Complexation of cations Pb(II), Cd(II) and Hg(II) by a new copolymer based on acrylic and crotonic acids in aqueous media.

Abstract

The complexes formation of divalent metal ions M(II) {M= Pb, Cd and Hg} with a new copolymer based on acrylic and crotonic acids was investigated in aqueous media with NaN[O.sub.3] (0.1 M) as supporting electrolyte. pH titrations of coplymer alone and in presence of metal were realized through the batch method to determine the properties of acid base and complexe-forming of ligand copolymer. The results show that for Pb(II), the predominant complex specie is Pb[A.sub.2] with a stability constant [pK.sub.2] = -8.82. On the contrary, with cadmium and mercury neither MA nor [MA.sub.2] becomes predominant and the stability increases as follows: Pb(II)> Cd(II)> Hg(II). The synthesized copolymer is a good fixation agent that can be used in the recovery of heavy metals, known for their high toxicity in hydric media.

Keywords: Complexation, Lead (II), Cadmium (II), Mercury (II), Poly (acrylic acid).

Introduction

The polyacrylic acids are used as complex ligands with regard to many cations such as alkaline, rare earth and transition elements which have a negative effect on the environment ecosystem and the safety problem. The literature on the subject is extensive and in recent time, a great deal of attention has been paid to advanced technologies for removing inorganic pollutants from water, wastewater and other effluents.

Environmental testing methods is a growing field of ecological engineering and we emphasize the importance of seeking new techniques for environmental protection. Several papers on devoted for recovery of heavy metals are encountered in the literature such as activated carbon method; coal derived organic acids based heavy metals sorbent or engineering processes [1]. The knowledge currently found in the literature concerned with complex interactions occurring between metal ions and the soil they occupy.

The first studies on the complexes of acrylic acid with metal ions were reported early by Gregor et al. [2] where the authors investigated the stability constants of acrylic acid based cuprates complexes by adopting the method of Bjerrum [3]. They showed that the complexes exhibit a high stability and at least two carboxylic groups were involved in the complexation mechanism such as simple dicarboxylic acids. The high stability of Cu-polymer was compared to that of analogous monomer (glutaric acid) reflecting the effect of powerful electrostatic field of polyelectrolyte chain (polymer). Later, Marinsky and et al. [4.5] proposed another experimental approach for studying the complexes equilibrium based on the model of two phases (Donnan) and concluded that Cu(II) forms complexes of type 1:2 with polyacrylic acid.

On the other hand, several studies were realized by using various methods including thermodynamic and spectroscopic measurements to determine the properties of acid base and complexing of polyacrylic and polymethacrylic acids which were frequently selected as models in such studies. As example, one can mention the works of Morlay and et al. [6-10].

It is well known that the equilibria depend strongly on the experimental conditions such as the concentrations of both the metal ion and polyelectrolyte and their ratio. They are also function of physical parameters such as pH, ionic strength or temperature. Indeed, for the occurrence of quantitative analysis of M(II) ions with the polyelectrolytes, two factors must be taken into consideration. The first is the electrostatic effect ascribed to the nature of polyelectrolyte, the complexing equilibrium is influenced by both the degree of dissociation of polyacid and the concentration of added salt. The second one is due to multidental properties of molecules of ligand polymer i.e. two adjacent carboxylic groups fixed on polyacrylic acid, they are very close to each other and can built bidentates complexes with some multivalent cation in addition to monodentate complexes [11].

The research has been directed to fundamental problems of environmental protection because of large scale reject of industrial effluents. The title ions are by far the most harmful with regard to toxicity concern.

This paper describes relatively a method for complexing some metals of group [II.sub.B] and preliminary insights of the phenomenon. It deals with the synthesis of copolymer based on acrylic and crotonic acids. In a second step, the acid base properties were investigated along with the ability complexing with regard to heavy metals namely, Pb(II), Cd(II) and Hg(II) well known for their toxicity. The stability constants were determined from pH titration by the batch method.

Experimental

Chemical reagents

Acrylic acid (AA; > 99%, Fluka, Switzerland), Crotonic acid (CA; > 97%, Fluka, Switzerland) monomers and azobisisobutyronitrile (AIBN, Merck) were used as received. Reagent-grade solvent toluene and petroleum ether were used without further purification. For viscosity measurements a solution of sodium nitrate (NaN[O.sub.3], Prolabo) was prepared. Distilled water was used in preparation of solutions.

