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Influence of acidic and basic treatments of activated carbon derived from waste rubber tires on adsorptive desulfurization of thiophenes
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Influence of acidic and basic treatments of activated carbon derived from waste rubber tires on adsorptive desulfurization of thiophenes Tawfik A. Saleh a,∗, Gaddafi I. Danmaliki b a b
Chemistry Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia Environmental Science Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 6 June 2015 Revised 8 November 2015 Accepted 13 November 2015 Available online xxx Keywords: Adsorptive desulfurization Fixed bed Waste tires Carbon
a b s t r a c t This work reports on the influence of treatment conditions on the waste tire-derived activated carbon for adsorptive desulfurization. The rubber tires were carbonized and activated. The obtained activated carbon (AC) was treated with HNO3 or NaOH at a temperature range of 30–90 °C. The morphology and surface properties of AC were characterized by surface pH, Boehm’s titration, N2 adsorption–desorption isotherms, Fouriertransform infrared spectroscopy, X-ray diffraction, and scanning electron microscope. The AC sample, treated with HNO3 at 90 °C, possess the highest surface oxygen containing functional groups (2.39 mmol/g), surface area (473.35 m2 /g) and pore volume (0.70 cm3 /g) and the more adsorption capacity to the refractory sulfur compounds. The Boehm’s titration experiments indicated that the amount of surface oxygen containing functional groups on the surface of the acid-treated AC increases with treatment temperatures. Acid-treated AC at 90 °C proves to be optimum for adsorptive desulfurization with the order of dibenzothiophene > benzothiophene > thiophene. © 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Combustion of fossil fuels is associated with the release of toxic gases that pose serious environmental and health related impacts. Sulfur and its refractory compounds found in fossil fuels are extremely reactive. They cause catalyst poisoning and corrode refining equipment. In order to reduce air pollution and technological problems, EU and US have enacted laws to regulate sulfur emissions to 10 and 15 ppm respectively [1,2]. Various technologies are currently in use to lower the organic sulfur compounds in fuels, these methods include: hydrodesulphurization (conventional method), oxidative desulfurization, ionic liquids desulfurization, adsorptive desulfurization and biodesulfurization [3–6]. Of all these technologies, adsorptive desulfurization, one of the most promising technologies is receiving much attention because of its facile and mild operating conditions. Much attention is focused on the development of porous adsorbents such as activated carbon, zeolites, alumina, and zirconia that are cheap, easily regenerated and possesses high selectivity for sulfur compounds [7–11]. The surface areas and porosities of AC are greatly influenced by the parent material and the method employed in the production. Better adsorption capacities and adsorption rates of AC are directly linked to
∗
Corresponding author. Tel.: +966 13 860 1734; fax: +966 13 860 1734. E-mail address: tawfi[email protected], tawfi[email protected] (T.A. Saleh). URL: http://faculty.kfupm.edu.sa/CHEM/tawfik/ (T.A. Saleh)
larger surface areas and pore size distributions [12]. Mesoporous and microporous volumes of AC also play a crucial role in the adsorption of larger molecules. In general, the selective adsorption of inorganic molecules on the surface of AC directly depends on the amount of oxygen containing complexes. The oxygen containing complexes are mostly created using dry or wet oxidation methods [13–15]. Adsorptive capability of AC modified with HNO3 at 120 °C was increased significantly and modification using HNO3 even at high temperatures showed a promising result in the removal of thiophenes from oil [16,17]. The aim of this work was to produce AC from end of life tires as a free-of-cost source, and to treat the AC using HNO3 and NaOH at various temperatures. The influence of acid/base treatment of AC for the removal of thiophenes was examined. The adsorption performance of the AC was evaluated. 2. Experimental 2.1. Development of the adsorbent Waste rubber tires were cleaned by removing the iron wires from the tire. Then, the waste rubber was cut into small pieces and thoroughly washed with deionized water, and then dried in an oven at 110 °C. The elemental analysis of the waste rubber indicated their initial elemental composition as 82.3 carbon, 9.2 oxygen, 0.7 nitrogen, 2.8 sulfur, 3.8 silicon and 1.2 silicon and silicon with different wt. %.
http://dx.doi.org/10.1016/j.jtice.2015.11.008 1876-1070/© 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: T.A. Saleh, G.I. Danmaliki, Influence of acidic and basic treatments of activated carbon derived from waste rubber tires on adsorptive desulfurization of thiophenes, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.11.008
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The granules were heated up to 300 °C to separate the black tire crude oil, distilled diesel oil and produced oil. Pyrolysis was carried out in a muffle furnace at 500 °C for 5 h to isolate the ash and carbon black from pyro-gas and other oils, followed by treatment with hydrogen peroxide solution to oxidize all adhering organic impurities. The material was washed with deionized water and dried in vacuum oven at 110 °C overnight. The char was then placed in the stainless tube, inserted into the furnace, heated under nitrogen flow through the sample bed at 200 ml/min at 20 °C/min to 900 °C and a holding time of 5 h. Then the reactor was cooled down to room temperature before the sample was removed, and kept in an oven overnight, then grounded to small particles using pestle and mortar. The produced black granules were sieved and the powdered carbon with a particle size of < 0.1 mm was used to allow good surface contact with acid or base reagent solution. Chemical activation was conducted on the prepared material using 4 M HNO3 and 4 M NaOH at three different temperatures respectively (30, 60, and 90 °C) and a duration of 3, 6 and 12 h. Then, the mixture was filtered and the obtained activated carbon (AC) was washed with deionized water and dried overnight. It should be mentioned that increasing the refluxing time from 3 h to 12 h has an overall net negative effect on the removal of the refractory sulfur compounds (see supporting information file, section S3.1.). Therefore, 3 h refluxing time was selected. 2.2. Boehm’s method for surface functional groups The amount of oxygen surface functional groups (both acidic and basic) was determined according to modified Boehm’s method [18]. 0.5 g of raw and treated carbon samples were added to beakers each containing 25 ml of the following 0.05 M solutions: NaOH, Na2 CO3 , NaHCO3 and HCl. The beakers were sealed and shaken for 24 h and then 10 ml of each filtrate was pipetted in an excess of 20 ml 0.05 M HCl for the determination of acidic functional groups or NaOH for the basic functional groups. The filtrate was titrated with 0.05 M NaOH or HCl, respectively, using phenolphthalein indicator and the volume required to reach the endpoint was noted. For Na2 CO3 reaction bases an excess of 30 ml 0.05 M HCl was added rather than 20 ml due to diprotic property of the base to ensure complete reaction with acid. The number of acidic sites was calculated under the assumption that NaOH neutralizes carboxylic, phenolic, and lactonic groups; Na2 CO3 neutralizes carboxylic and lactonic groups; and NaHCO3 neutralizes only carboxylic groups. The number of surface basic sites was calculated from the amount of hydrochloric acid required. The following formula was used to calculate the amount of surface acidic and basic groups neutralized with NaOH and HCl respectively [19]:
Moles of Carbon functionality ([R a or b] Vr − ([X] Vx − [T ] Vt )) = m where [R a or b] and Vr denote the concentration and volume of the reaction base or acid in mol/L and liters respectively. [X] and Vx denote the concentration and volume of the excess acid or base added to the aliquot. [T ] and Vt denote the concentration and volume of the titrant required to reach the endpoint. m stands for the mass of the carbon used in grams. The amount of surface acidic groups neutralized by Na2 CO3 and NaHCO3 was calculated using the following formula [20,21]:
ncsf =
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nHCl VB [B]VB − ([HCl]VHCl − [NaOH]VNaOH ) nB Va
where ncsf denotes the moles of carbon surface functionalities on the surface of the carbon that reacted with base during the mixn ing step. nHCl = molar ratio of acid to base [B] and VB are the conB centration and volume of the reaction base mixed with the carbon. [HCl] and VHCl are the concentration and volume of the acid added to
the aliquot taken from the original sample. [NaOH]VNaOH is the concentration and volume of the titrant in the back titration. Va is the volume of the aliquot taken from VB . Surface pH of the carbon samples was tested by HI 3512 bench-top pH meter equipped with a graphic LCD display to gain insight about the acidity and basicity of the samples. A suspension of 0.2 g of each sample was added to 10 ml of water and the suspension was stirred overnight to reach equilibrium. The samples were then filtrated and the pH of the each solution was measured. 2.3. Characterization of the adsorbent The FT-IR spectra of the samples were recorded using Nicolet 6700 spectrometer (Thermo electron, USA) with a resolution of 2.0 cm–1 well equipped with deuterated triglycine sulfate detector and an OMNIC program. The experiments were conducted on the powdered samples ground in an agate mortar to produce KBr pellets and spectra were obtained by adding 64 scans and corrected for the background noise. The spectra of the samples were recorded in transmission mode and the wavenumber range of 4000–400 cm–1 . Scanning electron microscopy (SEM) analysis of the treated samples was conducted by using Hitachi model S- 3500 N instrument. Energy dispersive X-ray spectroscopy (EDX) analysis was conducted using Oxford Instrument (England) and X-Max detector to determine the elemental composition of the treated samples. X ray diffraction patterns of the adsorbents were taken using (Rigaku Miniflex II desktop X-ray diffractometer) using Cu-Kα´ radiation and an X-ray gun operated at 40 kV (voltage) and 200 mA current. Data was collected from 2θ = 10°– 80° at a scan rate of 4°/min. The porous structure of the AC samples was characterized by adsorption/desorption of nitrogen at (−196 °C) on a Micromeritics ASAP 2020 surface area and porosimetry analyzer (Micromeritics, USA) to determine the surface area (BET) and pore volume of the treated sorbents. 2.4. Adsorption experiment 2.4.1. Batch mode In a typical run in the batch mode adsorption studies, various amounts, in the range between 0.1 and 0.5 g of adsorbents were introduced into 15 ml of the fuel solution containing thiophene (T), benzothiophene (BT) and dibenzothiophene (DBT) in hexane (85%) and toluene (15%) with initial concentration of 50 ppm each. The sulfur removal efficiency of the AC was calculated using the formula below:
x = (C0 − Ce )/C0 ∗ 100%
(1)
where x x = sulfur removal percentage (%) C0 = initial concentration of sulfur in model is (ppmw), Ce = final sulfur concentration. The amount of sulfur compounds adsorbed per unit mass of the adsorbent at equilibrium (qe mg/g) and at any time t (qt mg/g) termed as the adsorption capacity was calculated from the formulas:
qe =
V (C0 − Ce ) m
(2)
qt =
V (C0 − Ct ) m
(3)
where V (L) is the volume of the liquid phase, C0 (mg/L) is the initial concentration of the sulfur compounds before they come in contact with the adsorbent, Ce and Ct (mg/L) are the concentration at equilibrium and at any time t, and m (g) is the amount of the adsorbent. 2.4.2. Break-through experiments Fixed-bed flow system was used to test the simultaneous adsorptive desulfurization of the treated AC samples using a column system.
Please cite this article as: T.A. Saleh, G.I. Danmaliki, Influence of acidic and basic treatments of activated carbon derived from waste rubber tires on adsorptive desulfurization of thiophenes, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.11.008
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Table 1 Surface pH, total basic and acidic groups on the raw and treated AC. Sample
Raw AC AC-NaOH 30 °C AC-NaOH 60 °C AC-NaOH 90 °C AC-HNO3 30 °C AC-HNO3 60 °C AC-HNO3 90 °C
Surface pH
4.92 10.66 10.18 10.69 4.21 4.05 3.09
Total basic groups (mmol/g)
1.62 2.48 1.98 2.46 1.37 1.36 1.33
The adsorbent was packed inside the column. The model fuel sample was then passed through the column by a peristaltic pump with a controlled flow rate (150 rpm) at a temperature of 25 °C. Once the adsorption process started, treated fuels were periodically sampled at different time intervals for analysis. The experiments were repeated three times and the relative standard deviation was calculated and found to be < 1.5%. 2.5. Analysis The concentrations of the refractory sulfur compounds were analyzed by gas chromatography sulfur chemiluminescence detector (GC-SCD) (Model 7890A) system (Agilent) equipped with auto sampler (7693) and splitless injector. The column used was Agilent 19091S-001: 2638-45555 50 m × 0.2 mm dimensions and a film thickness of 0.5 μm methyl siloxane stationary phase. The injection volume was 1 μl with a flow rate of 0.839 ml/min. Inlet temperature was a 60 °C and oven program of 60 °C for 1 min, then 10 °C/min to 230 °C for 2 min. Other parameters and conditions of the method are listed in the Supporting information file, Table S1. 3. Results and discussion 3.1. Characterization 3.1.1. Boehm’s titration The acidic functional groups are due to the presence of phenols, lactols, lactones and carboxylic acids. The acidic groups are said to differ in their acidities that can be determined by reaction with 0.05 M NaOH, Na2 CO3 and NaHCO3 respectively [17,22]. Phenols are neutralized with NaOH, while lactones and carboxylic acids are neutralized by Na2 CO3 , and NaHCO3 neutralizes only carboxyl groups. The raw AC is acidic with relatively high amount of phenolic groups and total acidic groups of 1.71 mmol/g. However, upon treatment with NaOH at 30 °C the total acidic groups decreased to 1.23 mmol/g but the phenolic groups were greater than that of the raw samples (Table 1). The increase in phenolic groups can be attributed to the deformation of the lactone groups resulting in the formation of more phenolic groups [23]. This can be clearly observed by comparing the amount of the lactone groups on the base-treated ACs with that of the acidtreated ACs. Moreover, for the AC treated with NaOH at 90 °C, the carboxylic groups were reduced due to its significant deformation; but only the phenolic and lactonic groups were determined which contributed less acidity. The total basic functional groups on the surface of the AC-NaOH 30 °C were higher in all the samples modified. Although the influence of the temperature was not that significant, it could be noticed that the treatment at 60 °C seems to be the best temperature for NaOH modification giving the highest surface acidic functional groups (1.72 mmol/g) and the lowest surface basic groups (1.98 mmol/g). On the other side, the increase in the total acidity of the acid-treated ACs can be ascribed to the introduction of the new acidic functional groups on the surface of the ACs [17,22]. Total acidic functional groups increased with treatment temperature from
Surface acidic groups (mmol/g) Phenol
Lactone
Carboxyl
Total
0.81 1.07 1.21 1.13 0.80 0.82 0.69
0.22 0.16 0.33 0.25 0.64 0.71 0.81
0.68 0.00 0.18 0.00 0.67 0.69 0.89
1.71 1.23 1.72 1.38 2.11 2.22 2.39
