Composition and phase control of Ni/NiO nanoparticles for photocatalytic degradation of EDTA

Article abstract of DOI:10.1016/j.jallcom.2010.08.027

A sol-gel process followed by a calcination treatment was employed to synthesize magnetic Ni/NiO nanoparticles. The as-formed particles were characterized using XRD, TEM, UV-vis, surface area and VSM. The XRD results show that the patterns of the product synthesized using 1:1, 1:2, and 1:3 molar ratio of nickel nitrate/citric acid composed of a mixture of NiO and Ni, whereas the patterns recorded for 2:1 and 3:1 molar ratio exhibited a single NiO phase. All the prepared samples are in the form of nanoparticles with sizes locate in the range of 33 up to 118 nm for the samples prepared by different molar ratios. The photocatalytic tests were conducted for degradation of EDTA. The results show that the EDTA removal efficiency is 99.9% at 3 × 10^-3 M EDTA concentration. The reaction of Ni/NiO with EDTA follows first order kinetics with respect to EDTA. The rate constants and t_1/2 for Ni/NiO at 0.4 g/l catalyst/EDTA solution ratio were, respectively 460 min ^-1, 15.1 min.

Full text of DOI:10.1016/j.jallcom.2010.08.027

Journal of Alloys and Compounds 508 (2010) 133–140
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Composition and phase control of Ni/NiO nanoparticles for photocatalytic
degradation of EDTA
F.A. Harraz∗, R.M. Mohamed, A. Shawky, I.A. Ibrahim
Nanostructured Materials and Nanotechnology Division, Central Metallurgical Research and Development Institute, 1 El-Felezate Street, El-Tebbin,
P.O. Box 87, Helwan, Cairo 11421, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
A sol–gel process followed by a calcination treatment was employed to synthesize magnetic Ni/NiO
nanoparticles. The as-formed particles were characterized using XRD, TEM, UV–vis, surface area and VSM.
The XRD results show that the patterns of the product synthesized using 1:1, 1:2, and 1:3 molar ratio of
nickel nitrate/citric acid composed of a mixture of NiO and Ni, whereas the patterns recorded for 2:1 and
3:1 molar ratio exhibited a single NiO phase. All the prepared samples are in the form of nanoparticles
with sizes locate in the range of 33 up to 118 nm for the samples prepared by different molar ratios. The
photocatalytic tests were conducted for degradation of EDTA. The results show that the EDTA removal
efficiency is 99.9% at 3 × 10−3 M EDTA concentration. The reaction of Ni/NiO with EDTA follows first order
kinetics with respect to EDTA. The rate constants and t1/2 for Ni/NiO at 0.4 g/l catalyst/EDTA solution ratio
were, respectively 460 min−1, 15.1 min.
Received 29 March 2010
Received in revised form 5 August 2010
Accepted 6 August 2010
Available online 18 August 2010
Keywords:
Sol–gel technique
Nanoparticles
Ni/NiO
EDTA
© 2010 Elsevier B.V. All rights reserved.
Photodegradation
1. Introduction
are needed. Heterogeneous photocatalysis which is a category of
advanced oxidation method is therefore a promising method by
Ethylenediamminetetraactetic acid (EDTA) is a domestic and
industrial water contaminant. It is widely used as a decontaminat-
ing agent in nuclear industry. The presence of EDTA in radioactive
liquid waste can cause complexation of some of the target pre-
cipitant cations resulting in interference in their removal by
conventional treatment processes such as chemical precipitation
or ion exchange [1,2]. EDTA is not easily biodegradable [3], rarely
degradable by chlorine [4], hardly retained by activated carbon
fibres and resistant to ozone treatment [5,6]. Accordingly, it is a
crucial step to perform a pretreatment step for the removal of EDTA
for a better treatment of the liquid wastes.
