Chemical Engineering Journal 215–216 (2013) 691–698

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Chemical Engineering Journal

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Adsorption of paraquat from aqueous medium by Amberlite XAD-2 and XAD-4resins using dodecylsulfate as counter ion

Maurício P. Leite, Luís Gustavo T. dos Reis, Nicolle F. Robaina, Wagner F. Pacheco, Ricardo J. Cassella ⇑Departamento de Química Analítica, Universidade Federal Fluminense, Outeiro de São João Batista s/n, Centro, Niterói/RJ 24020-141, Brazil

h i g h l i g h t s

" The use of the Amberlite XAD resins for the removal of the herbicide paraquat is reported." The results show the fundamental role played by the sodium dodecylsulfate in the adsorption process." The adsorption process followed a pseudo first-order kinetic." Amberlite XAD resins presented high efficiency for the removal of paraquat from aqueous medium.

a r t i c l e i n f o

Article history:Received 14 July 2012Received in revised form 1 October 2012Accepted 4 October 2012Available online 23 November 2012

Keywords:ParaquatAdsorptionAmberlite XAD resinsSodium dodecylsulfate

1385-8947/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.cej.2012.10.087

⇑ Corresponding author. Tel.: +55 21 2629 2222; faE-mail address: cassella@vm.uff.br (R.J. Cassella).

a b s t r a c t

A study is reported about the adsorption of the herbicide paraquat (PQT2+) by the polymeric resinsAmberlite XAD-2 and XAD-4, in a medium containing sodium dodecylsulfate (SDS). The strategyemployed in this study was based on the adsorption of the ion-associate complex, formed between thecationic PQT2+ and the dodecylsulfate anion, which presents high affinity by the hydrophobic resins.The effect of several parameters that could affect the efficiency of adsorption (SDS concentration, pH,mass of adsorbent and initial concentration of PQT2+) was investigated. Also, a detailed kinetic character-ization of the system was performed. The SDS concentration added to the medium affected theadsorption efficiency, while the pH of the solution did not present any effect on the efficiency in the rangeof 2–9.5. The mass of adsorbent was studied in the range of 100–500 mg and did not influence theadsorption efficiency, changing only the adsorption rate. The kinetic evaluation of the system indicatedthat the adsorption of the PQT2+ followed a pseudo first-order model and that the adsorption rate wascontrolled by an intraparticle diffusion mechanism.

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1. Introduction

The use of herbicides dates back approximately 2000 years be-fore Christ, by the Romans, who used organic wastes to maintainroads. However, only in 1935 the first organic herbicide was devel-oped (dinitro-ortho-cresol, DNOC) [1]. Nowadays, the applicationof herbicides is an effective option in controlling weeds and theiruse has increased exponentially, primarily due to the intenseexpansion of the agriculture worldwide [2].

Paraquat (PQT2+, 1,1-dimethyl-4,4-dipyridinium chloride) is anon-selective herbicide that belongs to the class of the bipyridines(Fig. 1). It was firstly synthesized by Widel and Russian in 1882.However, at that moment, no one knew about the herbicidal prop-erties of the PQT2+ [3], which were only discovered in 1955. Thecommercialization of PQT2+ was initiated in 1962 by the Plant Pro-tection Division of the Imperial Chemical Industries (formerly ICI,

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x: +55 21 2629 2143.

now Syngenta). The first application of PQT2+ as an herbicide oc-curred in Malaysia in rubber plantations and, from that on, itsuse is widespread [3–5]. The great use of PQT2+ is related to its ra-pid effect at low concentrations, low cumulative effect on soil andlow price compared to other herbicides [4].

In the last years, special attention has been given to PQT2+,mainly due to the high rate of poisoning and fatalities attributedto it. The current literature reports that a significant number ofdeaths, deliberated or accidental, occurred after ingestion or der-mal exposure to PQT2+ [6] and, because of this, several countrieshave suspended or severely restricted its use [7]. The toxicity ofPQT2+ for humans is manifested in different organs, including liver,brain, kidneys, heart, adrenal glands and muscles. However, themain damage occurs in the lungs, where PQT2+ can transform theoxygen available into free radicals, culminating in respiratory fail-ure and death [8].

