Journal of Industrial and Engineering Chemistry 20 (2014) 2603–2609

Sorption kinetics of sulphate ions on quaternary ammonium-modifiedrice straw

Wei Cao a, Zhi Dang b,c,*, Bao-Ling Yuan a, Chun-Hua Shen a, Jin Kan a, Xiu-Ling Xue d

a College of Civil Engineering, Huaqiao University, Xiamen 361021, PR Chinab School of Environment and Energy, South China University of Technology, Guangzhou 510006, PR Chinac Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, Guangzhou 510006, PR Chinad College of Chemical Engineering, Huaqiao University, Xiamen 361021, PR China

A R T I C L E I N F O

Article history:

Received 14 April 2013

Accepted 23 October 2013

Available online 1 November 2013

Keywords:

Sulphate ions

Sorption kinetics

Quaternary ammonium modification

Rice straw

Sorption activated energy

A B S T R A C T

Batch sorption experiments with variable concentration of sulphate and temperature were conducted to

investigate sorption kinetics of sulphate ions on quaternary ammonium-modified rice straw (QMRS).

Kinetic data were discussed with pseudo first- and second-order rate equations and diffusion models.

The results showed the sorption equilibrium time is about 20 min and the activated energy Ea is 19.3 kJ/

mol. Increase of temperature and initial sulphate concentration favored sorption process. The sorption

kinetics followed pseudo second-order rate equation, and the overall rate was determined by film

diffusion. Additionally, characterization by 13C NMR and EDS confirmed the presence of quaternary

ammonium groups in QMRS.

� 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

1. Introduction

Sulphate (SO42�), as a common anionic ion, exists widely in

natural water and wastewater, such as acid mine drainage, effluentfrom chemical industry and so on. The main source of sulphate innatural water is chemical weathering of sulphur containingminerals and oxidation of sulfides and elemental sulphur [1].Sulphate is nontoxic, and sulphur is a necessary nutrient elementfor many kinds of living systems. However, improper disposal ofsulphate containing wastewater would cause pollution problemsto surrounding environments. High concentrations of sulphatecould break the balance of natural sulphur cycle [2]. It wasreported that excess ingestion of sulphate would bring manyphysiological damages to mammals [3,4]. Under anaerobicconditions, sulphate can be easily reduced to hydrogen sulfideby microorganism (sulphate-reducing bacteria, SRB). Hydrogensulfides are more dangerous to environmental ecosystem for itshigh reactivity, toxicity and corrosivity. Therefore, there is a needto remove sulphate from industrial wastewater before discharge.

The main methods for sulphate treatment include chemicalprecipitation, sorption, biological treatment and membrane

* Corresponding author at: School of Environment and Energy, South China

University of Technology, Guangzhou 510006, PR China. Tel.: +86 20 39380522;

fax: +86 20 39380508.

E-mail address: chzdang@scut.edu.cn (Z. Dang).

1226-086X/$ – see front matter � 2013 The Korean Society of Industrial and Engineer

http://dx.doi.org/10.1016/j.jiec.2013.10.047

separation, among which sorption techniques may be preferredbecause of its high selectivity and short time consumption [5].Sorbent plays a very important role in sorption treatment process.Ion exchange resins and zirconium oxides are commonly used assorbents to remove sulphate ions from aqueous solution.Unfortunately, they are too expensive to treat large volumes ofwastewater, resulting limitations for application in developingcountries or rural areas. In the past several years, studies showedthat cellulosic agricultural residues as low cost materials could beused to prepare low cost sorbent for removal of heavy metals anddyes from water [6,7]. Crop straw such as rice straw and corn stalkcontains many of cellulose, lignin and hemicelluloses, and a little ofextract and mineral residues. To prepare sorbent for anionicspecies, modification of cellulose, especially the grafting of cationicmonomers to the cellulose backbone was considered. Dimethy-lamine and diethylamine were selected to introduce amino groupsinto cellulosic materials in recent reports [8–10]. In fact, amongnitrogen-containing monomers, quaternary ammonium is a strongcationic group, which may imply an even better adsorption forsulphate anions. In our previous work, a quaternary ammonium-modified rice straw (QMRS) was prepared and this new typesorbent exhibited a good sorption capacity for sulphate anionsfrom water [11].

