JOURNAL OFENVIRONMENTALSCIENCES

ISSN 1001-0742

CN 11-2629/X

www.jesc.ac.cn

Available online at www.sciencedirect.com

Journal of Environmental Sciences 2012, 24(9) 16701678

Characterizing the optimal operation of photocatalytic degradation ofBDE-209 by nano-sized TiO2

Ka Lai Chow1, Yu Bon Man1, Jin Shu Zheng1, Yan Liang1,,Nora Fung Yee Tam2, Ming Hung Wong1,

1. Croucher Institute for Environmental Sciences, and Department of Biology, Hong Kong Baptist University, Hong Kong SAR, China2. Department of Biology and Chemistry, City University of Hong Kong, Hong Kong SAR, China

Received 15 November 2011; revised 27 February 2012; accepted 29 February 2012

AbstractBrominated ame retardants have been widely used in industry. There is a rapid growing public concern for their availabilities inthe environment. Advanced oxidation process (AOP) is a promising and ecient technology which may be used to remove emergingchemicals such as brominated ame retardants. This study aims at investigating optimal operational conditions for the removal ofBDE-209 using nano-scaled titanium(IV) oxide. The residual PBDE congeners after photocatalytical degradation of BDE-209 by TiO2were analysed by gas chromatography-mass spectrometry (GC-MS). It was found that the degradability of BDE-209 by TiO2 wasattributed to its photocatalytic activity but not the small size of the particles. The half-life of removing BDE-209 by TiO2 was 3.05 daysunder visible light. Tetra- and penta-BDEs were the major degraded products of BDE-209. Optimum conditions for photocatalyticaldegradation of BDE-209 was found to be at pH 12 (93% 1%), 5, 10, 20 mg/L (93.0% 1.70%, 91.6% 3.21%, 91.9% 0.952%,respectively), respectively of humic acid and in the form of anatase/rutile TiO2 (82% 3%). Hence, the eciency of removing BDE-209can be maximized while being cost eective at the said operating conditions.

Key words: PBDEs; TiO2; photocatalysis; hydroxyl radicalsDOI: 10.1016/S1001-0742(11)60992-3

Introduction

Since the 1970s, the demand of brominated ame retar-dants has been increasing (Wang et al., 2007). The annualglobal production of polybrominated diphenyl ethers (PB-DEs) increased from 40,000 tons (Arias, 1992), to around67,000 tons between 1992 to 2001 (BSEF, 2006). Asiancountries shared about 40% of the global demand ofPBDEs (BSEF, 2006). PBDEs are an important class ofbrominated ame retardants with a high production rate.They are generally used in polymer and textile productsas additive ame retardants (de Wit, 2002; Rahman et al.,2001). PBDEs added resins or polymers are also commoncomponents of electrical appliances, contributing to therelease of PBDEs from electronic waste (e-waste) (WHO,1994). Due to their persistent, lipophilic and bioaccumu-lating characteristics, the concern on their uses has beenincreasing since the 1990s (de Wit, 2002). PBDEs werereported to be potential endocrine disruptors (Lema et al.,2008) and neurotoxicants (Goodman, 2009).

In the past decade that the life-span of electronicappliances are becoming shorter and shorter due to theadvanced technology, many developed countries export

* Corresponding authors. E-mail: mhwong@hkbu.edu.hk (Ming HungWong); yliang@hkbu.edu.hk (Yan Liang)

e-waste to developing countries in East and SoutheastAsia for recycling. Illegal and inappropriate dumping andrecycling of e-waste are believed to be two major sourcesof PBDEs contamination in the environment (Wang et al.,2005). For example, PBDEs (27204250 ng/g dry weight)were found in soils from an acid leaching site (using strongacids to recover metals from electronic boards) of Guiyu(Leung et al., 2007). Although the European Union (EU)has banned the usage of all penta-BDE and octa-BDE inthe EU market since 2003 (EU, 2001), deca-BDE is still inuse in electrical and electronic equipments and may leadto contamination via acid leaching (Leung et al., 2007).It has been revealed that the soils in Guiyu, China (ane-waste recycling site) are polluted by PBDEs in whichBDE-209 is the dominant congener and occupied 35%to 82% of the total PBDEs content (Leung et al., 2007),BDE-209 was therefore chosen as the tested chemicalin this study. Tertiary-level wastewater treatment plants(WWTP) of modern wastewater treatment are insucientin removing PBDEs from wastewater. BDE-47, BDE-99, and BDE-209 were the major congeners identiedin sludge and euents from a tertiary sewage treatmentfacility that discharges into natural waters (North, 2004).PBDEs in wastewater may adsorp onto wastewater sludgeswhile some may enter natural water bodies resulting in a

