Applied Clay Science xxx (2014) xxx–xxx

CLAY-03011; No of Pages 9

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Applied Clay Science

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Research paper

Surface interactions of pyrene and phenanthrene on Cu-montmorillonite

Hadas Joseph-Ezra, Ahmed Nasser ⁎, Uri MingelgrinInstitute of Soils, Water and Environmental Sciences, ARO, Volcani Center, PO Box 6, Bet Dagan 50250, Israel

⁎ Corresponding author. Tel.: +972 39683306; fax: +9E-mail address: nasser@volcani.agri.gov.il (A. Nasser).

http://dx.doi.org/10.1016/j.clay.2014.04.0370169-1317/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Joseph-Ezra, H., ethttp://dx.doi.org/10.1016/j.clay.2014.04.037

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 December 2013Received in revised form 20 April 2014Accepted 29 April 2014Available online xxxx

Keywords:Abiotic degradationCu-montmorillonitePAHPhenanthrenePyrene

The feasibility of promoting degradation of pyrene and phenanthrene (PHE) at the surface of soil minerals isdemonstrated. Samples of magnetite, birnessite, Na-montmorillonite and Cu-montmorillonite (Cu-Mt) wereloaded with pyrene and PHE. While no significant degradation of the PAHs was observed on Na-montmorillonite, birnessite or magnetite, considerable degradation took place on Cu-Mt loaded with pyrene(60 ± 2% in 24 h). The initial load of pyrene affected the extent of degradation, with the highest degradationrate observed at a load of 1000 μg/g clay mineral. Pyrene degradation on Cu-Mt was inhibited by grinding theloaded mineral as well as by wetting it. Loading PHE on Cu-Mt resulted in little degradation of the PAH, butwhen the clay mineral was loaded with pyrene and PHE together, the degradation of pyrene was enhanced ascompared to its degradation in the absence of PHE. The degradation of PHE may be inhibited by its higher ioni-zation potential which reduces its tendency to participate in electron transfer reactions. The observed differencebetween the two PAHs in their affinity to various sites on themineral's surfacemay also contribute to the dispar-ity in their tendency to degrade at the surface. In experiments performed on soil samples mixed with Cu-Mt andloaded with the PAHs, patterns of degradation similar to those on the mineral itself were observed, but withlower degradation rates than those on neat Cu-Mt. LC–MSanalysis revealed that pyrene underwent on Cu-Mt ox-idation as well as dimerization. Elemental analysis conducted by SEM–EDS, revealed that both PHE and pyrenedisplayed a higher affinity to the edges of the clay mineral particles than to their planar surfaces. PHE displayeda higher affinity thanpyrene to Cu-Mt, both at the planar surfaces and at the edges of the particles of the claymin-eral. The surface sites which displayed the highest affinity to the PAHs were not catalytically active and hence,loading Cu-Mt with a PHE and pyrene mixture resulted in saturation of the more strongly adsorbing sites withPHE and the diversion of more pyrene molecules to the catalytically active sites.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Polycyclic aromatic hydrocarbons (PAHs) are widespread organicpollutants, created by the incomplete combustion of organic matter.PAHs pose a significant danger to human health due to their recalci-trance in the environment and their carcinogenic potential (Jenning,2012). The PAHs pyrene and phenanthrene (PHE) were frequently de-tected as contaminants in soils (e.g., Harayama, 1997). Removal ofPAHs from soils is often necessary because of their resistance to break-down by natural processes and their resultant slow attenuation(ATSDR, 1995). There aremanymethods for enhancing the degradationof organic pollutants in the soil, andmost are based on biological proce-dures (e.g., Khan et al., 2004). However, biological breakdownmethodsoften display shortcomings such as a slow removal of the organic sub-strate or relatively high concentrations of residues remaining in thesoil at the end of the process (Mulder et al., 2001). PAHs, due to their hy-drophobic nature and resultant low solubility, tend to sorb to the organ-ic fraction of soils. This sorption increases their resistance to microbial

72 39683301.

al., Surface interactions of pyr

degradation (Alexander, 2000; Mulder et al., 2001). It is expedient,therefore, to develop complementary, abiotic remediation methodsand those are being pursued by various groups (e.g., Manariotis et al.,2011; Usman et al., 2012; Yap et al., 2012).

Montmorillonite, manganese oxides and iron oxides are commonsoil minerals and are known to induce abiotic transformations of organ-ic pollutants, including hydrolysis, oxidation–reduction or rearrange-ment (e.g., Di Leo, 2000; Jia et al., 2012; Mingelgrin and Saltzman,1979; Mortland and Halloran, 1976; Nasser et al., 1997, 2000, 2009;Nasser et al., 2012; Pizzigallo et al., 1995). Hinedi et al. (1993) studiedthe degradation of benzene on Cu-montmorillonite (Cu-Mt) and report-ed that when benzene molecules came in contact with Cu ions, freeradicals formed, causing rapid oxidation and polymerization of the ben-zene. The possibility that a similar process occurs when PAH moleculesinteract with Cu-Mt is investigated in the present study.

The overall objective of this work is to demonstrate that it is possibleto degrade PAHs by bringing them in contact with common mineralsand to establish the potential of abiotic processes to remediate soils con-taminated by PAHs. The impact of applying amechanical force to Cu-Mtloaded with pyrene, PHE or a mixture of both, on the surface-enhanceddegradation of the PAHs was explored.

ene and phenanthrene on Cu-montmorillonite, Appl. Clay Sci. (2014),

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2. Materials and methods

2.1. Materials

Pyrene (C16H10), 99% purity, and PHE (C14H10), 98% purity, were ob-tained from Sigma-Aldrich, Switzerland. A pyrene stock solution(10,000 μg/ml), PHE stock solution (10,000 μg/ml) and pyrene–PHEstock solution (5000 μg/ml of each), were prepared in acetonitrile(ACN), and then stored in glass vials sealed with caps lined with Teflonsepta and kept refrigerated at 4 °C. These stock solutions were used toload the soil and mineral samples with a known concentration of thePAHs. All reagents were of analytical grade, purchased from Merck,Germany. All solvents were of HPLC grade and purchased from BioLabLtd., Israel.

