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Journal of Hazardous Materials 279 (2014) 296–301 Contents lists available at ScienceDirect Journal of Hazardous Materials jo ur nal ho me p ag e: www.elsevier.com/locate/jhazmat Oxidation of benzothiophene, dibenzothiophene, and methyl-dibenzothiophene by ferrate(VI) Abdullah Al-Abduly a , Virender K. Sharma b,a Department of Chemical Engineering and Advanced Materials, Newcastle University, Newcastle Upon Tyne NE1 7RU, United Kingdom b Department of Environmental and Occupational Health, School of Public Health, Texas A&M University, 1266 TAMU, College Station, TX 77843, USA h i g h l i g h t s Ferrate(VI) removed thiophenes completely in acetonitrile–phosphate buffer solution. The maximum removal efficiency was at pH 8.0. Thiophenes were oxidized by ferrate(VI) to sulfones. Ferrate(VI)-silica had higher removal efficiency than that of ferrate(VI) alone. a r t i c l e i n f o Article history: Received 23 April 2014 Received in revised form 11 June 2014 Accepted 27 June 2014 Available online 17 July 2014 Keywords: Ferrate Degradation Removal Thiophenes Sulfur Adsorption a b s t r a c t The reduction of sulfur content in liquid fuel is of a high concern environmentally, and oxydesulfurization approaches have shown high efficiency for removing thiophene-containing compounds from the liquid fuels. The present paper investigates the oxidation of benzothiophene (BT), dibenzothiophene (DBT), and 4-methyl-dibenzothiophene (4-MDBT) by ferrate(VI). The effects of reaction conditions such as the reaction medium pH, solvent type, and adsorbent on the reactivity of ferrate(VI) with the thiophene- containing compounds were investigated. The oxidation of DBT in phosphate–acetonitrile medium was found to be highly sensitive toward the reaction pH, and the highest removal efficiency was observed at the pH 8.0. The complete conversion of BT and DBT to their corresponding sulfones by ferrate(VI) was achieved at room temperature and [ferrate(VI)]/[BT/DBT] 7.5 while this molar ratio was found to be 8.5 for 4-MDBT. The addition of silica gel during the reaction was applied to enhance the oxidation of DBT by ferrate(VI). © 2014 Elsevier B.V. All rights reserved. 1. Introduction Fossil fuels are the most common source of energy in the world. More than 82% of energy comes from fossil fuels of which half comes from petroleum [1]. Crude oil is among the largest source of energy and the sulfur content in the crude oil derives its value. Sulfur compounds are undesirable in refining process because they cause problems such as catalysts deactivation and corrosions in pumping, pipelines, and refining instruments. Sulfur compounds also cause emission of sulfur oxide gases, which on reaction with water form sulfates and acid rain resulting in adverse effects on the environment, artifacts, and human health [1]. In crude oil sulfur exist in different forms such as sulfides, disulfides, mercaptans, and thiophenes. Importantly, more than 85% of the sulfur-containing Corresponding author. Tel.: +1 979 862 4941. E-mail address: [email protected] (V.K. Sharma). compounds in diesel fuel are thiophenes, and above 70% of the thiophene compounds are benzothiophene (BT) and dibenzothio- phene (DBT) [2]. Thiophene is a heterocyclic five-membered ring present as a separate ring, fused to one benzene ring (benzoth- iophene, BT), or two benzene rings (dibenzothiophene, DBT) (Fig. SM-1). In addition, several alkylated forms of DBT such as 4-methyl dibenzothiophene (4-MDBT) and 4,6-dimethyl dibenzothiophene (4,6-DMDBT) are found in liquid fuel (Fig. SM-1). Several methods including hydrodesulfurization (HDS), biodesulfurization (BDS), and oxidesulfurization (ODS) have been tested for fuel desulfurization [1]. The HDS process is based on reducing sulfur compounds to H 2 S over a catalyst by hydrogen at high pressure and temperature. The main drawback of the HDS approach was its low efficiency toward the desulfurization of thiophene compounds particularly the alkylated derivatives of the DBT, due to the alkyl groups, which hinder the effective interaction between the sulfur atoms of these compounds and the catalyst surface. Other concerns such as the severity of operational http://dx.doi.org/10.1016/j.jhazmat.2014.06.083 0304-3894/© 2014 Elsevier B.V. All rights reserved.

Oxidation of benzothiophene, dibenzothiophene, and methyl-dibenzothiophene by ferrate(VI)

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Page 1: Oxidation of benzothiophene, dibenzothiophene, and methyl-dibenzothiophene by ferrate(VI)

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Journal of Hazardous Materials 279 (2014) 296–301

Contents lists available at ScienceDirect

Journal of Hazardous Materials

jo ur nal ho me p ag e: www.elsev ier .com/ locate / jhazmat

xidation of benzothiophene, dibenzothiophene, andethyl-dibenzothiophene by ferrate(VI)

bdullah Al-Abdulya, Virender K. Sharmab,∗

Department of Chemical Engineering and Advanced Materials, Newcastle University, Newcastle Upon Tyne NE1 7RU, United KingdomDepartment of Environmental and Occupational Health, School of Public Health, Texas A&M University, 1266 TAMU, College Station, TX 77843, USA

i g h l i g h t s

Ferrate(VI) removed thiophenes completely in acetonitrile–phosphate buffer solution.The maximum removal efficiency was at pH 8.0.Thiophenes were oxidized by ferrate(VI) to sulfones.Ferrate(VI)-silica had higher removal efficiency than that of ferrate(VI) alone.

