The removal of toxic-hazardous metallic species presents in wastewaters, liquid effluents and, in general, aqueous environments is of the upmost importance due to its social impact and its implications on human health and the environment. Iron is one of these hazardous metals. In aqueous streams, iron can be present as iron(II), iron(III) or a mixture of both, and while both are essential for life, an excess in their ingesta (hyperferremia) is often accompanied with the increase of the formation of reactive oxygen species in cells, increasing, as a consequence of the above, the risk of cancer, heart disease, haemochromatosis; besides, iron accumulation in the central nervous system is connected with other serious illnesses.
The excess of the presence of iron(II) and (III) ions in the body comes from their use in agriculture and industry, that eventually ends into contaminated rivers, ponds, lakes, etc.
The presence of iron species in waters also increased the risk of undesirable bacterial growth within waterworks and water distribution system, resulting in the deposition of a slimy coating on the pippin system. The presence of iron in drinking-water is normally under 0.3 mg·L-1, but may be higher in communities where different iron salts are used as coagulating agents in water-treatment plants and where iron-bearing materials are used in the civil engineering associated with water distribution. Despite all the above, no health-based guideline value for iron is proposed (WHO, 2020WHO (2003). Iron in drinking-water. WHO guidelines for drinking-water quality. https://www.who.int/water_sanitation_health/dwq/chemicals/iron.pdf. www.who.int ).
The removal of iron(II) from aqueous solutions had been also of interest for various research groups, and particularly, several ion exchange resins and/or adsorbents were used recently in this role, i.e. 732-type strong acid cation exchange resin, in an ascorbic acid and EDTA medium, effectively loaded Fe(II) at acidic pH values (Zhou et al., 2018Zhou, G., Li, Q., Sun, P., Guan, W., Zhang, G., Cao, Z., Zeng, L. (2018). Removal of impurities from scandium chloride solution using 732-type resin. J. Rare Earths 36 (3), 311-316. https://doi.org/10.1016/j.jre.2017.09.009.), multiwalled carbon nanotubes modified with EDTA, and also containing amino and carboxyl groups removed Fe(II) from aqueous solutions (Desouky, 2018Desouky, A.M. (2018). Remove heavy metals from groundwater using carbon nanotubes grafted with amino compound. Sep. Sci. Technol. 53 (1), 1698-1702. https://doi.org/10.1080/01496395.2018.1441304.), whereas nanosized calcium deficient hydroxyapatite was other material used in the removal of this element from solutions (Van Dat et al., 2019Van Dat, D., Van Thuan, L., Hoang Sinh, L., Dinh Hien, T., Hoai Thuong, N. (2019). Effectiveness of calcium deficiency in nanosized hydroxyapatite for removal of Fe(II), Cu(II), Ni(II) and Cr(VI) ions from aqueous solutions. J. Nano Res. 56, 17-27. https://doi.org/10.4028/www.scientific.net/JNanoR.56.17.). MTS9100, C107E, MTS9570 and MTS9501, TP214, MTS9301 ion exchange resins were used in the removal of iron(II) (and other metals) from solutions (Bezzina et al., 2019Bezzina, J.P., Ruder, L.R., Dawson, R., Ogden, M.D. (2019). Ion exchange removal of Cu(II), Fe(II), Pb(II)and Zn(II)from acid extracted sewage sludge - Resin screening in weak acid media. Water Res. 158, 257-267. https://doi.org/10.1016/j.watres.2019.04.042.; Bezzina et al., 2020Bezzina, J.P., Robshaw, T., Dawson, R., Ogden, M.D. (2020). Single metal isotherm study of the ion exchange removal of Cu(II), Fe(II), Pb(II) and Zn(II) from synthetic acetic acid leachate. Chem. Eng. J. 394, 124862. https://doi.org/10.1016/j.cej.2020.124862.), in the case of these resins, the acid present in the aqueous solution had a definitive role in the removal (or not) of the various metal ions investigated. δ-MnO2/zeolite nanocomposites were used to simultaneously remove Fe2+, Mn2+, and NH4+-N from groundwater (Ma et al., 2019Ma, W.J., Chen, T.H., Chen, D., Liu, H.B., Cheng, P., Zhang, Z.X., Tao, Q., Zhang, Y.Z. (2019). Removal of Fe(II), Mn(II), and NH4+-N by using δ-MnO2 coated zeolite. Huanjing Kexue/Environ. Sci. 40 (10), 4553-4561. https://doi.org/10.13227/j.hjkx.201903257.). The commercially available Purolite S957 resin was other cationic exchange resin used in the removal of Fe(II) from solutions (Moghimi et al., 2020Moghimi, F., Jafari, A.H., Yoozbashizadeh, H., Askari, M. (2020). Adsorption behavior of Sb(III) in single and binary Sb(III)-Fe(II) systems on cationic ion exchange resin: Adsorption equilibrium, kinetic and thermodynamic aspects. Trans. Nonferr. Metal. Soc. China 30 (1), 236-248. https://doi.org/10.1016/S1003-6326(19)65195-2.), in this case, the investigation was oriented to the treatment of copper electrolytes. The effect of the presence of Fe(II) or Fe(III) on the removal of Cu(II) by a chelating resins was also investigated (Botelho Junior et al., 2020Botelho Junior, A.B., Vicente, A.D.A., Romano Espinosa, D.C., Soares Tenório, J.A. (2020). Effect of iron oxidation state for copper recovery from nickel laterite leach solution using chelating resin. Sep. Sci. Technol. 55 (4), 788-798. https://doi.org/10.1080/01496395.2019.1574828.), in this investigation, both batch and column operational modes were used.
