Experiments performed to investigate the influence of the stirring speed on Fe(III) uptake onto the resin was first conducted using aqueous solutions of 0.01 g^.L^-1 Fe(III) at pH 4 and resin dosage of 0.25 g^.L^-1, whereas the stirring speed was varied in the 600-1200 min^-1 range. It is worth to note here that previous experiments indicated that 0.01 g^.L^-1 iron(III) solutions at pH 4 were stable in the 4 h period. Results from this experiments were shown in Table 2, it can be seen that metal uptake increased progressively in the 600-900 min^-1 range, then becomes practically constant in the 900-1000 min^-1 range, and decreased at higher stirring speeds. The above results can be explained in terms that in the 600-900 min^-1 range, the increase of the stirring speed progressively decreased the thickness of the aqueous diffusion layer until it reached a minimum at 900-1000 min^-1, this minimum was accompanied by a maximum in iron(III) uptake onto the resin. At stirring speeds above 1000 min^-1, the decrease in the percentage of metal uptake can be explained by the formation of local equilibria between the resin beads and the surrounding solution, resulting in a non-efficient mixing between the aqueous and resin phases.

Processing of the experimental data, at 900 min^-1, demonstrated that the rate law governing Fe³⁺ uptake onto the resin is attributed (r²= 0.9917) to the moving boundary process (Lopez Diaz-Pavon et al., 2014):

where, F is the fractional approach to the equilibrium, defined as:

being [Fe]_r,t and [Fe]_r,e the iron(III) concentration in the resin at an elapsed time and at the equilibrium, respectively. The derived value of the rate constant k is 5x10^-3 min^-1.

The variation of the aqueous pH on the iron(III) uptake was investigated using aqueous solutions of 0.01 g^.L^-1 Fe(III) at pH values in the 1-4 range and resin dosages in the 0.25-1 g^.L^-1 range. The results from these experiments were shown in Fig. 2 and Table 3. Figure 2 showed the variation in the percentage of Fe(III) uptake onto the resin at various resin dosages. It can be seen that this percentage decreased, for every resin dosage investigated, as the pH of the solution was shifted to more acidic values. Since there were not protons taking part in the exchange process (see Eq. (1)), this decrease was attributable to a competitive reaction between Fe³⁺ and H⁺ to bond or exchange with the active group of the resin.

Table 3 showed the Fe(III) equilibrium loading values resulting from the experiments, accordingly with that which was expected this loading decreased with the decrease of the pH value; however, and for every pH value, there was not appreciable variation in the metal uptake concentration using the 0.25-0.5 g^.L^-1 resin dosage range, this almost near constant Fe(III) uptake concentration was extended, to the 0.25-1 g^.L^-1 resin dosage range, at pH 1.

Experimental data were best fitted (Table 4) to the Langmuir Type-I model (Al-Ghamdi et al., 2019):

in this equation, [Fe]_aq,e was the iron concentration in the aqueous solution at the equilibrium, [Fe]_r,m was the maximum iron(III) concentration in the resin, and K the Langmuir constant.

The separation factor R_L associated to the Lagmuir equation was estimated accordingly to the next relationship:

where [Fe]_aq,0 is the initial iron concentration in the aqueous solution. The values of R_L estimated for each pH are shown in Table 4, since they are included between zero and 1 values (0<R_L<1), in all the cases, the cationic exchange process is favourable.

The variation of the temperature in the 20-60 ºC was used to investigate the influence of this variable on the metal uptake onto the resin. In these experiments aqueous feed solutions of 0.01 g^.L^-1 Fe(III) at pH 2 and resin dosages of 0.25 g^.L^-1 were used. The contact time was of 4 h and the stirring speed of 900 min^-1. Results indicated that there was a continuous increase on the metal load onto the resin from 15 mg^.g^-1 (38% uptake) at 20 ºC to 21 mg^.g^-1 (52% uptake) at 60 ºC. Experimental results were fitted to the usual thermodynamic relationships to estimate the thermodynamic character of this cation exchange process. The iron(III) distribution coefficient between the resin and the aqueous solution was calculated accordingly to:

where [Fe]_r,e and [Fe]_aq,e were the iron concentrations in the resin and in the aqueous solution at the equilibrium, respectively. A plot of log D_Fe versus 1/T resulted in a straight line (r²= 1.0) to calculate the enthalpy and entropy associated to the exchange process. The positive enthalpy value (12 kJ^.mol^-1) was associated to an endothermic process, and its low value was consistent with the low energy characteristics of ion exchange processes. The positive entropy value (48 J·mol^-1K^-1) was an indication that the ion exchange process was associated with an increase of the process dissorded. The negative ΔGº value of -2 kJ^.mol^-1 was an indication of the spontaneous nature of the system.

