Kinetic Study and Equilibrium Isotherm Analysis of Reactive Dyes Adsorption onto Cotton Fiber

Transcription

1 ture nd Science 2010;8(11) Kinetic Study nd Equilirium Isotherm Anlysis of Rective Dyes Adsorption onto Cotton Fier A. M. Gml, S. A. Ao Frh, H. B. Sllm, G. E. A. Mhmoud nd L. F. M. Ismil* Al-Azhr University - Fculty of Science, Chemistry Deprtment, sr City, Ciro, Egypt. Lilfmi@yhoo.com Astrct: This study ws to evlute the dsorption cpility of fier to rective dyes (C.I. Rective Red 120 (RR120) nd C.I. Rective Blck 5 (RB5)) in queous solution. The experiments were crried out in tch system to optimise opertion vriles: dye concentrtion, ph, nd temperture. In ddition, dsorption of rective dyes onto fier with using sodium edte (SE), sodium citrte (SC) nd sodium sulphte (SS) ws lso investigted nd the results otined were compred. The results show tht the presence of orgnic slts (SE nd SC) significntly enhnce the dye uptke (high Q, nd Q f vlues) onto fier compred with the results otined in queous nd in the presence of sodium sulphte. Thus, presence of orgnic slts (SE nd SC) is n lterntive wy to susequently increse the ffinity etween RR120 nd RB5 dyes nd the fier. Moreover, the Lngmuir nd Freundlich dsorption models were pplied to descrie the equilirium isotherms t different opertion prmeters nd isotherm constnts. The Lngmuir model greed very well with experimentl dt. The pplicility of the Lngmuir isotherm suggests monolyer coverge of the two rective dyes on surfce of fier. The kinetics of the dsorption with respect to the initil dye concentrtion, temperture, ph nd different slts were investigted. The pseudo-first-order, second-order kinetic models nd intrprticle diffusion model were used to descrie the kinetic dt nd the rte constnts were evluted. The dynmicl dt fit well with the second-order kinetic model. Intr-prticle diffusion studies reveled tht, the dsorption rtes were not solely controlled y the diffusion step. Further thermodynmic investigtions showed tht, the dsorption is n endothermic process. The ctivtion energies (E ) for the pseudo second-order kinetics nd intrprticle diffusion kinetics for RR120 nd RB5 re energeticlly fvorle with different slts t different dye concentrtion nd the dsorption process is comintion of chemicl nd diffusion processes [ture nd Science 2010; 8(11):95-110] (ISS: ). Key words: Adsorption; Cotton fier; Isotherm; Kinetics; Thermodynmics. 1. Introduction: Cotton is n undnt nturl fier which consists of prcticlly pure cellulose (out 88-96%) [1,2]. turl cellulose fiers crry smll negtive chrge ( plteu = -11 mv) due to the presence of some croxylic cid groups from oxidtion t the primry hydroxylic sites [3]. At ph higher thn 8, some of the hydroxyl groups on the hydroxymethyl side chins my lso e ionized incresing the negtive chrge significntly [4]. The negtive chrges on the surfce of cellulose repel nionic dyes nd hence the efficiency of dye fixtion on cellulosic fiers is generlly low. To counter this prolem, numer of studies on dyeing hve een crried out to improve the dye uptke nd fstness properties [1] Exhustive dyeing of with rective dyes requires the presence of electrolytes (Cl or 2 SO 4 ), which suppresses negtive chrge uild-up t the fier surfce nd promotes incresed dye-uptke [5, 6]. Progress hs een mde in reducing slt requirements for some newer rective dyes, ut slt concentrtions re still too high [7]. Recently, reduced slt y the replcement of noniodegrdle inorgnic slt (sodium chloride) with iodegrdle orgnic one (tetrsodium edte nd trisodium citrte) in the exhustive dyeing of using rective, direct nd soluilised vt dyes hs een reported [5, 8]. Moreover, high wet fstness in dyeing of cellulosic textiles is est gined y the use of rective dyes; these re the only colornts which link covlently to cellulose. Fixtion is invrily chieved in the presence of queous lkli. Under these conditions, nucleophilic celluloste ion, Cell-O -, is in competition with hydroxide ion, OH -, for rection with the electrophilic rective group(s) of the dye [9]. It is shown tht the dye fixtion to the fric is controlled y solid liquid interfcil process; however, the rte of this rection is governed y the vilility of sites for the dsorption of dye molecules on the fric surfce [10]. The dsorption of dye molecules on the fric 95

