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Journal of Environmental Management 139 (2014) 50e58

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Journal of Environmental Management

journal homepage: www.elsevier .com/locate/ jenvman

Removal of organic compounds and trace metals from oil sandsprocess-affected water using zero valent iron enhanced by petroleumcoke

Parastoo Pourrezaei, Alla Alpatova, Kambiz Khosravi, Przemys1aw Drzewicz, Yuan Chen,Pamela Chelme-Ayala, Mohamed Gamal El-Din*

Department of Civil and Environmental Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2W2

a r t i c l e i n f o

Article history:Received 4 October 2013Received in revised form24 January 2014Accepted 2 March 2014Available online

Keywords:Zero valent ironPetroleum cokeOil sands process-affected waterAdsorptionOxidation

* Corresponding author. NSERC Senior IndustrialTailings Water treatment, Helmholtz e Alberta InitiMarkin/CNRL Natural Resources Engineering FacilitEnvironmental Engineering, University of Alberta, Ed2W2. Tel.: þ1 780 492 5124; fax: þ1 780 492 0249.

E-mail address: mgamalel-din@ualberta.ca (M. Ga

http://dx.doi.org/10.1016/j.jenvman.2014.03.0010301-4797/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The oil production generates large volumes of oil sands process-affected water (OSPW), referring to thewater that has been in contact with oil sands or released from tailings deposits. There are concerns aboutthe environmental impacts of the release of OSPW because of its toxicity. Zero valent iron alone (ZVI) andin combination with petroleum coke (CZVI) were investigated as environmentally friendly treatmentprocesses for the removal of naphthenic acids (NAs), acid-extractable fraction (AEF), fluorophore organiccompounds, and trace metals from OSPW. While the application of 25 g/L ZVI to OSPW resulted in 58.4%removal of NAs in the presence of oxygen, the addition of 25 g petroleum coke (PC) as an electronconductor enhanced the NAs removal up to 90.9%. The increase in ZVI concentration enhanced the re-movals of NAs, AEF, and fluorophore compounds from OSPW. It was suggested that the electronsgenerated from the oxidation of ZVI were transferred to oxygen, resulting in the production of hydroxylradicals and oxidation of NAs. When OSPW was de-oxygenated, the NAs removal decreased to 17.5% and65.4% during treatment with ZVI and CZVI, respectively. The removal of metals in ZVI samples wassimilar to that obtained during CZVI treatment. Although an increase in ZVI concentration did notenhance the removal of metals, their concentrations effectively decreased at all ZVI loadings. TheMicrotox� bioassay with Vibrio fischeri showed a decrease in the toxicity of ZVI- and CZVI-treated OSPW.The results obtained in this study showed that the application of ZVI in combination with PC is apromising technology for OSPW treatment.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The Athabasca oil sands in Alberta, Canada, contain one of thelargest oil deposits in the world (Allen, 2008). During the oilrefining process in the petroleum industry, a large amount of by-products, including process-affected water and petroleum coke(PC), are produced (Allen, 2008; Kannel and Gan, 2012). In Alberta,generated oil sands process-affected water (OSPW) and PC areaccumulated on site until suitable treatment technologies areadvanced (Energy Resources Conservation Board, 2011). Due to its

Research Chair in Oil Sandsative Lead (Theme 5), 3-093y, Department of Civil andmonton, Alberta, Canada T6G

mal El-Din).

toxicity, OSPW cannot be released to the receiving environmentand as a result, 840 million m3 of OSPW is currently stored in largesettling basins (Energy Resources Conservation Board, 2010).

OSPW contains high concentrations of inorganic salts and re-fractory organic compounds such as naphthenic acids (NAs) (Allen,2008). The presence of these contaminants makes OSPW corrosiveand toxic to a variety of aquatic biota and mammals (Garcia-Garciaet al., 2011; Jones et al., 2011; Pourrezaei et al., 2011). The slowdegradation rate of NAs in OSPW necessitates the development ofthe new advanced treatment methods to accelerate their decom-position. On the other hand, PC cannot serve as an efficient sourceof energy for the heat generation due to its high sulphur contentand low combustible volatiles (Friedrich et al., 1983). In turn, PC,which has shown promising results in the removal of contaminantsfrom OSPW, could be used as an abundant and free-of-chargeadsorbent for OSPW treatment (Zubot et al., 2012). PC at 300 g/Lhas shown to remove 60% of the dissolved organic carbon, 77% of

P. Pourrezaei et al. / Journal of Environmental Management 139 (2014) 50e58 51

the acid-extractable fraction (AEF), and 94% of the NAs from OSPW(Zubot et al., 2012). However, it has been found thatmetal ions (e.g.,vanadium, molybdenum, and nickel), accumulated in PC during thecoking process, could be released into the liquid phase upon theircontact with water. The leaching of metals from PC increased thetoxicity of treated OSPW (Puttaswamy et al., 2010), which in turnlimits the application of PC as an adsorbent.

Zero valent iron (ZVI) has been shown to be a cost-effective andan environmentally friendly reducing agent (Li et al., 2006; Shimizuet al., 2012). Recently, the application of ZVI and iron-based alloyshas drawn significant attention for the remediation of wastewatersand ground waters (Gillham and Ohannesin, 1994; Noubactep,2010). ZVI has been used for the removal of refractory organiccompounds (Mantha et al., 2001), nitroaromatics (Agrawal andTratnyek, 1996), nitrate (Alowitz and Scherer, 2002), and metals(Mak and Lo, 2011).

