Iron-Based Magnetic Nanoparticles for Removal of Heavy Metals from Electroplating and Metal-Finishing Wastewater - Nanotechnologies for Water Environment Applications - page 244

100

0.5 mM Cr + 0.5 mM anion
0.5 mM Cr + 1 mM anion

90

80

70

60

50

40

30

20

10

0

Cr alone K3PO4 K2SO4 KF KCN KNO3 KCl

Figure 9.18 Adsorption efficiency of Cr(VI) with competition of various anions.

In comparison with the above monovalent anions, F - shows the stronger
inhibition for the Cr(VI) adsorption onto -Fe 2 O 3 nanoparticles. Metal (hydr)oxides
adsorb F - ions on different sites. At low and intermediate concentrations, F - is attracted
onto the protonated surface or exchanged against OH(H) of singly coordinated surface
groups. At high F concentrations, doubly coordinated OH groups are also involved. The
F - ions replace surface hydroxyls according to the following reactions:

FeOH 2 + + F -1 = FeOH 2 + - F -1 (Eq. 9.7)
FeOH + H + + F -1 = FeF + H 2 O (Eq. 9.8)

The adsorption of F - may enhance the dissolution of metal (hydr)oxides
(Kraemer et al., 1998). It has been proposed that this is due to the increase of the ligand
exchange rate upon the presence of F - in the coordination sphere of the metal ion. Recent
Nuclear magnetic resonance (NMR) spectroscopy points to F - binding at singly and at
doubly coordinated surface sites, which affects the dissolution mechanism (Blesa et al.,
1994). At lower pH, the increased binding amount of F - at a higher concentration is
probably due to the precipitation of F - described by the following equation (Elrashidi
and Cammarata, 1996):

Fe 3+ + 3F - FeF 3 (Eq. 9.9)

The presence of PO 4 3- causes a significant decrease in Cr(VI) removal efficiency
(Figure 9.18). With an increase in the PO 4 3- concentration, the influence becomes more