PARAQUAT SORPTION ON CALCIUM ALGINATE GEL BEADS MONTSERRAT RUIZ (1), JESUS BARRON-ZAMBRANO (1), VICENTE RODILLA (2), AGATA SZYGULA (1), ANA MARIA SASTRE (3) (1) Department of Chemical Engineering Universitat Politècnica de Catalunya. E.P.S.E.V.G. Av. Victor Balaguer, s/n., E-88 Vilanova i la Geltrú SPAIN (2) Departament of Physiology, Pharmacology and Toxicology Universidad Cardenal Herrera-CEU Edificio Seminario s/n. 46113. Moncada, Valencia SPAIN (3) Department of Chemical Engineering, Universitat Politècnica de Catalunya. E.T.S.E.I.B. Diagonal 647, E-828 Barcelona SPAIN Abstract: Environmental pollution and accidental poisoning by agricultural chemicals have become a major social problem. The use of calcium alginate gel beads to remove paraquat dichloride (1,1 -dimethyl-4,4- bipyridinium dichloride) from wastewater under different batch experimental conditions was investigated. Special attention was paid to the effects of ph and the initial concentration of paraquat. We determined that the sorption of paraquat was ph-dependent and based on isotherm curves; the Langmuir model appeared to fit the sorption better than the Freundlich model. It was found that the rate of sorption of paraquat onto the calcium alginate gel beads increased with ph. In all cases, the maximum possible adsorption occurred in the first 12 min. The pseudo-second-order chemical reaction model appears to provide a good correlation, confirming that the dominant mechanism of calcium alginate gel bead sorption is chemical sorption. Key words: paraquat, calcium alginate gel beads, isotherm, kinetic model, sorption. 1 Introduction The presence of herbicides in the environment results from their wide use in many agricultural areas. These classes of pollutants are of particular importance, due to their toxicity and their role as environmental contaminants. Conventional methods for removing herbicides such as paraquat from aqueous solutions include UV treatment, chemical coagulation, sedimentation, filtration and sorption onto solid substrates [1]. Paraquat is a divalent contact herbicide. Although recently banned within the European Union due to its high toxicity, it is still used in many countries, as it has excellent herbicidal properties and a relatively low cost. The most commonly used adsorbent in the treatment of intoxication by herbicides is activated carbon. Paraquat, however, is a cationic herbicide and is thus strongly adsorbed by clay minerals. Montmorrillonite has been shown to be a particularly strong binding agent in vitro [2, 3]. The environmental impact of paraquat is highly influenced by its interaction with the clay components of the soil. Research on alternative sorbents remains a priority in toxicology. ISSN: 179-2769 3 ISBN: 978-96-474-14-7
The polysaccharide sodium alginate is the sodium salt of alginic acid. Its empirical chemical formula is NaC 6 H 7 O 6. Alginate solution has anionic properties, and may therfore be useful for removing paraquat from aqueous solutions, as paraquat has cationic properties. We used calcium to crosslink alginate into a gel form. In this study, we investigated the removal of paraquat from aqueous solutions under different batch experimental conditions using calcium alginate gel beads. 2 Experimental procedure 2.1 Materials Sodium alginate was supplied by Sigma-Aldrich. Paraquat solutions were prepared from paraquat purchased from Dr. Ehrenstorfer (Germany) as reference material. The other chemicals used in this study (i.e., HCl, NaOH and Ca(NO 3 ) 2 ) were all analytical grade and were supplied by Sigma- Aldrich. 2.2 Calcium alginate gel bead preparation Calcium alginate gel beads were prepared by dropping a 3.2 % (w/v) sodium alginate aqueous solution by means of a peristaltic pump into a.5 M solution of Ca(NO 3 ) 2. The drop of viscous sodium alginate solution became a translucent, semi-rigid sphere as soon as it came into contact with the surface of the calcium nitrate solution. A magnetic stirrer at the bottom of the beaker mildly agitated the solution. The spheres were soaked in the calcium nitrate solution and stored in the refrigerator for later experiments. The spheres became rigid and turned opaque white in colour after storage in the calcium nitrate solution overnight, indicating complete penetration of calcium into the spheres [4]. Each sphere contained an average of.6 mg sodium alginate. 