent to dsorption (surfce diffusion), nd diffusion in the solid mteril itself. Crnk [9] developed the following eqution for sphericl prticles: ) q t q e = 6 n n= exp ( Di n t R P 8 (7) where R P is the rdius of the dsorent prticle. This eqution provides nd exct solution for the sitution often descried s infinite th, which ssumes constnt surfce concentrtion for the solute, nd negligile externl film resistnce. For short times (St), or more precisely: q t /q e <.; D i ecomes D St, nd Eq. (7) my e pproximted to: ( qt ) R P D St = (8) q e 6t For long times (i.e. Lt; D Lt ), the highest terms in the series of Eq. (7) ecome extremely smll, nd only the first term hs prevlence. We hve [ ] D Lt = R P t Ln 6/ (9) (q t /q e ) Due to mthemticl resons D Lt vlues only my e positive if (q t /q e ) > [ (6/ )]. In the present work, we hve used Eqs. (8) nd (9) to determine the D i vlues (D St nd/or D Lt ) from the kinetic D (cm /s) E- E-.5.. Effective diffusion coefficients A numer of models to think out the min mss trnsfer resistnces (externl nd internl diffusion) hve een postulted, which differ in their description of the diffusion process into the pores [8]. The diffusion process in the homogeneous sphere model cn e expounded y mens of n eqution involving constnt diffusivity, E- 6 8 t (min) Fig.. Effective diffusion coefficients. Vrition with the contct time nd with the lod dsored (enclosed).

8 L.M. Cotoruelo et l. / Chemicl Engineering Journl 8 () dt showed in Fig. 9. The q t vlues corresponding to the different times were supplied y the second order exponentil decy model. Fig. plots the effective diffusivity vlues s function of the contct time nd the mount dsored. The equtions did not predict constnt vlues. After severl minutes of contct time, the pprent diffusion coefficients decresed, ccording to the incresing difficulty tht the molecules find to penetrte into the porous structures. Multiple collisions mong molecules nd the pore wlls, s well s the prtil ostruction of the pore mouths cn occur s the sme time. The surfce res of the pores ecome grdully less ttinle to dsorption.. Conclusions Activted crons otined from lignin, t different experimentl conditions, hve een successfully employed s dsorents for quntittive removl of PNP from queous solution. Chemicl nd physicl chrcteristics, s surfce res, pore volumes nd pore sizes, determine the yields nd rtes expected for the potentil use of the ACs s dsorents. It hs een found tht the ctivted crons urn off influences on the dsorption cpcity. The thermodynmic nlysis indicted tht the PNP dsorption ws spontneous nd exothermic. The results from the sorption energy prmeters estimtion showed the physisorption nture of the dsorption processes. The kinetic nlysis reveled dsorption rtes with second pprent order relted to the ville ctive sites on the ctivted crons surfces. The effective diffusion coefficients were low, in the rnge from to cm /s, decresing with the time, wht suggests tht the internl porosity controls the rte of the mss trnsfer towrd the ultimte equilirium stte. Acknowledgments The uthors cknowledge the Spnish DGICYT-MEC (Project CTQ 9-6) nd the Junt de Andlucí (TEP-8) for the finncil support. References [] K.E. Noll, V. Gounris, W. Hou, Adsorption Technology for Air nd Wter Pollution Control, Lewis Pu. Inc., Chelse, 99. [] A. Wolorsk, Adsorption on ctivted cron of p-nitrophenol from queous solution, Wter Res. (989) [] C. Moreno-Cstill, J. River-Utrill, M.V. López-Rmón, F. Crrsco-Mrín, Adsorption of some sustituted phenols on ctivted crons from ituminous col, Cron (995) [] S.K. Srivstv, R. Tygi, Competitive dsorption of sustituted phenols y ctivted cron developed from the fertilizer wste slurry, Wter Res. 9 (995) [5] R. Leod, B. Chrms, Evlution