Solar photocatalytic degradation of groundwater contaminated with petroleum hydrocarbons

Solar Photocatalytic Degradation ofGroundwater Contaminated withPetroleum HydrocarbonsIl-Hyoung Cho,a Lee-Hyung Kim,b Kyung-Duk Zoh,a Jae-Hong Park,c and Hyun-Yong Kimd

a Institute of Health and Environment, School of Public Health, Seoul National University, Seoul, 110-799, Korea; [email protected](for correspondence)b Department of Civil and Environmental Engineering, Kongju National University, Kongju-Si, Chungnam-Do, 341-702, Koreac National Institute of Environmental Research, Incheon, 404-708, Koread Environmental and Bio Korea, Daejon, 305-804, Korea

Published online 13 January 2006 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ep.10124

To evaluate the potential use for ex situ remediation,a solar-driven, photocatalyzed reactor system was con-structed and applied to the treatment of groundwatercontaminated with benzene, toluene, ethylbenzene,and xylene (BTEX) and total petroleum hydrocarbons(TPHs) near a gas station using selected advancedoxidation processes such as H2O2/solar light, TiO2

slurry/solar light, and immobilized TiO2/solar light.Groundwater samples containing BTEX and TPH,loaded with H2O2 or slurry and immobilized TiO2

catalyst, were exposed to solar light (37° N and 128° E)in winter with an average intensity of 1.6 mW/cm2

measured at 365 nm. Whereas the solar light/TiO2

slurry system achieved �70% degradation of BTEXand TPH within 4 h, the solar light/immobilized TiO2

and solar light/H2O2 systems did not show significantremoval within the same time. However, both TiO2

slurry and immobilized systems were able to reduceBTEX and TPH levels effectively if H2O2 (10 mM) wasadded. The degradation rates of low molecular weightgasoline (BTEX) and n-alkanes ranging from C10 toC15 were higher than those of n-alkanes ranging fromC16 to C20. The removal efficiency of BTEX and TPH inthe groundwater samples also increased with a largersolar collector area of the reactor. © 2006 AmericanInstitute of Chemical Engineers Environ Prog, 25: 99–109,2006

Keywords: TiO2, photocatalysis, H2O2, BTEX, TPH

INTRODUCTIONThe pollution of groundwater by hazardous organic

compounds is a matter of concern and has developedinto a major public policy issue since the 1990s in manycountries. Contamination of groundwater supplies bygasoline and other petroleum-derived hydrocarbonsreleased from underground storage tanks is a serious,widespread environmental problem [1]. Of these, pe-troleum hydrocarbons containing BTEX (benzene, tol-uene, ethylbenzene, and o,m,p-xylenes) and TPHs (to-tal petroleum hydrocarbons) are hazardous andregulated by many countries, and exist as non-aque-ous-phase liquids (NAPLs) when released to the sub-surface environment [2].

The conventional treatment techniques for ground-water contaminated with petroleum hydrocarbons arepacked tower air stripping, granular activated carbonadsorption, vapor extraction, and biodegradation. Un-fortunately, each of these processes has some disad-vantages [3]. In particular, biodegradation can be ap-plied to treat only low-level organic contamination andit is difficult to apply it to the remediation of highlycontaminated sites [4].

Advanced oxidation processes (AOPs) are com-monly used for remediating wastewater contaminatedwith recalcitrant organic pollutants [5, 6]. These meth-ods are attractive because they produce almost noby-product wastestreams and can be adjusted for thedegree of contaminant removal desired. The AOPs pro-duce stable, innocuous, and mineralized products,such as CO2 and H2O [7]. Also, some studies have© 2006 American Institute of Chemical Engineers

Environmental Progress (Vol.25, No.2) July 2006 99

sought to develop an efficient method of using sun-light, instead of artificial UV light, coupled with TiO2, todestroy toxic organic compounds [8, 9]. Recent re-searches in our laboratory have shown that solar light–induced photocatalysis efficiently removed E. coli [10],metal–EDTA complexes [11], and trichloroethylene [12].

Applications of the treatment to remove BTEX usingAOP processes including TiO2 photocatalysis werestudied [13–15]. However, the field application of solarphotocatalysis on the treatment of petroleum hydrocar-bon–contaminated groundwater in particular was notresearched.

In this study, we investigated the feasibility of selec-tive AOP processes combined with solar light for treat-ing groundwater highly contaminated with BTEX andTPH. The AOP processes used in this study were solarlight/H2O2, solar light/TiO2 slurry, and solar light/im-mobilized TiO2 processes. To demonstrate the feasibil-ity of applying these technologies, field tests were con-ducted using groundwater samples at a nearby gasstation site. Methods to increase the efficiency of BTEXand TPH degradation are also discussed.