2.3 Methods Paraquat solutions were prepared from paraquat dichloride in demineralised water. The ph of the solutions was controlled using hydrochloric acid and sodium hydroxide concentrated solutions (5M). For sorption isotherms, known volumes of paraquat solutions (1 ml) at a given concentration (1 to 4 mg/l) were placed in contact with.25 g of sorbent at room temperature (2±1ºC). After 3 days of agitation (15 rotations per min), in a reciprocal shaker, the solutions were filtered through 1.2 µm filtration membranes and the filtrates were analyzed using the dithionite spectrophotometric method [5, 6]. The concentration of paraquat in the solution was determined by absorption at 349.9 nm using a spectrophotometer (SHIMADZU 163 UVvisible spectrophotometer). The sorption capacity, was obtained using a mass balance equation, and was expressed as mg of paraquat g -1 of alginate, q ( C C m ) V f e = (1) where q e (mg g -1 ), and C f (mg L -1 ) represent the sorption capacity and the paraquat concentration in the solution at equilibrium, respectively; C is the initial concentration (mg L -1 ); V (L) is the volume of solution and m(g) is the mass of adsorbent. The concentration of calcium in the solution was determined by atomic absorption spectroscopy (VARIAN). The study of sorption kinetics was carried out using standard procedures [7]. One litre of paraquat solution at fixed ph (3, 7 and 1) was mixed with sorbent (.25 g) in a jar-test agitated system (15 rotations per min). Five millilitre samples were withdrawn at specified times, filtered through a 1.2 µm filtration membrane and analyzed as previously specified. In the study of effect of adsorbent mass, known volumes of paraquat solutions (1 ml) at 15. mg L -1 were placed in contact with different amounts of adsorbent (.5 to 22 g) at room temperature. The ph of the solutions (3 and 7) was controlled during the experiment. For analysis we used the same method that it was used in sorption isotherms. After 3 days of agitation (15 rotations per min) the solutions were filtered and the filtrates were analyzed using the dithionite spectrophotometric method. 3 Modeling 3.1 Sorption isotherms Equilibrium is described by the usual isotherm equations, such as those described by Freundlich, Langmuir, Brunauer Emmett and Teller [8]. Preliminary screening showed that the Langmuir and Freundlich models best fitted the experimental data, so the following standard equations were used to model the results: ISSN: 179-2769 31 ISBN: 978-96-474-14-7
The Langmuir model: qm b Ceq q = (2) (1 + bc ) The Freundlich model: q = k C 1/n F eq (3) Where q m is the maximum sorption capacity for a monolayer coverage (mg g -1 ), b is the affinity coefficient (L mg -1 ), and q (mg g -1 ), and C eq (mg L - 1 ) represent the sorption capacity and the paraquat concentration in the solution at equilibrium, respectively. K F and n represent the Freundlich coefficients (n, dimensionless; k F : mg 1-1/n g -1 l 1/n ). 3.2 Sorption kinetics Adsorption of paraquat with a variety of sorbents takes place in most cases due to the electrostatic interaction between the negatively charged surface of the adsorbent and the paraquat cation [9]. For this reason, we used a simple kinetic model, a pseudo-second order equation, to fit the experimental data [1, 11, 12, and 13]: dq t / dt = k (q e q t ) 2 (4) where k is the pseudo-second order rate constant (g/mg min -1 ), q e is the amount of paraquat adsorbed at equilibrium (mg/g) and q t is the amount of paraquat adsorbed at time t (mg g -1 ). Integrating Eq. (4) from t = to t > and rearranging it to obtain a linearized form, the result is: t 1 = q kq 2 h = kq e 2 e 1 + q eq (5) (6) where h is the initial sorption rate (mg g -1 min -1 ). 4 Results and discussion 4.1 Isotherms e t 4.1.1 Influence of the ph As can be seen from Figure 1, ph exerted a major influence on the adsorption capacity of paraquat onto calcium alginate gel beads. At ph 3, the high concentration of H + competed with paraquat for the binding sites. At ph 7, the lower concentration of H + resulted in a much better flow of cationic paraquat towards calcium alginate, since there was no need to compete with H +, thus resulting in better adsorption. At the same ph, and at low concentrations of paraquat, the relative proportion of calcium alginate gel beads increased because there was the same amount of adsorbent in all experiments, but different concentrations of paraquat. Theoretically, when there are more calcium alginate spheres, it should be possible for more dissolved paraquat to be adsorbed. However, as adsorption of paraquat proceeds and more calcium is exchanged, the increasing amount of calcium in the reactor fluid could, in turn, compete for the available binding