Poly (vinyl sulfate) potassium salt (PVSK) with molar mass of 170 KDa, was obtained from Sigma-Aldrich. The negative charges are attributed to sulfate group in each monomer. Flocculant (Trident 27506) that are commercial polyelectrolyte intended to be quantified in an industrial waste water treatment process. Concentration of PVSK is expressed in mg/L in order to compare with results obtained for Flocculant and also in molar concentration (mM) expressed on the basis of a monomeric unit which indicates the moles of ionic group per liter of the polymer in solution.

As the structure and identity are unknown for Flocculant concentration is expressed in mg/L.

Toluidine Blue O solution was used as an indicator and was obtained from Sigma-Aldrich. Aluminum, cadmium, chromium, copper, iron, potassium, nickel, lead, tin, zinc and magnesium standard solutions was obtained for AAS at 1000 mg/L (FLUKA).

Absorbance of solutions was measured at 400 - 800 nm with a UV-Vis spectrophotometer (Cary 100 Conc, Varian) using a 10 mm pathlength cell.

pH Meter was a Denver Instrument UP-5.

The FTIR spectra were recorded on a Spectrum 400 FT-IR/FT-NIR spectrometer (Perkin Elmer).

Synthetic PVS-OTB solutions were prepared mixing 1.0 mL of a 201.1 mg/L (1.24 mM) PVSK solution and different aliquots of a 82.6 mg/L (0.27 mM) OTB solution graduated to a 10 mL final volume.

The optimal OTB/PVSK ratio was determinated according to the following procedure: Solutions were prepared mixing 1.0 mL of a 201.1 mg/L PVSK solutions and different aliquots from 0.1 to 1.0 mL of 82.6 mg/L OTB solution and diluted to 10 mL final volume with deionized water. A solution was prepared with 1 mL of 82.6 mg/L mM OTB and diluted to 10 mL with deionized water; this solution was taken as a blank. UV-Vis absorbance of each solution was recorded and a graph of absorbance at 509 nm vs µg OTB was constructed. The same methodology was used to determine the optimal dose of OTB in the study of Flocculant-OTB interaction. The ignition concentration of Flocculant was 22.4 mg/L.

All titrations were performed in a batch model. The solutions were prepared mixing 1.0 mL of a 201.1 mg/L KPVS solution (1.24 μmoles) and 0.5 mL of a 82.6 mg/mL OTB solution (0.135 µmoles) with different aliquots of metal ion solutions such as Pb²⁺, Fe³⁺, Cd²⁺, Cr³⁺, Cu²⁺, K⁺, Mg²⁺, Ni³⁺ and Sn²⁺ at 10 mg/L. Then, UV-Vis spectra were obtained for all samples. The same procedure was followed with Flocculant, mixing 1.0 mL of a 22.4 mg/L solution with 0.5 mL of a 82.6 mg/L (0.27 mM) OTB solution with different aliquots of metal ion solutions 10 mg/L.

First, was determinate the optimal relationship between PVSK and OTB for the study. As can be seen in Figure 2, free OTB has a maximum of absorbance at 628 nm and decreases when OTB interacts with PVSK. The PVS-OTB complex formed by the association of these two species presents an absorbance band at 509 nm. In the first solution (0.20 mg of PVSK + 8.26 μg of OTB) the spectra shows almost exclusively the absorption band corresponding to the PVS-OTB complex at 509 nm. For

the rest of combinations the absorption band corresponding to the complex increases, but also the absorption band of free OTB indicating that at higher OTB concentration, PVSK has been saturated.

In Figure 2(b) are shown the bound and free OTB amount profiles versus the amount of OTB added. It is observed a change in slope at 50 µg of OTB added. Beyond this point the amount of bound OTB remains constant and free OTB increases gradually. This corresponds to the equivalence point titration of PVSK with OTB. In order to eliminate variations due to saturation phenomena was selected the amount of 41.30 µg OTB added (0.135 μmoles OTB/1.24 µmoles PVSK) for the titration study with metal ions. Also, at this relationship there are 24.65 µg (0.080 µmol) of bound OTB and 14.65 µg (0.048 µmol) of free OTB.

When K+ is exchanged by OTB on the PVS the solution has a pale pink coloration, but as the metal is added, OTB colorant is released and the solution turn to a blue, the typical color that indicates the presence of free OTB (Figure 3). In the absorbance spectra was observed that

as the metal ion amount increases, the absorbance band at 509 nm corresponding to the PVS-OTB complex gradually decreases and the absorbance band at 628 nm increases.

The UV-Vis spectra obtained in the titration of PVSOTB complex with Al³⁺ and Pb²⁺ are shown in Figure 4.

It is noteworthy that Al³⁺ causes a greater increase than Pb²⁺ in the absorption band at 628 nm because the higher displacement of the OTB complexed to the anionic polyelectrolyte. Another significant difference is the location of the isosbestic points, which are located at 573 nm for Al³⁺ and at 603 nm for Pb²⁺ complexes.

