Chemosphere 91 (2013) 717–723

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Chemosphere

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Technical Note

Performance and properties of nanoscale calcium peroxide for tolueneremoval

0045-6535/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.chemosphere.2013.01.049

⇑ Corresponding author. Tel.: +86 021 65980624; fax: +86 021 65989961.E-mail address: zhangyalei2003@163.com (Y. Zhang).

Yajie Qian a, Xuefei Zhou b, Yalei Zhang a,⇑, Weixian Zhang a, Jiabin Chen a

a State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, Chinab Key Laboratory of Yangtze River Water Environment for Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China

h i g h l i g h t s

" Nanoscale CaO2 was synthesized by nanoscale Ca(OH)2 and H2O2.

" Nanoscale CaO2 could eliminate toluene within 3 d at pH 6." The oxidation products include benzyl alcohol, benzaldehyde and cresol isomers." Hydroxyl and superoxide radicals appeared in the oxidation process.

a r t i c l e i n f o

Article history:Received 20 September 2012Received in revised form 17 January 2013Accepted 18 January 2013Available online 7 March 2013

Keywords:Nanoscale calcium peroxidePetroleum hydrocarbonTolueneRemoval

a b s t r a c t

Due to the large diameter and small surface, the contaminant degradation by conventional calcium per-oxide (CaO2) is slow with high dosage required. The aggregation of conventional CaO2 also makes it dif-ficult to operate. Nanoscale CaO2 was therefore synthesized and applied to remove toluene in this study.Prepared from nanoscale Ca(OH)2 and H2O2 in the ratio of 1:7, the finely dispersed nanoscale CaO2 par-ticles were confirmed by the scanning electron microscope to be in the range of 100–200 nm in size.Compared to their non nanoscale counterparts, the synthesized nanoscale CaO2 demonstrated a superiorperformance in the degradation of toluene, which could be eliminated in 3 d at pH 6. The oxidation prod-ucts of toluene were analyzed to include benzyl alcohol, benzaldehyde and three cresol isomers. With theaddition of 2-propanol, hydroxyl radicals were indicated as the main reactive oxygen species in the oxi-dation of toluene by nanoscale CaO2. Superoxide radicals were also investigated as the marker of nano-scale CaO2 in the solution. Our study thus provides an important insight into the application of nanoscaleCaO2 in the removal of toluene contaminants, which is significant, especially for controlling the petro-leum contaminations.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Calcium peroxide (CaO2) is a beige solid which can slowly re-lease oxygen at a ‘‘controlled’’ rate when dissolved in water (Eq.(1)). Besides its stable oxygen releasing capability, CaO2 also pos-sesses the capacities of bleaching, disinfection and deodorizing.Hence, CaO2 has been widely used in agriculture, aquiculture andmedicine (Ma et al., 2007). As an environmentally friendly oxidiz-ing agent, CaO2 cannot only degrade contaminants without nega-tive effect on the surroundings, but also provide microorganismswith oxygen which is beneficial for their growth, thus enhancingthe efficiency of bioremediation (Cassidy and Irvine, 1999; Kaoet al., 2003; Liu et al., 2006). In its natural state, CaO2 dissolvesin water to form H2O2 and Ca(OH)2 (Eq. (2)), liberating a maximum

of 0.47 g H2O2 g�1CaO2 (Northup and Cassidy, 2008) and thus con-sidered as a ‘‘solid form’’ of H2O2 (Tieckelmann and Steele, 1991).Commercial CaO2 has a variety of environmental application suchas remediation of contaminated soil and underground water, aswell as modification of aerobic bioremediation.

CaO2 þH2O! 0:5O2 þ CaðOHÞ2 ð1Þ

CaO2 þ 2H2O! H2O2 þ CaðOHÞ2 ð2Þ

Petroleum hydrocarbons contamination is a serious and wide-spread environmental problem. According to the United StatesEnvironmental Protection Agency, there are over 500000 LeakingUnderground Storage Tanks (LUSTs) in the U.S. (Anna and Dennis,2010), which have a potential risk to public health. Due to theinsidious nature of the LUST, gasoline leaks can contaminate soiland groundwater gradually, which has become one of the majorenvironmental problems in the world. Amongst many LUST

718 Y. Qian et al. / Chemosphere 91 (2013) 717–723

contaminants, the components of gasoline, such as benzene, tolu-ene, ethylbenzene and xylene, attract the most attention. Withthe characteristics of volatility and water-solubility, these com-pounds are not only toxic but also mobile (Jindrova et al., 2002;Bombach et al., 2009). In particular, toluene is carcinogenic, withthe Maximum Contaminant Levels ranging between 0.7 and1.0 ppm. Over long period exposure, toluene can damage the liver,kidney, and central nervous system (Meegoda and Hu, 2011).Therefore, the elimination of such pollutants is significant for safe-guarding of drinking water resources and groundwater.

