The performance of anionic exchange resin Dowex 1x8 in the removal of arsenic(V) from aqueous solutions was investigated. Batch experimentation was carried out under different variables, including, the stirring speed applied on the system, the pH of the aqueous solution, resin dosage and temperature. Due to the characteristic speciation of arsenic(V) in aqueous phases, the removal of this element from the solution is negligible at highly acidic or alkaline pH values, but it is possible at the aqueous pH range of 4-9, thus, both HAsO4 2- and H2AsO4 - species are loaded onto the resin. At the above pH range, arsenic(V) uptake is exothermic. Different models are fitted to the experimental values in order to gain knowledge about this ion exchange system: rate law, kinetics and solute loading onto the resin. This loading is compared against the yielded using non-functionalized multiwalled carbon nanotubes. The elution step is investigated using acidic solutions (HCl medium) as eluent, from the eluted solutions, arsenic(V) can be efficiently stabilized as ferric or calcium arsenates.
Se ha investigado la eliminación de arsénico(V) de disoluciones acuosas mediate la resina de intercambio aniónico Dowex 1x8. Los experimentos se han llevado a cabo bajo diferentes condiciones experimentales como, velocidad de agotación, pH de la disolución acuosa, concentración de la resina y temperatura. Debido a la especiación del arsénico(V) en medio acuoso, la eliminación de este no es posible en medios de bajo o alto valor del pH, pero es posible a valores de pH comprendidos entre 4 y 9, por lo tanto, las especies HAsO4 2- y H2AsO4 - son las responsables de la carga del elemento en la resina. En este margen, 4-9, de valores de pH, la eliminación del arsénico del medio acuoso tiene carácter exotérmico. Los datos experimentales se han adaptado a distintos modelos en relación con el mecanismo de la eliminación del arsénico de la disolución, cinética, y carga del elemento en la resina; asimismo, se ha comparado la carga del arsénico en la resina con la que se obtiene con el empleo de nanotubos de carbono de pared múltiple. La etapa de elución se ha investigado empleando disoluciones de ácido clorhídrico, de estos eluidos, el arsénico(V) se puede estabilizar precipitándolo como los arseniatos de hierro(III) o calcio.
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 (
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 (
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 (
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 (
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
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.
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
Stirring speed, min-1 | pH 1 | pH 4 | pH 9 | pH 13 |
---|---|---|---|---|
600 |
nil |
30 |
90 |
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 (
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
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
where DAs represented to the arsenic distribution coefficient and R is the gas constant.
Temperature, ºC | pH 4 | pH 9 |
---|---|---|
20 |
30a
|
95a
|
ΔHº, kJ·mol-1
|
-18 |
-28 |
Stirring speed: 800 min-1, Time: 4 h, aValues in % uptake
The arsenic distribution coefficient is calculated as:
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
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
whereas at pH 9, the equilibrium associated to the anion exchange process was:
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 (
Resin dosage, g·L-1 | pH | [As]r,e mg·g-1 | [As]aq,e, mg·L-1 |
---|---|---|---|
0.5 |
4 |
6.0 |
7.0 |
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 (
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
where [As]aq,0 is the initial arsenic concentration in the aqueous solution, the value of RL= 0.024 (separation factor) was derived, and thus, the ion exchange process was favourable.
pH | Isotherm | r2 | KF, mg·g-1 | 1/n | KL, L·mg-1 | [As]r,m, mg·g-1 |
---|---|---|---|---|---|---|
4 |
Freundlich |
0.9335 |
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
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.
pH | model | r2 | K, min-1 | [As]aq,0, mg·L-1 |
---|---|---|---|---|
4 |
first order |
0.9162 |
0.0146 |
9.1 |
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.
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.
Eluent | Aqueous/resin ratio, mL·g-1 | % Elution | [As] in the eluate, mg·L-1 |
---|---|---|---|
HCl 0.1 M |
2000a
|
99 |
8.9 |
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.
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 (
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 (
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.
To the CSIC (Spain) for support. To Mr, J.M. Medina, Mrs. E. Moroño and Mrs. M. López (CENIM) for assistance in the experimental and analytical work.