1. INTRODUCTION
⌅Iron(III) is another metallic element that usually accompanied to Mankind, and as others elements it is considered both essential and toxic to human life. Iron is an essential micronutrient and of importance for most life forms and is widely used in a variety of different proteins to carry out various functions (Geissler and Singh, 2011Geissler, C., Singh, M. (2011). Iron, meat and health. Nutrients 3 (3), 283-316. https://doi.org/10.3390/nu3030283.). The hazardousness of iron is demonstrated by its role in the development of diseases including cancer, ischemia, lung diseases, ageing as well as various neurodegenerative diseases (Recalcati et al., 2010Recalcati, S., Minotti, G., Cairo, G. (2010). Iron regulatory proteins: from molecular mechanisms to drug development. Antiox. Redox Sign. 13 (10), 1593-1616. https://doi.org/10.1089/ars.2009.2983.). Moreover, iron can induce cell death by generating free radicals as it interconverts between Fe2+ and Fe3+ forms (Nagla et al., 2017Nagla, R.E., Arab, T.T., Greenwood, M.T. (2017). Iron mediated toxicity and programmed cell death: A review and a re-examination of existing paradigms. Biochim. Biophys. Acta-Molecular Cell Res. 1864 (2), 399-430. https://doi.org/10.1016/j.bbamcr.2016.12.002.). The limited solubility of iron, especially iron(III) is a challenge as well as an advantage since it limits its toxicity.
However, a major iron related environmental health problem to humans, and much more common than iron toxicity, is iron-deficiency because of inadequate amounts of available iron in the diet (USEPA, 2004USEPA (2004). Iron. U.S. Environmental Protection Agency. EPA/600/1-78/017, Washington, D.C. ), leading to anemia due to a decreased production of functional hemoglobin.
Besides all the above issues in relation with iron, the presence of iron(III) in waters is accompanied by some characteristics as odor, taste and colour; its presence also causes corrosion and staining effects. It was established a secondary MCL (maximum contaminant level) of 0.3 mg.L-1 (USEPA, 2020USEPA (2020) Secondary drinking water standards: guidance for nuisance chemicals. U.S. Environmental Protection Agency. Accessed June 2021. www.epa.gov. https://www.epa.gov/sdwa/secondary-drinking-water-standards-guidance-nuisance-chemicals.), being noticeable effects above this value on the three waters characteristics mentioned previously; as an example, Fig. 1 shows water contaminated by iron(III) in CENIM-CSIC premises.
Together with the above health issues, iron is often found in the processing of other metals from raw materials or secondary wastes (Caravaca et al., 1994Caravaca, C., Cobo, A., Alguacil, F.J. (1994). Considerations about the recycling of EAF flue dusts as source for the recovery of valuable metals by hydrometallurgical processes. Resour. Conserv. Recycl. 10 (1-2), 35-41. https://doi.org/10.1016/0921-3449(94)90036-1.), and having little or non monetary value, its removal is also of importance in this field since its presence can be detrimental on the recovery of most valuable metals.
