Advanced Powder Technology 28 (2017) 122–130

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Advanced Powder Technology

journal homepage: www.elsevier.com/locate/apt

Original Research Paper

A novel green synthesis of zero valent iron nanoparticles (NZVI) using

three plant extracts and their efficient application for removal of Cr(VI)
from aqueous solutions
Mehdi Fazlzadeh a, Kourosh Rahmani a, Ahmad Zarei b, Hossein Abdoallahzadeh a, Fakhraddin Nasiri c,
Rasoul Khosravi d,⇑
a
Department of Environmental Health Engineering, School of Health, Ardabil University of Medical Sciences, Ardabil, Iran
b
Department of Environmental Health Engineering, School of Public Health, Gonabad University of Medical Sciences, Gonabad, Iran
c
Coordination and Monitoring Municipal Utilities Ardabil, Ardabil, Iran
d
Department of Environmental Health Engineering, School of Health, Shahid Sadoughi University of Medical Sciences, Yazd, Iran

a r t i c l e i n f o a b s t r a c t

Article history: In the present study, NZVI particles were synthesized from the plant extracts including Rosa damascene
Received 20 April 2016 (RD), Thymus vulgaris (TV), and Urtica dioica (UD). The FTIR arspectshowed that polyphenols, proteins
Received in revised form 18 September and organic acids which serve as reducing and stabilizing agents play a significant role in the synthesis
2016
of NPs and reduce the possibility of aggregation of NPs compared to chemical techniques of NPs synthe-
Accepted 20 September 2016
Available online 19 October 2016
sis. The amount and type of compounds in plant extracts affect the structure and also agglomeration of
NPs after adsorption process. Based on the results, the highest removal efficiency occurred at pH 2. With
increase in contact time and amount of dose, the percentage removal increases. Inversely, increase of ini-
Keywords:
Green synthesis
tial concentration of Cr(VI) decreases the removal efficiency of the contaminant. These nanoparticles have
Nanoparticles a high adsorption capacity. Accordingly, by applying a dose of 0.2 g/l and contact time of 10 min, the three
Cr(VI) NPs yielded >90% removal efficiency. Also, for 1 min contact time, the percentage removal was 94.87%,
Adsorption 83.48% and 86.8% for RD-Fe, UD-Fe and TV-Fe, respectively. By an increase to 25 min, the removal per-
centage reached to 100% for TV-Fe and UD-Fe. Moreover, 30 min was required to remove Cr(VI) com-
pletely by RD-F.
Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder
Technology Japan. All rights reserved.

1. Introduction have been used such as chemical precipitation, ion exchange,

reduction, membrane separation, reverse osmosis and adsorption.
Nowadays, the contamination of water, soil, and air with toxic However, most of these methods have been impeded by some
chemicals has become a high priority environmental problem. drawbacks, involving incomplete metal removal, expensive equip-
Rapid industrialization, extensive use of pesticides in agriculture ment, high maintenance cost, significant energy consumption,
and environment pollution with different chemicals has resulted toxic sludge generation, etc. [6]. Literature review shows that
in serious concerns [1–3]. Chromium as heavy metal is commonly adsorption process appears to be a reasonable and promising
found in huge quantities in effluent of different industries such as option for the removal of chromium from aqueous solutions. For
electroplating, mining, painting and dying, tannery, and fertilizer this reason, different types of natural and synthetic adsorbents
[4]. Chromium has a potential to bio-accumulate inside the human such as low cost wastes of fertilizer industry [7], granular activated
body and greatly affect the health of individuals. When chromium carbon [8], Boehmite [9], Pomace [10], activated carbon and other
level reaches 0.1 mg/g body weight, it can ultimately become low cost adsorbents [11], activated carbon derived from
lethal [5]. Therefore, proper treatment of chromium containing agricultural wastes [6], activated carbon of biological wastes and
effluents is normally required before discharge into environment. others [12–14] have been used. In recent years, zero valent iron
Various methods of Cr(VI) removal from industrial wastewater nanoparticles (NZVI) have attracted a great deal of attention due
to their efficiency in removal of different types of contaminants
from aqueous solutions [1]. Several physical and chemical
⇑ Corresponding author. Fax: +98 4533512004.
techniques [15] including sodium borohydride (NaBH4), ethylene
E-mail address: khosravi.r89@gmail.com (R. Khosravi).

