The removal of toxic metals from liquid effluents by ion exchange resins. Part XVIII: Vanadium(V)/H⁺/Amberlite 958

1. INTRODUCTION

Nowadays vanadium is used in many industries (steel-making, non-ferrous alloys, chemicals, aerospacial, catalysis); together the above, mining, smelting, and processing of ores containing this element are the main sources of vanadium in the environment. Also this metal is gaining more practical importance due to its use in vanadium redox flow batteries (Lee et al., 2021Lee, J.-C., Kurniawan, Kim, E.Y., Chung, K.W., Kim, R., Jeon, H.S. (2021). A review on the metallurgical recycling of vanadium from slags: Towards a sustainable vanadium production. J. Mater. Res. Technol. 12, 343-364. https://doi.org/10.1016/j.jmrt.2021.02.065.). However, the presence of this element in the environment, especially in its V oxidation state, is hazardous for life, producing several diseases when dietary intake exceeds certain values (Barceloux and Barceloux, 1999Barceloux, D.G., Barceloux, D. (1999). Vanadium. J. Toxicol. Clinic. Toxicol. 37 (2), 265-278. https://doi.org/10.1081/CLT-100102425.; Ma and Fu, 2009Ma, Z., Fu, Q. (2009). Comparison of hypoglycemic activity and toxicity of vanadium (IV) and vanadium (V) absorbed in fermented mushroom of Coprinus comatus. Biol. Trace Elem. Res. 132, 278-284. https://doi.org/10.1007/s12011-009-8394-x.), and also being suspicious to be carcinogenic (Luz at al., 2018Luz, A.L., Wu, X., Tokar, E.J. (2018). Toxicology of inorganic carcinogens. Adv. Mol. Toxicol. 12, 1- 46. https://doi.org/10.1016/B978-0-444-64199-1.00002-6.). Moreover, inorganic vanadium is considered more toxic than vanadium-bearing organic compounds (Ghosh et al., 2015Ghosh, S.K., Saha, R., Saha, B. (2015). Toxicity of inorganic vanadium compounds. Res. Chem. Intermed. 41, 4873-4897. https://doi.org/10.1007/s11164-014-1573-1.).

The above makes of a necessity its removal from aqueous effluents, being, for this purpose, various hydrometallurgical technologies recently reviewed (Le and Lee, 2021Le, M.N., Lee, M.S. (2021). A review on hydrometallurgical processes for the recovery of valuable metals from spent catalysts and life cycle analysis perspective. Min. Proc. Extract. Metall. Rev. 42 (5), 335-354. https://doi.org/10.1080/08827508.2020.1726914.) Most recent contributions aimed to the removal of this hazardous element from solutions included the use of ion exchange resins (Abhilasha et al., 2021Abhilasha, Agarwal, H., Meshram, P., Meshram, R.B., Jha, S., Patel, J.N., Soni, M., Rokkam, K., Mashruwala, S. (2021). Green process for recovery of vanadium from hazardous spent contact process catalyst by oxalic acid: kinetics and mechanism. Sep. Sci. Technol. 56 (18), 3183-3200. https://doi.org/10.1080/01496395.2021.1878222.; Morales et al., 2021Morales, D.V., Rivas, B.L. González, M. (2021). Poly(4-vinylbenzyl)trimethylammonium chloride) resin with removal properties for vanadium(v) and molybdenum(vi). a thermodynamic and kinetic study. J. Chil. Chem. Soc. 65, 5118-5124. https://doi.org/10.4067/S0717-97072021000105118.) adsorbents (Aregay et al., 2021Aregay, G.G., Ali, J., Shahzad, A., Ifthikar, J., Oyekunle, D.T., Chen, Z. (2021). Application of layered double hydroxide enriched with electron rich sulfide moieties (S₂O₄^2-) for efficient and selective removal of vanadium (V) from diverse aqueous medium. Sci. Total. Environ. 792, 148543. https://doi.org/10.1016/j.scitotenv.2021.148543.; Peng et al., 2021Peng, H., Qiu, H., Wang, C., Yuan, B., Huang, H., Li, B. (2021). Thermodynamic and kinetic studies on adsorption of vanadium with glutamic acid. ACS Omega 6 (33), 21563-21570. https://doi.org/10.1021/acsomega.1c02590.), ion exchange resin-activated carbon composites (Bao et al., 2021Bao, S., Chen, Q., Zhang, Y., Tian, X. (2021). Optimization of preparation conditions of composite electrodes for selective adsorption of vanadium in CDI by response surface methodology. Chem. Eng. Res. Des. 168, 37-45. https://doi.org/10.1016/j.cherd.2021.01.032.) and liquid-liquid extraction operation (Ju et al., 2021Ju, J., Feng, Y., Li, H., Liu, S., Xu, C. (2021). Separation and recovery of V, Ti, Fe and Ca from acidic wastewater and vanadium-bearing steel slag based on a collaborative utilization process. Sep. Purif. Technol. 276, 119335. https://doi.org/10.1016/j.seppur.2021.119335.; Ying et al., 2021Ying, Z., Huo, M., Wu, G., Li, J., Ju, Y., Wei, Q., Ren, X. (2021). Recovery of vanadium and chromium from leaching solution of sodium roasting vanadium slag by stepwise separation using amide and EHEHPA. Sep. Purif. Technol. 269, 118741. https://doi.org/10.1016/j.seppur.2021.118741.). Others suggested technologies included chemical precipitation, lectrokinetic remediation, photocatalysis reduction, coagulation, microbiological treatment, and membrane filtration. The advantages or disadvantages of each one of these operations are summarized elsewhere (Peng et al., 2022Peng, H., Guo, J., Li, B., Huang, H., Shi, W., Liu, Z. (2022). Removal and recovery of vanadium from waste by chemical precipitation, adsorption, solvent extraction, remediation, photo-catalyst reduction and membrane filtration. A review. Environ. Chem. Lett. 20, 1763-1776. https://doi.org/10.1007/s10311-022-01395-z.)