r t i c l e i n f o

rticle history:eceived 23 April 2014eceived in revised form 11 June 2014ccepted 27 June 2014vailable online 17 July 2014

eywords:

a b s t r a c t

The reduction of sulfur content in liquid fuel is of a high concern environmentally, and oxydesulfurizationapproaches have shown high efficiency for removing thiophene-containing compounds from the liquidfuels. The present paper investigates the oxidation of benzothiophene (BT), dibenzothiophene (DBT),and 4-methyl-dibenzothiophene (4-MDBT) by ferrate(VI). The effects of reaction conditions such as thereaction medium pH, solvent type, and adsorbent on the reactivity of ferrate(VI) with the thiophene-containing compounds were investigated. The oxidation of DBT in phosphate–acetonitrile medium was

errateegradationemovalhiophenesulfurdsorption

found to be highly sensitive toward the reaction pH, and the highest removal efficiency was observed atthe pH 8.0. The complete conversion of BT and DBT to their corresponding sulfones by ferrate(VI) wasachieved at room temperature and [ferrate(VI)]/[BT/DBT] ∼ 7.5 while this molar ratio was found to be∼8.5 for 4-MDBT. The addition of silica gel during the reaction was applied to enhance the oxidation ofDBT by ferrate(VI).

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Fossil fuels are the most common source of energy in the world.ore than 82% of energy comes from fossil fuels of which half

omes from petroleum [1]. Crude oil is among the largest sourcef energy and the sulfur content in the crude oil derives its value.ulfur compounds are undesirable in refining process because theyause problems such as catalysts deactivation and corrosions inumping, pipelines, and refining instruments. Sulfur compoundslso cause emission of sulfur oxide gases, which on reaction withater form sulfates and acid rain resulting in adverse effects on the

nvironment, artifacts, and human health [1]. In crude oil sulfurxist in different forms such as sulfides, disulfides, mercaptans, andhiophenes. Importantly, more than 85% of the sulfur-containing

∗ Corresponding author. Tel.: +1 979 862 4941.E-mail address: [email protected] (V.K. Sharma).

ttp://dx.doi.org/10.1016/j.jhazmat.2014.06.083304-3894/© 2014 Elsevier B.V. All rights reserved.

compounds in diesel fuel are thiophenes, and above 70% of thethiophene compounds are benzothiophene (BT) and dibenzothio-phene (DBT) [2]. Thiophene is a heterocyclic five-membered ringpresent as a separate ring, fused to one benzene ring (benzoth-iophene, BT), or two benzene rings (dibenzothiophene, DBT) (Fig.SM-1). In addition, several alkylated forms of DBT such as 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyl dibenzothiophene(4,6-DMDBT) are found in liquid fuel (Fig. SM-1).

Several methods including hydrodesulfurization (HDS),biodesulfurization (BDS), and oxidesulfurization (ODS) havebeen tested for fuel desulfurization [1]. The HDS process is basedon reducing sulfur compounds to H2S over a catalyst by hydrogenat high pressure and temperature. The main drawback of theHDS approach was its low efficiency toward the desulfurization

of thiophene compounds particularly the alkylated derivativesof the DBT, due to the alkyl groups, which hinder the effectiveinteraction between the sulfur atoms of these compounds and thecatalyst surface. Other concerns such as the severity of operational
Page 2: Oxidation of benzothiophene, dibenzothiophene, and methyl-dibenzothiophene by ferrate(VI)

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A. Al-Abduly, V.K. Sharma / Journal of

onditions, short life times of the catalysts employed and overallapital costs associated add more challenges toward the HDS.n case of the BDS method, the desulfurization process usuallychieved using biochemical, microbiological, or enzymatic meanso conduct the desulfurization [2–4]. This process has shown highelectivity toward thiophene compounds under milder opera-ional conditions in contrast to the HDS process [5–9]. However,

any challenges were found to affect the performance of theDS approach such as the limited number of enzymes that havehown the capability of removing sulfur atoms from heterocyclicolecules in liquid fuel without causing loss of carbon [4,7].

n addition, the BDS has shown slow reaction rates during theesulfurization of thiophene molecules; this was attributed to the

ow miscibility of bioactive species in organic solvents and to thearmful impact of organic solvents, which affect the growth ofost of the bio-organisms.The ODS process is based on the deep oxidation of thio-

henic compounds to their corresponding polar sulfoxides andulfones which then can be separated from the liquid fuel byxtraction with polar solvents or adsorption [10–14]. In con-rast to the HDS and BDS, the ODS approach has attracted

ore attention during the past few years due to its milderperational conditions, high selectivity and high reactions rates.everal oxidizing reagents were used for conducting the ODSuch as organic and inorganic peroxide acids, ozone, perchlo-ate, and inorganic peroxy-salts (NaClO4, K2CrO7, and KMnO4)1,15].

