**18. Complexing agents**

For over fifty years synthetic chelating agents from the group of aminopolycarboxylic acids (APCAs) have been the basis in many technological processes. Ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA) and diethylenetriaminepentaacetic acid (DTPA) are the best known traditional complexing agents. They are commonly applied in many branches of industry forming stable, water soluble complexes with various metal cations or as a masking agent. Nowadays there are a number of alternative products on the market which claim to be as effective as EDTA and NTA. Among them, IDS and MGDA should be listed.

Iminodisuccinic acid (IDS) also known as Baypure CX 100 is a medium-strong chelator consisting of: iminodisuccinic acid sodium salt > 32%, aspartic acid sodium salt < 7%, fumaric acid sodium salt < 3.5 %, hydroxysuccinic acid < 0.9 %, maleic acid sodium salt < 0.9 % (IDS Na-salz, 1998; Vasilev et al. 1996; Vasilev et al. 1998; Reinecke et al. 2000, Kołodyńska et al. 2009; Kołodyńska, 2009a). Iminodisuccinic acid sodium salt can form quintuplebonded complexes with metal ions. In this case, complexing occurs via the nitrogen and all four carboxyl groups. As a result of the octahedric structure of the complete complex, a water molecule is required for the sixth coordination point (Kołodyńska, 2009b). In the paper by Hyvönen it was found that for low pH conditions (less than 3), the tendency for M(II)/M(III) ions to form complexes with IDS may be assumed as: Cu(II)>Fe(III)>Zn(II)>Mn(II), whereas for pH >7 it can be as follows: Cu(II)>Zn(II)> Mn(II)>Fe(III) (Hyvönen et al. 2003; Hyvönen & Aksela, 2010). IDS is able to replace EDTA when rather moderate chelating agents are sufficient for masking alkaline earth or heavy metal ions. As a substitute for EDTA it is used in a variety of applications, including detergent formulations, corrosion inhibitors, production of pulp and paper, textiles, ceramics, photochemical processes, and as trace nutrient fertilizers in agriculture.

Methylglycinediacetic acid (MGDA) was patented by BASF and marketed under the brand name Trilon M. The active ingredient contained in Trilon M is the trisodium salt of methylglycinediacetic acid. The acid dissociation constants pKa of MGDA are as follows: pK1=1.6, pK2=2.5 pK3=10.5 (Jachuła et al. 2011; Jachuła et al. 2012). The most important property of Trilon M is the ability to form complexes (MGDA is a tetradentate chelating ligand where chelation involves three carboxylate groups and nitrogen atom) with metal ions, soluble in water in the large pH range 2-13. These complexes remain stable, especially in alkaline media and even at temperatures of up to 373 K. It is worth mentioning that MGDA chelating capacity was investigated by Tandy et al. (2004) in soil washing. It was found that 89-100% of MGDA can be degraded in 14 days, 90% of EDDS in 20 days while no EDTA was degraded in 30 days.

222 Ion Exchange Technologies

**17. Results**

**18. Complexing agents**

The pH values were measured with a PHM 84 pH meter (Radiometer, Copenhagen) with the glass REF 451 and calomel pHG 201-8 electrodes. The concentrations of heavy metals

As for the removal of toxic metal ions many different methods are available. Among them, the most commonly used are ion exchange, adsorption, reduction and precipitation. In many cases, the environmentally most compatible and cost-effective solutions include combination of two or more of these processes. From different waste waters those

For over fifty years synthetic chelating agents from the group of aminopolycarboxylic acids (APCAs) have been the basis in many technological processes. Ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA) and diethylenetriaminepentaacetic acid (DTPA) are the best known traditional complexing agents. They are commonly applied in many branches of industry forming stable, water soluble complexes with various metal cations or as a masking agent. Nowadays there are a number of alternative products on the market which claim to be as effective as EDTA and NTA. Among them, IDS and MGDA should be listed.