Synthesis of copolymers

Free-radical copolymerization was carried out in toluene (22.5 g), as solvent in the presence of AIBN (0.025 g) as initiator and then AA/CA monomers of 4.5/0.5; 4.0/1.0; 3.5/1.5; 3.0/2.0 g/g were added to the solution respectively. The mixture ([approximately equal to] 27.5 g) was taken in a 100 ml round bottom three-necked flask fitted with a mechanical stirrer and with a digital thermocouple to monitor the reaction temperature. The flask was immersed in a thermostatic oil bath at a temperature of 70[degrees]C under bubbling argon for 4 h. Stirring was continued until the reaction mixture became too viscous and gelled. The flask was then removed from the bath and allowed cool to room temperature. After cooling, the contents of the flask were filtered and washed several times by petroleum ether. The product obtained was dried in air and stored in glass containers at a refrigerator for characterization.

Characterization

Nuclear magnetic resonance spectroscopy

The copolymers were characterized by nuclear magnetic resonance (NMR) of proton [sup.1]H. The spectra was recorded in a Bruker spectrometer at 300 MHz by using ([D.sup.2]O as solvent).

Acid base titration

The aqueous solution was made by mixing lg material in 1 L of deionized water, stirred under magnetic stirring for 1 hr and finally stored at 20[degrees]C. The pH measurement were performed thanks to a pH meter type inoLab Multi level combined with a glass electrode (Schott), preliminary calibrated by two buffer solutions (pH 7 and 10). The solution was placed in a thermostated water bath whose temperature was regulated by a temperature controller. All solutions were thermostated at 25.0 [+ or -] 0.5 [degrees]C and contain high purity NaN[O.sub.3] (0.1 M) as supporting electrolyte.

Viscosity measurements and Estimation of molecular Weights

Viscosity measurements were made in a thermostatic water bath (JULABO) at 25.0[+ or -]0.1[degrees]C using a Ubbelohde viscosimeter of SCHOTT--GERATE AVS type 531. The viscosimeter employed had the following characteristics: Capillary diameter 0.36 mm; volume of bulb 10 ml. length of capillary 7.7 cm; efflux time for water 850 s. The synthesized copolymers were dissolved in distilled water with 1M NaN[O.sub.3] solution and the viscosity of four concentrations was measured. Intrinsic viscosity was obtained by extrapolation of plot of specific viscosity/concentration vs. Concentration to infinite dilution using linear least squares. Estimates of the copolymer molecular weight were obtained using the Mark--Houwink--Sakurada (MHS) equation [15]: [[eta]] = 8.3 x [10.sup.-5] [M.sub.V.sup.[degrees].758] were [[eta]] is the intrinsic viscosity and [M.sub.V], is the viscosity average molecular weight.

Results and discussion

[sup.1]H NMR spectra of copolymer

The [sup.1]H NMR spectra of copolymers given in Figure1, shows for example for copolymer AA-60 %/AC-40 % (fig. 1-D) a multiplet state at [delta]= 1.7 ppm which can be assigned to methylen groups (-C[H.sub.2]-) and a singlet at [delta]= 2.4 ppm attributed to methine groups (-CH-) [12]. A further peak positioned at [delta]= 3.7 ppm, attributed to proton of -CH-, belongs to the chain bonded to C[H.sub.3]. The peak at 4.3 ppm is due to residual water. Which correspond exactly to the expected product.

Acid base properties of the copolymer

According to its lowest n which measures the importance of intramolecular electrostatic forces tablel, the copolymer AA-60 %/AC-40 % gives the best interaction with the metal ions (II) where the complexation study is made with.

In a first step, we titrated the solution of prepared copolymer without M(II). A 20 [cm.sub.3]-aliquot containing 5 [cm.sup.3] of NaN[O.sub.3] was titrated by a standard solution of NaOH (0.1 M) at controlled temperature (25.0 [+ or -] 0.5[degrees]C). The titration was initiated when the temperature stabilized. The solution is stirred and the pH is measured when equilibrium is reached which in our case occurs in a few seconds. The variation of pH with [V.sub.NaOH] of copolymer alone and its derivative curve (The Gran method), depicted in Figure2., reveals one end point located around pH 8 which allow to determine the total concentration [C.sub.A] of reactive carboxylic sites of copolymer. [C.sub.A] was found to be 3.7 x [10.sup.-3] eq.[L.sup.-1].