Depending on the water characteristics, possible removalmechanisms in the ZVI/water system include direct or indirectreduction, adsorption and co-precipitation, and oxidation in thepresence of oxygen (Li et al., 2006; Shimizu et al., 2012; Stieberet al., 2011). Many researchers have focused on the reduction oforganic pollutants using ZVI, and only few reports have centred onthe oxidation of organic contaminants by ZVI and elucidation oftheir removal mechanisms (Shimizu et al., 2012; Joo et al., 2005).Depending on the water pH and the oxygen availability, ZVI isquickly oxidized to Fe (II) and Fe (III) and forms various species ofiron oxy/hydroxides (FeO, FeO(OH), and Fe2O3) (Aleksanyan et al.,2007; Noubactep, 2010). Surface corrosion of ZVI in the presenceof oxygen was suggested to produce Fe (II) and H2O2, with thesubsequent reaction of these two reagents in a Fenton-type reac-tion to generate highly reactive hydroxyl radicals (Stieber et al.,2011).

Carbonaceous materials, such as PC, have been known topossess electron conducting characteristics, which enable them toserve as electron mediators as well as adsorption sites (Tang et al.,2011). The reduction of 2,4-dinitrotoluene by ZVI (Oh et al., 2002)and hydrolysis of lindane and 1,1,2,2-tetrachloroethane (Mackenzieet al., 2005) were reported to significantly increase in the presenceof graphite and activated carbon, respectively. The oxidation ofterephthalic acid, which is a dicarboxylic acid, was shown to beeffectively enhanced in the presence of phosphotungstic acid as anelectron conductor (Lee et al., 2007).

The application of ZVI in combination with PC has not beenreported previously and this is the first time this process has beenstudied with respect to the removal of organic pollutants fromcontaminated water. The objectives of the present study were: (1)to evaluate the feasibility of using ZVI in combination with PC toremove organic compounds (fluorophores, NAs, and AEF) andmetals from OSPW; (2) to understand the potential role of PC in theimprovement of the organic compounds removal; (3) to investigateeffect of OSPW oxygen content on the removal of the organiccompounds; and (4) to determine the treatment efficiency in termsof acute toxicity removal by using Vibrio fischeri as test organisms.

2. Materials and methods

2.1. Materials

OSPW was collected from the West in-Pit tailings pond at Syn-crudeCanada Ltd., Alberta, Canada. Twobatches ofOSPW, received inJanuary 2011 and September 2012, were used for the experiments.OSPWwas stored at 4 �C andwas brought to room temperature (18e20 �C) before conducting the experiments. OSPW characteristics areshown in Tables S1 and S2 in the Supplementary Material.

PC, produced during fluid coking process in Syncrude CanadaLtd., was used as an adsorbent. Two different batches of PC,received in January 2010 and June 2011, were used. High-purity ZVIpowder (<100 mesh, >99%) was purchased from Fisher Scientific(Ottawa, ON, Canada) and was used as-received without anypretreatment.

2.2. Adsorption/oxidation experiments

In the first set of experiments, the adsorption/oxidation of thefluorophore organic compounds and the removal of metals fromOSPW were investigated as a function of the exposure time. A100mL of OSPWwas added to 2.5 g of ZVI alone, 25 g of PC alone, orZVI þ PC in a 250 mL Erlenmeyer flask. The flasks were left open tothe atmosphere and shaken vigorously at 270 rpm on an orbitalshaker (New Brunswick Scientific, Enfield, CT, USA) to maintain therequired oxygen supply. After pre-determined exposure times (4, 8,16, 32, 56, 80, 104 and 208 h), a 4 mL aliquot of each sample wascentrifuged (Eppendorf, Ontario, Canada) at 10,000 rpm for 5 minto settle the ZVI/PC. The supernatants were analysed for anychanges in the concentration of the fluorophore organic com-pounds. The supernatants were then filtered using 0.45 mm Nylonfilters (SUPELCO, Bellefonte, PA, USA) and analysed for metalscontent. After the optimal exposure time was determined, the ef-fect of ZVI loading on the removal of the organic compounds andmetals was investigated. The experiments were conducted in thesame way as described above, except that the ZVI loadings of 1.25,2.5, and 5 g in a 100 mL of OSPWwere used to result in the 12.5 g/L,25 g/L, and 50 g/L of ZVI. The mass of PC in CZVI samples was 25 g.Both treated and untreated samples were analyzed with respect tofluorophore organic compounds, AEF, NAs, and metals. The toxicityof untreated and treated OSPW toward V. fischeri was alsoevaluated.

Another set of experiments was performed to investigate theeffect of the dissolved oxygen on the removal/oxidation of thefluorophore organic compounds, AEF, and NAs. The concentrationof oxygen in OSPW was determined using the Azide-winklertitration method and the initial oxygen concentration was recor-ded to be 7.8� 0.2mg/L. Description of quality control methods andstatistics can be found in the Supplementary Material (SM).

