Advances in Environmental Biology, 10(4) April 2016, Pages: 204-219
AENSI Journals
Advances in Environmental Biology
ISSN-1995-0756
EISSN-1998-1066
Journal home page: http://www.aensiweb.com/AEB/
An Investigation Into The Adsorption Of Sulphur
Compounds Using Iron Oxide/Activated Carbon
Magnetic Composite
1Eman
Alzahrani, 2Hala M. Abo-Dief and 3Ashraf T. Mohamed
1
Chemistry Department, Faculty of Science, Taif University, 888-Taif, Kingdom of Saudi Arabia
The Egyptian Petroleum Research Institute, Nasr City, Cairo, Egypt and the now Department of Chemistry, Faculty of Science, Taif
University, Taif, KSA
3
The Mechanical Engineering Dept., Faculty of Eng., Al-Baha, KSA.
2
Address For Correspondence:
Eman Alzahrani: Chemistry Department, Faculty of Science, Taif University, 888-Taif, Kingdom of Saudi Arabia
E-mail: [email protected]
This work is licensed under the Creative Commons Attribution International License (CC BY).
http://creativecommons.org/licenses/by/4.0/
Received 12 February 2016; Accepted 28 April 2016; Available online 24 May 2016
ABSTRACT
Activated carbon (AC) is commonly used in the water treatment field. However, the usage of AC is limited by difficulties in separation from
the system. This study focused on preparation of iron oxide/activated carbon composite, which combined the adsorption feature of AC with
the magnetic properties of magnetic nanoparticles. Firstly, activated carbon was fabricated from coconut shells using chemical activation
with phosphoric acid (70%) followed by carbonization the materials at 400 ºC for 30 min, then the material was activated at 800 ºC for 10
min. Secondly, coconut shell-based activated carbon was immersed in ferric nitrate solution in presence of nitric acid (20%) as the carbon
modifying agent, followed by calcination the mixture at 600 ºC for 1h. The prepared materials were characterised using Brunauer–Emmett–
Teller (BET) instrument, scanning electron microscopy (SEM), and energy-dispersive X-ray (EDAX) measurement. The effect of the
fabricated composite adsorption of the sulphur compounds was evaluated using sodium sulphide (Na2S), dimethyldisulphide (CH3S2CH3),
and dimethylsulphoxide (CH3SOCH3). The sorption process using the prepared composites followed by the pseudo-second order kinetic
model and the adsorption isotherm date could be simulated with Langmuir model with maximum monolayer adsorption capacity of 135.13,
322.58 and 909 mg g-1 for Na2S, CH3S2CH3, and CH3SOCH3, respectively. These results suggest that prepared composite may be further
developed as potential for removal of sulfur compounds from petroleum water.
KEYWORDS: Activated carbon; iron oxide; composite; adsorption; sulphur compounds.
INTRODUCTION
Adsorption process is commonly used for removal of the pollutants, organic or inorganic materials. There
are different adsorbents that have been used for removal of pollutants. Activated carbon is known as adsorbent
that has been utilised for treatment due to its high specific surface area, chemical inertness, thermal stability, and
its capability for adsorbing a wide range of pollutants from air, soil, and liquids. In addition, rapid adsorption
kinetics onto the activated carbon has been found to be superior to other technique [1-5]. However,
commercially activated carbons are expensive and there are many problems with separation from the aquatic
system and regeneration of the used activated carbon; therefore, many researchers have produced activated
carbons using renewable and cheaper materials [6, 7]. The process of carbonization is commonly conducted in
order to prepare the activated carbon by heating the material at temperatures between 400 ºC and 900 ºC.
Commonly, the activating agent such as zinc chloride, potassium hydroxide, or phosphoric acid is mixed with
the source materials before carbonization in order to form the pores [8].
Copyright © 2016 by authors and Copyright, American-Eurasian Network for Scientific Information (AENSI Publication).