Recently, advanced oxidation method based on photocataly-
sis has been employed successfully for degradation of organic
pollutants [7–10]. For example, several works dedicated to the
photocatalytic degradation of reactive brillian red using boron-
doped titania [11], catalytic decomposition of methylene blue on
doped-TiO2 nanoparticles [12,13], photocatalytic activity of nanos-
tructured ZnO films for decolorization of malachite green [14]
are recently reported. The main advantage of applying such a
technique is its capability to convert the organic pollutants into
nontoxic species such as CO2 and H2O. Another big advantage
for this approach is the fact that no further separation processes
which a semiconductor catalyst under UV irradiation is able to
mineralize EDTA, for instant. In heterogeneous photocatalysis, OH•
radicals, already generated by UV light, are the essential species
in degradation of EDTA. This means, using this method, chemical
oxidants can be released in water as the result of catalyst irra-
diation. In reality, redox processes are likely taking place at the
catalyst/solution interface as long as the semiconductor is irra-
diated by a wavelength higher enough than the band gap of the
catalyst. Absorption of photons generates electron–hole pairs on
the catalyst surface which in turn can reduce or oxidize the organic
materials present in aqueous solutions. Reaction of holes with
water can lead to the formation of oxidizing radicals which can
attack EDTA and transform it to CO2 and H2O species.
Most of previous reports devoted to remove EDTA from water
using heterogeneous catalysis have been conducted using TiO2
semiconductor catalyst irradiated with wide range of spectral
lamps [15–21]. This means, to date there are challenges for
developing the controlling parameters of the heterogeneous pho-
tocatalysis, including developing new types of catalyst, deep
investigation on catalyst operation and its performance evalua-
tion and other relevant aspects as well. This in turn should enable
advances in such emerging area of catalysis.
On the other hand, NiO has recently received increased attention
due to its applications in a wide range of fields, including cataly-
sis [22], electrochromic films [23–26], fuel cell electrodes [27,28],
gas sensors [29,30], and smart windows [31–33]. Various meth-
∗
Corresponding author. Tel.: +20 2 25010 640, fax: +20 2 25010 639.
E-mail addresses: fharraz@cmrdi.sci.eg, fharraz68@yahoo.com (F.A. Harraz).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.08.027

134
F.A. Harraz et al. / Journal of Alloys and Compounds 508 (2010) 133–140
Table 1
ods have been known for synthesizing NiO including calcinations
of nickel compound in air at elevated temperature 1273 K, physical
and chemical vapor deposition and sol–gel process [24,34,35].
For example, Jiao et al. have recently reported a dip coat-
ing process combined with sol–gel method to fabricate NiO films
[24]. Wang et al. have synthesized NiO nanorods by a thermal
decomposition technique [36]. NiO nanowires with a polycrys-
talline nature have also been synthesized by an electrochemical
deposition approach [37], while single cubic crystal NiO nanowires
have been fabricated by means of a wet chemical method [38] and
by an oxidation route in a molten salt [39]. Importantly, some other
authors have reported and used NiO as a photocatalyst for degra-
dation of phenol [40]. Actually, the band gap of NiO is higher than
TiO2, but the degradation process is a bit higher than TiO2. The
main reason for the increase in degradation efficiency in case of
NiO is the release of lattice oxygen by which super-oxide radi-
cals are formed which greatly enhance degradation of phenol [40].
Furthermore, it was reported that NiO formed on AgNbO3 surface
and Ag-doped NiTiO3 could enhance the photocatalytic decompo-
sition of methylene blue [41,42]. As a recent report, a photocatalyst
LaNiO3 with perovskite structure was used successfully for methyl
orange degradation [43].
However, a definite understanding of the limits of NiO forma-
tion and its phase and morphology control is still lacking. To better
understand this, we report a simple sol–gel method followed by a
calcination process to synthesize Ni/NiO nanoparticles. A detailed
investigation was conducted in which the nanostructure phase and
morphology were correlated to a systematic variation in operating
parameters, with focusing to control the phase formation, and eval-
uate the applicability of the as-formed catalyst for the degradation
of EDTA.