The PQT2+ is classified as a highly toxic compound with a lethaldose for humans of 35 mg kg�1 [8]. In this context, some countrieshave established maximum contamination levels for PQT2+ in

Table 1Main properties of the Amberlite XAD-2 and XAD-4 resins employed in the presentwork.

Property XAD-2 XAD-4

Specific area (m2 g�1) 330 725Average pore diameter (Å) 90 50Density (g cm�3) 20–60 20–60Average pore volume (mL g�1) 1.07 1.08

Fig. 1. Structure of the (A) paraquat dichloride and (B) the proposed structure of the ion-associate.

692 M.P. Leite et al. / Chemical Engineering Journal 215–216 (2013) 691–698

drinking water. In both Taiwan and New Zealand, this level was setat 0.01 mg L�1 or 10 lg L�1 [9,10]. In spite of this fact, the contam-ination of surface and drinking waters with PQT2+ have been re-ported in the literature [11,12], making necessary thedevelopment of new strategies for its removal from waters, whichwas the main motivation of this work.

Basically, two processes are reported for the removal of PQT2+

from aqueous medium. The first process is related to the oxidationof PQT2+, which implies in the destruction of the structure of thepesticide [13–18]. Different reagents can be used for this purposeand, sometimes, the process is enhanced with the aid of ultravioletradiation, which increases the formation of highly reactive freeradicals. One problem observed in this kind of procedure is the pro-duction of toxic substances if the degradation process does notoperate at optimum conditions [18].

Another way to treat waters for the removal of PQT2+ is to pro-mote its adsorption on solid adsorbents. Various adsorbents suchas activated carbon [19], biological tissues [20,21] and modifiedmaterials [22–24] have been employed for the adsorption ofPQT2+ from aqueous solutions. Also, in this field, the use of clayminerals for retention of PQT2+ is widespread [25–30], which hap-pens probably due to the high capacity of this material to retaincharged species, as PQT2+.

The goal of this work was to evaluate the use of polymeric com-mercial resins (Amberlite XAD-2 and XAD-4) for the adsorption ofPQT2+ from aqueous solutions. Dodecylsulfate (from SDS) wasadded as counter ion in order to lead to the formation of an ion-associate complex (Fig. 1) between the cationic herbicide and theanionic surfactant, which presents high affinity by the resin.Although few papers described the use of XAD-2 in the presenceof SDS (none was found in relation to Amberlite XAD-4) for theseparation of paraquat, no information was given regarding the ef-fect of the experimental conditions on the process [31–34]. So, theretention process by both resins was studied in this work, takinginto account the possible variables that could influence the adsorp-tion process. Also, a detailed kinetic evaluation of the systems wasperformed.

2. Experimental

2.1. Reagents and solutions

All reagents used in the present work were of analytical gradeand employed without further purification. The solutions wereprepared with purified water (18.2 MX cm) obtained with a Sim-plicity Milli-Q System (Millipore, Bedford, USA) purificationsystem.

A 1000 mg L�1 aqueous solution of paraquat (PQT) was pre-pared by dissolving 69.1 mg of paraquat dichloride (Fluka, Stein-heim, Germany) in deionized water sufficient to complete 50 mLof solution in an amber volumetric flask. This solution was keptin the refrigerator and, under this condition, could be used forapproximately 60 days without observing degradation of PQT2+.

The PQT2+ solutions employed in this work were prepared by dilut-ing this stock solution immediately before use.

A 1000 mg L�1 sodium dodecylsulfate (SDS) stock solution wasprepared by dissolving 250 mg of SDS (Vetec, Rio de Janeiro, Brazil)in approximately 150 mL of water. Afterwards, the obtained solu-tion was transferred to a 250 mL volumetric flask and the volumewas made up to the mark with water.

