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

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].

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

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