Relation between nickel nitrate/citric acid molar ratios and the percentage of the
formed phases, band gap, and photocatalytic degradation of EDTA.
Nickel nitrate/
citric acid
molar ratio
% of formed
phase
Direct band
gap (eV)
Photocatalytic
degradation of
EDTA (%)
NiO
Ni
1:1
2:1
3:1
1:2
1:3
46
100
100
70
54
0
3.53
3.44
3.42
3.47
3.60
63.7
88.8
94.5
85.8
48.9
0
30
82
18
3. Results and discussion
3.1. Effect of nickel nitrate/citric acid molar ratio
A series of experiments has been carried out using different
nickel nitrate/citric acid molar ratio, mainly 2:1, 1:1, 1:3, 3:1 and
1:2. The as-formed products were characterized using XRD, surface
area, TEM, and UV–vis. The photocatalytic tests were consequently
performed for degradation of EDTA.
3.2. X-ray diffraction
Fig. 1 displays the XRD patterns of the samples prepared using
different nickel nitrate/citric molar ratios as 1:1, 1:2, 2:1, 1:3 and
3:1. The patterns of the product synthesized at 1:1, 1:2, and 1:3
molar ratio indicate that a mixture of NiO and Ni existed, whereas
the patterns of the product synthesized at 2:1 and 3:1 molar ratio
reveals a single NiO phase. From the patterns, it can be seen that
three distinct diffraction peaks at 2ꢀ = 37◦, 43◦ and 63◦ are observed
for nanosized NiO samples. These peak positions correspond well
with the three most intense peaks (1 1 1), (2 0 0), (2 2 0) of pure
NiO. Weaker intensity peaks detected at 2ꢀ = 75◦ and 79◦ can be
indexed, respectively to (3 1 1) and (2 2 2) crystal plans of NiO. The
observed diffraction peaks at 2ꢀ = 44◦, 52◦, and 76◦ are assigned to
(1 1 1), (2 0 0) and (2 2 0) of Ni, respectively. It is worth to note that
the peak positions are not shifted, but the shapes are broader than
the standard peaks, which can be essentially related to the nano-
size of as-synthesized particles. Table 1 shows the effect of nickel
nitrate/citric acid molar ratio on the percentage and the crystallite
size of the as-formed phases. The results show that increasing the
nickel nitrate/citric acid molar ratio in the order of 1:1, 2:1 and 3:1,
led to increasing the percentage of NiO in the formed phase from
46, 100 and 100, respectively. However, the above trend of increas-
ing nickel nitrate/citric acid molar ratio was found to decrease the
crystallite size (calculated by Scherer equation [45]) from 118, 94.3
and 75.6 nm, respectively. On the other hand, by decreasing the
nickel nitrate/citric acid molar ratio in the order of 3:1, 1:2 and 1:3,
one can observe a decrease of NiO% in the formed phases from 100,
70 and 18%, respectively.
2. Experimental details
2.1. Materials preparation and characterization
The sol–gel method was employed as follows: 5 g Ni(NO3)2·6H2O and x g citric
acid (x = 1.8, 3.61, 5, 6.9 and 9.025 g) were used to prepare different molar ratios
from Ni(NO3)2·6H2O:citric acid, mainly 2:1, 1:1, 1:3, 3:1 and 1:2. The precursor
was dissolved in 100 ml pure alcohol until a clear mixed solution is formed; and
then the solution was kept at 120 ◦C in an oven until the alcohol entirely evap-
orated and a green paste was obtained. After the sol–gel process, the paste was
placed into a tube furnace, and calcined at 750 ◦C for 5 h in air. The products were
then characterized using XRD, TEM, UV–vis, surface area and VSM. The magnetic
properties of the produced powders were measured, at room temperature, using a
vibrating sample magnetometer (VSM; 9600-1 LDJ, USA) in a maximum applied field
of 15 kOe. From the obtained hysteresis loops, the saturation magnetization (Ms) and
coercivity (Hc) were determined. X-ray diffraction (XRD) patterns of calcined cata-
lysts were measured using Rigaku RINT 2000. A Perkin-Elmer Lambda 800 UV/vis
spectrometer was used to determine the particle band-gap profile. During measure-
ment the samples were dispersed in aqueous systems placed into plastic cuvettes
for measurement. N2-adsorption measurement was carried out at 77 K using Nova
2000 series, Chromatech. Prior to analysis, the samples were outgased overnight at
250 ◦C. The morphology and particle size of the prepared samples were examined
by transmission electron microscopy (TEM; Hitachi H-9500 operated at 300 kV).
