Environmental Engineering Reference
In-Depth Information
nanoparticles reached equilibrium in the shortest time as compared to the other MeFe 2 O 4 ,
which may be due to the rapid redox reaction between the Cr species and the external
adsorbent surface. The second strongest magnetic particles, CoFe 2 O 4 showed the longest
adsorption time for reaching equilibrium, since longer contact time may be needed to
separate the aggregated nanoparticles and then mix themselves with the Cr(VI) anions.
On the other hand, when correlating the BET surface area data from Table 9.7
with the adsorption time listed above, it is not difficult to find that the higher the surface
area, the shorter the adsorption equilibrium time. Since the available active sites for
nanoparticles are mostly present outside of the surface, higher surface area means more
adsorption sites for Cr(VI). For a fixed number of adsorbate Cr(VI) anions and
nanoparticles with a relatively higher surface area, the ratio of the initial number of
Cr(VI) to the adsorption sites of the adsorbent becomes lower. Therefore, most of the
Cr(VI) anions can be adsorbed onto the exposed active sites faster, thereby shortening
equilibrium time. It is also noticed that MnFe 2 O 4 and NiFe 2 O 4 nanoparticles surpass our
previous studied nanoparticles such as Fe 3 O 4 and -Fe 2 O 3 on the adsorption equilibrium
time. By comparison, the Cr(VI) adsorption efficiency of MnFe 2 O 4 is much higher than
those of Fe 3 O 4 and -Fe 2 O 3 nanoparticles (Hu et al., 2004; Hu et al., 2005a).
Table 9.7 Magnetization, metal to Fe ratios and BET surface area of MeFe 2 O 4
Me:Fe 1
Saturation
Moment (emu/g)
BET Surface
Area (m 2 /g)
MeFe 2 O 4
pH zpc
CoFe 2 O 4
3.7
7.9
1: 2.0
55.1
MgFe 2 O 4
1.1
8.3
1: 2.5
70.3
ZnFe 2 O 4
1.2
8.2
1: 2.4
79.6
NiFe 2 O 4
2.2
8.0
1: 2.1
101.2
CuFe 2 O 4
3.2
8.5
1: 2.2
93.8
MnFe 2 O 4
4.6
6.8
1: 2.1
204.0
1 Me: Fe ratio is based on the findings from XPS
Effect of pH. Knowledge of the optimum pH is very important since pH affects
not only the surface charge of adsorption, but also the degree of ionization and
speciation of an adsorbate during the adsorption process. To examine the effect of pH on
the Cr(VI) adsorption efficiency, the solution pH was varied from 2.0 to 9.5. Figure 9.29
shows that the Cr(VI) removal by various MeFe 2 O 4 nanoparticles decreased sharply as
the pH increased. The effect of increasing pH on anion adsorption can be explained as
follows. The surface charge is neutral at pH pzc , which is 7.08.0 for all these MeFe 2 O 4 .
When pH is below the pH pzc , all of the adsorbent MeFe 2 O 4 surface is positively charged
(MeOH 2+ and MeOH groups), and the adsorption of anionic Cr(VI) existing as HCrO 4 -
 
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