To the pH adjustment of the solutions, different buffer systemswere employed, always with a total concentration of 0.01 mol L�1.Four buffer systems were used: (i) a phosphoric acid buffer for pH2.0, (ii) an acetate buffer for the pH 4.5, (iii) a phosphate buffer forthe pH 7.0 and (iv) an ammonium buffer system for pH 9.5.

Amberlite XAD-2 and XAD-4 resins, furnished by Sigma–Aldrich(São Paulo, Brazil) were employed as solid-phases. They are hydro-phobic cross linked polystyrene resins, characterized by their mac-roreticular porosity, broad size distribution and large surface area.The specifications of both Amberlite XAD-2 and XAD-4 resins aresummarized in Table 1. To perform this work, the resins were driedin an oven at 100 �C for a period of 4 h. After this time, the resinswere stored in a dry and moisture-free place.

2.2. Apparatus

The spectrophotometric determination of PQT2+ in the solutionswas performed with a Femto 800xi UV–vis spectrophotometer (SãoPaulo, Brazil) equipped with a quartz cuvette with 10 mm of opti-cal path. The spectrophotometer was set at 257 nm, which was thewavelength where maximum absorption was observed for thePQT2+ and PQT-DS2 ion-associate.

An Ika RW 20 DZM (Staufen, Germany) overhead stirrer withdigital adjustment of the stirring velocity was employed to agitatethe solutions in the adsorption experiments. It was equipped witha stirrer arm made of stainless steel furnished by the manufacturer.pH measurements were made with a Digimed 22-M (São Paulo,Brazil) potenciometer equipped with an Orion (Beverly, MA, USA)combined glass electrode.

2.3. General procedure

The experiments were carried out by stirring 200 mL of a solu-tion containing known concentrations of PQT2+ and SDS with aknown mass of the resins in a 250 mL beaker. During the stirringtime, aliquots of the solution were removed from the beaker, at

(A)

(B)

Fig. 2. Influence of the concentration of SDS on the adsorption of PQT2+ byAmberlite (A) XAD-2 and (B) XAD-4 resins. The pH was not adjusted, the mass ofresin was 200 mg and the concentration of PQT2+ was 5.0 mg L�1.

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intervals previously established, and the concentration of PQT2+

was determined by spectrophotometry at 257 nm, using the exter-nal calibration approach. The total time of stirring was always210 min. After the determination of the concentration of PQT2+,the aliquot used for this purpose was immediately returned tothe beaker in order to avoid variation in the volume of extractionduring the process. The extraction efficiency in the different exper-iments could be derived through the following equation:

Rð%Þ ¼ ðCo � CtÞCo

� 100� �

ð1Þ

where R is the efficiency of extraction (expressed as retention per-centage), Co is the initial concentration of PQT2+ in the solution andCt represents the concentration of PQT2+ at time t. All experimentswere carried out at laboratory ambient temperature (24 ± 2 �C)and the stirring speed was always 200 rpm.

3. Results and discussion

The main goal of this work was to establish experimental con-ditions for maximum extraction of PQT2+ by the Amberlite XAD-2and XAD-4 resins and to characterize the adsorption process.Hence, the addition of an anionic surfactant (SDS) was needed toconvert the cationic PQT2+, highly soluble in water, in an ion-asso-ciate with lower solubility in aqueous medium and higher affinityby the hydrophobic resin. The influence of various parameters (SDSconcentration, pH, mass of resin and initial concentration of PQT2+)on the efficiency of removal of the PQT2+ by the resins was studied.Finally, the kinetic characterization of the adsorption process wasperformed in order to model the system.

3.1. Effect of the SDS concentration

The extraction of PQT2+ depends on the formation of an ion-associate between the cationic PQT2+ and the dodecylsulfate anion,resulted from the dissociation of SDS. The surfactant employed wasthe sodium dodecylsulfate (SDS), which provided good results inthe adsorption of cationic dyes by polyurethane foams and Amber-lite XAD-2 and XAD-4 resins, using a similar approach [35–38]. So,the first parameter related to the extraction evaluated in this studywas the effect of the concentration of SDS added to the medium.This experiment was performed keeping the concentration ofPQT2+ constant (5.0 mg L�1) and varying the concentration of SDSbetween 0 and 200 mg L�1. The mass of resin was always 200 mgand the pH of the medium was not adjusted.