Iminodisuccinic acid (IDS) also known as Baypure CX 100 is a medium-strong chelator consisting of: iminodisuccinic acid sodium salt > 32%, aspartic acid sodium salt < 7%, fumaric acid sodium salt < 3.5 %, hydroxysuccinic acid < 0.9 %, maleic acid sodium salt < 0.9 % (IDS Na-salz, 1998; Vasilev et al. 1996; Vasilev et al. 1998; Reinecke et al. 2000, Kołodyńska et al. 2009; Kołodyńska, 2009a). Iminodisuccinic acid sodium salt can form quintuplebonded complexes with metal ions. In this case, complexing occurs via the nitrogen and all four carboxyl groups. As a result of the octahedric structure of the complete complex, a water molecule is required for the sixth coordination point (Kołodyńska, 2009b). In the paper by Hyvönen it was found that for low pH conditions (less than 3), the tendency for M(II)/M(III) ions to form complexes with IDS may be assumed as: Cu(II)>Fe(III)>Zn(II)>Mn(II), whereas for pH >7 it can be as follows: Cu(II)>Zn(II)> Mn(II)>Fe(III) (Hyvönen et al. 2003; Hyvönen & Aksela, 2010). IDS is able to replace EDTA when rather moderate chelating agents are sufficient for masking alkaline earth or heavy metal ions. As a substitute for EDTA it is used in a variety of applications, including detergent formulations, corrosion inhibitors, production of pulp and paper, textiles,

ceramics, photochemical processes, and as trace nutrient fertilizers in agriculture.

Methylglycinediacetic acid (MGDA) was patented by BASF and marketed under the brand name Trilon M. The active ingredient contained in Trilon M is the trisodium salt of methylglycinediacetic acid. The acid dissociation constants pKa of MGDA are as follows: pK1=1.6, pK2=2.5 pK3=10.5 (Jachuła et al. 2011; Jachuła et al. 2012). The most important property of Trilon M is the ability to form complexes (MGDA is a tetradentate chelating

were measured with the AAS spectrometer Spectra 240 FS (Varian, Australia).

containing heavy metal ions and complexing agents require special attention.

Fig. 5a-b shows the comparison of the logarithmic stability constants (log K) for the complexes of IDS and MGDA and selected metal ions with the stability constants for EDTA.

**Figure 5.** a-b. Comparison of conditional stability constants values of some complexes of metals with EDTA and IDS (a) as well as EDTA and MGDA (b).

A high or moderately high value for log K of Cu(II), Zn(II), Cd(II) and Pb(II) and first of all Fe(III) with IDS and MGDA indicates that these chelating agents have a high affinity for particular metal ions and they provide a preliminary indication of whether the chelating agent is suitable for the specific application.

As these complexing agents are widely applied, removal of their complexes with heavy metals is essential, especially when typical chemical precipitation methods are ineffective, even if solutions with high metal concentrations are treated. Therefore, more advanced techniques are required for cleaning up such contaminants and retardation of heavy metal ions mobility. Among these, the ion exchange with application of selective resins appears to be a more promising method for the treatment of such solutions.

Generally, chelating properties and selectivity of ion exchangers have been enhanced by: (i) immobilization of ligands with multiple coordinating sites such as bifunctional polymers or polyfunctional polymers possessing different functional groups, (ii) immobilization of low molecular weight complexing agents, (iii) by preparation of ion imprinted polymers (IIP), (iv) preparation of reactive ion exchangers (RIEX), (vi) immobilization of specific donor groups through application of Pearson's hard soft acid base theory, (vii) immobilization of macrocycles e.g. crown ethers, calixarenes, resorcinarenes etc. These approaches correspond to both chelating ion exchangers Dowex M 4195 and Diphonix Resin®. Additionally, their sorption selectivity can be affected by sorbate-sorbent and sorbate-solvent interactions. It has been well recognized that the resin matrix and the functional groups can strongly affect ion exchange capacity and selectivity (Clifford & Weber, 1983; Barron & Fritz, 1984; Li et al. 1998). Therefore, in the case of chelating ion exchangers, where the formation of coordination bonds is the basis of the sorption process, besides the parameters related to physicochemical properties of the resins, the effect of the presence of complexing agents should be also taken into account.