2.11 to 2.22 and finally 2.39 mmol/g for AC-HNO3 30, 60 and 90 °C respectively, and the total basic groups slightly reduced from 1.37 to 1.36 and finally to 1.33 mmol/g for the same samples respectively. 3.1.2. FTIR spectroscopy FTIR spectra of the raw, HNO3 , and NaOH treated AC at various temperatures (30, 60 and 90 °C) were taken to understand the functional groups present in the AC and they are given in Fig. 1. The Raw AC, NaOH and HNO3 treated AC have displayed varying degrees and intensities of their bands though they all have similar patterns but the bands increase with in treatment temperature. This implies that temperature has an effect on the development of oxygen containing functional groups and acidic and basic sites on the surface of the AC [24,25]. It can be seen that the samples showed a broad peak centering around 3430 cm−1 that can be ascribed to O–H stretching vibrations of hydroxyl groups or to chemisorbed water. There is a noticeable development peak at 2854 cm−1 and 2924 cm−1 as the temperature increases and these peaks can be due to the presence of aliphatic C–H stretch of CH, CH2 and CH3 . The peak at 1720 cm−1 is attributed to the carboxyl group. The peak at 1640 cm-1 is usually attributed to the carbonyl group. The peak around 1400 cm−1 appeared in HNO3 treated AC, may be attributed to the presence of C=O stretching vibrations of the C=O, and C–O [19,26]. This confirms the influence of temperature on the development of carbonyl groups [27]. The NaOH treated AC at (30, 60 and 90 °C) have shown similar patterns but the intensities are not well developed as compared to the HNO3 treated AC. The results also confirm the influence of temperature and HNO3 on the development of surface functional groups such as nitrate, carboxyl, and carbonyl groups. In addition to the absorption peaks already discussed, the peaks around 2200–2400 cm−1 are almost certainly due to residual CO2 , either in the chamber or adsorbed to the surface. The peaks at 500–700 cm–1 are noisy and difficult to attribute. 3.1.3. SEM/EDX result on the treated AC The SEM/EDX result of the treated AC is depicted in Fig. 2. The treatment with HNO3 at 30, 60, and 90 °C showed the presence of carbon and oxygen as the only elemental composition of the adsorbents and they showed a significant improvement trend in the weight% of oxygen from 11.01 to 11.86 and finally 14.39 meaning that oxygen containing functional groups kept on increasing with temperature upon acid treatment. This result is in accordance with the result obtained from the FT-IR, Boehm’s titration experiment and XRD results. EDX results on the base treated samples showed the presence of carbon, oxygen and sodium in different proportions. Increase in treatment temperature shows significant reduction in the wt. % of oxygen from 18.33 to 17.72 and finally 13.63 for AC-NaOH 30 °C, 60 °C and 90 °C respectively. 3.1.4. XRD patterns of the treated AC X-ray diffraction was conducted for the raw obtained AC, basetreated AC and acid-treated AC samples (Figs. 3 and 4). The X-ray
Please cite this article as: T.A. Saleh, G.I. Danmaliki, Influence of acidic and basic treatments of activated carbon derived from waste rubber tires on adsorptive desulfurization of thiophenes, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.11.008
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Fig. 1. FTIR spectra of raw AC and AC samples treated with sodium hydroxide or nitric acid at various temperatures.
diffraction profiles show two broad diffraction peaks of graphitic carbon with maxima at around 25° and 42° two theta. The two peaks are indexed to (002) and (100) or (101) planes of crystalline hexagonal graphite lattice respectively (JCPDS card no. 41-1487) [28,29]. The relative intensities increased upon treatment signifying the improvement in the degree of graphitization structure and crystallinity of the treated AC in addition to the removal of amorphous carbons [7,30]. The peak, at approximately 25° two theta, was broadened on increase of temperature, and this change in the width of the peak can be attributed to the widening or expansion of the graphite layers in the individual crystalline (disintegration to relatively smaller crystallites). It can be seen in Fig. 3 that the intensity of the peak, at approximately 25° two theta, of AC treated with the base decreased with increasing temperature from 60 to 90 °C indicating the induced irregularity into the layered graphitic structure of the AC after the activation process [31]. 3.1.5. Brunauer–Emmett–Teller (BET) surface area and porosity studies Nitrogen adsorption desorption isotherm is a useful tool in the characterization of carbon materials. AC samples express Type I isotherm according to Brunauer et al., 1940 [32,33], See Fig. S1 in the Supporting information file. A hysteresis loop can be observed in all the samples at the relative high pressure indicating the presence of mesopores. As shown in Table 2, the surface area and pore volumes were significantly enhanced upon NaOH and HNO3 treatments. The N2 adsorption capacity followed the order: Raw < AC-NaOH 30 °C < AC-NaOH 90 °C < AC-NaOH 60 °C < AC-HNO3 -30 °C < ACHNO3 -60 °C < AC-HNO3 -90 °C. Temperature also enhanced the development of the surface area in the acid treated AC. The change in surface area caused by different treatments is summarized in Table 2. It can be clearly seen that AC-NaOH-60 °C sample has the highest surface area (369 m2 /g), pore volume (0.69 cm3 /g) and pore size ˚ in the base treated AC. However, AC-HNO3 -90 °C has the (75.03 A) highest surface area (473.35 m2 /g) and pore volume (0.70 cm3 /g) of all the samples used which confirms the significance of acid treatment of AC in the development of surface area and porosity. The results are in agreement with the results obtained from the XRD, FT-IR and Boehm’s experiment.
can be seen that the removal is very rapid, nearly 95% of all the adsorption process occur in the first 5 min of the process. This is because of the available free sites for adsorption of the sulfur compounds at the beginning of the adsorption process. Equilibrium was achieved at around 30 min of adsorption process and afterwards the adsorption of the refractory sulfur compounds was noticed to decline due to the desorption process of the analytes from the adsorbent. The size of the adsorbent, concentration of the sulfur compounds, degree of mixing, the affinity of the adsorbent to the sulfur compounds and the diffusion coefficient of the adsorbent in bulk and solid phases play a cardinal role in the percentage of sulfur compounds adsorbed on the surface of the adsorbent [34]. The adsorption capacity followed the order: DBT > BT > T. The vast majority of the area of the samples is mesoporous, which makes it easier for adsorption at the initial time, however, when the contact time increases the refractory sulfur compounds are faced with much larger resistance to pass through the micropores. 3.2.2. Influence of adsorbent dosage on the performance of AC-HNO3 -90 °C and AC-NaOH-60 °C Adsorbent dosage is an important parameter in adsorptive studies because it determines the maximum adsorptive capacity of a given adsorbent using a starting concentration of the analyte of interest. Best NaOH and HNO3 treated samples were selected for testing the effect of adsorbent dosage. The effect of AC-HNO3 -90 °C and AC-NaOH60 °C dosages on the uptake of refractory sulfur compounds (Thiophene 52 ppm, BT 50 ppm and DBT 51 ppm at room temperature) were studied and the results are shown in Fig. 6a–b. The mass of the adsorbent was varied from 0.1 g to 0.5 g and the amount of refractory sulfur compounds adsorbed was found to be increasing with adsorbent dosage. The increase in adsorption capacity of sulfur compounds with dosage is expected because more active surface areas and sites for sulfur adsorption are introduced in the medium [35,36]. The removal efficiency after 0.25 g was less, becoming almost constant after 0.5 g; due to this, we fixed the rest of the experiments at an adsorbent dosage of 0.5 g. The trend of adsorption of the refractory sulfur compounds followed the order of DBT > BT > T. DBT was removed higher than all other refractory sulfur compounds which can be attributed to the dispersive interactions formed between the AC and DBT and due to the presence of double benzene rings in DBT [37,38].
3.2. Adsorption evaluation 3.2.1. Contact time The influence of contact time on the efficiency of AC-HNO3 -90 °C on the removal of refractory sulfur compounds is depicted in Fig. 5. It
3.2.3. Effect of treatment reagent The raw and treated AC both with NaOH and HNO3 were compared for their desulfurization efficiency. 0.5 g of each sample was taken and added to 50 ppm mixture of thiophene, BT and DBT in
Please cite this article as: T.A. Saleh, G.I. Danmaliki, Influence of acidic and basic treatments of activated carbon derived from waste rubber tires on adsorptive desulfurization of thiophenes, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.11.008
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Fig. 2. SEM image and EDX spectrum with an elemental analysis table of the prepared carbon treated with nitric acid at 30 °C (a), 60 °C (b), 90 °C (c), sodium hydroxide at 30 °C (d), 60 °C (e), 90 °C (f).