of surfce re of cron component of model cron-silic dsorents from dsorption dt of p-nitrophenol from queous solutions, Colloids Surf. A 5 (998) [6] S. Dutt, J.K. Bsu, R.N. Ghr, Studies on dsorption of p-nitrophenol on chrred sw-dust, Sep. Purif. Technol. () 7 5. [7] S. Hydr, M.A. Ferro-Grcí, J. River-Utrill, J.P. Joly, Adsorption of p- nitrophenol on n ctivted cron with different oxidtions, Cron () [8] A.P. Terzyk, M. Wisniewski, P.A. Guden, G. Rychlicki, S. Furmnik, Cron surfce chemicl composition in pr-nitrophenol dsorption determined under rel oxic nd noxic conditions, J. Colloid Interfce Sci. (8) 5. [9] M. Ahmruzzmn, S. Lxmi Gytri, Btch dsorption of -nitrophenol y cid ctivted jute stick chr: Equilirium, kinetic nd thermodynmic studies, Chem. Eng. J. 58 () 7 8. [] B. Petrov, B. Tsyntsrski, T. Budinov, N. Petrov, L.F. Velsco, C.O. Ani, Activted cron from col tr pitch nd furfurl for the removl of p-nitrophenol nd m-minophenol, Chem. Eng. J. 7 () 8. [] S. Suresh, V.C. Srivstv, I.M. Mizr, Adsorptive removl of phenol from inry queous solution with niline nd -nitrophenol y grnulr ctivted cron, Chem. Eng. J. 7 () 997. [] M.B. Hody, P.H.Y. Li, D.M. Crewdson, S.K. Bhrgv, The use of low rnk colsed dsorents for the removl of nitrophenol from queous solution, Fuel 7 (99) [] J. Rodríguez-Mirsol, T. Cordero, J.J. Rodríguez, Activted crons from CO prtil gsifiction of euclyptus krft lignin, Energy Fuels 7 (99) 8. [] J. Rodríguez-Mirsol, T. Cordero, J.J. Rodríguez, Preprtion nd chrcteriztion of ctivted cron from euclyptus krft lignin, Cron (99) [5] C.H. Giles, A.P. D Silv, A.S. Trivedi, Use of p-nitrophenol for specific surfce mesurement of grnulr solids nd fires, J. Appl. Chem. (97) 7. [6] C.H. Giles, T.H. McEwn, S.N. Nkhw, D. Smith, Studies in dsorption. Prt XI. A system of clssifiction of solution dsorption isotherms, nd its use in dignosis of dsorption mechnisms nd in mesurement of specific surfce res of solids, J. Chem. Soc. (96) 97. [7] C.H. Giles, S.N. Nkhw, Studies in dsorption. XVI. The mesurement of specific surfce res of finely divided solids y solution dsorption, J. Appl. Chem. (96) [8] L.M. Cotoruelo, M.D. Mrqués, F.J. Díz, J. Rodríguez-Mirsol, T. Cordero, J.J. Rodríguez, Activted crons from lignin: their ppliction in liquid phse dsorption, Sep. Sci. Technol. (7) [9] L.M. Cotoruelo, M.D. Mrqués, J. Rodríguez-Mirsol, T. Cordero, J.J. Rodríguez, Adsorption of romtic compounds on ctivted crons from lignin: Equilirium nd thermodynmic study, Ind. Eng. Chem. Res. 6 (7) [] E.A. Guggenheim, Appliction of Sttisticl Mechnics, Clrendon Press, Oxford, 966. [] D.D. Do, Adsorption Anlysis: Equiliri nd Kinetics, Series on Chemicl Engineering, vol., Imperil College Press, London, 998. [] J.M. Chern, Y.W. Chien, Adsorption of nitrophenol onto ctivted cron: isotherms nd rekthrough curves, Wter Res. 6 () [] D. Tng, Z. Zheng, K. Lin, J. Lun, J. Zhng, Adsorption of p-nitrophenol from queous solutions onto ctivted cron fier, J. Hzrd. Mter. (7) [] J.M. Li, X.G. Meng, C.W. Hu, J. Du, Adsorption of phenol, p-chlorophenol nd p-nitrophenol onto functionl chitosn, Bioresour. Technol. (9) [5] J.P. Bell, M. Tsezos, Removl of hzrdous orgnic pollutnts y iomss dsorption, J. Wter Pollut. Control Fed. 59 (987) [6] S. Lgergren, Aout the theory of so-clled dsorption of solule sustnces (Zur theorie der sogennnten dsorption geloester stoffe), K. Sven. Vetenskpskd Hndl. (898) 9. [7] Y.S. Ho, C.C. Ching, Sorption studies of cid dye y mixed sorents, Adsorption 7 () 9 7. [8] A. Leitão, E. Conceiç ão, R. Sntos, A. Rodrigues, Modelling of solid liquid dsorption: effect of dsorent lods on model prmeters, Cn. J. Chem. Eng. 7 (99) [9] J. Crnk, The Mthemtics of Diffusion, Clrendon Press, Oxford, 975.