In recent years, the manipulation of iron-based novel oxidant,errate(VI) (FeVIO4

2−) to oxidize compounds of environmentalnterests is forthcoming [16–19]. The experimental work reportedere stimulated from the work that have been carried out byiu et al. [20] to oxidize sulfur compounds in diesel using fer-ate(VI). The work of Liu et al. [20] showed a low oxidation activityf ferrate(VI) in aqueous-phase, even in the presence of mass-ransfer catalysts in the system. However, a substantial increasen the oxidation capacity of ferrate(VI) was seen in acetic acid,nd that was attributed to the enhancement of acidity distribu-ion within the organic phase. The present paper demonstrateshat the role of acetic acid was not related to acidic condition,ut rather causing aqueous dissolution of ferrate(VI) in order tocquire oxidation ability of the oxidant. Due to the requirement ofsing large amount of acetic acid, and the difficulties of its sep-ration and recycling after liquid fuel treatment, we have usedcetonitrile–phosphate mixture to investigate the performance oferrate(VI) toward the oxidesulfurization. This approach can bemployed in combination with the extraction of sulfur moleculesrom targeted liquid fuel either simultaneously or subsequentlysing acetonitrile. Acetonitrile can be recycled after performinghe required oxidation and reused for extracting the sulfur com-ounds.

The study of Liu et al. [20] also showed enhanced removal of sul-ur compound in the presence of manganese acetate, and that wasttributed to a catalytic effect of the latter compound. However, theuthors have not shown a direct effect of Mn(III) acetate on DBT inhe absence of ferrate(VI), and this was important to consider since

n(III) acetate is a well-known oxidant. Hence, Mn(III) acetate mayot act as a catalyst when it is used in a large amount and consumed

n the reaction system. Therefore, we have studied the oxidationf BT, DBT, and M-DBT by ferrate(VI) with the following aims: (i)emonstrate no catalytic activity of acetic acid and Mn(III) acetate

n oxidation of thiophenes, (ii) seek removal of thiophenes usingqueous phase chemistry of ferrate(VI) without applying additional

hemicals, (iii) identify and quantify oxidized compounds of sul-ur compounds, and (iv) show the improvement of the oxidationbility of ferrate(VI) using a solid surface (silica) in the reactionystem.

rdous Materials 279 (2014) 296–301 297

2. Experimental

2.1. Chemicals

DBT, DBTO2, 4-MDBT, and acetonitrile (HPLC grade) were pur-chased from Aldrich, while BT, Mn(III) acetate, and cyclohexane(99% plus) were obtained from Acros. All compounds were usedwithout further purification. Phosphate buffers were prepared froma sodium salt of phosphate (Na2HPO4) and adjusted with phos-phoric acid or sodium hydroxide to achieve the desired pH. Sodiumtetraborate (Na2B4O7·10H2O) was used for the preparation of theborate buffer. All buffer solutions were prepared using water thathad been distilled and passed through a 18 M� Milli-Q cm waterpurification system. Potassium ferrate (K2FeO4) of high purity(>98%) was synthesized by a wet chemical process [21]. The fer-rate(VI) solutions were prepared by dissolving ferrate(VI) salt intobuffer solution consisting 0.005 M Na2HPO4/0.001 M Na2B4O7 indistilled water, at pH 9.0. The pH of all substrate solutions wasadjusted using either phosphoric acid or sodium hydroxide beforeferrate(VI) addition. Phosphate was used as a pH buffer and as acomplexing reagent for Fe(III), to enhance the stability of ferrate(VI)toward self-decomposition reactions by minimizing the Fe(OH)3precipitation [22]. The concentration of ferrate(VI) was monitoredby measuring the absorbance at the wavelength 510 nm and molarabsorption coefficient of 1150 M−1 cm−1 at pH 9.0 [21]. A calibratedOrion 710A ion selective electrode system equipped with a glass pHelectrode was used for pH measurements.

2.2. Oxydesulfurization experiments

2.2.1. The oxidation of DBT by ferrate(VI) in bifacial systemIn order to examine the reactivity of ferrate(VI) with DBT

in petroleum ether, 12.5 ml of petroleum ether which consisted108.0 �M of DBT was placed into a 50 ml beaker containing a mag-netic stirrer. Then 12.5 ml of freshly prepared ferrate(VI) (753 �M)in buffered solution (0.005 M Na2HPO4/0.001 M Na2B4O7) wasadded to the same beaker. The mixture was stirred rapidly untilall ferrate(VI) color disappeared. The two layers were separated,and the organic layer was subjected to high performance liquidchromatography (HPLC) analysis.

2.2.2. The effect of acetic acid on ferrate(VI) reactivityThe effect of acetic acid on ferrate(VI) reactivity was investigated

by adding 12.5 ml of DBT (108 �M) stock prepared in petroleumether to 12.5 ml of acetic acid, and the solution was mixed beforethe addition of a calculated weight amount of ferrate(VI). The molarratio of [Fe(VI)]/[DBT]0 was fixed at 7.0. The mixture was stirreduntil no ferrate(VI) was seen in the solution. The mixture wasthen filtered and subjected to HPLC analysis, and the obtained DBTconcentration after the treatment with ferrate(VI) was comparedto a sample containing a mixture of 12.5 ml of DBT (108 �M) inpetroleum ether and 12.5 ml of acetic acid (control). The changein concentration between the ferrate(VI) treated sample and thecontrol was determined.