Next on the series (Alguacil et al., 2002Alguacil, F.J., Coedo, A.G., Dorado, T., Padilla, I. (2002). The removal of toxic metals from liquid effluents by ion exchange resins. Part I: chromium(VI)/sulphate/Dowex 1x8. Rev. Metal. 38 (4), 306-311. https://doi.org/10.3989/revmetalm.2002.v38.i4.412.; Alguacil, 2002Alguacil, F.J. (2002). The removal of toxic metals from liquid effluents by ion exchange resins. Part II: cadmium(II)/sulphate/Lewatit TP260. Rev. Metal. 38 (5), 348-352. https://doi.org/10.3989/revmetalm.2002.v38.i5.418.; Alguacil, 2003Alguacil, F.J. (2003). The removal of toxic metals from liquid effluents by ion exchange resins. Part III: copper(II)/sulphate/Amberlite 200. Rev. Metal. 39 (3), 205-209. https://doi.org/10.3989/revmetalm.2003.v39.i3.330.; Alguacil, 2017aAlguacil, F.J. (2017a). The removal of toxic metals from liquid effluents by ion exchange resins. Part IV: chromium(III)/H+/Lewatit SP112. Rev. Metal. 53 (2), e093. https://doi.org/10.3989/revmetalm.093.; Alguacil, 2017bAlguacil, F.J. (2017b). The removal of toxic metals from liquid effluents by ion exchange resins. Part V: nickel(II)/H+/Dowex C400. Rev. Metal. 53 (4), e105. https://doi.org/10.3989/revmetalm.105.; Alguacil, 2018aAlguacil, F.J. (2018a). The removal of toxic metals from liquid effluents by ion exchange resins. Part VI: manganese(II)/H+/Lewatit K2621. Rev. Metal. 54 (2), e116. https://doi.org/10.3989/revmetam.116.; Alguacil, 2018bAlguacil, F.J. (2018b). The removal of toxic metals from liquid effluents by ion exchange resins. Part VII: manganese(VII)/H+/Amberlite 958. Rev. Metal. 54 (3), e125. https://doi.org/10.3989/revmetalm.125.; Alguacil and Escudero, 2018Alguacil, F.J., Escudero, E. (2018). The removal of toxic metals from liquid effluents by ion exchange resins. Part VIII: arsenic(III)/OH/Dowex 1x8. Rev. Metal. 54 (4), e132. https://doi.org/10.3989/revmetalm.132.; Alguacil, 2019aAlguacil, F.J. (2019a). The removal of toxic metals from liquid effluents by ion exchange resins. Part IX: lead(II)/H+/Amberlite IR120. Rev. Metal. 55 (1), e138. https://doi.org/10.3989/revmetalm.138.; Alguacil, 2019bAlguacil, F.J. (2019b). The removal of toxic metals from liquid effluents by ion exchange resins. Part X: antimony(III)/H+/Ionac SR7. Rev. Metal. 55 (3), e152. https://doi.org/10.3989/revmetalm.152.; Alguacil, 2019cAlguacil, F.J. (2019c). The removal of toxic metals from liquid effluents by ion exchange resins. Part XI: Cobalt(II)/H+/Lewatit TP260. Rev. Metal. 55 (4), e154. https://doi.org/10.3989/revmetalm.154.; Alguacil and Escudero, 2020Alguacil, F.J., Escudero, E. (2020). The removal of toxic metals from liquid effluents by ion exchange resins. Part XII: Mercury(II)/H+/Lewatit SP112. Rev. Metal. 56 (1), e160. https://doi.org/10.3989/revmetalm.160.; Alguacil, 2020aAlguacil, F.J. (2020a). The removal of toxic metals from liquid effluents by ion exchange resins. Part XIII: zinc(II)/H+/ Lewatit OC-1026. Rev. Metal. 56 (3), e172. https://doi.org/10.3989/revmetalm.172.; Alguacil, 2020bAlguacil, F.J. (2020b). The removal of toxic metals from liquid effluents by ion exchange resins. Part XIV: Indium(III) /H+/Dowex-400. Rev. Metal. 56 (4), e184. https://doi.org/10.3989/revmetalm.184.), in the present work, the ion exchange process of Fe2+ from aqueous solutions of various pH using Lewatit TP208 cationic exchange resin is investigated. Experimental variables influencing Fe2+ uptake onto the resin are investigated, and the resin performance is evaluated against other ion exchange resins and multiwalled carbon nanotubes. Elution of the metal, from Fe2+-loaded resin, by HCl solutions is also presented.