2 ture nd Science 2010;8(11) surfce is sic step in the dyeing process. Consequently, the im of this work is to evlute the dsorption cpcity of fier to rective dyes, nmely C.I. Rective Red 120 (RR120) nd C.I. Rective Blck 5 (RB5). The reserch focused on evluting how the process opertion prmeters of initil dye concentrtion, ph, effect of noniodegrdle inorgnic slt (sodium sulphte) nd iodegrdle orgnic slts (tetrsodium edte nd trisodium citrte) nd temperture ffect the dsorption cpcity. The est-fit equilirium isotherms re determined y Freundlich nd Lngmuir models. Adsorption kinetic models re employed to nlyse the kinetics nd mechnisms of RR120 nd RB5 dsorption onto fier. 2. Experimentl : 2.1. Mterils nd chemicls Cotton fier preprtion Mill desized, scoured nd leched fric (Poplin) is supplied y Misr Compny for spinning nd weving, Mehl El-Kur, Egypt. To remove the wx nd impurities, the fier ws dded to oiling wter (2 l) which were dded to solution contining 4g/l sodium cronte nd 2g/l nonionic detergent (nstpon). The mixture ws then oiled for 4 hours. The ws then removed, wshed with hot wter nd cold wter nd ir dried t room temperture [11] Dyestuffs nd chemicls Two rective dyes (C.I. Rective Red 120 (Procion HE3B, RR120) nd C. I. Rective Blck 5 (Procion B, RB5)) re otined from ICI (Impericl Chemicl Industries Limited Dyestuffs Division Mnchesester, Englnd). The dyes re of commercil grde nd re used s received (Scheme 1). Other chemicls (Sodium Cronte, Sodium Sulphte (SS), Tetrsodiumedte SE) Trisodiumcitrte (SC), Detergent nstpon, Sodium hydroxide, nd itric cid) from BDH nd Merk re used s received Instruments The sorption spectr re recorded with JEWAY UV-Visile spectrophotometer. The sornce of solutions mesured using 1cm qurtz cell. (U.K.). A ph meter (DATALOGGER 6209; JECO ELECTROICS LTD, U.S.A.) is used to mesure the ph vlues of the dye solutions. A thermosttted shker th (Heto-Holten A/S Denmrk, Type SBD-50 cold), operted t 75 rpm, nd is used to study the kinetic dsorption of rective dyes onto fier. C.I. Rective Red 120 (RR120): Cl Cl O 3 S OH H H H H OH SO 3 O 3 S SO 3 O 3 S SO 3 C.I. Rective Blck 5 (RB5): O 3 SOCH 2 CH 2 O 2 S H 2 OH SO 2 CH 2 CH 2 OSO 3 O 3 S SO 3 Scheme 1: Chemicl structure of the studied dyes Adsorption experiment Adsorption of RR120 nd RB5 on fier is crried out in tch system. A desired mount of fier (1g) is dded to 30ml of known concentrtion of RR120 or RB5 solutions (L: R = 1:30). The dyes solutions re prepred to the desired concentrtions using Milli-Q wter. The ph is djusted with 0.1M HO3, nd 0.1M OH. The mixture is gitted t 150rpm in rotry shker (Rtek OM 15 oritl mixer, Austrli) t 30 C. The experiments re crried out over 4 hours to ensure tht dsorptive equilirium is otined. The smples re withdrwn from the experimentl flsk t pre-determined time intervls until dsorption equilirium is chieved. Then, the fier is 96

3 ture nd Science 2010;8(11) seprted. The residul dyes concentrtions re determined colorimetriclly. All experiments re crried out in triplicte, nd the verge vlues re tken to minimise rndom error Anlyticl method Dye concentrtion nd removl cpcity Dye concentrtion is determined colorimetriclly y mesuring t mximum sornce of the two dyes ((λ mx = 512 nm nd 597 nm for RR120 nd RB5, respectively)). Clirtion curve is plotted etween sornce nd concentrtion of the dye solution to otin the sornce-concentrtion profile for the two dyes. The mount of dye uptke per grm of ( ) t ny time ( ) is clculted y mss lnce reltionship Eq. (1) s follows [12]: =(C 0 -C t )(V/W) (1) where C 0 is the initil dye concentrtion (mg/l) nd C t is the dye concentrtion fter dyeing time t (mg/l), V is the volume of dye solution (ml) nd W is the weight of fier (g) used. The dsorption cpcity re determined t different opertion prmeters tht ffect the dsorption of RR120 nd RB5 onto fier (initil dye concentrtion ( mg/l for RR120 nd mg/l for RB5), ph ( ), effect of inorgnic slt (sodium sulphte [SS] = 0-60g/l in presence of 20 g/l 2 CO 3 ) nd orgnic slts (sodium edte [SE] = 0-60g/l nd sodium citrte [SC]= 0-60g/l) nd temperture(-90ºc). 3. Results nd Discussion: 3.1. Effect of contct time Adsorption isotherms re usully determined under equilirium conditions. A series of contct time experiments for the two rective dyes hve een crried out with initil dyes concentrtions of mg/l nd mg/l, L.R.1:30 nd ph nd in presence of 50g/l SS nd 20g/l sodium cronte used in fixtion t tempertures nd 60 o C for RR120 nd RB5, respectively (Figure 1). Moreover, dsorption in presence of g/l SE nd 25g/l SC is lso studied. Figures 1 nd 2 show the contct time necessry for rective dyes to rech sturtion is >100 mins. As cn e seen from Figure 1, the mount of the dsored dye onto fier increses with time nd, t some point in time, reches constnt vlue eyond which no more is dsored from solution. At this point, the mount of dye eing dsored onto fier is in stte of dynmic equilirium with the mount of the dye desoring from the fier. The time required to ttin this stte of equilirium is termed the equilirium time, nd the mount of dye dsored t the equilirium time reflects the mximum dsorption cpcity of the fier under those operting conditions [13, 14] Effect of temperture The effect of temperture on the dsorption of RR120 nd RB5 dyes on is investigted in the temperture rnge of -90 ºC t initil dye concentrtion mg/l nd mg/l, L.R.1:30 nd ph nd in presence of 50g/l SS nd 20g/l sodium cronte used in fixtion for RR120 nd RB5, respectively (Figure 1). Moreover, dsorption in presence of g/l SE nd 25g/l SC is lso studied. Figures 1 nd 2 show tht the dsorption of RR120 nd RB5 dyes on enhnced with rising the temperture up to ºC nd 60 ºC for oth dyes indicting tht high temperture fvored the dye dsorption onto fier. However, temperture higher thn o C nd 60 o C for RR120 nd RB5 dyes, respectively results in decrese in the dsorption, which my e ttriuted to decrese in dye molecule stility t higher tempertures [15]. The increse in temperture would increse the moility of the lrge dye ions s well s produce swelling effect with the internl structure of the fier, thus enling the lrge dye molecules to penetrte further [16, 17]. This my lso e result of n increse in the moility of the dye molecule with n increse in their kinetic energy, nd the enhnced rte of intrprticle diffusion of sorte with the rise of temperture. It is cler tht the sorption of RR120 nd RB5 dyes onto fier is n endothermic process nd RR120 nd RB5 dyes sorption my involve chemicl sorption [18]. 3.3 Effect of initil dyes concentrtions The dsorption cpcity of RR120 nd RB5 dyes on is function of the initil dye concentrtion. As expected, under equilirium conditions the mount of the dyes dsored on increses with n increse in the initil concentrtion of dyes solutions ( mg/l for RR120 nd mg/l for RB5, L.R.1:30 nd ph nd in presence of 50g/l SS nd 20g/l sodium cronte used in fixtion t tempertures nd 60 o C for RR120 nd RB5, respectively (Figure 2 for RR 120, similr ehviour is otined in cse of RB5). Moreover, dsorption in presence of g/l SE nd 25g/l SC is lso studied. It cn e proposed tht n increse in the initil dye concentrtion leds to n increse in mss grdient etween the solution nd dsorent ( fier), nd thus cts s driving force for the trnsfer of dye molecules from 97