http://dx.doi.org/10.1016/j.apt.2016.09.003
0921-8831/Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
 M. Fazlzadeh et al. / Advanced Powder Technology 28 (2017) 122–130 123

glycol, carbothermal synthesis, citric acid and chitosan have been 1 M NaOH. At this time, a black colored precipitate was appeared
employed for the preparation of NZVI particles [16]. A rapid agglom- which show the formation of Rosa damascene, Thymus vulgaris,
eration of nanoparticles due to van der Waals and electrostatic and Urtica dioica zero valent iron nanoparticles [17,18,20–22].
forces normally occurs which in general decreases their surface area The formed nanoparticles were separated by evaporation on a
to volume ratio. In order to provide more inter-particle electrostatic hot plate surface and collected by washing several times with
repulsions, and to overcome the obstacle of aggregation, organic deionized water and placed in nitrogen gas to avoid oxidation.
surfactants or capping agents can be used [15,16]. Green synthesis,
as an alternative and environmentally friendly technique for the 2.3. Sorption experiments
synthesis of NPs, has been attracting increasing attention in recent
years [16,17]. It has been reported that many plant extracts could Batch adsorption experiments were carried out in 250 ml Erlen-
be an alternative to existing chemical and physical techniques for meyer flasks inside an incubator container. The contents of all
the synthesis of nanoparticles [17]. Till date, several plants extracts Erlenmeyer flasks were mixed thoroughly using magnetic stirrers
including green tea, oolong tea, and black tea [18], grape marc, with a fixed setting to achieve a constant speed. For each test, at
black tea, and vine leaf extract [19], green tea and eucalyptus leaf first 40 ml of samples with determined concentration was added
[20] have been used for the synthesis of nanoparticles. Hence, plant into the Erlenmeyer flask. For adjusting the pH range, HCl or NaOH
extracts were shown to be nontoxic to living organisms, renewable 1 N were used. After that, a determined dose of NZVI was added
and being environmentally friendly and ecologically acceptable, into the Erlenmeyer flask and mixed immediately by shaker. As
when compared to other techniques of nanoparticles synthesis. the desired time passed, the samples were filtered through Whatt-
Polyphenol/caffeine content of the plant extracts were identified man No. 42 filter paper. The filtrates were analyzed for residual Cr
to be responsible for the stabilization of NZVI and serve as the (VI) concentration. The main process parameters considered were
reducing and capping agent [17]. The literature studies showed that pH (2, 4, 6, 7, 9), initial Cr(VI) concentration (10, 25, 50, 100, 200,
synthesized nanoparticles from different plants extracts are differ- 300 mg/l), NZVI dose (0.2, 0.4, 0.6, 0.8, 1 g/l), and contact time (1,
ent in shape, size and characteristics which change their ability in 5, 10, 15, 20, 25, 30 min). For the effectiveness and accuracy in
the removal of pollutants from aqueous solutions. For example, results, all the adsorption experiments were conducted at least in
green tea, oolang tea and black tea extracts have been used for the duplicate and average values were reported.
removal of Malachite green [21]. So this study aims at green synthe- The amount of Cr(VI) adsorbed onto the NZVI, qe (mg g1) was
sis of highly effective NZVI nanoparticles from native plants to Iran calculated as follows:
for the removal of pollutants from aqueous solution. The present
ðC 0  C t Þ  V
work explores the utility of NZVI nanoparticles, prepared using qe ¼ ð1Þ
m
three plants namely, Rosa damascene (RD), Thymus vulgaris (TV),
and Urtica dioica (UD) in regime of decontamination of aqueous where C0 (mg/l) and Ct (mg/l) are the initial and equilibrium con-
solution containing hexavalent chromium. This study also critically centrations of Cr(VI), respectively. qe (mg/g) is the amount of Cr
analyzes the effectiveness of the three synthesized nanoparticles (VI) adsorbed, M (g) is the mass of the NZVI and V(l) is the volume
and their comparative adsorption capacityies in the removal of of the liquid phase. Also, the removal percentage (R%) of chromium
chromium. A systematic characterization of NZVI was performed was calculated for each run by using Eq. (2):
using SEM and FTIR studies.  
Ct
R ð%Þ ¼ 1   100 ð2Þ
C0
2. Material and methods
where C0 and Ct were the initial and final concentration of chro-
mium in the solution, respectively.
2.1. Reagents and chemicals
2.4. Determination of residual Cr(VI) concentration
All the primary chemicals used in this study were of analytical
grade and were obtained from Merck, Germany. All solutions were
A scanning electron microscope (SEM) equipped with an energy
made with double distilled water. The pHs of the solutions were
dispersive X-ray microanalysis (LEO-1430 VP) was used to deter-
adjusted with hydrochloric acid and sodium hydroxide (0.1 N). A
mine the surface morphology of the NZVI. The surface functional
stock solution of 1000 mg/l Cr(VI) was prepared by dissolving
groups of NZVI were determined using Fourier Transform Infrared
2.8286 g K2Cr2O7 in 1000 ml double distilled water. Required con-
spectroscopy (FTIR-KBr) at wave numbers ranging from 400 to
centrations of Cr(VI) standards were prepared by appropriate dilu-
4000 cm1. To explore the structure of the NPs, X-ray diffraction
tion of the above stock Cr(VI) standard solution.
(XRD) patterns of synthesized NZVI before adsorption were per-
formed using a Philips X’Pert Pro instrument (Netherlands) within
2.2. Green synthesis of NZVI from Rosa damascene, Thymus vulgaris, the 2h range of 10–90°. The X-ray source was radioactive Cu Ka
and Urtica dioica extracts (k = 154 nm). A colorimetric method was used to analyze the Cr
(VI) concentration of the samples. Cr(VI) was measured at a wave-
In the beginning, dried leaves of Rosa damascene, Thymus vul- length of 540 nm using a double beam spectrophotometer (Model
garis, and Urtica dioica was purchased from market and then lambda 25- Perkin Elmer Company) according to method pre-
washed several times with double deionized water to remove sented in Standard Methods for the Examination of Water and
any dust and dried at room temperature. After that, the extracts Wastewater. Statistical analyses were performed using Excel
of Rosa damascene, Thymus vulgaris, and Urtica dioica were pre- software.
pared by boiling 60 g/l of the leaves of these plants at 80 °C for
1 h. After precipitation for 1 h, the extracts were filtered by a vac- 2.5. Determination of the zero point charge
uum pump. Then, a 0.1 M FeCl2
4H2O solution was prepared by
adding 19.9 g of solid FeCl2
4H2O (or 27.56 g of H2SO4
7H2O) into The zero point charge was determined using 0.01 M NaCl as an
1 liter of deionized water. 0.1 M FeCl2
4H2O solution was then electrolyte by adding 0.1 N NaOH or 0.1 N HCl solutions. To do the
added into 60 g/l Rosa damascene, Thymus vulgaris, and Urtica dioica test, 50 ml of the electrolyte was introduced into 8 beakers and
extracts in the ratio 2:3. The pH value was adjusted to 6 by adding then the pH was adjusted to the required value in the range
124 M. Fazlzadeh et al. / Advanced Powder Technology 28 (2017) 122–130