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There are different carbon containing source materials that have been successfully utilised for fabrication of
activated carbon; for example, bagasse [9], coir pith [10], oil palm shell and rice hulls [11], sunflower seed hull
[12], orange peel [13], pine fruit shell [14], coconut shells [15]. Coconut shells are cheap and abundant
agricultural waste. Fabrication of activated carbon from coconut shell can convert unwanted agricultural waste
into useful materials that can be used for wastewater treatment [16-19].
Recently, many researchers have been focused in the modification of activated carbon in order to enhance
its affinity with certain pollutants [20, 21]. Commonly, this can be performed by using chemical and thermal
treatments. The chemical treatments are performed by incubating the activated carbon with various chemicals
such as acid/base, strong oxidant, salts, or surfactants [22-24]. Thermal treatment are utilised to alter the surface
chemistry characteristics of the materials to be more effective [22]. The incorporation of the metal particles on
the surface of activated carbon can be considered as alternative method to alter and/or enhance their surface
properties while preserving the properties of the carbon matrix [25-27].
The corrosion of refinery equipment increasingly significant in recent years as the acid content in crude oil
being processed has progressively increased. Wang and Wateknson [28] concluded that sulphur is inherent in
crude oil, and is more concentrated in heavy fractions. It can be present in number of forms such as mercaptans,
sulphides, and the like. Crude oils, bitumen and their heavy fractions may have sulphur contents from less than
1% to over 6%. Al Zubaidy et al. [29] proposed that the Adsorption-desulfurization process of diesel fuel
removal of organo-sulfur compounds (ORS) from diesel fuel is an important aspect of all countries to reduce air
pollution by reducing the emission of toxic gases such as sulfur oxides and other polluted materials. Popoola et
al. [30] gives a comprehensive review of sulfur compound corrosion problems during oil and gas production and
its mitigation. They deduced that increasing concerns on the air quality regulation have urged the petroleum
refining industry to produce cleaner products by removing heteroatoms containing molecules from their major
products, diesel, and gasoline.
In this study, an environmentally friendly materials of iron oxide/ activated carbon composite are used to
overcome the separation of the sulphur compounds from the system. The activated carbon was prepared from
coconut shells as activated carbon precursor. Then, the coconut shell-based activated carbon soaked in a solution
mixture of Fe(NO3)3.9H2O and HNO3 (20%). The mixture stirred for 8h at 100 ºC and finally the mixture
calcinated at 600 ºC for 1 h in the presence of nitrogen to form iron oxide/activated carbon composite. The
fabricated materials were characterised using BET analysis and SEM/EDAX analysis. Electrochemical analysis
and Langmuir Adsorption Isotherm were used for investigating the removal of sulfur compounds; sodium
sulphide (Na2S), dimethyldisulphide (CH3S2CH3), and dimethylsulphoxide (CH3SOCH3) from aqueous
solutions with different concentrations and contact time.
Experiments:
2.1. Chemicals and materials:
Coconut shells were bought from a local shop in Taif, KSA. Iron (III) nitrate nonahydrate,
(Fe(NO3)3.9H2O), sodium sulphide (Na2S), dimethyldisulphide (CH3S2CH3), dimethylsulphoxide (CH3SOCH3),
and Whatman filter paper (diam. 15 mm) was purchased from Sigma-Aldrich (Nottingham, UK). Nitric acid
(HNO3) and phosphoric acid (H3PO4) was purchased from Fisher Scientific (Loughborough, UK). Distilled
water was employed for preparing all the solutions and reagents. Cylindrical rod magnets (40 mm diameter x 40
mm thick) for settlement of the composite was purchased from Magnet Expert Ltd. (Tuxford, UK).
2.2. Instrumentation:
A pH meter (Fisherman Hydrus 300, Thermo Orion, Beverly, MA, USA), a hot plate-stirrer from VWR
International LLC (West Chester, PA, USA), a scanning electron microscope (SEM) Cambridge S360 from
Cambridge Instruments (Cambridge, UK). A furnace (WiseTherm high temperature muffle furnaces, Wisd
Laboratory Instrument, Wertheim, Germany). Energy dispersive X-ray (EDAX) analysis was carried out using
INCA 350 EDX system from Oxford Instruments (Abingdon, UK). A Brunauer–Emmett–Teller (BET) model
using a Surface Area and Porosity Analyser was obtained from Micromeritics Ltd. (Dunstable, UK).
