The removal of toxic metals from liquid effluents by ion exchange resins. Part XVII: Arsenic(V)/H+/Dowex 1x8

1. INTRODUCTION

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Among the elements considered as toxic for humans, arsenic, both in its (V) and (III) oxidation states and both forming part of inorganic or organic compounds, has a pre-eminent position. The toxicity of this element is due to its carcinogenic character, promoting skin, liver, lung and kidney various cancers (Monteiro De Oliveira et al., 2021Monteiro De Oliveira, E.C., Siqueira Caixeta, E., Santana Vieira Santos, V., Barbosa Pereira, B. (2021). Arsenic exposure from groundwater: environmental contamination, human health effects, and sustainable solutions. J. Toxicol. Environ. Health -B: Crit. Rev. 24 (3), 119-135. https://doi.org/10.1080/10937404.2021.1898504.). Though there are a number of sites around the World affected by the presence of this hazardous element, its removal from landfills, waters, etc., is far to be completely finished, and still is of the upmost necessity in social, environmental and profitable terms.

The removal of arsenic(V) from waters, wastewaters, or generally speaking, aqueous solutions, is a topic of the utmost importance for the scientific community though there are few technologies proposed for this task (Alka et al., 2021Alka, S., Shahir, S., Ibrahim, N., Ndejiko, M.J., Vo, D.-V.N., Manan, F.A. (2021). Arsenic removal technologies and future trends: A mini review. J. Clean. Prod. 278, 123805. https://doi.org/10.1016/j.jclepro.2020.123805.; Weerasundara and Bundschuh, 2021Weerasundara, L., Ok, Y.-S., Bundschuh, J. (2021). Selective removal of arsenic in water: A critical review. Environ. Poll. 268, 115668. https://doi.org/10.1016/j.envpol.2020.115668.). Some recent investigations describe the use of micro-structured carbon-based (Islam et al., 2021Islam, A., Teo, S.H., Ahmed, M.T., Khandaker, S., Ibrahim, M.L., Vo, D.-V.N., Abdulkreem-Alsultan, G., Khan, A.S. (2021). Novel micro-structured carbon-based adsorbents for notorious arsenic removal from wastewater. Chemosphere 272, 129653. https://doi.org/10.1016/j.chemosphere.2021.129653.) and modified incinerated sewage sludge ash (Gao et al., 2021Gao, S., Wang, Q., Nie, J., Poon, C.S., Yin, H., Li, J.-S. (2021). Arsenate(V) removal from aqueous system by using modified incinerated sewage sludge ash (ISSA) as a novel adsorbent. Chemosphere 270, 129423. https://doi.org/10.1016/j.chemosphere.2020.129423.) adsorbents; ion exchange using various resins (Dadakhanov et al., 2021Dadakhanov, J., Marinova, A., Baimukhanova, A., Karaivanov, D., Temerbulatova, N., Kozempel, J., Roesch, F., Filosofov, D. (2021). Sorption of various elements on ion-exchange resins in acetic media. J. Radioanal. Nucl. Chem. 327, 1191-1199. https://doi.org/10.1007/s10967-021-07600-7.), resin-supported nano-hydrated zirconium oxide (Deng et al., 2021Deng, Z., Fang, Z., Liu, A., Xu, N., Zhang, X. (2021). From laboratory to large-scale manufacture of anion exchange resin-supported nano-hydrated zirconium oxide for As(V) removal from water solutions. Sci. Total Environ. 777, 146103. https://doi.org/10.1016/j.scitotenv.2021.146103.), and an electrochemical-ion exchange hybrid method (Rathi et al., 2021Rathi, B.S., Kumar, P.S., Ponprasath, R., Rohan, K., Jahnavi, N. (2021). An effective separation of toxic arsenic from aquatic environment using electrochemical ion exchange process. J. Hazard. Mater. 412, 125240. https://doi.org/10.1016/j.jhazmat.2021.125240.); various membrane processes (Moreira et al., 2021Moreira, V.R., Lebron, Y.A.R., Santos, L.V.S., Coutinho de Paula, E., Amaral, M.C.S. (2021). Arsenic contamination, effects and remediation techniques: A special look onto membrane separation processes. Process. Saf. Environ. Protec. 148, 604-623. https://doi.org/10.1016/j.psep.2020.11.033.; Sherugar et al., 2021Sherugar, P., Naik, N.S., Padaki, M., Nayak, V., Gangadharan, A., Nadig, A.R., Déon, S. (2021). Fabrication of zinc doped aluminium oxide/polysulfone mixed matrix membranes for enhanced antifouling property and heavy metal removal. Chemosphere 275, 130024. https://doi.org/10.1016/j.chemosphere.2021.130024.; Rojas-Challa et al., 2021Rojas-Challa, Y., de Gyves, J., Ortega-Muñoz, R., Montiel-Aguirre, F., González-Albarrán, R., Rodríguez de San Miguel, E. (2021). Comparative study of As (V) uptake in aqueous medium by a polymer inclusion membrane-based passive sampling device and two filamentous fungi (Aspergillus niger and Rhizopus sp.). Chemosphere 272, 129920. https://doi.org/10.1016/j.chemosphere.2021.129920.; Jarma et al., 2021Jarma, Y.A., Karaoğlu, A., Tekin, Ö., Baba, A., Ökten, H.E., Tomaszewska, B., Bostancı, K., Arda, M., Kabay, N. (2021). Assessment of different nanofiltration and reverse osmosis membranes for simultaneous removal of arsenic and boron from spent geothermal water. J. Hazard. Mater. 405, 124129. https://doi.org/10.1016/j.jhazmat.2020.124129.; Lopez et al., 2021López, J., Reig, M., Vecino, X., Cortina, J.L. (2021). Arsenic impact on the valorisation schemes of acidic mine waters of the Iberian Pyrite Belt: Integration of selective precipitation and spiral-wound nanofiltration processes. J. Hazard. Mater. 403, 123886. https://doi.org/10.1016/j.jhazmat.2020.123886.); and solvent extraction using a Triton X nonionic surfactant-choline chloride ionic liquid-water mixture (Silva et al., 2021Silva, S.J.B.E., Ferreira, G.M.D., Neves, H.P., de Lemos, L.R., Dias Rodrigues, G., Mageste, A.B. (2021). Use of aqueous two-phase systems formed by Triton X and choline chloride for extraction of organic and inorganic arsenic. Sep. Purif. Technol. 263, 118082. https://doi.org/10.1016/j.seppur.2020.118082.).