The investigated minerals were: magnetite (Fe3O4), provided bythe Geological Survey of Israel; Na-montmorillonite (Na-Mt), pur-chased from Fischer Scientific, USA; Cu-Mt, prepared as described inSection 2.2.1; and manganese oxide (birnessite, δ-MnO2), synthesizedas described by Nasser et al. (2009). The soil used was the surface hori-zon (0–20 cm) of a sandy loam soil (Hamra) from Bet Dagan, Israel.

2.2. Methods

All runs were performed in triplicates and the standard deviationwas calculated.

2.2.1. Preparation of loaded mineral and soil samplesCu-Mtwas prepared bywashing the original Na-Mt three timeswith

a 0.5 M solution of CuCl2, and then four times with distilled water. Ineach wash the mineral:solution ratio was 1:10 (w/v). While the pH ofthe CuCl2 solution was 3.5, the pH of the montmorillonite suspensionat the firstwashingwith the CuCl2 solutionwas 4.6 and at the lastwash-ing with the distilled water it reached 5.8. The empirical formula ofmontmorillonite is (Si4)IV(Al2)VIO10(OH)2 ∙ nH2O, where IV and VI de-note tetrahedral and octahedral sheets respectively. Isomorphous sub-stitution, namely the substitution of Al and Si atoms in the mineralmatrix by iron as well as other cations (Al may also substitute for theSi), occurs in montmorillonite, endowing the mineral with a net nega-tive charge which is compensated by exchangeable cations. The ex-changeable cation in the present study is Cu. The soil was air-dried,ground and sieved through a 2 mm mesh before being loaded withPAHs.

The loading procedure involved spiking the mineral or soil samplewith the pyrene and/or PHE stock solution to receive the desired finalload of PAH. Then, the spiked samples were mixed thoroughly andkept in a hood until a practically complete evaporation of the ACN(~1 h). The point in time at which themixturewas judged free of signif-icant residues of ACNwas defined as T0. The near complete evaporationof ACN was ascertained by the disappearance of all traces of gloss fromthe surface of the periodically mixed, thinly spread, spiked sample andby its return to room temperature after the cooling brought about bythe solvent's evaporation. The validity of this procedurewas ascertainedby quantifying the load of ACN left in selected samples by the procedureof Channing et al. (2001).

2.2.2. Extraction and analysisSamples of the loaded minerals (0.01 g) or soil (0.5 g) were extract-

ed with 3 ml ACN by sonication in an ultrasonic bath (MRC, ultrasoniccleaner), for 30 min and then centrifuged for 15 min at 1200 g, and fil-tered through a 0.4 μm syringe PTFE filter. The extracts were analyzedby high performance liquid chromatography (HPLC). The HPLC analysiswas carried out on a Shimadzu LC10AT HPLC, equipped with a photodi-ode array detector. The columnwas anHPLC cartridge LiChrocart® 250-4, LiChrospher® PAH (5 μm) (Merck, Germany), the mobile phase was20% double-distilled water and 80% ACN at a flow rate of 1 ml min−1,and the detector was set at 240 nm for pyrene and 251 nm for PHE.

Please cite this article as: Joseph-Ezra, H., et al., Surface interactions of pyrhttp://dx.doi.org/10.1016/j.clay.2014.04.037

The amount of pyrene or PHE that degraded was calculated by compar-ing the extracted amount to the amount of the PAH that was loaded onthe clay mineral.

2.2.3. Screening of the mineralsIn a preliminary experiment, samples of 0.25 g of eachmineral were

loadedwith 1000 μg/g pyrene. The sampleswere extracted at T0 and T24(24 h after T0). Based on the extent of degradation in the variouspreparates, Cu-Mt was selected for further study.

2.2.4. Preparates of Cu-montmorillonite loaded with PAHsCu-Mt samples containing 10, 100, 1000, 10,000 and 20,000 μg/g

pyrene, were studied to establish the effect of pyrene's initial concentra-tion on its rate of degradation.

Selected samples of 0.25 g Cu-Mt loaded with 1000 μg/g pyrenewere manually ground at T0 for 5 min, using a mortar (3 cm radius)and pestle.

To examine the influence of wetting on the degradation of pyreneloaded on Cu-Mt, 100 μl of distilled water were added to 0.25 g of Cu-Mt containing 1000 μg/g pyrene. Some of the wetted samples wereground as described above and the extent of degradation of pyrene inthe ground and non-ground samples was determined.

Cu-Mt samples were also loaded with 1000 μg/g and 100 μg/g PHE.Other Cu-Mt samples were loaded with both pyrene and PHE at a con-centration of 500 μg/g each and the degradation of the PAHs on thoseloaded samples was followed.

2.2.5. Particle size analysis and morphological determinationsThe size of Cu-Mt particles was determined in a dispersed state by a

laser-based particle size analyzer (PSA; HORIBA LA-910).Morphologicalfeatures of the various clay mineral-PAH systems were observed usingscanning electron microscopy (SEM; JSM-5410LV, JEOL Ltd.). Analyseswere run before and after grinding.

2.2.6. Degradation kinetics of pyrene sorbed on Cu-MtThe degradation kinetics of pyrene loaded on Cu-Mt was followed

for 30 days. Samples of Cu-Mt loaded with 1000 μg/g pyrene were ex-tracted (Section 2.2.2) at various times (0, 0.17, 0.5, 1, 2, 3, 4, 7 and 24 hafter loading, as well as after 14 and 30 days).