The results indicated that SDS plays an important role in theretention of PQT2+ by the resins under study (Fig. 2). This fact iscertainly related to the formation of an ion-associate betweenthe cationic PQT2+ and the anionic SDS. In the absence of the anio-nic surfactant, the extraction efficiency was extremely low, sincethe PQT2+ has high solubility in water (620 g L�1), showing no sig-nificant affinity by the resins. With the addition of SDS, there is aformation of a neutral substance through the association ofPQT2+ with dodecylsulfate anion. The ion-associate has lower solu-bility in water due to its large size and neutral charge, and a higheraffinity for the hydrophobic solid phase than the PQT2+.

The increased retention of PQT2+ with the increasing of the SDSconcentration can be better understood by analyzing the equilibriainvolved in the process:

2DS�ðaqÞþPQT2þðaqÞ¡PQT-DS2ðaqÞ ðion-associate formationÞPQT-DS2ðaqÞþXADðsÞ¡PQT-DS2-XADðsÞ ðion-associate adsorptionÞ

As mentioned previously, the increased retention of PQT2+ de-pends on the concentration of SDS. This occurs because, increasingthe concentration of SDS, the first equilibrium moves towards the

formation of the ion-associate, which is the substance actually re-tained by the resins. The increased conversion of PQT2+ into therespective ion-associate increases the amount of PQT2+ retained,since the equilibrium related to adsorption moves to the right sideof the equation. Obviously, the use of the excess of SDS facilitatesthis process, as can be evidenced by the results obtained in thisexperiment.

Maximum extraction efficiency of the pesticide of about 85 ± 5%was obtained using the Amberlite XAD-2 resin, while a maximumextraction efficiency of about 95 ± 4% was obtained using theAmberlite XAD-4 resin (Fig. 3). Although the maximum percentageof extraction are slightly different, in both cases the maximumadsorption can be obtained when the concentration of SDS addedis equal or greater than 50 mg L�1. Thus, in the further experi-ments, the concentration of SDS used was 200 mg L�1, in order toensure that maximum adsorption could be obtained with the res-ins even for higher concentrations of PQT2+.

3.2. Influence of the pH

The pH of the solution can play an important role in the processof solid phase extraction, since it is responsible for the distributionof species in solution and can affect the structure of the solid-phase. So, in order to determine the effect of the pH on the adsorp-

Fig. 3. Influence of the concentration of SDS on the efficiency of removal of PQT2+ atequilibrium conditions. The pH was not adjusted, the mass of resin was 200 mg andthe concentration of PQT2+ was 5.0 mg L�1.

(A)

(B)

Fig. 4. Influence of the mass of adsorbent on the adsorption of PQT2+ by theAmberlite (A) XAD-2 and (B) XAD-4 resins. The pH was not adjusted, theconcentration of SDS was 200 mg L�1 in both cases and the concentration ofPQT2+ was 5.0 mg L�1.

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tion of PQT2+ by the Amberlite XAD resins, experiments were per-formed with solutions containing 5 mg L�1 of PQT, 200 mg L�1 ofSDS and varying the pH from 2.0 to 9.5. It is important to note thatin all experiments a mass of 200 mg of resin was used. The buffersystems used for each pH adjustment were: pH = 2.0 (phosphoricacid buffer solution), pH = 4.5 (acetate buffer solution), pH = 7.0(phosphate buffer solution) and pH = 9.5 (ammonia buffer solu-tion). The total concentration of the buffer solutions was always0.01 mol L�1.