In the case of the presence of iron, as Fe3+, in liquid medium, several technologies are being using in the separation or elimination of the element from the aqueous solutions containing it. These technologies included solvent extraction with organic derivatives of phosphoric acid (Wang et al., 2020Wang, L., Wang, Y., Cui, L., Gao, J., Guo, Y., Cheng, F. (2020). A sustainable approach for advanced removal of iron from CFA sulfuric acid leach liquor by solvent extraction with P507. Sep. Purif. Technol. 251, 117371. https://doi.org/10.1016/j.seppur.2020.117371.) or mixtures of alcohols and octadecanamide (Zhu et al., 2021Zhu, K., Ren, X., Li, H., Wei, Q. (2021). Simultaneous extraction of Ti(IV) and Fe(III) in HCl solution containing multiple metals and the mechanism research. Sep. Purif. Technol. 257, 117897. https://doi.org/10.1016/j.seppur.2020.117897.), ion exchange using humic acids isolated from brown coals of Ekibastuz basin (Dauletbay et al., 2020Dauletbay, A. Serikbayev, B.A., Kamysbayev, D.K., Kudreeva, L.K. (2020). Interaction of metal ions with humic acids of brown coals of Kazakhstan. J. Exp. Nanosci. 15 (1), 406-416. https://doi.org/10.1080/17458080.2020.1810240.) or ion exchange resins (El-Hamid et al., 2020El-Hamid, A.M.A., Zahran, M.A., Ahmed, Y.M.Z., El-Sheikh, S.M. (2020). Separation of heavy metal ions from petroleum ash liquor using organic resins and FT-IR study of the process. Radiochemistry 62, 243-250. https://doi.org/10.1134/S1066362220020125.; Zhang et al., 2020Zhang, X., Zhou, K., Wu, Y., Lei, Q., Peng, C., Chen, W. (2020). Separation and recovery of iron and scandium from acid leaching solution of red mud using D201 resin. J. Rare Earths 38 (12), 1322-1329. https://doi.org/10.1016/j.jre.2019.12.005.), adsorption onto zeolite and bentonite (Bakalár et al., 2020Bakalár, T., Kaňuchová, M., Girová, A., Pavolová, H., Hromada, R., Hajduová, Z. (2020). Characterization of Fe(III) adsorption onto zeolite and bentonite. Int. J. Environ. Res. Public Health 17 (16), 5718. https://doi.org/10.3390/ijerph17165718.), natural or synthetic aluminosilicates (Flieger et al., 2020 Flieger, J., Kawka, J., Płaziński, W., Panek, R., Madej, J. (2020). Sorption of heavy metal ions of chromium, manganese, selenium, nickel, cobalt, iron from aqueous acidic solutions in batch and dynamic conditions on natural and synthetic aluminosilicate sorbents. Materials 13 (22), 5271. https://doi.org/10.3390/ma13225271.), and hydroxyapatite (Hamad et al., 2020Hamad, A.A., Hassouna, M.S., Shalaby, T.I., Elkady, M.F., Abd Elkawi, M.A., Hamad, H.A. (2020). Electrospun cellulose acetate nanofiber incorporated with hydroxyapatite for removal of heavy metals. Int. J. Biol. Macromol. 151, 1299-1313. https://doi.org/10.1016/j.ijbiomac.2019.10.176.), also solar technology was used to investigate its application on the removal of iron(III) from solutions (Arzate et al., 2020Arzate, S., Campos-Mañas, M.C., Miralles-Cuevas, S., Agüera, A., García Sánchez, J.L., Sánchez Pérez, J.A. (2020). Removal of contaminants of emerging concern by continuous flow solar photo-Fenton process at neutral pH in open reactors. J. Environ. Managem. 261, 110265. https://doi.org/10.1016/j.jenvman.2020.110265.).
Following the series of works devoted to the removal of toxic metals from aqueous solutions (Table 1), the present manuscript investigates about the cationic exchange process between Fe3+ and Lewatit TP208 resin. Different variables influencing metal ion uptake onto the resin are investigated, and models describing this uptake are also presented. The resin performance is compared against that of multiwalled carbon nanotubes, and data on the competitive exchange process of Fe3+ and Cu2+, Zn2+, Cr3+ or In3+ are given. Elution of the metal can be accomplished using acidic solutions.
Element | Resin | Reference |
---|---|---|
Cr(VI) | Dowex 1x8 | 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, 306-311. https://doi.org/10.3989/revmetalm.2002.v38.i4.412. |
Cd(II) | Lewatit TP260 | 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. |
Cu(II) | Amberlite 200 | 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. |
Cr(III) | Lewatit SP112 | 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. |
Ni(II) | Dowex C400 | 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. |
Mn(II) | Lewatit K2621 | 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/revmetalm.116. |
Mn(VII) | Amberlite 958 | 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. |
As(III) | Dowex 1x8 | 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. |
Pb(II) | Amberlite 120 | 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. |
Sb(III) | Ionac SR7 | 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. |
Co(II) | Lewatit TP260 | 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. |
Hg(II) | Lewatit SP112 | 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. |
Zn(II) | Lewatit OC-1026 | 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. |
In(III) | Dowex-400 | 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. |
Fe(II) | Lewatit TP208 | Alguacil, 2021Alguacil, F.J. (2021). The removal of toxic metals from liquid effluents by ion exchange resins. Part XV: iron(II)/H+/Lewatit TP208. Rev. Metal. 57 (1), e190. https://doi.org/10.3989/revmetalm.190. |
2. EXPERIMENTAL
⌅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. The characteristics of the multiwalled carbon nanotubes (MWCNTs, Sigma-Aldrich) used in the work are 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. Ecotox. Environ. Safety 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. Desal. Water Treat. 63, 351-356. https://doi.org/10.5004/dwt.2017.0264.). All the other chemicals used in this work are of AR grade.
