Iron-Based Magnetic Nanoparticles for Removal of Heavy Metals from Electroplating and Metal-Finishing Wastewater - Nanotechnologies for Water Environment Applications - page 240

Shaking Speed. At pH 2.5, removal of Cr(VI) from 20 mL of 100 mg/L Cr(VI)
solution by 0.1 g -Fe 2 O 3 nanoparticles at a shaking speed of 50, 100 and 200 rpm was
found to be 72.2%, 78.8% and 80.2%, respectively (Figure 9.16). The Cr(VI) removal
rate increased with an increase in shaking speed from 50 to 200 rpm but thereafter
remained unchanged. The results indicated that the rate of Cr(VI) removal was
controlled by the degree of shaking. The effect of increasing the shaking speed is to
decrease the boundary layer, and hence, the film resistance to mass transfer surrounding
the adsorbent particles. Consequently, at a mixing rate of 50, 100, 200 rpm the
respective equilibrium time was 60, 30, 15 minutes, elucidating a reduction in the
boundary layer surrounding the particles with increasing shaking speed from 50 to 200
rpm. Since the system was well-mixed under a higher shaking speed, the mass transfer
effect became insignificant. It is interesting to note that the percentage removal of Cr(VI)
at thermodynamic equilibrium increased with the increase of shaking speed from 50 to
200 rpm and was kept identical with a further increase in the mixing rate. It can be
explained by the fact that, at a relatively lower mixing rate, the system was incompletely
agitated, and hence, the poor dispersion of nanoparticles in the solution allowed only
part of the surface area of the adsorbents being exposed to Cr(VI) ions for their
adsorption. Thus, a good contact between the particle and fluid must be ensured under a
shaking speed equal to or higher than 200 rpm.

100

90

80

70

60

50

50rpm 100rpm

40

200rpm 300rpm

30

400rpm 500rpm

20

600rpm 700rpm

10

0

0 5 10 15 20 25 30 35 40 45 50 55 60 65

Time (min)

Figure 9.16 Effect of shaking speed on the removal of Cr(VI).

9.5.2.3 Effect of Aqueous Composition

Effect of pH. Because hydrogen and hydroxide ions often interact with
adsorbents commonly used in aqueous phase applications, the adsorption of other ions