It was observed that, throughout the range of pH studied, therewas no significant variation in the retention of the ion-associate byboth resins. This probably occurred because there was no change inthe structure of the Amberlite XAD resins or in the structure of thePQT2+, when the pH of the medium was changed. Also, the dissoci-ation of SDS was not affected by the modification of the pH, in thewhole range studied. In this context, the control of the pH duringthe adsorption of PQT2+ in the presence of SDS was not performed.It is also important to remember that in all further experiments thepH was not adjusted and the solutions remained with their originalpH, which was always in the range of 5.0–5.5.

3.3. Evaluation of the influence of the mass of resin

Typically, in solid-phase extraction procedures, the mass ofadsorbent has remarkable effect on the efficiency of solute re-moval. In general, the higher the mass of adsorbent, the greaterthe number of active sites and therefore greater amount of solutecan be extracted from the liquid phase. In order to verify the influ-ence of the mass of resin on the system under study, experimentswere carried out by varying the mass of resin from 100 to 500 mg,for both Amberlite XAD-2 and XAD-4 resins. The solutions em-ployed to investigate the effect of this parameter (200 mL) con-tained 5 mg L�1 of PQT2+ and 200 mg L�1 of SDS. The results canbe seen in the Fig. 4, for Amberlite XAD-2 and XAD-4.

Contrary to expectations, in both cases, there was no increase inthe efficiency of retention of PQT2+ by the resins with the increaseof the mass of adsorbent, a fact confirmed by the very similarextraction percentage obtained in all situations. This behavior indi-cated that the adsorption efficiency should not be associated withthe number of active sites of the resins, which seems to be suffi-cient to retain almost all the solute present in the solutions, inde-

pendently of the concentration of PQT2+ in solution (in the rangeof 2.5–10 mg L�1). However, it can be observed that the increaseof the mass of resin caused the increase of the retention rates, sincethe equilibrium was reached faster in both cases. The increaseof the mass of the resins leaded to the increase of the number ofeffective collisions between the solute and the active sites per unitof time, which accelerated the adsorption process.

3.4. Influence of the initial concentration of paraquat

To study the influence of the initial concentration of PQT2+ onthe extraction efficiency of the resins, solutions with different ini-tial concentrations of PQT2+ were investigated. For both resins,Amberlite XAD-2 and XAD-4, the initial concentrations tested ran-ged from 2.5 to 10 mg L�1 (1.34 � 10�5 to 5.37 � 10�5 mol L�1), al-ways keeping the concentration of SDS equal to 200 mg L�1. Theobtained results are presented in the Table 2.

The extraction efficiency was always close to the levels ob-served in other experiments, i.e., between 80% and 85% for theAmberlite XAD-2 resin, and between 95% and 100% for the Amber-lite XAD-4 resin. Similarly, the time required to reach the equilib-rium was the same, always close to 90 min. This result indicatedthat, probably, there is great availability of active sites in the resins

Table 2Influence of the initial concentration of paraquat on the adsorption efficiency andamount of solute retained in the solid phase. [SDS] = 200 mg L�1.

Initial concentration ofparaquat (mg L�1)

XAD-2 R(%)

XAD-4 R(%)

XAD-2(mg g�1)

XAD-4(mg g�1)

2.5 82.3 ± 2.6 99.2 ± 4.2 2.1 ± 0.1 2.5 ± 0.15.0 81.6 ± 3.5 96.4 ± 4.6 4.1 ± 0.2 4.8 ± 0.210 85.0 ± 2.7 98.2 ± 3.8 8.5 ± 0.3 9.8 ± 0.4

(A)

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in relation to the amount of solute present in the solutions, in thewhole range tested.

The efficiency of adsorption of the Amberlite XAD-4 resin washigher than that observed for the Amberlite XAD-2, probably be-cause of the highest surface area of the Amberlite XAD-4 resin.

(B)

Fig. 5. Application of the pseudo first-order model in the adsorption of PQT2+ by theAmberlite (A) XAD-2 and (B) XAD-4 resins.