The bound and free OTB amount profile obtained by titration of PVS-OTB showed that at 17 µg (0.628 µmol) of Al³⁺/mg PVSK, 5.55 µg (0.018 µmol) of OTB were released.

With the same amount of Pb²⁺ only 0.28 µg (0.0009 µmol) of OTB were displaced by the metal ion. In this sense, PVS has twenty times more affinity for Al³⁺ than Pb²⁺.

The absorbance profiles obtained by titration of PVSOTB complex with various metal ions are shown in Figure 5. In general, is possible to distinguish three groups according to the affinity that presents each metal ion with the anionic polyelectrolyte. Metal ions Al³⁺, Mg²⁺ and Cr³⁺ are the group with more affinity. The second group corresponds to Zn²⁺, Sn²⁺ and Fe³⁺ having intermediate affinity, and Ni²⁺, Cd²⁺, K⁺, Cu²⁺ and Pb²⁺ are the metal ions with the lower affinity. The OTB release ratio depends on the concentration, number of valence and affinity of polyelectrolyte toward the metal ion.

Observed affinity can be explained in terms of the well known Hard and Soft Acids and Bases principles (HSAB) proposed by Pearson.

Sulfate anion is a hard base and has more affinity toward hard acids such as Al³⁺, Mg²⁺ and Cr³⁺; an intermediate affinity for semihard acids as to Zn²⁺ and Fe³⁺and lower affinity toward soft acids as Pb²⁺.

Relationships metal ion/PE were variable due to metal ions with greater affinity favored formation of flocs or suspended solids at very low doses (17 µg Al³⁺/mg PVS), this is related to the effect of the metal ion valence because polyvalent ions can form ionic bonds with more than one anionic monomer unit of PVS and this promotes

the formation of flocs (see Figure 6), whereas with lower affinity ions higher ratios were achieved without presenting formation of solids or significant changes in absorbance even at relationships μg metal/mg PVSK equal to 60.

Equations (1) and (2) represent the equilibrium between PVSK and OTB to form the PVSK-OTB complex and the equilibrium in which the PVS-OTB complex is used to study the physicochemical behavior of anionic polyelectrolyte by exchange of OTB with metal ions. These two processes are in competition and are related with association constants K₁ and K₂, respectively. As was described, the amount of displaced OTB by metal ion depends on the affinity between polyelectrolyte and cation. The magnitude of K₂ is a relative measure of this affinity. In summary, metal ions which exhibit higher K₂ values should effect a greater displacement of OTB.

The association constant K₁ for PVSK-OTB complex was determined from the absorbance data profile at 509 nm, while the association constants of the PVS-metal ion complex were calculated indirectly using absorbance profiles at 628 nm. Calculation of the constants was performed by a curve fitting based on a non-linear leastsquares method [26] assuming that each monomeric unit of PVS interacts with a metal ion forming a 1:1 complex. This assumption is done considering that the equilibrium are achieved before the flocs formation. In Table 1 are reported the values obtained and is noteworthy that metal ions performing a higher displacement of OTB in PVS-OTB complex yields the greater magnitudes for K₂.

Table 1. Relative association constants of metal ions and PVSK.

Moreover, metal ions that generate less displacement OTB have lower K₂ values or even near the constant K₁.

Here, is interesting that association constant of PVSK⁺ complex is greater than PVS-OTB constant, this is the reason that OTB promotes low displacement of K⁺ in PVSK and when PVS-OTB complex is retitrated with K⁺ the latter displaces OTB.

The optimal relationship between Flocculant and OTB was determinate for the study. In Figure 7 free OTB band has a maximum absorbance at 628 nm and decreases when OTB interacts with Flocculant. In the first solution (0.224 mg of Flocculant + 8.26 μg of OTB) the spectrum shows an absorption band corresponding to the Flocculant-OTB complex at 555 nm. For the rest of combinations the absorbance increases as well as the absorbance of free OTB indicating that at higher OTB concentrations, Flocculant has not been saturated.

In Figure 7(b) is shown the bound and free OTB profile versus the total amount of OTB added. It is observed a slight change in slope at 50 µg of OTB added. This might correspond to the equivalence point titration, but unlike PVSK, Flocculant does not saturate at higher amounts of OTB. Then, the selected ratio for the titration study was 41.3 μg (0.135 μmoles) OTB/0.2236 mg Flocculant. At this ralationship there are 22.9 μg (0.075 μmol) of bound OTB and 18.4 μg (0.060 μmol) of free OTB.