EXPERIMENTAL

MaterialsThe TiO2 slurry used in this study was Degussa P-25,

which is mostly anatase, with a BET surface area of 50m2/g, and an average particle diameter of 30 nm.Ti[OCH(CH3)2]4 was supplied by Junsei Chemical Co.(Saitama, Japan) and 30% H2O2 was supplied by Merck(Darmstadt, Germany). Benzene, toluene, ethylben-zene, m,p,-xylene, and o-xylene, all of analytical grade,were purchased from Junsei Chemical Co. The standardsolutions of TPH [gasoline range organics (GROs) anddiesel range organics (DROs)] were prepared from com-mercial gasoline (C4–C10) and diesel (C10–C28) fuel. Thestandards for n-alkanes (individual hydrocarbons, C10–C20) and isoprenoid alkanes such as pristane (2,6,10,14-tetramethylpentadecane) and phytane (2,6,10,14-tetram-

ethylhexadecane) were obtained from Sigma HSL-15 andRadian, respectively. Analytical grades of sodium sulfate(Aldrich, Milwaukee, WI), methanol (Aldrich), methylenechloride (99%, ACS HPLC grade, Aldrich), and acetone(Merck) were used as purchased. All other chemicalswere of reagent grade quality or better and were used asreceived without further purification.

Preparation of TiO2 Immobilized SystemThe TiO2 immobilized system was prepared as fol-

lows: First, Ti[OCH(CH3)2]4 was added dropwise to asolution containing water and isopropanol (50:50 v/v%)and the chemical additives (acetyl acetone or dimethyl-amine). This mixture was stirred at moderate temperatureand pressure (100–150° C and 10–20 atm) for 4 h. Duringstirring, an aqueous solution of SiO2 sol was added to thesolution as the binder. The resulting TiO2 sol was dip-coated on ceramic beads 0.3 to 0.5 cm in diameter at aconstant withdrawal speed of 2 cm/min. The resultingTiO2 film was then annealed at room temperature.

To characterize the TiO2 film, the film crystallinitywas analyzed by X-ray diffraction (XRD) analysis using

Figure 1. XRD pattern of TiO2 films prepared with different chemical additives.

Figure 2. Scanning electron microscopic image ofTiO2 film.

100 July 2006 Environmental Progress (Vol.25, No.2)

Cu–K� radiation in a Siemens D5000 diffractometer.The XRD pattern of ceramic beads with a TiO2 coatingis shown in Figure 1, which shows that the TiO2 filmhad a sharp peak at 25.0°, which is the major peak forthe anatase form. The anatase TiO2 is known to havehigher band-gap energy than that of rutile TiO2; theanatase form is reported to have much higher photo-catalytic activity than that of the rutile form [16]. Thethickness of the TiO2 film was measured from thescanning electron microscope (SEM) image, as shownin Figure 2. The average thickness of the thin film was30–40 nm.

Monitoring Wells and Groundwater SamplingThe groundwater-sampling site was a fuel handling

and storage gas station, which was contaminated withspills and leaks from underground storage tanks. Thestorage tanks originally contained JP-4 jet fuel, which isa mixture of BTEX and diesel compounds. Floatingfuels were found under the water table in the contam-inated area.

A total of four monitoring wells were installed, asshown in Figure 3. Three (N1–N3) were for samplingand one was originally for air sparging and soil vaporextraction (MW4). These wells were sunk to depths of3 to 4 m from an underground storage tank of the gasstation. Because the study site is located in an almosthorizontal area, heights of the wells were not varied,with �1 m of difference in elevation across the site.

Groundwater samples collected from three wells(N1–N3) were analyzed. Three multilevel samplerswere installed along the groundwater flow direction todelineate the vertical distribution of the plume. Using aperistaltic pump (Eijkelkamp Agrisearch Equipment,Giesbeek, The Netherlands), groundwater from theNAPLs (non-aqueous-phase liquids) phase waspumped out of the monitoring wells into the storagereservoir tank. To minimize the volatilization loss aftersampling, the groundwater samples were collected in areservoir tank consisting of an air-free Teflon® bag.