2.2. Photocatalytic tests
Citrate ions can play multiple roles in the synthesis process,
including a reducing agent, a stabilizer and a complex agent. Citrate
ions could form a nickel complex with nickel ions in solution. The
formation of Ni(II)–citrate complexes depends essentially on the
molar ratio between Ni(II) and citric acid. Different molar ratios
between nickel ions and citric acid can greatly influence the reac-
tion rate and the final phases as well as the particle size. The
reducing ability of citrate ions is a function of concentration. Our
findings indicated that, as the concentration of citric acid increases
in solution, the amount of Ni metal phase increases. Low concentra-
tion of citrate ions favors the formation of NiO. A complete reaction
mechanism needs further spectrometry and kinetic analysis.
The photoactivity tests were carried out in a cylindrical Pyrex glass reactor con-
taining different catalyst loadings in 250 ml of aqueous solution of 5 × 10−3 M EDTA
at 30 ◦C for 60 min. A 150 W medium pressure Hg lamp (254 nm) immersed within
the photoreactor was used. The EDTA was determined by a comlexometric titration
with Zn2+ standard solution [44]. The removal efficiency of EDTA was calculated by
applying the following equation:
ꢀ
ꢁ
C0 − C
removal efficiency =
× 100%
C0
where C0 is the original EDTA content and C the retained EDTA in solution. Prior to
the photocatalytic tests, fresh solutions of 250 ml containing EDTA were adjusted to
a certain pH, and the catalyst was suspended at 0.4 g/l concentration. Suspensions
were kept in dark and continuously stirred at 30 ◦C for 60 min.

F.A. Harraz et al. / Journal of Alloys and Compounds 508 (2010) 133–140
135
Fig. 1. XRD of the prepared samples obtained by different nickel nitrate/citric acid molar ratios; nickel nitrate/citric acid molar ratio: (A) 1:1, (B) 1:2, (C) 2:1, (D) 3:1 and (E)
1:3.
Fig. 2. TEM images of the prepared samples obtained by different nickel nitrate/citric acid molar ratios; nickel nitrate/citric acid molar ratio: (A) 1:1, (B) 1:2, (C) 2:1, (D) 3:1
and (E) 1:3.

136
F.A. Harraz et al. / Journal of Alloys and Compounds 508 (2010) 133–140
impurities doping the crystalline network and the average crystal
size of the semiconductor. The band-gap values calculated for as-
formed samples are presented in Table 1. The results show a gradual
narrowing of the band gap with increment of nickel nitrate/citric
acid molar ratio, which in turn would increase the absorbency in
the visible light region.