In the presence of the complexing agents, IDS and MGDA, there are formed:

$$\rm M^{2+} + \rm H\_{nids} \rm n^{-4} \rightleftharpoons [M(H\_{nids})]^{n-2} \text{ where n=1,2,3}$$

and

$$\text{M}^{\text{2+}} + \text{H}\_{\text{n}}\text{mg}\text{da}^{\text{n}-3} \rightleftharpoons [\text{M}(\text{H}\_{\text{n}}\text{mg}\text{da})]^{\text{n-2}} \text{ where n=1,2.}$$

Therefore using selective chelating ion exchangers the sorption effectiveness will be dependent on the decomposition of neutral or anionic species of [MH2L], [MHL]- and [ML]2- type, where L=ids4-, mgda3-. Additionally, the 'sieve effect' is also important (Kołodyńska, 2010b; Kołodyńska 2010c; Kołodyńska 2011). In the case of the chelating resin Dowex M 4195 possessing the bis(2-pyridylmethyl)amine (bpa) functional groups, depending on the pH value the mechanism of sorption can be as presented earlier. Additionally, the ionic interaction mechanism between the protonated amines and the anionic complexes of the [ML]2- and [ML] is also possible (Kołodyńska 2011). Therefore, appropriate reactions can be as follows:

$$\begin{aligned} \text{R-HN^{+}(bpa)} \text{cCl} + [\text{ML}]^{2-} &\rightleftharpoons [\text{R-HN^{+}(bpa)}z] \text{[ML]}^{2-} + 2 \text{Cl}^{-} \\\\ \text{R-HN^{+}(bpa)} z \text{Cl} + [\text{MHL}] &\rightleftharpoons [\text{R-HN^{+}(bpa)z}] [\text{MHL}]^{-} + \text{Cl} \cdot \end{aligned}$$

or

$$\text{R-HN^{+}(bpa)}\\\text{2Cl} + \text{[ML]} \rightleftharpoons \text{[R-HN^{+}(bpa)}\\\text{2][ML]} + \text{Cl}^{+} $$

where: R is the Dowex M 4195 skeleton (PS-DVB), L is the ids4- or mgda3- ligand.

The analogous mechanism of sorption in the case of Diphonix chelating ion exchanger should be considered.

Kinetic studies

For the kinetic data, a simple kinetic analysis was performed using the pseudo first order and the pseudo second order equations:

$$
\log(q\_e - q\_t) = \log(q\_e) - \frac{k\_1 t}{2.303}
$$

$$
\frac{t}{q\_t} = \frac{t}{q\_e} + \frac{1}{k\_2 q\_e^2}
$$

2

where: qe is the amount of metal complexes sorbed at equilibrium (for the pseudo first order model also denoted as q1 and q2 for the pseudo second order model) (mg/g), qt is the amount of metal complexes sorbed at time t (mg/g), k1, k2 are the equilibrium rate constants (1/min), respectively.

224 Ion Exchange Technologies

and

or

should be considered.

and the pseudo second order equations:

Kinetic studies

should be also taken into account.

sorption selectivity can be affected by sorbate-sorbent and sorbate-solvent interactions. It has been well recognized that the resin matrix and the functional groups can strongly affect ion exchange capacity and selectivity (Clifford & Weber, 1983; Barron & Fritz, 1984; Li et al. 1998). Therefore, in the case of chelating ion exchangers, where the formation of coordination bonds is the basis of the sorption process, besides the parameters related to physicochemical properties of the resins, the effect of the presence of complexing agents

M2+ + Hnidsn-4 ⇄ [M(Hnids)]n-2 where n=1,2,3

M2+ + Hnmgdan-3 ⇄ [M(Hnmgda)]n-2, where n=1,2. Therefore using selective chelating ion exchangers the sorption effectiveness will be dependent on the decomposition of neutral or anionic species of [MH2L], [MHL]- and [ML]2- type, where L=ids4-, mgda3-. Additionally, the 'sieve effect' is also important (Kołodyńska, 2010b; Kołodyńska 2010c; Kołodyńska 2011). In the case of the chelating resin Dowex M 4195 possessing the bis(2-pyridylmethyl)amine (bpa) functional groups, depending on the pH value the mechanism of sorption can be as presented earlier. Additionally, the ionic interaction mechanism between the protonated amines and the anionic complexes of the [ML]2- and [ML]-

is also possible (Kołodyńska 2011). Therefore, appropriate reactions can be as follows:

2RHN+(bpa)2 Cl- + [ML]2- ⇄ [RHN+(bpa)2]2[ML]2- + 2Cl-

RHN+(bpa)2 Cl- + [MHL]- ⇄ [RHN+(bpa)2][MHL]-

RHN+(bpa)2 Cl- + [ML]- ⇄ [RHN+(bpa)2][ML]-

The analogous mechanism of sorption in the case of Diphonix chelating ion exchanger

For the kinetic data, a simple kinetic analysis was performed using the pseudo first order

1 log( ) log( ) 2.303 *et e*

*t e e*

*qq q*

*t t q q k q*

*k t*

2 2 1

where: R is the Dowex M 4195 skeleton (PS-DVB), L is the ids4- or mgda3- ligand.

 + Cl-

 + Cl-

In the presence of the complexing agents, IDS and MGDA, there are formed:

The sorption of Cu(II), Zn(II), Cd(II) and Pb(II) complexes with IDS on Dowex M 4195 in the M(II)-L=1:1 system is presented in Fig.6a. The analogous data for the Cu(II), Zn(II), Cd(II) and Pb(II) complexes with MGDA sorption on Dowex M 4195 are presented in Fig.6b and for Diphonix Resin® in Figs.6c and 6d.

**Figure 6.** a-b**.** The effect of the phase contact time on the sorption capacities of Cu(II), Zn(II), Cd(II) and Pb(II) complexes with IDS on Dowex M 4195 (a) and Diphonix Resin® (c) as well as the Cu(II), Zn(II), Cd(II) and Pb(II) complexes with MGDA on Dowex M 4195 (b) and Diphonix Resin® (d) (c0 110-3 mol/dm3, shaking speed 180 rpm, shaking time 1-120 min, room temperature).

The straight lines of t/qt vs. t suggest the applicability of the pseudo second kinetic model to determine the qe, k2 and h parameters (from the intercept and the slope of the plots). These kinetic parameters are presented in Tables 3 and 4.

It was shown that the equilibrium was reached very quickly. More than 90% of metal ions were bound to Dowex M 4195 and Diphonix Resin® within 10-20 min of the phase contact time and therefore a slight increase until a plateau was reached after about 2 h was observed. The values of the theoretical qe for the studied resins were in good agreement with those obtained experimentally (qe,exp). On Dowex M 4195 about 95 %, 100 %, 99 % and

**Table 3.** The pseudo second order kinetic parameters for the sorption of Cu(II), Zn(II), Cd(II) and Pb(II) complexes with IDS and MGDA on Dowex M 4195.

**Table 4.** The pseudo second order kinetic parameters for the sorption of Cu(II), Zn(II), Cd(II) and Pb(II) complexes with IDS and MGDA on Diphonix Resin®.

97.5 % of the Cu(II), Zn(II), Cd(II) and Pb(II) complexes with IDS and 94 %, 98 %, 96 % and 95 % complexes with MGDA are sorbed at this time, respectively. On Diphonix Resin® for the Cu(II), Zn(II), Cd(II) and Pb(II) complexes with IDS and MGDA the adequate values are as follows: 94 %, 89 %, 97 % and 98 % as well as 97 %, 86 %, 99 % and 96 %. These results indicate that the sorption process of metal ions in the presence of IDS and MGDA on Dowex M 4195 and Diphonix Resin® followed a pseudo second order kinetics, which meant that both the external mass transfer and intraparticle diffusion together were involved in the sorption process. The correlation coefficients (R2) obtained for the pseudo second order kinetic model are in the range 0.9991 -1.000 for all metal complexes. The pseudo first order parameters were not shown because the correlation coefficients for this model are low (0.7438-0.8745 for the IDS complexes and 0.919-0.986 for the MGDA complexes on Diphonix Resin®.