Despite many studies of the lignocelluloses-based sorbentspublished in this area, only a few have been investigated about theanionic sorbent from crop straw [8–10,12,13], and even fewerabout the sorption kinetics of quaternary ammonium modified

ing Chemistry. Published by Elsevier B.V. All rights reserved.

(1)

(2)

W. Cao et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 2603–26092604

lignocelluloses [9,10]. The sorption rate also plays an importantrole in sorption technology. Rapid sorption is significant tominimize the consumption of time and cost in wastewatertreatment. It is very necessary to understand the sorption kineticsof the quaternary ammonium modified lignocelluloses. Therefore,the present study focuses on the sorption kinetics of sulphateanions on the QMRS. Sorption experiments were carried out withvariable concentrations of sulphate and temperatures. The datawere analyzed with pseudo first- and second-order rate equations,and especially a mathematical model developed by Reichenbergand Boyd [14,15] was used to determine film diffusion and particlediffusion controlled sorption

2. Materials and methods

2.1. Materials and chemical reagents

Rice straw was obtained from the countryside aroundGuangzhou, China. Raw rice straw was washed with tap waterand with deionized water. Then it was shattered and sieved toobtain particles in range of 0.2–0.9 mm (80–20 mesh). These strawparticles will be used as raw materials to prepare QMRS.Trimethylamine water solution and epichlorohydrin used in thiswork were bought from Sinopharm Chemical Reagent Co. Ltd(Shanghai, China). The stock solution of sulphate anion wasprepared by dissolving certain weight dried sodium sulphate(Na2SO4) in deionized water. Sulphate anion solutions withrequired concentrations were diluted from this stock solution.The pH of solution was adjusted with 0.1 M sodium hydroxide(NaOH) and 0.1 M hydrochloric acid (HCl) solution. All thechemicals are analytical grade.

2.2. Preparation of QMRS

Six-gram virginal rice straw particles were firstly treated with10% (w/v) NaOH solution at room temperature for 2 h to exposecellulose and to form sodium cellulose. Secondly, the alkalitreated rice straw was reacted with 60 mL epichlorohydrin in athree-neck flask for 6 h at 65 8C to obtain epoxypropyl-cellulose.The excess epichlorohydrin was removed from reaction systemby filtration. Thirdly, 60 mL 33% trimethylamine water solutionwas added into the flask and reacted for 3 h at 80 8C to introducequaternary ammonium groups into rice straw. The product waswashed with 1:1 ethanol, 0.1 M HCl solution, and deionizedwater to convert it into chlorine resident form. After being driedat 60 8C, the dried product as quaternary ammonium modifiedrice straw (QMRS) would be used in following sorptionexperiments. During modification process, the main chemicalreactions of cellulose from rice straw were simply shown informulae (1) and (2).

2.3. Characterization of rice straw and QMRS

To assess the modification efficiency and the sorption potentialof QMRS, we have tested the nitrogen content (N%) of the rice strawbefore and after being modified. The results showed QMRS

contained more nitrogen element (2.75%) in contrast with rawrice straw (0.45%). Further, in this study, the cross polarization/magic angle spinning (CP/MAS) 13C solid-state NMR wereemployed in order to confirm the presence of quaternaryammonium groups of QMRS. The experiments were performedon a Bruker AVANCE 400 spectrometer with static field strength of2.3 T (100 MHz 1H) at 298 K. The proton 908 pulse time was 5.5 ms.The acquisition time was 0.0304 s and the delay time afteracquisition of signals was 3Ys. The surface element content of rawand modified rice straw was analyzed by using energy dispersivespectroscopy (S3700N, Hitachi Limited, Japan).