3.2.3. Surface area measurement at different nickel nitrate/citric
acid molar ratios
The N2 adsorption–desorption measurement at a liquid N2 tem-
perature of −196 ◦C was used to study mesoporosity and textural
properties of the synthesized NiO nanoparticles. Fig. 4 depicts
the as-recorded N2 adsorption–desorption isotherm. The isotherm
shows a typical type IV sorption behavior with type H2 hysteresis
loop, representing the mesoporous structure according to the clas-
sification of IUPAC [49]. The apparent step in the adsorption branch
combined with the sharp decline in the desorption branch is an
obvious indication of mesoporosity. An abrupt increase in adsorp-
tion volume of adsorbed N2 was observed and located in the P/P0
value greater than 0.6. This sharp increase is generally associated
with the capillary condensation, indicating the good homogene-
ity of the sample and fairly small pore size since the P/P0 position
of the inflection point is related to the pore size. From the N2
adsorption–desorption, TEM and XRD analysis, the as-synthesized
Ni/NiO possesses nanocrystalline mesoporous structure. The tex-
tural properties of the nanocrystalline mesoporous Ni/NiO samples
are given in Table 2. The results show that BET surface area of 123,
107, 61, 31 and 26 m2 g−1 with total pore volume of 0.16, 0.34, 0.93,
0.34 and 0.46 cm3 g−1 for nickel nitrate/citric acid molar ratio equal
3:1, 2:1, 1:2, 1:1 and 1:3, are achieved respectively.
Fig. 3. UV–vis of the prepared samples obtained by different nickel nitrate/citric
acid molar ratios; nickel nitrate/citric acid molar ratio: (A) 1:1, (B) 1:2, (C) 2:1, (D)
3:1 and (E) 1:3.
3.2.1. Transmission electron microscopy (TEM)
Fig. 2 shows the TEM images of the prepared samples using dif-
ferent molar ratios of nickel nitrate/citric acid. The micrographs
show that all the prepared samples are in the form of nanoparti-
cles with sizes in the range of 33–97, 48–80, 28–50, 42–119 and
40–90 nm for samples prepared using 1:1, 1:2, 2:1, 3:1 and 1:3
molar ratios, respectively. In complementary to the above XRD
spectra of as-formed samples, it can be realized that the nanoparti-
cles are formed as a mixture of hexagonal and cubic in case of 1:1,
1:2 and 1:3 molar ratio, whereas they are hexagonal in case of 2:1
and 3:1 molar ratios.
3.2.4. Effect of nickel nitrate/citric acid molar ratio on
photocatalytic degradation of EDTA
A series of experiments has been conducted to study the effect
of nickel nitrate/citric acid molar ratio on EDTA removal efficiency
under the following conditions: 0.4 g/l catalyst/EDTA solution ratio;
5 × 10−3 M EDTA; 1 h reaction time and at pH 3. The results indi-
cate that increasing nickel nitrate/citric acid molar ratio in the
order 1:2, 2:1 and 3:1 leads to increasing EDTA removal efficiency
from 85.8, 88.8 and 94.5%, respectively. This is attributed to an
increase in the % of NiO from 70, 100 and 100% and surface area
from 61, 107 and 123 m2/g, respectively and a decrease in the
band gap in the order of 3.47, 3.44 and 3.42 eV, respectively. How-
ever, by decreasing the nickel nitrate/citric acid molar ratio in the
order 1:1, 1:2, and 1:3, one can find a significant change in EDTA
removal efficiency with the following values: 63.7, 85.8 and 48.9,
respectively. From the photocatalytic point of view, the optimum
condition of nickel nitrate/citric acid molar ratio is 3:1 at which
the prepared catalyst has 123 m2/g BET surface area, 3.42 eV band
gap, 100% NiO and 94.5% EDTA removal efficiency. However, for
the sake of achieving a magnetic photocatalyst, the sample with
nickel nitrate/citric acid molar ratio of 1:2 at which the prepared
catalyst has 61 m2/g BET surface area, 3.47 eV band gap, 70% NiO,
3.2.2. UV–vis
The DR-UV spectra of the samples prepared by different nickel
nitrate/citric acid molar ratios are shown in Fig. 3. Inspection of the
spectra reveals the existence of absorption signals around 365 nm,
which is likely related to NiO. The origin of such absorption is
attributed to intra-3d transition of Ni2+ in the hexagonal struc-
ture of NiO [46,47]. For the samples prepared using different nickel
nitrate/citric acid molar ratios, one can observe that the absorption
signals would gradually shift to shorter wavelength with moving
from sample (A) to sample (E). An estimate of the optical band gap
is obtained using the following equation:
˛(hꢁ) = A(hꢁ − Eg)m/2
where A is a constant, ˛ is the absorption coefficient, and m equals
1 for a direct transition. The energy intercept of a plot of (˛hꢁ)2
vs. (hꢁ) yields Eg for a direct transition [48]. The band-gap value
usually reported for NiO phase is 3.5 eV [35]. However, these values
are generally influenced by the synthesis method, the existence of
Table 2
Textural, surface and morphology of prepared samples by different molar ratios of nickel nitrate/citric acid.