2.2.7. Elucidation of the spatial distribution of pyrene and PHE on Cu-MtCu-Mt samples loaded with pyrene or with PHE were examined by

X-ray diffraction (XRD) and underwent elemental analysis using a scan-ning electron microscope (SEM) equipped with a SEM–EDS (scanningelectron microscopy–energy dispersive spectroscopy) microprobe. TheXRD was performed with a Philips PW 1830/3710/3020 diffractometerwith a Cu-Kα1 radiation source, wavelength of 1.541 Å and at only firstorder reflection (d001). SEM readings were made on a JEOL Ltd.(Tokyo Japan), model JSM-5410LV micrograph. Microprobe analyses(Link ISIS X-ray analysis; Oxford Instruments, High Wycombe, UK)were used to extract the distribution of the PAHs at the mineral surfacefrom the density distribution of carbon atoms across the surface (limitof lateral resolution ~1–2 μm in diameter).

2.2.8. Abiotic degradation of pyrene and PHE loaded on soilPyrene and PHE were loaded on soil samples (Section 2.1) in the

presence of Cu-Mt. Soil samples containing 1000 μg/g pyreneunderwent four treatments: 1) no grinding no Cu-Mt added; 2)manualgrinding, noCu-Mt added; 3) no grinding, 3% Cu-Mt added; and 4)man-ual grinding, 3% Cu-Mt added. Soil samples already containing Cu-Mtbefore pyrene addition were also studied. Soil mixed with 10% Cu-Mtwas loaded with 1000 μg/g pyrene and then 1 g samples of the loadedmixture was ground in a mortar (7 cm radius). Also, pyrene and PHEwere loaded together at a concentration of 500 μg/g each on soil sam-ples containing 10% Cu-Mt, and 1 g samples of the loaded mixture wasground manually. The samples were extracted at T0 and T24.

ene and phenanthrene on Cu-montmorillonite, Appl. Clay Sci. (2014),

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Fig. 1. Particle diameter analysis of Cu-Mt before and after manual grinding for 5 min.

3H. Joseph-Ezra et al. / Applied Clay Science xxx (2014) xxx–xxx

2.2.9. Degradation productsExtracts (Section 2.2.2) from samples in which degradation of the

PAHs was detected were analyzed by HPLC (ibid) and LC–MS in searchfor degradation products of pyrene and PHE. LC–MS analysis was per-formed on an Accela high speed LC system (Thermo Fisher Scientific,Inc.), equipped with an Accela PDA detector. The chromatographic sep-arations were performed using a Phenomenex Synergi Fusion-RP col-umn, 2 × 100 mm and 2.5 μm particle size. Isocratic conditions weremaintained with the mobile phase being MeOH:H2O 80:20. Columntemperature was maintained at 30 °C. MS detection was performedwith a LTQ Orbitrap Discovery hybrid FT mass spectrometer equippedwith an electrospray ionization ion source. Ionization parameterswere: spray voltage of 3.5 kV, capillary temperature of 250 °C, capillaryvoltage of 35 V and scanning range (m/z) of 100–1000Da. Xcalibur soft-ware (Thermo Fisher Scientific, Inc.) was used.

3. Results and discussion

3.1. Mineral screening

The different minerals (magnetite, birnessite, Na-Mt and Cu-Mt)loaded with 1000 μg/g pyrene or PHE were tested for their capacity toenhance degradation of the PAHs. No significant degradation was ob-served on magnetite, birnessite or Na-Mt. On Cu-Mt, however, pyreneconcentration was reduced to 745 ± 35 μg/g at T0 and to 400 ±20 μg/g at T24. T0 (Section 2.2.1), is a point in time of ~1 h after spikingthe mineral with the PAH and it seems that degradation had com-menced at the instance of spiking, so that at T0, ~25% of the appliedpyrene already degraded. Cu-Mt did not enhance significant PHE degra-dation (Section 3.7).

3.1.1. Cu-Mt/PAH interactionsPyrene loaded on Cu-Mt degraded spontaneously and efficiently,

though such degradation did not occur when the PAH was loaded onthe other minerals tested. Cu-Mt consists of alumino-silicate layerswith copper serving as a counter cation which is located both betweenthe layers and on the clay mineral particles' outer surface. Copper hasa catalytic capacity that may have brought about the observed fast deg-radation of pyrene just as it did the breakdown (and polymerization) ofother aromatic species (e.g., Boyd and Mortland, 1986; Doner andMortland, 1969; Mortland and Halloran, 1976). Accordingly, Hinediet al. (1993), reported thatwhen benzene came in contactwith the cop-per ions on the surface of Cu-Mt, the benzene molecules transformedinto cation radicals, which in turn produced oxidized and polymerizedspecies. A similar reaction may occur with pyrene, which like benzene,is an aromatic CH system (the observed resistance of PHE to degrada-tion is discussed in Section 3.7). Accordingly, an attempt was made toelucidate the processes driving the degradation reactions of the PAHsand the effect of various factors on the reactions.

3.2. Manual grinding

There are numerous reports on the enhancement of surface degra-dation of organic compounds by mechanochemical means such asgrinding (e.g., Napola et al., 2006; Nasser and Mingelgrin, 2012;Nasser et al., 2000; Nomura et al., 2012; Pizzigallo et al., 2004). Conse-quently, the effect of grinding on the degradation of the PAHs on thesurface of Cu-Mt was explored. As expected (since grinding was per-formed at T0; Section 2.2.4), in ground samples of Cu-Mt loaded with1000 μg/g pyrene, 299 ± 35 μg/g pyrene disappeared until T0 (as com-pared to 255 ± 35 μg/g in the non-ground samples). However, nofurther degradation was observed after grinding. The slightly lowerpyrene concentration at T0 in the ground samples (in which T0 wasdefined as the point in time at which grindingwas terminated) as com-pared to the non-ground samplesmay be accounted for by the degrada-tion that occurred during the time that passed between the end of

Please cite this article as: Joseph-Ezra, H., et al., Surface interactions of pyrhttp://dx.doi.org/10.1016/j.clay.2014.04.037

solvent evaporation (Section 2.2.1) and the end of grinding (~5 min).While the above difference in the rate of pyrene degradation at T0between the ground and non-ground samples is, as it should be, barelysignificant, the results presented in this section and in Section 3.1demonstrate, perhaps surprisingly, that while pyrene loaded on Cu-Mtdegraded spontaneously and rather rapidly (60% in 24 h), grindingstrongly suppressed the degradation. A possible explanation for the in-hibitory effect of grinding on the degradation of pyrene is offered below(Section 3.3).

