Environmental Engineering Reference
In-Depth Information
38.3 Preparation and Stability of Pure Al
13
Through a
SO
2−
-Ba
2+
method (Xu et al., 2003), a series of puriied Al
13
solutions can be
prepared as shown in Table 38.1. It can be seen that the samples prepared at concentra-
tions of 0.01-2.0 mol Al/L contain mainly Al
b
species (based on the ferron reactivity
as shown in the ferron assay, the species can be divided into three classes: the rapid
reactive Al
a
, the moderate reactive Al
b
, and the inert species Al
c
), i.e., >97%. No Al
c
frac-
tion exists signiicantly as the ferron method is analytically erroneous around 1%. It
seems also that the Al
a
fraction could be contributed from the surface Al of Al
13
, i.e., as
the result of rapid dissolution of the surface Al by ferron. It needs to be noted that the
speciation characterization by the ferron method is operationally deined, i.e., Al
a
is the
fraction reacted suddenly in 1 min. Some researchers deined Al
a
as the fraction reacted
within 30 s (Bertsch, 1996). Therefore, the Al
a
calculated here is signiicantly higher.
The samples obtained therefore contain mainly Al
13
species. It indicates that the Al
13
samples prepared can be diluted into various concentrations. High-concentration Al
13
solutions are relatively stable and therefore become the valid proof for a commercial
preparation. Characterization using
27
Al-nuclear magnetic resonance (NMR) analysis
exhibits only a sharp single response at 63.0 ppm (Wang et al., 2011). No peak of Al
m
(monomer) at 0 ppm or other species can be observed for all the samples. The peak area
at 63.0 ppm increases rapidly with the increase of concentration. A very good linear
equation with an
R
2
of 0.9996 can be attained, i.e.,
y
= 4.085
x
+ 0.0231, where
y
is the
peak area and
x
is the concentration. It indicates that the only species observed in the
above samples are Al
13
. The results show that the Al
13
detected by
27
Al-NMR has a good
relation with Al
b
under these conditions (Bi et al., 2014).
Al
13
solutions at concentrations between 0.11 and 2.1 mol Al/L were further aged at room
temperature to investigate the effect of concentration on the stability of Al
13
. The results
of the ferron assay are shown in Table 38.2. It seems that the Al
13
solution is in a state of
pseudo-stability and tends to aggregate with aging. Concentration has a signiicant role for
its stability at >0.5 mol Al/L. At a higher concentration of 2.1 mol Al/L, the Al
13
solution
undergoes slow aggregation with aging and becomes turbid after 1 week, and only 87% of
Al
b
remains after 1 month of aging. The 0.1 mol Al/L solution shows quite stable features
and undergoes only a minor change in 1 month of aging. It has also been conirmed by
NMR analysis. It indicates that the decreased Al
13
in the
27
Al-NMR spectrum contributes
partially to those remaining in the Al
b
fraction during aging.
TABLE 38.1
Speciation Distribution of Puriied Al
13
Solutions by Ferron Assay
Al
total
(mol Al/L)
Al
a
%
Al
b
%
Al
c
%
0.01
2.35
97.44
0.20
0.055
2.20
97.77
0.03
0.11
2.31
97.33
0.36
0.42
2.06
97.38
0.56
1.06
2.02
96.80
1.18
2.11
1.91
96.59
1.60
