The cationic exchange resin Dowex C400 was used to remove nickel(II) from aqueous solutions of different pH values and under various experimental conditions: stirring speed of the aqueous solution/resin system, temperature, resin dosage and aqueous ionic strength. The selectivity of the resin was investigated against the presence of various metals in the aqueous solution, and the removal of nickel(II) from aqueous solutions was also compared with results obtained using multiwalled carbon nanotubes or functionalized (carboxylic groups) multiwalled carbon nanotubes as adsorbents. According to batch experimental data, best fit of the results is obtained with the Freundlich model, whereas the ion exchange process is best explained by the pseudo-first order model. Experimental data fit well to the moving boundary controlled model. Elution of the nickel(II) loaded onto Dowex C400 resin is fully possible using acidic solutions.
Though nickel is considered a hazardous element, there is not a clear evidence that soluble nickel salts alone are responsible for cancer, or whether when they appeared in conjunction with insoluble nickel form. In fact, this element does not appear in the list of harmful elements in primary and secondary drinking water standards (USEPA,
The overall consideration of nickel toxicology has lead to a recent number of reports about the removal of this metal from aqueous solutions using different technologies (El-Bahy and El-Bahy,
Dowex C400 (Fluka) having the characteristics shown in
Characteristics of Dowex C400 resin
Active group | Sulfonic in H+ form |
Matrix | Crosslinked styrene-DVB |
Particle form and mean size | Spheres, 410 μm |
All the loading experiments were carried out using 200 mL of the aqueous solution, 800 rpm and 20 ºC, except when these variables were investigated.
Metals in solution were analysed by AAS spectrometry, whereas metals load in the resin was calculated by the mass balance.
Being Dowex C400 a cationic exchanger, in which the sulfonic group is the active one, the equilibrium which describes the removal of nickel(II) from the solutions in the 0-5 pH range is represented by (
where, R is the non-reactive part of the resin, and r and aq represented the resin and the aqueous solution, respectively.
In batch ion exchange-adsorption experiments, the stirring speed could play a key-role in the performance of the system, since the correct stirring speed allows to gain a maximum in the metal loaded in the corresponding solid resin-adsorbent; however, this variable is often neglected in many scientific reports (AlOmar
Influence of stirring speed on nickel loaded onto the resin.
The influence of the temperature on metal load onto the resin was investigated using a resin dose of 0.25 gL-1 and aqueous solution of 0.01 gL-1 Ni(II) at pH 5. The results from
Influence of the temperature on nickel(II)l loading
T (ºC) | D (Lg-1) | log D |
---|---|---|
20 | 156 | 2.2 |
40 | 794 | 2.9 |
60 | 3996 | 3.6 |
Time: 5 h
where [Ni]r and [Ni]aq represented the nickel(II) concentrations at equilibrium in the resin and in the aqueous solution, respectively, increases as the operational temperature increases.
Thermodynamic parameters were estimated using conventional relationships, indicating that the reaction is endothermic (ΔHº 65 kJmol-1), and ΔSº and ΔGº are 111 kJmol-1K and 33 kJmol-1, respectively. The positive ΔGº value indicated that the exchange process is not spontaneous, and the also positive ΔSº value is an indication of increasing randomness at the liquid-resin interface during the exchange process.
The influence of the aqueous pH, ranging 0-5, on metal load onto the resin was investigated using 0.25 gL-1 of resin and an aqueous solution containing 0.01 gL-1 Ni(II). As can be seen seen from
Influence of pH on metal uptake
pH | Ni(II) (mgL-1) | Ni(II) (mg.g-1) |
---|---|---|
5 | 0.25 | 39.0 |
3 | 0.4 | 38.4 |
1 | 0.5 | 38.0 |
0 | 9.6 | 1.6 |
Resin: 0.25 gL-1
Influence of the aqueous pH on nickel removal from solution.
The influence of the resin dosage on nickel uptake onto the resin was also investigated, when the resin dosage was changed between 0.5 and 0.05 gL-1, and the results obtained are summarized in
Influence of resin dosage on metal uptake
Resin (gL-1) | [Ni(II)] (mgL-1) | [Ni(II)] (mg.g-1) |
---|---|---|
0.5 | 0.03 | 19.9 |
0.25 | 0.25 | 67.2 |
0.13 | 1.6 | 67.2 |
0.05 | 4.5 | 110.0 |
The aqueous ionic strength is another experimental variable which could influence the metal loading onto the resins, and its effect on the present system was investigated by the use of different concentrations of lithium chloride in the aqueous solution. After 3 h of contact between 0.25 gL-1resin and an aqueous solution containing 0.01 gL-1Ni(II), the results presented in
Metal uptake at various aqueous ionic strengths
I (M) | ||
---|---|---|
1 | - | - |
0.5 | 0.9 | 0.37 |
0.25 | 15.0 | 6.0 |
0.13 | 40.5 | 16.2 |
0 | 93.4 | 37.4 |
After 3 h
It is important to remark that when
Performance of Dowex C400 resin in binary solutions at 1:1 molar metals relationship
System | Elements pair | D (L g-1) | βNi/M |
---|---|---|---|
1 | Ni(II) | 196 | 0.99 |
Mn(II) | 198 | ||
2 | Ni(II) | 196 | 0.98 |
Cu(II) | 199 | ||
3 | Ni(II) | 196 | 2.6 |
Co(II) | 75.9 | ||
4 | Ni(II) | 196 | 8.7 |
Zn(II) | 22.6 | ||
5 | Ni(II) | 196 | <0.005 |
In(III) | >40000 |
Aqueous solution: 0.17 mmolL-1 each metal at pH 5. Resin dosage: 0.25 gL-1. Temperature: 20 ºC. Time: 5 h
It can be seen from this
Nickel(II) loaded onto the resin from binary solutions.