2.3. Analytical determination

An ultra performance liquid chromatograph (UPLC, Waters, MA,USA) with a Phenyl BEH column (150 � 1 mm, 1.7 mm) (Waters,Milford, MA, USA) was used for the chromatographic separationand analysis of NAs before and after treatments. A Fourier trans-form infrared (FT-IR) spectrometer (PerkinElmer Spectrum, 100 FT-IR Spectrometer, Waltham, USA) was used to measure the AEFconcentrations in OSPW according to the method described byJivraj et al. (1996). The fluorophore organic compounds in OSPWwere analysed using Fluorescence Spectrophotometer (Varian CaryEclipse, Ontario, Canada). Synchronous fluorescence spectroscopy(SFS), a semi-quantitative analysis, was used to target a group ofcompounds (e.g., one ring, two rings, etc.) and not specific com-pounds. A model 500 Microtox� analyzer (Strategic Diagnostic Inc.)was used tomeasure the light emitted by the V. fischeri bacteria as aresult of their normal metabolic processes to assess the toxicity ofthe samples before and after treatments (Chelme-Ayala et al.,2011a,b). The decrease in bacterial luminescence was measuredafter 5 and 15 min of exposure.

The X-ray photoelectron spectroscopy (XPS) was performedusing an AXIS 165 spectrometer (Kratos Analytical, Manchester,UK). An Elan 6000 ICP Mass Spectrometer (PerkinElmer, MA, USA)was used to quantify the concentration of the trace metals.

P. Pourrezaei et al. / Journal of Environmental Management 139 (2014) 50e5852

Ultrapure water (18 MU cm), prepared by a Synergy� UV instru-ment (Millipore, Molsheim, France), was used in the entire set ofexperiments. The detailed description of these methods can befound in SM.

3. Results and discussion

3.1. The effect of the exposure time on fluorophore organiccompounds

The SFS analysis of OSPW samples showed peaks at 270e274 nm, 308e310 nm, 324e326 nm, and 406e415 nm (Fig. 1). Theposition of each peak is pointed by arrows. To simplify furtherdiscussion, these peaks are herein called as (I), (II), (III), and (IV),respectively. The peaks (I), and [(II), (III)] represent one and twomember ring compounds, respectively, and peak (IV) was related toabove four member rings in the structure of the aromatic acidcompounds (Kannel and Gan, 2012; Kavanagh et al., 2009). OSPWcontrol samples showed the highest intensity of the peaks, fol-lowed by ZVI, PC, and CZVI, respectively, indicating the highestremoval of the related compounds during CZVI treatment. Therewas no change in the intensity of the peaks (I), (II), and (III) in OSPWcontrol samples, except the peak (IV), for which the intensitydecreased with time (Fig. 1a). One possible explanation for theobserved trend is that these compounds could be removed fromOSPW due to their higher hydrophobicity. Shaking of the samplecontainers causes these very hydrophobic molecules tomove to thesurface of the solutions; thereby, leaving the solution due to surfaceevaporation during the intense shaking. This phenomenonwas notobserved in the presence of PC and ZVI, because of their attractionto these compounds. Therefore, the concentration of compounds in

Fig. 1. SFS spectra of (a) OSPW control, (b) PC-, (c) ZVI-, and (d) CZVI-treated OSPW as arespectively. The pH of the solution was 8.5.

peak IV decreased at lower rate. For PC (Fig. 1b), the decrease in theintensity of peaks (II) and (III) was higher than that of peak (I).Higher number of the rings and carbon numbers in the structure ofthe organic compounds were shown to result in their higher hy-drophobicity and less solubility. This, in turn, increased the hy-drophobic interactions between the organic compounds and theadsorbents. The compounds in group (I) are more soluble; there-fore, they have less affinity to be adsorbed on the PC than com-pounds (II) and (III). The decrease in the intensity of peak (IV)followed the same trend as in OSPW control, with a completeremoval at >100 h exposure time. The compounds related to thesepeaks were removed completely by ZVI and CZVI at all exposuretimes (Fig. 1c and d). The decrease in intensity of peaks (I), (II) and(III) during CZVI treatment was higher than that observed duringZVI and PC treatments.

The removal of compounds corresponding to peaks (I), (II), and(III) using ZVI alone after 208 h exposure indicated the oxidative/adsorptive ability of ZVI. The ability of ZVI to reduce the concen-tration of organic compounds was also reported in other studies(Joo et al., 2005; Lee et al., 2007). As shown in Fig. S1, during theCZVI treatment, the reduction in the peaks intensity exhibitedbiphasic kinetics, with a rapid removal of the compounds in thefirst 56 h, followed by a slower removal. Rasheed et al. (2011) alsoshowed that the degradation of organic compounds from awastewater using nanoscale ZVI followed a similar trend. Based onthis observation, 56 h exposure time was chosen for further ex-periments. Additionally, the removal of the compounds corre-sponding to each peak was calculated and is shown in Fig. S1. Theremovals of the peaks (II) and (III) were almost similar, and theywere higher than that of peak (I) for CZVI treatment, at all theexposure times. The higher removal of the compounds