2–12. 0.1 g portion of nanoparticles was then added into each bea- electron which attributes to the free radical scavenging potential of
ker and shaken for 48 h. After 48 h agitation, the adsorbent was fil- the plant extracts. Moreover, it is found that the antiradical activity
tered and the final pH of the filtrate was measured. By plotting the of any molecule depends on the abundance of phenolic OH groups,
initial pH versus the pH after 48 h, the zero point charge of and there is a positive correlation between the antioxidant activity
nanoparticles was determined. and the number of the phenolic OH groups [21]. On the other hand,
increasing the number of the phenolic hydroxyl groups can
enhance the anti-oxidant capacity. The second strong peak in
3. Results and discussion region E in the range of 1126–1190 cm1, corresponds to carbonyl
group which shows heterocyclic compounds resulting from pro-
3.1. Characterization of the synthesized NPs teins of plant extracts. These compounds produce capping ligands
in nanoparticles. Furthermore, the existing proteins inhibit
FTIR spectra analysis (400–4000 cm1) was carried out in order agglomeration of NPs and stabilize NPs by forming a capping layer
to characterize NPs and determine the presence of surface func- onto NPs [23]. Accordingly, the FTIR spectra of NPs exhibited
tional groups in the extracts of Rosa damascene, Thymus vulgaris, another prominent peak in region C between 1628 cm1 and
and Urtica dioica. The spectra for synthesized nanoparticles before 1640 cm1. This peak corresponds to resonance absorption of the
adsorption are shown in Fig. 1. In this figure, the most distinguish- C@C in alkene groups belonging to the family of no-saturated
able peaks are shown for the NPs which are coded separately. Fur- hydrocarbon compounds [17]. Another strong peak is observed in
thermore, the functional groups corresponding to each region is region G within the range of 615–617 cm1, indicating aromatic
identified which is given in Table 1. As can be seen from Fig. 1, compounds of alkanes. Other peaks can also be seen in regions F
all synthesized nanoparticles have relatively same peaks. In range and D, which all correspond to organic, aromatic compounds, and
between wave number 3400 and 3430 cm1, the highest peak was also derivations of these compounds including polyphenols, alco-
appeared in region A which corresponds to polyphenols. But it is hol or terpenoids, proteins and organic acids in plant extracts
generally found that the presence of phenolic compounds can [23–25]. According to Fig. 1, the intensity of peaks corresponding
reduce Fe2+ to Fe0 [1,17,23]. This indicates the existence of stronger to TV-Fe is stronger than other synthesized NPs in almost all peaks,
functional groups on the three NPs. while for RD-Fe NPs the relationship is opposite. This shows the
These peaks show the prominent phenolic functional groups in difference between stabilizing and reducing agents in the extracts
FTIR analysis. Abundant phenolic groups provide a favorable of different plants. The intensity of RD-Fe peak in region E is less
molecular structure for the effective delocalization of the unpaired than other NPs due to the presence of NPs which inhibit aggrega-