´
˚
Nickel
nitrate/
citric acid
SBET
(m2/g)
St
Vm
(mL/g)
VP
(mL/g)
Total
r (A)
Sex.
(m2/g)
VP (mL/g)
micro
VP (mL/g)
meso
VP (%)
micro
VP (%)
meso
Smicro
(m2/g)
Smeso
(m2/g)
CBET
(m2/g)
3:1
2: 1
1:2
1:1
1:3
123.1
107.5
61.00
31.00
26.00
74.98
24.49
70.00
40.00
30.00
28.15
24.59
14.00
7.00
0.16
0.34
0.930
0.340
0.460
33.38
193.46
108.71
33.00
31.00
35.00
34.77
39.58
0.003
0.010
0.080
0.09
0.05
0.930
0.320
0.370
0.07
0.29
0.33
5.70
18.00
59.43
16.05
100.00
94.00
82.00
40.57
83.95
0.160
9.00
58.52
13.77
49.00
23.00
17.00
39.95
79.98
25.00
11.00
6.00
79.30
381.00
275.00
443.00
1.50
7.00
SBET: BET surface area; St: surface area derived from Vl−t plots: Smicro: surface area of micropores; Vmicro: pore volume of micropores; Smeso: surface area of micropores; r:
mean pore radius; Sex.: external surface area; VP: total pore volume; Vmeso: pore volume of mesopores.

F.A. Harraz et al. / Journal of Alloys and Compounds 508 (2010) 133–140
137
Fig. 4. N2 adsorption–desorption isotherm of the prepared Ni/NiO nanoparticles prepared by different nickel nitrate/citric acid molar ratios; nickel nitrate/citric acid molar
ratio: (A) 1:1, (B) 1:2, (C) 2:1, (D) 3:1 and (F) 1:3.
30% Ni and 85.8% EDTA removal efficiency is selected for further
investigation.
tallite size of the formed phases. As can be seen, the crystallite size
increases gradually (61.8, 90.4, 95.0 and 101.7 nm) as the calcina-
tion temperature increases, respectively from 550, 650, 750 and
850 ◦C. It is believed that higher temperature enhances the growth
of the formed clusters, which results in faster growth speed.
Fig. 6 shows the TEM images of as-prepared samples that were
subjected to heat treatment at different calcination temperatures,
mainly 550, 650, 750 and 850 ◦C. The images reveal the formation of
nanoparticles with sizes ranging from 15 to 30, 15 to 65, 50 to 70 and
70 to 120 nm for the samples treated at 550, 650, 750 and 850 ◦C,
respectively; note the different scale bars of TEM image D. It can
be realized that the average particle size increases with calcination
temperature. According to the XRD results, it can be realized that
the nanoparticles are a mixture of hexagonal and cubic in case of
550, 650, and 750 ◦C, whereas a hexagonal NiO is achieved in case
of 850 ◦C calcination temperature.
3.3. Effect of calcination temperature
The optimum catalyst prepared using 1:2 nickel nitrate/citric
acid molar ratio is subsequently calcined at different temperatures
ranging from 550 to 850 ◦C. The samples after heat treatment were
characterized using XRD, surface area, TEM. Then, the photocat-
alytic tests were carried out for degradation of EDTA to understand
the catalytic performance of our catalyst before and after calcina-
tion.