Please cite this article as: T.A. Saleh, G.I. Danmaliki, Influence of acidic and basic treatments of activated carbon derived from waste rubber tires on adsorptive desulfurization of thiophenes, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.11.008
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Fig. 3. XRD pattern of Raw AC and NaOH treated AC at 30, 60 and 90 °C.
Fig. 4. XRD pattern of AC treated with HNO3 at 30, 60 and 90 °C. Table 2 Surface area and pore volume analysis. Adsorbents
BET surface area (m2 /g)
Pore volume (cm3 /g)
˚ Pore size (A)
Raw AC AC-NaOH 30 °C AC-NaOH 60 °C AC-NaOH 90 °C AC-HNO3 30 °C AC-HNO3 60 °C AC-HNO3 90 °C
183.21 220.48 369.27 268.11 429.26 454.80 473.35
0.30 0.44 0.69 0.47 0.66 0.67 0.70
77.54 79.83 75.03 70.47 62.10 59.60 60.22
20 ml (85% hexane and 15% toluene). The set ups were stirred and samples were taken after 30 mins and analyzed with GC-SCD. The result is shown in Fig. 7 and the percentage removals of thiophene, BT and DBT from the raw and treated samples at various temperatures were reported. It can be clearly seen that raw sample performed the least. AC-NaOH-60 °C showed the best performance in the base treated samples and this can be attributed to the presence of oxygen containing functional groups, high surface area, pore volume, and pore size exhibited by the sample compared with other base treated samples. AC-HNO3 -90 °C showed the best performance regardless of treatment conditions because it showed the highest total acidic groups, the largest intensity in XRD analysis, and the highest
in % transmittance in FT-IR. It also possesses the highest surface area and pore volume. It should be noted that the reagent type plays a role while the temperature has a subtle effect. The adsorbents performance and adsorptive capacities followed the order: AC-HNO3 90 °C > AC-HNO3 -60 °C > AC-HNO3 -30 °C > AC-NaOH-60 °C > ACNaOH-90 °C > AC-NaOH-30 °C > Raw AC. This order is consistent with the increase in the surface area and pore volume of the adsorbents and their total acidic groups [40,41]. This implies that both surface area and surface chemistry play an important role in adsorption capacity. The adsorption capacities were calculated and listed in Table S2.
Please cite this article as: T.A. Saleh, G.I. Danmaliki, Influence of acidic and basic treatments of activated carbon derived from waste rubber tires on adsorptive desulfurization of thiophenes, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.11.008
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Fig. 5. Influence of contact time on removal of thiophene, BT and DBT by AC-HNO3 90 °C (adsorbent dosage 0.5 g, 20 ml volume of model fuel containing 52 ppm thiophene, 50 ppm BT and 51 ppm DBT). Fig. 7. Percentage removal of thiophene, BT, and DBT (initial concentration of 50 ppm each) on acid and base treated AC samples after 30 min (dosage of each sample was 0.5 g in 20 ml initial solution volume) at 25 °C.
The percentage removal of the refractory sulfur compounds in all the ACs followed the order: DBT > BT > T. The treated ACs showed at least 80% percentage removal or greater of DBT and this is due to the size of the molecule and acidic functional groups on the surfaces of the larger pores which contributes to polar interactions [42,43]. Dispersive interactions between the delocalized π -electrons within the benzene rings of DBT and the electron rich region of the nanoporous carbon aromatic ring also play a major role in the adsorption. The results also confirm the influence of treatment conditions on the overall performance in adsorptive studies. 3.3. Breakthrough curves
Fig. 6. Influence of adsorbent dosage AC-HNO3 -90 °C (a) and AC-NaOH-60 °C (b) on the adsorptive removal of thiophene, BT and DBT (initial concentrations 51, 50 and 52 ppm respectively, 20 ml volume of solution and 30 min contact time).
The decrease in the efficiency of the AC-NaOH-90 °C compared with AC-NaOH-60 °C can be attributed to the increase in irregularity of graphitic layer structures of AC treated with the base at 90 °C as it has been discussed under the section of XRD results (see Fig. 3). On the other side, the adsorption efficiency of the AC treated with acid increases with treatment temperature. This trend is in consistence with the XRD patterns of these samples, which indicated the improvement in the crystallinity and graphitic structure of the AC (see Fig. 4). From this point of view, it is conceivable that increasing behaviors of the adsorption by acid-treated AC could reflect the contribution of π –π interaction between the basal plane of carbon and the aromatic ring of the adsorbate [39,40].
The performance of AC-NaOH-60 °C and AC-HNO3 -90 °C were further studied in a fixed bed mode. Breakthrough curves were generated by plotting transient total sulfur concentration normalized by the feed total sulfur concentration (C/C0 ) versus cumulative time. The breakthroughs curves for thiophene, BT and DBT in the base and acid treated samples are depicted in Fig. 8a–b. The starting concentrations of thiophene, BT and DBT were 50 ppm, 51 ppm and 52 ppm respectively. The acid treated sample showed higher breakthrough times compared to the base treated sample. DBT was lowered to 0 ppm after 5 min of adsorption in all the samples tested; however, breakthrough was not achieved even after 400 min of adsorption. This indicates the suitability of acid treatment in the adsorption of this compound. A breakthrough was achieved after 180 min for DBT using AC-NaOH60 °C. The breakthroughs of thiophene and BT in both AC-NaOH-60 °C and AC-HNO3 -90 °C were achieved after 5 min and 10 min respectively. For AC-NaOH-60 °C, thiophene returned to its initial concentration after the first 120 min while BT returned to its initial concentration after 50 min signifying no further adsorption that has taken place after this time. This is in contrast with the AC-HNO3 -90 °C performance, both thiophene and BT did not reach their initial concentration even after 400 min of the column adsorption process. The adsorption capacities in column mode for all the adsorbents followed the order DBT > BT > T. This means that AC treated with HNO3 is good in both batch and fixed bed mode. 3.4. Regeneration of the used adsorbent The synthesized activated carbon (AC-HNO3 -90C) was evaluated for its regeneration abilities. The regeneration was done by heating the adsorbent in air for 3 h at 350 °C. The temperature is said to vaporize all the sulfur compounds from the adsorbent. As results showed in Fig. 9, the percentage removal of the sulfur
Please cite this article as: T.A. Saleh, G.I. Danmaliki, Influence of acidic and basic treatments of activated carbon derived from waste rubber tires on adsorptive desulfurization of thiophenes, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.11.008
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obtained for AC-HNO3 at 90 °C. The adsorption of dibenzothiophene, benzothiophene and thiophene by the AC-HNO3 90 °C were higher than that of the raw AC by about 2.2, 3.4 and 3.8 times respectively. This can be explained according to Lewis acid–base theory, most thiophene sulfur compounds are Lewis base, which are easy to be adsorbed at acid sites. The study has the advantages of solving doubly environmental problems by converting such negative-value waste tires to value-added activated carbon for adsorptive desulfurization toward clean fuel. Acknowledgments The authors gratefully acknowledge the support provided by King Fahd University of Petroleum & Minerals (KFUPM). This project was funded by the National Plan for Science, Technology and Innovation (MAARIFAH)—King AbdulAziz City for Science and Technology— through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM)—the Kingdom of Saudi Arabia, award number (13-PET393-04). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2015.11.008. References
Fig. 8. Breakthrough curve for AC-NaOH-60 °C (a) and AC-HNO3 -90 °C (b).