2.2.3. Mn(III) acetate effect on DBT–cyclohexane systemThe Mn(III) acetate effect on DBT oxidation was tested in

the absence of ferrate(VI). A 10.0 ml of cyclohexane containing108.0 �M DBT was mixed with 10.0 ml of acetic acid using mag-netic stirring. A weighed amount of Mn(III) acetate was added to

the solution. The [Mn(III)]/[DBT] was set at 7.0. The mixture wascovered and heated to 35 ◦C and stirred for 34 min. The mixture wasthen allowed to cool down, and the organic layer was collected andsubjected to HPLC analysis. The DBT concentration was compared
Page 3: Oxidation of benzothiophene, dibenzothiophene, and methyl-dibenzothiophene by ferrate(VI)

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o a control sample treated under the same conditions except forhe addition of Mn(III) acetate.

.2.4. The oxidation of DBT by ferrate(VI) in homogenousolutions

Stock solutions of BT, DBT, and 4-MDBT were prepared byissolving weighed amounts of each compound into 60% acetoni-rile in a 0.005 M Na2HPO4/0.001 M Na4B4O7·10H2O. The stockolutions were subjected to further dilutions to obtain desired con-entrations. A fixed volume (12.5 ml) of each thiophenic compoundample was placed into a 50 ml beaker containing a magnetic stir-er, and the pH was adjusted to the desired level. This solutionas mixed with 12.5 ml of freshly prepared ferrate(VI) solution.

he mixed solution was covered with Parafilm and stirred untilo distinctive ferrate(VI) color was seen. The time required forhe treatment was found to vary with the pH of the mixture; fornstance, the mixing time at the pHs 3.0, 8.0 and 9.0 were 2.0,0.0, and 80 min, respectively. The concentrations of BT, DBT, and-MDBT were determined by HPLC.

.2.5. Fe(OH)3 effectFerric hydroxide was produced via the decomposition of

errate(VI) in water (4K2FeO4 + 10H2O → 4Fe(OH)3 + 3O2 + 8OH−)23]). Specific concentrations of ferrate(VI) were prepared in a.005 M Na2HPO4/0.001 M Na2B4O7·10H2O, and 12.5 ml of each ofhese solutions was placed into a 50 ml beaker. The solutions pHas adjusted to pH 9.0 and covered by Parafilm and allowed to reactntil all solutions became brownish in color. A 12.5 ml of 108.0 �MBT prepared in 60% acetonitrile and 0.005 M Na2HPO4/0.001 Ma2B4O7 buffer and was added to each beaker containing 12.5 mlf Fe(OH)3. The mixtures were then re-covered with Parafilmnd mixed for 30 min. The total volume of each solution (25 ml)as subjected to vigorous extraction by 25 ml of cyclohexane for

min. This extraction procedure was found to be the optimal forecovering 99% of non-oxidized DBT from the acetonitrile bufferedolutions. After the separation, the organic layer was collected fromach sample and subjected to analysis.

.2.6. The oxidation of DBT in the presence of silica gelThe catalytic effect of silica gel on ferrate(VI) reaction with DBT

as evaluated. No activation was performed on the silica gel. Theeighted amounts of silica gel were added to a 25 ml of solution

ontaining 12.5 ml of 102 �M of DBT in 60% acetonitrile in borate-hosphate buffer and 12.5 ml of ferrate(VI) in borate-phosphateuffer. The molar ratio of [Fe(VI)]/[DBT] was set at 3.0. The samexperiment was performed in the absence of ferrate(VI) in order tonvestigate the effect, if any, of silica gel on DBT oxidation.

.3. Analytical techniques

.3.1. High performance liquid chromatographyHigh performance liquid chromatography (HPLC) technique was

mployed to determine the concentrations of BT, DBT, 4-MDBT, andBTO2 before and after treatment with ferrate(VI). An Atlantis dC18.6 �m × 150 �m, 5 �m column was used in a Water Alliance 2695nalytical HPLC connected to a Water 996 photodiode array detec-or. A flow rate of 0.5 ml/min, an injection volume of 30 �l, and aavelength range of 297–323 nm were used. One liter of eluent wasrepared by mixing 750 ml of ethanol with 250 ml Milli-Q water

n a volumetric flask which was then filtered using 0.22 �m GV

embrane filters. The detection limits of analytes were <1.0 �M.

he main product of DBT oxidation, DBTO2, was also identified inhe chromatogram, by matching the product peak with a standardhromatogram of DBTO2.

rdous Materials 279 (2014) 296–301

2.3.2. Real Time-Mass SpectroscopyDirect Analysis in Real Time-Mass Spectroscopy (DART-MS)

approach was used to perform direct mass scanning for the mainproducts of thiophenic compounds after the oxidation reactions.A JEOL AccuTOF JMS-T100LC coupled with an Ion Sense modelDART100 was used to conduct the products analysis. The tempera-ture was fixed at 450 ◦C with a flow rate of 4.94 l/min of Helium gas.The peak voltage was 1500 V, while the detector was set at 2100 V.The Capillary Scrape method was employed with no additives tothe samples.