4 ture nd Science 2010;8(11) ulk solution to the fier surfce. The increse in the proportionl dye dsorption is ttriuted to the equilirium shift during the dsorption process [12, 19 nd 20]. Moreover, RR120 hs greter moleculr weight thn RB5; it gives etter dsorption cpcity. It is ovious tht the dye fixtion is relted to dye molecule structure. Lrger molecule usully mens more intermoleculr interction due to vn der Wls force etween the dye nd fric, contriuted gret prt in the dye fixtion [21] Effect of ph The ph of dye th is n importnt influencing fctor for the dsorption of RR120 nd RB5 dyes on. In this study, the ph of RR120 nd RB5 dyes solutions is vried in the ph rnge of 6.04 to with initil dye concentrtion mg/l nd mg/l nd L.R.1:30 for 2 hours dyeing time in presence of 50g/l SS nd 20g/l sodium cronte used in fixtion t tempertures nd 60 o C for RR120 nd RB5, respectively (Figure 3). Moreover, dsorption in presence of g/l SE nd 25g/l SC is lso studied. The mximum dsorption cpcity is oserved t high ph vlues (10.02 nd 11.01) for RR120 nd RB5 in presence of 50g/l SS nd 20g/l sodium cronte. In presence of g/l SE nd 25g/l SC the mximum sorption cpcity ws oserved t ph 9.52 nd 8.31 for RR120 nd RB5, respectively. The increse of RR120 nd RB5 dyes dsorption onto fier with incresing ph vlues my e explined y the electrosttic interction etween dye molecules nd negtively fier surfce. This result emphsizes the fct tht lkli is necessry for covlent ond fixtion using SS s this slt is neutrl electrolyte. Wheres SE nd SC re lkline polycroxylic sodium slts. Rective dyes contining chlorotrizinyl groups (RR120) rect with nucleophiles y n ctivted heteroromtic nucleophilic sustitution mechnism [9]. Chlorotrizine rective dye fixtion involves the formtion of covlent ond etween hydroxyl group, under lkline conditions in the fric, nd the dye molecule, y the nucleophilic displcement of chloride ion [10]. Moreover, nucleophilic ddition chrcterizes the dye-fric rection in which nucleophilic group in the fric dds cross n ctivted cron-cron doule ond in the rective group contin vinylsulphone moiety (RB5) [22] Effect of sodium sulphte Due to the presence of hydroxyl nd some croxylic cid groups, the nturl cellulose fiers crry negtive chrge [1, 3] wheres RR120 nd RB5 dyes re lso nion hence negtively chrged; there is electrosttic nion-nion repulsion etween dye nd cellulose. This mens tht electricl repulsion etween the RR120 nd RB5 dyes nd cellulose fiers in the dyeing process must e overcome y other forces of ttrction. For this reson, electrolytes such s sodium chloride or sodium sulphte hve een dded into the dye th to promote the dsorption of nionic dye on cellulose fiers [23]. Therefore, the effect of sodium sulphte (concentrtion rnge 0-60g/l) on the dsorption of RR120 nd RB5 dyes on is investigted in this study nd the results re shown in Figure 4. It is found tht n increse in the sodium sulphte concentrtion over the rnge 0-50 g/l led to n increse in the dye dsored on nd then remined constnt in the concentrtion rnge when fier is dyed with RR120 nd RB5 dyes solution. The results otined in our study cn e explined y the Donnn model [24]. According to this model, cellulose fier re negtively chrged [3, 4], nd dyeing occurs y the trnsfer of dye from the externl to the internl solution. Upon ddition of sodium sulphte (s n electrolyte) to the dye solution, the sodium ions ( + ) distriute etween the externl solution nd internl solution so tht the negtive chrge on the cellulose surfce is neutrlized or shielded, llowing the dye molecules to e dsored on the [3, 25] Effect of sodium edte nd sodium citrte The generlly ccepted model for dyeing of rective dyes into cellulosic frics consists of two phses i.e. dsorption nd diffusion phse nd fixtion phse [26]. Electrolytes such s sodium chloride or sodium sulphte hve een dded into the dye th to promote the dsorption of nionic dye on cellulose fiers [23]. Recently, reduced slt y the replcement of noniodegrdle inorgnic slt (sodium chloride) with iodegrdle orgnic one (trisodium citrte) in the exhustive dyeing of using rective, direct nd soluilised vt dyes hs een reported [8]. Moreover, the use of polycroxylic cid slts hve proved the most effective clss evluted, promoting higher exhustion nd fixtion of rective dyes on thn sodium chloride or sodium sulpht [5]. Sodium edte, ethylenediminetetrcetic cid, tetrsodium slt is sequestering gent tht works y complextion mechnism. The resulting complex remins solule nd innocuous under the condition of processing. This phenomenon hs een widely exploited in textile industries such s in dyeing to prevent precipittion of dyes y clcium nd mgnesium slts present in hrd wter nd thus producing rillint dyed goods The effect of SE nd SC concentrtion on 98