No. 9 Characterizing the optimal operation of photocatalytic degradation of BDE-209 by nano-sized TiO2 1671

large PBDEs ux into receiving waters and thus, posingpotential hazards to drinking water sources and sheriesresources (Rayne and Ikonomou, 2005), and there is a needto investigate eective removal of these pollutants, so as toprotect wildlife and human health.

To cope with an uprising application of PBDEs andits release into the environment, dierent remediationtechnologies should be developed. Plant uptake and dis-sipation of PBDEs by plants were investigated by Italianryegrass, pumpkin, and maize. The reduction rates of thetotal PBDEs in the soils were low (13.3% to 21.7%).Although it is a relatively environmental friendly methodfor remediation of PBDEs, but it usually takes longer time(e.g. 60 days) and requires more space (Huang et al., 2011).Photolytic debromination was attempted for remediationof deca-BDE. Nevertheless, the half-life (150200 hr) ofusing articial UV light was not ideal for treatment ofhighly PBDEs-contaminated soil (Soderstrom et al., 2004).Therefore, more advanced remediation technology suchas advanced oxidation processes (AOP) was developedfor removing trace levels of emerging chemicals (e.g.acetaminophen, antipyrine, atrazine, caeine and proges-terone) in wastewater (Klamerth et al., 2009). It wasreported that more than 90% of BDE-209 in acetonitrilewas degraded by TiO2 after 7.5 min of irradiation ofultraviolet (UV) radiation (Sun et al., 2009). During AOP,reactive free radicals (e.g. hydroxyl radicals) are producedand are able to reduce the toxicity and complexity oforganic chemicals. Under suitable conditions, the pollu-tants (e.g. total organic carbon) may even be mineralizedto the end product as CO2 (Gultekin and Ince, 2007).For example, it was reported that 92.5% of total organiccarbon was mineralized after advanced oxidation with UVlight and hydrogen peroxide in 90 min (Ince and Apikyan,2000). TiO2 is a prominent photocatalyst used in AOPfor treating organic pollutants. Nanosized TiO2 is evenmore eective in photocatalytical activity than other sized-TiO2 (Jiang et al., 2008). Previous studies all focusedon the characterization of kinetics and mechanisms ofphotocatalytical degradation of PBDEs in organic solventand on natural matrices other than water (Soderstrom et al.,2004; Sun et al., 2009).

Photocatalytical degradation of emerging contaminants(e.g. synthetic hormones) by TiO2 has been investigated(Ohta et al., 2002; Sun et al., 2009; Panchangam et al.,2009) but very few studies focused on the degradation ofPBDEs by TiO2. Photoreductive debromination of BDE-209 by TiO2 under UV light, via debromination pathwayhas been reported (Sun et al., 2009). However, high opera-tional costs are associated with the degradation technologyof PBDEs by TiO2 under UV light and may not beecient enough to treat large-scale loadings of wastewaterin modern cities. Consequently, there is an urgent needfor characterization of an ideal removal system of PBDEs,especially BDE-209 by TiO2, which can provide infor-mation for further developments of advanced wastewatertreatment plant.

The present study was aimed at investigating the abilityof photocatalyst (TiO2) to degrade BDE-209. The time

course of BDE-209 degradation by TiO2 was studied in0.1% DMSO. In addition, the eciencies of photocatalyt-ical degradation of BDE-209 by TiO2 at dierent pHs,humic acid concentrations and crystalline forms of TiO2were examined for the identication of optimal operationparameters, providing further information for future ad-vanced remediation of wastewater containing PBDEs.

