The removal of toxic metals from liquid effluents by ion exchange resins. Part XIV: Indium(III)/H⁺/Dowex-400

Francisco José Alguacil

Centro Nacional de Investigaciones Metalúrgicas (CENIM, CSIC), Avda. Gregorio del Amo 8, 28040 Madrid, España

(*Corresponding author: fjalgua@cenim.csic.es)

1. INTRODUCTIONTOP

As a unit operation in Hydrometallurgy, the ion exchange with resins technique has been widely used to the recovery/separation/removal of practically all the metals from different resources or wastes. Moreover, ion exchange processing using resins has the advantages, over other separation technologies, i.e. solvent extraction and liquid membranes, that it can be use on highly diluted solutions and/or non-clarified ones. Thus, and being an unit operation used from the late 40 s of the past century; its uses in the field of the aqueous processing of hazardous and profitable metals is of current interest (Araucz et al., 2020; Bezzina et al., 2020; Jacukowicz-Sobala et al., 2020; Lee et al., 2020; Simonescu et al., 2020)

Indium, with a global production of 760 ton in 2019 year (Schuyler Anderson, 2020), is a metal with wide uses in smart technologies, but at the same time, and as many metals, it is hazardous to humans if it is inadvertently ingested from waters, in fact, indium is being recognized as a potential carcinogenic (Luz et al., 2018), thus, its recovery from these liquid streams is of a practical necessity. In aqueous solutions, indium is stable in the (III) oxidation state as In³⁺ as well as forming complexes, i.e. InCl₄⁻.

Following on the series (Alguacil, 2002; Alguacil et al., 2002; Alguacil, 2003; Alguacil, 2017a; Alguacil, 2017b; Alguacil, 2018a; Alguacil 2018b; Alguacil and Escudero, 2018; Alguacil 2019a; Alguacil 2019b; Alguacil 2019c; Alguacil and Escudero, 2020; Alguacil 2020), in this work, the adsorption of In³⁺ from acidic aqueous solutions by using Dowex-400 cationic exchange resin is investigated. Different experimental variables influencing metal uptake onto the resin are investigated. The elution of the metal from In³⁺-loaded resin by acidic solutions is also carried out.

2. EXPERIMENTALTOP

The commercially available cationic exchange resin Dowex-400 (Fluka) presented a crosslinked styrene-DVB matrix, containing sulfonic groups, and having the form of spherical beds of 410 μm mean size. All the other chemicals used in this work were of AR grade.

Metal uptake experiments were carried out by the following procedure: 200 mL of the aqueous solutions and the corresponding resin dosage were placed in a glass reactor and mechanically shaken via a for blades glass impeller (2.3 cm diameter), and samples were withdrawn at elapsed times. Elution experiments were carried out on the same basis.

The concentration of indium in the aqueous solutions was analyzed by AAS, and the concentration of the metal in the resin was determined by the mass balance.

3. RESULTS AND DISCUSSIONTOP

3.1. Indium uptake onto the resin TOP

Indium(III), acting as In³⁺, is loaded onto the resin by the following equilibrium:

where the subscript r and aq represented to the resin and aqueous phases, respectively. Accordingly, metal uptake onto the resin responded to a cationic exchange reaction.

3.1.1. Influence of the stirring speedTOP

To study the influence of the stirring speed on indium(III) removal from the solution by the resin, several experiments were carried out using a resin dosage of 0.5 g·L⁻¹ and aqueous phases of 0.01 g·L⁻¹ In³⁺ at pH 4. The results (Fig. 1) show that, under these experimental conditions, the metal removal is not dependent upon the stirring speed, and equilibrium (95% In(III) removal from the solution) is reached after 4–5 h contact. These results demonstrated that within these stirring speeds range, a minimum in the thickness of the aqueous diffusion layer is reached.

Results were used to estimate the rate law associated to the exchange process, being the best fit (r²= 0.997) of the experimental data to a film-diffusion controlled process (Lopez Diaz-Pavon et al., 2014):

with rate constant (k) estimated as 0.03 min⁻¹. In the above expression, F represented the factorial approach to the equilibrium, calculated as:

being [In]_r,t and [In]_r,e, the metal concentrations in the resin at an elapsed time t and at the equilibrium, respectively

3.1.2. Influence of the temperatureTOP

The influence of this variable on the removal of indium(III) by the resin was investigated by the same aqueous solution as above, but using a resin dosage of 0.25 g·L⁻¹. Results, summarized in Table 1, show that the change of temperature, between 20 and 40 °C, has a negligible effect on the percentage of indium removed from the solution, but that there is a decrease in indium removal as the temperature is increased from 40 to 60 °C, thus, in these range of temperatures metal uptake onto the resin is exothermic with ∆H°= - 90 kJ·mol⁻¹. The values of other thermodynamic values estimated for this process are ∆G°= -13 kJ·mol⁻¹ and ∆S°= -0.26 kJ·mol⁻¹ K; thus, the exchange process is spontaneous and with a decreased randomness.