In this manuscript, and following the series of investigations proposed by the authors on the use of ion exchange resins in the treatment of solutions containing hazardous elements (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 et al., 2002aAlguacil, F.J., Coedo, A.G., Dorado, T., Padilla, I. (2002a). 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, 2002bAlguacil, F.J. (2002b). 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, 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.), Dowex 1x8 resin is used to investigate its performance in the removal of arsenic(V), from solutions of different pH values, and under various experimental conditions, with some of the results compared with that obtained multiwalled carbon nanotubes as adsorbent for the element. The ion exchange experimental data are fitted to various models to explain the arsenic(V) uptake onto the resin. From the As(V)-loaded resin, the elution step under various acidic conditions is investigated, and from the eluted solution the removal of arsenic(V), as the correspondent iron(III) or calcium(II) arsenates, is proposed.

2. EXPERIMENTAL

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Dowex1x8 (Fluka) is a strong basic resin having the trimethylammonium cation, as active group, and being chloride the counteranion. It contained a matrix of styrene-divinylbenzene in the form of spherical beads, with particle size in the 50-100 mesh (150-300 µm) range. Other chemicals used in the experimental work are of AR grade, whereas the multiwalled carbon nanotubes (MWCNTs) have the characteristics given in a previous work (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.).

The experiments performed in batch form, both in arsenic uptake and elution operations, were carried out in a glass reactor vessel (250 mL), containing the arsenic(V) aqueous solution to which the corresponding resin/MWCNTs dosages were added. Stirring was provided via a four blades glass impeller at 800 min^-1 and 20 ºC, except when these variables were investigated. pH values were adjusted with the appropriate HCl or NaOH solutions, and being measured by a Crison 506-pHmeter. In the case of the elution step and due to the lower solution volume used, the stirring speed was maintained at 300 min^-1, in order to avoid undesirable ejections from the reaction vessel.