function of exposure time. The concentrations of ZVI and PC were 25 and 200 g/L,

P. Pourrezaei et al. / Journal of Environmental Management 139 (2014) 50e58 53

corresponding to these peaks may be due to their affinity to PCbased on their hydrophobicity. The removal of the fluorophoreorganic compounds using CZVI was higher than the sum of theremovals observed using ZVI and PC for most of the exposure times,which showed the synergistic effect of the PC and ZVI additions.Upon addition of ZVI to the samples, they turned orange due to theoxidation of iron. The lower removal rate may indicate the buildupof the oxide products on the surface of ZVI (Joo et al., 2005),resulting in a lower availability of reactive surfaces. The XPS spec-troscopy revealed a peak at 724.3 eV (Fig. S2), indicating thepresence of FeO(OH)/Fe2O3 on the surface of ZVI (Chen et al., 2012).This is consistent with previous study by Stieber et al. (2011) whichalso reported the formation of FeO(OH) on the surface of ZVI duringthe degradation of azo-dyes and pharmaceuticals at pH > 5.Therefore, the lower removal of peaks I and II in ZVI may be due tothe loss of active sites on the ZVI upon oxidation.

3.2. The effect of exposure time on metals removal

Fig. 2 shows that vanadium (V), manganese (Mn), nickel (Ni),and molybdenum (Mo) were released from PC upon contact withOSPW. The concentration of the released V increased withincreasing exposure time, while no significant changes in theconcentrations of Mn, Ni, andMowere observed. This phenomenonis due to the lower solubility of vanadium oxide or its ions incomparison to the other metals studied in this research. Therefore,a gradual increase in the concentration of vanadium in solutionwasobserved. When ZVI was added to OSPW, the concentrations of V,Mn, Ni, and Mo decreased below their original concentration inOSPW in both ZVI and CZVI samples (Fig. 2). After 208 h, the re-ductions in concentration of metals in ZVI and CZVI samples were24 � 6% and 87 � 2%, 81 � 3% and 98 � 1%, 76 � 1% and 78 � 3%,96� 0% and 93� 8% for V, Mn, Ni, andMo, respectively. No leachingfrom PC was observed for arsenic (As), cadmium (Cd), cobalt (Co),antimony (Sb), strontium (Sr), and uranium (U) (Fig. 2). The con-centration of these elements also decreased below their initiallevels in untreated OSPW during ZVI and CZVI treatments. Theremoval of As, chromium (Cr), and zinc (Zn) from ground waterusing ZVI was also reported (Abedin et al., 2011). The authorsattributed the observed effect to the adsorption of the metals ontothe iron hydroxides and co-precipitation. Additionally, the depo-sition of the metals on the ZVI may form a bimetallic complex withFe0 on the ZVI surface. This complex (Fe0/Metal), in turn, mayenhance the activity of the modified ZVI. The results of the presentstudy indicate that although the addition of PC results in the releaseof some toxic metals into aqueous phase, its consequences could besuccessfully eliminated by the addition of ZVI.

3.3. The effect of ZVI loading on SFS, AEF, NAs, and metals removal

The intensity of peaks (I), (II), and (III) decreased with increasingZVI concentration in both ZVI and CZVI treatments (Fig. S3). Thelarger decrease in the intensity of the peaks was observed at 50 gZVI/L in ZVI and CZVI treatments. This phenomenon only happenedat the highest concentrations of ZVI at 50 mg/L, which was not theoptimum condition. In addition, CZVI treatment resulted in lowerpeaks intensity as compared to samples containing ZVI for all threeapplied ZVI concentrations. The reduction in the intensity of thepeaks (II) and (III) was higher than that of peak (I) for both 12.5 g/Land 25 g/L ZVI in ZVI and CZVI treatments, showing higher removalof the compounds with two rings. This reduction in concentrationof compounds with larger number of rings is due to their hydro-phobicity and stronger attraction toward PC than that observed forone member ring compounds. However, this phenomenonwas notobserved at 50 g/L of ZVI. The observed peaks with the higher

intensity in the region of 380e406 nm at 50 g/L of ZVI could be dueto the shift in the positions of the peaks caused by the presence ofhigh concentration of hydroxides (Senesi, 1990), enhancement ofthe fluorescence due to the formation of metal-complexes withorganic degradation products (Elkins and Nelson, 2002), or changeof the OSPW matrix resulting in the redistribution of the signalintensity in the treated OSPW (Henderson et al., 2009). The mo-lecular structure of the fluorophore organic compounds, the con-centration of metal ions relative to the concentration of thefluorophore organic compounds, the metal ion speciation, and thepH conditions can also affect the fluorescence intensity (Hendersonet al., 2009; Patel-Sorrentino et al., 2002).

The results of the AEF and NAs removals during the ZVI and CZVItreatments as a function of ZVI concentration are shown in Table 1.The difference between the addition of 12.5 g/L and 25 g/L ZVI toOSPWwas not statistically significant (p¼ 0.05) in the ZVI samples;whereas, the addition of 50 g/L ZVI resulted in a significant increasein the AEF and NAs removals. In the CZVI samples, the increase inZVI concentration from 12.5 g/L to 25 g/L resulted in a notable in-crease in the AEF and NAs removals, perhaps due to a dramaticincrease in the surface area, as the concentration increases from 25to 50 g/L.When ZVI concentrationwas further increased to 50 g/L, itdid not result in a significant improvement in the removal effi-ciency (p ¼ 0.04). This behaviour can be explained by the fact thatPC has more salient effect in reacting with AEF and NA due to itshydrophobicity; therefore, the synergistic effect of the added ZVIwas limited. This trend is consistent with the results observedduring the removal of fluorophore organic compounds by CZVItreatment. Kavanagh et al. (2009) observed a linear correlationbetween the concentration of fluorophore organic compounds andAEF in OSPW. The authors suggested that the aromatic acids, withsimilar chemical properties to NAs, could co-isolate with NAs andthus be accounted in the SFS analysis. This explains the similaritiesin the removals of fluorophore, NAs, and AEF compounds.