1 Experimental

1.1 Materials

BDE-209 was obtained from Dr. Ehrenstorfer (Germany),while 13C-labeled surrogate and internal standard solutionsof PBDEs were purchased from Wellington Laborato-ries Inc. (Canada). TiO2, mixture of rutile and anatase(nanopowder < 100 nm particle size, 99.5% trace metalsbasis), TiO2, anatase (nanopowder < 25 nm particle size,99.7% trace metals basis) and TiO2, rutile (nanopowder< 100 nm particle size, 99.5% trace metals basis) wereobtained from Sigma-Aldrich (USA).

1.2 Reactive oxygen species (ROS) production by TiO2with BDE-209

Nano-sized TiO2 solution (1% mixture of anatase andrutile crystalline forms) was prepared in 0.1% DMSO inpH 7 buer solution. The solution was then sonicated for35 min in an ultrasonic cleaner (Branson Model 3510,40 kHz). The negative control did not contain TiO2 andthe particle control was added with nano-sized SiO2 in-stead of TiO2. BDE-209 in 0.1% DMSO (75 ppb) wasspiked into the solutions. Dichlorofuorescein diacetate(H2DCF-DA) (Invitrogen D-399) is a common probeused for detecting hydroxyl radicals (LeBel et al., 1992).The hydroxyl radicals generated by TiO2 and SiO2 weremeasured by dichlorouorescein (DCF) assay. First, 2,7-dichlorouorescin diacetate (DCFH-DA) (2.5 mmol/L)was hydrolyzed in 0.01 mol/L NaOH for 30 min in thedark at room temperature (25C) for the preparation ofDCFH stock solution. The mixture was then neutralizedwith 0.1 mol/L PBS to pH 7.4, followed by centrifugationat 3000 r/min for 10 min. The supernatant was removedand resuspended in 500 L DMSO. DCFH solution (25mol/L) was added to 500 L of the samples for 30 min.Finally, 200 L of the solutions were injected into 96-well plates for uorescence measurements by a microplatereader (excitation = 498 nm; emission = 522 nm) (TECANinnite F200) (Foucaud et al., 2007).

Interaction between the ROS production by TiO2 duringdierent time intervals was studied. The control and treat-ment solutions were placed in a uorescent lamp chamber(103 mol/(m2sec)) and shaken for 4 hr, and 1, 2, 3, 4,5 and 6 days using a shaker. After that, 500 L of thesamples were sampled and ROS measured according to theprocedure stated previously (Foucaud et al., 2007).

The ROS productivity of TiO2 on BDE-209 at dierentpH levels was examined by adjusting the control andtreatment solutions to pH 4, 6, 7, 8 and 12 with HCl(1 mol/L) and NaOH (1 mol/L), following the work by

1672 Journal of Environmental Sciences 2012, 24(9) 16701678 / Ka Lai Chow et al. Vol. 24

Zhang et al. (2008). The solutions were shaken in the lightchamber for 4 hr using a shaker. After that, 500 L of thesamples were extracted and ROS measured according tothe procedure stated previously (Foucaud et al., 2007).

The eect of humic acid concentration on ROS produc-tivity of TiO2 on BDE-209 was analyzed by adding varioushumic acid concentrations (5, 10, 20, 40 mg/L) accordingto Zhang et al. (2008), into the control and treatmentsolutions. The stock solution was prepared by dissolving100 g of humic acid in 0.1 mol/L NaOH solution, followedby dilution with 1000 mL distilled water. Desired testingconcentrations were then prepared by further dilutions withdistilled water. The solutions were shaken in a uorescentlamp chamber for 4 hr. After that, 500 L of the sampleswere extracted and ROS measured according to the proce-dure stated previously (Foucaud et al., 2007).

TiO2 occurs in nature as three dierent forms, anatase,rutile and brookite (Greenwood and Earnshaw, 1984).The eects of distinct structures of TiO2 (anatase, rutile,and mixture of anatase and rutile) on ROS productivitywere evaluated by adding dierent forms of TiO2 into thecotrol and treatment solutions. The solutions were shakenin a light chamber for 4 hr. After that, 500 L of thesamples were extracted and ROS measured according tothe procedure stated previously (Foucaud et al., 2007).

