The removal of toxic metals from liquid effluents by ion exchange resins. Part XIII: Zinc(II)/H⁺/Lewatit OC-1026

Francisco José Alguacil*

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

*Corresponding Author: fjalgua@cenim.csic.es

1. INTRODUCTIONTOP

Zinc is an essential element for life, though and excess in zinc exposure (ingesta) is detrimental for human health because it causes nausea, diarrhea, headaches and losses of essential copper and iron in human body (Chasapis et al., 2012; Maret, 2013).

There are not stringent regulations about maximum zinc allowance in drinking water, though from a zinc concentration above 3 mg·L⁻¹, water tends to be opalescent, develops a greasy film when boiled, and has an undesirable astringent taste (WHO, 2020). In the USA, there was not also a general rule about zinc allowances in waters, i.e. i) if the effluent comes from an electroplating facility, the daily limit for zinc is 0.6 mg·L⁻¹, ii) if the facility is engaged in chemical etching and milling, the daily limit is 164 mg·L⁻¹, and iii) in the steam electric power generating industry, the daily limit is 1.0 mg·L⁻¹. These differences are a function of the ability of the specific industry to treat its wastewater and remove the pollutants using technology applicable to that industry at the time that USEPA studied the industry (Walker Jr., 2020).

Despite the above, several procedures aimed to the removal of this element from aqueous solutions had been already reported (Abdelrahman et al., 2019; Aliyu, 2019; Gümüş, 2019; Sakalova et al., 2019; Mirjavadi et al., 2019; Zand and Abyaneh, 2019; Ahmad et al., 2020; Wieszczycka et al., 2020; Zhang et al., 2020), all of them are based in the adsorption of Zn²⁺onto different adsorbents.

Following on 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; Alguacil 2019c; Alguacil and Escudero, 2020), in this work, the adsorption of zinc(II) from acidic aqueous solutions by using Lewatit OC-1026 resin is investigated. This resin has an organic phosphorous derivative (di-2ethylhexyl phosphoric acid) adsorbed in the resin matrix. Various experimental variables influencing zinc(II) loading onto the resin are studied, as well as the performance of various resins and multiwalled carbon nanotubes on zinc(II) uptake. The elution of zinc(II) from metal-loaded resin by acidic solutions is also described.

2. EXPERIMENTALTOP

Lewatit OC-1026 resin, consisted in white spherical beads of styrene-DVB matrix with adsorbed di-2-ethylhexyl phosphate groups and particle size in the 0.3–1.25 mm range. Other resins used in this investigation were purchased from Fluka, whereas all the chemicals used in the experimental work are of AR grade. The characteristics of the multiwalled carbon nanotubes (MWCNTs) and oxidized multiwalled carbon nanotubes (ox-MWCNTs) were given elsewhere (Alguacil et al., 2016; Alguacil et al., 2017).

Batch, loading and elution, experiments were done in a glass reactor vessel (250 mL), containing the Zn(II) (metals) solution and the corresponding resins/adsorbents dosage. Agitation of the system was provided via a four blades glass impeller.

The analysis of zinc and the accompanying metals in the aqueous solutions was done by Atomic Absorption Spectrometry, and the metal loaded onto the resins/adsorbent was calculated by the mass balance.

3. RESULTS AND DISCUSSIONTOP

A distinctive difference of this resin with others used in these series of works, is that in the present case, Lewatit OC-1026 has a liquid-liquid extraction reagent adsorbed in the resin matrix as functional group, which in the present case (Fig. 1) is an organic phosphorous derivative (D2EHPA), thus, in the case of zinc(II), metal uptake onto the resin is represented by the next equilibrium:

which is a cation exchange reaction, releasing protons to the aqueous solution. In the above equation, r and aq subscripts represented to the species in the resin and the aqueous phase, respectively, and R represented the 2-ethylhexyl chain associated to the phosphorous derivative.

3.1. Zinc(II) uptake onto LewatitOC-1026 resinTOP

The stirring speed applied to the system is a experimental variable often neglected in many ion exchange/adsorption investigations, however, it can varies the quantitative data of a given system. In this work, and to investigate the influence of this variable on zinc(II) uptake onto the resin, aqueous solutions of 0.01 g·L⁻¹ Zn(II) at pH 4, were put into contact with a resin dosage of 0.25 g·L⁻¹, at 20 °C, and various stirring speeds ranging from 400 to 1200 min⁻¹. The results derived from this investigation, Table 1, showed that there was a continuous increase of the metal uptake with the increase of the stirring speed from 400 to 800 min⁻¹, and then zinc uptake levels off at about 21.6 mg·g⁻¹, with a further decrease of this values for stirring speeds around 1200 min⁻¹. These results can be explained by a first decrease of the thickness of the aqueous boundary layer, until a minimum was obtained at 800–1000 min⁻¹. The further decrease in metal uptake are the result of the formation of local equilibria between the solution and the resin beads, causing the decrease of zinc(II) loaded onto the resin. Based in the above, a stirring speed of 900 min⁻¹ was used throughout all the experimentation.