Fig. S4 shows the effect of ZVI concentration on the metalsremoval. For all applied ZVI concentrations, the removals of theleached V, Mn, Ni, and Mo from PC were 97.7 � 0.08%, 98.2 � 1.2%,93.4 � 6.2%, and 97.6 � 1.6%, respectively. The removals of As, Cd,Co, and Sr were 69.0 � 1.7%, 100.0 � 0.0%, 91.1 � 6.3%, and98.8� 0.5%, respectively. The removal of metals in ZVI samples wassimilar to that of CZVI samples. Although an increase in ZVI con-centration did not enhance the removal of metals; their concen-trations effectively decreased at all ZVI loadings. Since PC did notcontribute to the metal removal, there was no significant change inthe concentrations of the metals between the ZVI and CZVItreatments.

3.4. Toxicity study on V. fischeri

Table 1 displays the effect of the ZVI concentration on thetoxicity of the samples to luminescent V. fischeri. The untreatedOSPWwas found to be toxic to the tested bacteriawith a 58.9� 1.0%inhibition. The OSPW treated with ZVI, CZVI, and PC showed lowerinhibition as compared to the untreated OSPW. The inhibitiondecreased to 6.1 � 1.4% during CZVI treatment, as compared to ZVI(32.0 � 6.0%) and PC (28.3 � 3.0%) treatments. The observed effectimplies the synergistic effect of the ZVI and PC on the decreasedtoxicity of the treated OSPW. Due to the small change in the con-centration of the metals between ZVI and CZVI treatments, thedecrease in the toxicity could be related to the higher removal ofthe organic compounds by the addition of the ZVI into PC samples.The inhibition concentration (IC) of OSPW and ZVI at 25 g/L after15 min of exposure was also calculated. The IC20 and IC50 for OSPWwere found to be 27.8 � 1.0% (v v�1) and 69.5 � 3.0% (v v�1),respectively. For ZVI, the IC20 and IC50 were found to be 51.2 � 3.0%

Fig. 2. Changes in the distribution of NAs species for (a) OSPW control, (b) OSPW treated with 25 g/L ZVI, (c) OSPW treated with ZVI þ PC at 25 g/L of ZVI, and (d) OSPW treated with200 g/L of PC. The exposure time was 56 h and the pH of the solution was 8.5.

P. Pourrezaei et al. / Journal of Environmental Management 139 (2014) 50e5854

Table 1UPLC-MS analysis of the removal of acid-extractable fraction (AEF) and NAs from as-received and de-oxygenated samples along with the levels of inhibition towards Vibriofischeri as a function of ZVI concentration at 56 h exposure time. The pH of OSPW was 8.5.

Treatment type AEF NAs Oxidized NAs Inhibition towardsVibrio fischeri

mg/L % mg/L % mg/L % %

OSPW 61 � 2 e 26.2 � 3.6 e 12.6 � 1.5 e 58.9 � 1.0ZVI (12.5 g/L) 13.8 � 1.9 22.6 � 3.1 9.9 � 1.6 37.7 � 6 e e 39.3 � 2.1ZVI (25 g/L) 17.6 � 1.3 28.8 � 2.2 15.3 � 0.2 58.4 � 0.9 4.3 � 0.04 33.9 � 0.3 32.0 � 6.0ZVI (50 g/L) 42.0 � 3.6 68.9 � 5.9 25.6 � 0.1 97.7 � 0.4 e e 26.8 � 1.0CZVI (12.5 g/L) 21.3 � 0.1 34.9 � 0.2 16.3 � 1.4 62.1 � 5.5 e e 9.7 � 3.5CZVI (25 g/L) 45.4 � 0.5 74.4 � 0.8 23.8 � 0.1 90.9 � 0.5 5.6 � 0.01 44.2 � 0.1 6.1 � 1.4CZVI (50 g/L) 50.9 � 3.8 83.4 � 6.2 25.9 � 0.0 98.9 � 0.0 e e 5.5 � 4.0PC (200 g/L) 8.5 � 0.0 13.9 � 0.0 4.3 � 2.3 16.3 � 8.8 1.0 � 0.06 8.2 � 0.5 28.3 � 3.0ZVI (25 g/L) d.o. 1.9 � 0.9 3.1 � 1.5 4.6 � 0.1 17.5 � 0.4 2.1 � 0.10 16.3 � 0.8 e

CZVI (25 g/L) d.o. 14.2 � 2.7 23.2 � 4.5 17.1 � 1.2 65.4 � 4.5 5.1 � 0.10 40.8 � 0.8 e

PC (25 g/L) d.o. 9.8 � 0.6 16.1 � 1.0 17.0 � 1.2 64.7 � 4.7 7.3 � 0.42 58 � 3.3 e

ZVI: Zero valent iron, CZVI: ZVI combined with petroleum coke; PC: petroleum coke, d.o.: de-oxygenated.