5 ture nd Science 2010;8(11) the dsorption of RR120 nd RB5 onto frics re conducted t different concentrtions (0-60 g/l). The dsorption cpcity on fier increses with incresing SE nd SC concentrtion (Figure 5). It is cler tht incresing the slt concentrtion from 0 to 50 g/l leds to pronounced effect on the dye uptke for oth dyes nd further increses, however, leds to mrginl increse. This result reflects the findings tht SE nd SC cts s fixing gent. The results show tht etter dsorption cpcity cn e otined using sodium edte nd sodium citrte Adsorption isotherms The Lngmuir isotherm which hs een successfully pplied to mny other rel sorption processes cn e used to explin the sorption of RR120 nd RB5 onto fier. A sic ssumption of the Lngmuir theory is tht sorption tkes plce t specific sites within the dsorent [27,28]. The most widely used two-prmeter eqution descriing the dsorption process hs the liner form [26]: C e /q e =1/Q+(1/Q)C e (2) For lower concentrtions, the following form of the Lngmuir eqution is found to e more stisfctory [20, 27, 33]: 1/q e =(1/Q)+(1/QC e ) (3) In the ove equtions, Q is the mximum mount of the dye sored per unit weight of fier to form complete monolyer coverge on the surfce ound t high equilirium dye concentrtion C e, q e is the mount of dye dsored per unit weight of fier t equilirium, nd is the Lngmuir constnt relted to the ffinity of the inding sites. The vlue of Q represents prcticl limiting dsorption cpcity when the surfce is fully covered with dye molecules nd ssists in the comprison of dsorption performnce [21]. The vlues of Q nd re clculted from the intercepts nd slopes of the stright lines of plot of 1/q e versus 1/C e. The dt otined from the dsorption experiment conducted during the present investigtion is fitted using different temperture (-90ºC) nd different slts (SS, SE nd SC) into the isotherm eqution. A plot of (1/q e versus 1/C e ) results is liner grphicl reltion indicting the pplicility of the ove model for different temperture in presence of 50 g/l SS nd 20 g/l sodium cronte s shown in Figure 6 nd for different slts (SE nd SC). The vlues re clculted from the slope nd intercept of different stright lines representing the different temperture nd different slts. The oserved liner reltionship is sttisticlly significnt s evidenced y the r vlues (which re close to unity). This indictes the pplicility of the isotherm (Lngmuir isotherm) nd the surfce. The Lngmuir isotherm constnts long with correction coefficients re reported in Tle 1. Dt in Tle 1 shows tht Q nd vlues increse with incresing temperture confirming the endothermic nture of the dsorption. Incresing the temperture reduces the viscosity of the solution nd increses the rte of diffusion of dye molecules [29]. The essentil chrcteristic of the Lngmuir isotherm cn e expressed in terms of dimensionless equilirium prmeter, such s the seprtion fctor (or) equilirium fctor (R L ) used in the following eqution [30]: R L =1/(1+C o ) (4) where is the Lngmuir constnt nd C o is the initil concentrtion of the dsorte in solution. This prmeter indictes tht isotherm will e shped ccording to the following dsorption chrcteristics: RL > 1 unfvourle; RL = 1 corresponds to liner; 0 < RL < 1 is fvourle nd RL =0 is irreversile [30] s given in Tle 1. It cn e seen tht the dsorption of RR120 nd RB5 onto fier will e fvorle. The Freundlich isotherm model is lso used to fit the experimentl dt for the dsorption of RR120 nd RB5 dyes on t ph nd 11.01, respectively t different tempertures (-90 ºC) nd in presence of different slts (SS, SE nd SC). The empiricl Freundlich eqution sed on heterogeneous surfce is given y Eq. (5)[31]: 1/n q e =Q f C e (5) where Q f is roughly n indictor of the dsorption cpcity nd 1/n of the dsorption intensity. The liner form of the model cn e expressed s follows: lnq e =lnq F +1/n(lnC e ) (6) C e, q e re the dye concentrtion (mg/l) nd the mount of dye dsored per grm of ( ) t equilirium, respectively. Q f nd n re the Freundlich constnts which re relted to dsorption cpcity nd intensity, respectively. Liner plots of ln q e versus ln C e (Figure 7) showed tht the dsorption follows the Freundlich isotherm model well with high correltion coefficients (r > 0.99) (Tle 1). The vrious constnts ssocited with the isotherm re the intercept, which is roughly n indictor of sorption cpcity (Q f ) nd the slope (1/n) sorption intensity. The vlues re recorded in Tle 1. The Freundlich isotherm hs een illustrted to e specil cse of heterogeneous surfce energies ut it cn e esily extended to this cse. The mgnitude of the exponent 1/n gives n indiction of the fvorility nd cpcity of the dsorent/dsorte system. Vlues where n > 1 represent fvorle 99

