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Advances in Environmental Biology, 10(4) April 2016, Pages: 204-219

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Advances in Environmental Biology

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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 1Chemistry Department, Faculty of Science, Taif University, 888-Taif, Kingdom of Saudi Arabia 2 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. 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].

205 Eman Alzahrani et al, 2016 Advances in Environmental Biology, 10(4) April 2016, Pages: 204-219

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.


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 gold-platinum 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


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


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)

Material Surface area(m2 g-1)± RSD (%)

Pore volume(cm3 g-1) ± RSD (%)

Activated carbon 823±4.1 0.65±4.3 Iron oxide /activated carbon composite 533±4.4 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).


Fig. 2: EDAX spectra of (A) activated carbon, and (B) iron oxide/activated carbon composite.

CK

aO

Ka

A

B


Fig. 3:Sulphur compound removal percentage/contact time relation.

Fig. 4: Sulphur compound removal/sulphur compound concentration.


Figure 5.A. Poteniostat current/concentraion relation of Na2S before treatment (above), and after

treatment (below).


Figure 5.B. Poteniostat current/concentraion relation of CH3S2CH3 before treatment (above), and after treatment (down).


0

2

4

6

8

10

12

14

16

18

20

0.0005 0.0001 0.00001 1E-06 0.00000001

Concentration, g/L

Cur

rent

, µA

Fig. 5.C: Poteniostat current/concentraion relation of CH3SOCH3 before treatment (above), and after treatment (down).


Fig. 6.A: Poteniostat current/potential relation of Na2S before treatment (above),and after treatment (down).


Fig. 6.B: Poteniostat current/potential relation of H3S2CH3 before treatment (above), and after treatment

(down).


Fig. 6.C: Poteniostat current/potential relation of CH3SOCH3 before treatment (above), and after treatment

(down).


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 Qmax. (mg g-1) Na2S 1/qe = 5.1754 * (1/Ce) + 0.0074* (0.9996) 135.13 H3S2CH3 1/qe = 4.8219 * (1/Ce) + 0.0031* (0.9994) 322.58 CH3SOCH3 1/qe = 4.9337* (1/Ce) + 0.0011* (0.9999) 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


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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