Samples of Cu-Mt loadedwith pyrene that did not undergo grindingchanged color from gray to a reddish hue, while the ground samples didnot change color. It is possible that the above color change might haveresulted from the reduction of Cu+2 to its elemental form. This sugges-tion, though speculative, is in agreementwith the observed oxidation ofthe PAH (Section 3.10.2) and the assertion that Cu ions served as elec-tron acceptors (e.g., Hinedi et al., 1993).

3.3. Particle size and morphology of Cu-Mt

Grinding reduced the particle size of the clay mineral drastically(Fig. 1). The diameter of 90% of the particles was b50 μm after grindingas compared to 240 μm before grinding. Fig. 2 demonstrates that grind-ing also caused a significant change in the inner structure of the Cu-Mtparticles, namely in the way the Cu-Mt tactoids are arranged within theclay mineral's particles. Grinding resulted in a collapse of the particles,analogous to the collapse of a house of cards (Fig. 2) and those PAHmol-ecules that were originally adsorbed to an external planar surface of astack of layers (or tactoid), became trapped between surfaces of two ad-jacent tactoids that caved in on top of each other. This change in the PAHmolecules' position vis a vis the Cu-Mt surfaces had to affect the interac-tion of the molecules with potentially active (e.g., charged or highlypolar) surface sites and hence the tendency of themolecules to undergoa surface reaction. The changes in themorphology of the Cu-Mt particlesupon grinding and the resultant change in the orientation of the sorbedPAH molecules relative to the surfaces of the clay mineral layers maythus explain why grinding inhibited pyrene degradation on Cu-Mt.

3.4. Effect of the load on pyrene degradation

Loading pyrene on Cu-Mt at different initial concentrations affectedthe PAH's degradation rate. The initial concentrations tested were: 10,100, 1000, 10,000 and 20,000 μg/g. The respective percentages of degra-dation at T24 were: 0%, 31%, 63%, 58% and 34% respectively (Fig. 3) andthe corresponding quantities of pyrene degraded were: 0, 31, 631,5860 and 6787 μg/g, respectively. Thus, initial surface concentrationaround 1000 μg/g resulted in the highest percent degradation.

Pyrene degrades apparently only at specific active sites on themineral's surface, but it also adsorbs at sites on which no degradationoccurs. The observed pattern of degradation rates (Fig. 3), suggests thefollowing sequence of adsorption of pyrene on the different sites thatexist on the heterogeneous Cu-Mt surface: the sites on which pyrene

ene and phenanthrene on Cu-montmorillonite, Appl. Clay Sci. (2014),

Fig. 2. SEM images (×5000) of Cu-Mt + 1000 μg/g pyrene, (a) without grinding and (b) after manual grinding.

4 H. Joseph-Ezra et al. / Applied Clay Science xxx (2014) xxx–xxx

adsorbs preferentially do not enhance degradation and only after thesesites are nearing saturation, the sorption on the catalytically active sitesbegins to dominate. This is why at the lower pyrene loads, the percentdegradation is low (Fig. 3), and it increases up to a maximum at around1000 μg/g, decreasing again at higher loads. The decrease in degrada-tion at the highest loads suggests a saturation of the catalytically activesites and sorption of the surplus load of pyrene at different, non-activesurface sites or at a different configuration vis a vis the same surfacesite (e.g., a multilayer of the PAH).

3.5. Kinetics of degradation of pyrene on Cu-Mt

Since the concentration of pyrene loaded on Cu-Mt changed signifi-cantly between T0 and T24, amore detailed observation of the kinetics ofdegradation was undertaken.

Cu-Mt loaded with 1000 μg/g pyrene was analyzed for pyrene con-centration at T0 and 0.17, 0.5, 1, 2, 3, 4, 7, and 24 h afterwards, as wellas two weeks and one month later (Fig. 4a, b). The results indicatethat surface degradation begins immediately upon the loading ofpyrene. Most of the degradation occurred in thefirst 24 h, and then con-tinued at a much slower rate. The rate of reaction slowed down consid-erably after about 60% of the loaded pyrene broke down (Fig. 4a, b). Asexpected for a surface reaction in the absence of a liquid phase, the ki-netics was not a simple integral order. The fraction of the PAH that de-graded during the first, fast degradation phase is, most likely, thefraction that adsorbed at the catalytically-active surface sites.

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5-10

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Fig. 3. Initial concentration of pyrene loaded on Cu-Mt vs. percentage of pyrene degradedat T24.

Please cite this article as: Joseph-Ezra, H., et al., Surface interactions of pyrhttp://dx.doi.org/10.1016/j.clay.2014.04.037

3.6. Effect of wetting on pyrene degradation

If copper ions are responsible for the degradation of pyrene, the ad-dition of water may interfere with this process, as it competes withpyrene for adsorption on the strongly hydrophilic Cu ions, even as anouter shell ligand of the already hydrated ions. Accordingly, the effecton pyrene degradation of adding water to Cu-Mt loaded with 1000 μg/gpyrene was studied.

Addition ofwater (40%w/w), did not result in the formation of a freeliquid phase yet it inhibited almost completely the degradation ofpyrene. These results are in agreement with those of Hinedi et al.(1993) who suggested that in the breakdown of benzene sorbed onCu-Mt a single electron transfer from the benzene molecule to the Cuion occurred. When multilayers of water were present at the claymineral's surface (and accordingly, the sorbed Cu ions possessed a num-ber of hydration shells), the reaction was totally inhibited. It is likelythat in the case of pyrene, a similar process occurs and accordinglypyrene degradation is also inhibited by the presence of excess water.