1.3 Photodegradation of BDE-209 by nano-sized TiO2

Nano-sized TiO2 solution (1% mixture of anatase and ru-tile crystalline forms) was prepared in 0.1% DMSO in pH7. The solution was then sonicated for 35 min in an ultra-sonic cleaner (Branson Model 3510, 40 kHz). The negativecontrol did not contain TiO2 and the particle control wasadded with nano-sized SiO2 instead of TiO2. BDE-209 in0.1% DMSO was spiked into the solutions with dierentoperation conditions (pH, humic acid concentrations andcrystalline forms of TiO2) to nal concentration of 75 ppb.The solutions were shaken in the light chamber for 6 days.

The residual products and BDE-209 left after the pho-todegradation were extracted by liquid-liquid extractionand quantied by a gas chromatography-mass spectrome-try (GC-MS). The mixture solutions mentioned above (20mL) were shaken with 20 mL dichloromethane (DCM)in a separation funnel in order to extract PBDEs. Thelower immiscible part of solution (PBDEs dissolved in 20mL of DCM) was then drained into a round bottle ask.This step was repeated for 3 times and totally 60 mL ofDCM was obtained from each treatment. The extracts werethen concentrated by a rotary evaporator and replaced byhexane (nearly 2 mL) (US EPA, 1996). GC-MS analysis ofthe samples was determined according to US EPA standardmethod 1614 (US EPA, 2007).

1.4 Quality control

Extraction and analysis were conducted in dark, to mini-mize the exposure to light. A procedure blank was includedin each batch of extraction. The extracts were analyzedusing an Agilent 7890A GC-MS instrument connectedwith an Agilent 5975C inert MSD triple-axis detector(Agilent Technologies, USA). The method detection limit

(MDL), which was calculated as a mean of the backgroundsignal plus three times the standard deviation in the blanksamples, was 0.5 to 1 g/L for BDE-3 to BDE-191, 4to 5 g/L for BDE-197 to BDE-206, 100 g/L for BDE-209. Surrogate standards were added into samples prior toextraction and the recoveries ranged from 76.1% to 112%.

1.5 Data analysis

The statistical analyses of the data were performed byusing the SPSS version 16 software package for windows.The statistical values were all calculated for triplicates.A 95% condence limit (p < 0.05) was applied for theindication of signicant dierences between samples.

2 Results and discussion

2.1 Time course of BDE-209 degradation

Nano-sized TiO2 prominently enhanced the degradation ofBDE-209, compared with the control (Fig. 1a). Figure 1billustrates the photocatalytical degradation of BDE-209 byTiO2 during the 14 days. The reduction rate of BDE-209increased with time at the beginning until day 6 and thenleveled o at day 14. With initially only BDE-209 in thesolution, the photocatalytical degradation of BDE-209 inthe current study actually followed the rst-order rate lawin the beginning 6 days (Fig. 1c) (Eq. (1)).

y = 1.88 0.0945t (1)

where, y is the log concentration of BDE-209 and t (day)is the reaction time. According to Eq. (1), the half-life forBDE-209 degradation in this study was 3.05 days.

There is a lack of information concerning the naturaldegradation pathway of BDE-209 in water as no prior stud-ies were conducted to investigate its degradation kineticsin water under visible light. However, the results of naturalphotodegradation of BDE-209 in other environmental ma-trixes by sunlight from previous studies may provide someclues for interpreting the results of this study (Table 1).It indicated that the half-life of photocatalytical degradedBDE-209 in this study was much lower than those ofnaturally degraded BDE-209 adsorbed onto other naturalmatrixes (Table 1). This suggested that photocatalyticTiO2 may enhance the photodegradation of BDE-209 andshorten their half lives.

It was suggested that reactive oxygen species may playa signicant role in photocatalytical degradation of PBDEs(Ra and Hites, 2006). The present results showed thatthe hydroxyl radicals produced by TiO2 increased from4 to 8 hr and then decreased until day 6. Although therewas a drop after 8 hr, the levels of hydroxyl radicalsstill remained at double of the control level (Fig. 1d).Debromination of PBDEs may be attributed to the produc-tion of hydroxyl radicals. When the TiO2 is photoexcited,an electron-hole pair is formed. The holes are scavengedby the hydroxyl groups of adsorbed water, yielding .OHradicals which add to the aromatic ring of BDE-209or the reaction products. The addition of the hydroxylradical weakens the aryl-Br bond leading to a subsequent

No. 9 Characterizing the optimal operation of photocatalytic degradation of BDE-209 by nano-sized TiO2 1673

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Fig. 1 Photocatalytical degradation of BDE-209 and induction of ROS by TiO2. (a) degradation rates of control with no nanoparticles and treatmentwith TiO2; (b) degradation of BDE-209 and formation of dierent congeners by TiO2 at dierent time intervals; (c) photocatalytical degradation rate ofBDE-209 by TiO2; (d) inuence of time intervals on hydroxyl radicals production by nano-sized TiO2. Points with the same letter at the top were notsignicantly dierent (p > 0.05) according to one-way ANOVA test.