100 TV UD RD

90
D

80 B F

70
T%

60
A G

50

40 C

30
4000 3600 3200 2800 2400 2000 1600 1200 800 400
Wavenumbers (cm-1)
Fig. 1. FTIR spectra of synthesized NPs from plants extracts, Rosa damascene (RD), Thymus vulgaris (TV), and Urtica dioica (UD) before adsorption.

Table 1
Prominent peaks and related functional groups.

Region code TV-Fe peaks (cm1) UD-Fe peaks (cm1) RD-Fe peaks (cm1) Functional groups Compounds indicated
A 3400 3415 3430 AOH stretch Polyphenols
B 2940 2940 2940 CAH stretch Alkanes
C 1630 1628 1640 C@C stretch Alkenes
D – 1387 1365 CH3 bend Alkanes
1430 – 1430 CAC stretch In rings
Aromatics
E 1126 1130 1190 CACoAC stretchig Carbonyl compounds
F 822 822 830 CAH bending Aromatics
– – 770 CAH bending Aromatics
G 615 615 617 CAH bending Alkanes (acetylen)
 M. Fazlzadeh et al. / Advanced Powder Technology 28 (2017) 122–130 125

(a) 100 TV-Fe before

TV-Fe after 492
90 822

80

70
3885 1268 507
T% 60 1435
2360 2330 615
2940
50 1435
1630
40
2930 1630 1050
1137
30 1126
3400 3290

20
4000 3600 3200 2800 2400 2000 1600 1200 800 400
-1
Wavenumbers (cm )

(b) 100 UD-Fe before

UD-Fe after 492
822

80 507
670
2940
1387
615
T%

60
2865 1426

1628
2935
40
341 1120
1145
1655 1130
3390

20
3900 3400 2900 2400 1900 1400 900 400
Wavenumbers (cm -1 )

(c) 100 RD-Fe before 770

RD-Fe after 830
1430 1365 530

80
615 550

617
T%

2940 1428
60 1640

2935 1180
1640
40 3430
1190
3360

20
4000 3600 3200 2800 2400 2000 1600 1200 800 400
-1
Wavenumbers (cm )
Fig. 2. FTIR spectra before and after adsorption for TV-Fe (a), UD-Fe (b), and RD-Fe (c) at wave numbers from 400 to 4000 cm1.

