The removal of toxic metals from liquid effluents by ion exchange resins. Part XI: Cobalt(II) /H⁺/Lewatit TP260

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

Cobalt is an element necessary for human life, in fact, a biochemical key cobalt-bearing product is vitamin B₁₂ or cyanocobalamin. However in excess, cobalt became toxic for humans. Upon ingesta, cobalt distributed in all tissues and liver, kidney and bones, thus it is responsible for a series of diseases, and also based on the animal data, the International Agency for Research on Cancer (IARC) had considered that this metal has a great possibility to produce cancer in humans (ATSDR, 2004; Kim et al., 2006; Leyssens et al., 2017).

Thus, the removal of this element from different environments is of a practical necessity. In aqueous media, cobalt is normally found as cobalt(II) or Co²⁺, and the treatment of solutions containing it, included, as recent literature shows, ion exchange, adsorption, solvent extraction and liquid membranes processing (Ashtari and Pourghahramani, 2018; Bozecka et al., 2018; Devi et al., 2018; Farag et al., 2018; Hayati et al., 2018; Kara et al., 2018; Ma et al., 2018; Omelchuk and Chagnes, 2018; Song et al., 2018; Vafaei et al., 2018; Xavier et al., 2018; Yuan et al., 2018; Zherebtsov et al., 2018; Anirudhan et al., 2019; Rodriguez et al., 2019).

In this new article of the series (Alguacil et al., 2002; Alguacil, 2002; Alguacil, 2003; Alguacil, 2017a; Alguacil, 2017b; Alguacil, 2018a; Alguacil 2018b; Alguacil and Escudero, 2018; Alguacil 2019a; Alguacil 2019b), the removal of cobalt(II) from aqueous solutions by the use of the cationic exchanger Lewatit TP260 resin is investigated. Several experimental parameters affecting the metal loading onto the resin are considered, and also competitive cobalt-metals systems as well as the use of various resins and smart adsorbents are studied in terms of cobalt uptake. The elution of this metal from cobalt-loaded resin by different eluants is also investigated.

2. EXPERIMENTALTOP

Lewatit TP260 (Fluka) is a macroporous weakly acidic resin with di-Na⁺ substituted (aminomethyl)phosphonic acid groups. Other cations exchange resins and chemicals used in the experimentation are of AR grade. The oxidized and non-oxidized multiwalled carbon nanotubes (MWCNTs) have the characteristics given elsewhere (Alguacil et al., 2016; Alguacil et al., 2017).

Batch experiments (loading and elution) were carried out in a glass reactor (250 mL), containing the aqueous solution of cobalt(II) and the resins/adsorbents, and was stirred via a four blades glass impeller at 1200 min⁻¹ and 20 ºC, except when these variables were investigated.

Cobalt (and metals) in the aqueous solutions were analysed by AAS, whereas cobalt (and metals) loaded onto the resins/adsorbent were calculated by the mass balance.

3. RESULTS AND DISCUSSIONTOP

Since Lewatit TP260 is a cationic exchanger resin, it is logical to attribute the metal uptake onto the resin to the next equilibrium:

where [- represented the non-reactive part of the resin, and r and aq refereed to the species in the resin and in the aqueous solution, respectively.

3.1. Cobalt(II) loading onto Lewatit TP260TOP

The variation of the stirring speed may have a key influence in the load of a given metal onto a given resin/adsorbent, though the investigation of this variable in these systems is very often neglected by researchers. Considering its experimental importance, in the present system the influence of this variable on cobalt(II) uptake onto the resins was first investigated by the use of aqueous solutions containing 0.01 g·L⁻¹ Co(II) at pH 4 and resin dosages of 0.25 g·L⁻¹. The results obtained from this investigation being summarized in Table 1. It can be seen that the metal uptake increases from 28 mg·g⁻¹ to 32 mg·g⁻¹ when the stirring speed of the system increases from 300 to 1200 min⁻¹, and then remained constant. Thus, in the 1200-1400 min⁻¹ range, the system reached a minimum in the thickness of the feed solution boundary layer and the metal uptake maximizes.

Using the results at 1200 min⁻¹, it was estimated the rate law for cobalt(II) uptake. The best fit (r²= 0.9795) corresponded to the aqueous diffusion (Lopez Diaz-Pavon et al., 2014):

with k estimated as 0.012 min⁻¹. In the above equation, t was the elapsed time, and F was calculated as:

being [Co(II)]_r,t and [Co(II)]_r,e the cobalt concentrations in the resin at an elapsed time and at equilibrium, respectively.

The influence of the temperature (20-60 ºC range) on cobalt(II) loading onto the resin was investigated using the same aqueous solution and resin dose as above, and stirring speeds of 1200 min⁻¹. After 5 hours contact time, time in which the system achieved the equilibrium for all the range of temperatures investigated, the percentage of metal loading onto the resin is increased with the increase of the temperature, i.e., 80% at 20 ºC and 92% at 60 ºC, which corresponded to cobalt(II) uptakes of 32.0 and 36.8 mg·g⁻¹, respectively. Thus, it is concluded that the metal loading onto the resin is endothermic, with ∆Hº estimated as 21 kJ·mol⁻¹, whereas ∆Sº is 95 J·mol⁻¹ K⁻¹, representative of a process which incremented its randomness, and ∆Gº -7 kJ·mol⁻¹, indicative of an spontaneous equilibrium. Moreover, the variation of the temperature indicated that the system reached the equilibrium at shorter contact times as the temperature is increased, i.e. 5 h at 20 ºC against 3 h at 60 ºC, and the results were fitted to the usual kinetics models. The results from this fit indicated that at 20 ºC, the pseudo-first order kinetic model (Hemavathy et al., 2019) best represented the experimental results (r²= 0.9763):

with k of 0.012 min⁻¹ and ln [Co(II)]_r,t of 3.4, value which compared well with the experimentally obtained of 3.5. At 60 ºC, the experimental results were best fitted to the pseudo-second order kinetic model (Alguacil, 2018c) (r²= 0.9718):

with k in the 5x10⁻⁴ min⁻¹ magnitude order.