Table 1 Degradation of BDE-209 in dierent matrixes with or without TiO2

Solid matrixes/solvents Half-lives (t1/2) Rate constant (k) Reference

SunlightSediment 990 days 0.0007 0.0003 day1 Ahn et al., 2006

80 hr Soderstrom et al., 200481 hr Sellstrom et al.,1998

Sand 37 hr Soderstrom et al., 2004533 hr Hua et al., 2003

UV0.1% TiO2 in hexane < 10 min 0.12 0.0049 min1 Sun et al., 20090.1% TiO2 in acetonitrile 2.1 min 0.33 0.02 min1 Sun et al., 2009methanol 51 min Eriksson et al., 2004

Visible light0.1% DMSO 4.05 days This study1% TiO2 in 0.1% DMSO 3.05 days This study

cleavage, and the hydroxyl group is then replaced by thebromine atom. The hydroxyl radicals will then generate aseries of oxidation in the aromatic ring system (An et al.,2008).

2.2 Eect of pH levels, concentrations of humic acids,and crystalline forms of TiO2 on photocatalyticaldegradation of BDE-209

In general, the majority of photocatalytical degradationproducts after treatment with nano-sized TiO2 under threedierent operational conditions were dominated by penta-

and tetra-BDEs (Fig. 2a). They contributed to 25.7% and33.7%, respectively of the photocatalytical degradationproducts at pH 12 (Fig. 2a). Photocatalytic reaction takesplace on the surface of the semiconductor photocatalyst,TiO2. Hence, the pH level of the solution is important tothe photocatalytic function of TiO2, because it determinesthe surface charge property and dispersion of TiO2 (Haqueand Muneer, 2007). Under acidic conditions, TiO2 willbe protonated as TiOH2+, creating a positive charge onthe surface of the catalyst. In contrast, TiO2 will bedeprotonated as TiO under alkaline conditions, creating

1674 Journal of Environmental Sciences 2012, 24(9) 16701678 / Ka Lai Chow et al. Vol. 24

a negative charge on the surface of the catalyst (Sun et al.,2006). Since the majority of the TiO2 particles are mono-charged at these extreme pH levels, they will repel eachother and become dispersed. This can probably increasethe eective surface area in contact with BDE-209 and thusenhance the degradation at pH 12.

Figure 2b shows the distribution of PBDEs productsfrom photocatalytical degradation of BDE-209. A higherproportion of BDE-209 was reduced in humic acids, con-centrations ranging from 520 mg/L. Penta- (40.6%) andtetra-BDEs (38.6%) were the most dominant congenersobserved in 20 mg/L of humic acids. It has been suggestedthat the adsorption between humic acids and TiO2 ispH-dependent (Cho and Choi, 2002; Li et al., 2002).Adsorption of humic acids with TiO2 increased at lowerpH levels. At pH 7, adsorption of humic acids was nearsaturation at 10 mg/L of the initial concentration of humicacids. An increase of the initial concentration to 25 mg/Lonly contributed to a slight rise of adsorption isotherms(Cho and Choi, 2002). Therefore, the adsorption of humicacids with TiO2 was concentration-dependent at pH 7.When the initial concentration went beyond 20 mg/L,adsorption between humic acids and TiO2 declined andthus inhibited the photosensitization eect of humic acidson TiO2. Accordingly, the photocatalytical degradability ofBDE-209 also declined at 40 mg/L.