3. Results and discussion

3.1. The oxidation of DBT in bifacial system

The oxidation of DBT in petroleum ether using a buffered solu-tion of ferrate(VI) was tested under similar experimental conditionsto those reported in the previous work [20]. The mixed solutionsproduced two layers: organic layer composed of DBT and an aque-ous layer contains ferrate(VI) in buffer. After the treatment withferrate(VI), the analysis of the organic layer showed that the con-centration of DBT was lowered by 8.6%. This removal percentagewas found to be in good agreement with that reported by Liuet al. [20]. Therefore, the limited conversion of DBT in petroleumether/phosphate buffer-ferrate(VI) system may be due to the lowinteraction between ferrate(VI) and the targeted sulfur compound.

3.2. The effect of acetic acid on ferrate(VI) reactivity

The oxidation of DBT by ferrate(VI) in petroleum ether-aceticacid, performed previously [20], showed a removal of 58.0% of DBToxidation at [Fe(VI)]/[DBT]0 = 7.0 and 35 ◦C. The researchers haveattributed this to a potential catalytic effect of acetic acid on fer-rate(VI) reactivity [20].

In order to understand the role of acetic acid on ferrate(VI) reac-tivity with DBT, similar experimental conditions were employed inthe present research, and several observations were made: (i) a sin-gle layer homogenous mixture of acetic acid and petroleum etherwas observed, (ii) potassium ferrate(VI) salt solubility in aceticacid was lower than that in water and (iii) the distinctive colorof ferrate(VI) could not be observed within the mixture during thisexperiment. Under these experimental conditions, a 53.3% of theinitial DBT was removed after the treatment with ferrate(VI), andthat was found to agree with the results of a previous study [20].From the observations made during this set of experiments, it maybe concluded that the acetic acid performed the role of enhanc-ing the distribution of ferrate(VI) oxidant within the organic liquidfuel rather than catalytic activity to enhance the DBT oxidation. Thehigh volumetric ratio of acetic acid added to liquid fuel, and therequirement of additional treatments to recover acetic acid fromthe system after the treatment are major limitations. Therefore,further experiments were carried out without using acetic acid.

3.3. Mn(III) acetate effect on DBT oxidation

In order to understand the role of Mn(III) acetate on DBT oxi-dation by ferrate(VI), studied previously [20], the direct effect ofMn(III) acetate on DBT was tested in the absence of ferrate(VI). Theresults have shown that the initial DBT concentration was loweredby 30% after the treatment with Mn (III) acetate. Therefore, it is

clear that Mn(III) acetate had a direct effect on DBT present in thesolution, and can increase the oxidation of DBT as an oxidant ratherthan a catalyst. This agrees with additional removal obtained usinga mixture of Mn(III) acetate and ferrate(VI) [20].
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A. Al-Abduly, V.K. Sharma / Journal of Hazardous Materials 279 (2014) 296–301 299

pH2.0 4.0 6.0 8.0 10.0 12.0

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ig. 1. The effect of pH on ferrate(VI) reactivity DBT concentration in 60% acetonitrilen borate-phosphate buffer at 25 ◦C. ([DBT]0 = 116 �M, and [Fe(VI)]0/[DBT]0 = 4.83)solid line represents sulfur the trend of the experimental data points).

.4. The oxidation of DBT in a homogenous system

In order to evaluate the reactivity of ferrate(VI) with DBT in more precise manner, the highest possible interaction betweenhese compounds must be provided. Therefore, petroleum etheras replaced with other solvents: methanol, ethanol, acetone, and

cetonitrile. These solvents were found to be capable of dissolvingBT and form homogenous solutions when mixed with ferrate(VI)queous solutions. The reactivity of aqueous ferrate(VI) solutionsith these solvents in an alkaline medium at pH 9.5 was tested

o ensure minimum reactions between ferrate(VI) and solvent. Forhat purpose, the time required for distinctive color of ferrate(VI) toisappear was used as an indicator to assess the reactivity betweene(VI) and each solvent. Reactions of methanol and ethanol witherrate(VI) were the fastest and the ferrate(VI) color disappearedithin a minute. In case of acetone, the time required for the fer-

ate(VI) color to disappear was ca. 6.5 min. The reaction was foundo be the slowest, and the ferrate(VI) color remained for more than0 min. Thus, acetonitrile was chosen as a co-solvent with a phos-hate buffer to conduct further oxidation studies.

The optimal composition of the mixed solvents was foundo be between 75.0 and 60.0% of acetonitrile in aqueous bufferolution. In amounts of acetonitrile lower than 60.0%, the sul-ur compounds started to precipitate out, while pure solventad no buffering capacity. Therefore, the 60% acetonitrile in.005 M Na2HPO4/0.001 M Na2B4O7 was employed to dissolve thehiophenic compounds, and to maintain an adjustable pH envi-onment. The 60% acetonitrile–borate-phosphate system overcamehe limited interaction observed between petroleum ether and fer-ate(VI) solution. The reactivity of ferrate(VI) with 108 �M DBTn 60% acetonitrile–borate phosphate was studied using a molaratio of [Fe(VI)]/[DBT] = 7.0 at pH 3.0 and room temperature. Thebtained results from acetonitrile system showed that more than3% of DBT was oxidized by ferrate(VI). This confirmed that thexidation enhancement observed by adding acetic acid to theetroleum ether system was mainly due to the improved distri-ution of the oxidant.