3.7. Cu-Mt loaded with PHE and with a pyrene/PHE mixture

PHE was loaded on Cu-Mt at a concentration of 1000 μg/g and sub-samples were then ground manually for 5 min. Only little degradationof PHE (~15% at T24) was observed (Fig. 5), both after and withoutgrinding. The difference in the rate of degradation between pyreneand PHE obviously stems from the difference between their molecularstructures (Scheme 1). PHE contains three benzene rings and pyrenefour. In general, the larger the number of rings in a polyaromatic system,the smaller is the gap between the ground and the excited energy levelsand the ionization potential tends to be lower (e.g., Dabestani andIvanov, 1999). Thus, pyrene (ionization potential 7.4 eV, ibid) is morelikely to transfer an electron to a Cu ion (thereby initiating the degrada-tion) than PHE (ionization potential 7.9 eV, ibid). This suggested mech-anism of electron transfer from the PAH to a Cu ion is substantiated bythe nature of the observed degradation products (Section 3.10). Unlikepyrene, PHEwas resistant to protonation in the positive ionization pro-cess employed in the LC–MS analysis (Section 3.10.2). This observationsupports the assertion that it is the higher energy required for the PHE–Cu electron transfer that retards the degradation of PHE on Cu-Mt. Thestructure of a molecule also affects its affinity to various surface sitesand its preferred orientation vis a vis the surface. The mode of adsorp-tion, in turn, may determine the rate of a surface reaction. Hence, thedifference in the rate of degradation at the clay mineral's surface be-tween the PAHs may also arise from a different adsorption behavior.The spatial distribution of each of the two investigated PAHs at the sur-face of Cu-Mt is discussed in Section 3.8.

When pyrene and PHE were loaded together on Cu-Mt at a concen-tration of 500 μg/g each, little PHE degradation occurred, with or with-out grinding, just as when it was loaded alone (Fig. 6a). On the otherhand, degradation of pyrene when loaded together with PHE, reached

ene and phenanthrene on Cu-montmorillonite, Appl. Clay Sci. (2014),

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Fig. 4. Degradation kinetics of pyrene loaded on Cu-Mt at an initial concentration of 1000 μg/g (a) from T0 to T24 and (b) from T0 to 42 days.

5H. Joseph-Ezra et al. / Applied Clay Science xxx (2014) xxx–xxx

in non-ground samples 31.2 ± 1.2% at T0 and 89.9 ± 3% at T24. Aftergrinding, pyrene degradation reached 75.5±4.5% at T0 (namely, imme-diately following 5 min of grinding) and 94.5 ± 1.3% at T24 (Fig. 6a).After a month, PHE still remained practically unaffected, while pyrenelost 97% of its original load in the non-ground samples and over 99%in the ground ones (Fig. 6b).

The above results support the hypothesis that pyrene and PHE differin their affinity to the various types of adsorption sites at the Cu-Mt sur-face. It seems that PHE preferentially adsorbs on catalytically inactivesites, thus reducing the availability of such sites to pyrene, causing alarger fraction of the loaded pyrene to adsorb on the catalytically activesites and thus resulting in a higher rate of pyrene degradation as com-pared to that in the absence of PHE.

3.8. Spatial distribution of pyrene and PHE at the surface of Cu-Mt

Adsorption can occur at diverse sites on Cu-Mt particles (e.g., at in-terlayer spaces, outer planar surfaces or broken edges), thus exposingthe sorbed molecules to very different environments. PAH-loaded Cu-

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Fig. 5. Surface concentration of PHE loaded on Cu-Mt at an initial concentration of 1000 μg/gat T0 and T24, without and after manual grinding.

Please cite this article as: Joseph-Ezra, H., et al., Surface interactions of pyrhttp://dx.doi.org/10.1016/j.clay.2014.04.037

Mtwas analyzed byXRD and by SEM–EDS to try and determine the pre-ferred adsorption sites of pyrene and PHE.

3.8.1. XRD determinationsThe XRD diffraction patterns showed that all Cu-Mt samples loaded

with pyrene or PHE had a similar c-spacing of 12.5 Å. The spacing of theunloaded Cu-Mt was 13.4 Å. Namely, the spacing between the layers ina tactoid did not increase by addition of the PAHs. The somewhat largerspacing of the unloaded Cu-Mt may have resulted from a partial dehy-dration of the clay mineral during the loading of the PAHs due to wet-ting with ACN (Section 2.2.1). Hinedi et al. (1993) assumed thatbenzene molecules adsorb on Cu-Mt mainly in the interlayer spaces,and indeed, interlayer sorption of organic and specifically aromatic spe-cies onmontmorillonitewaswidely reported (e.g., Greene-Kelly, 1955).However, if the PAH molecules were adsorbed between the Cu-Mtlayers, it would have been expected that the c-spacing increase by atleast 1.4 Å, the covalent diameter of carbon atoms making up the aro-matic rings, while loading with PAHs did not result in any increase inthe interlayer spacing. There are two possible explanations for this:1) the PAH molecules are located in the interlayer spaces between(rather than on top of) the Cu ions. The measured 12.5 Å c-spacing im-plies a sufficient opening between the layers to accommodate the PAH

Pyrene Phenenthrene

Scheme 1.

ene and phenanthrene on Cu-montmorillonite, Appl. Clay Sci. (2014),

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PHE-nogrinding

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T0 T24

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b

Fig. 6. Surface concentration of PHE and pyrene loaded on Cu-Mt at an initial concentra-tion of 500 μg/g each, with or without manual grinding: (a) at T0 and T24 and (b) afterone month.

50 µm

6 H. Joseph-Ezra et al. / Applied Clay Science xxx (2014) xxx–xxx

molecules in a side by side arrangement without further expanding thespacing. However, a side by side orientation of the PAH molecules andthe Cu ions would have reduced the likelihood of the electron transferreaction that was observed. 2) There is little penetration of PAH mole-cules into the interlayer spaces. In that case, the catalytically activesites must be located outside the interlayer spaces.

3.8.2. SEM analysesThe hypothesis that on Cu-Mt pyrene adsorbsmore readily than PHE

to sites which enhance degradation while PHE adsorbs preferentially tosites which do not enhance degradation, was tested using elementalanalysis with SEM–EDS. The load of pyrene or PHE at a given locationon the surface was indicated by the atomic percent of carbon, takinginto account the carbon content of the unloaded Cu-Mt.