Electrochemical measurements were performed in a conventional three-electrode glass cell. Electrode potentials
were measured against a saturated calomel electrode (SCE), the counter electrode was a mesh of Pt (purity
99.9%) and the working electrode was made of glassy carbon electrode. Tests were performed at 25±1°C and
thermostatically controlled. The electrochemical analysis was performed using a potentiostat/galvanostat (Model
73022) Metrohm UK Ltd. (Runcorn, Cheshire, UK).
2.3. Fabrication of activated carbon:
The activated carbon was prepared from coconut shells using our previous work [2]. Coconut shells were
washed with distilled water, dried in sunlight for 24 h, and ground. The dried powder was sieved to 250-500 µm
in size. Then, it was soaked with a boiling solution of 70% H3PO4 solution. After 24 h, the excess amount of
solution decanted off and air dried. The material was carbonized at 400 ºC for 30 min using a muffle furnace.
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The dried material was activated in a muffle furnace at 800 ºC for 10 min. Subsequently, the carbonized
materials produced was taken out and washed with plenty of distilled water to remove the residual acid. Finally,
the coconut shell-based activated carbon was dried at 60 ºC for 24 h and stored.
2.4. Preparation of iron oxide/ activated carbon composite:
20 g of the prepared coconut shell-based activated carbon was mixed with a solution mixture of
Fe(NO3)3.9H2O and HNO3 (20%). The mixture was magnetically stirring for 8 h at 100 ºC. The mixture was
treated at 600 ºC for 1 h in the presence of nitrogen in order to form iron oxide/activated carbon composite.
Finally, the fabricated materials were washed with distilled water in order to remove unreacted materials.
2.5. Characterisation of the fabricated materials:
The surface area and pore volume of the prepared activated carbon and composite were performed by
measuring N2 isotherms at 77 K. All samples were outgassed at 400 K under vacuum for 6 hours. The surface
morphology of prepared materials was observed by scanning electron microscopy (SEM). The sample was gold
coated prior to SEM observation using a SEMPREP 2 sputter coater, and the thickness of the layer of goldplatinum was around 2 nm. In addition, the materials were studied using energy dispersive X-ray (EDAX)
analysis. The sample was poured into a disposable sample container to three quarter of its capacity. This is to
ensure the X- ray passes through the test sample in order to give accurate sulphur counts. The sample was then
covered with X- ray transparent plastic film window. The power was switched on and light up the X- ray lamp
within seconds. The analyzer gives three different readings at 30seconds intervals. The readings were recorded
and average sulphur content was determined in percentage sulphur by weight. X-ray diffraction patterns were
recorded using a PANalytical X'Pert Pro diffractometer operating with the following parameters: Cu Kα
radiation (λ = 1.5405 Å), 45 mA, 40 kV, Ni filter, 2θ scanning range 5–80°, and scan step size of 0.03. Phase
identification was made using the reference database supplied with the equipment.
2.6. Sulphur compounds (SC) adsorption:
0.1 g of the fabricated material was mixed with 10 mL of each sulphur compound (Na2S, CH3S2CH3,
CH3SOCH3) with different concentration at 25 °C and test period ranging from 1min to 60min at 350 rpm
magnetic stirrer. The material was separated from the solution using magnet and the remained sulphur
compound was measured using cyclic voltammetry electrochemical analysis.