Solar Photocatalysis ReactorThe field-test solar photocatalytic reactor used to

degrade BTEX and TPH in groundwater is shown in

Figure 4. The solar reactor had eight quartz tubes (di-ameter: 0.6 in.), modules connected, and the modulewas 0.75 � 1 m (length � width) with UV-transparenttubular receivers and a radiation area of 0.75 m2. Thequartz tubes were packed with either 1.0 wt % of TiO2slurry or TiO2-coated ceramic beads, as a substitute for0.1 wt % of TiO2 slurry. The total volume of the eightquartz tubes was approximately 730 mL, the reservoirtank volume was 10 L, and the flow rate was 3 L/min.The surface of the reactor consisted of aluminized filmhighly reflective in the UV range (300–380 nm) manu-factured by 3M (St. Paul, MN). The eight modules wereconnected in series and the water flowed directly fromone to the next and finally to the reservoir tank. Arotary pump (Cole Parmer Instrument Co., VernonHills, IL) continuously circulated the groundwater sam-ples between the reservoir tank and solar reactor. Theacceptance angle for the solar reactor was 52° eitherside of normal. The solar light intensity was monitoredusing a radiometer (VLX-3W Radiometer 9811-50, ColePalmer) at 365 nm, mounted at the same inclinationangle (38°) to the plate. This wavelength was selectedbecause it is very close to that of the TiO2 band gap.

Next, the solar reactor, connected in series with foursolar reactors instead of one reactor, was constructedwith the radiation area of 3 m2. This reactor was used toinvestigate the influence of the solar collector area onthe removal of BTEX and TPH.

At regular time intervals, aliquots were withdrawnthrough a three-way valve. Samples were pumped outof the monitoring wells into the reservoir tank. Toavoid contamination and minimize evaporation andloss of BTEX by volatilization from the reactor, theconnected pipe line, reservoir tank, connection parts ofeach module, and sampling port were covered uptightly by Teflon® films and the groundwater was col-lected in an air-free tank. All the experiments, unlessspecified, were performed under random weather con-ditions between noon and 4 p.m. at a gas station sitelocated in Korea (38° N and 128° E).

Extraction of Groundwater SamplesA liquid–liquid extraction technique was used to

extract BTEX and TPH compounds from the ground-water samples. At regular time intervals during thereaction, groundwater samples were withdrawn fromthe reactor using a spring-loaded adjustable syringe (20mL), and then immediately transferred to 30-mL glassvials containing 5 mL of methylene chloride with aTeflon®-lined rubber septum. An aluminum crimp capwas used to minimize loss of the samples by volatiliza-tion. The samples were then extracted using a mechan-ical shaker for 2 h at ambient temperature (winter 4° C).After shaking, the extracts were sonicated using anultrasound bath (Branson 5210 R-DTH, Yamato, To-kyo, Japan) for 10 min. The two phases were separatedby centrifugation (5 min at 1500 rpm), and the meth-ylene chloride phase, transferred into a 2-mL vial, wasanalyzed using gas chromatography.

AnalysisThe term “total petroleum hydrocarbons” (TPHs) is

generally used to describe the measurable amount of

Figure 3. Cross-sectional view of the groundwatermonitoring wells near the gas station.


petroleum-based hydrocarbons; and thus the TPH in-formation obtained depends on the analytical methodused. One of the difficulties with TPH analysis is thatthe scope of the methods varies significantly. In thisstudy, the concentration of TPH having from C4 to C28should be analyzed because the main contaminant (in theKorean gas station) is gasoline and diesel. Therefore theanalytical methods [17, 18] for measuring GROs (C4–C10)and DROs (C10–C28) were used to measure TPH.

The BTEX and TPH compounds were analyzed us-ing a gas chromatograph equipped with a flame ion-ization detector, an autosampler/autoinjector (HP-6890II, Hewlett–Packard, Palo Alto, CA), and anintegrator (HP-3396II). BTEX and TPH compoundswere separated chromatographically using a capillarycolumn (HP-5, 5% phenyl methyl siloxane, 30 m � 0.32mm, 0.25 �m). The initial temperature was kept at 40°C for 4 min, then increased at 25° C/min to a finaltemperature of 300° C, and maintained at this temper-ature for 2 min to ensure the column was clean. Theinjector and detector temperatures were 280 and 300°C, respectively. The H2 gas and air-flow rates of the

flame ionization detector were 40 and 400 mL/min,respectively. N2 carrier gas was delivered at a rate of 30mL/min. Total organic carbon (TOC) was measuredusing a TOC analyzer (TOC 5000A, Shimadzu, Kyoto,Japan). The concentrations of BTEX, TPH, and TOC ineach well sample (N1–N3) are shown in Table 1.

The cations and anions in the groundwater sampleswere analyzed using an ion chromatograph (LC-20,Dionex Corp., Sunnyvale, CA). The column was anIonpac AS14 (Dionex) for the anion analysis and anIonpac CS 12-A column (Dionex) for the cation analy-sis. The flow rate was 1 L/min and the injection volumewas 25 �L of the filtered reaction samples. The eluentconsisted of a mixture of 3.5 mM Na2CO3 and 1 mMNaHCO3 for the anion analysis and 20 mM methanesulfonic acid for the cation analysis.