Fig. 9. Regeneration efficiency of AC-HNO3 -90 °C on the adsorption of sulfur compounds.
compounds showed some decrease in the adsorptive capacity of the adsorbent. However, it still showed great promise in the removal DBT even after three cycles of regeneration. The order of desulfurization after three regeneration cycles followed the sequence: DBT (59%) > T (6%) > BT (5%). 4. Conclusion Waste tires can be effectively used as a raw material for the preparation of high-surface area AC after treatment. Upon treatment with NaOH or HNO3 , the surface area and functional groups, such as carboxyl, lactone and phenol groups were enhanced. The effect of the treatment temperature on the adsorption performance was limited. The highest total acidic functional groups of 2.39 mmol/g was
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Influence of acidic and basic treatments of activated carbon derived from waste rubber tires on adsorptive desulfurization of thiophenes Tawfik A. Saleh a,∗, Gaddafi I. Danmaliki b a b
Chemistry Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia Environmental Science Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 6 June 2015 Revised 8 November 2015 Accepted 13 November 2015 Available online xxx Keywords: Adsorptive desulfurization Fixed bed Waste tires Carbon
a b s t r a c t This work reports on the influence of treatment conditions on the waste tire-derived activated carbon for adsorptive desulfurization. The rubber tires were carbonized and activated. The obtained activated carbon (AC) was treated with HNO3 or NaOH at a temperature range of 30–90 °C. The morphology and surface properties of AC were characterized by surface pH, Boehm’s titration, N2 adsorption–desorption isotherms, Fouriertransform infrared spectroscopy, X-ray diffraction, and scanning electron microscope. The AC sample, treated with HNO3 at 90 °C, possess the highest surface oxygen containing functional groups (2.39 mmol/g), surface area (473.35 m2 /g) and pore volume (0.70 cm3 /g) and the more adsorption capacity to the refractory sulfur compounds. The Boehm’s titration experiments indicated that the amount of surface oxygen containing functional groups on the surface of the acid-treated AC increases with treatment temperatures. Acid-treated AC at 90 °C proves to be optimum for adsorptive desulfurization with the order of dibenzothiophene > benzothiophene > thiophene. © 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Combustion of fossil fuels is associated with the release of toxic gases that pose serious environmental and health related impacts. Sulfur and its refractory compounds found in fossil fuels are extremely reactive. They cause catalyst poisoning and corrode refining equipment. In order to reduce air pollution and technological problems, EU and US have enacted laws to regulate sulfur emissions to 10 and 15 ppm respectively [1,2]. Various technologies are currently in use to lower the organic sulfur compounds in fuels, these methods include: hydrodesulphurization (conventional method), oxidative desulfurization, ionic liquids desulfurization, adsorptive desulfurization and biodesulfurization [3–6]. Of all these technologies, adsorptive desulfurization, one of the most promising technologies is receiving much attention because of its facile and mild operating conditions. Much attention is focused on the development of porous adsorbents such as activated carbon, zeolites, alumina, and zirconia that are cheap, easily regenerated and possesses high selectivity for sulfur compounds [7–11]. The surface areas and porosities of AC are greatly influenced by the parent material and the method employed in the production. Better adsorption capacities and adsorption rates of AC are directly linked to
∗
Corresponding author. Tel.: +966 13 860 1734; fax: +966 13 860 1734. E-mail address: tawfi[email protected], tawfi[email protected] (T.A. Saleh). URL: http://faculty.kfupm.edu.sa/CHEM/tawfik/ (T.A. Saleh)
larger surface areas and pore size distributions [12]. Mesoporous and microporous volumes of AC also play a crucial role in the adsorption of larger molecules. In general, the selective adsorption of inorganic molecules on the surface of AC directly depends on the amount of oxygen containing complexes. The oxygen containing complexes are mostly created using dry or wet oxidation methods [13–15]. Adsorptive capability of AC modified with HNO3 at 120 °C was increased significantly and modification using HNO3 even at high temperatures showed a promising result in the removal of thiophenes from oil [16,17]. The aim of this work was to produce AC from end of life tires as a free-of-cost source, and to treat the AC using HNO3 and NaOH at various temperatures. The influence of acid/base treatment of AC for the removal of thiophenes was examined. The adsorption performance of the AC was evaluated. 2. Experimental 2.1. Development of the adsorbent Waste rubber tires were cleaned by removing the iron wires from the tire. Then, the waste rubber was cut into small pieces and thoroughly washed with deionized water, and then dried in an oven at 110 °C. The elemental analysis of the waste rubber indicated their initial elemental composition as 82.3 carbon, 9.2 oxygen, 0.7 nitrogen, 2.8 sulfur, 3.8 silicon and 1.2 silicon and silicon with different wt. %.
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The granules were heated up to 300 °C to separate the black tire crude oil, distilled diesel oil and produced oil. Pyrolysis was carried out in a muffle furnace at 500 °C for 5 h to isolate the ash and carbon black from pyro-gas and other oils, followed by treatment with hydrogen peroxide solution to oxidize all adhering organic impurities. The material was washed with deionized water and dried in vacuum oven at 110 °C overnight. The char was then placed in the stainless tube, inserted into the furnace, heated under nitrogen flow through the sample bed at 200 ml/min at 20 °C/min to 900 °C and a holding time of 5 h. Then the reactor was cooled down to room temperature before the sample was removed, and kept in an oven overnight, then grounded to small particles using pestle and mortar. The produced black granules were sieved and the powdered carbon with a particle size of < 0.1 mm was used to allow good surface contact with acid or base reagent solution. Chemical activation was conducted on the prepared material using 4 M HNO3 and 4 M NaOH at three different temperatures respectively (30, 60, and 90 °C) and a duration of 3, 6 and 12 h. Then, the mixture was filtered and the obtained activated carbon (AC) was washed with deionized water and dried overnight. It should be mentioned that increasing the refluxing time from 3 h to 12 h has an overall net negative effect on the removal of the refractory sulfur compounds (see supporting information file, section S3.1.). Therefore, 3 h refluxing time was selected. 2.2. Boehm’s method for surface functional groups The amount of oxygen surface functional groups (both acidic and basic) was determined according to modified Boehm’s method [18]. 0.5 g of raw and treated carbon samples were added to beakers each containing 25 ml of the following 0.05 M solutions: NaOH, Na2 CO3 , NaHCO3 and HCl. The beakers were sealed and shaken for 24 h and then 10 ml of each filtrate was pipetted in an excess of 20 ml 0.05 M HCl for the determination of acidic functional groups or NaOH for the basic functional groups. The filtrate was titrated with 0.05 M NaOH or HCl, respectively, using phenolphthalein indicator and the volume required to reach the endpoint was noted. For Na2 CO3 reaction bases an excess of 30 ml 0.05 M HCl was added rather than 20 ml due to diprotic property of the base to ensure complete reaction with acid. The number of acidic sites was calculated under the assumption that NaOH neutralizes carboxylic, phenolic, and lactonic groups; Na2 CO3 neutralizes carboxylic and lactonic groups; and NaHCO3 neutralizes only carboxylic groups. The number of surface basic sites was calculated from the amount of hydrochloric acid required. The following formula was used to calculate the amount of surface acidic and basic groups neutralized with NaOH and HCl respectively [19]:
Moles of Carbon functionality ([R a or b] Vr − ([X] Vx − [T ] Vt )) = m where [R a or b] and Vr denote the concentration and volume of the reaction base or acid in mol/L and liters respectively. [X] and Vx denote the concentration and volume of the excess acid or base added to the aliquot. [T ] and Vt denote the concentration and volume of the titrant required to reach the endpoint. m stands for the mass of the carbon used in grams. The amount of surface acidic groups neutralized by Na2 CO3 and NaHCO3 was calculated using the following formula [20,21]:
ncsf =
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nHCl VB [B]VB − ([HCl]VHCl − [NaOH]VNaOH ) nB Va
where ncsf denotes the moles of carbon surface functionalities on the surface of the carbon that reacted with base during the mixn ing step. nHCl = molar ratio of acid to base [B] and VB are the conB centration and volume of the reaction base mixed with the carbon. [HCl] and VHCl are the concentration and volume of the acid added to
the aliquot taken from the original sample. [NaOH]VNaOH is the concentration and volume of the titrant in the back titration. Va is the volume of the aliquot taken from VB . Surface pH of the carbon samples was tested by HI 3512 bench-top pH meter equipped with a graphic LCD display to gain insight about the acidity and basicity of the samples. A suspension of 0.2 g of each sample was added to 10 ml of water and the suspension was stirred overnight to reach equilibrium. The samples were then filtrated and the pH of the each solution was measured. 2.3. Characterization of the adsorbent The FT-IR spectra of the samples were recorded using Nicolet 6700 spectrometer (Thermo electron, USA) with a resolution of 2.0 cm–1 well equipped with deuterated triglycine sulfate detector and an OMNIC program. The experiments were conducted on the powdered samples ground in an agate mortar to produce KBr pellets and spectra were obtained by adding 64 scans and corrected for the background noise. The spectra of the samples were recorded in transmission mode and the wavenumber range of 4000–400 cm–1 . Scanning electron microscopy (SEM) analysis of the treated samples was conducted by using Hitachi model S- 3500 N instrument. Energy dispersive X-ray spectroscopy (EDX) analysis was conducted using Oxford Instrument (England) and X-Max detector to determine the elemental composition of the treated samples. X ray diffraction patterns of the adsorbents were taken using (Rigaku Miniflex II desktop X-ray diffractometer) using Cu-Kα´ radiation and an X-ray gun operated at 40 kV (voltage) and 200 mA current. Data was collected from 2θ = 10°– 80° at a scan rate of 4°/min. The porous structure of the AC samples was characterized by adsorption/desorption of nitrogen at (−196 °C) on a Micromeritics ASAP 2020 surface area and porosimetry analyzer (Micromeritics, USA) to determine the surface area (BET) and pore volume of the treated sorbents. 2.4. Adsorption experiment 2.4.1. Batch mode In a typical run in the batch mode adsorption studies, various amounts, in the range between 0.1 and 0.5 g of adsorbents were introduced into 15 ml of the fuel solution containing thiophene (T), benzothiophene (BT) and dibenzothiophene (DBT) in hexane (85%) and toluene (15%) with initial concentration of 50 ppm each. The sulfur removal efficiency of the AC was calculated using the formula below:
x = (C0 − Ce )/C0 ∗ 100%
(1)
where x x = sulfur removal percentage (%) C0 = initial concentration of sulfur in model is (ppmw), Ce = final sulfur concentration. The amount of sulfur compounds adsorbed per unit mass of the adsorbent at equilibrium (qe mg/g) and at any time t (qt mg/g) termed as the adsorption capacity was calculated from the formulas:
qe =
V (C0 − Ce ) m
(2)
qt =
V (C0 − Ct ) m
(3)
where V (L) is the volume of the liquid phase, C0 (mg/L) is the initial concentration of the sulfur compounds before they come in contact with the adsorbent, Ce and Ct (mg/L) are the concentration at equilibrium and at any time t, and m (g) is the amount of the adsorbent. 2.4.2. Break-through experiments Fixed-bed flow system was used to test the simultaneous adsorptive desulfurization of the treated AC samples using a column system.
Please cite this article as: T.A. Saleh, G.I. Danmaliki, Influence of acidic and basic treatments of activated carbon derived from waste rubber tires on adsorptive desulfurization of thiophenes, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.11.008
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Table 1 Surface pH, total basic and acidic groups on the raw and treated AC. Sample
Raw AC AC-NaOH 30 °C AC-NaOH 60 °C AC-NaOH 90 °C AC-HNO3 30 °C AC-HNO3 60 °C AC-HNO3 90 °C
Surface pH
4.92 10.66 10.18 10.69 4.21 4.05 3.09
Total basic groups (mmol/g)
1.62 2.48 1.98 2.46 1.37 1.36 1.33
The adsorbent was packed inside the column. The model fuel sample was then passed through the column by a peristaltic pump with a controlled flow rate (150 rpm) at a temperature of 25 °C. Once the adsorption process started, treated fuels were periodically sampled at different time intervals for analysis. The experiments were repeated three times and the relative standard deviation was calculated and found to be < 1.5%. 2.5. Analysis The concentrations of the refractory sulfur compounds were analyzed by gas chromatography sulfur chemiluminescence detector (GC-SCD) (Model 7890A) system (Agilent) equipped with auto sampler (7693) and splitless injector. The column used was Agilent 19091S-001: 2638-45555 50 m × 0.2 mm dimensions and a film thickness of 0.5 μm methyl siloxane stationary phase. The injection volume was 1 μl with a flow rate of 0.839 ml/min. Inlet temperature was a 60 °C and oven program of 60 °C for 1 min, then 10 °C/min to 230 °C for 2 min. Other parameters and conditions of the method are listed in the Supporting information file, Table S1. 3. Results and discussion 3.1. Characterization 3.1.1. Boehm’s titration The acidic functional groups are due to the presence of phenols, lactols, lactones and carboxylic acids. The acidic groups are said to differ in their acidities that can be determined by reaction with 0.05 M NaOH, Na2 CO3 and NaHCO3 respectively [17,22]. Phenols are neutralized with NaOH, while lactones and carboxylic acids are neutralized by Na2 CO3 , and NaHCO3 neutralizes only carboxyl groups. The raw AC is acidic with relatively high amount of phenolic groups and total acidic groups of 1.71 mmol/g. However, upon treatment with NaOH at 30 °C the total acidic groups decreased to 1.23 mmol/g but the phenolic groups were greater than that of the raw samples (Table 1). The increase in phenolic groups can be attributed to the deformation of the lactone groups resulting in the formation of more phenolic groups [23]. This can be clearly observed by comparing the amount of the lactone groups on the base-treated ACs with that of the acidtreated ACs. Moreover, for the AC treated with NaOH at 90 °C, the carboxylic groups were reduced due to its significant deformation; but only the phenolic and lactonic groups were determined which contributed less acidity. The total basic functional groups on the surface of the AC-NaOH 30 °C were higher in all the samples modified. Although the influence of the temperature was not that significant, it could be noticed that the treatment at 60 °C seems to be the best temperature for NaOH modification giving the highest surface acidic functional groups (1.72 mmol/g) and the lowest surface basic groups (1.98 mmol/g). On the other side, the increase in the total acidity of the acid-treated ACs can be ascribed to the introduction of the new acidic functional groups on the surface of the ACs [17,22]. Total acidic functional groups increased with treatment temperature from
Surface acidic groups (mmol/g) Phenol
Lactone
Carboxyl
Total
0.81 1.07 1.21 1.13 0.80 0.82 0.69
0.22 0.16 0.33 0.25 0.64 0.71 0.81
0.68 0.00 0.18 0.00 0.67 0.69 0.89
1.71 1.23 1.72 1.38 2.11 2.22 2.39