Despite that mentioned above, about the non-influence of the temperature, in the 20–40 °C, on In(III) removal from the solution, there is an influence of this variable on the time to reach equilibrium, as values in Table 2 show. Thus, this variation is used to estimate the kinetic model associate to each temperature. The results from these fittings show that at 20 °C, the experimental results are best represented (r²= 0.999) by the pseudo-first order kinetic model:

with the rate constant k estimated as 0.016 min⁻¹, and [In]_r,e, that is the metal concentration in the resin at the equilibrium, of 33 mg·g⁻¹ (experimental 35 mg·g⁻¹).

At 30 °C, the 2nd order model, best fits (r²= 0.999) the experimental data:

with k of 3x10⁻³ L·mg⁻¹ min. In this expression, [In]_aq,0 and [In]_aq,t represented the indium concentrations in the initial solution and at elapsed time, respectively.

At 40 °C, the pseudo-second order kinetic model equation fits (r²= 0.989) the experimental data:

with k of 1x10⁻³ g/mg min. Furthermore, the fit of the data to the pseudo-second order model should be indicative of chemical activation between the resin and the indium (III) ions (Bayazit et al., 2020), whereas the pseudo-second order kinetic model is often associated to uptake processes obeying the aqueous film diffusion rate law (Hubbe et al., 2019).

3.1.3. Influence of the aqueous pH valueTOP

The series of experiments performed, to investigate the influence of the aqueous pH value on the removal of In(III) from the solution, used aqueous solutions of 0.01 g·L⁻¹ In(III) at various pH values (0–4), and resin doses of 0.25 g·L⁻¹. The results derived from this investigation are shown in Fig. 2, plotting the percentage of indium removed from the solution versus the pH of the solution.

It can be seen that the pH has a markedly influence on the removal of indium from the solution, decreasing this value as the pH of the solution shifted to more acidic values (Eq. (1)). From this Eq. (1):

and considering the definition of the distribution coefficient as:

by substituting this value in Eq. (7) and rearranging:

where C is a term involving the other terms of Eq. (7). Thus, a plot of log D versus pH shall give a straight line of slope 3; from the results show in Fig, 2, effectively a slope near 3 results, indicating that Eq. (1) is a true representation of the equilibrium involved in the removal of indium(III) from the solution using this resin.

3.1.4. Influence of resin dosageTOP

The effect of the variation in the dosage of Dowex-400 on the removal of indium(III) from the solution was studied, and the results are summarized in Table 3. As it is logical, the increase of the resin dosage promoted the removal of indium from the solution.

Experimental data are fitted to various Langmuir and Freundlich isotherm models, and the best fit (r²= 0.995) corresponded to the Langmuir Type-2 isotherm:

being [In]_r,m the maximum indium uptake onto the resin (100 mg·g⁻¹) and k estimated as 0.45 L·mg⁻¹. The separation factor:

is estimated as 0.18, thus, being this value comprised between 0 and 1, the cationic exchange process is favourable.

3.2. Indium(II) elutionTOP

The equilibrium represented in Eq. (1), indicated that indium(III) elution is possible by shifting it to the left, thus, acidic solutions are used to elute indium from the resin. Resin with indium uptakes of 19 mg·g⁻¹ are used in the experiments, whereas hydrochloric acid solutions are used as eluting agent, both to reach these acidic medium and to promote the formation of anionic species, i.e. InCl₄⁻, which evidently are not loaded by this type of resins. The results from these experiments are summarized in Table 4, and it can be seen that indium is effectively eluted from the resin using these acidic solutions, and under different experimental conditions. Based in these results, it seemed possible to concentrate the indium solution at least six times with respect the initial metal aqueous solution feeding the loading step, and at the same time, the resin regenerates. From these concentrated solutions, indium can be further recovered even as zero valent metal (Alguacil and Escudero, 2019).

4. CONCLUSIONSTOP

The results indicate that Dowex-400 resin remove indium(III) from aqueous solutions. Metal uptake onto the resin corresponds to a cation exchange, with apparent formulation ([-SO₃⁻)₃In³⁺ in the resin phase, releasing at the same time H⁺ ions to the aqueous phase. Thus, the removal of indium from the solution is pH-dependent, decreasing as the pH of the solution shifts to more acidic values. Whereas the stirring speed applied to the system does not affect to the exchange process, indium(III) removal from the solution is exothermic (∆H° = -90 kJ·mol⁻¹). Various models are used to fit the experimental data, from these fit it can be concluded that:

ACKNOWLEDGEMENTTOP

To the CSIC (Spain) for support.

REFERENCESTOP