The breakthrough curves for Cu(II) ions in the presence of MGDA on Dowex M4195 from single metal ion solutions of a concentration 1x10–3 M are shown in Fig. 7. Typical 'S' shaped curves were obtained in the experiments. Analogous results were obtained on Diphonix Resin®. It should be mentioned the UV exposition does not have a significant effect on the decomposition of the complexes in the resin phase.

complexes with IDS and MGDA on Dowex M 4195.

complexes with IDS and MGDA on Diphonix Resin®.

decomposition of the complexes in the resin phase.

Resin®.

**System qe.exp [mg/g] q2 [mg/g] k2 h R2** Cu(II)-IDS=1:1 5.63 5.61 1.012 5.789 0.9987 Zn(II)-IDS=1:1 5.91 5.88 1.007 4.897 0.9988 Cd(II)-IDS=1:1 9.81 9.89 0.987 12.456 0.9999 Pb(II)-IDS=1:1 19.10 19.00 0.845 16.789 0.9992 Cu(II)-MGDA=1:1 6.05 5.98 2.335 9.237 0.9999 Zn(II)-MGDA=1:1 4.17 4.03 1.017 16.783 0.9996 Cd(II)-MGDA=1:1 12.55 12.23 0.924 10.123 0.9999 Pb(II)-MGDA=1:1 17.77 17.46 0.688 7.525 0.9999 **Table 3.** The pseudo second order kinetic parameters for the sorption of Cu(II), Zn(II), Cd(II) and Pb(II)

**System qe.exp [mg/g] q2 [mg/g] k2 h R2** Cu(II)-IDS=1:1 6.12 6.21 2.211 10.207 0.9999 Zn(II)-IDS=1:1 6.01 6.09 1.345 7.123 0.9991 Cd(II)-IDS=1:1 10.21 10.11 0.988 23.434 0.9999 Pb(II)-IDS=1:1 20.39 20.26 0.876 37.551 0.9998 Cu(II)-MGDA=1:1 5.66 5.61 3.469 11.111 0.9999 Zn(II)-MGDA=1:1 4.48 4.48 2.395 48.077 0.9999 Cd(II)-MGDA=1:1 10.23 10.24 0.024 2.475 0.9999 Pb(II)-MGDA=1:1 18.94 18.93 0.188 67.568 0.9999 **Table 4.** The pseudo second order kinetic parameters for the sorption of Cu(II), Zn(II), Cd(II) and Pb(II)

97.5 % of the Cu(II), Zn(II), Cd(II) and Pb(II) complexes with IDS and 94 %, 98 %, 96 % and 95 % complexes with MGDA are sorbed at this time, respectively. On Diphonix Resin® for the Cu(II), Zn(II), Cd(II) and Pb(II) complexes with IDS and MGDA the adequate values are as follows: 94 %, 89 %, 97 % and 98 % as well as 97 %, 86 %, 99 % and 96 %. These results indicate that the sorption process of metal ions in the presence of IDS and MGDA on Dowex M 4195 and Diphonix Resin® followed a pseudo second order kinetics, which meant that both the external mass transfer and intraparticle diffusion together were involved in the sorption process. The correlation coefficients (R2) obtained for the pseudo second order kinetic model are in the range 0.9991 -1.000 for all metal complexes. The pseudo first order parameters were not shown because the correlation coefficients for this model are low (0.7438-0.8745 for the IDS complexes and 0.919-0.986 for the MGDA complexes on Diphonix

The breakthrough curves for Cu(II) ions in the presence of MGDA on Dowex M4195 from single metal ion solutions of a concentration 1x10–3 M are shown in Fig. 7. Typical 'S' shaped curves were obtained in the experiments. Analogous results were obtained on Diphonix Resin®. It should be mentioned the UV exposition does not have a significant effect on the

**Figure 7.** The breakthrough curves of Cu(II) complexes with MGDA on Dowex M 4195 without and with UV exposition (c0 110-3 mol/dm3, bed volume 10 cm3, flow rate 0.6 cm3/min)

It is well known that the particle size of ion exchange resins influences the time required to establish equilibrium conditions and two types of diffusion must be considered in an ion exchange equilibrium e.g. the film diffusion (the movement of ions from a surrounding solution to the surface of an ion exchange particle) and the internal diffusion (the movement of ions from the surface to the interior of an ion exchange particle). Film diffusion is usually the controlling reaction in dilute solutions whereas the internal diffusion is controlling in more concentrated solutions. The particle size of an ion exchange resin affects both the film diffusion and the internal diffusion (Kołodyńska, 2011).

