D. Shi et al. / Journal of Alloys and Compounds 782 (2019) 183e192
185
ꢁ
2
0 mL of 10 mg L 1 Cr(VI) solution in a 50 mL conical flask. In each
p-nitrophenol concentrations. The minimum time of reaction was
recorded from the addition of reactants to the vanishing of the
original yellow color of the reaction solution.
of two procedures, the two samples were taken at the same iron
content of 0.050 g L . In addition, in the control experiments, NH
ꢁ1
2
-
SiO (0.01 g) and commercial micro-scale SiO (0.01 g) was also
2
2
used alone in a separate experiment. The experiments were carried
out in a thermostatic shaker bath operating at 20 ± 0.5 C for 5 min,
and the rotation speed was 180 rpm. To monitor the progress of the
reaction, the reaction solution of 2 mL was taken for spectroscopic
analysis. The concentration of Cr(VI) in supernatant was deter-
3. Results and discussion
ꢀ
0
3.1. Nanostructure of Fe @NH
2
-SiO
2
Typical TEM images of the NZVI products were shown in Fig. 1.
mined by using the diphenylcarbohydrazide method [22]. After the
The images at lower magnification (Fig. 1A and B) indicate that the
loading assemblages on NH -SiO nanospheres were spherical as-
2 2
0
removal of Cr(VI) by Fe @NH
2
-SiO
2
and NH
2
-SiO
2
, the solid samples
were washed by deaerated ethanol twice with inert gas protection
and then analyzed by XPS to test the composition of final chromium
products. Furthermore, to evaluate the effect of pH value on the
Cr(VI) removal, the initial pH value of the Cr(VI) solution was
adjusted to 1, 2, 3, 4, 5 and 6, respectively. The experimental pro-
cedures for the removal of Cr(VI) under different pH were the same
as mentioned above. The removal percentage of Cr(VI) was calcu-
semblages and strikingly uniform in morphology with an average
diameter of about 50 nm. Besides, it can be seen from higher
magnification images (Fig. 1C and D) that every spherical assem-
blage was assembled with plentiful of dispersed nanoparticles. The
average diameter of these nanoparticles was measured to be 6.1 nm
by counting more than 100 visual particles. Furthermore, The SAED
pattern presented in Fig. 1D suggests the presence of vague diffuse
lated by the equation: removal (%) ¼ (C
0
ꢁCe)/C
0
ꢂ 100%, where C
0
rings, which can be indexed as (200) and (110) planes of a-Fe phase
from the surface to the inside [24]. There is an absence of lattice
fringe in Fig. 1D and perfect diffuse rings in SAED pattern, sug-
and Ce are the initial and equilibrium concentration of Cr(VI),
respectively. The experimental data were obtained from triplicate
0
samples. The loading amount of Fe on the surface of NH
2
-SiO
2
gesting that the nanoparticles on the surface of NH
spheres are poorly ordered and amorphous because of being
encapsulated in the hydrocarbon chains of the NH -SiO . Careful
2 2
-SiO nano-
nanospheres was analyzed by the 1, 10-phenanthroline colori-
metric method [23].
2
2
examination of Fig. 2 suggests that there is a strong and broad
ꢀ
2
.5. Removal of p-nitrophenol from aqueous solution
diffraction peak at around 2
q
¼ 23 , which can be ascribed to the
amorphous silica component [25,26]. In addition, the peak at
ꢀ
0
The reduction of p-nitrophenol (0.62 mM) was respectively
2q
¼ 44.72 corresponds to the (110) plane of Fe [27], and
0
0
performed using Fe @NH
2
-SiO
NZVI (Fe , 3.46 mM) with the aid of NaBH
environment. In the control experiments, NH
2
(Fe , 3.46 mM) and unsupported
(30 mM) in an oxygen
-SiO was also used
compared with unsupported NZVI, the XRD peak intensity
0
ꢀ
0
4
(2
q
¼ 44.72 ) of Fe @NH
2 2
-SiO obviously decreases, which may be
associated with the disturbance from the increased organic coating
on the surface of metal particles [28].