The present manuscript followed the series of investigations on the use of ion exchange resins in the treatment of solutions containing hazardous elements (Alguacil et al., 2002Alguacil, F.J., Cobo, A., Alonso, M. (2002). Copper separation from nitrate/nitric acid media using Acorga M5640 extractant Part I: solvent extraction study. Chem. Eng. J. 85 (2-3), 259-263. https://doi.org/10.1016/S1385-8947(01)00166-8.; 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.; Alguacil 2021aAlguacil, F.J. (2021a). 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.; Alguacil 2021bAlguacil, F.J. (2021b). The removal of toxic metals from liquid effluents by ion exchange resins. Part XVI: iron(III)/H⁺/Lewatit TP208. Rev. Metal. 57 (3), e203. https://doi.org/10.3989/revmetalm.203.; Alguacil and Escudero, 2021Alguacil, F.J., Escudero, E. (2021). The removal of toxic metals from liquid effluents by ion exchange resins. Part XVII: arsenic(V))/H+/Dowex 1x8. Rev. Metal. 58(2), e221. https://doi.org/10.3989/revmetalm.221.). In the present investigation, Amberlite 958 resin was used to study its performance on the removal of vanadium(V), under various experimental conditions and compared some of the results with that obtained on the use of multiwalled carbon nanotubes as adsorbent for this element. The results derived from the ion exchange experiments were fitted to various models to explain the various aspects of the vanadium(V) uptake onto the resin. The elution of vanadium(V) loaded onto the resin under various acidic conditions was also investigated.

2. EXPERIMENTAL

Amberlite 958 (Fluka) had chloride ion as exchange anion, it was formed by a macroreticular acrylic copolymer of 0.67-0.87 mm particle size. Being an anion exchanger, this resin removes metal-anionic complexes from aqueous solutions accordingly to the general equilibrium, Eq. (1):

where R represented to the non-reactive part of the resin, X^- to the anionic metal complex, and the subscripts r and aq to the resin and aqueous phases, respectively.

Other chemicals used in the experimentation were of AR grade. Vanadium solutions were prepared from ammonium vanadate, whereas the multiwalled carbon nanotubes (MWCNTs) had the characteristics 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. Saf. 130, 81-86. https://doi.org/10.1016/j.ecoenv.2016.04.008.).