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

3. RESULTS AND DISCUSSION

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3.1. Arsenic(V) loading onto Dowex 1x8 resin

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The variation of stirring speed was examined in order to investigate its influence on arsenic(V) uptake onto the resin. Experiments were carried out using aqueous solutions of 0.01 g·L^-1 As(V) at pH values of 1, 4, 9, and 13, whereas the resin dosage was of 1 g·L^-1. The results derived from these experiments were summarized in Table 1. It can be seen that the removal of As(V) from the solutions of pH 1 or 13 was non-existent. It is also observed that, at pH 4, the variation of the stirring speed had not influence on the arsenic(V) uptake onto the resin, but reached a maximum in the 700-900 min^-1 range when the experiments were carried out with solutions at pH 9. Moreover, after increasing the stirring speed at 1000 min^-1 an evident decrease in arsenic(V) uptake was found; this result was consistent with the formation of local equilibria between the resin bead and the surrounded solution which decreased the arsenic(V) load onto the resin. These results indicated that, at pH 4 and in the whole range of the investigated stirring speeds, the system had reached a minimum in the thickness of the aqueous boundary layer and, thus, arsenic(V) maximized. The same behaviour, using the solution of pH 9 and in the 700-900 min^-1 range, was found. From Table 1, it was noticed that the percentage of arsenic(V) onto the resin was by far greater at pH 9 than in the solution of pH 4.

Table 1. Influence of the stirring speed on As(V) uptake onto the resin

Stirring speed, min^-1	pH 1	pH 4	pH 9	pH 13
600 700 800 900 1000	nil nil nil nil nil	30 30 30 30 30	90 94 95 93 84	nil nil nil nil nil

Temperature: 20 ºC, Time: 4 h, Values in % uptake

Using the corresponding experimental data, the rate law governing the loading of As(V) onto the resin was determined by plotting (Fig. 1) the value of F versus time, being F defined as:

F = \frac{{[A s]}_{r, t}}{{[A s]}_{r, e}}

(1)

Figure 1. Variation of F with time. Stirring speed: 800 min^-1.

In the above equation, [As]_r,t and [As]_r,e were the arsenic concentrations in the resin at an elapsed time and at the equilibrium, respectively. The results represented in Fig. 1 showed the apparent difference in the resin performance during arsenic(V) uptake at the various pH values. At pH 4, 80% (F= 0.8) of the corresponding maximum arsenic uptake was achieved in about 10 min, whereas at pH 9, this exchange rate was reached in about 50 min. Further, the fit of the experimental data to the various models indicated that, for both solutions at pH 4 or 9, the best fitting was to the aqueous diffusion model (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.):

\ln (1 - F) = - k t

(2)

with rate constants k of 0.21 min^-1 (r²= 0.9206) and 0.03 min^-1 (r²= 0.9833) in the case of the solution at pH 4 and pH 9, respectively.

The influence of the temperature on arsenic(V) uptake onto the resin was also investigated; in this case, the experiments were performed on the solutions of pH 4 and 9 and the same resin dosage than above. The results derived from these series of experiments were shown in Table 2. It can be seen that, for the two solutions, the increase of the temperature produced a decrease in the arsenic(V) uptake onto the resin. Thermodynamic values were estimated from a plot of log D_As versus temperature and the slope (ΔHº) and intercept (ΔSº) of the expression:

\log D_{A s} = \frac{△ S^{\circ}}{2.3 R} - \frac{△ H^{\circ}}{2.3 R} \frac{1}{R}

(3)

where D_As represented to the arsenic distribution coefficient and R is the gas constant.

Table 2. Influence of temperature on As(V) uptake onto the resin

Temperature, ºC	pH 4	pH 9
20 40 60	30^a 21^a 15^a	95^a 90^a 83^a
ΔHº, kJ·mol^-1 ΔSº, kJ·mol^-1 K^-1 ΔGº, kJ·mol^-1	-18 -6x10^-2 0.4	-28 -6x10^-2 -9

Stirring speed: 800 min^-1, Time: 4 h, ^aValues in % uptake

The arsenic distribution coefficient is calculated as:

D_{A s} = \frac{{[A s]}_{r, e}}{{[A s]}_{a q, e}}

(4)

where [As]_r,e and [As]_aq,e are the arsenic concentrations in the equilibrium in the resin and in the aqueous solution, respectively.

