3.2 Results and discussion
3.2.3 Three-phase SLM study
3.2. Results and discussion 67
0 10 20 30 40 50 60 70 80 90 100
0 10 20 30 40 50 60 70 80 90 100
0 10 20 30 40 50 60 70 80
% Recovery of Chromium
% Extraction of Chromium
Time (h)
%Extraction
%Recovery
Figure 3.12: Efficiency of extraction and recovery of Cr(VI) in three phase LM based separation process during fed-batch operation.
at 0.03 M Na2-EDTA and at pH 6.5. Extraction and recovery were 67.78% and 58.25% respectively for 0.03 M Na2-EDTA and 82.5% and 71.54% respectively at pH 6.5. However, the %recovery got enhanced as compared to that in BLM operation in both the cases. This was possible due to the structural advantage in SLM where the thickness of membrane is very small compared to that in BLM and hence the solute needs to diffuse through lesser distance across the two membrane interfaces.
The amount of hold up of solute in the LM phase is also small as compared to that in BLM. Nevertheless the %extraction had been reduced in both the cases. It however is not a matter of concern as the final recovery in the strip phase is the factor which is of prime importance in case of three phase LM operation.
0 10 20 30 40 50 60 70 80
0 10 20 30 40 50 60 70 80
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045
% Recovery of Chromium
% Extraction of Chromium
Strip phase concentration (M)
% Extraction
% Recovery
Figure 3.13: Efficiency of extraction and recovery of Cr(VI) in three phase SLM based separation process at various concentrations of strippant in strip phase; solvent = sunflower oil, concentration of extractant = 1% (vol/vol), stirring speed = 120 rpm, run time = 48 h.
3.2. Results and discussion 69
30 40 50 60 70 80 90
30 40 50 60 70 80 90
3 4 5 6 7 8 9 10
% Recovery of Chromium
% Extraction of Chromium
Strip phase pH
% Extraction
% Recovery
Figure 3.14: Efficiency of extraction and recovery of Cr(VI) in three phase SLM based separation process at various pH in strip phase; solvent = sunflower oil, strip phase conc. = 0.03 M, concentration of extractant = 1% (vol/vol), stirring speed = 120 rpm, run time = 48 h.
3.2.3.2 Effect of speed of stirring
Unlike in BLM the stirring speed could not be maintained at 400 rpm in the strip phase of SLM because higher turbulence disturbs the stability of SLM whereby the organic phase oozes out of the pores of PVDF membrane. The experiments were thus carried out with stirring speed in range of 60-200 rpm in both the compartments.
The results are shown in Figure 3.15. As expected, the increase in stirring speed from 60 to 120 rpm manifest in higher %extraction and %recovery of Cr(VI) and reached over 80% extraction and over 70% recovery of Cr(VI), much higher recovery than in BLM. The speed of stirring was set at 120 rpm for all further experimentations.
0 10 20 30 40 50 60 70 80 90
0 10 20 30 40 50 60 70 80 90
0 50 100 150 200 250
% Recovery of Chromium
% Extraction of Chromium
RPM
% Extraction
% Recovery
Figure 3.15: Efficiency of extraction and recovery of Cr(VI) in three phase SLM based separation process at various speed of stirring in strip phase; solvent = sunflower oil, strip phase conc. = 0.03 M, strip phase pH = 6.5, concentration of extractant = 1%
(vol/vol), run time = 48 h.
3.2.3.3 Effect of period of operation
As the agitation (i.e. speed of stirring) in SLM operation is lower than that in BLM one requires higher period of operation in SLM if one desires to achieve the same level of performance. The experiment in SLM was thus performed for a duration of 78 h. The results are shown in Figure 3.16. Maximum extraction and recovery of Cr(VI) can be seen in between 48 h and 68 h respectively. The percent extraction and recovery reached over 80% and 70% respectively. Interestingly it is also observed that %extraction and %recovery show declining trend after 68 h of operation. This is due to the fact that SLM loses its stability after prolonged period of operation whereby the organic phase oozes out of the pores of PVDF membrane. This yields a direct channelling between two aqueous phases of feed and strip sides. It is also noticed that %extraction and %recovery of Cr(VI) do not significantly increase after
3.2. Results and discussion 71
48 h. Hence the run time of 48 h was selected for rest of the study.
0 10 20 30 40 50 60 70 80 90
0 10 20 30 40 50 60 70 80 90
0 10 20 30 40 50 60 70 80
% Recovery of Chromium
% Extraction of Chromium
Time (h)
% Extraction
% Recovery
Figure 3.16: Efficiency of extraction and recovery of Cr(VI) in three phase SLM based separation process at various periods of operation; solvent = sunflower oil, strip phase conc. = 0.03 M, strip phase pH = 6.5, concentration of extractant = 1%
(vol/vol), speed of stirring = 120 rpm.
3.2.3.4 Effect of concentration of carrier
The effect of concentration of aliquat 336 in organic phase on extraction and recovery were studied at concentration in the range of 0-3%. The results are presented in Figure 3.17. The trend of profile is similar to that observed in three phase BLM operation and the reasons are already stated in Section 3.2.2.5. However, the extent of recovery is nearly twice (71.54%) of that observed in BLM operation (35.33%) due to thinner membrane size.
0 10 20 30 40 50 60 70 80 90
0 10 20 30 40 50 60 70 80 90
0 0.5 1 1.5 2 2.5 3 3.5
% Recovery of Chromium
% Extraction of Chromium
Carrier concentration (% volume)
% Extraction
% Recovery
Figure 3.17: Efficiency of extraction and recovery of Cr(VI) in three phase SLM based separation process at variousconcentrations of aliquat 336; solvent = sunflower oil, strip phase conc. = 0.03 M, strip phase pH = 6.5, period ofoperation = 48 h, speed of stirring = 120 rpm.
3.2.3.5 Fed batch system
The fed batch runs were carried out in SLM in the same fashion as they were carried out in BLM. The results are shown in Figure 3.18. The trends of profile are similar in both BLM and SLM operations, however, the concentration of Cr(VI) in strip phase reaches 90.68 mg L−1 in case of SLM operation within 72 h period as compared to mere 43.3 mg L−1 in case of BLM operation in the same time duration. Unlike in BLM, it didn’t reach any saturation point either.
3.2. Results and discussion 73
0 20 40 60 80 100
0 20 40 60 80 100
0 10 20 30 40 50 60 70 80
Chromium concentration in strip phase (mg L-1) Chromium concentration in feed phase (mg L-1)
Time (h)
Cr(VI) in feed Cr(VI) in strip
Figure 3.18: Efficiency of extraction and recovery of Cr(VI) in three phase SLM based separation process during fed-batch operation; solvent = sunflower oil, strip phase conc. = 0.03 M, strip phase pH = 6.5, concentration of extractant = 1% (vol/vol), speed of stirring = 120 rpm.