6 ture nd Science 2010;8(11) dsorption conditions. In most cses the exponent etween 1 < n < 10 shows eneficil dsorption [30, 32, 33]. Bsed on the correltion coefficient (r 2 ) shown in Tle 1 the dsorption isotherm cn e etter descried y Lngmuir eqution. Also, the Lngmuir eqution yields etter fit of the experimentl dt thn the Freundlich eqution. The pplicility of the Lngmuir isotherm suggests monolyer coverge of the two rective dyes on surfces of fier. Moreover, Tle 1 shows tht the presence of orgnic slts (SE nd SC) significntly enhnce the dye uptke (high Q, nd Q f vlues) onto fier compred with the results otined in queous nd in the presence of inorgnic slt (SS). Thus, presence of orgnic slts (SE nd SC) is n lterntive wy to susequently increse the ffinity etween RR120 nd RB5 dyes nd the fier Adsorption kinetics In order to investigte the mechnism of dsorption nd potentil rte controlling steps such s mss trnsfer nd chemicl rection, processes re used to test experimentl dt. Mny models such s homogeneous surfce diffusion model (lso known s pore nd diffusion model) hve een extensively utilized for tch rectors to descrie the trnsport of solutes inside dsorent prticles; however, the mthemticl complexity of there models mkes them inconvenient for prcticl use. Any kinetic or mss trnsfer representtion is likely to e glol. From system design viewpoint, lumped nlysis of kinetic dt is hence sufficient for prcticl opertion [14, 19, 21, 34-36]. The Lgergren s eqution, pseudo first-order eqution, is given s [36]: (d /dt)=k 1 (q e - ) (7) fter integrtion y pplying the oundry conditions = 0 t t = 0 nd = t t = t, Eq. (7) ecomes: ln(q e /(q e - )=k 1 t (8) Eq.(8) cn e rerrnged to otin liner form: ln(q e - )=lnq e k 1 t (9) where (q e )nd ( ) re the mounts of dye sored t equilirium nd t time (t) ( ), respectively, nd (k 1 ) is the equilirium rte constnt of pseudo-first-order sorption (min -1 ). A stright line of ln(qe-qt ) versus t (Figure 8) in presence of 50 g/l SS nd 20 g/l sodium cronte, suggests the pplicility of this kinetic model to fit the experimentl dt. The dt otined from the dsorption experiment conducted during the present investigtion is fitted using different temperture (-90ºC), different orgnic slts (SE nd SC) nd different dye concentrtions ( mg/l for RR120 nd mg/l for RB5). The rte constnts (k 1 ) using the Lgergren eqution, the equilirium dsorption density (q e ) clculted from the plots nd its corresponding regression correltion coefficient vlues re shown in Tles 2 nd 3. The equilirium dsorption cpcity, q e, is required to fit the dt, ut in mny cses q e remins unknown due to slow dsorption processes. For this reson, it is necessry to otin the rel equilirium dsorption cpcity, q e, y extrpolting the experimentl dt to t = or y using tril nd error method. Also, in mny cses, the first-order eqution of Lgergren does not fit well for the whole rnge of contct time nd is generlly pplicle over the initil stge of dsorption processes [19, 21, 37 ]. On the other hnd, pseudo second-order eqution sed on equilirium dsorption [19, 21, 35, 37 nd 38] is expressed s: d /dt=k 2 (q e2 - ) 2 (10) Integrting Eq. (10) for the oundry conditions t = 0 to t = t nd = 0 to = gives [39]: 1/(q e2 - )=1/ +k 2 t (11) where k 2 (g/mg min) is the rte constnt of second-order dsorption. If second-order kinetics re pplicle, the plot of t/qt versus t should show liner reltionship. There is no need to know ny prmeter eforehnd nd q e2 nd k 2 cn e determined from the slope nd intercept of the plot (Figure 9). Also, this procedure is more likely to predict the ehvior over the whole rnge of dsorption nd is in greement with chemicl sorption eing the rte-controlling step [21, 37], which my involve vlency forces through shring or exchnge of electrons etween dye nions nd dsorent. The clculted q e2 nd k 2 vlues for RR120 nd RB5 t different temperture, different dye concentrtion nd different slts re listed in Tles 2 nd 3. In order to nlyze the dsorption kinetics for the two rective dyes, the pseudo first- nd second-order kinetic models re used to nlyze the dt. Figure 8 shows plot of linerised form of the pseudo first-order model t mg/l nd mg/l concentrtions of RR120 nd RB5, respectively in presence of 50g/l SS nd 20g/l sodium cronte concentrtion used for fixtion for the initil 120 min of dsorption. The slopes nd intercepts of plots of ln(qe - qt ) versus t re used to determine the first-order rte constnt k 1 nd equilirium dsorption density q e1. Tles 2 nd 3 list the clculted results with the correltion coefficients. The clculted q e vlues otined from the first-order kinetic model do not give resonle vlues, which re different from experimentl q e vlues. This finding suggests tht the dsorption of rective dyes 100