2.4. Sorption kinetic experiments

Sorption kinetic experiments were carried out in a 100 mLconical flask containing 0.1 g RS-AE and 50 mL sulphate solution.The flasks were put into a thermostat orbital shaker at rotation rateof 150 rpm and desired temperature (�1 K). The pH value of sulphatesolution was 6.8 without any adjustment during experiment process.The effect of initial concentration on sulphate sorption kinetics wasstudied by varying initial sulphate concentration from 50 to 200 mg/L(50, 100, 150 and 200 mg/L) at temperature of 298 K. While to study theeffect of temperature on sulphate sorption kinetics, experiments wereperformed at temperature of 291, 298, 308 and 318 K with 100 mg/Lsulphate initial concentration. The contact time intervals (1, 2, 5, 10, 20,30, 40, 50, 60, 70, 80 and 90 min) were adopted to obtain sorptionkinetic data. The supernatant liquid in the conical flask was separatedfrom adsorbent by filtration, and the filtrate was collected for chemicalanalysis. Sulphate concentration in this study was measured by ionchromatography (ICS 900, Dionex Co. Ltd., USA). The samples were atleast duplicated, and the mean values were used.

The sorption amount of sulphate at time t, qt (mg/g), wascalculated from mass balance equation in the following form (3):

qt ¼ðCo � CtÞ � V

m; (3)

where Co (mg/L) and Ct (mg/L) are in the places of initial sulphateconcentration and sulphate concentration at time t, respectively; V

(0.05YL) is the volume of sorption solution and m (0.1 g) is the massof adsorbent.

2.5. Sorption kinetic models

In order to describe sorption process, various overall reactionrate equations were proposed, such as pseudo first- and second-order rate equations. These two models were widely used forpollutants sorption from wastewater in recent years [16–18].Lagergren proposed the pseudo first-order equation as follows:

dqt

dt¼ k p1ðqe � qtÞ; (4)

where qe and qt (mg/g) are the sulphate sorption amount atequilibrium and time t, respectively; kp1 (1/min) is the pseudofirst-order rate constant [19,20]. After being integrated at theboundary conditions of qt = 0 at t = 0 and qt = qt at t = t, the pseudo

Fig. 2. The EDS analysis of rice straw before and after modification.

W. Cao et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 2603–2609 2605

first-order equation yields

logðqe � qtÞ ¼ logqe �k p1

2:303� t: (5)

The pseudo second-order equation was presented as

dqt

dt¼ k p2ðqe � qtÞ

2; (6)

where kp2 (g/(mg min)) is the pseudo second-order rate constantof sorption process [21,22]. Integrating Eq. (6) with boundaryconditions t = 0 to t = t and qt = 0 to qt = qt, gives

1

ðqe � qtÞ¼ 1

qe

þ k p2 � t: (7)

Separating the variables qt and t, Eq. (7) was rearranged into alinear form

1

qt

¼ 1

qe

þ 1

k p2 � q2E

� 1t: (8)

The plot of 1/qt to 1/t should be a straight line if the sorptionkinetics follows pseudo second-order rate equation. In this study,the linear form of pseudo first- and second-order models (Eqs. (5)and (8)) will be used to fit the sorption kinetic data of sulphate onQMRS.

3. Results and discussion

3.1. Characterization of sorbents

Fig. 1 shows the CP/MAS 13C NMR spectra of raw rice straw andQMRS. The main signals in both raw rice straw and QMRS areconsistent with that of native cellulose. The signals at 105 ppmarise from C-1 of cellulose. The signals at 84 and 89 ppm areattributed to C-4 of amorphous cellulose and crystalline cellulose,respectively; and the signal from 72 to 75 ppm is related to C-2, C-3and C-5 [23–25]. These results suggest the structural skeleton ofQMRS is native cellulose. Compared to the 13C NMR spectra of rawrice straw and QMRS, the largest difference is a new sharp peak at55.6 ppm observed in the spectrum of QMRS. This signal isattributed to the carbon atom combined to quaternary ammonium(C–N(CH3)3) [26,27]. The presence of this signal at 55.6 ppmprovides evidence to the existence of quaternary ammoniumgroups in QMRS. In addition, the results of EDS analysis (Fig. 2)also showed nitrogen content (%) of rice straw highly increasedafter quaternary ammonium modification. The main elementalcomposition of raw rice straw and QMRS were 25.78% C, 66.21% O,6.49% Si, and 31.29% C, 3.67% N, 60.61% O, 1.64% Si, 2.78% Cl,