P. Pourrezaei et al. / Journal of Environmental Management 139 (2014) 50e58 55

(v v�1) and >100.0% (v v�1), respectively, showing the reduction inthe toxicity of the OSPW toward V. fischeri after ZVI treatment.

3.5. The effect of ZVI, PC and CZVI on the NAs removal

The distribution of the NAs species present in OSPW before andafter treatments with ZVI, PC, and CZVI (at 25 g ZVI/L) is shown inFig. 3 as a function of the carbon number (n) and cyclicity (-Z). Theremoval of the total NAs was 58.4� 0.9% in ZVI treatment; whereas,90.9� 0.5% removal was achieved in CZVI treatment. Similar resultswere reported by Gamal El-Din et al. (2011) inwhich 76% removal ofNAs was observed using petroleum-coke/ozonation process, and55% removal of model compound was obtained in the UV/H2O2process (Afzal et al., 2012a; Gamal El-Din et al., 2011). However, theZVI=S2O8

2� method showed only 50% removal of NAs at ambienttemperature, although, higher removal was reached at highertemperature (Drzewicz et al., 2012).

In ZVI, the removal of the NAs species were in the range of 35e100% (�Z ¼ 2, n ¼ 10e16), 39e100% (�Z ¼ 4, n ¼ 10e18), 25e67%(�Z ¼ 6, n ¼ 11e18), 21e100% (�Z ¼ 8, n ¼ 13e19), 51e100%(�Z ¼ 10, n ¼ 15e20), and 57e67% (�Z ¼ 12, n ¼ 17e21), respec-tively. These findings are confirmed by Gamal El-Din et al. (2011)that compounds with higher carbons are more favourable todegrade (Afzal et al., 2012b). Other study reported only 41% NAsremoval using diethylaminoethyl-cellulose method (Frank et al.,2006). The removal of the individual NAs species was higher inCZVI (Fig. 3c) as compared to ZVI treatment. The NAs removal inCZVI treatment was in the range of 84e100% (�Z ¼ 2, n ¼ 10e16),50e100% (�Z ¼ 4, n ¼ 10e18), 41e100% (�Z ¼ 6, n ¼ 11e18), 35e100% (�Z ¼ 8, n ¼ 13e19), 82e100% (�Z ¼ 10, n ¼ 15e20), and 94e100% (�Z¼ 12, n¼ 17e21). The concentration of the individual NAsspecies before and after treatment is shown in Tables S3 to S12. Theremoval of the NAs species increased with the increase in n for each�Z group of compounds due to increase in their hydrophobicity(Drzewicz et al., 2012). This, in turn, enhances the adsorption of NAson the surface of ZVI/PC, facilitating their further oxidation. Theconcentration of the total oxidized NAs increased by 34% aftertreatment using ZVI due to the oxidation of NAs. In contrary, theconcentration of the oxidized NAs decreased by 44% as compared tountreated OSPW during CZVI treatment. There was no generaltrend observed for the change in the concentration of the oxidizedNAs in ZVI and CZVI treatments with respect to n and -Z (Tables S6eS9). However, increase in the concentration of the oxidized NAswith two (�Z ¼ 4) and three (�Z ¼ 6) rings was slightly higher.Previous studies showed that the concentration of NAs with two orthree rings was generally higher as compared to other species(Drzewicz et al., 2012; Jones et al., 2011). Therefore, it can be

expected that oxidized NAs would also contain higher number ofthese species.

3.6. The effect of oxygen on the removals of NAs, AEF, andfluorophore organics

NAs and AEF removals in ZVI and CZVI treatments for as-received and de-oxygenated OSPW are shown in Table 1. The NAsand AEF removal for the OSPW in ZVI and CZVI were significantlyhigher, 17.6 and 45.4% in oxygenated solution, compared to 1.9 and14.2% de-oxygenated for ZVI and CZVI respectively. Fig. S3b showsthe effect of de-oxygenation on the SFS spectra of the treatedsamples. Similar to NAs and AEF, the dissolved oxygen in sampleshad a strong effect on the efficiency of the removal of fluorophoreorganic compounds fromOSPW. At the same applied treatment, theintensity of the peaks corresponding to de-oxygenated sampleswas higher as compared to their non-oxygenated counterparts.

The removal of NAs did not changewith the increase in n and�Zin the sample treated with ZVI (Table S11); whereas, in CZVItreatment, increasing n resulted in the increase in the removal ofNA species in each �Z group (Table S12). These results clearlyindicated that oxygen enhanced the removal of organic compoundslikely because of the formation of hydroxyl radicals (Abedin et al.,2011). According to Joo et al. (2004), the removal of molinate us-ing nanoscale ZVI in the absence of oxygenwas negligible; whereas,70% removal was achieved in the aerated or vigorously shakensamples. The turbulence created by aeration or shaking can facili-tate the continuous removal of the passivating layer on the ZVIsurface, resulting in more effective electron transfer from ZVI to thecontaminants/oxygen. In addition, vigorous mixing may facilitatethe oxygen uptake and increase the oxygen transfer to the ZVIsurface for the formation of hydroxyl radicals. Table 1 shows thatthere was no significant increase in the concentration of oxidizedNAs in de-oxygenated OSPW, indicating that the NAs were notoxidized when the sample was purged.