Lewatit TP208 resin (Fluka) has a crosslinked styrene-DVB matrix, containing sulfonic groups in Na+-form, and having the form of spherical beds of 410 µm mean size. Other cationic exchange resins used in the present work were purchased from the Fluka catalogue. The characteristics of the multiwalled carbon nanotubes (Sigma-Aldrich) used in the work were given elsewhere (Alguacil et al., 2016Alguacil, F.J., Lopez, F.A., Rodriguez, O., Martinez-Ramirez, S., Garcia-Diaz, I. (2016). Sorption of indium (III) onto carbon nanotubes. Ecotoxicol. Environ. Safe. 130, 81-86. https://doi.org/10.1016/j.ecoenv.2016.04.008.; Alguacil et al., 2017Alguacil, F.J., Garcia-Diaz, I., Lopez, F., Rodriguez, O. (2017). Removal of Cr(VI) and Au(III) from aqueous streams by the use of carbon nanoadsorption technology. Desalin. Water Treat. 63, 351-356. https://doi.org/10.5004/dwt.2017.0264.). All the other chemicals used in this investigation were of AR grade.
Iron(II) uptake and elution experiments were carried out by the standard procedure mentioned in other works of this series, whereas iron(II) concentration in the aqueous solutions was analyzed by AAS, and the metal uptake onto the resin was calculated by the mass balance.
3. RESULTS AND DISCUSSION⌅
3.1. Iron(II) uptake onto the resin⌅
Figure 1 showed the speciation of iron(II) in aqueous solutions against the pH values of this phase. It can be seen, that in solution, iron(II) existed as the predominant species, from pH 0 to pH vales near 8, in the form of the Fe2+ cation. The same speciation characteristics were obtained when the counter-anions were chloride or nitrate.
Thus, in the 0-8 pH range, iron(II) was loaded onto the resin by the next equilibrium:
where R was the non-exchangeable part of the resin, and the subscript aq and r represented the aqueous and resin phases, respectively. Accordingly, with the above, Fe2+ uptake onto the resin responded to a cationic exchange reaction.
3.1.1. Influence of the stirring speed⌅
To achieve effective loading of Fe2+ onto the resin, it is convenient to investigate the effect of the stirring speed applied on the system and its influence on the metal uptake. In the present investigation, stirring of the system was carried out from 300 to 1200 min-1, and results derived from this experimentation showed that maximum metal uptake at the equilibrium (32 mg·g-1) was not influence by the stirring speed. These results indicated that the aqueous boundary layer become constant and a minimum in the above range of stirring speeds. However, the stirring speed influenced the time in which the system reached the equilibrium (Fig. 2), in this figure the y-axe is the fractional achievement of the equilibrium, defined as:
where [Fe]r,t and [Fe]aq,t are the iron concentration in the resin at an elapsed time and at the equilibrium, respectively. From this figure it can be observed that from 600 to 1200 min-1 the equilibrium was reached after approximately 180 min, whereas in the case of 300 min-1, this state was reached after 300 min.