The systems checked were: Cu-Mt; Cu-Mt/PHE (1000 μg/g) and Cu-Mt/pyrene (1000 μg/g and 10,000 μg/g). Elemental analyses were per-formed on both of extended patches across the planar surfaces of theclay mineral's particles (~60 × 40 μm in area), and at specific locations(~1–2 μm in diameter, the limit of lateral resolution of the analyticalsystem). The latter point-analyses were performed both at the planarsurfaces and the edges of the Cu-Mt particles.

Table 1 summarizes the atomic percent of carbon measured at thebroad patches on the planar surface. As is evident from the chemicalcomposition of Cu-montmorillonite (Section 2.2.1), the exact ratio of

Table 1Gross elemental analysis of the planar surfaces of Cu-Mt loaded with PHE or pyrene(atomic %).

Element Cu-Mt Cu-Mt/PHE1000 μg/g

Cu-Mt/pyrene1000 μg/g

Cu-Mt/pyrene10,000 μg/g

Carbon 5.4 27.5 12.2 38.4Copper 1.1 1.2 1.0 2.8Others 93.5 71.3 86.8 58.8

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cations (and hence the formula weight of the mineral) varies with thespecific montmorillonite sample. When converting the atomic percentto aweight percent, the different atomicweights of the elements shouldbe considered. In the case of carbon loaded on montmorillonite, theconversion factor for carbon from atomic to weight percent is, to a closeapproximation, 0.6. The percent weight of carbon obtained for Cu-Mt,Cu-Mt/PHE (1000 μg/g), Cu-Mt/pyrene (1000 μg/g) and Cu-Mt/pyrene(10,000 μg/g) was thus, 3.2%, 16.5%, 7.3% and 23.0%, respectively.

The above measured carbon weight percentages are much higher(even after taking into account the initial carbon content of themineralbefore the PAHs were loaded— 3.2% by weight), than those that wouldhave been expected had the PAH load been evenly distributed through-out the volume of the clay mineral's particle. For example, when themineral was loaded with 10,000 μg/g pyrene, 1% carbon content byweight and when the load was 1000 μg/g pyrene, 0.1% carbon contentwould have been expected had the added PAH been evenly distributedthroughout the volume of the Cu-Mt particle. The penetration depth ofthe electron beam used in the elemental analysis is ~1–2 μm and henceconsiderably larger than the thickness of the individual clay minerallayer (c-spacing ~12.5 Å), but much smaller than the average thicknessof the analyzed clay mineral's particles (e.g., Fig. 7). Consequently, therecorded carbon percentages clearly indicate that the loaded PAH mol-ecules concentrated at the outermost part of the claymineral's particles,but the analysis could not distinguish between sorption in the interlayerspaces and sorption at the external surfaces of the tactoids located nearthe outer borders of the clay mineral particle. The other elements de-tected by SEM–EDS are distributed evenly throughout the volume ofthe Cu-Mt particles and indeed a correct reading of the relative shareof each element in the crystal structure was obtained.

The carbon percentmeasured in samples loadedwith 1000 μg/g PHEwas much higher than in samples loaded with 1000 μg/g pyrene(Table 1), indicating a different affinity of each PAH to the various sorp-tion sites. Thus, a larger fraction of pyrene than of PHE either adsorbedat the particles' edges or penetrated deeper into the Cu-Mt particle.

Point elemental analysis was performed on the surface of Cu-Mt par-ticles (Fig. 7), comparing the elements' distribution at the planar sur-faces of the clay mineral's particles to that at the particle's edges. Acomparison of particles loaded with 1000 μg/g pyrene to those contain-ing a similar load of PHE (Table 2), reveals that particles loaded withPHE contain a higher percentage of C both at the external planar sur-faces and at the edges of the Cu-Mt particles. In the case of both PAHsthe carbon content at the particles' edges was significantly higher thanthat at the planar surfaces. Since the content of pyrene was lower thanthat of PHE, both at the planar surfaces and at the edges, when both

Fig. 7. Specific points on the planar surface and on edges of a Cu-Mt particle analyzed bySEM microprobe (stars mark the probed spots: blank star — edges; full star — planarsurface).

ene and phenanthrene on Cu-montmorillonite, Appl. Clay Sci. (2014),

Table 2Elemental analysis of Cu-Mt loaded with PAHs probed at specific spots at planar surfacesand edges of the clay mineral particles (atomic %).

Loaded PAH Element Planar surface Edges

PHE (1000 μg/g) Ca 19.2 (0.3)b 28.3 (7.6)Cu 1.7 (0.1) 1.7 (0.2)Others 73.8 64.7C/Cu 11.3 16.6

Pyrene (1000 μg/g) Ca 10.1 (4.2) 14.7 (1.0)Cu 1.4 (0.5) 1.7 (0.4)Others 83.2 78.3C/Cu 7.2 8.6

Pyrene (10,000 μg/g) Ca 40.5 (3.0) 31.0 (2.4)Cu 2.5 (0.2) 1.5 (0.1)Others 51.7 62.2C/Cu 16.2 20.7

a Percentage C after subtracting carbon content of PAH-free clay mineral.b Values in parentheses are standard deviations.

y = 0.6131x -0.1328r2 = 0.59

y = 1.0282x + 0.1355r2 = 0.62

0

1

2

3

4

0 1 2 3 4

%P

AH

%Cu

y = 0.4368x + 0.0266r2 = 0.74

y = 1.1919x -0.4963r2 = 0.75

0

0.5

1

1.5

2

2.5

3

3.5

0 0.5 1 1.5 2 2.5 3

%P

AH

%Cu

pyrene edges PHE edges

a

pyrene planar surface PHE planar surface

b

Fig. 8.Molecular percent of pyrene and PHE vs. atomic percent of Cu at the edges (a) andon the planar surfaces (b) of Cu-Mt at a PAH load of 1000 μg/g.

7H. Joseph-Ezra et al. / Applied Clay Science xxx (2014) xxx–xxx

PAHs were loaded at the same rate (Table 2), a significant amount ofpyrene must have been located deeper inside the clay mineral particle,hidden from the electron beam.