These experiments were carried out using resin dosages of 0.5 g^.L^-1 and aqueous solutions containing 0.005-0.08 g^.L^-1 Fe(III) at pH 2. The results derived from the experiments were shown in Fig. 3, plotting the factorial approach to the equilibrium F versus time. These F values were calculated as in Eq. (3). From Fig. 3, it can be seen that there was little variation on the change of F value with time in the 0.01-0.08 g^.L^-1 Fe(III) concentrations range, whereas with the lowest iron(III) concentration of 0.005 g^.L^-1, the 50% of metal uptake was reached at shorter elapsed time (about 30 min) than with the higher iron(III) concentrations (near 60 min). This tendency was maintained for every elapsed time.

The kinetic equation associated to the Fe³⁺ uptake onto the resin was estimated using the two extreme metal concentrations, 0.005 and 0.08 g^.L^-1, used in this investigation. At the lowest metal concentration, the results best responded (r²= 0.9977) to the pseudo-second order kinetic model (Mohagheghian et al., 2017):

At the highest metal concentration (0.08 g^.L^-1), the experimental data best fitted (r²= 0.9961) to the pseudo-first order kinetic model (Hamza et al., 2019):

The results derived from both fits were summarized in Table 5. It also can be seen that the equilibrium iron(III) concentration in the resin, calculated from both models, can be compared reasonably well with the experimental data.

The uptake results using the resin were compared with experiments carried out using multiwalled carbon nanotubes. In these series of experiments aqueous solutions of 0.01 g^.L^-1 Fe(III) at pH values of 2 or 4 were used, whereas resin or MWCNTs dosages were of 1 g^.L^-1. The results, summarized in Table 6, showed that iron(III)-loading onto Lewatit TP208 was better, at the two pH values investigated, than that of MWCNTs, and thus, that under the present experimental process, the cation exchange process was a more efficient process, in terms of iron(III) removal from aqueous wastes, than the adsorption process.

The performance of Lewatit TP208 resin, in terms of iron(III) uptake, was compared to that when the aqueous feed solution contained an accompanying cation. Thus, binary solutions containing 0.01 g^.L^-1 Fe³⁺ and 0.01 g^.L^-1 of Cu²⁺, Zn²⁺, Cr³⁺ or In³⁺ at pH 2 were put into contact with a resin dose of 0.25 g^.L^-1. The results derived from the set of experiments were given in Table 7. The corresponding values of the distribution coefficients were calculated as in eq. (6), whereas the values of the separation factor, ß_Fe/M, were calculated as:

Thus, and from the results present in the above Table, it can be deducted that Fe³⁺ was loaded onto the resin, under the present experimental conditions, preferably to Cu²⁺, Zn²⁺ and Cr³⁺, and it can be separate from these elements, since ß_Fe/M values were greater than 1. In the case of In³⁺, the experimental ß_Fe/In value indicated that Fe³⁺ can not be separate from In³⁺, again under the present experimental conditions, but In³⁺ can be separate from Fe³⁺, as the ß_In/Fe value of near 1.5 indicated.

Fe³⁺ uptake was compared to that of Fe²⁺ uptake onto the resin, but in this case using monoelemental solutions. The aqueous feed phase contained 0.01 g^.L^-1 Fe³⁺ or Fe²⁺ at pH 2, and the resin dosage was of 0.25 g^.L^-1. The results indicated that Fe³⁺ uptake was greater than that of Fe²⁺, 15 mg^.g^-1 versus 4 mg^.g^-1, and thus, Fe³⁺ can be separate from Fe²⁺.