Also, was studied the effect of pH on the FlocculantOTB interaction due to the composition and structure of Flocculant is unknown. As is shown in Figure 8 the interaction Flocculant-OTB is dependent of pH. At lower pH (2 - 4) is observed the highest absorbance for free OTB. Beyond a pH 4 appears the absorbtion band at 555 of Flocculant-OTB complex. At the same time, the absorbance at 628 nm decreases into the range of pH 4 - 6.5 and remain constant at higher pHs. These results demonstrated that at low pH the functional groups in the Floc-

culant are not in anionic form and there is not interaction with OTB. As the pH increases the groups becomes anionic and these favored the formation of Flocculant-OTB complex. Indirectly, this experiment represents the spectroscopic titration of the ionizable groups in Flocculant, which allowed to determinate a pKa 5.45 from these experimental data [26].

In order to elucidate the functional groups in Flocculant, it was analyzed by infrared spectroscopy (Figure 9). The spectra showed significant vibrations at 3338, 3185, 2925 and 1653 cm⁻¹. Considering the experimental data of pKa and the infrared vibrations is suspected the presence of aliphatic carboxylic groups bearing in the Flocculant.

Moreover, Figure 10 shows the UV-Vis spectra obtained by titration of Flocculant-OTB complex with Sn²⁺ and Zn²⁺.

Instead, the absorbance increases gradually with other metal ions reaching the maximum absorbance of OTB at ratios beyond 6.7 µg Mⁿ⁺/mg Flocculant (Figure 11).

For example, 14.13 µg (0.048 µmol) of OTB are released at 2.24 µg Zn²⁺/mg Flocculant ratio, but it continues releasing until 19.01 µg (0.062 µmol) at 6.7 µg Zn²⁺/mg Flocculant. Otherwise, only 8.79 µg (0.029 µmol) of OTB are exchange at 2.24 µg Pb²⁺/mg Flocculant. These results indicates the degree of interaction between Flocculant and each metal ion studied, being Sn²⁺ the one with the most favorable interaction.

In Figure 11 is presented the OTB absorbance profile as function of the titration of Flocculant-OTB complex with metal ions, which is representative of affinity degree between metal ions and Flocculant. The experimental data obtained for Sn²⁺ indicates has more interaction with Flocculant than other metal ions, due to the fact that a lower dose of this cation was necessary for removal OTB from flocculant-OTB complex. Moreover, for Sn²⁺ the absorbance decrease for µg metal/mg Flocculant > 2.3 confirming the higher Sn-flocculant affinity. This same behavior occurred in the interaction between aluminum and PVSK relations larger than 17 µg metal/mg PVSK.

Equations (3) and (4) represent the equilibrium between Flocculant and OTB to form Flocculant-OTB and the equilibrium in which the Flocculant-OTB complex is used to study the physicochemical behavior of the polyelectrolyte by exchange of OTB with metal ions. These two processes are in competition and are associated with equilibrium constants K₃ and K₄, respectively. The amount of OTB displaced by the metal ion depends on the affinity of the polyelectrolyte toward the cation. The magnitude of the K₄ is a relative measure of this affinity. Metal ions which exhibit higher K₄ values should effect a greater displacement of OTB.

The association constant K₃ of Flocculant-OTB complex was determined from the absorbance data at 555 nm, while association constants for Flocculant-metal ion complexes were calculated indirectly using absorbance data at 628 nm. Calculation of constants was performed by a curve fitting based on a non-linear least-squares method [26] assuming that each monomeric unit of the polymer reacts with a metal cation forming a 1:1 complex. In Table 2 are shown the association constants values obtained, as we expected the metal ions that performed a greater displacement of OTB in FlocculantOTB complex presented greater magnitudes for K₄. Here highlight the 10⁴ and 10³ values for Flocculant-Sn²⁺ and Flocculant-Zn²⁺ complexes respectively, which mean the Flocculant forms a highly stable complexes with these metal ions and it is in agreement with their observed experimental behavior. Moreover, the association constants K₄ for rest of metal ions are above of K₃ value due to their ability to displace OTB and the affinity of Flucculant for metal ions.

Comparing all results about physicochemical characteristics of PVSK and Flocculant is noteworthy that the interaction PVS-OTB and Flocculant-OTB are much different. In the first one, saturation occurs at very small relationship OTB/PVSK indicating a strong interaction; while with the second one, saturation was not observed even with higher amounts of total OTB added.

Also, metal ion titrations require ratios higher than 20 µg Mⁿ⁺/mg PVSK for maximum displacement of the bound OTB, and with all that is not totally displaced. While Flocculant-metal ion interaction relationships are less than 10 µg Mⁿ⁺/mg Flocculant and metal ions with

(a)                                                      (b)

(c)                                                        (d)

Figura 10. UV-Vis spectra obtained by titration of Flocculant-OTB complex with Sn²⁺ and Zn²⁺ (a) and (c) and bound and free OTB amount profiles (b) and (d).

Table 2. Relative association constants of Flocculant-metal ion complexes.

high affinity exchange or displaced almost all the bound OTB. In general, relative association constants for PEmetal ion complexes were higher with the Flocculant in comparison with PVS. These results are in agreement with the observed experimental behavior.