Up to date, several methods have been developed for remedia-tion of water resources contaminated by these gasoline derivedhazards (Zein et al., 2006; Garoma et al., 2008; Liang et al., 2008;Berlendis et al., 2010; Yu et al., 2011). Amongst all, nanoscaleCaO2 has turned to be one effective and environmentally friendlyapproach which attracts increasing attention (Khodaveisi et al.,2011). Compared with conventional CaO2, nanoscale CaO2 has abetter dispersion and transportation capacity. The tremendous sur-face area of nanoscale CaO2 leads to rapid hydrocarbon oxidationand consequently low nanoscale CaO2 consumption for sites reme-diation. The relatively low cost is also beneficial. Not surprisingly,nano-sized CaO2 has recently been utilized for oil spills clean-upfrom LUST. Several projects have been conducted in New Jersey,USA. Furthermore, two American companies have also usednano-sized CaO2 as oxidant to remediate the soil contaminatedwith various organic contaminants, such as gasoline, heating oil,methyl tertiary butyl ether, ethylene glycol and solvents (Muellerand Nowack, 2010).

Previous studies have reported conventional CaO2 as oxidant foroil contaminated soils remediation (Arienzo, 2000; Bogan et al.,2003; Northup and Cassidy, 2008; Goi et al., 2011; Xu et al.,2011). However, the consensus has not been reached as regard tothe efficiency of CaO2 and H2O2, the influence of pH, and the mech-anism of CaO2 effects. Moreover, at present time, the application ofnano-sized CaO2 is still limited to the field projects. Therefore, theimplementation of this new technology of nanoscale CaO2 urgesfurther understandings of its roles in contaminants degradationas well as the background mechanism behind it.

2. Materials and procedures

2.1. Chemical reagents

Toluene (99.9%) and its oxidation products were purchasedfrom Sigma–Aldrich (St. Louis, MO, USA). Ultra-pure water was ac-quired from a Milli-Q water purification system (Millipore, Bed-ford, MA, USA). Ca(OH)2 (98%), H2O2 (non-stabilized, 30%) andHCl (37% solution in water) were all purchased from Fisher(USA). The powder of Ca(OH)2 was wet-grinded to nanoscale parti-cles by NETZSCH (Germany). GC-grade n-hexane and ethyl acetatewere applied as extraction solvents for the analysis of toluene. Ni-tro blue tetrazolium (NBT) was purchased from Nakalai Tesque(Kyoto, Japan). All other reagents (e.g., thiourea) were obtainedfrom Fisher Scientific or Aldrich at analytic grade and used withoutfurther purification. Microporous membrane (0.45 lm � 50 mm)was obtained from CNW (Germany). All the aqueous solutionswere prepared in Milli Q-water, with pH adjusted by HCl.

2.2. Preparation of CaO2 and its characteristics analysis

For the preparation of CaO2, an aliquot of H2O2 was first placedin a 500 mL beaker maintained in an ice bath. With vigorously stir-ring, a known amount of nanoscale Ca(OH)2 slurry was addedslowly, followed by adequate incubation to allow complete reac-

tion between the two compounds. According to the followingreaction:

CaðOHÞ2 þH2O2 ! CaO2 þ 2H2O ð3Þ

1:1 molar ratio of Ca(OH)2 and H2O2 was needed. In our study,excessive H2O2 was provided to maximize the production of CaO2.CaO2 was unstable and released O2 easily at the pH above 9, thus,the solution of the above reaction should be kept in an acidic statevia adjusting pH to 6 with HCl.

Above samples were passed through a microporous membrane(0.45 lm � 50 mm) to remove water, then dried in the vacuumdesiccator for 24 h at 30 �C. A VEGATS 5136MM SEM (TESCAN,Czech) was used to analyze CaO2 nanoparticles size and distribu-tion. The material structure was recorded with a Bruker D8 Ad-vance Power XRD (Germany) at 40 kV and 40 mA.