tion. Therefore, it can be stated that the stability of the RD-Fe NPs tion of Cr(VI) by NPs. Moreover, the obvious changes in the inten-
against aggregation is more than two other NPs. Fig. 2 displays FTIR sity of the peaks show the important role of the functional groups
spectra of synthesized NPs. Fig. 2a–c shows FTIR spectra before and in the adsorption of Cr(VI). After adsorption, the peaks of phenolic
after adsorption for TV-Fe, UD-Fe, and RD-Fe nanoparticles, respec- groups for all three NPs became broader and more changes in
tively. It can be clearly seen from the figure that the peaks have wavenumbers occurred in these regions. These changes were more
changed after adsorption process. This change is due to the adsorp- significant for TV-Fe than RD-Fe, indicating more activity of the
126 M. Fazlzadeh et al. / Advanced Powder Technology 28 (2017) 122–130

phenolic groups in TV-Fe compared to RD-Fe, therefore, it can be in functional groups is relatively higher than other two NPs which
stated that TV-Fe is more efficient in chromium adsorption than may contribute to more aggregation during adsorption process for
RD-Fe. Moreover, the intensity of peaks had a little change for three RD-Fe NPs.
NPs, indicating the important role of polyphenol groups in the pre- According to BET analysis, it is evident that specific surface area
vention of reaction [26]. From Fig. 2, it was observed that the inten- of NPs are relatively small and about 1.63, 2.42 and 1.42 m/g, for
sity of peaks in region E (shown in Fig. 1), which inhibits TV-Fe, UD-Fe and RD-Fe, respectively. The analysis also showed
aggregation of NPs, has been declined after adsorption for all that total pore volume for these NPs was 4.52  102,
NPs. By decreasing the number of functional groups serving as 2.97  102 and 2.08  102 cm3/g, respectively. The results
aggregation inhibitor, more aggregation of NPs occurs. Fig. 2 shows demonstrated that pore volume is effective in Cr(VI) removal by
FTIR spectra of Rosa damascene. As is evident, the rate of decrement NPs. Accordingly, pore volume of TV-Fe was twice as that of RD-Fe.

Fig. 3. SEM images of synthesized NPs, TV-Fe NPs (a), TV-Fe NPs (b) after adsorption, UD-Fe NPs (c), UD-Fe NPs (d) after adsorption, RD-Fe NPs (e) and RD-Fe NPs (f) after
adsorption.
 M. Fazlzadeh et al. / Advanced Powder Technology 28 (2017) 122–130 127