.5. Effect of pH

Due to the high dependency of ferrate(VI) oxidation propertiesn the pH [25], the pH effect was studied by mixing solutions oferrate(VI) with solutions containing DBT at fixed concentrationsithin the pH range from acidic to alkaline (Fig. 1). In the study

arried out by Liu et al. [20], no significant impact of the pH onerrate(VI) reactivity with the thiophenic compounds was seen inmmiscible petroleum ether solvent. However, the results of Fig. 1hows that the highest removal percentages were achieved at pH

Fig. 2. Oxidation of organo sulfur compounds (S) by ferrate(VI) in 60% acetoni-trile in borate-phosphate buffer at 25 ◦C. ([BT]0 = 135 �M, [DBT]0 = 98 �M, and[4-MDBT]0 = 97 �M).

8.0–9.0, while at pH 10.0 and pH 7.0, the removal was lower thanthat at pH 9.0 and 8.0. In acidic media, at pH 5.0 and 3.0, the removalpercentages were almost twice as low as than those absorbed at pH9.0 and 8.0 which reflects a significant effect of pH on the removalof DBT by ferrate(VI).

The high sensitivity of the oxidation reaction toward the reac-tion pH can be explained considering two critical factors, thestability of ferrate(VI) ions at different pH values, and the reactivityof ferrate(VI) in these media. Ferrate(VI) undergoes a spontaneousdecomposition in water which leads to oxygen formation [26,27]:

2FeO42− + 5H2O → 2Fe3+ + 3

2O2 + 10OH−

and the rate of this reaction was found to be very slow atpH 9.0–10.0 and was faster above and below these values [22].The reactivity of ferrate(VI) usually increases as pH decreases[18]. This is related to the protonation equilibrium of ferrate(VI)(H3FeO4

+ � H+ + H2FeO4, pKa1 = 1.9; H2FeO4 � H+ + HFeO4−, pKa2

3.5; HFeO4− � H+ + FeO4

2−, pKa3 = 7.23) [23].The protonated Fe(VI) species (e.g. HFeO4

−) is more reactivethan the non-protonated species (FeO4

2−) [22–24]. The oxidationefficiency depends on cumulative effect of two processes occur-ring simultaneously in solution and both processes are highly pHdependent [19,25,26]. The maxima in the pH range of pH 8.0–9.0in Fig. 1 indicates that the rates of the oxidation of DBT by fer-rate(VI) in this pH range were higher than the self-decompositionof Fe(VI) under studied conditions. This resulted in maximumremoval of DBT in this pH range. It also appears that the rate ofself-decomposition in acidic media is significant compared to thereactivity of ferrate(VI) with DBT, resulting in the lowering of theoxidative efficiency of ferrate(VI) (Fig. 1).

3.6. Oxidation and product study

Quantitative analysis of sulfur compounds (S) before and afterthe oxidation by ferrate(VI) is presented in Fig. 2. As can beseen from Fig. 2, the concentration of the thiophenic compoundsdecreased as the concentration of ferrate(VI) increased in a reason-ably linear relationship. Significantly, nearly complete removals ofBT and DBT were observed by ferrate(VI) with [Fe(VI)]/[BT]0 = 7.5

and 7.0, respectively. The complete oxidation of 4-MDBT requireda higher concentration of ferrate(VI) than that for DBT. Removal of4-MDBT needed a molar ratio ([Fe(VI)]/[4-MDBT]0) of 8.5 at roomtemperature to achieve the complete removal. Such behavior can be
Page 5: Oxidation of benzothiophene, dibenzothiophene, and methyl-dibenzothiophene by ferrate(VI)

300 A. Al-Abduly, V.K. Sharma / Journal of Hazardous Materials 279 (2014) 296–301

[Ferra te]0/[DBT]0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

[Spe

cies

] (%

)

0

20

40

60

80

100

DBT DBTO2

Ft

roabiDoit

rcgDrt

ufwMsTmacttoacTot

3

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DBT + Solid + Fe(VIDBT + SolidDBT + Fe(VI)

ig. 3. The decrease in concentration of DBT and the formation of DBTO2 as a func-ion of ferrate(VI) concentration. ([DBT]0 = 106 �M, pH 8.0 and t = 25 ◦C).

elated to the potential steric hindrance caused by the methyl groupn 4-MDBT, which may hinder the interaction between the sulfurtom and the oxidant. The importance of steric hindrance causedy methyl groups at position 4 was also observed in other studies

n which there was less oxidation of 4-MDBT, and 4,6-DMDBT thanBT using metal oxide (V2O5) or Fe-TAML [27,28]. Therefore, thexidation of BT, DBT, and 4-MDBT may be occurring through directnteraction between the thiophenic compounds and FeO4

2− ions inhe solution.

The influence of Fe(III), produced from the reduction of fer-ate(VI), on the removal of selected thiophenes was ruled out byarrying out separate experiments. Removal of DBT was investi-ated in alkaline media (pH 8.0). Several molar ratios of Fe(III) toBT were applied at a fixed concentration of DBT (108.0 �M) at

oom temperature (Fig. SM-2). The results obtained indicated thathere was no removal effect of DBT caused by Fe(III) (Fig. SM-2).