The average values of the ions found in the ground-water samples are summarized in Table 2. The ground-water samples contained high concentrations of cal-cium, magnesium, chloride, sulfate, and bicarbonateions, which are the ions typically found in mostgroundwater. Using the data of Table 2, the electroneu-trality was checked. Total concentrations of cations and

Figure 4. Schematic view of a solar photocatalytic reactor.

Table 1. Average concentrations of BTEX, TPH, andTOC in groundwater samples taken from the wells.*

Contaminant

Well number

N1 (mg/L) N2 (mg/L) N3 (mg/L)

Benzene 9.4 � 0.1 26.4 � 1.3 21.9 � 0.3Toluene 20.5 � 2.4 18.8 � 0.5 58.5 � 1.1Ethylbenzene 3.2 � 0.2 0.8 � 0.1 1.2 � 0.1m,p-Xylene 5.9 � 0.4 4.2 � 0.2 8.4 � 0.2o-Xylene 7.5 � 0.5 10.5 � 0.3 12.6 � 0.3Total BTEX 46.9 � 3.4 60.8 � 2.4 103.0 � 2.3TPH 584.0 � 3.2 850.0 � 5.1 954.0 � 5.3TOC 380.0 � 5.1 420.0 � 4.2 460.0 � 3.5

*From three samples analyzed.

Table 2. Average concentrations of cations andanions in groundwater samples.*

Ion concentration mg/L mM

Na� 32.0 � 1.2 1.39 � 0.05K� 0.5 � 0.01 0.0128 � 0.0003Ca2� 56.0 � 2.4 1.40 � 0.06Mg2� 20.0 � 1.6 0.82 � 0.07NO3

� 21.7 � 1.3 0.35 � 0.02Cl� 37.2 � 1.4 1.05 � 0.04SO4

2� 12.1 � 0.9 0.126 � 0.01CO3

� 0.3 � 0.1 0.005 � 0.001HCO3

� 290.0 � 3.4 4.75 � 0.056

*From three samples analyzed.


anions were 5.83 and 6.41 mM, respectively. This resultshows that the electroneutrality condition is close, butnot satisfied, indicating that there are some unmea-sured cations in the groundwater samples.

RESULTS AND DISCUSSION

Solar Degradation of BTEX and TPHFirst, the degradation of BTEX and TPH using one

solar reactor was examined. Three different experi-ments were conducted to examine the degradationpattern of groundwater samples under similar sunnyconditions in winter using solar/H2O2, solar/immobi-lized TiO2, and solar/TiO2 slurry systems. Figure 5shows the typical solar light intensity from noon to4 p.m. on a sunny day during these experiments. Asshown in Figure 5, the light intensity on a sunny dayfluctuated slowly and smoothly from 1.5 to 1.8 mW/cm2 between noon and 3 p.m., and then decreased to0.8 mW/cm2 after 3 p.m. The temperature changesduring the testing day were �1° C. The application ofthe three AOPs was investigated under these weatherconditions.

The result of solar light/H2O2 treatment of ground-water samples from well N2 is shown in Figure 6. It isshown that the solar light/(10 mM) H2O2 system wasnot effective in the degradation of groundwater sam-ples, and only a small proportion of BTEX, TPH, andTOC was degraded. This poor degradation is attributedto the slow photolysis of H2O2 (10 mM) and the pro-duction of hydroxyl radical in the presence of solarlight, as shown in the following equation:

H2O2 � solar light 3 2OH � (1)

Because solar UV radiation is only a very small part ofthe total solar spectrum, between 3.5 and 8%, this slow

production of hydroxyl radical from H2O2 is to beexpected, compared with that for artificial UV light [19].

Next, the solar/immobilized system, in which TiO2coated on ceramic beads was placed, was used to treatgroundwater samples (N1 well samples). Preliminaryexperiments using this immobilized system had shownthe complete destruction of gas-phase BTEX com-pounds in the presence of UV light [20], and otherrecalcitrant compounds using solar light [21]. There-fore, the removal of BTEX and TPH was expected withthis immobilized system. However, Figure 7 shows thatthe TiO2 immobilized system also was not effective inthe degradation of BTEX, TPH, or TOC.

Table 2 shows that the groundwater samples containhigh concentrations of ions, especially chloride (Cl�),sulfate (SO4

2�), and bicarbonate (HCO3�). The presence

of inorganic ions in the groundwater samples can affectTiO2 photocatalysis. A bicarbonate concentration of 50mM is at the extreme high end of the range normallyfound in groundwater [22]. The concentration of bicar-bonate (4.75 mM) found in the groundwater samples,however, is much lower than the range normally foundin typical groundwater, where it cannot efficiently actas a strong buffer. In fact, the initial measured pH ofaverage groundwater samples was 7.2, and the solutionpH decreased to about 5.9 as the reaction proceeded,resulting in an acidic range of pH.