The influence of the aqueous pH value on cobalt(II) uptake onto the resin was investigated using aqueous solutions of 0.01 mg·L⁻¹ Co(II) at various pH values (1-5) and resin dosages of 0.5 g·L⁻¹. The results from these set of experiments were ­summarized in Table 2, in which it can be seen that in the 3-5 pH interval range the metal loading was practically the same, but at the more acidic pH value the metal loads was nil, indicating that cobalt(II) was not recovered from the aqueous solution.

The influence of the continuous variation of the resin dosage onto the metal loads was investigated using the same aqueous solution that in previous experiments and resin dosages in the 0.07-0.5 g·L⁻¹ range. Table 3 showed the results obtained from these experiments, and it was concluded that the variation of the resin dosage produced an increment in the percentage of cobalt(II) loaded onto the resin, but at the same time the metal uptake decreased. These results were used to estimate the equilibrium isotherm, and the results from this fit indicated that the experimental results were best represented by the Langmuir model (r²= 0.9612) (Daraei and Mittal, 2017; Rahmani et al., 2017):

being [Co(II)]_aq,e the metal concentration in the aqueous solution at the equilibrium. The value of b was estimated as 0.13 L·mg⁻¹, and the constant separation factor or equilibrium parameter, R_L was estimated using the next equation:

being [Co(II)]₀ the initial metal concentration in the aqueous solution. Thus, R_L, from the present system, is of 0.44, indicating a favourable system.

The increase the ionic strength (I) of the aqueous solution and its effect on cobalt(II) uptake was investigated using aqueous solutions of 0.01 g·L⁻¹ Co(II) and various LiCl concentrations, and resin dosages of 0.25 g·L⁻¹. Table 4 showed the decrease in metal loading as the ionic strength of the solution was increased.

In order to compare the performance of Lewatit TP260 with respect to cobalt(II) removal from aqueous solutions, different cationic exchangers were examined and also non-oxidized and oxidized multiwalled carbon nanotubes. The experimental conditions used were of aqueous solutions containing 0.01 g·L⁻¹ Co(II) at pH 4, exchangers/adsorbents dosages of 0.5 g·L⁻¹ and temperature of 20 ºC. Table 5 showed the results obtained from these sets of experiments. It can be seen that metal uptake is similar in the case of Lewatit SP112 and Lewatit TP260 resins, both acidic and in Na⁺ form though with different active group, and with near half the metal uptake in the case of oxidized-multiwalled carbon nanotubes. In the case of Lewatit OC1026 resin, cobalt uptake is about six times lower than those exhibited with SP112 and TP260 resin, and this uptake is even lower in the case of using multiwalled carbon nanotubes as adsorbent for cobalt(II).

The competitive removal of cobalt(II) in presence of other cations in the aqueous solution was also investigated. In this case, the aqueous solutions contained 1.7x10⁻⁴ M of each element (binary solutions) at pH 4 and the Lewatit TP260 dosage was of 0.25 g·L⁻¹. The results obtained in these sets of experiments being summarized in Table 6 in the form of the separation factor Co/Metal (β_Co/M), calculated. as:

and where, D is the distribution coefficient (cobalt or metals) between the resin and the aqueous solutions, defined as:

and where [Co]_r,e and [Co]_r,aq are the cobalt (metals) concentrations in the resin and in the aqueous solution at equilibrium, respectively. Accordingly with the results presented in Table 6, only in the case of the Co-Mn and Co-Zn pairs, cobalt is separated selectively from the accompanying metal (β>1). In all the other cases, the accompanying metal is exchanged preferably to cobalt(II) (β<1). In these cases, it should be better accomplish a non-selective metals load onto the resin, and then separate them by selective or controlled elution process (Cerpa et al., 2017). It should be noted here that in the case of cobalt(II), these multielmental systems demonstrated that the cobalt(II) uptake decreased with respect to the uptake resulted in the removal of the metal from a cobalt monoelemental solution, this should be attributed to interactions between the ions presented in the aqueous solution.

3.2. Cobalt (II) elution from Co(II)-loaded Lewatit TP260 resinTOP

Elution experiments were performed with resin loaded with 14 mg Co/g under various experimental conditions. Table 7 showed that the variation of the volume of eluant versus resin weight had a negligible effect on the percentage of cobalt eluted using 1 M HCl solutions as eluant. In the same Table, was presented the result obtained when the eluant solution was changed from HCl to H₂SO₄ or NaCl solutions. This change in the solution composition produced a dramatic change in the results, since with 1 M sulphuric acid the percentage of cobalt eluted is the same than that obtained with 1 M HCl solution, but when 1 M NaCl solution was used, the percentage of cobalt recovered in the solution was of a mere 5%. From the above results, it is seemed clear that the elution responded to the next equilibrium:

where the subscript aq, represented the elution phase. Thus, a washing of the resin with NaOH solution is needed in order to recycle back to the Na⁺ form.

4. CONCLUSIONSTOP

ACKNOWLEDGEMENTTOP

To the CSIC (Spain) for support.

REFERENCESTOP