Iron-Based Magnetic Nanoparticles for Removal of Heavy Metals from Electroplating and Metal-Finishing Wastewater - Nanotechnologies for Water Environment Applications - page 256

and CrO 4 2- occurs. On the other hand, at pH higher than pH pzc , the adsorption surface is
negatively charged, and increasing electrostatic repulsion between negatively charged
Cr(VI) species and negatively charged adsorbent particles results in a release of the
adsorbed HCrO 4 - and CrO 4 2- . When the electrostatics (i.e., nonspecific adsorption)
governs the adsorption, the surface must have an overall positive charge in order to
allow Cr(VI) adsorption to take place. It should be noted that acidic pH is especially
favored for the occurrence of the chemical redox reaction between Cr(VI) and Mn(II)
during the adsorption process, and thus the adsorption of Cr(VI) onto MnFe 2 O 4
decreased more sharply with higher pH compared to the other MeFe 2 O 4 .

100

MnFe2O4
MgFe2O4
ZnFe2O4
CuFe2O4
NiFe2O4
CoFe2O4

90

80

70

60

50

40

30

20

10

0

1 2 3 4 5 6 7 8 9 10

pH

Figure 9.29 Cr(VI) removal by various MeFe 2 O 4 at various pH ((Hu et al., 2007a)

Effect of Shaking Speed. Since an optimum shaking speed is essentially
required to maximize the interactions between metal ions and adsorption sites of the
adsorbent in the solution, the effect of shaking speed on Cr(VI) adsorption was
investigated herein. At pH 2.0, the percentage removal of Cr(VI) from 20 mL of 100
mg/L K 2 CrO 4 solution by 0.1 g magnetic nanoparticles under various shaking speeds are
shown in Figure 9.30. It is found that Cr(VI) removal efficiency gradually increased
with an increase in shaking rate from 100 to 400 rpm and then remained almost constant
with further increasing speed till 800 rpm. This phenomenon is expected because at a
relatively lower shaking speed, the system is incompletely mixed; hence the poor
dispersion of nanoparticles in solution resulted in only a small portion of the surface area
of the adsorbent being exposed and reacted with the Cr(VI) ions. With further increasing
the shaking speed from 400 to 800 rpm, the effect of shaking speed on the Cr(VI)
adsorption became comparatively insignificant since the system was well-mixed under a