The value of ΔGº was estimated by the next expression:

△ G^{\circ} = △ H^{\circ} - T △ S^{\circ}

(5)

Thus, using both solutions the exchange reaction was associated to an exothermic process (negative change of enthalpy) and a non-spontaneous (positive ΔGº) or spontaneous (negative ΔGº) ion exchange process at pH 9 and pH 4, respectively. The negative value of ΔSº indicated a process which decreased its randomness.

As it was described from the data presented in Table 1, the removal of arsenic(V) from solutions at pH values of 1 or 13 was negligible, thus it can be ascertain that arsenic(V) uptake onto the resin was greatly dependent on the pH of the aqueous solution. Moreover, two defined zones of arsenic uptake can be described. One at pH 4 with a percentage of arsenic uptake onto the resin of 30%. The other at pH 9, in which the loading of this element reached about 95% and, thus, the arsenic(V) uptake onto Dowex 1x8 resin was associated to the exchange of chloride ions from the resin with the corresponding predominant arsenic(V) species within the above pH values. Then, at pH 4, the exchange reaction was:

R - {C l}_{r} + H_{2} A s O_{4 a q}^{-} ⟺ R - H_{2} A s {O_{4 r} + C l}_{a q}^{-}

(6)

whereas at pH 9, the equilibrium associated to the anion exchange process was:

{2 (R - C l)}_{r} + H_{2} A s O_{4 a q}^{2 -} ⟺ R_{2} - H A s {O_{4 r} + 2 C l}_{a q}^{-}

(7)

It was also apparent the preference of the resin towards the divalent anion (greater uptake) and against the monovalent arsenic species.

Studying the influence of the variation in resin dosage on arsenic(V) solutions, it was found that both, in the solutions of pH 4 and 9, the increase in the resin dosage decreased the arsenic concentration in the resin, but decreased the equilibrium arsenic concentration in the solution (Table 3). Experimental data were fitted to various isotherms models, and the results of these fitting indicated that at pH 4, 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. Molec. Liq. 277, 584-593. https://doi.org/10.1016/j.molliq.2018.12.134.) best represented the loading data:

l n {[A s]}_{r, e} = l n K_{f} + \frac{1}{n} l n {[A s]}_{a q, e}

(8)

Table 3. Influence of the resin dosage on As(V) equilibrium data

Resin dosage, g·L^-1	pH	[As]_r,e mg·g^-1	[As]_aq,e, mg·L^-1
0.5 0.25 2.5 5 0.25 0.38 0.5 0.65 1.25	4 4 4 4 9 9 9 9 9	6.0 3.7 2.3 1.2 25.6 24.5 19.0 15.0 7.9	7.0 5.4 4.2 3.8 3.6 0.8 0.5 0.25 0.15

Aqueous phase: 0.01 g·L^-1 As(V) at pH 4 or pH 9, Temperature: 20 ºC, Time: 4 h, Stirring speed: 800 min^-1

This isotherm is an empirical model which assumed that the uptake surface sites are heterogeneous presenting a non-uniform distribution of heat of uptake over the resin surface.

Using the solution of pH 9, the best fit corresponded to the linear form of the Langmuir Type-1 isotherm (Ayawei et al., 2017Ayawei, N., Ebelegi, A.N., Wankasi, D. (2017). Modelling and interpretation of adsorption isotherms. J. Chem. 2017, 3039817. https://doi.org/10.1155/2017/3039817.):

\frac{{[A s]}_{a q, e}}{{[A s]}_{r, e}} = \frac{1}{K_{L} {[A s]}_{r, m}} + \frac{1}{{[A S]}_{r, m}} {[A s]}_{a q, e}

(9)

where [As]_r,m represented to the maximum concentration of arsenic loaded onto the resin. The Langmuir model or monolayer model is based on assumption that the exchange sites are homogenously distributed over the resin surface, there is not interaction between loaded molecules, and all the active sites have the same affinity for exchange of a single molecular layer, the values of the parameters corresponding to the two fittings were shown in Table 4. Furthermore, and taking into account the next relationship (Jamali and Akbari, 2021Jamali, M., Akbari, A. (2021). Facile fabrication of magnetic chitosan hydrogel beads and modified by interfacial polymerization method and study of adsorption of cationic/anionic dyes from aqueous solution. J. Environ. Chem. Eng. 9 (3), 105175. https://doi.org/10.1016/j.jece.2021.105175.):

R_{L} = \frac{1}{1 + K_{L} {]A s[}_{a q, 0}}

(10)

where [As]_aq,0 is the initial arsenic concentration in the aqueous solution, the value of R_L= 0.024 (separation factor) was derived, and thus, the ion exchange process was favourable.

