The effect of sorbent dose

Sorbent dose is an important parameter which influences the extent of metal uptake from the solution by varying the sorption mass. For the various metal ions (Mg²⁺, Ca²⁺, K⁺ and Na⁺), the initial concentration of the aqueous feed solution was 500 mg/L. The experiment was conducted with the metal ion solutions being shaken separately with the various masses of the resin or the nanofibres for 8 hours (the procedure can be seen in section 3.6.5).

The results of the effect of the Purolite S950 resin dosage on the sorption of Mg²⁺, Ca²⁺, K⁺ and Na⁺ ions from their separate solutions are shown in Table 4.1 and Figure 4.13.

Table 4.1: The effect of resin mass on the sorption of Mg²⁺, Ca²⁺, K⁺ and Na⁺ ions from the individual

Figure 4.13: The effect of resin mass on the sorption of Mg²⁺, Ca²⁺, K⁺ and Na⁺ by Purolite S950 resin (Metal ion concentration, 500 mg/L; Temperature, 25 ^○C; volume, 25 ml; contact time 8 hours)

In this experiment, the metal ion concentration (500 mg/L) and the time (8 hrs) were kept constant while the resin mass was varied between 0.1 and 0.5 g. The amount of metal ion uptake increased from 25.4 and 11.4 to 97.4 and 97.8 % sorption for Mg²⁺ and Ca²⁺ ions respectively, as a result of the increase in the resin mass from 0.1 g to 0.5 g. A similar trend was observed for K⁺ and Na⁺ ions where the uptake of metal ions increased from 0.4 and 11.4 to 0.8 and 13.4 % sorption respectively. The K⁺ ion sorption by the resin was limited to between 0.3 g and 0.5 g / 25 mL, while Ca²⁺ and Mg²⁺ ions reached the zenith for the resin amount of 0.5 g / 25 mL. Based on these results, the optimum resin amount for both Ca²⁺ and Mg²⁺ was determined to be 20 g resin/L. Figure 4.13 shows clearly that the percentage sorption of the metal ions increased as the sorption dose increased from 0.1 g to 0.5 g. This increase can be attributed to the increase in the number of available sites as the dose of the sorbent is increased. The removal of metal ions increased with the increase in the Purolite S950 resin mass which reached its maximum at 0.5 g / 25 mL or 20 g resin/L. It is however reasonable that the near 100 % sorption of Mg²⁺ and Ca²⁺ from the solution is due to the availability of more exchange sites than the cations available in the solution. Maximum sorption is achieved when the exchange sites are exhausted; this means there are still more available exchange sites left on the resin capable of more sorption.

The nanofibre mass was also varied from 0.1 to 0.5 g in 25 mL metal ion solutions, the

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sorbent dose. From these results, it can therefore be inferred that the optimum sorption amount was found to be 12 g sorbent/L for the sorption of Mg²⁺, Ca²⁺, K⁺ and Na⁺ ions by PAN nanofibres. Any further increase in the sorption dose showed little or no effect upon the percentage sorption of the metal ions from the solution, this is perhaps due to the saturation of the available exchange sites.

Table 4.2: Effect of the sorption dose on the sorption of Mg²⁺, Ca²⁺, K⁺ and Na⁺ ions from individual solutions by PAN nanofibres.

PAN mass (g / 25 mL)

Mg²⁺ sorption (%)

Ca²⁺ sorption (%)

K⁺ sorption (%)

Na⁺ sorption (%)

0.1 22.2 4.0 1.0 9.4

0.2 23.0 5.0 1.4 10.4

0.3 23.4 5.4 2.0 10.6

0.4 23.6 5.5 2.0 10.6

0.5 23.8 5.5 2.0 10.6

Figure 4.14: The percentage sorption of Mg²⁺, Ca²⁺, K⁺ and Na⁺ ions with PAN nanofibres as a function of sorbent mass.

(Metal ion concentration, 500 mg/L; Temperature, 25 ^○C; volume, 25 mL; contact time 8 hours).

In the case of PAN+TiO2 nanofibres, the results in Table 4.3 and Figure 4.15 reveal that as the sorbent dose increased, there was a slight increase in the sorption of metal ions from the solution. From these results, the optimum sorbent amount was found to be 12 g sorbent/L for the sorption of Mg²⁺, Ca²⁺, K⁺ and Na⁺ ions by PAN+TiO2 nanofibres. The subsequent increase in the sorbent dose showed an insignificant increase in the percentage sorption of calcium, magnesium and sodium ions from the solution, this is perhaps due to exchange sites’

saturation.

Table 4.3: Effect of the sorption dose on the sorption of Mg²⁺, Ca²⁺, K⁺ and Na⁺ ions from individual solutions by PAN+TiO₂ nanofibres.

PAN+TiO2

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Figure 4.15: The percentage sorption of Mg²⁺, Ca²⁺, K⁺ and Na⁺ ions with PAN+TiO₂ nanofibres as a function of sorbent mass.

(Metal ion concentration, 500 mg/L; Temperature, 25 ^○C; volume, 25 mL; contact time 8 hours).

After the doping of PAN with TiO2, the PAN+TiO2 nanofibre when used for sorption showed a slight increase in the percentage of sorption of all the metal ions in solution compared to PAN nanofibres. Although there was a negligible increase in the sorption of metal ions when the various masses of the PAN+TiO2 nanofibre were used in the sorbent dose experiments.

This could be linked to exchange sites saturation, therefore, no sorption after a mass of 0.3 g sorbent.

PAN was also doped with zeolite and thereafter, it was used to carry out sorption for the individual metal ion solutions. It can be observed in Table 4.4 and Figure 4.16 that there was an increase in the percentage sorption of metal ions generally, compared with the PAN and PAN+TiO2 respectively. PAN+ZEOLITE showed an increased percentage sorption for both magnesium and calcium than what was obtained with PAN and PAN+TiO₂. The optimum sorbent dose for PAN+ZEOLITE nanofibre was found to be 8 g sorbent/L for the sorption of Mg²⁺, Ca²⁺, K⁺ and Na⁺ ions. Further increase in the sorbent dose showed no significant increase in the percentage of sorption.

0 5 10 15 20 25 30

0 0.1 0.2 0.3 0.4 0.5

% Sorption

Sorbent dose (g)

%Ca Ads

%K Ads

%Mg Ads

%Na Ads

Table 4.4: Effect of the sorption dose on the sorption of Mg²⁺, Ca²⁺, K⁺ and Na⁺ ions from individual

(Metal ion concentration, 500 mg/L; Temperature, 25 ^○C; volume, 25 mL; contact time 8 hours).

0

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