3.7. Removal mechanisms of organic compounds

In aqueous solutions, ZVI is quickly oxidized to form Fe (II) andFe (III) products (Joo et al., 2005; Lee et al., 2007). FeO is alsoformed during this redox reaction, which is adsorbed and forms athin film on the surface of unreacted ZVI (Liu et al., 2008). Posi-tively charged irons of oxy/hydroxide precipitate at pH > 5(Aleksanyan et al., 2007; Noubactep, 2010), and they are the pre-dominant species generated in ZVI/water system (Mak and Lo,2011; Noubactep, 2010). The presence of these species wasconfirmed by the XPS analysis of ZVI (Fig. S2). The deconvolution of

Fig. 3. ICP-MS analysis of the metal contents of OSPW based on the effect of the exposure time on OSPW treated with PC, ZVI, and CZVI. The concentrations of ZVI and PC were 25and 200 g/L, respectively. The pH of the solution was 8.5.

P. Pourrezaei et al. / Journal of Environmental Management 139 (2014) 50e5856

Fe2p (Fig. S2) showed peaks at 710.8, 713.4, and 724.3 eV, indicatingthe presence of the Fe (III), Fe (II), and FeO(OH)/Fe2O3 compoundson the surface of ZVI, respectively (Chen et al., 2012; Drzewiczet al., 2012; Li et al., 2006). The peak which indicates the pres-ence of the Fe0 on the surface of the ZVI was not observed (Li et al.,2006). The formation of these iron oxy/hydroxide products couldcontribute to the adsorption/co-precipitation of the organic com-pounds through surface adsorption and inner sphere complexformation (Gu et al., 1994), resulting in their removal from OSPW.Adsorption of organic compounds on the surface of iron oxy/hy-droxides at pH > 5 has been reported by several studies (Mak andLo, 2011; Noubactep, 2010; Shimizu et al., 2012). In addition, car-boxylic functional groups of the NAs may form complexes with thedissolved Fe (II) (Drzewicz et al., 2012; Kannel and Gan, 2012; Makand Lo, 2011), resulting in the increased hydrophobicity of the NAsand enhanced adsorption onto the surface of PC or ZVI (Drzewiczet al., 2012). Increasing concentrations of the iron complexes(such as FeeNAs), on the other hand, may facilitate their aggre-gation (Mak and Lo, 2011), and these aggregates could furtherattach to the surface of the precipitates to form complex oxideproducts containing iron and NAs. Organic and inorganic con-taminants could also be trapped in the matrix of the forming ironhydroxide precipitates (Noubactep, 2010). As long as the hydrox-ides are not dissolved due to high pH of OSPW, the contaminantsare immobilized and removed together with the precipitates(Noubactep, 2010). Significantly lower NAs and AEF removals andthe higher peak intensity of SFS spectra in the de-oxygenatedsamples also highlight the important role of oxygen in theremoval of the organic compounds. The iron hydroxide precipitatesformed on the ZVI surface upon its contact with water (Joo et al.,2005; Shimizu et al., 2012) may decrease electron transfer to theZVI surface and the decomposition rate of the contaminant(Shimizu et al., 2012). As the reaction proceeds, the thickness of the

passivating layer would increase, which explains the decrease inthe removal efficiency of the organic compounds with time(Fig. S1). The availability of ZVI and oxygen, and the continuousoxidation of ZVI to generate Fe(II) and H2O2 could enhance theformation of hydroxyl radicals (Joo et al., 2004), which translatedinto high removal of the organic compounds. Although thedegradation of the organic compounds was a slow process, itseffectiveness at high pH suggests the possibility of the ZVI appli-cation for in situ OSPW remediation without any pH adjustment.

In low oxygen environments, ZVI is quickly oxidized to formFe(II) and Fe(III) products according to reactions (1) and (2), whichcould contribute to the adsorption/co-precipitation of the con-taminants. Additionally, generated electrons and hydrogen couldresult in a series of reductive reactions. However, in the presence ofhigh concentrations of oxygen in the solution, which is the case inthis study, besides the adsorption/co-precipitation of the contam-inants, oxidation is another removal mechanism. Reactions (3) and(4) show the possible pathways in the presence of oxygen and ZVIas suggested in a number of studies (Chen et al., 2012; Drzewiczet al., 2012; Joo et al., 2005, 2004; Lee et al., 2007; Shimizu et al.,2012; Stieber et al., 2011).The generated electrons from the oxida-tion of ZVI are transferred to oxygen, which results in the produc-tion of H2O2 (reaction (2)).

Fe0 þ 2 H2O4Fe2þ þ H2 þ 2 OH� (1)

Fe2þ/Fe3þþe� (2)

Fe0 þ O2/Fe2þ þ H2O2þ2OH� (3)

Fe2þ þH2O2/Fe3þ þ $OHþ OH� (4)

P. Pourrezaei et al. / Journal of Environmental Management 139 (2014) 50e58 57

The hydroxyl radical formed in reaction (4) reacts unselectivelywith organic compounds present in solution according to thefollowing mechanism (Drzewicz et al., 2012).