RESULTS AND DISCUSSION
3.1. Fabrication of the composite:
The aim of this work was to fabricate a composite that consists of amorphous porous carbon as a matrix and
dispersed nanoparticles in order to use them for removal of sulphur compounds. The activated carbon was
fabricated from available raw materials that was coconut shells since they are abundant and inexpensive natural
resource. The activated carbon was fabricated using chemical activation with dehydrating and stabilising agent
that was H3PO4 solution because it can produce activated carbon with high porosity [31-33]. In this study, the
carbonization temperature was 400 ºC for 30 min, and the activation temperature was 800 ºC for 10 min. During
activation, as the temperature increase endothermic reactions between carbon atoms and water vapor are
happened that can form macrpores, mespores, and micropores, and allow removal of organic matter such as the
hemicellulose, cellulose, and lignin in the coconut shells and form graphite like carbon structure [19, 34, 35].
After fabrication of activated carbon using coconut shell, it was dispersed in aqueous Fe(NO3)3.9H2O
solution that was used as a magnetic precursor in order to form iron oxide formed onto carbon matrix. Nitric
acid was utilised as carbon modifying agent [3]. The sample was calcinated at 800 ºC for 10 min in order to get
a carbonous composite [36]. After formation of the iron oxide/activated carbon composite material, they were
washed with distilled water to remove unreacted materials. After formation of the composite, the materials were
attracted to a conventional laboratory magnet and a clear solution was obtained that was removed.
3.2. Characterization:
3.2.1. Measurement of pore volume and BET surface area:
The prepared coconut shell-based activated carbon and iron oxide/ activated carbon composite were
physically characterised using a BET instrument, as can be seen in Table 1. The prepared activated carbon was
found to have a surface area of 823±4.1 m2 g-1. It was found that the surface area of coconut shell-based
activated carbon was higher than the coconut shell-based activated carbon produced by Tan et al. [16] with
surface area 525 m2 g-1. It was speculated that Tan et al used lower carbonization temperature (325 ºC) while in
this study the carbonization temperature was 400 ºC, and the same activation temperature (800 ºC) was used in
both study. Using lower carbonization temperature can cause fewer volatile substance and less tar to be released,
and form under developed coke structures. In contrast, Azevedoa et al. [37] fabricated activated carbon from
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coconut shells using zinc chloride (ZnCl2) with surface area of 1266 m2 g-1, which is higher than the values in
this work. It was speculated that due to using of ZnCl2 as an activating agent resulting in increased the surface
area while in this study H3PO4 was used. However, it was not preferred to utilise ZnCl2 since it can produce
hazardous gases for example chlorine and hydrochloric acid.
It was found that the surface area of the prepared activated carbon was decreased to 533±4.4 m2 g-1 after
formation of the composite. The experimental results were consistent with those reported by Oliveria et al. [38]
who found that the surface area of commercial activated carbon (933 m2 g-1) was decreased to 658 m2 g-1 after
precipitation of iron oxide onto a carbon surface. The reduction of the surface could be due to presence of iron
oxide particles in the composite. In addition, it was found that the average pore volume of the prepared activated
carbon was decreased from 0.65±4.3 cm3 g-1 to 0.54±4.8 cm3 g-1 and this was due to the formation of iron oxide
particles inside the pores.
3.2.2. SEM analysis:
The crystalline nature of the nanoparticle formed in the carbon matrix was studied and the morphology of
the fabricated materials were characterised by SEM analysis since it is the primary tool for studying the surface
morphology and the physical properties of the adsorbent surface [39]. Fig. 1 (A, C, and E) shows the SEM
images of activated carbon and Fig. 1 (B, D, and F) shows the SEM images of the iron oxide/activated carbon
composite. It was found that the roughness of the materials was increased after forming the composite. These
micrographs gave an a good indication that the obtained composite had high porosity which can hold more
solute from the solution during adsorption [32]. However, the presence of crystalline iron oxides in the structure
of the composite was not easily observed in the SEM images. This is because the most of the iron oxide
nanoparticles was incorporated inside the pores of the activated carbon. The same result was observed with
other group [3].