The spherical assemblages were also verified by the SEM image
(Fig. 3A). The EDX elemental mapping images of Fe @NH
2
2
alone in a separate experiment. The reduction of p-nitrophenol was
probed by UVeVis spectroscopy (T6, Beijing Purkinje General In-
strument Co. Ltd, China) in the range of 250e550 nm and the
change in p-nitrophenol concentration was monitored at 400 nm
wavelength. In order to determine the progress of the reaction,
0
2
-SiO
2
were presented in Fig. 3. On the basis of the Fe-K, O-K and C-K
signals and their distribution area, it can be concluded that the
2
mL of reaction solution was sampled for UVevis spectroscopic
highly disperse and spherical assemblages decorated on the surface
ꢀ
0
analysis. The reaction was conducted at 20 ± 0.5 C. P-nitrophenol
and its final reductive products were determined using a HPLC
system (Waters, e2695, USA) equipped with a UVeVis detector
of NH
2
-SiO
2
nanospheres were Fe nanoparticles.
0
3.2. Synthesis mechanism of Fe @NH
2
-SiO
2
2
489 and a reversed phase column C-18 (250 mm ꢂ 4.6 mm) with
ꢀ
the column temperature of 35 C. The mobile phase was methanol/
As illustrated in Scheme 1, NH
2
-SiO
2
nanospheres were syn-
water with a ratio of 60/40 (v/v), and the flow rate was
thesized by the hydrolysis polymerization of APTMS and TEOS in a
mixture of water and alcohol at 20 C. It is known that in the syn-
ꢁ
1
ꢀ
0
.65 mL min . The injection volume was 20
m
L and the detection
wavelength was set at 280 nm.
thesis of SiO
2
spheres by St o€ ber method, emulsion droplets are first
formed through the hydrogen bonding of the silanylating reagents,
and then hydrolysis polymerization occurs in the inside of droplets
under the catalytic action of ammonia, leading to uniform colloidal
spheres [29]. In this study, the hydrolysis polymerization appears
immediately via co-condensation reaction from the addition of the
silica precursor (TEOS) into APTMS solution, which results in the
generation of amino-group modified silica. Compared with the
above classical St o€ ber method for the synthesis of silica spheres,
0
2
.6. The recycling of the Fe @NH
2
-SiO
2
0
To recycle the Fe @NH
2
-SiO
-SiO was magnetically collected at the
bottom of the conical flask by removing the aqueous solution. Then,
mL of aqueous solution of NaBH and 24 mL of p-nitrophenol
2
nanoparticles with the aid of
0
NaBH
4
, the used Fe @NH
2
2
2
4
solution were successively added into the conical flask for recycling
test. In this way, the contents of Fe (3.46 mM) and p-nitrophenol
0.62 mM) were remained the same at the beginning of every cycle.
To test the effect of NaBH concentration in the reaction solution on
-SiO , the initial concentrations of NaBH
4
0
without the use of ammonia, amino-groups of APTMS not only acts
as a catalyst in the hydrolysis and polycondensation reaction, but
(
4
also makes SiO
2
amino-functionalization easy. It should be noted
0
the recycling of Fe @NH
2
2
that loading mass of amino-group is important, since it relates to
3
þ
were respectively adjusted to 10, 20, 30 and 40 mM while keeping
the number of binding sites for the adsorption of Fe . In amino-
functionalized process of SiO spheres, Guan et al. [30] found that
0
initial concentrations of Fe and p-nitrophenol unchanged. Also, to
2
examine the influence of p-nitrophenol concentration on the
recycling, the initial concentrations of p-nitrophenol were adjusted
to 0.77 and 0.88 mM but keeping other conditions unchanged. The
the resulting loading mass of N (wt%) was 0.23, 0.34, 0.38 and 0.44,
respectively when 0.1, 0.2, 0.3 and 0.4 mL APTMS/mL TOES was
used. Taking into account the change in loading mass of N with
increasing ratio of APTMS to TEOS, the volumetric ratio of 0.3 (mL/
experimental procedures of recycling for p-nitrophenol removal
0
were the same as mentioned above. The same Fe @NH
2
-SiO
4
and
2
mL) was selected for subsequent formation of NH
2 2
-SiO
nanoparticles were recycled for many times at different NaBH
nanospheres.