sites [4]. This effect was observed in our study, since the overall binding efficiency at low concentrations of paraquat was not as high as expected. At ph 3 in all concentration ranges, the average percentage of adsorbed paraquat was 3%, using 2.5 g L -1 for adsorbent (35% at low concentrations of paraquat to 25 % at high concentrations). A higher difference in adsorption percentage was observed at ph 7 when different concentrations were used. For low concentrations of paraquat 55% of adsorption was observed, but it fell to 33% for high concentrations. Figure 2 shows the relation between the ratio of calcium released from the alginate gel beads and the paraquat adsorbed with the initial concentration in the solution at different ph values. We can observe that at low concentrations of paraquat in the solution, the ratio of calcium release from the alginate gel beads was very high, especially at ph 3. As the concentration of paraquat in the solution increased, the ratio tended to one. These results explain the decreased efficiency at low concentrations of paraquat in the solution: a larger amount of calcium in the solution hampered the adsorption of paraquat onto alginate gel beads. Table 1 shows the values of the Langmuir and the Freundlich model parameters for paraquat sorption on calcium alginate gel beads at different ph. The parameter b obtained for the application of the Langmuir model shows the affinity of paraquat for calcium alginate gel beads. The same affinity was shown by the Freundlich parameter n. Table 1. Langmuir and Freundlich parameters for the removal of paraquat from aqueous solutions by alginate gel beads. ISSN: 179-2769 32 ISBN: 978-96-474-14-7
MSR: Mean sum residuals according to: where F is the function representative of the sorption equation (the Langmuir or Freundlich equation). q (mg/g) 16 12 8 ph =3 4 ph = 7 Lagmuir Model 1 2 3 4 5 6 Ce (mg/l) Fig. 1. Influence of ph on paraquat sorption using calcium alginate gel beads as the adsorbent Ratio (calcium relased/paraquat adsorbed) 12 1 MSR 8 6 4 2 = n ( Fexp. (C i ) - Fcalc. (C i )) i=1 ph 3 ph 7 1 3 5 75 1 15 2 25 3 35 4 Initial concentration of paraquat (mmol/l) Fig. 2. Ratio between calcium released from the alginate gel beads and the paraquat adsorbed, compared with the initial concentration of paraquat in the solution at different ph. The affinity coefficient, which is proportional to coefficient b, increases with ph. As can be seen from Figure 2, according to the slope of the initial portion of the curves and the values of parameter b, calcium alginate gel beads had low affinity for this herbicide. Lower MSR were obtained in the Langmuir model than the Freundlich one. The best fitting of the experimental data to the mathematical expression of the Langmuir model allows us to assume that the sorption of paraquat was practically homogeneous on the calcium alginate gel beads. However, it is not sufficient to describe the sorption mechanism. n 2 4.2 Kinetics 4.2.1 Effect of ph The effect of ph (i.e., 3, 7 and 1) on the paraquat uptake rate by calcium alginate gel beads is shown in Fig. 3. The sorption capacity increased with increasing ph. The data presented in Table 2 indicate that the sorption capacity (i.e., q e ) and the initial rate of adsorption (h) both increased as the ph increased from 3 to 1. The correlation coefficient r 2 for the pseudo-second order adsorption model had an extremely high value (>.997), and its calculated equilibrium adsorption capacities, q e, fitted well with the experimental data. Taken together, our results suggest that the pseudo-second order adsorption mechanism is predominant and that the overall rate of the paraquat adsorption process appears to be controlled by the chemical process [14]. It is recognized that chemisorption (ionexchange, electrostatic attractions) is the most prevalent mechanism, with ph as the main factor affecting adsorption. q (mg/g) Parameters of the Langmuir model q m b MSR (mg g -1 ) (L mg -1 ) ph 3 149.71.13 1.35 7 171.48.17 2.39 Parameters of the Freundlich model ph k N MSR (mg 1-1/n g -1 l -1/n ) 3.36 1.24 13.39 7.98 1.46 1.19 1 8 6 4 2 15 3 45 t (min) ISSN: 179-2769 33 ISBN: 978-96-474-14-7