The mass analysis was employed to evaluate the oxidation prod-cts of BT, DBT and 4-MDBT (Fig. SM-3–SM-5). As may be seenrom these results, the main products of BT, DBT, and 4-MDBTere found to be oxidized quantitatively to BTO2, DBTO2, and 4-DBTO2, respectively. Interestingly, the mass scan of BT treated

amples showed a small fraction of a BTO compound (Fig. SM-4).his suggested that the oxidation mechanism goes through the for-ation of the sulfoxides prior to the formation of the sulfones. In

ddition, the presence of BTO, but not DBTO and 4-MDBTO in thease of oxidation of DBT and 4-MDBT, respectively, indicates thathe oxidations rates of DBTO and 4-MDBTO by ferrate(VI) were rela-ively higher than their non-oxidized forms. A quantitative analysisf the oxidation products may suggest the formation of sulfoness the main product of thiophenes. Fig. 3 shows the quantitativeonversion of DBT to DBTO2 as a function of Fe(VI) molar ratio.he formation of DBTO2 had an equivalent reversed trend to thatbserved for the decrease of DBT, and therefore it can be concludedhat thiophene-sulfones were the ultimate products.

.7. Effect of silica

The effect of adding silica gel on DBT oxidation by ferrate(VI) wastudied. In the absence of ferrate(VI), the addition of silica gel to theeaction medium was found to cause no effect on DBT concentra-ion. On the other hand, when ferrate(VI) was added to the systemn the presence of silica gel, the conversion percentage was found

o be higher than the corresponding value when no silica gel wasdded (Fig. 4). In addition, the conversion percentage was found toncrease with the amount of silica gel added, and the highest con-ersion improvement was ∼22% when the amount of silica gel in

Fig. 4. The effect of silica gel addition on DBT oxidation by ferrate(VI)([DBT]0 = 106 �M, [Fe(VI)]/[DBT]0 = 3.3, pH 8.0, t = 25 ◦C).

the system reached 100 mg/25 ml (Fig. 4). The presence of the solidin the solution mixture created a heterogeneous system in whichsilica acted as a catalyst to enhance the oxidation efficiency. How-ever, further investigation will be needed to fully understand thecatalytic role of silica that led to the observed enhancement of DBToxidation.

4. Conclusions

The removal of DBT by ferrate(VI) in petroleum ether was notsignificant due to the poor interaction between ferrate(VI) andDBT. DBT oxidation by ferrate(VI) in petroleum ether–acetic acid(v/v) = 1, and molar ratio [Fe(VI)]/[DBT] = 7.0 at room temperaturewas 53.0% and was related to the enhanced solubility of ferrate(VI)in the system. There was no catalytic contribution of acetic acidin the oxidation of DBT by ferrate(VI). Mn(III) acetate was foundto be an effective oxidant to increase the removal of DBT fromthe petroleum ether system; again suggesting that Mn(III) had nocatalytic effect on the DBT oxidation. Ferrate(VI) had a high effi-ciency for removing BT, DBT, 4-MDBT in a homogeneous system(60% acetonitrile in 0.005 M Na2HPO4/0.001 M Na2B4O7). However,the removal of DBT by ferrate(VI) in a homogeneous system wasdependent upon the pH and the molar ratio of ferrate(VI) to DBT.Ferrate(VI) oxidized BT, DBT, and 4-MDBT to their correspondingsulfones. Enhanced removal of DBT by ferrate(VI) was observed byadding silica gel in the reaction system.

Acknowledgments

This work was carried out at the Center of Ferrate Excellence,Florida Institute of Technology, Melbourne, FL, USA and the supportof the Center is acknowledged. We wish to thank the reviewers fortheir comments which improved the paper greatly.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2014.06.083.

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eferences

[1] V. Chandra Srivastava, An evaluation of desulfurization technologies for sulfurremoval from liquid fuels, RSC Adv. 2 (2012) 759–783.

[2] A. Ates, G. Azimi, K.-H. Choi, K.-H.W.H. Green, M.T. Timko, The role ofcatalyst in supercritical water desulfurization, Appl. Catal., B 147 (2014)144–155.

[3] G. Mohebali, A.S. Ball, Biocatalytic desulfurization (BDS) of petrodiesel fuels,Microbiology 154 (2008) 2169–2183.

[4] M. Soleimani, A. Bassi, A. Margaritis, Biodesulfurization of refractoryorganic sulfur compounds in fossil fuels, Biotechnol. Adv. 25 (2007)570–596.

[5] A. Abin-Fuentes, M.E. Mohamed, D.I.C. Wang, K.L.J. Prather, Exploring the mech-anism of biocatalyst inhibition in microbial desulfurization, Appl. Environ.Microbiol. 79 (2013) 7807–7817.

[6] B. Galan, E. Diaz, J.L. Garcia, Enhancing desulphurization by engineering a flavinreductase-encoding gene cassette in recombinant biocatalysts, Environ. Micro-biol. 2 (2000) 687–694.

[7] L. da Silva Madeira, V.S. Ferreira-Leitao, E.P. da Silva Bon, Dibenzothio-phene oxidation by horseradish peroxidase in organic media: effect of theDBT:H2O2 molar ratio and H2O2 addition mode, Chemosphere 71 (2008)189–194.