The pH drop with degradation can be explained bythe decrease in concentration of hydroxide ion (OH�)from the production of OH radicals. Consequently, theTiO2 is hydrated rapidly in aqueous solution accordingto the following equation:

2TiO2� � 2TiIV�� O � H2O7 (2)

2TiO2� � 2TiIV�� OH]

where TiO2 represents titanium dioxide in bulk solid

Figure 5. Typical solar light intensity at 365 nm (nW/cm2) vs. exposed time on a sunny day from noon to 4:00p.m. in the treatment of groundwater using several AOP processes.


phase and Ti(IV) is surface titanium. The hydrated TiO2surface is amphoteric and has pH-dependent specia-tion accordingly.

The equilibrium among the surface species can bedepicted by

TiIV�� OH2� ¢O¡

� H �

TiIV�� (3)

�OH ¢O¡� OH �

TiIV�� O � � H2O

The point of zero charge (pzc) value of the TiO2 is atpH 6.25 [23]. The TiO2 surface is positively charged inacid media (pH � 6.25), whereas it is negativelycharged under alkaline conditions (pH � 6.25). Be-cause TiO2 exhibits an amphoteric character with azero charge in the pH range around 6.2–6.4, as shownin Eq. 3, the hydration of titanium hydroxide occurs atwhich the reaction proceeds. Because the solution pHdecreased as the reaction proceeded, anions such aschloride and sulfate can attach to the surface of TiO2 atacidic pH.

Figure 6. Removal of BTEX, TPH, and TOC (samples from well N2) using the solar light/H2O2 system.(Experimental conditions: 0.2 wt % TiO2, [BTEX]initial 60.8 mg/L, [TPH]initial 850.0 mg/L, [TOC]initial 420.0mg/L.)

Figure 7. Removal of BTEX, TPH, and TOC (samples from well N1) using the solar light/TiO2 immobilizedsystem. (Experimental conditions: 0.2 wt % TiO2, [BTEX]initial 46.9 mg/L, [TPH]initial 584 mg/L, [TOC]initial 380 mg/L.)


Abdullah et al. [24] observed that the reaction ratesdecreased substantially by chloride, sulfate, and phos-phate at lower pH values. Kormann et al. [25] alsoinvestigated the influence of chloride and carbonate onTiO2 photocatalytic oxidation of chloroform, and pro-posed that the degree to which these anions inhibiteddegradation was correlated strongly with the extent towhich they adsorbed onto the catalyst surface. There-fore, ionic species such as chloride and sulfate may bethe main cause of the inhibition of BTEX, TPH, andTOC degradation in the immobilized system.

Another reason for the low degradation in the im-mobilized system can be the radical-scavenging effectof bicarbonate ions in the groundwater samples. Bicar-bonate is a well-known radical scavenger [26, 27],which has a strong influence on AOP processes. How-ever, because the concentration of bicarbonate ion(290.0 mg/L) is lower than that of the sum of BTEX(46.9 mg/L) and TPH (584.0 mg/L) in the groundwatersamples, it is likely that the scavenging effect of bicar-bonate is not large compared to the ionic adsorptioneffect on TiO2.

Compared with the solar light/TiO2 immobilizedsystem, the solar light/TiO2 slurry system showed asignificant reduction in the concentration of BTEX,TPH, and TOC (samples from N1 well), as shown inFigure 8. This may be explained by the fact that theTiO2 slurry contains more surface area than the TiO2immobilized system, resulting in overcoming the ef-fects of anionic adsorption on the TiO2, and the radicalscavenging by bicarbonate ion. Although the TiO2slurry system can effectively treat petroleum-contami-nated groundwater, the full-scale application of theslurry system is not feasible because of the necessity forTiO2 filtration. Therefore, the method to increase theremoval efficiency in the immobilized system isneeded.

Effect of Adding H2O2 OxidantsThe addition of oxidizing species, such as H2O2,

during TiO2 photocatalysis often leads to an increase inthe rate of photo-oxidation [28, 29]. As shown in Eq. 4,the illumination of TiO2 with solar light generates holesin the valence bands (hvb

� ), and the electrons in theconduction band (ecb

� ). However, one of the practicalproblems in the TiO2 photocatalysis system in aqueoussolutions is the hole–electron recombination process(Eq. 5). This behavior can limit the use of the photo-catalysis system.