From the above results, it was derived that the rate law associated with the load of iron(II) onto the resin best fitted to the film-diffusion controlled model (Table 1). The expression of such model was described elsewhere (Lopez Diaz-Pavon, 2014Lopez Diaz-Pavon, A., Cerpa, A., Alguacil, F.J. (2014). Processing of indium(III) solutions via ion exchange with Lewatit K-2621 resin. Rev. Metal. 50 (2), e010. https://doi.org/10.3989/revmetalm.010.):
in the expression, F was the fractional achievement of the equilibrium (Eq. (2)) and k was the rate constant.
|Stirring speed, min-1||r2||k , min-1|
3.1.2. Influence of the temperature⌅
Using aqueous solutions of 0.01 g·L-1 Fe2+ at pH 3, and resin dosages of 0.13 g·L-1, the influence of the temperature (20-60 ºC) on Fe2+ uptake onto the resin was investigated. In these series of experiments, the reaction time was of 5 h, and the stirring speed of 1000 min-1. Experimental results indicated that the increase of the temperature increased the metal loading onto the resin from 9.6 mg·g-1 at 20 ºC to 30 mg·g-1 at 60 ºC. If the metal distribution coefficient between the resin and the aqueous phases was estimated as:
in this range of temperature, iron(II) uptake onto the resin was endothermic with ΔHº= 31 kJ·mol-1. The values of other thermodynamic values estimated for the exchange process were ΔGº= 1 kJ·mol-1 and ΔSº= 103 J·mol-1 K-1; thus, the Fe2+ uptake was non-spontaneous and with increasing randomness. In Eq. (4), [Fe]r,e had the same significance that in Eq. (2), and [Fe]aq,e was the corresponding metal concentration in the aqueous solution at the equilibrium.
3.1.3. Influence of the aqueous pH value⌅
To assess the role of the aqueous solution pH on Fe2+ uptake onto the resin, the variation of this variable in the range of 1 to 5 was carried out using aqueous phases of 0.01 g·L-1 Fe(II) at these various pH values and resin doses in the 0.13-1 g·L-1. It is clear from Fig. 3 that the metal uptake decreases with a decrease in pH from 5 to 1. The decrease of the metal uptake onto the resin with the decrease of the pH value can be attributed to the interaction of H+ and Fe2+ with the active sites of the resin, and their respective affinity to be exchanged with the Na+ ions of the active groups of the resin
3.1.4. Influence of resin dosage⌅
To investigate the effect of the resin dosage on Fe2+ uptake onto the cationic exchanger, several experiments were performed at pH 5. The results (Table 2) show that the percentage of Fe2+ removal from the solution increases when increasing the resin dosage. These results can be attributed to that increasing the cationic exchanger dosage large number of exchange sites for metal uptake is provided, resulting the above in an increase in the percentage of iron(II) uptake onto the resin.
|Resin dosage, g·L-1||% Fe2+ removal||[Fe]r,e, mg·g-1||[Fe]aq,e, mg·L-1|
Aqueous phase: 0.01 g·L-1 Fe2+ at pH 5; Temperature: 20 ºC; Stirring speed 1000 rpm; Time: 5 h
Experimental data best fitted (r2= 0.964) to the Freundlich isotherm model (Frohlich et al., 2019Frohlich, A.C., Foletto, E.L., Dotto, G.L. (2019). Preparation and characterization of NiFe2O4/activated carbon composite potential magnetic adsorbent for removal of ibuprofen and ketoprofen pharmaceuticals from aqueous solutions. J. Clean. Prod. 229, 828-837. https://doi.org/10.1016/j.jclepro.2019.05.037.):
with ln K= 2.3 and 1/n of 1.4. The Freundlich model may be indicative that the exchange process is performed on heterogeneous surfaces, and since n<1, the exchange process is undesirable.
The kinetics models associated to the present system were evaluated using the two extreme resin dosages, 0.13 and 1 g·L-1, used in this investigation. The results from this fit were shown in Table 3. At the lower resin dosage, the kinetics best responded to the first-order kinetic model (Wust el al., 1999Wust, W.F., Kober, R., Schlicker, O., Dahme, A. (1999). Combined zero- and first-order kinetic model of the degradation of TCE and cis-DCE with commercial iron. Environ. Sci. Technol. 33, 4304-4309. https://doi.org/10.1021/es980439f.):
|Resin dosage, g·L-1||Model||r2||k|
|1||pseudo-second order||0.999||7x10-3 g·mg-1 min-1|
being [Fe]aq,0 and [Fe]aq,t the iron concentrations in the solution at the initial and at elapsed time, respectively. From the model, it is obtained that [Fe]aq,0 is 9.97 mg·L-1, value which fits notably well with the experimental of 10 mg·L-1.