Vanadium(V) loading batch experiments were carried out in a glass reactor vessel (250 mL), containing the corresponding metal aqueous solutions to which resin or MWCNTs dosages were added. Stirring was provided via a four blades glass impeller and pH values were adjusted with the appropriate HCl or NaOH solutions, and being measured by a Crison 506-pHmeter. Elution step was investigated on the same basis as above.

Vanadium was analysed in the various aqueous solutions by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), within 2% associated error, whereas metal loaded onto the resin or MWCNTs was calculated by the mass balance.

3. RESULTS AND DISCUSSION

3.1. Vanadium(V) uptake onto Amberlite 958

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3.1.1. Influence of stirring speed

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In order to achieve effective uptake of vanadium(V) on the resin, it was convenient to investigate the influence of the stirring speed on metal uptake. Diffusional resistances encountered during the metal uptake can had an influence on the metal load onto the resin, and also on the time to reach metal distribution equilibrium between the resin and the aqueous phase. In the present investigation, stirring of the aqueous phase-resin system was carried out from 600 to 1000 min^-1 (Fig. 1). The factorial approach to the equilibrium (F) was calculated as (Eq. 2):

$F=\frac{{\left[V\right]}_{r,t}}{{\left[V\right]}_{r,e}}$  (2)

Figure 1.  Variation of F versus time at various stirring speeds. Aqueous phase: 0.01 g·L^-1 V(V) at pH 4; Resin dosage: 1.25 g·L^-1; Temperature: 20 ºC.

indicate not appreciable variation in vanadium(V) uptake with the various stirring speeds. This indicated that the aqueous boundary layer was a minimum in this range of stirring speeds, and the resistance due to it was minimized. In the above Eq. (2), [V]_r,t and [V]_r,e represented the vanadium(V) concentrations in the resin at an elapsed time and at the equilibrium, respectively.

The kinetics order associated to vanadium(V) uptake onto the resin was modelled using different kinetics models: first order, second order, pseudo-first order and pseudo-second order, from this modelling process, it was found that the experimental data best fit (r²= 0.9963) to the pseudo-first order kinetic equation (Hemavathy et al., 2019Hemavathy, R.R.V., Kumar, P.S., Suganya, S., Swetha, V., Varjani, S.J. (2019). Modelling on the removal of toxic metal ions from aquatic system by different surface modified Cassia fistula seeds. Bioresour. Technol. 281, 1-9. https://doi.org/10.1016/j.biortech.2019.02.070.), Eq. (3):

$\ln \left({\left[V\right]}_{r,e}-{\left[V\right]}_{r,t}\right)={\ln \left[V\right]}_{r,e}-k_{ps1}t$  (3)

with k_ps1 (constant rate) estimated as 0.026 min^-1, and [V]_r,e of 7.2 mgV·g^-1 resin, value with matched well with the experimental value of 7.6 mgV·g^-1 resin.

3.1.2. Influence of temperature

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The variation of temperature on vanadium(V) uptake was also investigated. In the present investigation, the resin dosage was of 0.16 g·L^-1, whereas the aqueous solution had the same composition than above. The results from the investigation was resumed in Table 1, it can be observed that metal uptake increased with the increase of the temperature, being the thermodynamic values of ΔHº, ΔSº and ΔGº estimated from a plot of log D versus 1/T and the corresponding equations (Alguacil and Cobo, 1998Alguacil, F.J., Cobo, A. (1998). Solvent extraction equilibrium of nickel with LIX 54. Hydrometallurgy 48 (3), 291-299. https://doi.org/10.1016/S0304-386X(97)00103-5.; 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.). From this plot, it was shown that the ion exchange process presented and endothermic character (ΔHº= 13 kJ·mol^-1). Other thermodynamics parameters derived from the experimental data and the convenient mathematical relationships indicated that ΔGº= -5 kJ·mol^-1, indicative of a spontaneous process, and ΔSº= 61 J/mol K, this positive value should be explained by an increase in the system randomness. In Table 1, D represented the vanadium distribution coefficient, calculated as:

Table 1.  Variation of vanadium(V) uptake with temperature

T, ºC	% V uptake	log D
20	55	1.2
40	62	1.0
60	70	0.91

Time: 3 h. Stirring speed: 800 min^-1

$D=\frac{{\left[V\right]}_{r,e}}{{\left[V\right]}_{aq,e}}$  (4)

where [V]_r,e and [V]_aq,e were the vanadium(V) concentrations, at the equilibrium, in the resin and in the aqueous solution, respectively.