2.11 to 2.22 and finally 2.39 mmol/g for AC-HNO3 30, 60 and 90 °C respectively, and the total basic groups slightly reduced from 1.37 to 1.36 and finally to 1.33 mmol/g for the same samples respectively. 3.1.2. FTIR spectroscopy FTIR spectra of the raw, HNO3 , and NaOH treated AC at various temperatures (30, 60 and 90 °C) were taken to understand the functional groups present in the AC and they are given in Fig. 1. The Raw AC, NaOH and HNO3 treated AC have displayed varying degrees and intensities of their bands though they all have similar patterns but the bands increase with in treatment temperature. This implies that temperature has an effect on the development of oxygen containing functional groups and acidic and basic sites on the surface of the AC [24,25]. It can be seen that the samples showed a broad peak centering around 3430 cm−1 that can be ascribed to O–H stretching vibrations of hydroxyl groups or to chemisorbed water. There is a noticeable development peak at 2854 cm−1 and 2924 cm−1 as the temperature increases and these peaks can be due to the presence of aliphatic C–H stretch of CH, CH2 and CH3 . The peak at 1720 cm−1 is attributed to the carboxyl group. The peak at 1640 cm-1 is usually attributed to the carbonyl group. The peak around 1400 cm−1 appeared in HNO3 treated AC, may be attributed to the presence of C=O stretching vibrations of the C=O, and C–O [19,26]. This confirms the influence of temperature on the development of carbonyl groups [27]. The NaOH treated AC at (30, 60 and 90 °C) have shown similar patterns but the intensities are not well developed as compared to the HNO3 treated AC. The results also confirm the influence of temperature and HNO3 on the development of surface functional groups such as nitrate, carboxyl, and carbonyl groups. In addition to the absorption peaks already discussed, the peaks around 2200–2400 cm−1 are almost certainly due to residual CO2 , either in the chamber or adsorbed to the surface. The peaks at 500–700 cm–1 are noisy and difficult to attribute. 3.1.3. SEM/EDX result on the treated AC The SEM/EDX result of the treated AC is depicted in Fig. 2. The treatment with HNO3 at 30, 60, and 90 °C showed the presence of carbon and oxygen as the only elemental composition of the adsorbents and they showed a significant improvement trend in the weight% of oxygen from 11.01 to 11.86 and finally 14.39 meaning that oxygen containing functional groups kept on increasing with temperature upon acid treatment. This result is in accordance with the result obtained from the FT-IR, Boehm’s titration experiment and XRD results. EDX results on the base treated samples showed the presence of carbon, oxygen and sodium in different proportions. Increase in treatment temperature shows significant reduction in the wt. % of oxygen from 18.33 to 17.72 and finally 13.63 for AC-NaOH 30 °C, 60 °C and 90 °C respectively. 3.1.4. XRD patterns of the treated AC X-ray diffraction was conducted for the raw obtained AC, basetreated AC and acid-treated AC samples (Figs. 3 and 4). The X-ray
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Fig. 1. FTIR spectra of raw AC and AC samples treated with sodium hydroxide or nitric acid at various temperatures.
diffraction profiles show two broad diffraction peaks of graphitic carbon with maxima at around 25° and 42° two theta. The two peaks are indexed to (002) and (100) or (101) planes of crystalline hexagonal graphite lattice respectively (JCPDS card no. 41-1487) [28,29]. The relative intensities increased upon treatment signifying the improvement in the degree of graphitization structure and crystallinity of the treated AC in addition to the removal of amorphous carbons [7,30]. The peak, at approximately 25° two theta, was broadened on increase of temperature, and this change in the width of the peak can be attributed to the widening or expansion of the graphite layers in the individual crystalline (disintegration to relatively smaller crystallites). It can be seen in Fig. 3 that the intensity of the peak, at approximately 25° two theta, of AC treated with the base decreased with increasing temperature from 60 to 90 °C indicating the induced irregularity into the layered graphitic structure of the AC after the activation process [31]. 3.1.5. Brunauer–Emmett–Teller (BET) surface area and porosity studies Nitrogen adsorption desorption isotherm is a useful tool in the characterization of carbon materials. AC samples express Type I isotherm according to Brunauer et al., 1940 [32,33], See Fig. S1 in the Supporting information file. A hysteresis loop can be observed in all the samples at the relative high pressure indicating the presence of mesopores. As shown in Table 2, the surface area and pore volumes were significantly enhanced upon NaOH and HNO3 treatments. The N2 adsorption capacity followed the order: Raw < AC-NaOH 30 °C < AC-NaOH 90 °C < AC-NaOH 60 °C < AC-HNO3 -30 °C < ACHNO3 -60 °C < AC-HNO3 -90 °C. Temperature also enhanced the development of the surface area in the acid treated AC. The change in surface area caused by different treatments is summarized in Table 2. It can be clearly seen that AC-NaOH-60 °C sample has the highest surface area (369 m2 /g), pore volume (0.69 cm3 /g) and pore size ˚ in the base treated AC. However, AC-HNO3 -90 °C has the (75.03 A) highest surface area (473.35 m2 /g) and pore volume (0.70 cm3 /g) of all the samples used which confirms the significance of acid treatment of AC in the development of surface area and porosity. The results are in agreement with the results obtained from the XRD, FT-IR and Boehm’s experiment.
can be seen that the removal is very rapid, nearly 95% of all the adsorption process occur in the first 5 min of the process. This is because of the available free sites for adsorption of the sulfur compounds at the beginning of the adsorption process. Equilibrium was achieved at around 30 min of adsorption process and afterwards the adsorption of the refractory sulfur compounds was noticed to decline due to the desorption process of the analytes from the adsorbent. The size of the adsorbent, concentration of the sulfur compounds, degree of mixing, the affinity of the adsorbent to the sulfur compounds and the diffusion coefficient of the adsorbent in bulk and solid phases play a cardinal role in the percentage of sulfur compounds adsorbed on the surface of the adsorbent [34]. The adsorption capacity followed the order: DBT > BT > T. The vast majority of the area of the samples is mesoporous, which makes it easier for adsorption at the initial time, however, when the contact time increases the refractory sulfur compounds are faced with much larger resistance to pass through the micropores. 3.2.2. Influence of adsorbent dosage on the performance of AC-HNO3 -90 °C and AC-NaOH-60 °C Adsorbent dosage is an important parameter in adsorptive studies because it determines the maximum adsorptive capacity of a given adsorbent using a starting concentration of the analyte of interest. Best NaOH and HNO3 treated samples were selected for testing the effect of adsorbent dosage. The effect of AC-HNO3 -90 °C and AC-NaOH60 °C dosages on the uptake of refractory sulfur compounds (Thiophene 52 ppm, BT 50 ppm and DBT 51 ppm at room temperature) were studied and the results are shown in Fig. 6a–b. The mass of the adsorbent was varied from 0.1 g to 0.5 g and the amount of refractory sulfur compounds adsorbed was found to be increasing with adsorbent dosage. The increase in adsorption capacity of sulfur compounds with dosage is expected because more active surface areas and sites for sulfur adsorption are introduced in the medium [35,36]. The removal efficiency after 0.25 g was less, becoming almost constant after 0.5 g; due to this, we fixed the rest of the experiments at an adsorbent dosage of 0.5 g. The trend of adsorption of the refractory sulfur compounds followed the order of DBT > BT > T. DBT was removed higher than all other refractory sulfur compounds which can be attributed to the dispersive interactions formed between the AC and DBT and due to the presence of double benzene rings in DBT [37,38].
3.2. Adsorption evaluation 3.2.1. Contact time The influence of contact time on the efficiency of AC-HNO3 -90 °C on the removal of refractory sulfur compounds is depicted in Fig. 5. It
3.2.3. Effect of treatment reagent The raw and treated AC both with NaOH and HNO3 were compared for their desulfurization efficiency. 0.5 g of each sample was taken and added to 50 ppm mixture of thiophene, BT and DBT in
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Fig. 2. SEM image and EDX spectrum with an elemental analysis table of the prepared carbon treated with nitric acid at 30 °C (a), 60 °C (b), 90 °C (c), sodium hydroxide at 30 °C (d), 60 °C (e), 90 °C (f).
