Anal. Methods Environ. Chem. J. 5 (1) (2022) 22-35
Research Article, Issue 1
Analytical Methods in Environmental Chemistry Journal
Journal home page: www.amecj.com/ir
AMECJ
A new analytical method based on Co-Mo nanoparticles
supported by carbon nanotubes for removal of mercury vapor
from the air by the amalgamation of solid-phase air removal
Danial Soleymani-ghoozhdi a, Rouhollah Parvari a, Yunes Jahani b, Morteza Mehdipour-Raboury a
and Ali Faghihi-Zarandi a, *
a Department of Occupational Health and Safety at Work, Kerman University of Medical Sciences, Kerman, Iran
b Department of Biostatistics and Epidemiology, School of Public Health, Kerman University of Medical Sciences, Kerman, Iran
ABSTRACT
Heavy metals are a major cause of environmental pollution, and
mercury is a well-known toxicant that is extremely harmful to the
environment and human health. In this study, new carbon nanotubes
coated with cobalt and molybdenum nanoparticles (Co-Mo/MWCNT)
were used for Hg0 removal from the air by the amalgamation of solid-
phase air removal method (ASPAR). In the bench-scale setup, the
mercury vapor in air composition was produced by the mercury vapor
generation system (HgGS) and restored in a polyethylene airbag (5
Li). In optimized conditions, the mercury vapor in the airbag passed
through Co-Mo/MWCNT and was absorbed on it. Then, the mercury
was completely desorbed from Co-Mo/MWCNT by increasing
temperature up to 220 °C and online determined by cold vapor atomic
absorption spectrometry (CV-AAS). The recovery and capacity of Co-
Mo/MWCNT were obtained at 98% and 191.3 mg g-1, respectively.
The Repeatability of the method was 32 times. The mercury vapors
absorbed on Co-Mo/MWCNT adsorbent could be maintained at
7 days at the refrigerator temperature. The Co-Mo/MWCNT as a
sorbent has many advantages such as; high capacity, renewable, good
repeatability and chemical adsorption (amalgamation) of mercury
removal from the air. The method was successfully validated by a
mercury preconcentrator analyzer (MCA) and spiking of real samples.
Keywords:
Mercury removal,
Air,
Adsorption,
Cobalt and molybdenum nanoparticles,
Multiwalled Carbon nanotube,
Amalgamation solid-phase air removal
ARTICLE INFO:
Received 5 Dec 2021
Revised form 10 Feb 2022
Accepted 26 Feb 2022
Available online 28 Mar 2022
*Corresponding Author: Ali Faghihi-Zarandi
Email: Alifaghihi60@yahoo.com
https://doi.org/10.24200/amecj.v5.i01.163
------------------------
1. Introduction
Heavy metals are a major cause of environmental
pollution, and mercury (Hg) is a well-known
toxicant that is extremely harmful to the
environment and human health because of its
persistence, bioaccumulation, and neurological
toxicity [1, 2]. Hg can affect many organs and
cause a variety of symptoms in the body, although it
targets the nervous system, it may also have serious
toxicological effects on the kidney. In addition to the
nervous and kidney system, other systems such as
the cardiovascular system can also be damaged by
exposure to mercury [3, 4]. Mercury has been used
in various products and processes due to its unique
properties. It is utilized in industrial processes that
produce chlorine, sodium hydroxide (Chlor-alkali
plants), the vinyl chloride monomer for polyvinyl
chloride (PVC) production, and polyurethane
elastomers. Mercury is also released from coal-
red power plants and cement production [5, 6].
Therefore, Hg emissions have attracted worldwide
attention. Minamata Convention on mercury, which
23
Co-Mo@MWCNTs for removal of mercury from air Danial Soleymani-ghoozhdi et al
aims is to protect human health and the environment
from anthropogenic emissions and releases of
mercury and mercury compounds, entered into
force on 16 August 2017 [7, 8]. Recently, the
different methods have been introduced for the
sampling and analysis of mercury. NIOSH 6009
and OSHA 140 are the recommended methods
for the sampling of mercury. In these methods,
sample preparation depends on the applied nitric
acid and hydrochloric acid which can be hazardous
to the environment and human health [9, 10].