R �Hþ OH�/R

� þH2O (5)

R� þ O2/RO2

�(6)

2 RO2�/ROþ ROH (7)

Furthermore, a secondary reaction may also take place toremove more organic contaminants through the adsorption on thesurface of ZVI (Drzewicz et al., 2012).

Fe2þ þ OH$/Fe3þþOH� (8)

Fe3þ þ H2O2/ FeOOH2þþHþ (9)

In addition to the effect of the oxygen content, the adsorption ofcontaminants onto ZVI is also affected by ionic strength, tempera-ture, pH of the solution, and the presence of competitive ions(Boparai et al., 2013). In particular, the ion strength of the solutionshas a great impact on the adsorption process by affecting theelectrostatic repulsion between the adsorbent surface and theadsorbate molecules (Rangsivek and Jekel, 2005). A detailed studyof the effect of these parameters on the different removal mecha-nisms in warranted.

3.8. Removal mechanisms of trace metals

The removal mechanisms of the metals from ground waterswere reported to be based on the adsorption of the metals on thesurface of ZVI and co-precipitation with the iron oxy/hydroxidesduring the oxidation of ZVI (Abedin et al., 2011; Mak and Lo, 2011).Karthikeyan et al. (1997) showed that freshly formed iron hy-droxides have the binding sites for Cu adsorption. Cr (III) (Charletand Manceau, 1992), As (V) (Waychunas et al., 1993), and Cr (III),Ni, and Zn (Crawford et al., 1993) were shown to be removed by co-precipitation using iron oxy/hydroxides. The observed effect duringthe removal of these elements was attributed to the higher avail-able surface area of iron oxy/hydroxides, which in turn, containhigher binding sites for the adsorption. Metal ions could also beincorporated into the hydroxide lattice to form mixed-crystals,resulting in their co-precipitation (Crawford et al., 1993;Karthikeyan et al., 1997; Noubactep, 2010).

ZVI interaction with metal ions content in a solution is alsocomplex and follows different pathways. The first pathway is basedon the transformation of toxic metals to non-toxic forms andfurther precipitation or co-precipitation in terms of mixed Fe3þ andhydroxides (Blowes et al., 1997). Reactions (10)e(12) show thetransformation of toxic metals to nontoxic form.

Cr6þ þ Fe�/Cr3þþFe3þ (10)

ð1� xÞFe3þ þ ðxÞCr3þ þ 3H2O/CrxFeð1�xÞðOHÞ3ðsÞ þ3Hþ

(11)

ð1� xÞFe3þ þ ðxÞ Cr3þ þ 2H2O/Feð1� xÞCrxOOHðsÞþ3Hþ

(12)

However, the removal of other metals such as As by ZVI involvessurface complexation (Daus et al., 2004). The iron hydroxideformed on the surface of ZVI activate the As bond formation under

oxidizing conditions (Daus et al., 2004). It has been reported thatNi, Cu, V also form complexes with the hydroxide on the surface ofZVI and their consequence precipitation/co-precipitation in solu-tion (Shokes and Moller, 1999). Uranium reacts with ZVI in a redoxreaction in the proximity of ZVI in which U(VI) is reduced to U(IV)followed by precipitation on the surface of solid iron according toreactions (13) and (14) (Groza et al., 2009), and Se is removedthrough dissociative adsorption on the surface of ZVI.

UO22þ þ 4Hþ þ 2e�/U4þþH2O (13)

U4þþ2H2O/UO2ðsÞþ4Hþ (14)

4. Conclusion

The results of the present study clearly indicated that thepresence of PC enhanced the removal of the organic compoundsin OSPW by facilitating the electron transfer from ZVI to the re-actants. ZVI/PC could be successfully applied for the removal ofthe organic compounds, including NAs and fluorophore organiccompounds from OSPW. PC could serve as simultaneous adsorp-tion and oxidation sites for the organic compounds. This is theadvantage of the PC as an electron conductor over the non-conductive adsorbents such as polymers for OSPW remediation.Furthermore, CZVI-treated OSPW showed less toxicity toward V.fischeri as compared to non-treated OSPW. The promising resultsobtained in this study showed that the proposed treatment has apotential for use as an in situ process on an industrial scale to treatthe large volumes of OSPW. In addition, this process will utilizethe PC, which is available free of charge and is stockpiled on theoil production sites. The study of residual management, includingthe disposal and/or re-use of treatment wastes as well as theassessment of the by-products formed during CZVI and ZVItreatments, by using model NAs and OSPW samples are subjectsof our next studies. Further studies are also warranted to elucidatethe removal mechanisms of the different organic species and tracemetals present in OSPW.

Acknowledgements

The authors acknowledge the financial supports provided by theresearch grants from Alberta Innovates-Energy and EnvironmentSolutions, Syncrude Canada Ltd., Helmholtz-Alberta Initiative (HAI)(RES0006289), NSERC research grant for the research tools andinstruments, and NSERC Industrial Research Chair in Oil SandsTailings Water Treatment (RES0008827). The authors acknowledgethe Alberta Centre for Surface Engineering and Science at theUniversity of Alberta for XPS analysis.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jenvman.2014.03.001

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