The anionic exchange resin Amberlite 958 was used to remove manganese(VII), as permanganate ion, from aqueous solutions of different pH values. Besides the aqueous pH, other variables involved in the exchange process were investigated, including, stirring speed of the system, temperature, ionic strength and resin concentrations. The non-lineal fit of the experimental points, indicated that the equilibrium isotherm is best explained by the Langmuir equation, whereas the anion exchange equilibrium is endothermic and non-spontaneous. The Mn(VII) loading performance of Amberlite 958 resin was compared with that of other anion exchangers and multiwalled carbon nanotubes. The elution of the Mn(VII) loaded onto Amberlite 958 was investigated using hydrazine sulphate solutions, which render manganese to the eluate as the somewhat less hazardous form of Mn(II).
Manganese occurs naturally in the II or III oxidation states. Being manganese(II) the only manganese oxidation state stable in waters, the presence of manganese(VII) in the environment is only due to antropogenic causes.
The manganese(VII) oxidation state has a double character, to one side, its oxidative powder makes to this element a powerful decontaminant for wastes containing harmful compounds (Zhang
Following in the series of articles published in this Journal about the use of ion exchange resins in the removal of hazardous metals (Alguacil
Amberlite 958 (Fluka) has the characteristics shown in
Characteristics of Amberlite 958 resin
Active group | quaternary ammonium salt in chloride form |
Matrix | macroreticular acrylic copolymer |
Particle form and particle size | Spheres, 13-45 mesh |
All the loading experiments were carried out in a glass reactor using 200 mL of the aqueous solution, 1200 rpm and 20 °C, except when these variables were investigated.
Metals in solution were analysed by AAS spectrophotometry, whereas metals load in the resin was calculated by the mass balance.
Manganese(VII) or the permanganate ion MnO4
−, which is the form in which Mn(VII) is normally found in aqueous solutions, loads onto the resin
where the subscripts aq and r refereed to the aqueous and resin phases, respectively, and ]- to the non-reactive matrix of the resin.
The effect of varying the stirring speed on Mn(VII) loading onto the resin was investigated using aqueous phases of 0.01 g·L−1 Mn(VII) at pH 5, and resin doses of 0.5 g·L−1. The stirring speed was varied between 350 and 1200 rpm. The results derived from this investigation were shown in
Influence of stirring speed on Mn(VII) loading onto the resin. Aqueous phase: 0.01 g·L−1 Mn(VII) at pH 5. Resin dosage: 0.5 g·L−1. Temperature: 20 °C.
In the case of the temperature, the influence of this variable on metal uptake by the resin was investigated using the same aqueous solution as above, but with a resin dose of 0.25 g·L−1. The results from this study were summarized in
Manganese(VII) load at various temperatures
nT, °C | [Mn(VII)] (mg·L−1) | [Mn(VII)] (mg·g−1) | D |
---|---|---|---|
20 | 1.25 | 35.0 | 28.0 |
40 | 0.70 | 37.2 | 53.1 |
60 | 0.28 | 38.9 | 138.9 |
Aqueous phase: 0.01 g·L−1 Mn(VII) at pH 5, resin dosage: 0.25 g·L−1, Time: 5 h
Calculated as [Mn(VII)]e,r/[Mn(VII)]e,aq
The influence of the pH on metal load onto the resin at various aqueous pH was investigated with resin doses of 0.5 g·L−1 and aqueous solutions of 0.01 g·L−1 Mn(VII) at pH values from 1 to 5. The results were shown in
Influence of aqueous pH on Mn(VII) loading onto the resin. Aqueous phase: 0.01 g·L−1 Mn(VII) at various pH values. Resin dosage: 0.5 g·L−1. Temperature: 20 °C. Stirring speed: 1000 rpm.
The variation of the resin concentration and its influence on metal uptake were investigated with resin doses of 0.075-1 g·L−1 and aqueous solutions of 0.01 g·L−1 Mn(VII) at pH 5. The results showed in
Influence of resin dosage on Mn(VII) uptake. Rest of experimental conditions as in Fig. 2.
The kinetics of manganese(VII) loaded onto the resin was derived from the results obtained with the resin dose of 0.25 g·L−1. The model fit to the experimental data indicated that under the given experimental conditions, the kinetics of manganese(VII) uptake is governed by the pseudo-first order rate law (Hallajiqomi and Eisazadeh,
where [Mn(VII)]e,r and [Mn(VII)]t,r are the manganese concentrations in the resin at equilibrium and at elapsed time, respectively, t is the time and k1 is the rate constant. From the fit data, k1 is 0.011 min−1 and ln[Mn(VII)]e,r= 3.59 which compares well with the experimental value of 3.56.