Emissions from different sources, mercury release
in different forms, including elemental mercury
(Hg0), oxidized mercury (Hg2+), and particulate
bond mercury (Hgp) [11, 12]. Among of various
states of mercury, Hg0 is difcult to remove due to
its stability, long persistence time, high volatility
and insolubility in water [13, 14]. Therefore,
effective Hg0 control technologies are immediately
needed. Several control technologies for Hg0,
including catalytic oxidation [15], photocatalytic
oxidation [16], photochemical removal [17], wet
oxidation [18], and adsorption method [19] have
been developed. Among the various Hg0 removal
methods, the adsorption technique has been widely
studied because of its simplicity, economical,
and good removing efciency [20, 21]. In recent
years, novel carbon-based materials, such as
graphene and graphene oxide, carbon nanotubes
and nanobers, carbon spheres, and metal-organic
frameworks, have been applied for Hg0 removal.
Carbon nanotubes (CNTs) are one type of one-
dimensional nanomaterials, which have been used
for Hg0 removal from water and air due to their
unique physicochemical properties. Carbon-based
materials Because of their large surface area,
exible surface chemistry, and variety diversity, are
the most widely studied adsorbents for Hg0 removal
from ue gases and air [21–23]. Because of its high
removal efciency, the activated carbon (AC) based
adsorption process is considered one of the most
effective technologies for mercury removal, but high
operation costs and adsorbent loss have impeded its
further development [22, 23]. Therefore, developing
more cost-effective carbon-based sorbents for Hg0
removal has signicance [21]. In recent years,
novel carbon-based materials, such as bio-chars
[24], graphene and graphene oxide [25, 26], carbon
nanotubes and nanobers [27, 28], metal-organic
frameworks [29], have been applied for Hg0 removal
by analytical methods. Carbon nanotubes (CNTs) are
one type of one-dimensional nanomaterials which
have been used for Hg0 removal from water and
air due to their unique physicochemical properties
[30-32]. Also, to improve the performance of Hg0
adsorption, some modication methods have been
studied which mainly improve the surface pore
structure of adsorbents and/or increase the active
sites on the surface of adsorbents [33]. Metal or
metal oxide loaded on the surface of CNTs and
other carbon-based materials were a type of catalyst
with both high adsorption and catalytic capability.
Consequently, these types of catalysts can be an
effective material for Hg0 removal from the air.
Shen et al. reported that the surface area (BET) of
activated carbon (AC) was decreased after loading of
Mn or Co on AC, but the on the other hand, the metal
oxide functionalized on the AC surface can promote
Hg0 catalytic oxidation [34]. Ma et al used the
analytical method based on Fe-Ce decorated multi-
walled carbon nanotube (MWCNT) for removal of
Hg0 from ue gas. The results showed that Fe-Ce/
MWCNT had good Hg0 removal performance [32].
Liu et al Suggested the adsorption of Co/TiO2 for
Hg0. The results showed that the high oxidation
activities for Hg0 was obtained by this catalyst [35].
Molybdenum (Mo) is commonly added as a promoter
to vanadium-based catalysts in Hg0 oxidation, but its
catalytic oxidation activity is poor [36].
In this work, Hg0 was removed from the air by using
Co-Mo/MWCNTs. Brunauer−Emmett−Teller (BET)
analysis, X-ray diffraction (XRD), scanning electron
microscopy (SEM) and transmission electron
microscopy (TEM) were employed to analyze
the characteristics of the samples. Experimental
parameters affecting the Hg0 removal process from
the air such as temperature and ow rate were
investigated and optimized. Also, comparisons
between the proposed method and previous methods
were obtained.
24 Anal. Methods Environ. Chem. J. 5 (1) (2022) 22-35
2. Experimental
2.1. Materials and Chemical reagents
Mercury standard was used in the mercury vapor
generation system (HgGS). It was prepared by
dilution of 1 ppm (1000 mg L-1) Hg (II) standard
solution (CAS Number.: 7487-94-7) which was
purchased from Fluka, Germany. Deionized water
(DW) was prepared by water purication system
from RIPI. The stannous chloride (SnCl2, CAS
Number: 7772-99-8) and the NaBH4 (CAs Number:
6940-66-2) analytical grade were purchased from
Merck and Sigma (Germany) which was diluted
with DW. The SnCl2 or the NaBH4 as reducing
agents was used by dissolving in HCl and NaOH/
DW, respectively. The reducing agents was added
to 100 mL deionized water (DW) and mixed well.