3.3.1. Phase, morphology and magnetic properties evaluation
Fig. 5 shows the XRD patterns of calcined samples prepared at
different temperatures, mainly 550, 650, 750 and 850 ◦C. The XRD
patterns of samples treated at 550, 650, and 750 ◦C calcination tem-
peratures revealed a mixture of NiO and Ni phases; however the
pattern related to the sample synthesized at 850 ◦C shows a sin-
gle phase of NiO with a good crystalline state. Table 3 shows the
effect of calcination temperature on the percentage and the crys-
The magnetic properties recorded by the vibrating magne-
tometer at a room temperature for different produced catalysts
thermally treated from 550 to 850 ◦C are listed in Table 4. The
results show that the as-formed catalyst treated at 550 ◦C cal-
cination temperature has high saturation magnetization, high

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F.A. Harraz et al. / Journal of Alloys and Compounds 508 (2010) 133–140
Fig. 5. XRD of the prepared samples obtained by different calcination temperatures, ◦C; calcination temperature: (A) 550 ◦C, (B) 650 ◦C, (C) 750 ◦C, and (D) 850 ◦C.
Fig. 6. TEM images of the prepared samples obtained by different calcination temperatures, ◦C; calcination temperature: : (A) 550 ◦C, (B) 650 ◦C, (C) 750 ◦C, and (D) 850 ◦C.
Table 3
Effect of calcination temperature on the percentage and the crystallite size of the formed phases, photocatalytic degradation of EDTA and the measured surface area.
Calcination temperature (◦C)
% of formed phases
NiO Ni
94.4 5.6
Crystallite size (nm)
Photocatalytic degradation of EDTA (%)
BET (m2/g)
550
650
750
850
61.8
90.4
95.0
92.1
90.5
85.8
80.4
99.5
82.5
61.0
42.0
97.1
70
2.9
30.0
0.0
100
101.7

F.A. Harraz et al. / Journal of Alloys and Compounds 508 (2010) 133–140
139
Table 4
Magnetic properties of the catalyst prepared at different calcination temperatures.
Calcination temperature (◦C)
Magnetic properties
Saturation magnetization, Ms (emu/g)
Remanent magnetization, Mr (emu/g)
Coercivity, Hc (Oe)
Remanence ratio, Mr/Ms
550
650
750
850
45.34
38.64
20.81
–
1.471
0.7726
0.7393
–
6.463
3.972
8.517
–
0.0324
0.0199
0.0355
–
Table 5
coercively and high remnant magnetization. Also, the surface area
was found to decrease from 99.5, 82.5, 61.0 and 42.0 m2/g by
increasing the calcination temperature from 550, 650, 750 and 850◦
C, see Table 3. Therefore the optimum condition for calcination tem-
perature to prepare our magnetic photocatalyst is 550 ◦C at which
99.5 m2/g BET surface area, 45.34 emu/g saturation magnetization,
1.471 emu/g remanent magnetization and 6.463 Oe coercivity were
achieved.
Rate constant of reaction kinetics of EDTA with Ni/NiO sample.
Catalyst/EDTA solution ratio (g/l)
K (×10−4 min−1
)
t1/2 (min)
0.2
0.3
0.4
0.6
190
222
460
579
36.5
31.2
15.1
11.9
that an increase in catalyst/EDTA solution ratio from 0.2 to 0.4 g/l,
leads to an increase in EDTA removal efficiency from 97.1 to 99.9%
respectively, whereas at a catalyst/EDTA solution ratio more than
0.4 g/l, the EDTA removal efficiency almost remains unchanged. The
optimum condition of catalyst/EDTA solution ratio is 0.4 g/l giving
a value of 99.9% EDTA removal efficiency.