Whereas the electrons are consumed by the reactionwith oxygen (O2) to convert it to the superoxide radical(O2

��) (Eq. 7), the holes can react with hydroxide ion onthe TiO2 surface to form hydroxyl radicals (Eq. 6).Therefore, the photocatalytic reaction needs sufficientoxygen (O2) to suppress this recombination process.H2O2 is considered to have two functions in the pro-cess of photocatalytic oxidation. It accepts a photoge-nerated electron from the conduction band, therebypromoting the charge separation (Eq. 8). H2O2 alsoforms hydroxyl radicals according to Eqs. 8 and 9 [8,23].

TiO2 � solar light 3 TiO2Hvb� (4)

�ecb� ) UV light absorption�

TiO2hvb� � ecb

� � 3 heat Recombination� (5)

TiO2hvb� � � OHsur

� 3 OH � (6)

TiO2ecb� � � O2 3 O2

� � (7)

TiO2ecb� � � H2O2 3 OH � � OH � (8)

Figure 8. Removal of BTEX, TPH, and TOC (samples from well N1) using the solar light/TiO2 slurry system.(Experimental conditions: 0.2 wt % TiO2, [BTEX]initial 46.9 mg/L, [TPH]initial 584 mg/L, [TOC]initial 380mg/L.)


H2O2 � O2� � 3 OH � � OH � � O2 (9)

The effect of adding H2O2 (10 mM) on the removalof BTEX and TPH in the TiO2 slurry and immobilizedTiO2 systems was then investigated. The light intensitypattern during these experiments was similar with Fig-ure 5 except that the temperature during these runs wasa little higher (from 0.1 to 1.2 ° C). Under these weatherconditions, Figures 9 and 10 show the degradationefficiency of BTEX, TPH, and TOC using TiO2 immo-

bilized and slurry systems in the presence of H2O2.Samples from well N1 were used in both TiO2 systemsin the absence of H2O2 (Figures 7 and 8), and samplesfrom well N2 were used in both TiO2 systems in thepresence of 10 mM H2O2 (Figures 9 and 10). Therefore,the concentrations of BTEX, TPH, and TOC in the twoexperiments differ. Nevertheless, by comparing Figure9 with Figure 7 for the immobilized system, and Figure10 with Figure 8 for the slurry system, it is still evidentthat the addition of H2O2 (10 mM) significantly in-

Figure 9. Removal of BTEX, TPH, and TOC (samples from well N2) in the solar light/TiO2 immobilized systemin the presence of H2O2. (Experimental conditions: 0.2 wt % TiO2, [H2O2] 10 mM, [BTEX]initial 60.8 mg/L,[TPH]initial 850.0 mg/L, [TOC]initial 420.0 mg/L.)

Figure 10. Removal of BTEX, TPH, and TOC (samples from well N2) using the solar light/TiO2 slurry system inthe presence of H2O2. (Experimental conditions: 0.2 wt % TiO2, [H2O2] 10 mM, [BTEX]initial 60.8 mg/L,[TPH]initial 850.0 mg/L, [TOC]initial 420.0 mg/L.)


creased the degradation efficiency in both the slurryand immobilized systems.

The Degradation Pattern of Carbon Chains duringPhotocatalysis

Next, the concentration changes of n-alkanes andisoprenoids during 4 h of treatment in the solar light/TiO2 slurry/H2O2 system is presented in Figure 11. Asshown in Figure 11, whereas long-chain hydrocarbons(high molecular weight: �C15) were detected after 4 hof treatment, peaks of the lighter n-alkanes (C10–C15)and isoprenoids (pristine and phytane) disappearedcompletely.

Figure 10 also demonstrates that, whereas TOC andTPH were degraded incompletely during 4 h of irradi-ation, BTEX was almost completely removed in �90min in the solar/TiO2 slurry/H2O2 system. These resultsindicate that it was relatively easy to remove lightern-alkanes (C10–C15) and low molecular weight com-pounds such as BTEX and long-chain alkanes (�C15),although isoprenoids (pristine and phytane) were rel-atively difficult to decompose in TiO2 photocatalysis.This also suggests that mineralization, usually ex-pressed by TOC reduction, is achieved mainly fromBTEX compounds during TiO2 photocatalysis.

Influence of the Collector Area of the ReactorFinally, as another method to increase the treatment

efficiency, especially in the TiO2 immobilized system,the solar reactor system was upgraded to four solarcollectors instead of one collector. An experiment thenexamined the effect of upgrading to a large-scale TiO2immobilized system in the presence of H2O2 undersunny conditions with samples from well N3. Figure 12shows that 10–15% more degradations of BTEX, TPH,and TOC were achieved during 4-h treatment using the

system consisting of four solar collectors. This resultimplicates that, when upgraded to larger scale, moregroundwater can be treated within the same time.