At the highest resin dosage (1 g·L-1), the experimental data fitted to the pseudo-second order kinetic model (Hao el al., 2017Hao, J., Dai, C., Liu, Y., Yang, Q. (2017). Removal of copper through adsorption by magnesium hydroxide nanorod. Desalin. Water Treat. 90, 252-261. https://doi.org/10.5004/dwt.2017.21420.):
being [Fe]r,e and [Fe]r,t, defined as in eq. (2). In this case, the value of [Fe]r,e was 10 mg·g-1 which was practically the same that the experimental value found as 9 mg·g-1.
3.2. Iron(II) uptake onto different resins and multiwalled carbon nanotubes⌅
The performance of Lewatit TP-208 resin on Fe(II) uptake was compared against other cationic exchange resins and also when multiwalled carbon nanotubes are used to remove Fe2+ from the aqueous solution. Table 4 resumed the results derived from the investigation. It can be seen, that higher Fe2+ uptakes were obtained with resins containing sulfonic acid groups (H+ or Na+ form) as active exchange sites. In the case of multiwalled carbon nanotubes, iron(II) uptake onto the adsorbent is 4.5 times lower than the metal loaded onto TP-208 resin.
|Exchanger/adsorbent||Functional group||[Fe]r or MWCNT, e, mg·g-1|
|Lewatit K2621||sulfonic group, H+||33|
|Amberlite 200||sulfonic group, H+||38|
|Lewatit SP112||sulfonic group, Na+||33|
|Lewatit TP260||phosphonic acid, Na+||8|
|Lewatit TP208||iminodiacetic acid, Na+||4|
|Lewatit TP 208||iminodiacetic acid, Na+||9a|
Aqueous phase: 0.01 g·L-1 Fe(II) at pH 2 except aat pH 5; Resin dosage: 0.25 g·L-1; aLewatit TP208 and MWCNT dosage: 1 g·L-1; Temperature: 20 ºC; Stirring speed: 1000 min-1; Time: 5 h
3.3. Fe2+ elution⌅
Since as one can be seen from Fig. 3, the percentage of iron(II) loaded onto the resin decreased as the value of the pH of the aqueous solution decreased, the elution of iron(II) loaded onto the resin was investigated using acidic solutions. Thus, a 1 M HCl solution was used as eluent on the resin loaded with 16 mg·g-1 Fe(II), HCl was chosen as acid medium due to the possibility of the reuse of the solutions to yield an end-product of pharmaceutical interest.
The results from this investigation were summarized in Table 5. From them, it can be seen that the variation in the volume of solution (Vs)/resin weight relationship had little influence on the percentage of Fe(II) elution (averaging 75%), however, the iron(II) concentration in the resulting solution can be increased more than ten times with respect to the initial feed solution. The resin can be regenerated to its Na+ form by washing it with NaOH solutions.
|Vs/resin weight, mL·g-1||% Fe(II) elution||[Fe]aq, mg·L-1|
Temperature: 20 ºC; Time: 0.5 h
Lewatit TP208 resin removes Fe2+ from aqueous solutions by a cationic exchange reaction releasing Na+ ions to the liquid waste; however, metal uptake onto the resin is pH-dependent. The stirring speed applied on the system has not any influence on the iron(II) concentration load, in the resin, at the equilibrium, but influences the time to achieve it. In the 300-1200 min-1 range, the rate law associated to the exchange process is best represented by the film-diffusion controlled model. The increase of the temperature increases the percentage of iron(II) loaded onto the resin, resulting in an endothermic process (ΔHº= 31 kJ·mol-1). The increase in the resin dosage results in a decrease of the metal concentration load onto the resin, but the equilibrium iron(II) concentration, in the corresponding solution, decreases as the resin dosage increases. The kinetic models associated to the exchange process are dependent of the resin dosage:
The exchange process is best represented by the Freundlich isotherm model, being the kinetics. At acidic conditions of pH 2, resins containing sulfonic groups present better iron(II) removal results than TP208 resin, though at pH 5 this resin presents best results with respect to the famed multiwalled carbon nanotubes.
Iron(II) elution from loaded resin is accomplished using hydrochloric acid.