It is worth to brief mention here, that when the population or crowding effect, in a given system, appeared, it means that the value of the metal uptake of a target element in, i.e. the binary solution, decreased in comparison with the same value obtained when the given element is alone in solution.
The performance of Dowex C400 was compared with results obtained in the removal of nickel(II) by other smart adsorbents like multiwalled carbon nanotubes (MWCN) and carboxylic functionalized multiwalled carbon nanotubes (C-MWCN) are. Results for these series of experiments are shown in
Comparison of nickel exchanger-adsorbents
Exchanger-adsorbent | Ni uptake (mg g-1) |
---|---|
Dowex C400 | 19.9 |
MCN | <1.0 |
CMCN | 4.4 |
Aqueous solution: 0.01 gL-1 Ni(II) at pH 5. Resin or carbon nanotubes dosage: 0.5 gL-1.
Temperature: 20 ºC. Time: 5 h
The adsorption data were used to examine their fit to the Langmuir or Freundlich models (Alguacil
with 1/n 0.33 and ln Kf 4.1. In the above equation, [Ni]r,e and [Ni]aq,e represented the nickel concentrations at equilibrium in the resin and in the aqueous solution, respectively, whereas Kf is a Freundlich constant.
Adsorption or exchange kinetics was also evaluated. The pseudo-first and pseudo-second order rate equations, Eqs. (
were used to estimate their fit to the experimental data. In the above equations, [Ni]r,e represented the nickel concentration in the resin at equilibrium, [Ni]r,t represented the metal concentration in the resin at an elapsed time, t is the elapsed time and k1 and k2 are the respective rate constant for each model. Numerical results,
Results of the numerical calculations of the adsorption kinetics models.
Model | r2 | Constant rate |
---|---|---|
Pseudo-first order | 0.993 | k1= 1.4x10-2 min-1 |
Pseudo-second order | 0.991 | k2= 4.9x10-4 g mg-1 min-1 |
Moreover, the exchange process was evaluated by the use of the models described elsewhere (López Díaz-Pavón
with rate constant k 3.5x10-3 min-1 and r2 0.988. In the above equation, the fractional attainment to the equilibrium, F, was calculated by the next relationship:
where, [Ni]r,t and [Ni]r,e are the nickel concentrations in the resin at and elapsed time t and at equilibrium, respectively.
As it is mentioned above, and since nickel is poorly loaded onto the resin at pH values in the zero range, it seemed logical the use of mineral acids to accomplish both the nickel elution from the resin and the resin regeneration in the same step. Results obtained from these series of experiments are summarized in
Results of elution experiments
Eluant | Resin (mg) | Solution volume (mL) | Ni(II) eluted (%) | Ni(II) (mg L-1) |
---|---|---|---|---|
HCl 1M | 0.1 | 25 | 91.3 | 56.4 |
H2SO4 1M | 0.1 | 25 | 98.5 | 60.9 |
H2SO4 1M | 0.1 | 12.5 | >99 | 123-124 |
Time: 1 h
The removal of nickel(II) from aqueous solutions, in the 1-5 pH range, can be successfully done using the resin Dowex C400, however, at zero pH this removal falls off, the increase of the aqueous ionic strength also decreases the metal uptake onto the resin. The endothermic exchange process (between H+ from the resin and Ni2+ from the solution) is best explained by the moving boundary model, being the kinetics best explained by the pseudo-first order kinetic rate.
From binary solutions, nickel(II) can be separate from cobalt(II) and zinc(II), almost equally uptake onto the resin than manganese(II) and copper(II), whereas indium(III) is best loaded onto the resin than nickel. In any case, the uptake of nickel onto the resin is independent of the accompanying metal in the solution and also almost equal to that when only nickel is present in this solution (at 0.25 gL-1resin dosage, nickel uptake of 39.2 mg.g-1 from binary solutions versus 39 mg.g-1 for nickel monoelemental solution).
With respect to nickel removal from solutions, the performance of Dowex C400 resin is better than that of the multiwalled carbon nanotubes or carboxylic functionalized multiwalled carbon nanotubes.
To the CSIC (Spain) for support. To Mr. J.M. Medina for assistance in part of the experimental work.