The experimental results of pH titrations of copolymer as a function of log {[[alpha].sub.A]/(1-[[alpha].sub.A])} lead to concentrations [[NaOH].sub.a], [H] and [OH], given in Figure3., [[NaOH].sub.a], [H] and [OH] being respectively the concentrations of added NaOH solution, free [H.sub.3][O.sup.+] and O[H.sup.-]. The degree of copolymer dissociation [[alpha].sub.A] was calculated from the pH value and the degree of neutralization, as given by added amount of NaOH solution (Eq. 1) [6]:

[[alpha].sub.A]=[([NaOH].sub.a]+[H]-[OH])/[C.sub.A] (Eq.1)

[OH] was deduced from the ionic product of water. [[alpha].sub.A] shows that the modified equation of Henderson-Hasselbalch (Eq. 2):

pH = pK[m.sup.H] + n.log([[alpha].sub.A]/(1 - [[alpha].sub.A])) (Eq.2)

is valid when the quantity log([[alpha].sub.A]/(1-([[alpha].sub.A])) lies between -0.5 and +0.5 i.e. in the linear part of plot corresponding respectively to pH 3.6 and 5.4. The values of apparent dissociation constants [pK.sub.m.sup.H] at [alpha]= 0.5 and the empirical n-value, given in table 3, in good agreement with those of the literature data [2-6]. n measure the importance of intramolecular electrostatic forces and needs to be taken into consideration.

Figure 4 gives the variation of the apparent dissociation constant [pK.sub.app.] against the dissociation coefficient [[alpha].sub.A] of copolymer. The curve shows that the variation of [pK.sub.app] is practically constant up to [[alpha].sub.A] ~ 0.4 above which it increases linearly with [[alpha].sub.A]. Indeed, during the dissociation of copolymer, the ionization of the carboxylic groups induces electrostatic attraction forces with respect to the protons. Consequently, the deprotonation of the functional groups depends on dissociation of all the other functional groups on the polyelectrolyte chain which leads to a variation of [pK.sub.app] with [[alpha].sub.A]. When [[alpha].sub.A] increases, the protons remain more attracted towards polyacid and become less and less mobile, which leads to a continuous reduction in the strengh of acidity of the polyelectrolyte [7-14]. This phenomenon is more accentuated for [[alpha].sub.A]-values larger than 0.4.

Complexation equilibrium of metal ions by the copolymer

The equilibrium of copolymer fixation AA-60 %/AC-40 % soluble in water with the heavy metals, namely Pb(II), Cd(II) and Hg(II) were studied by the batch method. Two R-values were investigated, R (= CA/CM) being the ratio of ligand and metal concentrations. A volume V ([cm.sup.3]) of metal (2 x [10.sup.-2] M) was added to 5 [cm.sup.3] of NaN[O.sub.3] and completed with distilled water to make 50 [cm.sup.3] of final volume. The solution was then stirred under constant magnetic stirring and subsequently titrated by NaOH 0.1 M. All experiments were performed at 25 [+ or -] 0.5 [degrees]C. The results of pH titration are gathered in Figure 5.

At the beginning, the titration curve of Pb(II) exhibits a slight decrease compared to unloaded copolymer suggesting the formation of stable complexes [16].. The discrepancy increases with pH and peaks in the range 6.5-10. In alkaline media (pH ~11), the convergence of the two curves implies a quasi total complexation of Pb(II). Concerning Cd(II), beyond pH 4 one can observe a decrease with a net difference around pH 6.8 followed by a convergence near pH 11 indicating the absence of free Cd(II) species (concentration < [10.sup.-8] M) and consequently a total complexation. Nearly the same behavior can be noticed with Hg(II)

According to Gregor et al. [2], the drop of pH in the titration plots of copolymer in the region 6.5-10 with the various M(II) is ascribed to the equilibria displacement (1 [right arrow] 3) in the forward directions:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (1)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (2)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (3)

The reaction (3) is the combination of (1) and (2). The value K2 of synthesized complexes is calculated from the relation (Eq. 3):

log [K.sub.2] = log [B.sub.2] + 2 pK [m.sup.H] (Eq.3)

The stability constant is deduced from the variation curves of the average coordinance n against p([HA]/[[H.sup.+]]), illustrated in Figure 6., [pK.sub.m.sup.H] being the apparent dissociation constante of copolymer at cc-- 0.5. It is worth mentioning that during titration of systems ligand-M(II), none precipitate has been observed. The present study was restricted to two kinds of complexes, namely MA and [MA.sub.2] as suggested by Gregor et al. [2], a represent the ligand copolymer (RCO[O.sup.-]). In this model, the contribution of formed complexes by carboxylic sites for a number exceeding two have not been taken into account.