Fig. 1. The 13C NMR spectra of raw rice straw and QMRS.

respectively. Quaternary ammonium, as a strong basic group, ispositively charged in aqueous solution and it could bind negativelycharged species through electrostatic attraction [10,28,29]. There-fore, the formation of quaternary ammonium groups could highlyimprove the sorption of anionic sulphate by QMRS.

3.2. Sorption kinetics with different initial sulphate concentration

The variations of sulphate sorption amount with contact timeunder different initial sulphate concentrations were shown inFig. 3a. The sorption amount qt increases fast within initial 20 min,and then its growth slows down. Accordingly, the sorption processcan be divided into two stages, the rapid increase stage and theslow increase stage, which was also called near equilibrium stage[30]. The rapid increase stage is possibly derived by electrostaticforce between SO4

2� and active sorption sites (quaternaryammonium groups) on the surface of QMRS [11]. The slow nearequilibrium stage may be resulted from the decrease of electrostaticattraction after the adsorbent being saturated by SO4

2� anions. Italso can be seen that the equilibrium sorption capacity (qe) ofsulphate on QMRS is enhanced as initial sulphate concentrationincreases from 50 to 200 mg/L.

The experimental data are analyzed with pseudo first- andsecond-order sorption kinetic models, and the results are plotted inFig. 3b and c, respectively. The calculated parameters of the twokinetic models are presented in Table 1. The correlationcoefficients of pseudo second-order equation are greater thanthat of pseudo first-order. The experimental equilibrium sorptionamounts (qe,Exp) agree perfectly with the theoretical values (qe, Cal)calculated from pseudo second-order equation. Nevertheless, qe,Exp

Table 1The parameters of pseudo first- and second-order kinetic models for sulphate sorption on QMRS under different initial sulphate concentration.

C0 (mg/L) qe,Exp (mg/g) Pseudo first-order equation Pseudo second-order equation

qe,Cal (mg/g) kp1 (1/min) R2 qe,Cal (mg/g) kp2 (g/(mg min)) R2

50 24.2 12.1 0.0610 0.927 26.9 0.0111 0.998

100 46.8 26.6 0.0829 0.943 44.4 0.0113 0.996

150 72.4 44.8 0.0949 0.986 73.6 0.016 0.995

200 74.8 46.9 0.117 0.960 74.6 0.0124 0.996

W. Cao et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 2603–26092606

deviates considerably from that of pseudo first-order rateequation. These results suggest that sulphate sorption on QMRSfollows pseudo second-order kinetic models, and the sorption rateis proportional to the square of sorption driving force (qe – qt).From the variation of the pseudo second-order rate constant kp2, itcan be seen that the pseudo second order rate constant kp2 isenhanced as the initial sulphate concentration increase from 50 to200 mg/L. This result indicates that increase of initial sulphateconcentration accelerates the sorption of sulphate on QMRS. Tounderstand this phenomenon, collision theory of chemical reactionperhaps should be considered. According to reaction collisiontheory, the reaction rate is proportional to activated collisionfraction [31]. Increase of initial sulphate concentration improvesthe possibility of activated collision between sorbate (SO4

2�) andsorption site. Therefore, the sorption rate of sulphate is enhancedwith initial sulphate concentration.

3.3. Sorption kinetics at different temperature

The sorption kinetics of sulphate on QMRS at 291 K, 298 K,308 K and 318 K are plotted in Fig. 4. From Fig. 4a, it finds that thesorption amount of sulphate increases with contact time, and alarge fraction of sulphate is removed in the first 20 min, whichwas called rapid increase stage in above. Then the sorption amountof sulphate reached a limit value in the left time called nearequilibrium stage. From Fig. 4a, we also can see that theequilibrium sorption amount of sulphate is slightly improvedwith increase of sorption temperature. It preliminarily shows thatincrease of temperature facilitates the sorption of sulphate onQMRS.