2.3. Reaction setup

Batch reactions were conducted to study the reaction betweentoluene and nano-sized CaO2 at different conditions in 100 mLheadspace vials with headspace caps and PTFE stoppers at roomtemperature. The synthesized CaO2 solution which was preparedby Ca(OH)2 and H2O2 with the molar ratio of 1:7 was added intoa series of 100 mL headspace vails, subsequently with 53.1 lL tol-uene added, and the final volume adjusted to 100 mL with deion-ized (DI) water. Control experiments with DI water only or equalamount of H2O2 without the addition of Ca(OH)2 were also con-ducted. Reaction solution was constantly stirred. Samples (3 mLeach) were taken at pre-selected time intervals, and the reactionwas immediately quenched with HCl. After extraction with 2 mLn-hexane for three times, samples were transferred into 2 mL am-ber vials and stored at �18 �C before analysis. All experimentswere performed in duplicate.

2.4. Analysis of toluene and the products

Toluene was analyzed by Focus GC (Thermo Fisher Scientific,USA) with a flame ionization detector. Details were provided inSupplementary Material (SM).

The products were determined using a Trace GC2000 coupledwith DSQII MSD. The concentration of nano-sized CaO2 in the solu-tion was set at 400 mM and the solution pH was at 6.0. Detailedinformation was also provided in SM.

2.5. The mechanism determination

To explore the mechanism of toluene degraded by nanoscaleCaO2, two individual experiments were designed. 2-Propanol wasapplied for hydroxyl radicals identification due to its effectivenessas a scavenger of �OH (Ndjou’ou et al., 2006). For superoxide radi-cals identification, NBT was used for detecting �O�2 (Goto et al.,1998). Since NBT could be reduced to its diformazan form by �O�2 ,which was only slightly dissolved in aqueous solution, shiftingthe maximum absorptive wavelength (kmax) from original visibleto UV spectrum. 5 mL of samples was withdrawn at the pre-se-lected time intervals, with 5 mL NBT (1 mM) added immediately.After 5 min, the reaction mixture was centrifuged and the precipi-tate was dissolved into 5 mL anhydrous ethanol, followed by anal-ysis on the UV-2600 spectrophotometer (Techcomp, China).

3. Results and discussion

3.1. Characterization of CaO2

As shown in Fig. 1a and b, conventional CaO2 particles have un-even size distributions, with most particles greater than 1 lm, and

Fig. 1. Scanning electron microscope image of synthesized CaO2 particles. (a and b) The image of conventional CaO2; and (c and d) the image of nanoscale CaO2.

0 2 4 6 8 10

0

100

200

300

400

Con

cent

ratio

n of

tolu

ene

(mg.

L-1)

Time (d)

blank Ca(OH)2:H2O2 = 1:1

Ca(OH)2:H

2O

2 = 1:3

Ca(OH)2:H2O2 = 1:5

Ca(OH)2:H

2O

2 = 1:7

H2O2

Fig. 2. Effect of different Ca(OH)2 and H2O2 ratios on toluene degradation. InitialpH = 6. [Ca(OH)2] = 400 mM.

Y. Qian et al. / Chemosphere 91 (2013) 717–723 719

some even larger than 20 lm. They demonstrate irregular shapeswith clumped structures caused by the aggregation of small CaO2

particles. These properties account for the challenges of the appli-cation conventional CaO2 in the groundwater remediation. Thesynthesized nanoscale CaO2, however, as indicated from SEM im-age in Fig. 1c and d, revealed relatively uniform particle sizes at100–200 nm, with high percentage of acquiring granular and cubicstructures. The individual particles could be obviously identifiedfrom Fig. 1d. These results indicated relatively high dispersion ofthe synthesized CaO2 nanoparticles.

The XRD images of the standard and synthesized CaO2 are pre-sented in Fig. SM-1. There was no great difference between themindicating CaO2 being the major component in each sample. Themain dominant peaks: 2h = 30.1, 35.6, 47.3 match the XRD forCaO2 (JCPDS 03-0865). However, certain amounts of Ca(OH)2 andCaCO3 were identified in the synthesized products, along withsmall quantities of inorganic CaH2 as impurities.

3.2. Effect of Ca(OH)2 and H2O2 ratio on the degradation of toluene

To study the effect of the ratio between Ca(OH)2 and H2O2 onCaO2 synthesis, different H2O2 concentrations were used in thereaction (3). The resultant toluene removal corresponding to differ-ent ratios was measured and shown in Fig. 2. In the control groups,negligible toluene removal was observed using DI water, and lowlevel of toluene degradation in the presence of 30% liquid H2O2.