3.5. Kinetic evaluation of the system

Kinetic models are of fundamental importance for the charac-terization of adsorption. They can describe, in detail, what happensin the course of the process and are able to supply important infor-mation regarding the amount of solute that is adsorbed and theamount of solute that remains in solution at any moment of theexperiment. Also, the establishment of a correct kinetic modelmakes possible the prediction of the equilibrium conditions.

In this work, the data obtained in the adsorption of the ion-associate complex of PQT2+ were analyzed according to the kineticmodels of pseudo-first order (Lagergren), pseudo-second order andintraparticle diffusion (Morris–Weber). Firstly, a pseudo-first ordermodel was considered. This model can be described by the follow-ing equation:

logðqe � qtÞ ¼ log qe �k1

2:303t ð2Þ

where qe is the amount of solute adsorbed at equilibrium (mol g�1),qt is the amount of solute adsorbed at time t (mol g�1) and k1 is thepseudo-first order overall rate constant (min�1).

Applying the experimental data obtained in the experiments ofthe influence of the initial concentration of PQT2+ to Eq. (2), it waspossible to observe the variation of the amount of solute adsorbedversus time (Fig. 5), which provided quantitative information onthe rate with which the solute is transferred to the solid phase, un-der the experimental conditions set in the previous experiments. Ifthe adsorption process follows a pseudo-first order model, a signif-icant correlation must be observed between the quantities consid-ered. However, the analysis of the coefficients of determination (r2)does not provide enough information about the quality of the mod-el, which should, additionally, provide good prediction capacity ofthe concentrations adsorbed and remained in solution. The analy-sis of the system using the pseudo-first order model for the tworesins under study is shown in the Table 3.

As it can be seen in Table 3, the correlation between log (qe � qt)and time was observed (r2 always higher than 0.962) for any of thetested concentrations, indicating that the system could follow apseudo-first order model. In order to prove definitely that thepseudo-first order model was adequate to describe the adsorptionof PQT2+ (as an ion-associate with dodecylsulfate), the constants ofthe pseudo-first order (k1) were calculated and predictions weremade regarding the amount of solute adsorbed at the equilibriumðqp

eÞ. The predicted values were compared with those obtainedexperimentally ðqe

eÞ and the difference between them was calcu-lated. For the different concentrations of PQT2+ tested, the pseu-do-first order models showed very good ability to predict thevalues of qe, with differences varying from 0.19% to 13%. These data

confirm the hypothesis that the adsorption of PQT2+ follows apseudo-first order kinetic model.

Another aspect to be highlighted is the proximity of the valuesof the kinetic constants (k1) obtained at the different concentra-tions, for each resin. This indicates that there should be no signif-icant difference in the speed of adsorption due to the initialconcentration of the PQT2+ in solution.

Even with a good indication that the adsorption of PQT2+ by theAmberlite XAD resins followed a pseudo-first order model, thepseudo-second order model was tested. In this case, it could bemodeled using the following equation:

tqt¼ 1

k2 � q2eþ 1

qet ð3Þ

where qe is the amount of solute adsorbed at equilibrium (mol g�1),qt is the amount of solute adsorbed at time t (mol g�1) and k2 is thepseudo-second order constant (L min�1 mol�1).

The plot of t versus t/qt, obtained from the experimental data,yielded straight lines with excellent coefficients of determination(r2 always higher than 0.99) for all concentrations (Fig. 6). Also,the kinetic constants were calculated and predictions about theconcentrations at the equilibrium were made, using the slopes(1/qe) and the interceptions ð1=k2 q2

e Þ of the lines. As it can be seen

Table 3Results obtained in the evaluation of the pseudo-first order model to characterize the kinetics of adsorption of paraquat by the Amberlite XAD-2 and XAD-4 resin.