[8] A. Caro, K. Boltes, P. Leton, E. Garcia-Calvo, Biodesulfurization of dibenzothio-phene by growing cells of Pseudomonas putida CECT 5279 in biphasic media,Chemosphere 73 (2008) 663–669.

[9] F. Tao, B. Yu, P. Xu, C.Q. Ma, Biodesulfurization in biphasic systems containingorganic solvents, Appl. Environ. Microbiol. 72 (2006) 4604–4609.

10] S. Otsuki, T. Nonaka, N. Takashima, W. Qian, A. Ishihara, T. Imai, T. Kabe, Oxida-tive desulfurization of light gas oil and vacuum gas oil by oxidation and solventextraction, Energy Fuels 14 (2000) 1232–1239.

11] M.C. Capel-Sanchez, J.M. Campos-Martin, J.L.G. Fierro, Highly efficient deepdesulfurization of fuels by chemical oxidation, in: Abstracts of Papers, 235thACS National Meeting, New Orleans, LA, United States, April 6–10, 2008, 2008,FUEL-031.

12] A. Ishihara, D. Wang, F. Dumeignil, H. Amano, E.W. Qian, T. Kabe, Oxida-tive desulfurization and denitrogenation of a light gas oil using an

oxidation/adsorption continuous flow process, Appl. Catal., A 279 (2005)279–287.

13] W.F. De Souza, I.R. Guimaraes, M.C. Guerreiro, L.C.A. Oliveira, Catalytic oxidationof sulfur and nitrogen compounds from diesel fuel, Appl. Catal., A 360 (2009)205–209.

[

rdous Materials 279 (2014) 296–301 301

14] J. Zhang, A. Wang, Y. Wang, H. Wang, J. Gui, Heterogeneous oxidative desulfu-rization of diesel oil by hydrogen peroxide: catalysis of an amphipathic hybridmaterial supported on SiO2, Chem. Eng. J. 245 (2014) 65–70.

15] H. Song, J. Gao, X. Chen, J. He, C. Li, Catalytic oxidation-extractive desulfurizationfor model oil using inorganic oxysalts as oxidant and Lewis acid-organic acidmixture as catalyst and extractant, Appl. Catal. A: Gen. 456 (2013) 67–74.

16] J.Q. Jiang, Advances in the development and application of ferrate(VI) for waterand wastewater treatment, J. Chem. Technol. Biotechnol. 89 (2014) 165–177.

17] Y. Lee, B.I. Escher, U. von Gunten, Efficient removal of estrogenic activity dur-ing oxidative treatment of waters containing steroid estrogens, Environ. Sci.Technol. 42 (2008) 6333–6339.

18] V.K. Sharma, Ferrate(VI) and ferrate(V) oxidation of organic compounds: kinet-ics and mechanism, Coord. Chem. Rev. 257 (2013) 495–510.

19] E.M. Casbeer, V.K. Sharma, Z. Zajickova, D.D. Dionysiou, Kinetics and mecha-nism of oxidation of tryptophan by ferrate(VI), Environ. Sci. Technol. 47 (2013)4572–4580.

20] B. S. Liu, B. Wang, L. Cui, Deep desulfurization of diesel oil oxidized by Fe (VI)systems, Fuel 87 (2008) 422–428.

21] Z. Luo, M. Strouse, J.Q. Jiang, V.K. Sharma, Methodologies for the analyticaldetermination of ferrate(VI): a review, J. Environ. Sci. Health –A Toxic/Hazard.Subs. Environ. Eng. 46 (2011) 453–460.

22] V.K. Sharma, Oxidation of inorganic compounds by ferrate (VI) and Ferrate(V):one-electron and two-electron transfer steps, Environ. Sci. Technol. 44 (2010)5148–5152.

23] V.K. Sharma, Oxidation of nitrogen-containing pollutants by novel ferrate(VI)technology: a review, J. Environ. Sci. Health – A Toxic/Hazard. Subs. Environ.Eng. 45 (2010) 645–667.

24] N. Noorhasan, V.K. Sharma, Kinetics of the reaction of aqueous iron(VI) withethylediaminetetraacetic acid, Dalton Trans. (2008) 1883–1887.

25] H. Goff, R.K. Murmann, Studies on the mechanism of isotopic oxygen exchangeand reduction of ferrate(VI) ion (FeO4

2−), J. Am. Chem. Soc. 93 (1971)6058–6065.

26] V.K. Sharma, W. Rivera, J.O. Smith, B. O’Brien, Ferrate(VI) oxidation of aqueouscyanide, Environ. Sci. Technol. 32 (1998) 2608–2613.

27] S. Mondal, Y. Hangun-Balkir, L. Alexandrova, D. Link, B. Howard, P. Zandhuis,A. Cugini, C.P. Horwitz, T.J. Collins, Oxidation of sulfur components in diesel

fuel using Fe-TAML® catalysts and hydrogen peroxide, Catal. Today 116 (2006)5540561.

28] D. Xu, W. Zhu, H. Li, J. Zhang, F. Zou, H. Shi, Y. Yan, Oxidative desulfurization offuels catalyzed by V2O5 in ionic liquids at room temperature, Energy Fuels 23(2009) 5929–5933.