7 ture nd Science 2010;8(11) onto fier is not first-order rection. The plot of the linerised form of the second-order model t mg/l nd mg/l concentrtions of RR120 nd RB5, respectively in presence of 50g/l SS nd 20g/l sodium cronte concentrtion used for fixtion re shown in Figure 9. The stright lines in plot of t/qt versus t show good greement of experimentl dt with the second-order kinetic model for the two dyes. The slopes nd intercepts of plots of the t/qt versus t re used to clculte the k 2 nd q e2. The vlues of the prmeters k 2 nd clculted nd experimentl q e nd of correltion coefficients re lso presented in Tles 2 nd 3. The correltion coefficients for the second-order kinetic model re greter thn for the two dyes. The clculted q e2 vlues lso gree very well with the experimentl dt in the cse of pseudo second-order kinetics. These results suggest tht ech of the dye sorption systems is not first-order rection nd tht the second-order model, sed on the ssumption tht the rte limiting step my e chemisorption involving vlence forces through shring or exchnge of electrons [12]. 3.9.Adsorption mechnism The dye sorption is usully governed y either the liquid phse mss trnsport rte or the intr-prticle mss trnsport rte [12]. However, most of the dsorption processes my e descried y either the externl mss trnsfer (oundry lyer diffusion) or intrprticle diffusion model. The lter is given y the Weer Morris eqution [, 41]. =k i t 1/2 (12) where the prmeter k i is the diffusion coefficient vlue, t the time nd is the mount of dye dsored. Figure 10 shows the dt fitting to the ove eqution for RR120 nd RB5 dyes, wheres, the diffusion coefficient rte vlues, k i, otined from this eqution re tulted in Tles 2 nd 3 for the two dyes investigted in the present study. The intrprticle diffusion constnts (k i ) vlues re otined from the slope of the stright line portions of plot of versus t 1/2 for vrious RR120 nd RB5 concentrtions nd solutions tempertures s shown in Figure 10 nd Tles 2 nd 3. It is oserved tht intrprticle rte constnt vlues (k i ) increse with initil dye concentrtion. The oserved increse in k i vlues with incresing initil dye concentrtion in different slts (SS, SE, nd SC) t ph nd nd L:R 1:30 cn e explined y the growing effect of driving force resulted in reducing the diffusion of dye species in the oundry lyer nd enhncing the diffusion in the solid [42]. Also, s show in Tles 2 nd 3, incresing the temperture promote the pore diffusion in sorent prticles ( fric) nd enhnce the intrprticle diffusion rte. It is likely tht lrge numer of ions diffuse into the pore efore eing dsored [29]. Figure 10 shows tht the dsorption plots of fier re not liner over the whole time rnge nd cn e seprted into few liner regions. This my revel tht there re two or three dsorption stges tking plce. Lorenc-Growsk nd Gryglewicz [43] explined such multi-linerity stges tht the initil portion cn e ttriuted to externl surfce dsorption tht the dsorte diffuses through the solution to the externl surfce of the dsorent or the oundry lyer diffusion of solute molecules, where the dsorption rte is high. The second portion illustrtes the grdul dsorption stge, where intr-prticle diffusion rte is rte-controlling. The lst portion refers to the finl equilirium stge in which the intr-prticle diffusion strts to slow down nd level out s the extremely low dye concentrtion remins in the solution or mximum dsorption ws ttined. Generlly, when dsorption steps re not dependent of one nother; the plot of ginst t 1/2 should give two or more intercepting lines depending on the ctul mechnism [44]. The intr-prticle diffusion plots of the two dyes provide liner reltionship; however, none of the lines pssed through the origin. This indictes tht the intrprticle diffusion is involved in the dsorption process ut not the only rte-controlling step. Tht is, some other mechnisms such s complextion or ion-exchnge my lso control the rte of dsorption [45, 46] Activtion Prmeters The pseudo second-order model nd intrprticle diffusion model re identified s the est kinetic models for the dsorption of RR120 nd RB5 onto fric. Accordingly, the pseudo second-order rte constnts (k 2 ) nd intrprticle diffusion rte constnts (k i ) re dopted to clculte the ctivtion energy of the dsorption process of rective dyes onto fric using Arrhenius eqution (13) [29, 47]: lnk=lna-e /RT (13) where k, refer to either the rte constnts (k 2 ) of pseudo second- order model or intrprticle diffusion rte constnts (k i ) of intrprticle diffusion model, E is the ctivtion energy of sorption (kj/mol), R is the gs constnt (8.314 J/mol K), T is the solute temperture ( o K) nd A the Arrhenius fctor which is temperture independent fctor, respectively. The ctivtion energy is determined from the slope of the plot of lnk 2 or lnk i versus 1/T t ph nd nd L: R 1:30 in queous medium nd in presence of optimum concentrtion from different slts (SS), (SE) nd (SC) with constnt initil dye concentrtions nd for RR120, nd RB5 101

8 ture nd Science 2010;8(11) respectively (Figures 11 nd 12). The clculted ctivtion energy vlues re listed in Tles 4 nd 5. Moreover, the enthlpy (ΔH # ) nd entropy (ΔS # ) of ctivtion re clculted using the Eyring Eqution (14) [48, 49]: Ln(k/T)=ln(K /h)+δs # /R-ΔH # /RT (14) where (k ) nd (h) refer to Boltzmnn's constnt nd Plnck's constnt, respectively. The enthlpy (ΔH # ) nd entropy (ΔS # ) of ctivtion re clculted from the slope nd intercept of the plot of ln(k/t) versus 1/T. Gis energy of ctivtion (ΔG # ), cn e written in terms of enthlpy nd entropy of ctivtion (Eqution 15) [50-52]: ΔG # =ΔH # -TΔS # (15) The clculted vlues of (ΔG # ), (ΔH # ) nd (ΔS # ) for RR120 nd RB5 with initil dye concentrtions mg/l nd mg/l nd L.R. 1:30 in queous medium, in presence of 50g/l slt concentrtion (SS) nd 20g/l sodium cronte concentrtion used for fixtion t ph nd nd in presence of g/l dd t ph 9.52 nd 25g/l dd t ph 8.31 for SE nd SC, respectively (Figure 13 nd 14 ) re listed in Tles 4 nd 5. From Tles 4 nd 5, the positive ΔG # vlue suggests tht dsorption rections require energy to convert rectnts into products. The positive vlue of ΔH # confirms the endothermic process, mening the rection consume energy. The negtive vlue of ΔS # indictes tht the dsorption leds to order through the formtion of ctivted complex suggesting tht RR120 nd RB5 dsorption on fier surfce is n ssocited mechnism. Also the negtive vlue of ΔS # normlly reflects tht no significnt chnge occurs in the internl structure of the dsorent during the dsorption process [53, 54]. The mgnitude of the ctivtion energy (E ) yields informtion on whether the dsorption process is minly physicl or chemicl. Unuonh et l. [55] reported tht the ctivtion energy of physisorption is normlly not more thn 4.20kJ/mol. Consequently, the ctivtion energies (E ) for the pseudo second-order kinetics nd intrprticle diffusion kinetics listed in Tles 4 nd 5, for RR120 nd RB5 re energeticlly fvorle with different slts nd the dsorption process is comintion of chemicl nd diffusion processes [56]. inorgnic slt (sodium sulphte (SS)) nd orgnic slts (sodium edte (SE) nd sodium citrte (SC)) nd temperture. The dsored mount of RR120 nd RB5 incresed with incresing dye concentrtion, ph nd temperture. Moreover, presence of SE nd SC increses the dsored mount. The dsorption of RR120 nd RB5 onto fier in presence of SE nd SC ws successively chieved, nd these results re lso promising for the use SE nd SC to increse dsorption cpcity nd offer the potentil s n exhusting nd fixing gent for rective dyeing of. The dt otined from dsorption isotherms re well fitted to Lngmuir model. The pplicility of the Lngmuir isotherm suggests monolyer coverge of the two rective dyes on surfces of fier. The results otined in tch dsorption of RR120 nd RB5 onto fier showed tht the dsorption kinetics cn e explined y second order eqution etter thn Lgergren s first order. Kinetic studies showed tht dsorption profiles of ll mterils followed pseudo-second order model with multi-step diffusion process. The vlues of k 2, q e,exp nd q e,cl ll incresed with the temperture, suggesting tht incresing the temperture incresed the dsorption cpcity nd the dsorption rte. The regression results of the intrprticle diffusion model suggested tht intrprticle diffusion is not the only rte-controlling step. The ctivtion energies (E ) for the pseudo second-order kinetics nd intrprticle diffusion kinetics for RR120 nd RB5 re energeticlly fvorle with different slts t different dye concentrtions nd the dsorption process is comintion of chemicl nd diffusion processes. Corresponding uthor: L. F. M. Ismil Al-Azhr University - Fculty of Science, Chemistry Deprtment, sr City, Ciro, Egypt. Lilfmi@yhoo.com 4. Conclusion This investigtion exmined the equilirium nd the dynmic dsorption of two rective dyes (C.I. Rective Red 120 (RR120) nd C. I. Rective Blck 5 (RB5)) onto fier. The dsorption cpcity is determined t different opertion prmeters tht ffect the dsorption of RR120 nd RB5 onto fier (initil dye concentrtion, ph, effect of 102