Figure 2c illustrates the percentage distribution of PBDEcongeners after being photocatalysed by TiO2 in dierentcrystalline forms. Again, tetra- (33.3%) and penta-BDE(34.5%) were the most dominant congeners found in thedegraded residues with the mixture of anatase and rutile. It

has been reported that the crystalline forms of TiO2 wouldaect the morphology and microstructure characteristics,and thus the photocatalytical activity of TiO2 particles(Tayadea et al., 2007; Collins-Martnez et al., 2007).Anatase and rutile, the two basic catalytic crystalline formsof TiO2, contributed to dierences in microstructure, andthus varied in photocatalytical eciencies. For example,anatase has a higher adsorptive ability towards organiccompounds than rutile (Staord et al., 1993). In addition,anatase also has a larger band gap than rutile (Mo andChing, 1995). Due to the interplay of these characteristics,the mixture of anatase and rutile usually displayed moreadvanced photocatalytical eects (Bojinova et al., 2007).In mixed-phase TiO2 catalysts, electrons are transferredfrom anatase to the electron trapping sites of rutile which isat a lower energy state. The recombination rate of anataseis thus lowered, resulting in more ecient electron-holeseparation and greater catalytic reactivity (Bickley, 1991).Therefore, most BDE-209 was degraded in the mixture ofanatase and rutile (residual portion = 17.1%).

2.3 Relations between ROS production and photo-catalytic degradation of BDE-209 under dierentoperational conditions

2.3.1 At dierent pH levelsSignicant dierences were observed between treatmentswith TiO2 and the controls with SiO2, indicating thatthe degradability of BDE-209 was mainly due to thephotocatalytic activity, but not the nano-size of the nano-materials. It was found that the relative uorescence units

4C4S4T 6C6S6T 7C7S7T 8C8S8T 12C12S12T

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Fig. 2 Degradation prole of BDE-209 by TiO2 in 0.1 % DMSO under dierent operating conditions. (a) after photocatalytical degradation by TiO2at dierent pHs, 4C: control at pH 4; 4S: particle control using SiO2 at pH 4; 4T: treatment using TiO2 at pH 4; (b) after photocatalytical degradationby TiO2 in dierent concentrations of humic acid, 0C: control with 0 mg/L humic acid; 0S: particle control using SiO2 with 0 mg/L humic acid; 0T:treatment using TiO2 with 0 mg/L humic acid; (c) after photocatalytical degradation by TiO2 in dierent crystalline forms.

No. 9 Characterizing the optimal operation of photocatalytic degradation of BDE-209 by nano-sized TiO2 1675

(RFU) ratio of treatments over controls was the highest atpH 8 (3.98 0.169) and pH 12 (4.20 0.423) (Fig. 3a).This means that induction of the hydroxyl radicals was thehighest at pH 8 and 12, and can be attributed to the higherconcentration of OH ions in the solution (Herrmann etal., 1993). Meanwhile, the production of BDE-47 (11.5 1.10 nmol/L) and BDE-99 (6.82 0.547 nmol/L), andthe reduction percentage of BDE-209 (93% 1%) werethe greatest at pH 12 (Fig. 4). There was a signicantpositive correlation between ROS induction and BDE-47(r = 0.652, p < 0.01) and BDE-99 (r = 0.835, p < 0.01)production, which revealed that the generation of thesecongeners may be related to the increase of ROS.

Hydroxyl radicals were reported to be responsible forphotolysis of organic pollutants (Peterson et al., 1991). Thegeneration of hydroxyl radicals actually depends on the pHlevel of the solution. The productions of hydroxyl radicalsin neutral, alkaline, and acidic solutions can be illustratedby the following reactions.

In neutral and acidic solution (Fujishima and Honda,1972):

H2O + h+ .OH + H+ (2)In alkaline solution (Sato and White, 1980):

OH + h+ .OH (3)Under alkaline conditions, hydroxyl radicals occupied

most of the ROS (Lair et al., 2008), as indicated by the

results of the current study. Hydroxyl radicals are morereadily generated at higher pH (Zheng et al., 1997) whichmay explain why photocatalytic degradation of BDE-209and production of lower congeners were the most vigorousat pH 12.