Fig. 3 shows the SEM images of these three nanoparticles before bonded onto NPs during synthesis of plant extracts. The presence
and after adsorption. This image represents that plants extracts of the acidic compounds enhances the resistance of NPs against
have been applied successfully for the synthesis of nanoparticles. pH increase. Among these, the extract of Rosa damascene had lower
The average size of synthesized NPs was 100 nm. The synthesized amounts of acids. The rise in adsorption efficiency at low pH values
NPs were non uniform and exhibit different shapes and void space. can be attributed to the existence of various forms of hexavalent
Fig. 3e and f displays synthesized NPs of Rosa damascene extracts chromium in solution such as H2CrO4, HCrO 2 2
4 , CrO4 , Cr2O7 , which
before and after adsorption. Before adsorption, as can be seen obvi- the solubility is strongly pH dependent. Dominant form of chro-
ously, the synthesized NPs were completely dispersed which can mium for adsorption at pH value <6.4 is HCrO 4 which decreases
significantly increase the absorption rate. But in other NPs this uni- with any increase in pH value [27]. HCrO 2
4 and Cr2O7 are domi-
formity in dispersion is lower which generally reduce the rate of nant forms in solution at optimum pH. HCrO 4 results from the
adsorption. But, as can be seen in Fig. 3f, a large amount of RD-Fe hydrolysis of dichromate ions and Cr2O2 7 formation is results of
NPs is agglomerated resulting in a loss of effectiveness in adsorp- HCrO4 , that these interactions are dependent on pH fluctuations
tion and extends the reaction time. This agglomeration tendency [28,29]. The similar results were obtained by other studies using
of TV-Fe NPs can also be seen in FTIR analysis. From the above dis- Ocimum americanum L. seed pods [30] and low cost bio-char [31].
cussion, it can be expected that RD-Fe NPs had a higher adsorption 
rate in comparison to TV-Fe and UD-Fe NPs. But the adsorption rate Cr2 O2
7 þ H2 O ! 2HCrO4 ð3Þ
occurs in shorter time which decreases significantly with time.
The XRD patterns of the three nanoparticles are presented in HCrO4 ! CrO2
4 þH
þ
ð4Þ
Fig. 4. There are some prominent peaks in XRD which can perfectly
index crystalline Fe. The reflections in the diagram were identified HCrO4 þ 7Hþ þ 3e ! Cr3þ þ 4H2 O ð5Þ
to belong to iron oxide (Fe3O4 and Fe2O3), zero valent iron (Fe (0)),
organic matter and sodium chloride (NaCl). Presence of organic þ 
CrO2
4 þ 8H þ 3e ! Cr
3þ
þ 4H2 O ð6Þ
matter indicates the important role of plant extract in nano synthe-
sis which can be determined by FTIR analysis. In these three
þ 
nanoparticles it can be seen Fe (0) that approved the synthesis of Cr2 O2
7 þ 14H þ 6e ! 2Cr
3þ
þ 7H2 O ð7Þ
zero valent iron with three plant extract.
3.3. Effect of dose
3.2. Effect of initial pH
The effect of adsorbent dose on the removal of hexavalent chro-
Fig. 5 displays the results of initial pH influence. It can be mium is shown in Fig. 6. From the figure it can be shown that the
observed from the figure that the maximum removal efficiency removal efficiency of hexavalent chromium increases with increas-
was found to be at pH = 2. With increase in pH value, the percent- ing the adsorbent dose. Accordingly, at adsorbent dosage of 0.2 g/l,
age removal decreases for these three adsorbents. The decrease is the removal efficiencies of TV-Fe, UD-Fe and RD-Fe were 93.2%,
more prominent for RD-Fe NPs. Accordingly, by increasing the pH 92.55% and 90.74%, respectively. Again for 1 g/l, the removal
value from 2 to 9, the removal efficiency decreased from 91.75% reached 97.5%, 97.1% and 96.94%, respectively. From these results
to 60.95%, receptively. But the rate is less for other synthesized it can be stated that the removal percentage increases slightly with
NPs herein. This shows that different compounds in various plant dose and never reach 100%. This represents that the availability of
extracts have different potential with pH variation. The decrement surface sorption area is not alone the only reason for enhanced
in removal efficiencies for the three NPs with pH increase is as: RD- adsorption but also interaction during adsorption process are
Fe > UD-Fe > TV-Fe. This also shows that more organic acids were important. The rise in adsorption efficiency with dose increase is

1: FeOOH
TV UD RD
2: Organic matter
1 3 3: Fe3O4
4 4: Fe2O3
2 5 3 5: Fe (0)
1 6 6: NaCl

1
1
2 3 4 6
5 3

1
2 3 4 6
5 3

20 30 40 50 60 70 80
2 (Degree)
Fig. 4. XRD pattern of the three green nanoparticles.
128 M. Fazlzadeh et al. / Advanced Powder Technology 28 (2017) 122–130

100 NPs. By increasing the initial concentration of Cr(VI), the number

of collisions with NPs increases which consequently enhance the
Removal efficiency %

90 rate aggregation and provides more surface area available for Cr

(VI) adsorption. As is evident from the results, for all concentration
80 of Cr(VI), a removal efficiency above 90% was obtained at 10 min.