9 ture nd Science 2010;8(11) Slt dd Aqueous nd ph [SS] = 50g/l, [ 2CO 3 ] = 20g/l nd ph 10.02(R R120) nd (RB5) [SE] = g/l nd ph 9.52 [SC] = 25g/l nd ph 8.31 Temp ºC Tle 1: Lngmuir nd Freundlich isotherm constnts of dsorption of RR120 nd RB5 onto fric t different tempertures, nd different slts. Q () Lngmuir isotherm (l/mg) R L RR120 r Freundlich isotherm Q f () n r Q () Lngmuir isotherm (l/mg) R L RB5 r Freundlich isotherm Q f () n ٩ r Tle 2: Kinetic prmeters for RR120 ( mg/l) nd RB55 ( mg/l) t different tempertures nd L.R. 1:30 y using different slts. RR120 RB5 Slt dd Temp. o C qe,exp qe1, cl Pseudo first-order model k1 x10 2 min -1 r1 qe2, cl Pseudo second-order model k2 x10 2 g / mg min h x10 min r2 Intrprticle diffusion mode ki x10 g/m min -1 ri qe,exp qe1, cl Pseudo first-order model k1 x10 2 min -1 r1 qe2, cl Pseudo second-order model k2 x10 2 g / mg min h x10 min r2 Intrprticle diffusion mode ki x10 g/m min -1 ri Aqueous nd ph [SS] = 50g/l, [2CO3] = 20g/l nd ph 10.02(RR120) nd 11.01(RB5) [SE] = g/l nd ph [SC] = 25g/l nd ph

10 ture nd Science 2010;8(11) Slt dd Aqueous nd ph [SS] = 50g/l, [ 2CO 3] = 20g/l nd ph (RR120) nd (RB5) [SE] = g/l nd ph 9.52 [SC] = 25g/l nd ph 8.31 [Dye] mg/l Slt dd Tle 3: Kinetic prmeters for RR120 nd RB5 with different initil dye concentrtions t temperture o C nd 60 o C nd L.R. 1:30 y using different slts. qe,exp Aqueous medium nd ph [S.S.] = 50g/l, [ 2CO 3] = 20g/l nd ph (RR120) nd (RB5) [S.E.] = g/l nd ph 9.52 [S.C.] = 25g/l nd ph 8.31 qe1, cl Pseudo first-order model k1 x10 2 min r qe2, cl RR120 Pseudo second-order model k2 x10 2 g / mg min h x10 min r Intrprticle diffusion mode ki x10 g/m min ri [Dye] mg/l qe,exp qe1, cl Pseudo first-order model k1 x10 2 min r qe2, cl RB5 Pseudo second-order model k2 x10 2 g / mg min h x10 min Tle 4: Activtion prmeters for pseudo second-order model of RR120 nd RB5 with initil dye concentrtion mg/l nd mg/l nd L.R. 1:30 y using different slts. Temp. o C ΔG # 2 (kj/mol) ΔH # 2 (kj/mol) RR120 ΔS # 2 (j/mol K) r E 2 (kj/mol) r 2 ΔG # 2 (kj/mol) ΔH # 2 (kj/mol) RB5 ΔS # 2 (j/mol K) Tle 5: Activtion prmeters for intrprticle diffusion model of RR120 nd RB5 with initil dye concentrtion mg/l nd mg/l nd L.R. 1:30 y using different slts. Slt dd Aqueous medium nd ph Temp. o C ΔG # i (kj/mol) ΔH # i (kj/mol) RR120 ΔS # i (j/mol K) r i E i (kj/mol) r i ΔG # i (kj/mol) ΔH # i (kj/mol) RB5 ΔS # i (j/mol K) r i r E i (kj/m ol) r2 E 2 (kj/mol) r i Intrprticle diffusion mode ki x10 g/m ri min r 2 104

11 ture nd Science 2010;8(11) [S.S.] = 50g/l, [ 2CO 3] = 20g/l nd ph (RR120) nd (RB5) [S.E.] = g/l nd ph 9.52 [S.C.] = 25g/l nd ph ( ) Time (min) ( ) Time (min) Figure 1: The effect of contct time nd temperture of RR120 () nd RB5 () on fric with initil dye concentrtion mg/l, mg/l nd L.R. 1:30 t ph nd in queous medium ( ) nd in presence of 50g/l slt concentrtion (SS) nd 20g/l sodium cronte concentrtion used for fixtion ( ). 35 E F G H I 3.2 (m g /g co tto n ) A 1 B 2 C 3 D in SS for 2.045X10-5 in SS for X10-5 in SS for 7.070X10-5 in SS for 1.002X10-4 in SS for 5.021X10-4 in SS for 1.002X10-3 in SE for 2.045X10-5 in SE for X10-5 in SE for 7.070X10-5 in SE for 1.002X10-4 in SE for 5.021X10-4 in SE for 1.002X10-3 in SC for 2.045X10-5 in SC for X10-5 in SC for 7.070X10-5 in SC for 1.002X in SC for 5.021X10-4 A in SC for 1.002X10-3 ( ) in SS for RR 120 in SS for RB Time (min) ph Figure 2: Effect of different RR120 concentrtions on t L.R.1:30 for 2 hours dyeing time in presence of 50g/l SS in presence of 20 g/l sodium cronte used in fixtion t ph 10.02, g/l SE t ph 9.52 nd 25 g/l SC t ph Figure 3: Effect of ph on in presence of SS of RR120 ( ) nd RB5 (..) on fric with initil dye concentrtion mg/l nd mg/l for RR120 nd RB5 respectively nd L.R.1:30 for 2 hours dyeing time t tempertures nd 60 o C for RR120 nd RB5 respectively. 105

