Environmental Engineering Reference
In-Depth Information
9.3.3.1 Surface-Coating Method
Feroxyhyte is a poorly ordered form of -FeOOH with weak magnetism and
comparatively amorphous at temperatures lower than 100 o C (Schwertmann and Cornell,
2000). Poorly crystalline -FeOOH transforms directly to hematite under a vacuum or in
air at 150 o C (Mackay, 1960). Since the amorphous material is easy to be coated on the
surface of nanoparticles, herein -FeOOH was considered as the coating material for
enhancement of heavy metal adsorption. Specifically, 2 g of dry -Fe 2 O 3 and 5 g of
ferric salt were added into 300 mL of deoxygenated ultrapure water under mechanical
stirring. The solution pH was decreased to around 3.2, and then 5 M of NaOH was added
gradually into the mixture till pH 8.0. Green precipitate was formed and 40 mL of H 2 O 2
(30%) was added rapidly into the solution. The green precipitate gradually became
reddish brown, and the pH dropped to around 3.0 owing to the release of proton. With
stirring, 5 M NaOH was added dropwisely into the mixture till pH 8.0. The -FeOOH-
coated -Fe 2 O 3 nanoparticles were formed and separated from the solution via an
external magnetic field and rinsed three times to remove unwanted ions. To ensure that
the -FeOOH is stably and tightly coated on the surface of the -Fe 2 O 3 , the above
material was heated in an oven at 80 o C for 24 h for aging. Finally, the -FeOOH-coated
-Fe 2 O 3 nanoparticles with mass ratio of -FeOOH to -Fe 2 O 3 equal to 1 were collected.
To synthesize the -FeOOH-coated -Fe 2 O 3 nanoparticles with ratios ranging from 0 to
2, the -Fe 2 O 3 nucleus was fixed at a mass of 2 g and the coated -FeOOH was changed
from 0 to 3 g through changing the mass of the chemical reagents during the
synthesizing process.
9.3.3.2 Metal-Doping Method
In this study, five common metals such as Al(III), Cu(II), Ni(II), Zn(II) and
Mg(II) were considered as the doping materials on the -Fe 2 O 3 nanoparticles.
Substitution of a large range of cations can be readily induced in Fe 3 O 4 and -Fe 2 O 3
because tetrahedral as well as octahedral positions are available. A common laboratory
method to synthesize metal-doped oxides is to add base to the mixed Me-Fe salt
solutions to precipitate metal-associated iron oxide (Edelstein and Cammarata, 1996).
First, 200 mL of deoxygenated ultrapure water was prepared with N 2 gas bubbling for 30
minutes. For synthesizing pure Fe 3 O 4 , varying amounts of FeCl 2 and FeCl 3 salts with a
molar ratio of 1:2 were needed. However, for producing divalent metal-doped Fe 2 O 4 ,
part of FeCl 2 was replaced by MeCl 2 (Me = Cu, Mg, Zn, Ni). Thus, MeCl 2 , FeCl 2 and
FeCl 3 salts with a molar ratio of 1:3:6 were dissolved. Under vigorous mechanical
stirring, 1.0 M NaOH was added by drops into the above solution. Black precipitate was
gradually formed when pH reached 10. The precipitate was isolated and rinsed twice to
remove any unwanted chemicals. The chemical reaction is expressed as follows:
0.33MeCl 2 + FeCl 2 + 2FeCl 3 + 8NaOH FeMe 0.33 Fe 2 O 4 (s) + 4H 2 O (Eq. 9.5)
 
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