CONCLUSIONSThis field study treated real groundwater contami-

nated with BTEX and TPH through the use of selectiveAOPs using solar light. The experimental results ob-tained from this study were as follows:1. The solar light/TiO2 slurry system achieved �70%

degradation of BTEX and TPH during 4 h of treat-ment, whereas the solar light/TiO2 immobilized andsolar light/H2O2 systems were not effective in thedegradations.

2. The synergistic effect of TiO2 with the oxidant H2O2led to a substantial increase in the rate of removal ofBTEX and TPH, and the extent of mineralization incomparison with TiO2 alone in both the TiO2 slurryand immobilized systems.

3. Although it was relatively easy to remove short-chain carbon compounds, such as BTEX and lightern-alkanes, long-chain alkanes (�C15) were rela-tively difficult to decompose in TiO2 photocatalysis.

4. The removal efficiency of BTEX, TPH, and TOC withTiO2 photocatalysis increased with increasing solarcollector areas in the reactor.This study investigated the feasibility of using solar

light for the ex situ treatment of groundwater, espe-cially highly contaminated with BTEX and TPH, so thatbioremediation techniques cannot be applied. The ex-perimental results gave the useful information whenapplying photocatalysis into the field treatment ofgroundwater contaminated by petroleum hydrocar-bons. Based on the results of this study, it is found that,when applying TiO2 photocatalysis to the treatment ofpetroleum-contaminated groundwater, the effects of

Figure 11. Concentration changes of n-alkanes and isoprenoids during 4 h of treatment using solar light/TiO2slurry/H2O2 system.


increasing removal efficiency such as solar collectorareas and the amounts of H2O2 must to be consideredalong with the ionic composition of the groundwater totreat contaminated groundwater cost effectively.

LITERATURE CITED1. Air Force Center for Environmental Exchange

(AFCEE). (1994). Use of risk-based standards forcleanup of petroleum contaminated soil, San An-tonio, TX: Brooks Air Force Base, Armstrong Lab-oratory, and Air Force Institute of Technology.

2. Huntleya, D., & Beckettb, G.D. (2002). Persistenceof LNAPL sources: Relationship between risk re-duction and LNAPL recovery, Journal of Contami-nant Hydrology, 59, 3–26.

3. Nadim, F., Hoag, G.E., Liu, S., Carley, R.J., & Zack,P. (2000). Detection and remediation of soil andaquifer systems contaminated with petroleumproducts: An overview, Journal of Petroleum Sci-ence Engineering, 26, 169–178.

4. Schreiber, M.E., & Bahr, J.M. (2002). Nitrate-en-hanced bioremediation of BTEX-contaminatedgroundwater: Parameter estimation from natural-gradient tracer experiments, Journal of Contami-nant Hydrology, 55, 29–56.

5. Sedlak, D.L., & Andren, A.W. (1991). Oxidation ofchlorobenzene with Fenton’s reagent, Journal ofEnvironmental Engineering and Science, 25, 777–782.

6. Watts, R.J., Udell, M.D., Rauch, P.A., & Leung, S.W.(1990). Treatment of pentachlorophenol (PCP)contaminated soils using Fenton’s reagent, Hazard-ous Waste and Hazardous Materials, 7, 335–345.

7. De Heredia, J.B., Torregrosa, J., Dominguez, J.R., &Peres, J.A. (2001). Oxidation of p-hydroxy benzoicacid by UV radiation and by TiO2/UV radiation:

Comparison and modeling of reaction kinetics,Journal of Hazardous Materials B, 38, 255–264.

8. Malato, S., Blanco, J., Maldonado, M., Ibanez, P., &Campos, A. (2000). Optimising solar photocatalyticmineralisation of pesticides by adding inorganicoxidizing species; application to the recycling ofpesticide containers, Applied Catalysis B: Environ-ment, 28, 163–174.

9. Mehos, M.S., & Turchi, C.S. (1993). Field testingsolar photocatalytic detoxification on TCE contam-inated groundwater, Environmental Progress, 12,194–199.

10. Cho, I.H., Moon, I.Y., Chung, M.H., Lee, H.K., &Zoh, K.D. (2002a). Disinfection effects on E. coliusing TiO2/UV and solar light system, Water Sci-ence and Technology: Water Supply, 2, 181–190.

11. Cho, I.H., Kim, H.Y., Zoh, K.D., & Lee, H.K.(2002b). A study on the removal of toxic metal–EDTA complex using solar light/TiO2 system, Wa-ter Science and Technology: Water Supply, 2, 299–304.