3.2.3. EDAX analysis:
In addition to SEM analysis, the formation of composite was further studied using EDAX analysis to
identify the chemical composition of the synthesized materials. Fig. 2 shows the elements of the activated
carbon, (A), and the composite material, (B). The EDAX spectrum of activated carbon showed strong peaks of
C and O. As can be seen in Fig.2 (B), there were new peaks in the composite sample that were Fe, which
confirms forming of the composite.
3.3. Removal of sulphur compounds:
3.3.1. Effect of contact time:
Fig. 3 illustrates the three sulphur compounds that were sodium sulphide (Na2S), dimethyldisulphide
(CH3S2CH3), and dimethylsulphoxide (CH3SOCH3) removal percentage/contact time relation at various types of
adsorbent compounds at 10g L-1 concentration and 350 rpm magnetic stirring. It is clear that as the contact time
increases, the sulphur removal percent increases. It was found that the CH3SOCH3 compound trend was the
highest compared to both CH3S2CH3 and Na2S. This was due to CH3SOCH3 contains oxygen that faculty react
with sulphur and adsorbed it (Hosseini and Hamidi, 2014). Moreover, it was found that with the progression of
time, the difference between the three trends diminishes until the effect of each compound on the sulphur
removal percent was the same.
3.3.2. Effect of sulphur compounds concentration:
Fig. 4 illustrates the variation of sulphur compounds removal with sulphur concentrations using the three
adsorbents of constant concentration. It is clear that at lower sulfur compound concentration, the three
adsorbents were effective in adsorbing the sulphur compound; however, it was found that with increasing the
sulphur compound concentration, the effect of the adsorbents decreases. It was found that the CH3SOCH3
compound trend was higher compared to both CH3S2CH3 and Na2S in agreement with the previous work [40].
3.4. Potentiostat results:
Fig. 5 (A, B and C) illustrate the relation between the sulphur concentration and the current before and after
adsorption treatment. It is clear that the sulphur concentration precipitates after adsorption is lower than that
before adsorption in agreement with the amount of current produced. It is clear that the amount of current after
adsorption is higher than that before adsorption due to the amount of current consumed in adsorption. Moreover,
it is clear that the precipitates sulphur at the case of CH3SOCH3 compound is lower than that of CH3S2CH3 and
Na2S respectively due to its higher adsorption. Fig. 6 (A, B, and C) show the current / potential relation for the
three adsorbent compound the drawn by the potentiostat. It is clear from the figures that the sulphur
concentration value before the adsorption is higher than after concentration declared by the lower amount of
current produced. It is clear that the sulphur concentration after adsorption in the case of CH3SOCH3 compound
is lower than that of CH3S2CH3 and Na2S, respectively, in agreement with the previous results. Also, it is clear
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that the amount of current consumed in carrying the adsorption of sulphur using CH3SOCH3 have the lowest
value compared to the other compounds values illustrating its higher effect in sulphur adsorption followed by
CH3S2CH3 and Na2S, respectively.
3.5. Langmuir Adsorption Isotherm:
The uptake of Na2S, H3S2CH3, and CH3SOCH3 by the adsorbent is the result of the physical attraction or
chemical coordination between sulphur and the chemical moiety on the three adsorbents then the maximum
number of such sites would be finite. When the adsorbent and adsorbate come in contact with each other,
dynamic equilibrium is established between the adsorbate concentration in both the phases. The state is dynamic
in nature, as the amount of adsorbate migrating onto the adsorbent would be counter balanced by the amount of
adsorbate migrating back into the solution. When all the sites available achieve equilibrium, the adsorptive
capacity would be maximum. A plot of equilibrium concentration (Ce) and adsorptive capacity (Qe) was drawn
for each adsorbent. The adsorption isotherms for different adsorbents are shown in Fig. 7. To estimate maximum
adsorption capacity (Qmax) linearized forms of Langmuir and was prepared based on the following equations;
1/qr = (1/Qmax.) + (1/b)Qmax. (1/CR)
(1)
Log qe = (1/n) (logCe) + log Kf
(2)
where Ce is the solute concentration at equilibrium in aqueous phase in g L-1, qe is the solute adsorbed per
unit weight of adsorbent in mg g-1. Qmax is the maximum solute adsorbed per unit weight of adsorbents in mg g-1
and b, n and Kf are constants in Eqs. (1) and (2). Separate curves were drawn for all adsorbents by plotting 1/Qe
against 1/Ce to get the corresponding Langmuir isotherms, and from the equations Qmax was obtained. The
equations obtained for Langmuir adsorption isotherms for the chemical and natural adsorbents are shown in
Table 2.