3.1.3. Influence of the resin dosage and pH variation

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The influence of these two variables on vanadium(V) removal from the aqueous solution was investigated using resin dosages varying from 0.15 g·L^-1 to 1.25 g·L^-1, and solutions containing 0.01 g·L^-1 V(V) at pH 1, 4, 7, and 11. The results from this set of experiments were resumed in Table 2. These results showed that in the 4-11 pH range the increase of the resin dosage resulted in a decrease of the vanadium(V) remaining in the solution; results showed in this same Table also demonstrated that vanadium(V) uptake onto the resin was also pH-dependent, though protons did not appear in the overall ion exchange mechanism showed in Eq. (1). This dependence should be attributable to the various vanadium(V) species presented in the aqueous solutions at the various pH values (Fig. 2). At pH 1, and being VO₂ ⁺ the predominant species in the solution, it was logical that vanadium(V) was not loaded onto the resin due to the anion exchanger character of Amberlite 958 resin, and all the vanadium(V) remained in the equilibrated solution. From pH 4 to 11, the percentage of vanadium(V) in the equilibrated solution progressively increased. The electrostatic attractions between the positively charged active groups of the resin and the anionic vanadium species favour vanadium loading onto the resin (Li et al., 2020Li, M., Zhang, B., Zou, S., Liu, Q., Yang, M. (2020). Highly selective adsorption of vanadium(V) by nano-hydrous zirconium oxide-modified anion exchange resin. J. Hazard. Mater. 384, 121386. https://doi.org/10.1016/j.jhazmat.2019.121386.), also, it seemed that the uptake of polyvanadate species was favoured against the presence, at higher pH values, of mononuclear vanadium species in the solution. Within all the resin dosages tested, maximum vanadium(V) uptake was yielded at pH 4, being this result in accordance which it was described in the literature (Wołowicz and Hubicki, 2022Wołowicz, A., Hubicki, Z. (2022). Vanadium(V) Removal from aqueous solutions and real wastewaters onto anion exchangers and Lewatit AF5. Molecules 27 (17), 5432. https://doi.org/10.3390/molecules27175432.).

Table 2.  Percentage of vanadium(V) remaining in the solution at various pH values and resin dosages

Resin dosage, g·L-1	pH 1	pH 4	pH 7	pH 11
0.15	100	45	48	76
0.30	100	23	30	49
0.65	100	14	22	38
1.25	100	5	12	27

Temperature: 20 ºC; Time: 3 h; Stirring speed: 800 min^-1

Figure 2.  Vanadium(V) speciation in the aqueous phase (Puigdomenech, 2021Puigdomenech, I. (2021). MEDUSA PROGRAM. www.kth.se/che/medusa.).

Considering the above, vanadium uptake onto the resin can be expressed by the next equilibria, at pH 4:

$5{\left(R^{+}C{l}^{-}\right)}_r+V_{10}{O}_{27}O{H}_{aq}^{5-}\iff R_5^{+}{V}_{10}{O}_{27}O{H}_r^{5-}+5C{l}_{aq}^{-}$  (5)

at pH 7:

$R^{+}C{l}_r^{-}+V{O}_2{\left(OH\right)}_{2_{aq}}^{-}\iff R^{+}V{O}_2{\left(OH\right)}_{2_r}^{-}+C{l}_{aq}^{-}$  (6)

and at pH 11:

$2{\left(R^{+}C{l}^{-}\right)}_r+V{O}_{3}O{H}_{aq}^{2-}\iff R_2^{+}V{O}_{3}O{H}_r^{2-}+2C{l}_{aq}^{-}$  (7)

At this pH 11, the decrease in vanadium uptake can be attributable to the lesser exchangeability of the VO₃OH^2- species, and competition of OH^-, presented in the aqueous solution, to be loaded onto the resin (Zhang and Leiviskä, 2020Zhang, R., Leiviskä, T. (2020). Surface modification of pine bark with quaternary ammonium groups and its use for vanadium removal. Chem. Eng. J. 385, 123967. https://doi.org/10.1016/j.cej.2019.123967.):

$R^{+}C{l}_r^{-}+O{H}_{aq}^{-}\iff R^{+}O{H}_r^{-}+C{l}_{aq}^{-}$  (8)

In eqs (5-8)eqs 5, 6, 7, 8, R represented the non-exchangeable moiety of the resin, whereas subscripts r and aq represented to the equilibrated resin and aqueous phases.