Given that Cu-Mt particles are poorly organized aggregates oftactoids stacked in a card house-like, porous structure (e.g., Fig. 2a),the following organization of the PAH molecules in the Cu-Mt particlesis proposed: the PAH molecules sorb, most likely, on the outer surfacesof the tactoids, since XRD did not reveal any expansion in c-spacingupon PAH addition (Section 3.8.1). Apparently PHE, when added dis-solved in ACN, sorbs more readily than pyrene and hence more of theapplied PHE is retained closer to the outer border of the clay mineralparticle and less of it than of pyrene penetrates into the inner cavitiesof the particle. This is possibly due to the PHE molecule's smaller sizeand the occurrence of a process akin to size exclusion chromatography.

While the concentration of pyrene loaded at 10,000 μg/g was lowerat the edges than at the planar surfaces (Table 2), the concentration ofboth pyrene and PHE loaded at 1000 μg/g, was higher at the edges(ibid). At lower loads, the edges (which seem to be the location of thehigher-affinity adsorption sites), took up much of the applied PAH, butas the amount of PAH added increased, the preferred sites at the edgesapproached saturation and a larger fraction of the added PAHmoleculesadsorbed at lower-affinity (or less accessible) sites on the planarsurfaces. The dependence of the degradation of pyrene on its load(Section 3.4, Fig. 3), indicates that the sorption sites with the highest af-finity are not the catalytically active ones. Hence, it may be hypothe-sized that the bulk of pyrene degradation takes place on the planarsurfaces rather than at the broken edges.

Elemental analysis revealed a good correlation between the contentof carbon (and hence of PAH molecules) and the content of copper(Fig. 8). This correlation is consistent with the suggestion that copperions are themajor sorption sites of the PAHs. Dividing the ratio betweenthe atomic percentages of carbon and copper by the number of carbonatoms in a PAHmolecule, yields the numerical ratio between the mole-cules of that PAH and copper ions at a given spot at the surface. Fig. 8demonstrates that at a 1000 μg/g load, the ratio of PHEmolecules to cop-per ions is almost twice that of pyrene.

Loading Cu-Mtwith amixture of pyrene and PHE enhanced the deg-radation of pyrene (Section 3.7), although PHE seems to sorb preferen-tially to theCu ions. This outcomemay be explained as follows: Cu existsat the clay mineral's surface in various chemical forms. In the prepara-tion of Cu-Mt, Na-Mt and CuCl2 are used. Cu+2 tends to hydrolyze inaqueous solution, part of it turning, for example, toCu(OH)+, a reactionleading to a drop in pH (Section 2.2.1). The presence of hydroxy-copperspecies results in the accumulation and possibly the formation ofvarious Cu-oxyhydroxides at the clay mineral's surface. Since the sur-face charge at the tactoid's broken edges is pH dependent, the lowerthe pH, the less negative the charge at the edges and the moreoxyhydroxides, whose positive charge is lower than that of Cu+2, will

Please cite this article as: Joseph-Ezra, H., et al., Surface interactions of pyrhttp://dx.doi.org/10.1016/j.clay.2014.04.037

tend to accumulate there (e.g., Bahranowski et al., 1998; Ding andFrost, 2004; Morton et al., 2001). At the planar surfaces, where the neg-ative charge density is pH-independent, hydrated Cu+2 ions are likely tobe more prevalent. It is possible that PHEmolecules which adsorb pref-erentially at the edges of the Cu-Mt particles on Cu specieswhich do notenhance PAH degradation, block these Cu species to pyrene. As a result,pyrenemay sorb to a larger extent on the degradation-inducing copperspecies in the presence of PHE than in its absence. Hinedi et al. (1993)assigned hydrated Cu+2 ions as the benzene-degrading species on theCu-Mt surface, in agreement with the above suggested prevalence ofCu+2 ions at the planar surfaces and the proposition that these surfacesare the major location of the degradation-inducing sorption sites.

3.9. Abiotic degradation of pyrene and PHE in a soil containing Cu-Mt

No degradation was observed when 3% (W/W) Cu-Mt was added tosoil samples loadedwith pyrene, either after or without grinding.Whenthe Cu-Mt was added, the pyrene loaded on the soil was already sorbedto soil components, so that degradation of pyrene could hardly be ex-pected. Similarly, no degradation was observed in a blank in whichsoil loaded with the PAH was incubated without addition of Cu-Mt.When pyrenewas loaded on pre-mixed Cu-Mt and soil, and the fractionof the Cu-Mt was increased to 10%, the pattern of pyrene degradationwas very different. While non-ground samples displayed only insignifi-cant degradation of pyrene at T0 and 27% degradation at T24, grindingthe samples resulted in 11% degradation at T0, and 19% degradation atT24 (Fig. 9). Loading pyrene when a sufficient amount of Cu-Mt is al-ready present in the soil enabled a significant amount of pyrene tocome in contact with the clay mineral's surface and thus promote deg-radation. Grinding increased the amount of pyrene that came in contactwith Cu-Mt on the one hand but lowered the degradable fraction of thesorbed pyrene (Section 3.2) on the other. Consequently, grinding in-creased the amount of pyrene degraded at T0 but not of that degradedat T24 (Fig. 9).

ene and phenanthrene on Cu-montmorillonite, Appl. Clay Sci. (2014),

Fig. 9. Pyrene concentration in a soil to which 10% Cu-Mt was added, with and withoutgrinding.