The Qmax for the adsorbate of Na2S, H3S2CH3, and CH3SOCH3 were found to be 135.13, 322.58, and 909.09
mg g-1. Maximum adsorbent capacities (Qmax) was obtained by the natural adsorbent than chemical adsorbent.
The CH3SOCH3 was the highest adsorbate because it contains oxygen, which react fastly with the iron in the
adsorption followed by H3S2CH3 because these two sulphur compounds are organic where the organic
compounds adsorbate fastly compared to the nonorganic sulphur compound like Na2S.
Table 1: The surface area and total pore volume for activated carbon and composite calculated using the BJH method, and RSD (n= 3)
Pore volume(cm3 g-1) ± RSD (%)
Material
Surface area(m2 g-1)±
RSD (%)
Activated carbon
Iron oxide /activated carbon composite
823±4.1
533±4.4
0.65±4.3
0.54±5.8
Fig. 1: SEM micrographs of activated carbon (A, C, and E), and iron oxide/activated carbon composite (B, D,
and F).
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A
OKa
CKa
B
Fig. 2: EDAX spectra of (A) activated carbon, and (B) iron oxide/activated carbon composite.
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Fig. 3:Sulphur compound removal percentage/contact time relation.
Fig. 4: Sulphur compound removal/sulphur compound concentration.
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Figure 5.A. Poteniostat current/concentraion relation of Na2S before treatment (above), and after
treatment (below).
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Figure 5.B. Poteniostat current/concentraion relation of CH3S2CH3 before treatment (above), and
after treatment (down).
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20
18
16
Current, µA
14
12
10
8
6
4
2
0
0.0005
0.0001
0.00001
1E-06
0.00000001
Concentration, g/L
Fig. 5.C: Poteniostat current/concentraion relation of CH3SOCH3 before treatment (above), and after treatment
(down).
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Fig. 6.A: Poteniostat current/potential relation of Na2S before treatment (above),and after treatment (down).
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Fig. 6.B: Poteniostat current/potential relation of H3S2CH3 before treatment (above),
(down).
and after treatment
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Fig. 6.C: Poteniostat current/potential relation of CH3SOCH3 before treatment (above), and after treatment
(down).
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Fig. 7: Langumir isotherm of (A) Na2S, (B) CH3S2CH3, and (C) CH3SOCH3.
Table 2: Langmuir adsorption isotherms parameters for the sulphur adsorbate.
Adsorbate
Langmuir isotherm equation
Na2S
1/qe = 5.1754 * (1/Ce) + 0.0074* (0.9996)
H3S2CH3
1/qe = 4.8219 * (1/Ce) + 0.0031* (0.9994)
CH3SOCH3
1/qe = 4.9337* (1/Ce) + 0.0011* (0.9999)
Qmax. (mg g-1)
135.13
322.58
909.09
Conclusion:
In summary, this work focused on producing of activated carbon from coconut shell waste as activated
carbon precursor, followed by production of iron oxide/ activated carbon composite. The properties of coconut
shell AC materials and the formed composite were studied using different techniques that were BET analysis,
and SEM/EDAX analysis. There was reduction in the surface area of the composite compared with the prepared
activated carbon due to the formation of iron oxide inside the pores. In addition, the valuable effect of iron oxide
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on the prepared activated carbon was confirmed through the EDAX analysis spectrum. It is clear that the
fabricated composite can be utilised as sorbents for the sulfur compounds removal from the petroleum water.
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