The constants [B.sub.2] and [K.sub.2] for investigated M(II) are given in table2. The large value of [K.sub.2] has the same order of magnitude than that cited in the literature [10] and indicate the formation of very stable complexes between the copolymer and M(II); the stability order is the following: Pb(II)> Cd(II)> Hg(II). Figure 6 depicts the formation of M(II) for a ratio R= 5. The observed plateau around [n.sub.av.]= 2, where the slope decreases considerably, suggest that for Pb(II), the predominant specie in this pH range was found to be Pb[A.sub.2]. Concerning the remaining cations, the plots for R= 5 not exhibit any plateau for [n.sub.av.] in the 0-2 range and one can conclude that none of species Cd[A.sub.2], CdA, Hg[A.sub.2] and HgA become predominant. For M(II), an augmentation can be easily seen beyond n,,,,.= 2 suggesting in this pH range the formation of complexes other than [MA.sub.2], however their structure has not been determined [2]

Viscosity measurements

The viscosity measurements table 3, show that the incorporation of crotonic acid with acrylic acid with a quantity going from 0.5 g to 2.0 g decreases the overall molecular weights of the copolymers and makes fall the molecular weight from 54000 to 25000 g/mol, i.e. a decreases of approximately 54% which agree with the literature table 4. As a consequence there is also a decrease in the reduced viscosity figure 7.

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Conclusion

The main goal of the present work was to investigate the acid base behavior and the complexes properties of acrylic and crotonic acids copolymer with regard to divalent cations of group IIB in aqueous media, known for their toxicity and an issue of environmental field. The results of pH titration of copolymer alone indicated the presence of only one type of acidity with a turning point around pH8. The constants [pK.sub.m.sup.H] and n, characterizing the copolymer agree with that reported in the literature data. The notable stability of copolymer complexes with different metal cations, evidenced from high constants [K.sub.2]) show that this material work as good agent fixation and can be used in the recovery of heavy metals present in contaminated water.

References

[1] Querol X., Alastuey A., Moreno N., Alvarez-Ayuso E., Garcia-Sanchez Cama J, Ayora C. and Simon M., 2006, "Immobilization of heavy metals in polluted soils by the addition of zeolitic material synthesized from coal fly ash", Chemosphere, 62, pp. 171.

[2] Gregor, H. P., Luttinger, L.B. and Loebl, E.M., 1955,. "Metal-polyelectrolyte complexes. I. The polyacrylic acid-copper complex", J. Phys. Chem., 59, pp. 34-39.

[3] Bjerrum J., 1941, "In metal ammine formation in aqueous solution", Haase, Copenhagen

[4] Marinsky, J. A. and Anspachlb, W. M., 1975, "Complexation of Copper(II) by a Polymethacrylic Acid Gelia", J. of Phys. Chem., 79, pp. 439-444.

[5] Anspachlb, W. M. and Marinsky, J. A., 1975, "Complexing of Nickel(l1) and Cobalt(l1) by a Polymethacrylic Acid Gel and Its Linear Polyelectrolyte Analog", J. Phys. Chem., 79, pp. 433-439.

[6] Morlay, C., Cromer, M., Mouginot, Y. And Vittori, O., 1998, "Potentiometric study of Cu(II) and Ni(II) complexation with two high molecular weight poly(acrylic acids)", Talanta, 45, pp. 1177-1188.

[7] Morlay, C., Cromer, M., Mouginot, Y. and Vittori, O., 1999, "Potentiometric study of Cu(II) and Pb(II) complexation with two high molecular weight poly(acrylic acids); comparison with Cu(II) and Ni(II), Talanta, 48, pp. 1159-1166.

[8] Morlay, C., Cromer, M., and Vittori, O., 2000, "The removal of copper(II) and nickel(II) from dilute aqueuous solution by a synthetic flocculant: a polarographic study of the complexation with a high molecular weight poly(acrylic acids)for different pH values.", Wat. Res, 34, pp. 455-462.

[9] Morlay, C., Mouginot, Y., Cromer, M. and Vittori, O., 2001, "Determination of the complexation properties of a cross-linked poly(acrylic acid) gel with copper(II), nickel(II) and lead(II) in dilute aqueous solution.", Can. J.Chem., 79, pp. 370-376.

[10] Morlay, C., Mouginot, Y., Cromer, M. and Vittori, O., 2002, "The removal of Cu(II), Cd(II), Ni(II) and Pb(II) from dilute aqueous solution by a poly(acrylic acid) flocculant and its cross-linked analogue.", Wat. Scie. and Tech, 2, pp. 27-34.