At pH 4, the equilibrium data were presented in Table 3, these values were used to estimate the loading isotherm. The results of the various fittings indicated that at this pH value, vanadium(V) uptake was best represented (r²= 0.9679) by the linear form of the Freundlich isotherm (Elbadawy, 2019Elbadawy, H.A. (2019). Adsorption and structural study of the chelating resin, 1,8-(3,6-dithiaoctyl)-4-polyvinyl benzenesulphonate (dpvbs) performance towards aqueous Hg(II). J. Mol. Liq. 277, 584-593. https://doi.org/10.1016/j.molliq.2018.12.134.):

${\ln \left[V\right]}_{r,e}={\mathrm{lnK}}_F+\frac{1}{n}{\ln \left[V\right]}_{aq,e}$  (9)

Table 3.  Equilibrium values at pH 4

Resin dosage, g·L-1	[V]aq,e, mg·L-1	[V]r,e, mg V/g resin
1.25	0.5	7.6
0.65	1.4	13
0.30	2.3	26
0.15	4.5	37

Temperature: 20 ºC; Time: 3 h; Stirring speed: 800 min^-1

being the values of ln K_F and 1/n of 2.49 and 0.74, respectively. K_F and n represented relative indicators of uptake capacity and the energy associated to this uptake or the uptake intensity of a given system, respectively. If 1/n relationship was less than 1, as it was in the present system, it was favoured the load of the solute onto the resin.

3.1.4. Influence of vanadium(V) concentration in the aqueous solution

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In this case, solutions containing 0.01 or 0.1 g·L^-1 vanadium(V) at pH 4 were put into contact with a resin dosage of 1.25 g·L^-1. The results obtained from this set of experiments were shown in Fig. 3, where F (factorial approach to the equilibrium) values were calculated as in Eq. (2). It can be seen, that the increase of vanadium(V) concentration in the solution had an influence on the time to approach the equilibrium, i.e. at 0.01 g·L^-1 V(V) in the solution 80% (F= 0.8) of the equilibrium was reached within one hour of contact between the solution and the resin, whereas using the more concentrated metal solution, this value (80%) was reached within 1.5 hours of reaction. The experimental results were used to estimate the rate law associated to the metal uptake onto Amberlite 958 resin. The results from this fit indicated that, using both metal solutions, diffusion through the aqueous layer dominated vanadium(V) load onto the resin (Lopez Diaz-Pavon et al., 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.):

Figure 3.  Variation of F versus time at two vanadium(V) concentrations in the aqueous solution. Temperature: 20 ºC; Stirring speed: 800 min^-1.

$\ln \left(1-F\right)=-kt$  (10)

with k (rate constant) values of 0.028 min^-1 (r²= 0.9931) and 0.017 min^-1 (r²= 0.9699) for the initial metal concentrations of 0.01 g·L^-1 and 0.1 g·L^-1, respectively.

3.1.5. Vanadium(V) uptake onto multiwalled carbon nanotubes: a comparison

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The use of the resin in the removal of vanadium(V) from the solution was also compared with the use of multiwalled carbon nanotubes as adsorbent for the metal. In these experiments, the aqueous solution was of 0.01 g·L^-1 at pH 4, whereas the resin or the multiwalled carbon nanotubes dosages were of 0.3 g·L^-1. Other experimental variables were 20 ºC temperature, 800 min^-1 stirring speed, and 3 hours of equilibration time. The results indicated that when the adsorbent was used, metal load was of 12 mg V/g carbon nanotubes, whereas in the case of the resin this uptake was of near 26 mg V/g resin. Thus, at a first approach, it was apparent that, under the present experimental conditions, the resin was more effective than the carbon nanotubes on the metal uptake, and thus, in the removal of this hazardous metal from the solution.

4. CONCLUSIONS