This was consistent with the previous report (Kong et al., 1998)that H2O2 has limited effect under any pH conditions. This con-firmed that the reduction of toluene in the experiment was mainlycaused by nanoscale CaO2 rather than H2O2. With the increasing of

400 450 500 550 600 650 700

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1.6

1.8 (b) 0.25h 0.5h 1h 1.5h 2h 3h 6h 10h 22h 34hblank

Abs

orba

nce

Wavelength (nm)

(a)

400 450 500 550 600 650 700

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09 0.25h 0.5h 1h 1.5h 2h 3h 6h 10h 22h 34h blank

Wavelength (nm)

Fig. 3. The UV–Vis absorption spectra of the NBT solution with CaO2 and H2O2 in the previous 34 h. (a) The image of CaO2; and (b) the image of H2O2.

0

100

200

300

400

blank 200mM 400mM 600mM H

2O

2

(a)

0 2 4 6 8 10 12

0

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400

Time (d)

blank pH=5 pH=6 pH=7 pH=8 pH=9

(b)

Con

cent

ratio

n of

tolu

ene

mg.

L-1

Fig. 4. (a) Kinetics of toluene degradation at different CaO2 concentrations; InitialpH = 6; and (b) effect of pH on toluene degradation by nanoscale CaO2 particles.[CaO2] = 400 mM.

720 Y. Qian et al. / Chemosphere 91 (2013) 717–723

H2O2 concentration in the synthesized process, the removal of tol-uene increased dramatically. At the molar ratio of 1:1 and 1:3about Ca(OH)2 to H2O2, toluene could not be removed completelyin 9 d, with the final removal efficiency of 82%. But when the molarratio raised up to 1:5 and 1:7, the reaction rate increased rapidlyand toluene could be removed completely in 7 and 3 d, respec-tively. Thus the optimal molar ratio about Ca(OH)2 and H2O2 inthe synthesis of CaO2 was supposed to be 1:7.

Fig. 2 shows that compared to the control group of liquid H2O2,the experimental groups still had great effect on the reduction oftoluene. It indicated that the addition of Ca(OH)2 markedly im-pacted the efficiency of contaminants removal. This was also con-firmed by the study of �O�2 in the solution. In comparison withCaO2, the amount of free radicals generated from H2O2 was verylimited as shown in Fig. 3a and b, indicating that the reactive oxy-gen species (ROS) in CaO2 system was mainly responsible for tolu-ene removal. According to the study of Northup and Cassidy(2008), 30% liquid H2O2 would disproportionate to oxygen in watereasily thus exhausted quickly while Ca(OH)2 could retain H2O2

with synthesized CaO2 which extended the action time with con-taminants. Previous studies (Watts et al., 1999; Kremer, 2003) re-ported that the most serious limitation of the application of H2O2

to contaminated soil remediation was the instability of H2O2 atneutral pH which half-life was only minutes to hours. This advan-tage of nanoscale CaO2 had a great potential in soil remediation. Athigh molar ratio of Ca(OH)2 to H2O2 which was at 1:5 and 1:7, withthe increasing of H2O2 concentration, the toluene degradation rateincreased. Zhai et al. (2003) reported the products of CaO2 weremaximized when the ratio of Ca(OH)2 to H2O2 reached 1:7. At thisratio, the reaction between Ca(OH)2 and H2O2 was complete. Withthe maximal CaO2 products, the degradation rate of toluene was upto the peak in our experiment.

3.3. Kinetic studies of toluene oxidative transformation

The typical reaction profile is presented in Fig. 4a for variousinitial nanoscale CaO2 concentrations at pH 6.0. In our experiment,the amount of Ca(OH)2 was controlled at 200, 400, 600 mM and theratio of Ca(OH)2 and H2O2 was set at the optimal value 1:7 for syn-thesis of CaO2. As indicated in Fig. 4a, the removal efficiency of tol-uene increased rapidly with the increase of initial nanoscale CaO2

concentrations. The removal of toluene occurred in 9, 3 and 2 d,

for the concentration of nanoscale CaO2 at 200, 400 and 600 mM,respectively. Within 2 d, the removal efficiency of toluene reached67%, 95% and 100% by 200, 400 and 600 mM CaO2, respectively.Thus the removal of toluene raised notably with the concentrationof CaO2 from 200 to 400 mM, while that was very low from 400 to600 mM. We proposed that 400 mM CaO2 was the best choice tak-ing the amount of toluene degradation and the concentration ofnano-sized CaO2 into consideration.