70 TV-Fe 3.5. Effect of contact time

UD-Fe
60 Fig. 8 shows the effect of contact time on the removal of hex-
RD-Fe
avalent chromium by the synthesized NPs. As can be seen from this
50 figure, with increase in contact time, the percentage removal
0 2 4 6 8 10 increases. Accordingly, for 1 min contact time, the removal per-
Initial pH centages of chromium were 94.87%, 83.48% and 86.8% for RD-Fe,
UD-Fe and TV-Fe, respectively. By an increase to 25 min, the
Fig. 5. Effect of initial pH on Cr(VI) adsorption using NZVI at an initial Cr removal percentage reached to 100% for TV-Fe and UD-Fe. A
concentration of 100 mg/l, NPs dose of 0.2 g/l, contact time of 10 min and at mixing 30 min was needed to remove Cr(VI) completely by RD-F. The rea-
speed of 200 rpm.
son for the higher removal efficiency of Cr(VI) with time is that the
adsorbate molecules have enough time to be captured by the
adsorbents [27]. In other studies of chromium removal, Vikrant
in agreement with the results of Veng et al. in 2013 [17]. Moreover,
and Pant [32] by adsorption onto eucalyptus bark, Sharma et al.
adsorption capacity decreases with dose increase mainly due to the
[33] by adsorption onto PTPS fly ash and Pillay et al. [34] by
unsaturation of adsorption sites through adsorption process. The
adsorption onto multi-walled carbon nanotubes, also reported
adsorption capacity of Cr(VI) is significantly high for the three
increase in removal efficiency with increase in contact time. The
adsorbents. Accordingly, at a dose 0.2 g/l, the adsorption capacities
change in the removal efficiencies of the used NPs might be due
for TV-Fe, UD-Fe and RD-Fe were 466, 462 and 453.7 mg/g, respec-
to different structures of produced NPs resulted from these plants
tively. More interesting is the high removal percentage above 90%
which affecting the capping and reducing capability. Therefore, the
in this dose which shows the high potential of these nanoparticles
active and functional groups of each NPs is different. The results
in the removal of Cr(VI). To the best of our knowledge, this is the
showed that only the increase in contact time can enhance the
first report of NPs synthesis having high adsorption capacities.
removal efficiency to nearby 100%. Therefore, optimum pH was

3.4. Effect of initial concentration of chromium

100
According to Fig. 7, the removal efficiency decreases with
Removal efficiency %

increase in initial concentration of Cr(VI). Accordingly, removal

efficiencies of Cr(VI) were 99.5%, 100% and 97% for TV-Fe, UD-Fe 95
and RD-Fe, respectively at initial concentration of 10 mg/l. Also at
initial concentration of 300 mg/l, the removal efficiencies declined
90
to 94.48%, 93.36% and 91.29%, respectively. Levankumar et al. in TV-Fe
2009 conducted a study on Ocimum americanum L. seed pods and
UD-Fe
showed that by increasing the concentration from 100 to 85
150 mg/l and then 200 mg/l, the removal efficiency increases RD-Fe

[30]. This result is consistent with the results reported by Gupta 80

et al. who used fertilizer production wastes for chromium removal 0 50 100 150 200 250 300 350
[7]. As it is seen from the figures, the removal efficiency of Cr(VI) Initial Cr(VI) Concentration mg/L
decreases slightly by increase at initial concentration of the con-
taminant indicating high adsorption capacity of the synthesized Fig. 7. Effect of initial Cr concentration on Cr(VI) adsorption by NZVI.

550 100

450
Removal efficiency %

95

TV-Fe qt
350
qt mg/g

UD-Fe qt
90
RD-Fe qt
250 TV-Fe %
UD-Fe %
85
150 RD-Fe %

50 80
0 0.2 0.4 0.6 0.8 1 1.2
ZVNI concentration g/L
Fig. 6. Effect of NPs dosage on of Cr(VI) adsorption onto NZVI at an initial pH of 2, initial Cr concentration of 100 mg/l, contact time of 10 min and at mixing speed of 200 rpm.
 M. Fazlzadeh et al. / Advanced Powder Technology 28 (2017) 122–130 129

the pseudo first order rate constant. The linear form of pseudo-
100 second order equation can be expressed in Eq. (9):
Removal efficiency %

t 1 t
95 ¼ þ ð9Þ
qt K 2 qe2 qe
where K2 (mg/g min) is the rate constant of second order adsorp-
90
TV-Fe tion. The best fit kinetic model was selected by considering the
UD-Fe regression correlation coefficient (R2) which is a measure of how
85 RD-Fe well the predicted values from a forecast model match with the
experimental data. The studies on the adsorption of Cr(VI) onto
80 TV-Fe, UD-Fe, RD-Fe NPs revealed that the adsorption process
0 5 10 15 20 25 30 35 obeyed the pseudo-second order kinetic model. The R2 values
Time (min) obtained from the models were 0.9981, 0.9956 and 0.9953 for TV-
Fe, UD-Fe, RD-Fe NPs, respectively which all are more than R2 values
Fig. 8. Effect of contact time on Cr(VI) adsorption by NZVI. of the pseudo first order model (Fig. 9).