[11] Miyajima, T., Mori, M. and Ishiguro, S. I., 1997, "Analysis of complexation equilibria of polyacrylic acid by a Donnan-based concept.", J. of coll. and inter. Sci., 187, pp. 259-266.

[12] Flores, M., Rodriguez, R. and Arroyo, R., 1999, "Synthesis and spectroscopic characterization of [Eu.sup.3+]-doped poly(acrylic acid).", Materials Letters, 39, pp. 329-334.

[13] McLaren J. V., Watts J.D., Gilbert A., J. Polym. Sci., 16C (1967) 1900-1915

[14] Rodolfo D. Porasso, Julio C. Bengas and Marc A. G. T. Van Hoop, 1999, "Chemical and Electrostatic Association of varios metal ions by poly(acrylic acid) and poly(methacrylic acid) as studied by potentiometry" , J. Phys. Chem. B, 103, pp. 2361-2365.

[15] Oth A. and Doty P., Macro-ions II. Polymethacrylic acid, (1951), J. Am. Chem. Soc., 56, pp. 43-50.

[16] R. Roma-Luciow et al, European Polymer Journal, 37, "Complexes of poly(acrylic acid) with some divalent, trivalent and tetravalent metal ions" (2001) pp. 1741-1745.

Salah Bassaid and Messaoud Chaib * Laboratoire Chimie et Environnement, B.P 78 Universite Ibn-Khaldoun Tiaret- 14000 Algeria

* E-mail: [e-mail]

Table 1: values of [C.sub.A], [pK.sub.m.sup.H]
and n of various materials copolymers (concentration
of the supporting electrolyte [C.sub.E] = 0,1 M ; temperature t =
25,0[+ or -]0,5[degrees]C).

Materials         [pK.sub.m.sup.H]          n

AA-90%/CA-10%                  4.9        2.09
AA-80%/CA-20%                  4.8        2.04
AA-70%/CA-30%                  4.7        2.01
AA-60%/CA-40%                  4.5        2.00

Table 2: The values of constants [B.sub.2] and [K.sub.2] with R for
the copolymer complexes

CATION      R (*)    [logB.sub.2]   [logK.sub.2]   [K.sub.2]

Pb(II)         10        - 0,60           8,40     2,51.[10.sup.8]
                5        - 0,18           8,82     6,61.[10.sup.8]

Cd(II)         10         - (a)          - (a)              - (a)
                5        - 2,54           6,46     2,88.[10.sup.6]

Hg(II)         10        - 3,32           5,68     4,78.[10.sup.5]
                5        - 2,90           6,10     1,26.[10.sup.6]

(*) R = [C.sub.A]/[C.sub.M] ([C.sub.A]: total concentration of the
carboxylic sites of the ligand ; [C.sub.M]
total concentration of the metal cation).

(a) Non given value.

Table 3: Molecular weights and intrinsic viscosities of copolymers.

                                                   Molecular
            Monomers      Intrinsic                weights
Sample No   composition   viscosity                [M.sub.v]
            AA/CA (g/g)   [[eta]] (dl.[g.sup.-1])  (g.[mol.sup.-1])

        1   4.5/0.5                        0.32    54 000
        2   4.0/1.0                        0.27    43 000
        3   3.5/1.5                        0.22    33 000
        4   3.0/2.0                        0.18    25 000

Table 4: Values of pK[alpha]=0.5H and n, found in literature for some
acrylic polmers study

Average molecular      [C.sub.A]          [C.sub.NaN03]
weight                 (eq/l)             (mol/l)

[3.10.sup.4]             [10.sup.-2]           0.2
                         [10.sup.-2]           1.0
                         [10.sup.-2]           2.0
                         [10.sup.-2]           3.0
3,52.[10.sup.5]          [10.sup.-2]           0.25
2,50.[10.sup.4] (a)    3,7.[10.sup.-3]         0.1

[C.sub.A]              [pK[alpha]=
(eq/l)                 0.5.sup.H]         n                Reference

    [10.sup.-2]                  4.91     1.69             [2]
    [10.sup.-2]                  4.48     -
    [10.sup.-2]                  4.30     1.39
    [10.sup.-2]                  4.23     -                [13]
    [10.sup.-2]                   4.9     -
  3,7.[10.sup.-3]                4.50     2.00             our study

(a) : determined by viscosity measurements