By the use of aqueous solutions of 0.01 g·L⁻¹ Zn(II) at pH 4, and resin doses of 0.25 g·L⁻¹, the effect of the variation of the temperature on zinc(II) uptake was investigated. The results from this set of experiments were summarized in Table 2. It can be seen that there was a continuous decrease in the zinc(II) loaded onto the resin with the increase of the temperature. From the experimental values it was estimated that the ion exchange reaction represented in Eq. (1) had an exothermic character (∆H°= -16 kJ·mol⁻¹) in an spontaneous process, and with ∆Sº estimated as -44 J/mol K, thus, the exchange process decreased its randomness. This variable also influenced the time in which the system reached the equilibrium, i.e. at 20 °C, this time was of 5 h, at 40 °C decreased to 30 min, and at 60 °C 15 min of reaction time was needed to reach equilibrium conditions. At 20 °C, the kinetics was best represented (r²= 0.9966) by the pseudo-second order model (Chen et al., 2019):

being [Zn]_r,t and [Zn]_r,e the zinc concentrations in the resin at an elapsed time and at the equilibrium respectively, and k the rate constant, estimated as 1.6×10⁻³ g/mg min. Moreover, from the model it was estimated that [Zn]_r,e was of 22 mg·g⁻¹, which fitted very well with the experimentally found value of 21.6 mg·g⁻¹. At 60 °C, the experimental data fitted well (r²= 1.00) with the second order kinetic model (Ho, 2006):

where [Zn]_aq,t and [Zn]_aq,0 represented the zinc concentrations in the aqueous solution at an elapsed time t and the initial one, respectively, and k being the corresponding kinetic constant, estimated as 4×10⁻³ L/mg min. In this case, the model estimated [Zn]_aq,0 as 0.01 g·L⁻¹, which again matched to the experimental value.

The pH of the aqueous solutions is another parameter to consider in these ion exchange/adsorption systems, and in the removal of zinc(II) from solutions using Lewatit OC-1026 resin, the influence of this variable on zinc uptake was investigated using aqueous solutions of 0.01 g·L⁻¹ Zn(II) at various pH values (1 to 4) and a resin dosage of 0.5 g·L⁻¹. Figure 2 showed the variation of the metal loaded onto the resin versus the pH of the aqueous solution, and it was evident that a decrease of this pH value resulted, accordingly to the equilibrium of Eq. (1), in a decrease of the metal uptake. Probably, this effect, again accordingly with Eq. (1), will be less evident as the resin dosage increased. At pH 4, the experimental results best fitted (r²= 0.9900) to the particle diffusion model (López Díaz-Pavón et al., 2014):

with F calculated as:

and k, the rate constant, estimated as 9.5×10⁻³ min⁻¹.

The influence of the variation of the aqueous ionic strength on zinc(II) uptake was also investigated, and in this case the resin dosage was of 0.25 g·L⁻¹ and the aqueous solutions contained 0.01 g·L⁻¹ Zn(II) and various concentrations of LiCl. As it can be seen from Table 3, the increase of the ionic strength also decreased the metal uptake onto the resin.

Zinc(II) uptake onto the resin was also ­investigated using various resin concentrations (0.05–1.0 g·L⁻¹). The results derived from this experimentation were resumed in Table 4, and as it was somewhat expected, the metal uptake onto the resin decreased as the resin dosage increased. These data were used to estimate the loading isotherm which represented the equilibrium of Eq. (1). It was found, that this best fit (r²= 0.9861) was on the Langmuir type-2 isotherm (Abd El Salam and Zaki, 2019):

being [Zn]_r,m the maximum zinc loading onto the resin (46 mg·g⁻¹) and k the corresponding estimated Langmuir constant. (0.25 L·mg⁻¹). The separation factor:

was estimated as 0.29, thus, the exchange process was favourable, since R value was lower than 1 and greater than zero.

Different resins and multiwalled carbon nanotubes were used to investigate their performance in zinc(II) uptake, expressed as the distribution coefficient:

and the results showed in Table 5. Generally speaking, ion exchange resins resulted in greater distribution coefficient values, thus better zinc uptakes, than multiwalled carbon nanotubes.

Several experiments were done in order to evaluate the use of Lewatit OC-1026 in the separation of zinc(II) from other accompanying metals in the aqueous solutions. Thus, experiments were carried out on binary solutions having zinc/metal equimolar concentrations of 1.5×10⁻⁴ M, and resin dosages of 0.25 g·L⁻¹. The results from this set of experiments were given in Table 6, in the form of the separation factor, defined as:

It was shown, that under the present experimental conditions, zinc(II) can be separated, β values greater than 1, from Cu(II), Ni(II), Cr(III), Mn(II) and Co(II), however, Fe(III) and In(III) were loaded onto the resin preferably to zinc(II).

3.2. Zinc(II) elutionTOP

Accordingly with the equilibrium represented in Eq. (1), zinc(II) elution is possible by shifting it to the left, thus acidic solutions are used to elute zinc from the resin. Resin with zinc uptakes of 1.4 mg·g⁻¹ were used in the experiments, and both sulphuric and hydrochloric acid solutions were used as eluting agents. The results from these experiments were shown in Table 7, and it can be seen that zinc(II) was effectively eluted from the resin using these acidic solutions, whereas neither the change of the acid nor the variation in the liquid/solid ration had a great influence on the recovery of zinc from the metal-loaded resin. Based in these results, it appeared to be possible to concentrate the zinc solution at least five times with respect the initial zinc aqueous solution feeding the loading step.

4. CONCLUSIONSTOP

Lewatit OC-1026 is useful in the removal of zinc(II) aqueous solutions. The influence of several variables on this removal can be summarized as:

ACKNOWLEDGEMENTTOP

To the CSIC (Spain) for support.

REFERENCESTOP