12. Cho, I.H., Kim, H.Y., & Zoh, K.D. (2003). Detoxi-fication of trichloroethylene (TCE) using solarlight/TiO2 in a UV concentrating radiation system,Journal of Water and Environment Technology, 1,37–42.

13. Goswami, D.Y., Klausner, J.F., Mathur, G.D., Mar-tin, A., Wyness, P., Schanze, K., Turchi, C., & Marc-hand, E. (1993). Solar photocatalytic treatment ofgroundwater at Tyndall AFB: Field test results, Pro-ceedings of the 1993 Annual Conference of theAmerican Solar Energy Society (pp. 235–239),Washington, DC, April.

14. Madabushi, S., Goswami, D.Y., Klausner, J.F., &Jotshi, C. (1998). Design construction and testing ofa solar photocatalytic treatment facility for BTEX

Figure 12. Effect of the solar collector area on the removal of BTEX, TPH, and TOC removal after 4 h oftreatment in the sunlight/immobilized TiO2/H2O2 system.


contaminated groundwater, Proceedings of the1998 Solar Energy Conference, Albuquerque, NM.

15. Zhu, K., Chen, H., Li, G., & Liu, Z. (1998). In situremediation of petroleum compounds in ground-water quifer with chlorine dioxide, Water Re-search, 32, 1471–1480.

16. Tang, H., Levy, F., Berger, H., & Schmid, P.E.(1995). Urbach tail of anatase TiO2, Physical Re-view B: Condensed Matter, 52, 7771–7774.

17. American Society for Testing and Materials (ASTM).(2003). ASTM D3921-96. Standard test method foroil and grease and petroleum hydrocarbons in wa-ter, West Conshohocken, PA: ASTM.

18. Weisman, W. (1998). Analysis of petroleum hydro-carbons in environmental media (Volume 1). Avail-able from http://www.aehs.com/publications/catalog/contents/tph.htm

19. Matthews, R.W., & McEvoy, S.R. (1992). A compar-ison of 254nm and 350nm excitation of TiO2 insimple photocatalytic reactors, Journal of Photo-chemistry and Photobiology A: Chemistry, 66, 355–366.

20. Son, H.S., Yang, W.H., Kim, H.Y., Lee, S.J., & Zoh,K.D. (2003). The photocatalytic removal of gasphase BTEX using TiO2 coated on glass and ce-ramic bead, Journal of Korean Society for Atmo-spheric Environment, 19, 57–66.

21. Zoh, K.D., Lee, S.J., Son, H.S., & Kim, H.Y. (2002).Photocatalysis of TNT (2,4,6-trinitrotoluene) usingTiO2 slurry and immobilized systems, Proceedingsof the 3rd World Water Congress (pp. 1152–1160),Melbourne, Australia.

22. James M. Montgomery, Consulting Engineers, Inc.(1985). Water treatment principles and design,New York, NY: Wiley.

23. Poulios, I., & Tsachpinis, I. (1999). Photodegrada-tion of the textile dye Reactive Black 5 in thepresence of semiconducting oxides, Journal ofChemical Technology and Biotechnology, 74, 349–357.

24. Abdullah, M., Low, C., & Matthews, R.W. (1990).Effects of common inorganic anions on rates ofphotocatalytic oxidation organic carbon over illu-minated titanium dioxide, Journal of PhysicalChemistry, 94, 6820–6825.

25. Kormann, C., Mahnemann, D.W., & Hoffmann,M.R. (1991). Photolysis of chloroform and otherorganic molecules in aqueous TiO2 suspensions,Environmental Science and Technology, 25, 494–500.

26. Hoigne, J., & Bader, H. (1985). Rate constants ofreactions of ozone with organic and inorganiccompounds in water. III: Inorganic compoundsand radicals, Water Research, 19, 993.

27. American Water Works Association (AWWA) Re-search Foundation. (1997). Effect of bicarbonatealkalinity on performance of advanced oxidationprocesses, Denver, CO: AWWA Research Founda-tion.

28. Wolfrum, E.J., & Ollis, D.F. (1994). Hydrogen per-oxide in heterogeneous photocatalysis. In G. Helz,R. Zepp, & D. Crosby (Eds.), Aquatic and surfacephotochemistry (pp. 451–465), Boca Raton, FL:Lewis Publishers/CRC Press.

29. Malato, S., Balnco, J., Richter, C., Braun, B., &Maldonado, M.I. (1998). Enhancement of the rateof solar photocatalytic mineralization of organicpollutants by inorganic oxidizing species, AppliedCatalysis B: Environment, 17, 347–356.


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Solar photocatalytic degradation of groundwater contaminated with petroleum hydrocarbons