Table 4. Parameters of the isotherms fitting

pH	Isotherm	r²	K_F, mg·g^-1	1/n	K_L, L·mg^-1	[As]_r,m, mg·g^-1
4 9	Freundlich Langmuir	0.9335 0.9957	0.056	2.4	4	27

It was examined the variation in the arsenic(V) uptake onto the resin at various elapsed times, when resin dosage was of 0.5 g·L^-1 or 1.25 g·L^-1. The results of these experiments were shown in Fig. 2, and they were used to estimate the fit of the data to several kinetics models. The results of the corresponding fittings were shown in Table 5, as it was indicated in this Table and using both arsenic-bearing solutions, the best fit corresponded to the first-order kinetic model (Wust et al., 1999Wust, W.F., Kober, R., Schlicker, O., Dahmke, A. (1999). Combined zero- and first-order kinetic model of the degradation of TCE and cis-DCE with commercial iron. Environ. Sci. Technol. 33 (23), 4304-4309. https://doi.org/10.1021/es980439f.):

\ln {[A s]}_{a q, t} = \ln {[A s]}_{a q, 0} - k t

(11)

in the above equation, [As]_aq,0 and [As]_aq,t represented to the arsenic concentrations, in the aqueous solution, at time zero and at elapsed time, respectively.

Table 5. Results of the fit to kinetic models

pH	model	r²	K, min^-1	[As]_aq,0, mg·L^-1
4 9	first order first order	0.9162 0.9942	0.0146 0.0695	9.1 8.9

Figure 2. Variation of the arsenic concentration in the aqueous solution with the reaction time. Aqueous phase: 0.01 g·L-1 As(V) at pH 4 or pH 9; Resin dosage: 0.5 g·L^-1 (pH 4) or 1.25 g·L^-1 (pH 9); Temperature: 20º C; Stirring speed: 800 min^-1.

The performance of Dowex 1x8 resin was compared to that of multiwalled carbon nanotubes. In these series of experiments aqueous solutions contained 0.01 g·L^-1 As(V) at pH 1, 4, 9, or 13, whereas the resin or the MWCNTs dosages were of 1.25 g·L^-1, other experimental variables were temperature of 20º C, time of 4 hours and stirring speed of 800 min^-1. The results from these experiments showed that both and MWCNTs were ineffective in the removal of arsenic from solutions of pH 1 and 13, however, the same performance was found for the carbon nanotubes in the treatment of the solutions of pH 4 or 9, with the arsenic concentration in the respective aqueous solutions maintained fixed at 0.01 g·L^-1. As it was shown above, in these conditions of pH 4 and 9, the arsenic removal rate, using 1.25 g·L^-1 resin dosage, was of 46% or 98.5%, respectively.

3.2. Arsenic(V) elution

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Considering the resin performance with the pH values of the aqueous solutions, it was showed that arsenic(V) was not loaded onto the resin either at highly acidic or alkaline pH values, thus, the elution step was considered using solutions with these two extreme pH values. Table 6 summarized the results of these investigations; firstly, it can be seen that in alkaline medium the percentage of arsenic elution is very low, thus, this medium was not adequate for the recovery of arsenic(V) from the arsenic-loaded resin, against these results, acidic solutions in HCl medium were adequate for the removal of the element from the loaded resin. Using HAsO₄^2- as the species loaded onto the resin (Eq. (7)), the elution process can be represented by the next equilibrium:

R_{2} - H A s O_{4 r} + 2 H {C l}_{a q} \Leftrightarrow 2 R - {C l}_{r} + H_{3} {A s O}_{4 a q}

(12)

Table 6. Elution results

Eluent	Aqueous/resin ratio, mL·g^-1	% Elution	[As] in the eluate, mg·L^-1
HCl 0.1 M HCl 0.1 M HCl 0.1 M HCl 0.01 M NaOH 0.1 M NaOH 0.1 M NaOH 0.1 M	2000^a 1000^a 500^a 500^a 800^b 400^b 200^b	99 99 99 85 14 8 7	8.9 17.7 35.4 30.4 1.2 1.3 2.3

^{^a}Resin loaded with 17.9 mg·g^-1; ^bResin loaded with 16.7 mg·g^-1; Temperature: 20º C; Stirring speed: 300 min^-1. Time: 30 min

At the same time that arsenic was recovered and concentrated in the eluate, the resin was regenerated.