3.3.2. Calcined samples and photocatalytic degradation of EDTA
This section is devoted to understand the effect of calcination
temperature on the catalytic performance of the as-formed cata-
lysts for EDTA degradation. A series of experiments was conducted
accordingly under the following conditions: 0.4 g/l catalyst/EDTA
solution ratio; 5 × 10−3 M EDTA; 1 h reaction time and at pH 3. The
results are summarized in Table 3. The results indicate that increas-
ing calcination temperature in the order 550, 650, 750, and 850 ◦C
leads to a decrease in EDTA removal efficiency from 92.1, 90.5, 85.8
and 80.4%, respectively. This can be attributed to a decrease in BET
values from 99.5, 82.5, 61.0 and 42.0 m2/g, respectively. Based on
this result, the most appropriate calcination temperature is 550 ◦C
at which the prepared catalyst has 99.5 m2/g BET surface area,
3.44 eV band gap, 94.4% NiO and 92.1% EDTA removal efficiency.
3.4.4. Kinetics of EDTA with Ni/NiO catalyst
The reaction order with respect to EDTA was determined by
plotting the reaction time vs. log[EDTA] according to the following
equation for various ratios of catalyst/EDTA solution.
Log[C]t = −kt + Log[C]0
0.693
k
t1/2
=
3.4. Parameters of photocatalytic process
where [C]0 and [C]t represent the concentration of the substance
in solution at zero time and t time of illumination respectively,
and k and t1/2 represent the apparent rate constant (min−1) and
half life time of the reaction (min), respectively. The findings are
illustrated in Fig. 8 and the apparent rate constants and t1/2 are sum-
marized in Table 5. The results show that the reaction follows first
order kinetics with respect to EDTA and the rate constants ranged
from 190 × 10−4 to 579 × 10−4 min−1. In addition, t1/2 was found to
decrease from 36.5 to 11.9 min by increasing catalyst/EDTA solu-
tion ratio from 2 × 10−4 to 6 × 10−4. The first order rate equation
for EDTA is given by: R = k[EDTA].
3.4.1. Effect of pH
A series of experiments has been carried out to study the effect
of pH on EDTA removal efficiency under the following conditions:
0.4 g/l catalyst/EDTA solution ratio; 5 × 10−3 M EDTA and 1 h reac-
tion time. The obtained results indicate that increasing the pH of
EDTA solution from 3 to 7 leads to an increase in EDTA removal effi-
ciency from 92.1 to 97.1%, however by exceeding the pH 7, the EDTA
removal efficiency almost remains unchanged. The possible reason
for this behavior is that alkaline pH value favors the formation of
more OH radical due to the presence of large quantity of OH− ions
in the alkaline medium, which enhances the photocatalytic degra-
dation of EDTA significantly [50]. The optimum condition for pH is
7 at which a value of 97.1% photodegradation of EDTA is achieved.
3.4.2. Effect of EDTA concentration
The effect of EDTA concentration on EDTA removal efficiency
operating under the above conditions at pH 7 was investigated.
The results indicate that an increase in EDTA concentration from
5 × 10−5 to 5 × 10−3 M has no significant effect on EDTA removal
efficiency, while at a concentration more than 5 × 10−3 M, the EDTA
removal efficiency was significantly decreased. Values of EDTA
removal efficiency of 97.1, 80.0, and 50.0% are respectively obtained
at EDTA concentrations of 5 × 10−3, 7.5 × 10−3 and 5 × 10−2 M. The
optimum condition for EDTA concentration is 5 × 10−3 M at EDTA
removal efficiency of 97.1%.
3.4.3. Effect of catalyst/EDTA solution ratio
Fig. 7 depicts the results representing the effect of catalyst/EDTA
solution ratio on EDTA removal efficiency under aforementioned
conditions at EDTA concentration 5 × 10−3 M. The results indicate
Fig. 7. Effect of catalyst concentration, g/l on EDTA removal efficiency.

140
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4. Conclusions
A simple sol–gel approach was employed successfully to synthe-
size magnetic nanoparticles of Ni/NiO with controlled composition
and phase. The as-formed catalyst with and without heat treatment
was tested for EDTA degradation in an aqueous solution. Based on
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