2.3.2 At dierent concentrations of humic acidThe present study showed that 20 mg/L of humic acidhad the highest RFU ratio of treatments over the controls(2.68 0.0907) (Fig. 3b). This was an uprising trend from0 mg/L but it dropped at 40 mg/L of humic acid (1.64 0.0616). A similar observation was found for BDE-47(23.6 2.20, 30.9 3.31 nmol/L) with peaks presentedat 10 and 20 mg/L of humic acid. BDE-99 production(36.8 2.97 nmol/L) was only signicantly higher thanthe control at 20 mg/L of humic acid (p < 0.05), whileBDE-209 reduction peaked at 5, 10 and 20 mg/L of humicacid (93.0% 1.70%, 91.6% 3.21%, 91.9% 0.952%,respectively) which were all signicantly (p < 0.05) higherthan the controls (Fig. 5). There was a signicant corre-lation between ROS induction and production of BDE-47(r = 0.703, p < 0.01) and BDE-99 (r = 0.696, p < 0.01),which illustrates that the increase of ROS may be relatedto the production of these congeners.

Dissolved organic matter has been proven to promotethe photodegradation of organic pollutants such as car-boxin and oxycarboxin in aqueous condition (Aguer etal., 2002; Hustert et al., 1999). This may be the resultof photosensitization which extends the response of TiO2

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Fig. 3 Production of ROS by TiO2 in 0.1% DMSO under dierent operating conditions. (a) eect of pH levels on hydroxyl radicals production by TiO2(RFU ratio of treatment over controls); (b) eect of concentrations of humic acid on hydroxyl radicals production by TiO2 (RFU ratio of treatment overcontrols); (c) eect of dierent crystalline structures of TiO2 on hydroxyl radicals production by TiO2 (RFU ratio of treatment over controls). Pointswith the same letter at the top were not signicantly dierent (p > 0.05) according to one-way ANOVA test.

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1676 Journal of Environmental Sciences 2012, 24(9) 16701678 / Ka Lai Chow et al. Vol. 24

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0 mg/L 5 mg/L 10 mg/L 20 mg/L 40 mg/L

b a a a aa a a a a

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Control SiO2 TiO2 Control SiO2 TiO2 Control SiO2 TiO2

Fig. 5 Photocatalytical degradation by TiO2 in 0.1% DMSO with dierent concentrations of humic acid. (a) concentration of BDE-47 in 0.1% DMSO;(b) concentration of BDE-99 in 0.1% DMSO; (c) reduction percentage of BDE-209 in 0.1% DMSO. Points with the same letters at the top were notsignicantly dierent (p > 0.05) according to one-way ANOVA test.

Control SiO2 Anatase Rutile Mixture Control SiO2 Anatase Rutile Mixture Control SiO2 Anatase Rutile MixtureConce

ntr

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Fig. 6 Photocatalytical degradation by dierent crystalline forms of TiO2 in 0.1% DMSO. (a) concentration of BDE-47 in 0.1% DMSO; (b)concentration of BDE-99 in 0.1% DMSO; (c) reduction percentage of BDE-209 in 0.1% DMSO. Points with the same letter at the top were notsignicantly dierent (p > 0.05) according to one-way ANOVA test.

(Zhao et al., 2004). A photosensitizer can transfer absorbedenergy to a chemical or generate oxygen reactive species toenhance the breakdown of organic pollutants (Takahashi etal., 1988). Humic acid has been used as a photosensitizerin photodegradation of DDTs in the presence of TiO2(Zhao et al., 2004). This may be due to the ability ofproduction of ROS by humic acids, such as hydrogenperoxide, hydroxyl radicals and singlet oxygen (Aguer etal., 1999; Sandvik et al., 2000). In addition, the detectionof hydroxyl radicals and BDE-209 reduction increasedwith the concentration of humic acid until 20 mg/L, thissuggests that the photosensitization eect of humic acidon TiO2 may be concentration dependent.

2.3.3 In dierent crystalline structures of TiO2TiO2 can exist naturally as several crystalline forms, in-cluding anatase, rutile and mixture of anatase and rutile(Tsuji et al., 2006). The inuence of dierent crystallineforms of the catalyst on ROS production and degradationof BDE-209 were investigated. Results showed that themixture of anatase and rutile produced the most hydroxylradicals (RFU ratio = 4.21 0.528) (Fig. 3c). The mixturealso generated the highest amount of BDE-47 (12.0 1.81 nmol/L) among the three crystalline forms (Fig. 6).Signicant reduction of BDE-209 compared with controlcould only be observed in the anatase/rutile mixture (82% 3%). A signicant negative correlation (r = 0.696, p