An equilibrium isotherm was generated from the experimental loading results, and this isotherm is shown in
Equilibrium loading isotherm of Mn(VII) onto Amberlite 958 resin and its fit to the Langmuir and Freundlich models. Experimental points: ∆, Langmuir model: continuous line, Freundlich model: broken line. Temperature: 20 °C. Stirring speed: 1000 rpm.
where [Mn(VII)]e,r and [Mn(VII)]e,aq are the manganese concentrations in the resin and in the aqueous solution at equilibrium, respectively, whereas Q0 represented the amount of solute required to form a monolayer, accordingly to the present fit 66.3 mg·g−1, and b is a constant, in the present case with a value of 1.07.
The manganese (VII) load onto the resin can be also affected by the ionic strength presented by the aqueous solution. Thus, this variable was also investigated using aqueous solutions of 0.01 g·L−1 Mn(VII) and different amount of inorganic salts, the resin dose was in all the cases of 0.25 g·L−1. The results of these set of experiments are summarized in
Influence of the feed ionic strength on Mn(VII) exchange
Ionic strength (M) | Mn(VII) load |
---|---|
1M LiCl | 7.8 |
0.1 LiCl | 25.0 |
0.1 Li NO3 | 32.0 |
0.1 NaCl | 36.1 |
0.1 NaClO4 | 26.2 |
0.1 Li2SO4 | 31.2 |
Time: 5 h
Several others exchangers or adsorbents were tested in order to compare the capacity of Amberlite 958 to load manganese(VII) with that of these. In this case, the exchange resins or the adsorbents doses were of 0.25 g with aqueous feeds of 0.01 g·L−1 Mn(VII) at pH 5. The results obtained from these set of experiments being shown in
Mn(VII) load with different types of exchanger-adsorbents
Exchanger-adsorbent | Active group |
Mn (VII) load (%) | Mn(VII) uptake (mg·g−1) |
---|---|---|---|
Amberlite 958 | QAS | 87.5 | 35.0 |
MWCNT | none | 93.0 | 37.2 |
Ionac SR7 | QAS | 69.0 | 27.6 |
Lewatit EP-63 | none | nil | nil |
Lewatit MP-64 | QAS | 20.3 | 8.1 |
Temperature: 20 °C, Time: 5 h,
QAS: quaternary ammonium salt
The experimental data from which manganese(VII) is uploaded onto Amberlite 958, Ionac SR7 and the MWCNT was tested against various models in order to gain knowledge of how this metal species is loaded onto the above three. From this fit, it is obtained that with either with Amberlite 958 and Ionac SR7, both acting as anionic exchangers, the metal load can be explained by the film-diffusion controlled process (Amberlite 958 r2 = 0.997, rate constant = 0.011 min−1, Ionac SR7 r2 = 0.990, rate constant = 0.014 min−1):
where as in the case of the non-functionalized MWCNT adsorbent, the manganese(VII) load is best explained by the particle-diffusion controlled model (r2 = 0.985, rate constant 0.13 min−1)
In the above equations, F is the factorial approach to the equilibrium, calculated as:
where both manganese concentrations have the same significance as above, and k is the rate constant (López Díaz-Pavón
Different elution solutions were examines, in the case of 1M NaCl solution, there is not any appreciable removal from the metal loaded onto the resin well after 1 hour of reaction. In the case of 1 M HCl solution, the situation is a little better but the yield is not above 30% manganese recovery in the solution after 2 hours of reaction. The above is not the case, if solutions of hydrazine sulphate are used as eluant for the present system. In these cases, using various experimental conditions, i.e. 5 g·L−1 hydrazine sulphate solution, resin loaded with 14 mg Mn(VII)/g resin and liquid/resin ratios of 100-200 mL·g−1, the percentage of manganese recovery in the eluate is quantitative even at 15 min of contact. In the same experimental conditions that above, but using a 2.5 g·L−1 hydrazine sulphate solution as eluant, the recovery is 80% at 7.5 min and quantitative at 10 min of reaction. Moreover, using these hydrazine sulphate solutions, manganese is obtained in the eluate as the less hazardous form of Mn(II):
Various experimental conditions that influences the manganese(VII) uptake onto Amberlite 958 resin were investigated. With a stirring speed of 1000 rpm, maximum metal loading was achieved, indicating that at this speed a minimum in the thickness of the aqueous layer is attaining, whereas an increase of the temperature from 20 °C to 60 °C increased the metal uptake in the resin, being this indicative of an endothermic process (∆H°= 31.1 kJ·mol−1). An equilibrium loading isotherm was experimentally obtained, and its better fit is to the Langmuir model, whereas the metal loading process responded well to the pseudo-first order kinetic law. The manganese(VII) uptake had been compared between various anion exchanger resins and adsorbents without any specific interchangeable group such as multiwalled carbon nanotubes and Lewatit EP-63 resin. The experimental results indicated that best manganese(VII) uptake results were obtained with the carbon nanotubes and Amberlite 958 resin, and metal loading responded to the particle-diffusion (carbon nanotubes) or film-diffusion (Amberlite 959) controlled models. Best elution results were obtained with hydrazine sulphate solutions, and manganese released in the eluant as the less toxic form of Mn(II).
To the CSIC (Spain) for support. To Mr. J.M. Medina for assistance in part of the experimental work.