According to the manufacturer data the particle size of Dowex M 4195 is 0.300-1.200 mm. However, Diphonix Resin® available on the commercial scale is in the range 0.30-0.85 mm, 0.15-0.30 mm and 0.075-0.15 mm.

In the presented paper Diphonix Resin® with the particle size 0.075-0.150 mm was used to study the sorption process of Cu(II), Zn(II), CdII) and Pb(II) in the presence of IDS and MGDA. In the paper by Cavaco et al. (2009) it was found that for the range 0.15-0.30 mm, 50 % of the particles have diameters less than 0.223 mm. As follows from the obtained results, the bead size of the used chelating ion exchangers has also approximately the Gaussian distribution (Fig. 8 a-b). It was found that with the increase of bead dimensions, the volume fractions of disc-similar beads decrease and the beads are more spherical (Kołodyńska, 2011).

A decrease in the particle size thus shortens the time required for equilibration of particle size and pore characteristics have an effect on equilibrium concentration and influence sorption kinetics. Therefore this factor is essential, especially when the sorption of metal complexes, not metal ions is taken into account. In the case of large complexes the sieve effect is observed.

Kinetic sorption experiments were also carried out with the increased complexes concentrations from 110-3 mol/dm3 to 210-2 mol/dm3 and these results were presented in

**Figure 8.** a-b. Comparison of the distribution of the bead size of Dowex M 4195 (a) and Diphonix Resin® (b) based on the Zingg classification.

(Kołodyńska, 2011). It was found that with an increase of metal complexes concentrations a continuous increase in the amount adsorbed per unit mass of ion exchanger was observed till the equilibrium was achieved. For the pseudo second order kinetic model, the rate k2 values decrease with the increasing initial concentrations, while h increases.

### **19. pH effects**

The effect of pH was studied for the Cu(II), Zn(II), Cd(II) and Pb(II) in the M(II)-IDS=1:1 and M(II)-MGDA=1:1 systems at the pH varied from 2 to 12. The optimal sorption range of the Cu(II), Zn(II), Cd(II) and Pb(II) complexes with IDS practically does not change in the pH range from 4 to 10 both on Dowex M 4195 and Diphonix Resin® whereas, at high pH values, decrease in removal efficiency is observed. In the case of the Cu(II), Zn(II), Cd(II) and Pb(II) complexes with MGDA a slight decrease in sorption efficiency with the increasing pH was also shown.

### **20. Adsorption studies**

The Langmuir equation was applicable to the homogeneous adsorption system, while the Freundlich equation was the non-empirical one employed to describe the heterogeneous systems and was not restricted to the formation of the monolayer. The well-known Langmuir equation was represented as:

$$\frac{1}{q\_e} = \frac{1}{bq\_0c\_e} + \frac{1}{q\_0}$$

where: qe is the equilibrium M(II) ions concentration on the ion exchanger, (mg/g), ce is the equilibrium M(II) ions concentration in solution (mg/dm3), q0 is the monolayer capacity of ion exchanger (mg/g), b is the Langmuir adsorption constant (L/g) related to the free energy of adsorption.

The values of q0 and b were calculated from the slope and the intercept of the linear plots ce/qe vs. ce. On the other hand, the Freundlich equation was represented as:

$$q\_e = K\_F \mathcal{C}\_e^{\delta \bar{\mu}}$$

where: KF and 1/n are the Freundlich constants corresponding to the adsorption capacity and the adsorption intensity.