The removal efficiency of toluene by conventional CaO2 wasalso studied at pH 6 with 5 mM toluene and 400 mM CaO2. As

0 50 100 150 200 250

0.00E+000

2.00E+007

4.00E+007

6.00E+007

8.00E+007

1.00E+008

peak

are

a

Time (h)

benzyl alcohol benzaldehyde 2-cresol 3-cresol 4-cresol

Fig. 5. Products of toluene oxidation by CaO2 nanoparticles. [CaO2] = 400 mM,intitial pH = 6.

Y. Qian et al. / Chemosphere 91 (2013) 717–723 721

shown in Fig. SM-2, toluene could be removed completely in 3 d bynanoscale CaO2, while toluene residues could be still detected after9 d with conventional CaO2. This difference was due to the muchlarger surface area of nanoscale CaO2, providing more active sur-face sites which could increase the removal of toluene. Thus, com-pared with conventional CaO2, lower dosage of nanoscale CaO2

would be needed to reach the same degree of pollutants removal.

3.4. Effect of pH

Fig. 4b presents the toluene degradation in different pH condi-tions. In the system of pH 6, toluene could be removed totally in3 d. Whereas there were still 15% and 19% toluene residues inthe solution of pH 5 and 7. Compared with the acidic and neturalconditions, toluene removal in alkaline conditions was very lim-ited. The removal efficiency of toluene increased quickly from pH5 to 6, and peaked at pH 6. With further increase of pH, removalefficiency decreased rapidly and little reaction rate variation wasobserved from pH 8 to 9. Therefore, pH plays an important rolein CaO2 reaction with pollutants. The change of toluene concentra-tion at pH 8 and 9 illustrated a two stage contaminants removalwhich contained a volatilization dominated stage in the first 10 hand a degradation stage afterwards.

At low pH conditions, CaO2 had a high stability and the exhaus-tion of reactive species by releasing O2 could be reduced markedly.Nonetheless, further decreasing pH would also increase the disso-lution of CaO2 sharply thus exhausting the reactive CaO2 particlesquickly and then led to the low speed of toluene degradation. Athigh pH conditions, CaO2 was difficult to dissolve but easy to re-lease O2. The observation of pH between 8 and 9 indicated thatthe release of O2 raised up to the highest level in 10 and 6 h,respectively. With pH rising, O2 was fast librated from CaO2. Com-bined with the high volatilization of toluene, it was responsible forthe first stage of toluene removal. Due to the loss of reactive CaO2,the reaction rate obtained from the second stage of pH 8 and 9 wasdetermined much lower than other pHs. The optimization of tolu-ene degradation rate by nanoscale CaO2 was near neutral condi-tions, with the highest rate at pH 6. In contrast, removal ratesunder both acidic and alkaline conditions were comparably lower.These results provide good modification exploration for the com-bined process of chemical oxidation and bioremediation.

3.5. Products analysis and reaction mechanism

By GC-MS analysis, benzyl alcohol, benzaldehyde, 2-cresol, 3-cresol, 4-cresol were identified to be the major products from tol-uene oxidation. The products and their formation rates are shownin Fig. 5. At pH of 6.0, these five products were simultaneouslyaccumulated continuously in the first 22 h, reaching peaks at22 h when 83% of toluene was degraded, and thereafter decreasedslowly. Among all the products, the largest amount formed wasbenzyl alcohol, and the smallest benzaldehyde. The amount ofthree cresol isomers products followed the decreasing order of:p-cresol > o-cresol > m-cresol.

As presented in Fig. SM-3, in contrast to the complete removalof toluene without 2-propanol, very little degradation was ob-served when adding 2-propanol, indicating �OH from CaO2 as dom-inant oxidative species. This has been reported by previous studies(Northup and Cassidy, 2008). In our experiment, the amount ofsuperoxide radicals (�O�2 ) in the solution was investigated. For theshort lifetime of �O�2 (�0.1 s), it serves as the marker of CaO2 inthe systems. As in Fig. 3a, within the first 34 h, kmax decreased withtime, indicating the reduction of �O�2 . But beyond 22 h the reactivespecies �O�2 could hardly be detected. This means the CaO2 particlesin the solution have been almost exhausted. The amount of oxida-tion products in Fig. 5 shows a fast increase in the first 22 h and

then drops dramatically. This tendency should also owe to thechange of nanoscale CaO2 in the solution marked by �O�2 .