Fig. 3. Kinetic of paraquat sorption using calcium alginate gel beads as an adsorbent (ph 1: ; ph 7: ; ph 3: ). C o =1 mg/l;.25 g adsorbent; V = 1 L. Table 2. Kinetics parameters for paraquat sorption onto calcium alginate gel beads at different ph. ph k 1 3 (g mg -1 min -1 ) q e (mg g -1 ) h r 2 (mg g -1 min -1) 3 2.8 33.11 3.54.9975 7.6 81.96 3.86.9994 1.9 9.91 7.58.9998 4.3 Effect of adsorbent mass The effect of varying the alginate mass on paraquat sorption with an initial paraquat concentration of 15 mg L -1 and mixing speed of 15 rpm is shown in Fig. 3. In Fig. 3 we can observed that a concentration of 22 g L -1 was needed to remove 83% of paraquat from the initial solution (15 mg L -1 ) at ph 3. In contrast, similar amounts at ph 7 removed practically 93% of the total paraquat. Adsorption (%) 1 9 8 7 6 5 4 3 2 1 ph 3 ph 7,5 1 2 4 6 7 9 11 16 18 2 22 [Calcium alginate] (g/l) Fig. 4. Effect of alginate dosage on paraquat sorption at different ph. Co = 15 mg L -1. 5 Conclusions Our results confirm that paraquat can be adsorbed onto calcium alginate gel beads under selected experimental conditions. Sorption capacities in the range of 14 to 171 mg g -1 have been obtained for high concentrations of paraquat in aqueous solutions. The sorption of paraquat was ph-dependent. Effective paraquat binding was reduced by increasing amounts of calcium in the solution (introduced with the beads) or by decreasing both the initial dissolved paraquat concentration and ph. A high dose of adsorbent, 22 g L -1, was used to remove 83 and 93% of paraquat from a solution of 15 mg L -1 of paraquat at ph 3 and ph 7, respectively. The classical empirical approach for adsorption data modelling relies on simple mathematical equations, such as the Langmuir and Freundlich isotherms, that can be applied only to represent data collected at constant ph. In our study, the Langmuir equation showed a better fit to the experimental data for sorption isotherms. The pseudo-second order chemical reaction model provides a good correlation of the paraquat sorption kinetics. The correlation of this adsorbent to a pseudo-second order model confirms the important effect of the functional groups of the adsorbent in the adsorption of paraquat, with chemisorption being the principal mechanism. ACKNOWLEGDMENTS This work was supported by the MEC (Ref. CTQ25-943) and DEU of the Autonomous Government of Catalonia (SGR25-934). M. Ruiz acknowledges the financial support of the Spanish Ministry of Science and Education. J. Barron-Zambrano acknowledges the financial support of the Spanish Ministry of Science and Education through the Juan de la Cierva programme. References: [1] Hamadi, N. K., Swaminathan, S., Chen, X. D., Adsorption of Paraquat dichloride from aqueous solution by actived carbon derived from used tires, Journal of Hazardous Materials, Vol. 112, 24, pp. 133-141. [2] Hague, R., Lilley, S., Coshow, W.R., Mechanism of adsorption of diquat and paraquat on montmorrillonita surface. Journal ISSN: 179-2769 34 ISBN: 978-96-474-14-7
of colloid and interface science, Vol. 33, 197, pp. 185-188. [3] Rytwo, G., Nir, S., Margulies, L., Adsorption and interactions of diquat and paraquat with Montmorrillonita, Soil Science Society of America Journal, Vol.6, No.2, 1996, pp. 61-61. [4] Jang, L.K., Brand, W., Resong, M., Mainieri, W., Feasibility of using alginate to absorb dissolved copper from aqueous media, Environmental Progress, Vol. 9, No.4, 199, pp. 269-274. [5] Yuen, S. H., Bagnem, S. E., Myles, D., Spectrophotometric determination of diquat and paraquat in aqueous herbicide formulations, Analyst, Vol. 92, 1967, pp. 375-381. [6] Berry, D. J., Grove, J., The determination of paraquat in urine, Clinica Chimica Acta, Vol. 34, 1971, pp. 5-11. [7] Ruiz, M., Sastre, A.M., Guibal, E., Palladium sorption on gluteraldehiyde-crosslinked chitosan, Reactive &Functional Polymers, Vol. 45, 2, pp.155-173. [8] Kinniburgh, D. G., General Purpose Adsorption Isotherms, Environ. Sci. Technol, Vol. 2, No. 9, 1986, pp. 895-94. [9] Tsai, W.T., Lai, C. W., Hsien, K.J., Adsorption kinetics of herbicide paraquat from aqueous solution onto activated bleaching earth, Chemosphere, Vol. 55, 24, pp. 829-837. [1] Ho, Y. S., Chiang, C. C., Sorption studies of acid dye by mixed sorbents, Adsorption, Vol. 7, 21, pp. 139-147. [11] Ho, Y.S., McKay, G., The kinetics of sorption of divalent metal ions onto sphagnum moss peat, Water Res., Vol. 34, 2, pp. 735-742. [12] Ho, Y.S., Chiang, C.C., Hsu, Y.C., Sorption kinetics for dye removal from aqueous solution using activated clay, Sep. Sci. Technol., Vol. 36, 21, pp. 2473-2488. [13] Wu, F.C., Tseng, R.L., Juang, R.S., Kinetics modelling of liquid-phase adsorption of reactive dyes and metal ions on chitosan, Water Res., Vol. 35, 21, pp. 613-618. [14] McKay, G., Ho, Y. S., Pseudo-second- order model for sorption processes, Process Biochem., Vol. 34, 1999, pp. 451-465. ISSN: 179-2769 35 ISBN: 978-96-474-14-7