8 H. Joseph-Ezra et al. / Applied Clay Science xxx (2014) xxx–xxx

WhenPHEwas added to the soil–claymineralmixture togetherwithpyrene, no PHEdegradationwas observed at T0 and only 7%degradationoccurred up to T24 in the non-ground samples, (Fig. 10a). In the groundsamples, 15%degradationwasmeasured at T0 and remained the sameatT24. In the case of pyrene, again almost no degradation was observed inthe non-ground samples at T0, but 46% degradation occurred until T24.In the ground samples, 23% degradation was measured at T0 and 26%at T24 (ibid). Thus, in the non-ground samples the degradation of bothPAHs proceeded at an even rate, while in the ground samples, the rateof degradation diminished significantly after grinding. Grinding appar-ently increased the fraction of the loaded PAHs that came in contactwith Cu-Mt, but at the same time also inhibited the degradation of thePAH molecules that were sorbed on the clay mineral (Sections 3.2 and

0

100

200

300

400

500

600

PHE-no grinding PHE-manualgrinding

PYR-no grinding PYR-manualgrinding

PA

H r

emai

nin

g (

µg

/g)

0

100

200

300

400

500

600

PHE PYR

PA

H r

emai

nin

g (

µg

/g)

b

T0 T24

T0 T720

a

Fig. 10. Pyrene (PYR) and PHE concentrations in soil pre-mixed with 10% Cu-Mt, (a) withand without grinding at T0 and T24 and (b) no grinding at T0 and after 1 month (T720).

Please cite this article as: Joseph-Ezra, H., et al., Surface interactions of pyrhttp://dx.doi.org/10.1016/j.clay.2014.04.037

3.3). After a month, 83% of the loaded pyrene and only 12% of the PHEdegraded in the non-ground samples (Fig. 10b). The degradation kinet-ics of both pyrene and PHE in the soil–clay mineral mixture was thussimilar to that in the pure Cu-Mt, including the enhancing effect thepresence of PHE had on pyrene degradation (Section 3.7).

3.10. Degradation products

3.10.1. HPLCHPLC analysis produced peakswhich indicated the formation of deg-

radation products in all samples in which PAH disappearance was ob-served. Since the identity of the degradation products was unknown,no calibration curves to determine their concentration could be con-structed. The areas of the peaks assigned to the various degradationproducts were therefore used to extract relative changes in quantitywhich were compared to the corresponding changes in the concentra-tion of the parent PAH.

A comparison of the UV spectra of pyrene and of its main degrada-tion product when loaded on Cu-Mt, suggests that the degradationproduct lost much of the aromatic nature of the parent PAH. This is in-dicated by the sharp peaks in pyrene's UV spectrum (at 240.6, 273.1and 334.7 nm) which disappeared in the spectrum of the degradationproduct in which only rather diffused peaks were observed. The maindegradation product (Section 3.10.2) appeared already at T0, and itsarea grewwith time simultaneouslywith the decrease in the concentra-tion of pyrene.

3.10.2. LC–MSLC–MS analysis revealed that pyrene underwent both oxidation and

dimerization at the surface of Cu-Mt (Table 3). Oxidation seems to bethe main degradation pathway, but dimerization was also observedand since them/z range analyzed in the present studywas not sufficientto check for it, higher polymerization reactions might have also oc-curred. In the main degradation product of pyrene, two oxygen atomsreplaced protons. Such oxidation fits the hypothesis that electron trans-fer to copper ions brought about the oxidation of the pyrene moleculesas proposed by Hinedi et al. (1993) for benzene degradation at the sur-face of Cu-Mt (Sections 3.1.1 and 3.6). The reaction products identifiedin the present study thus indicate a mechanism for the degradation ofPAHs at the surface of Cu-Mt in which a cation radical transition speciesis formed that can undergo both dimerization and a reaction withoxygen.

While the protonation of pyrenewas easily accomplished, PHEhard-ly reacted with the protons in the positive ionization process. Thisobservation is in agreement with the hypothesis that the larger ioniza-tion potential of phenanthrene as compared to that of pyrene is at leastin part responsible for the low efficiency of Cu-Mt in degrading PHE ascompared to its efficiency in degrading pyrene (Section 3.7), or morespecifically, for the higher tendency of pyrene to transfer an electronto a Cu ion as compared with the resistance of PHE to this process.

4. Conclusions

Cu-Mt enhances the abiotic degradation of pyrene and to a muchlesser extent that of PHE. Degradation of pyrene loaded on Cu-Mt is arelatively fast surface reaction and the presence of PHE further aug-ments this degradation. The fact that Cu-Mt enhanced the degradationof pyrene but hardly that of PHE can be explained by the difference inthe molecular structure between the two PAHs. Pyrene is more suscep-tible to electron transfer reactions than PHE and also differs from PHE inits affinity to the various types of sorption sites at the Cu-Mt surface. Al-though both PAHs sorb preferentially to surface sites which do not en-hance degradation, PHE has a higher affinity to those sites and henceits presence diverts more pyrene molecules to degradation-promotingsorption sites. The nature of the degradation products indicates that

ene and phenanthrene on Cu-montmorillonite, Appl. Clay Sci. (2014),

Table 3Transformation products of pyrene loaded on Cu-Mt.a

Peak Measured mass Calculated composition Comments Probable parent compound

1 233.1 C16H9O2 Protonated molecular ion C16H8O2

205.1 C15H9O \CO177.1 C14H9 \CO

2 433.1 C32H17O2 Protonated molecular ion C32H16O2

432.1 C32H16O2 Cation radical416.1 C32H16O [M + H]+\(OH) = cation radical405.1 C31H17O [M + H]+\(CO)388.1 C31H16 [M + H]+\(OH)\(CO)

3 202.1 C16H10 Cation radical (Pyrene) C16H10

4 432.1 C32H16O2 Cation radical C32H16O2

5 403.1 C32H19 Protonated dimer pyrene C32H18

401.1 C32H17 Dimer pyrene — H6 400.1 C32H16 ? C32H18

401.1 C32H17 Dimer pyrene — H402.1 C32H18 Cation radical dimer pyrene

a Error varied between 0.7 and 3.9 ppm.

9H. Joseph-Ezra et al. / Applied Clay Science xxx (2014) xxx–xxx

oxidation and dimerization are the leading pathways for pyrene degra-dation on Cu-Mt.

Acknowledgments

The authors are indebted to Dr. Julius Benari for executing the LC–MS analyses and his interpretation of the data and to Dr. Naomi Bahatfor performing the SEM analyses.

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