Iron(III), and metals, uptake onto the resin and iron elution experiments were carried out by the same procedure described in other investigations of these series; iron(III) and other metals concentrations in the aqueous solutions were analyzed by AAS, and the metal uptake onto the resin, or the carbon nanotubes, was calculated by the mass balance.
3. RESULTS AND DISCUSSION
⌅3.1. Iron(III) uptake onto the resin
⌅Being Lewatit TP208 a cationic exchange resin it is logical to attribute Fe3+ uptake onto the resin to the next equilibrium:
In the above equation, R represented the non-reactive part of the resin, and the subscript aq and r represented the aqueous and resin phases, respectively.
3.1.1. Influence of the stirring speed
⌅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.
Stirring speed, min-1 | % Fe(III) uptake |
---|---|
600 | 23 |
800 | 45 |
900 | 52 |
1000 | 50 |
1100 | 36 |
1200 | 27 |
Temperature: 20 ºC. Time: 4 h
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., 2014López Díaz-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.):
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.
3.1.2. Influence of the aqueous pH and resin dosage
⌅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.
pH | 0.25 g.L-1 | 0.38 g.L-1 | 0.5 g.L-1 | 1 g.L-1 |
---|---|---|---|---|
4 | 21 | 19 | 19 | 9.9 |
3 | 20 | 19 | 19 | 9.9 |
2 | 15 | 17 | 18 | 9.6 |
1 | 4.8 | 4.8 | 5.0 | 4.7 |
Experimental conditions as in Fig. 2
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., 2019Al-Ghamdi, Y.O., Alamry, K.A., Hussein, M.A., Marwani, H.M., Asiri, A.M. (2019). Sulfone-modified chitosan as selective adsorbent for the extraction of toxic Hg(II) metal ions. Adsorpt. Sci. Technol. 37 (1-2), 139-159. https://doi.org/10.1177/0263617418818957.):
pH | r2 | K, L.mg-1 | [Fe]r,m, mg.g-1 | RL |
---|---|---|---|---|
4 | 0.9982 | 8.3 | 21 | 0.012 |
3 | 0.9986 | 10 | 20 | 9.9x10-3 |
2 | 0.9905 | 345 | 16 | 3.0x10-4 |
1 | 0.9810 | 3.6 | 5 | 0.027 |
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 RL 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 RL estimated for each pH are shown in Table 4, since they are included between zero and 1 values (0<RL<1), in all the cases, the cationic exchange process is favourable.
3.1.3. Influence of the temperature on Fe(III) uptake
⌅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 DFe 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.
3.1.4. Influence of the initial metal concentration in the aqueous feed solution
⌅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., 2017Mohaghehian, A., Pourmosheni, M., Vahidi-Kolur, R., Yang, J.-K., Shirzad-Siboni, M. (2017). Preparation and characterization of kaolin coated with Fe3O4 nanoparticles for the removal of hexavalent chromium: kinetic, equilibrium and thermodynamic studies. Desal. Water Treat. 90, 262-272. https://doi.org/10.5004/dwt.2017.21426.):
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., 2019Hamza, M.F., Wei, Y., Benettayeb, A., Wang, X.P., Guibal, E. (2019). Efficient removal of uranium, cadmium and mercury from aqueous solutions using grafted hydrazide-micro-magnetite chitosan derivative. J. Mater. Sci. 55, 4193-4212. https://doi.org/10.1007/s10853-019-04235-8.):
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.