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Fig. 3. XRD pattern of Raw AC and NaOH treated AC at 30, 60 and 90 °C.
Fig. 4. XRD pattern of AC treated with HNO3 at 30, 60 and 90 °C. Table 2 Surface area and pore volume analysis. Adsorbents
BET surface area (m2 /g)
Pore volume (cm3 /g)
˚ Pore size (A)
Raw AC AC-NaOH 30 °C AC-NaOH 60 °C AC-NaOH 90 °C AC-HNO3 30 °C AC-HNO3 60 °C AC-HNO3 90 °C
183.21 220.48 369.27 268.11 429.26 454.80 473.35
0.30 0.44 0.69 0.47 0.66 0.67 0.70
77.54 79.83 75.03 70.47 62.10 59.60 60.22
20 ml (85% hexane and 15% toluene). The set ups were stirred and samples were taken after 30 mins and analyzed with GC-SCD. The result is shown in Fig. 7 and the percentage removals of thiophene, BT and DBT from the raw and treated samples at various temperatures were reported. It can be clearly seen that raw sample performed the least. AC-NaOH-60 °C showed the best performance in the base treated samples and this can be attributed to the presence of oxygen containing functional groups, high surface area, pore volume, and pore size exhibited by the sample compared with other base treated samples. AC-HNO3 -90 °C showed the best performance regardless of treatment conditions because it showed the highest total acidic groups, the largest intensity in XRD analysis, and the highest
in % transmittance in FT-IR. It also possesses the highest surface area and pore volume. It should be noted that the reagent type plays a role while the temperature has a subtle effect. The adsorbents performance and adsorptive capacities followed the order: AC-HNO3 90 °C > AC-HNO3 -60 °C > AC-HNO3 -30 °C > AC-NaOH-60 °C > ACNaOH-90 °C > AC-NaOH-30 °C > Raw AC. This order is consistent with the increase in the surface area and pore volume of the adsorbents and their total acidic groups [40,41]. This implies that both surface area and surface chemistry play an important role in adsorption capacity. The adsorption capacities were calculated and listed in Table S2.
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Fig. 5. Influence of contact time on removal of thiophene, BT and DBT by AC-HNO3 90 °C (adsorbent dosage 0.5 g, 20 ml volume of model fuel containing 52 ppm thiophene, 50 ppm BT and 51 ppm DBT). Fig. 7. Percentage removal of thiophene, BT, and DBT (initial concentration of 50 ppm each) on acid and base treated AC samples after 30 min (dosage of each sample was 0.5 g in 20 ml initial solution volume) at 25 °C.
The percentage removal of the refractory sulfur compounds in all the ACs followed the order: DBT > BT > T. The treated ACs showed at least 80% percentage removal or greater of DBT and this is due to the size of the molecule and acidic functional groups on the surfaces of the larger pores which contributes to polar interactions [42,43]. Dispersive interactions between the delocalized π -electrons within the benzene rings of DBT and the electron rich region of the nanoporous carbon aromatic ring also play a major role in the adsorption. The results also confirm the influence of treatment conditions on the overall performance in adsorptive studies. 3.3. Breakthrough curves
Fig. 6. Influence of adsorbent dosage AC-HNO3 -90 °C (a) and AC-NaOH-60 °C (b) on the adsorptive removal of thiophene, BT and DBT (initial concentrations 51, 50 and 52 ppm respectively, 20 ml volume of solution and 30 min contact time).
The decrease in the efficiency of the AC-NaOH-90 °C compared with AC-NaOH-60 °C can be attributed to the increase in irregularity of graphitic layer structures of AC treated with the base at 90 °C as it has been discussed under the section of XRD results (see Fig. 3). On the other side, the adsorption efficiency of the AC treated with acid increases with treatment temperature. This trend is in consistence with the XRD patterns of these samples, which indicated the improvement in the crystallinity and graphitic structure of the AC (see Fig. 4). From this point of view, it is conceivable that increasing behaviors of the adsorption by acid-treated AC could reflect the contribution of π –π interaction between the basal plane of carbon and the aromatic ring of the adsorbate [39,40].
The performance of AC-NaOH-60 °C and AC-HNO3 -90 °C were further studied in a fixed bed mode. Breakthrough curves were generated by plotting transient total sulfur concentration normalized by the feed total sulfur concentration (C/C0 ) versus cumulative time. The breakthroughs curves for thiophene, BT and DBT in the base and acid treated samples are depicted in Fig. 8a–b. The starting concentrations of thiophene, BT and DBT were 50 ppm, 51 ppm and 52 ppm respectively. The acid treated sample showed higher breakthrough times compared to the base treated sample. DBT was lowered to 0 ppm after 5 min of adsorption in all the samples tested; however, breakthrough was not achieved even after 400 min of adsorption. This indicates the suitability of acid treatment in the adsorption of this compound. A breakthrough was achieved after 180 min for DBT using AC-NaOH60 °C. The breakthroughs of thiophene and BT in both AC-NaOH-60 °C and AC-HNO3 -90 °C were achieved after 5 min and 10 min respectively. For AC-NaOH-60 °C, thiophene returned to its initial concentration after the first 120 min while BT returned to its initial concentration after 50 min signifying no further adsorption that has taken place after this time. This is in contrast with the AC-HNO3 -90 °C performance, both thiophene and BT did not reach their initial concentration even after 400 min of the column adsorption process. The adsorption capacities in column mode for all the adsorbents followed the order DBT > BT > T. This means that AC treated with HNO3 is good in both batch and fixed bed mode. 3.4. Regeneration of the used adsorbent The synthesized activated carbon (AC-HNO3 -90C) was evaluated for its regeneration abilities. The regeneration was done by heating the adsorbent in air for 3 h at 350 °C. The temperature is said to vaporize all the sulfur compounds from the adsorbent. As results showed in Fig. 9, the percentage removal of the sulfur
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obtained for AC-HNO3 at 90 °C. The adsorption of dibenzothiophene, benzothiophene and thiophene by the AC-HNO3 90 °C were higher than that of the raw AC by about 2.2, 3.4 and 3.8 times respectively. This can be explained according to Lewis acid–base theory, most thiophene sulfur compounds are Lewis base, which are easy to be adsorbed at acid sites. The study has the advantages of solving doubly environmental problems by converting such negative-value waste tires to value-added activated carbon for adsorptive desulfurization toward clean fuel. Acknowledgments The authors gratefully acknowledge the support provided by King Fahd University of Petroleum & Minerals (KFUPM). This project was funded by the National Plan for Science, Technology and Innovation (MAARIFAH)—King AbdulAziz City for Science and Technology— through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM)—the Kingdom of Saudi Arabia, award number (13-PET393-04). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2015.11.008. References
Fig. 8. Breakthrough curve for AC-NaOH-60 °C (a) and AC-HNO3 -90 °C (b).
Fig. 9. Regeneration efficiency of AC-HNO3 -90 °C on the adsorption of sulfur compounds.
compounds showed some decrease in the adsorptive capacity of the adsorbent. However, it still showed great promise in the removal DBT even after three cycles of regeneration. The order of desulfurization after three regeneration cycles followed the sequence: DBT (59%) > T (6%) > BT (5%). 4. Conclusion Waste tires can be effectively used as a raw material for the preparation of high-surface area AC after treatment. Upon treatment with NaOH or HNO3 , the surface area and functional groups, such as carboxyl, lactone and phenol groups were enhanced. The effect of the treatment temperature on the adsorption performance was limited. The highest total acidic functional groups of 2.39 mmol/g was
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