The sorption kinetic curves at different temperature arealso analyzed with pseudo first- and second-order sorptionrate equations. The related parameters of kinetic models arepresented in Table 2. From Fig. 4b, it can be seen that pseudo first-order equation is not applicable with the entire contact time ofsulphate sorption on QMRS. Fig. 4c shows that the experimentalplots agree well with pseudo second-order kinetic equation, ofwhich the correlation coefficient is much greater than that ofpseudo first-order kinetic equation. These results confirm thesorption kinetics of sulphate anions on QMRS follows pseudo-second order kinetic model. From Table 2, it can also be seen thatthe pseudo second-order rate constant kp2 increases with theenhancement of sorption temperature from 291 K to 318 K. Itsuggests the increase of temperature can accelerate the sorptionof sulphate on QMRS.

Table 2Parameters of pseudo first- and second-order kinetic models for sulphate sorption on

T (K) qe,Exp (mg/g) Pseudo first-order equation

qe,Cal (mg/g) kp1 (1/min)

291 41.2 28.8 0.0759

298 46.8 26.6 0.0829

308 48.1 27.4 0.0997

318 48.8 25.4 0.108

3.4. Sorption activation energy Ea

It is known that the temperature dependence of the rate of mostchemical reactions can be fit successfully with Arrhenius equation[32]. So the activation energy Ea of the sorption of sulphate anion isevaluated by using the linearized Arrhenius equation in thefollowing form [33]:

lnk ¼ lnA � Ea

R � T (9)

where Ea is the sorption activation energy (kJ/mol); k is thesorption rate constant A is the Arrhenius constant; R (8.314 J/(K mol)) and T (K) were the ideal gas constant and Kelvintemperature, respectively. The sorption of sulphate anion onQMRS follows pseudo second-order kinetic model, so here thesorption rate constant k is replaced by pseudo second-order rateconstant kp2. The magnitude of activation energy was often usedto distinguish physical sorption and chemical sorption. Theenergy requirement for physical sorption usually is no more than4.2 kJ/mol since the forces involved in physical sorption are weak[34,35]. Chemical sorption involves chemical bond force, which ismuch stronger than that of physical sorption. The rate for activatedchemical sorption varies with temperature according to a finiteactivation energy from 8.4 to 83.7 kJ/mol [36].

The activation energy Ea for sorption of sulphate anion on QMRSis 19.3 kJ/mol, which is calculated from the slope of correspondingln kp2 versus 1/T (shown in Fig. 5). This result suggests that thesorption of sulphate anion mainly involves chemical sorptionmechanism. The positive value of Ea indicates that the sorption isan endothermal process and the rise of temperature can accelerateits occurrence. Furthermore, the magnitude of activation energyalso gives information about whether the sorption rate is governedby the sorption reaction or the by the diffusion process. The Ea ofdiffusion controlled sorption process is usually less than 25 kJ/mol,while reaction kinetics controlled sorption process is greater than30 kJ/mol [33,35,37]. Therefore, the rate of sulphate sorption byQMRS is possibly controlled by diffusion process of SO4

2�. Whetherthe sorption rate is controlled by film diffusion or by particlediffusion needs deeper analysis.

3.5. Film diffusion and particle diffusion

The sorption process in liquid system usually includes threeconsecutive steps according to previous studies [38,39]: (1)transport of the adsorbate to penetrate the boundary film

QMRS at different temperature.