All the laboratory glassware (Sigma) and PVC
plastics were cleaned by nitric acid (10% ,v/v)
for at least 2 days and then washed for many
times with DW. Cobalt (II) nitrate hexahydrate
(Co (NO3)2.6 H2O; CAS Number: 10026-22-
9) and Molybdenum powder (10 μm, ≥99.95%,
CAS Number: 7439-98-7) were purchased from
Sigma Aldrich (Germany). The MWCNTs and Co/
Mo-MWCNTs adsorbents was synthesized and
prepared from nano center of RIPI. In this study,
the Co-Mo/MWCNTs adsorbent was used for
mercury removal from air.
2.2. Apparatus
The mercury standard (Hg0) was generated by
the mercury vapor generation system (HgGS) in
chamber. The bench scale included of HgGS for
HgH2, chamber, PVC bags, the quartz tubes as a
column, the heater accessory (220 AC Voltage, 35-
450 °C), the digital ow meter control (50-500 ml
min Ar/air), Pure air accessory, O2 and water digital
detectors, the digital temperature control, the
MC-3000 as trace mercury analyser (Germany),
and the CV-AAS for determining the mercury
concentration. The pure air pushed with owrate
of 50-250 ml min-1 to chamber and mixed with
mercury vapour at 100 °C. The air lines (tubes)
and PVC bags were covered with heating jackets.
The quartz tubes with outer diameter of 0.35 inch,
inner diameter of 0.2 inch and length of 4.0 inch
was used as a column for the Co-Mo/MWCNTs
adsorbent. The Hg0 determined by a cold vapor
atomic absorption spectrometer (CV-AAS, GBC
Plus 932, AUS). A mercury hollow-cathode lamp
with a current of 8 mA, the wavelength of 253.7
nm based on a spectral band width (0.5 nm) was
used. Argon (99.99%) was used as a carrier gas for
mixer of CV-AAS and glass separator. The SKC air
sampling pump (USA), 50 to 2000 ml min-1 was
used.
2.3. Co and Mo Catalyst preparation
The sol-gel method has been extensively used in the
preparation of supported metal catalysts because it
typically results in highly homogeneous materials
with high degree of metal dispersion. In this sense,
catalysts were supported on silica sol-gel with the
metal to 50 percent based on silica added. To obtain
metallic catalyst supported on high-surface area
silica by the sol-gel method, the polymerization of
an alkoxy-silane such as tetrathoxysilane (TEOS),
also known as tetraethyl orthosilicate, is carried out
in the presence of the appropriate metal precursors.
In our case, catalyst nanoparticles were prepared
from high purity salts of the transition metals:
Co (NO3)2.6H2O and (NH4)6Mo7O4. 4H2O, from
Baker Co. To accelerate the polymerization, an
increase in pH can be brought about by addition of
a base, which causes a rapid hydrolysis followed
by polymerization. Simultaneously with this
polymerization process, the metallic ions (Co and
Mo) precipitate, thus forming a homogeneous and
well-dispersed mixture (Fig.1).
2.4. Co-Mo/MWCNTs synthesis
As Figure 1, After placing the catalyst inside a
quartz tube, a continuous nitrogen ow rate of 1 L
min-1 was passed through the reactor for removing
the oxygen. Subsequently, the reduction process
was accomplished within at 600 °C. The reduction
process was kept for 30 minutes in an atmosphere
of 90 % v/v of N2 and 10 % v/v of H2. Next, the
temperature was increased up to 700 °C for the
nucleation and growing of CNTs [37-39].