3.3. Post-treatment of the As-bearing eluate

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After the elution stage, the arsenic(V) from the concentrated eluate can be recovered for safe dumping by precipitation with iron(III) or calcium(II) salts, to form the corresponding iron(III) or calcium(II) arsenates but in amorphous form. The various conditions for such precipitations were sufficiently described in the literature (Navarro et al., 2004Navarro, P., Vargas, C., Araya, E., Martín, I., Alguacil, F.J. (2004). Arsenic precipitation from metallurgical effluents. Rev. Metal. 40 (6), 409-412. https://doi.org/10.3989/revmetalm.2004.v40.i6.297.; De Klerk et al., 2015De Klerk, R.J., Feldmann, T., Daenzer, R., Demopoulos, G.P. (2015). Continuous circuit coprecipitation of arsenic(V) with ferric iron by lime neutralization: The effect of circuit staging, co-ions and equilibration pH on long-term arsenic retention. Hydrometallurgy 151, 42-50. https://doi.org/10.1016/j.hydromet.2014.11.003.; Doerfelt et al., 2016Doerfelt, C., Feldmann, T., Roy, R., Demopoulos, G.P. (2016). Stability of arsenate-bearing Fe(III)/Al(III) co-precipitates in the presence of sulfide as reducing agent under anoxic conditions. Chemosphere 151, 318-323. https://doi.org/10.1016/j.chemosphere.2016.02.087.), and basically consisted in the addition of the iron/calcium salt to the arsenic(V) solution, raising the pH value to the adequate pH value for the precipitation of the arsenate salt, filtration, drying and storage. Depending of the overall conditions, sometimes seemed adequate to use temperature and pressure in the precipitation stage (Monhemius and Swash, 1999Monhemius, A.J. Swash, P.M. (1999). Removing and stabilizing As from copper refining circuits by hydrothermal processing. JOM 51, 30-33. https://doi.org/10.1007/s11837-999-0155-y.), using this procedure, crystalline escorodite (iron(III) arsenate) was formed, being this salt much stable than the corresponding amorphous scorodite.

Figure 3. Arsenic(V) species distribution versus pH. Diagram generated by MEDUSA^® software package and Hydra database.

4. CONCLUSIONS

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Under a wide range of pH values, from 4 to 9, Dowex 1x8 resin seemed adequate to remove toxic arsenic(V) from aqueous solutions. This pH-dependence is consequence to the form in which arsenic(V) is present in the solutions (Fig. 3), thus, H₂AsO₄ ^- and HAsO₄ ^2- being the arsenic species which are loaded onto the resin, moreover, the divalent species are loaded onto the resin at higher percentages than the monovalent one. Thus, two defined pH values are used in the investigation, pH 4, being H₂AsO₄ ^- the predominant species, and pH 9 with the HAsO₄ ^2- predominance.
At pH 4, the variation of the stirring speed (600-1000 min^-1) has negligible influence in the arsenic uptake onto the resin, being the rate law dominated by aqueous diffusion; the anion exchange process is exothermic, and the arsenic loading onto the resin responded to the Freundlich isotherm model and the first-order kinetic model.
At pH 9, maximum arsenic uptake onto the resin is obtained in the 700-900 min^-1 range, and the process best fits to the aqueous diffusion law; the anion exchange process is also exothermic, and the arsenic uptake onto the resin followed the Langmuir Type-1 isotherm model.
In the 4-9 range of pH values, and against the resin performance, multiwalled carbon nanotubes does not remove arsenic(V) from the solution at any extent.
The elution of the arsenic loaded onto the resin is best performed using HCl solutions, and at the same time that arsenic is removed from the resin, this is regenerated. From the eluate, arsenic can be precipitated in the form of amorphous iron(III) or calcium(II) arsenates.

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The removal of toxic metals from liquid effluents by ion exchange resins. Part XVII: Arsenic(V)/H⁺/Dowex 1x8

La eliminación de metales tóxicos presentes en efluentes líquidos mediante resinas de cambio iónico. Parte XVII: Arsénico(V)/H⁺/Dowex 1x8