Pseudo second-order equation

R2 qe,Cal (mg/g) kp2 (g/(mg min)) R2

0.986 42.4 0.00856 0.979

0.943 44.4 0.0113 0.996

0.978 47.8 0.0142 0.999

0.959 48.5 0.0169 0.995

Fig. 3. Sorption kinetics of sulphate on QMRS under different initial sulphate

concentration: (a) experimental plots; (b) pseudo first-order kinetic plots; (c)

pseudo second-order plots (conditions: pH 6.8; temperature = 298YK; sorbent

dose = 2Yg/L).Fig. 4. Sorption kinetics of sulphate on QMRS at different temperature: (a)

experimental plots; (b) pseudo first-order kinetic plots; (c) pseudo second-order

plots (conditions: pH 6.8; initial sulphate concentration = 100Ymg/L; sorbent

dose = 2Yg/L).

W. Cao et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 2603–2609 2607

surrounding the sorbent and to reach the external surface of thesorbent (film diffusion); (2) transport of the adsorbate from theexternal surface to the active sorption sites existing on the internalsurface of the sorbent (particle diffusion); (3) sorption reactionbetween the adsorbate and the active sorption site. For an ionexchange process, the transport of ions being exchanged should be

considered because they also affect the rate of the sorption process.Generally, sorption or ion exchange reaction (step (3)) is so rapidthat it does not represent the limiting step of overall sorption rate.Therefore, the overall sorption rate is determined by film diffusion

Fig. 5. Arrhenius plots for the sorption of sulphate on QMRS.

Fig. 6. Bt vs. time plots for sorption of sulphate anion on QMRS under different initial

sulphate concentration (a) and different temperature (b).

W. Cao et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 2603–26092608

(step (1)), otherwise by particle diffusion (step (2)). A mathemati-cal model of Eqs. (10)–(12) has been developed and used todistinguish between the film diffusion and particle diffusioncontrolled sorption [14,18,38,40,41].

F ¼ 1 � 6

p2

X1

n¼1

exp½�n2Bt�n2

; (10)

F ¼ qt

qe

; (11)

B ¼ p2Di

r2; (12)

where F is the fractional attainment of equilibrium at time t and itis calculated as the ratio of the sorption amount at time t and atequilibrium. B is a time constant determined by the effectivediffusion coefficient Di and the radius r of the sorbent particle. The‘‘n’’ means an integer, which defines the infinite series solution.

The linearity test of Bt versus t can be used to distinguishwhether the film diffusion or the particle diffusion controls thesorption rate. If the plot of Bt vs. t is a straight line passing throughthe origin, then the sorption rate is controlled by particle diffusionmechanism otherwise by film diffusion mechanism. Bt values ateach observed F can be obtained from Reichenberg’s table [14].Fig. 6 shows the Bt plots vs. time for sulphate sorption on QMRS atdifferent temperature and concentration. It can be seen, under allexperimental conditions, the Bt vs. time plots do not pass throughthe origin. The results indicate that sorption rate of sulphate byQMRS is controlled by the film diffusion of sulphate ions. It alsoshould be noticed that, under lower initial sulphate concentrationand lower temperature, the plot is linear and nearly passes thoughthe origin. Perhaps, we can infer that particle diffusion of SO4

2�willturn to be the rate limiting process when the initial sulphateconcentration and the sorption temperature are decreased to someextent.

4. Conclusion

The sorption of sulphate anions by QMRS is fast and theequilibrium time is about 20 min, which is an advantage forcontinuous treatment of wastewater containing sulphate. Thesorption kinetics of sulphate on QMRS follows pseudo second-order rate equation, and an increase of initial sulphate concentra-tion and sorption temperature facilitates the sorption. The sorptionactivated energy Ea is 19.3 kJ/mol indicating that sulphate sorptionis an endothermic and chemical sorption process. Under experi-mental conditions, the over sorption rate is controlled by filmdiffusion of sulphate anions. In addition, the 13C NMR spectrum ofQMRS confirms the presence of quaternary ammonium groups asfunctional groups in QMRS.

Acknowledgements

This work was financially supported by the National NaturalScience Foundation of China (Nos. 41073088 and 21207043), andthe Starting Scientific Research Fund for Imported Talents ofHuaqiao University (No. 12BS211), and the Key Lab of PollutionControl and Ecosystem Restoration in Industry Clusters, Ministryof Education, China.

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