Metal concentration, g.L-1 | Model | a[Fe]r,e, mg.g-1 | k |
---|---|---|---|
0.005 | pseudo-2nd order | 11 | 2.5x10-3 g·mg-1 min-1 |
0.08 | pseudo-1st order | 48 | 0.011 min-1 |
aExperimental values: 9.9 and 51 mg·g-1
3.1.5. Iron(III) uptake using multiwalled carbon nanotubes: a comparison with resin results
⌅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.
pH 2 | pH 4 | |
---|---|---|
Lewatit TP208 | 9.8 | 9.9 |
MWCNTs | nil | 4.2 |
Temperature: 20 ºC. Time: 4 h. Stirring speed: 900 min-1
3.1.6. Iron(III) competitive uptake versus other cations
⌅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, ßFe/M, were calculated as:
System | [M]aq,e, mmol.L-1 | [M]r,e, mmol.g-1 | DFe/M | ßFe/M |
---|---|---|---|---|
Fe3+-Zn2+ | 0.11-0.12 | 0.26-0.14 | 2.4-1.2 | 2.0 |
Fe3+-Cu2+ | 0.11-0.11 | 0.27-0.15 | 2.5-1.4 | 1.8 |
Fe3+-Cr3+ | 0.11-0.19 | 0.28-0.070 | 2.5-0.37 | 6.8 |
Fe3+-In3+ | 0.11-0.045 | 0.29-0.17 | 2.6-3.8 | 0.68 |
Temperature: 20 ºC. Time: 4 h. Stirring speed: 900 min-1
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 ßFe/M values were greater than 1. In the case of In3+, the experimental ßFe/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 ßIn/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+.
3.2. Iron(III) elution
⌅Accordingly, with the results obtained in subsection 3.1.2., a shift of the aqueous pH value towards more acidic values leaded to a decrease in the percentage of iron(III) uptake onto the resin, thus, it was logical to use acid solutions as eluants for the present systems. The elution experiments were carried out using a resin dosage of 0.5 g.L-1 loaded with 7 mg.g-1 of Fe3+, as elution solutions 1M sulphuric acid or hydrochloric acid were used. The results were showed in Table 8, and whereas the reaction time had not effect on the elution yields, it can be seen that the HCl solution performed, in terms of the percentage of Fe3+ elution, better than the sulphuric acid one, also the increase of the volume of elution solution/resin weight (Vaq/Wr) relationship increased this percentage.
Eluant | Vaq/Wr, mL.mg-1 | time, min | % elution | Fe(III) mg.L-1 |
---|---|---|---|---|
1 M H2SO4 | 200 | 15-60 | 52 | 18 |
1 M HCl | 200 | 15-60 | 67 | 23 |
1 M HCl | 400 | 15-60 | 82 | 14 |
Temperature: 20 ºC. Stirring speed: 300 min-1.
It is expected that these elution results will be improve under continuous implementation, i.e. columns. Once iron(III) is eluted, the resin is returned to its Na+ form by washing it with the adequate NaOH solution
4. CONCLUSIONS
⌅Iron(III), in the form of Fe3+ cation, can be remove from liquid solutions or effluents by the use of Lewatit TP208 resin. Maximum metal loading onto the resin is achieved at a stirring speed of 900 min-1, and a minimum in the thickness of the aqueous boundary layer is reached. Fe3+ uptake onto Lewatit TP208 responded to a cation exchange reaction, which released Na+ cations to the aqueous phase, however, the metal loading process is pH-dependent, and this dependence is attributable to a competitive reaction between H+ and Fe3+ of the aqueous medium to link with the active group of the resin. Also, the percentage of metal uptake onto the resin is dependent on the resin dosage. The increase of the temperature leads to an increase of the percentage of Fe3+ uptake, thus resulting in an endothermic exchange process. The variation of the initial metal concentration in the aqueous solution also influences the percentage of metal uptake, decreasing this value as the initial iron(III) concentration in the solution increases from 0.005 to 0.08 g.L-1. By the use of Lewatit TP208 resin, Fe3+ can be separate from Cu2+, Zn2+ and Cr3+, but not from In3+, which presented a distribution coefficient value higher than that of Fe3+. Fe3+ is loaded onto the resin far better than Fe2+. With respect to multiwalled carbon nanotubes, Fe3+ load onto the resin is higher, at pH values of 2-5, than that of the one yield by the carbon nanotubes. Fe3+ elution, from the metal-loaded resin, can be accomplished by the use of acidic solutions.