Previous studies revealed that ROS was generated from theCaO2 system, which mainly contained �OH and �O�2 (Tomat andRigo, 1976; Kurata et al., 1988; Navio et al., 1996). Based uponour results, the possible mechanism of ROS production was pro-posed as follows:

CaO2 þ 2H2O! CaðOHÞ2 þH2O2 ð4Þ

H2O2 þ e� ! �OHþ OH� ð5Þ

�OHþH2O2 ! HO�2 þH2O ð6Þ

HO�2 þ OH� ! �O�2 þH2O ð7Þ

As in reaction (5), H2O2 obtained a single electron to produce�OH in the suspension of alkaline-earth metal peroxides whichwas proved by the addition of the scavenger 2-propanol. The reac-tion (7) indicated that the �O�2 was also produced, consistent withseveral previous reports (Stefan et al., 1996; Ma et al., 2005). Tolu-ene in the ROS system therefore could be oxidized through twopathways, which were shown in Fig. 6.

In the first pathway, three cresol isomers were generated byROS in the system. Navio’s research also showed similar resultsfrom toluene oxidation by UV-illuminated TiO2 (Navio et al.,1996). Hydroxyl radicals could be attached to the aromatic ringof toluene, leading to the formation of an intermediate radicalwhich is further aromatized (Fujihira et al., 1981). Due to the var-iation of structural stability, there was a quantitative distinctionamong the three intermediates. Fig. 6 shows the different struc-tures of the three intermediates. Ia Ib Ic, IIa IIb IIc and IIIa IIIb IIIcwere the different structures of ortho-, meta- and pare-intermedi-ates respectively. Among the three isomers of ortho-intermediates,Ib possessed a positive charge in the tertiary carbon which was arelatively stable structure. Similarly, pare-intermediates also ac-quired a stable structure represented by IIIc. As a result, o-cresoland p-cresol were the major products in the first pathway. Afterlosing a hydrogen ion, the percentages of o-, m-, and p-isomerswere 44%, 24% and 32% respectively. Under the influence of ben-zene-ring, a-H would be relatively active. On the other hand, weproposed a second pathway by which methyl carbons were at-tacked by ROS, promoting benzyl radicals formation from a CH3

group (Dixon and Norman, 1964; Davis et al., 1975). With furtheroxidation of benzyl radicals, alkoxy radicals would be generatedin the system, subsequently reacted with H2O2 or lost an electronto produce benzyl alcohol or benzaldehyde correspondingly. Nev-

Fig. 6. Proposed reaction mechanism for toluene by nanoscale CaO2 particles.

722 Y. Qian et al. / Chemosphere 91 (2013) 717–723

ertheless, we do not exclude the possibilities of other unidentifiedproducts formation in the reaction process.

4. Conclusions

We have explored a novel CaO2 synthesis method which waseasy to handle in engineering in this study. The highest yield ofnanoscale CaO2 was obtained from proper ratio of 1:7 betweengrinded nanoscale Ca(OH)2 and H2O2. The results of SEM revealedthe sizes of 100–200 nm for synthesized CaO2 nanoparticles withbetter dispersion than that of conventional CaO2. In the investiga-tion of the degradation efficiency of 5 mM toluene at pH 6, the bestinitial CaO2 concentration was 400 mM. The synthesis of CaO2 wasalso significantly influenced by pH, with pH 6 being optimal, underwhich condition toluene could be eliminated in 3 d. Benzyl alcohol,

benzaldehyde and three cresol isomers were the major productsduring the toluene oxidation, owing to the attack of benzene ringcarbons and methyl carbons. Hydroxyl radicals and superoxideradicals were confirmed to be present in the system.

Acknowledgements

This research was financially supported by the National KeyTechnologies R & D Program (2012BAJ25B04), the National NaturalScience Foundation of China (41072172, 51138009), State Key Lab-oratory of Pollution Control and Resource Reuse Foundation (No.PCRRY11004), Research Foundation of Shanghai Committee of Sci-ence and Technology (11QH1402600), and New Century ExcellentTalents in University (NCET-11-0391).

Y. Qian et al. / Chemosphere 91 (2013) 717–723 723

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.chemosphere.2013.01.049.

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