The plot of ln qe vs. ln ce was employed to generate the intercept KF and the slope 1/n.

228 Ion Exchange Technologies

(b) based on the Zingg classification.

**[%] (a)**

**19. pH effects**

also shown.

of adsorption.

**20. Adsorption studies**

Langmuir equation was represented as:

**Figure 8.** a-b. Comparison of the distribution of the bead size of Dowex M 4195 (a) and Diphonix Resin®

**[-]**

**[%] (b)**

**0,00 0,2 0,4 0,6 0,8 1,0**

**[-]**

(Kołodyńska, 2011). It was found that with an increase of metal complexes concentrations a continuous increase in the amount adsorbed per unit mass of ion exchanger was observed till the equilibrium was achieved. For the pseudo second order kinetic model, the rate k2

The effect of pH was studied for the Cu(II), Zn(II), Cd(II) and Pb(II) in the M(II)-IDS=1:1 and M(II)-MGDA=1:1 systems at the pH varied from 2 to 12. The optimal sorption range of the Cu(II), Zn(II), Cd(II) and Pb(II) complexes with IDS practically does not change in the pH range from 4 to 10 both on Dowex M 4195 and Diphonix Resin® whereas, at high pH values, decrease in removal efficiency is observed. In the case of the Cu(II), Zn(II), Cd(II) and Pb(II) complexes with MGDA a slight decrease in sorption efficiency with the increasing pH was

The Langmuir equation was applicable to the homogeneous adsorption system, while the Freundlich equation was the non-empirical one employed to describe the heterogeneous systems and was not restricted to the formation of the monolayer. The well-known

> 111 *e e q bq c q*

where: qe is the equilibrium M(II) ions concentration on the ion exchanger, (mg/g), ce is the equilibrium M(II) ions concentration in solution (mg/dm3), q0 is the monolayer capacity of ion exchanger (mg/g), b is the Langmuir adsorption constant (L/g) related to the free energy

The values of q0 and b were calculated from the slope and the intercept of the linear plots

ce/qe vs. ce. On the other hand, the Freundlich equation was represented as:

0 0

values decrease with the increasing initial concentrations, while h increases.

**0,00 0,2 0,4 0,6 0,8 1,0**

The exemplary results presented in Fig.9a-b indicate that for the studied range of concentration of Cu(II) complexes with MGDA (110-3 M - 210-2 M) the sorption capacity of Dowex 4195 and Diphonix Resin® increases.

**Figure 9. a-b.** The effect of the concentration on the sorption capacities of Cu(II) complexes with MGDA on Dowex M 4195 (a) and Diphonix (b) (c0 110-3 -20x10-3 mol/dm3, shaking speed 180 rpm, shaking time 1-120 min, room temperature).

The experimental data obtained for the sorption of Cu(II), Zn(II), Cd(II) and Pb(II) in the presence of IDS and MGDA on Dowex M 4195 and Diphonix Resin® were well represented by the Langmuir isotherm model (Table 5). The correlation coefficients of the linear plot of ce/qe vs. ce obtained from them were high, ranging from 0.9512 to 0.9999 (Kołodyńska, 2011). The highest values of the Langmuir parameter q0 were obtained in the case of Pb(II)

**Table 5.** The Langmuir and Freundlich isotherm parameter values for the sorption of Cu(II), Zn(II), Cd(II) and Pb(II) ions in the presence of IDS and MGDA on Dowex M4195.

complexes with IDS and MGDA on Dowex M 4195 and Diphonix Resin®. They are equal to 121.58 mg/g and 97.64 mg/g on Dowex M 4195 and 112.37 mg/g and 100.20 mg/g on Diphonix Resin®, respectively.

For the studied systems regeneration tests were conducted using HCl, HNO3, H2SO4 and NaCl at 1M and 2M concentrations. Based on the series of five experiments using known amounts of Cu(II) complexes with IDS and MGDA sorbed, it was established that the overall recoveries of Cu(II) eluted from Dowex M 4195 and Diphonix Resin® by 2M HCl and H2SO4 were above 98 %, suggesting that the recovery is quantitative.