Parameter Initial concentration of paraquat (mg L�1)a

XAD-2 XAD-4

2.5 5.0 10 2.5 5.0 10

r2 0.962 0.989 0.990 0.999 0.998 0.997k1 (min�1) 3.06 � 10�2 4.47 � 10�2 4.47 � 10�2 2.69 � 10�2 2.60 � 10�2 2.65 � 10�2

qpe (mol g�1) 6.80 � 10�6 1.80 � 10�5 3.60 � 10�5 9.55 � 10�6 1.88 � 10�5 3.90 � 10�5

qee (mol g�1) 7.81 � 10�6 1.84 � 10�5 3.68 � 10�5 9.56 � 10�6 1.84 � 10�5 3.77 � 10�5

Difference (%) 13 2.1 2.1 0.19 2.4 3.4

a 2.5 mg L�1 = 9.70 � 10�6 mol L�1; 5.0 mg L�1 = 1.94 � 10�5 mol L�1 and 10 mg L�1 = 3.88 � 10�5 mol L�1.

(A)

(B)

Fig. 6. Application of the pseudo second-order model in the adsorption of PQT2+ bythe Amberlite (A) XAD-2 and (B) XAD-4 resins.

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in Table 4, although very good correlations were verified, the pre-diction of the amount of solute retained on the resins in the equi-librium was not efficient, being observed differences in relation tothe experimental values ranging from 11.3% to 24.4%, worse thanthose observed when the pseudo-first order model was used forthe same purpose. This result confirmed the hypothesis that theadsorption of PQT2+ by the Amberlite XAD-2 and XAD-4 resins pos-sibly followed a pseudo-first order model, with an adsorption ratenot dependent on the initial concentration of solute.

From the mechanistic point of view, the adsorption process wasevaluated taking into account three stages. The first stage is thetransference of the solute from the bulk solution until the bound-ary layer film, which represents the interface between the solutionand the solid-phase. The transference is strongly improved withthe stirring of the solution, which increases the number of effectivecollisions among the solute molecules (the ion-associate PQT-DS2)and the adsorbent. The second stage is the diffusion of the mole-cules through the interface, a process called film diffusion. It isimportant to highlight that just after crossing the interfacial film,the solute molecules achieve the adsorbent surface and are re-tained by the resin. Finally, in the last stage, the solute migratesinto the porous structure of the resin, leaving the more externalsites free for the adsorption of other molecules from solution.The adsorption rate is given by the slowest step, which, in fact,controls the time spend in the whole process [39].

The importance of the diffusion process on the adsorption of thePQT2+ by the Amberlite XAD-2 and XAD-4 resins was verified withthe application of the Morris–Weber model (Eq. (4)). It indicates ifa diffusion process (intraparticle or film diffusion) controls theadsorption rate, which happens when a straight line passingthrough the origin is obtained when plotting t1/2 against qt [40].

qt ¼ Ridt1=2 þ C ð4Þ

where qt (mol g�1) is the amount of solute adsorbed at time t (min),Rid is the rate constant of diffusion (mol g�1 min�1/2), and C(mol g�1) is the intercept.

The model in question was applied to the data obtained in theadsorption of PQT2+ (5 mg L�1) by the resins under study in theinterval of 0–210 min. The concentration of SDS in the study was200 mg L�1 and the mass of adsorbent was 200 mg. The results ob-tained (Fig. 7) confirmed that a diffusion process played an impor-tant role on the kinetic of adsorption and that the two resinsstudied showed similar behavior, despite of small differences inthe speed at which the solute diffuses to the pores of the resins,evidenced by the small differences that were observed betweenthe diffusion rate constants for XAD-2 and XAD-4 (1.61 � 10�6

and 1.88 � 10�6 mol g�1 min�1/2, respectively).As mentioned previously, for both cases (XAD-2 and XAD-4),

the adsorption of PQT2+ seemed to occur through a similar mecha-nism. In the beginning of the process a rapid increase of qt with t1/2

was observed, probably because the agitation of the system wasefficient, enhancing the solute transportation from solution tothe interface. The straight line obtained when plotting qt versust1/2 passed through the origin, evidenced that a diffusion processwas controlling the adsorption rate. According to Crini et al. [39],this fact also suggests that the resistance of the interfacial filmwas very low, since the value of C, in the Morris–Weber equationwas close to zero. The low resistance of the interfacial film proba-bly resulted in a high rate of diffusion through the film and madethe film diffusion not relevant to control the adsorption rate. So,the adsorption rate must be controlled by an intraparticle diffusionprocess.