25
Co-Mo@MWCNTs for removal of mercury from air Danial Soleymani-ghoozhdi
2.5. Characterization
The high-resolution images were obtained using
a high-resolution transmission electron microscope
JEOL JEM-2010, operated at 200 kV and a
scanning electron microscopy (SEM) JEOL JSM
5300 operated at 5 kV. Complementary RAMAN
spectroscopy was performed. The Co-Mo/
MWCNTs samples were deposited onto a sample
holder with an adhesive carbon foil and sputtered
with Au before imaging. The morphology of Co-Mo/
MWCNTs was obtained by a transmission electron
microscopy (TEM, Zeiss, Germany). For the TEM
analysis, the samples were dispersed in C2H5OH
and a drop was used. The chemical analysis for
the determination of Co and Mo concentration in
synthesized samples was performed using F-AAS.
2.6. General Procedure
The mercury vapor removal was performed using
a bench-scale setup (Fig. 2). First, 40 mg of Co-
Mo/MWCNTs nanoadsorbent was put onto the the
quartz tubes. Then, the end of the adsorbent were
tied by re-proof linen. The pure air was mixed
with mercury vapor in chamber containing 0.1-10
μg Hg0 per liter air (21% O2, 0.2% H2O) at 25 °C.
By the procedure, 0.1─10 μg of Hg0 was generated
by the mercury vapor generation system (HgGS)
and restored in a PVC bag. The value of mercury
in PVC bag was validated using MC analyzer. Due
to procedure, the mercury standard solution (1-2
mL min-1), HCl (5% v/v, 5 mL min-1), and SnCl2
as reducing agent (2.5 mL min-1) were mixed with
pure air in mixer and pass through a peristaltic
pumps. Elemental mercury vapor was generated
in the reaction loop, and pumped into a 5 L
polyethylene (PE) bag, as a bulk container. Finaly,
the the mercury concentration was obtained 0.1-10
μg Hg0 per liter air in the polyethylene bag (5 L)
was mixed with 21 % O2 and 0.2 % H2O vapor
at 25 °C (10─100×TLV OSHA). Then The mixure
Hg0 and pure air passed through 40 mg of the Co-
Mo/MWCNTs adsorbents, at optimized air ow
rate 250 ml min. After amalgamation/adsorption
process, the elemental mercury was released from
the Co-Mo/MWCNTs adsorbents by a thermal
desorption accessory at 220 °C, under Ar ow rate
and transffered to the absorption cell of CV-AAS
(Fig.2). Finally, Hg0 concentration was determined
by CV-AAS. The conditions were presented in
Table 1.
Fig.1. Synthesis of Co-Mo/MWCNTs by Sol gel method and CVD procedure
26
3. Results and Discussion
3.1. Co-Mo/MWCNTs Raman Spectra
Figure 3a shows the Raman spectra for CNTs-
Co, in which the ratio ID/IG is 0. 26, relating a
high purity material. On the other hand, with Mo
the quality is decreased in a high level (Figure
3b), mainly with Mo (ID/IG ~ 0.59). This is due
to the solubility of C in Mo. In order to obtain
a better quality, in this case the CVD process
must performed to high temperatures (~900°C).
In our experiments, for comparison purposes,
the temperature was always the same for the
different metal-catalyst (~700°C). According
to previous reports, the increase of the D band
intensity (characteristic peak at ~1350 cm-1)
with decreasing multiwalled carbon nanotubes
(MWCNT) content, is a direct result of the addition
of carbonaceous by-products. In the same sense,
a decrease in the G’ band intensity (characteristic
peak at ~2700 cm-1) is observed as the MWCNT
mass fraction decreases. The G’ band on Figures
a reects the well-structured carbon walls in the
samples with Co catalyst, while the Figure 3b
(CNTs-Mo), indicate a less ordered structures,
due to the carbonaceous byproducts.
Anal. Methods Environ. Chem. J. 5 (1) (2022) 22-35
Fig.2. The procedure for removal mercury vapor from air based on Co-Mo/MWCNTs by the ASPAR procedure
Table1. Method conditions for mercury vapor removal with the Co-Mo/MWCNTs
Chamber Conditions Value
Hg0 values 0.1─10 μg per liter
O2 (g) 21%
H2O (g) 0.2%
PVC bag 5 L
Ar ow rate 0.2 L min-1
Air owrate 0.25 L min-1
Heat 220 °C
Removal efciency with air More than 95%
Absorption capacity 191.3 mg g-1 (2% Co and 2% Mo)
Adsorbent amount 40 mg