3.7. Reduction of hexavalent chromium to trivalent chromium

(a) 8 y = -0.0673x + 6.0542

R² = 0.9882 TV-Fe An atomic adsorption spectrophotometry (Model Spectr AA
y = -0.0889x + 6.1725
220-Varian Company) was used in order to determine the chro-
6 R² = 0.9815 UD-Fe mium concentration in the solution. The experiments on the
y = -0.0805x + 6.0935 adsorption of Cr(VI) by the NPs were carried out under optimum
ln (qe -qt)

R² = 0.988 RD-Fe conditions. The obtained results were the same as the results mea-
4
sured by spectrophotometer. Based on the results, it can be stated
that the removal of Cr(VI) was done by adsorption not by
2 reduction.

3.8. Removal mechanisms of Cr(VI)

0
0 10 20 30 40 50 60 70 80
Herein, three possible mechanisms can be attributed to Cr(VI)
t (min) removal by the green NPs:

(b) 0.3 1. Considering small sizes of iron NPs, Cr(VI) ions adsorption
y = 0.0019x + 0.0122 TV-Fe
R² = 0.9981 occurred in monolayer behavior from the solution and by sedi-
y = 0.0018x + 0.0115 mentation and separation of the NPs, Cr(VI) can be removed.
UD-Fe
0.2 R² = 0.9956 Based on BET analysis, it is evident that the surface area of
the NPs is not significant, therefore it can be stated that this
y = 0.0018x + 0.012 RD-Fe
t/qt

R² = 0.9953 removal mechanism is in lower rank compared to other possi-

ble mechanisms.
0.1
2. The pores on the NPs could have a considerable role in the
removal of Cr(VI) ions. The analysis showed that the pore vol-
ume of TV-Fe is twice as that in RD-Fe. Therefore, it can be
0 ascertained that higher pore volume in TV-Fe is responsible
0 20 40 60 80 100 120 140 for more Cr(VI) removal by the adsorbent compared to RD-Fe.
t (min) 3. As the synthesized NPs are prone to agglomeration, these
agglomerates can provide an ideal place for Cr(VI) ions removal
Fig. 9. Kinetic models of (a) pseudo-first order model and (b) pseudo-second order available in the solution with increasing initial Cr(VI) concen-
model for adsorption Cr(VI) onto TV-Fe, UD-Fe and RD-Fe NPs.
tration. The agglomerates also cause that Cr(VI) ions attach in
multilayer behavior to the spaces between NPs and longer time
be available for NPs penetration into smaller pores and more
found to be 2 at a dose of 0.2 g/l. For TV-Fe and UD-Fe, a contact internal adsorption sites. The mechanism is more predominant
time of 25 min was required in a concentration of 100 mg/l Cr in higher Cr(VI) concentrations.
(VI). But RD-F required a longer contact time, 30 min (see Fig. 8).
4. Conclusions

3.6. Adsorption kinetics Synthesis of nanoparticles by use of green plants including Rosa
damascene, Thymus vulgaris, and Urtica dioica is an easy and eco-
Adsorption kinetics herein describe the rate of adsorption of Cr friendly method which produces nanoparticles in size of 100 nm.
(VI) onto TV-Fe, UD-Fe, RD-Fe NPs. The experimental data obtained The SEM images show that synthesized NPs are irregular in shape
in this study were analyzed using the pseudo-first and pseudo- and their tendency to aggregation change after adsorption. FTIR
second order kinetic models. The pseudo-first order equation in analysis revealed the existence of functional polyphenols and car-
linear form can be expressed in Eq. (8): boxylic groups and also proteins and organic acids which in gen-
lnðqe  qtÞ ¼ ln qe  K 1 t ð8Þ eral minimize aggregation of NPs and serve as reducing agents.
The highest peaks are attributed to polyphenols which show strong
where qe (mg/g) is amount of Cr(VI) adsorbed at equilibrium, qt reducing capacity of used plant extracts. The results of the present
(mg/g) is amount of Cr(VI) adsorbed at time t and K1(1/min) is study show that the synthesized NPs have the ability to adsorb
130 M. Fazlzadeh et al. / Advanced Powder Technology 28 (2017) 122–130

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