Table 4Results obtained in the evaluation of the pseudo-second order model to characterize the kinetics of adsorption of paraquat by the Amberlite XAD-2 and XAD-4 resins.

Parameter Initial concentration of paraquat (mg L�1)a

XAD-2 XAD-4

2.5 5.0 10 2.5 5.0 10

r2 0.997 0.994 0.994 0.991 0.989 0.993k2 (min�1) 5.64 � 103 2.67 � 103 1.34 � 103 2.34 � 10�3 1.04 � 10�3 5.91 � 10�2

qpe (mol g�1) 8.74 � 10�6 2.05 � 10�5 4.09 � 10�5 1.17 � 10�5 2.29 � 10�5 4.56 � 10�5

qee (mol g�1) 7.81 � 10�6 1.84 � 10�5 3.68 � 10�5 9.56 � 10�6 1.84 � 10�5 3.77 � 10�5

Difference (%) 11.9 11.3 11.3 21.9 24.4 20.9

a 2.5 mg L�1 = 9.70 � 10�6 mol L�1; 5.0 mg L�1 = 1.94 � 10�5 mol L�1 and 10 mg L�1 = 3.88 � 10�5 mol L�1.

Fig. 7. Application of the intraparticle diffusion model (Morris–Weber) in theadsorption of PQT2+ by the Amberlite XAD-2 and XAD-4 resins.

M.P. Leite et al. / Chemical Engineering Journal 215–216 (2013) 691–698 697

After passed the first 90 min, the slopes decreased abruptly andthe lines became almost parallel to the t1/2 axis. This fact indicatedthat the system achieved the equilibrium and the adsorption pro-cess was no longer dependent on the intraparticle diffusion. Also,the value of C was maximum, which would prevent the diffusionof new molecules of the solute through the interfacial film. Thisphenomenon happened, probably due to the low concentrationof solute remained in the solution and its high concentration onthe surface of the resins. A similar behavior was verified in theadsorption of malachite green by cyclodextrins [39] and in theadsorption of ionic-pairs formed of cationic dyes and sodium dode-cylsulfate by polyurethane foam [35–36].

4. Conclusions

The use of Amberlite XAD-2 and XAD-4 resins appeared as asimple alternative for the removal of paraquat present in aqueoussolution due to the affinity observed between the ion-associateformed between the dodecylsulfate anion and the cationic pesti-cide. The adsorption of paraquat by the Amberlite XAD-2 andXAD-4 resins showed satisfactory results throughout this presentwork in terms of extraction efficiency, and the Amberlite XAD-4 re-sin presented a slightly higher efficiency than Amberlite XAD-2,probably because of its higher specific area.

Also, it was found that the retention of paraquat is dependenton the concentration of surfactant added to the medium and doesnot vary with the initial concentration of the pesticide in solution.It was verified higher efficiency of retention of paraquat when the

concentration of SDS was higher than 50 mg L�1, which representsa molar concentration of about ten times higher than the concen-tration of pesticide. In relation to the chemical variables studied, itwas clear that the change of pH had no significant effect on theretention efficiency.

The data obtained allowed that the system could be modeledfrom the standpoint of its kinetic behavior. The overall process ofadsorption of paraquat by the Amberlite XAD-2 and XAD-4 resinsobeyed a pseudo-first order kinetic. Also, the application of theMorris–Weber model indicated that the adsorption rate is proba-bly dependent on the intraparticle diffusion process.

Acknowledgments

The authors are grateful to Coordenação de Aperfeiçoamento dePessoal de Nível Superior (CAPES), Fundação Carlos Chagas Filho deAmparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) and toConselho Nacional de Desenvolvimento Científico e Tecnológico(CNPq) for providing grants, scholarships and financial supports.

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