12 ture nd Science 2010;8(11) ( ) in SS for RR 120 in SS for RB 5 ( ) in SE for RR 120 in SE for RB 5 in SC for RR 120 in SC for RB [SS]g/l [SE] nd [SC] (g/l) Figure 4: Effect of different SS concentrtions on of RR120 ( ) nd RB5 (..) on fric with initil dye concentrtion mg/l nd mg/l for RR120 nd RB5 respectively, L.R.1:30 nd ph 7.03 for 2 hours dyeing time t tempertures nd 60 o C for RR120 nd RB5 respectively. Figure 5: Effect of different SE nd SC concentrtions on of RR120 ( ) nd RB5 (..) on fric with initil dye concentrtion mg/l nd mg/l for RR120 nd RB5 respectively, L.R.1:30 nd ph 7.03 for 2 hours dyeing time t tempertures nd 60 o C for RR120 nd RB5 respectively. 1/q e (g /mg) /q e (g /mg) /C e (ml/mg) /C e (ml/mg) Figure 6: Lngmuir dsorption isotherm of RR120 () nd RB5 () onto fric t different tempertures with L.R.1:30 nd ph nd in queous medium ( ) nd in presence of 50g/l slt concentrtion (SS) nd 20g/l sodium cronte used for fixtion ( ). ln q e ln q e ln C e ln C e Figure 7: Freundlich dsorption isotherm of RR120 () nd RB5 () onto fric t different tempertures with L.R.1:30 nd ph nd in queous medium ( ) nd in presence of 50g/l slt concentrtion (SS) nd 20g/l sodium cronte used for fixtion ( ). 106

13 ture nd Science 2010;8(11) Ln (q e - ) Ln (q e - ) Time (min) Time (min) Figure 8: The pseudo first-order plot of RR120 () nd RB5 () on fric t different tempertures with initil dye concentrtion mg/l nd mg/l, L.R. 1:30 nd ph nd used in queous medium ( ) nd in presence of 50g/l slt concentrtion (SS) nd 20g/l sodium cronte concentrtion used for fixtion ( ). t/ (g min/mg) Time (min) t/ (g min/mg) Time (min) Figure 9: The pseudo second-order plot of RR120 () nd RB5 () on fric t different tempertures with initil dye concentrtion mg/l nd mg/l L.R. 1:30 nd ph nd used in queous medium ( ) nd in presence of 50g/l slt concentrtion (SS) nd 20g/l sodium cronte concentrtion used for fixtion ( ). ( ) ( ) Time 1/2 ( min 1/2 ) Time 1/2 ( min 1/2 ) Figure 10: The intrprticle diffusion kinetics plot of RR120 () nd RB5 () on fric t different tempertures with initil dye concentrtion mg/l nd mg/l, L.R. 1:30 nd ph nd used in queous medium ( ) nd in presence of 50g/l slt concentrtion (SS) nd 20g/l sodium cronte concentrtion used for fixtion ( ). 107

14 ture nd Science 2010;8(11) Ln K Dyeing in Aq. Dyeing in S.S. Dyeing in S.E. Dyeing in S.C. Ln K Dyeing in Aq. Dyeing in S.S. Dyeing in S.E. Dyeing in S.C /T (1/ o K) /T (1/ o K ) Figure 11: Arrhenius plot for pseudo second-order model of RR120 () nd RB5 () with initil dye concentrtion mg/l nd mg/l nd L.R 1:30 in queous medium, in presence of 50g/l slt concentrtion (SS) nd 20g/l sodium cronte concentrtion used for fixtion t ph nd nd in presence of g/l dd t ph 9.52 nd 25g/l dd t ph 8.31 from SE nd SC, respectively. Ln K i Dyeing in Aq. Dyeing in S.S. Dyeing in S.E. Dyeing in S.C. Ln K i Dyeing in Aq. Dyeing in S.S. Dyeing in S.E. Dyeing in S.C /T (1/ o K) /T (1/ o K) Figure 12: Arrhenius plot for intrprticle diffusion model of RR120 () nd RB5 () initil dye concentrtion mg/l nd mg/l nd L.R. 1:30 in queous medium, in presence of 50g/l slt concentrtion (SS) nd 20g/l sodium cronte concentrtion used for fixtion t ph nd nd in presence of g/l dd t ph 9.52 nd 25g/l dd t ph 8.31 for SE nd SC, respectively. Ln K 2 /T Dyeing in Aq. Dyeing in S.S. Dyeing in S.E. Dyeing in S.C. Ln K 2 /T Dyeing in Aq. Dyeing in S.S. Dyeing in S.E. Dyeing in S.C /T (1/ o K) /T (1/ o K) Figure 13: Eyring plot for pseudo second-order model of RR120 () nd RB5 () with initil dye concentrtion mg/l nd mg/l nd L.R. 1:30 in queous medium, in presence of 50g/l slt concentrtion (SS) nd 20g/l sodium cronte concentrtion used for fixtion t ph nd nd in presence of g/l dd t ph 9.52 nd 25g/l dd t ph 8.31 for SE nd SC, respectively. 108