Removal of polycyclic aromatic hydrocarbons

Environ Sci Pollut Res DOI 10.1007/s11356-015-5676-z

RESEARCH ARTICLE

Removal of polycyclic aromatic hydrocarbons (PAHs) from inorganic clay mineral: Bentonite Gizem Karaca 1 & Hüseyin S. Baskaya 1 & Yücel Tasdemir 1

Received: 5 December 2014 / Accepted: 22 October 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract There has been limited study of the removal of polycyclic aromatic hydrocarbons (PAHs) from inorganic clay minerals. Determining the amount of PAH removal is important in predicting their environmental fate. This study was carried out to the degradation and evaporation of PAHs from bentonite, which is an inorganic clay mineral. UV apparatus was designed specifically for the experiments. The impacts of temperature, UV, titanium dioxide (TiO2), and diethylamine (DEA) on PAH removal were determined. After 24 h, 75 and 44 % of ∑12 PAH in the bentonite were removed with and without UV rays, respectively. DEA was more effective as a photocatalyst than TiO2 during UV application. The ∑12 PAH removal ratio reached 88 % with the addition of DEA to the bentonite. It was concluded that PAHs were photodegraded at high ratios when the bentonite samples were exposed to UV radiation in the presence of a photocatalyst. At the end of all Responsible editor: Philippe Garrigues Highlights • Removal of PAHs from bentonite was investigated here for the first time • A device was designed and applied successfully during PAH removal experiments • Effects of UV and photo-catalysts on PAH removal were investigated • During PAH removal applications, evaporated PAHs amounts were determined • Removed and evaporated PAH ratios varied between 44–88 and 0.3– 2.5 %, respectively * Gizem Karaca [email protected] Hüseyin S. Baskaya [email protected] Yücel Tasdemir [email protected] 1

Department of Environmental Engineering, Faculty of Engineering, Uludag University, 16059, Nilüfer Bursa, Turkey

the PAH removal applications, higher evaporation ratios were obtained for 3-ring compounds than for heavier ones. More than 60 % of the amount of ∑12 PAH evaporated consisted of 3-ring compounds. Keywords UV . Photodegradation . Evaporation . TiO2 . Diethylamine

Introduction Polycyclic aromatic hydrocarbons (PAHs) are semi-volatile organic compounds (SVOCs) that are released to the environment as a result of incomplete fossil fuel combustion. PAHs have been quantified in various matrices, and PAH removal alternatives have been evaluated by several researchers due to their carcinogenic and mutagenic effects on the human body (Zhang et al. 2008; Flotron et al. 2005; Jones et al. 1989; Dong et al. 2010; Karaca 2013; Li et al. 2001). Studies on the removal of PAHs from different matrices are important in understanding the movement of PAHs among environmental media. PAH compounds present in sludge can be degraded using biological methods such as aerobic treatment (Trably and Patureau 2006), chemical methods such as the Fenton process (Flotron et al. 2005), electrochemical processes (Zheng et al. 2007), UV applications (Karaca and Tasdemir 2013a, 2014; Salihoglu et al. 2012), and composting (Cai et al. 2007a, b). PAH removal from soils by UV light (Zhang et al. 2006b, 2008, 2010; Dong et al. 2010; Wang et al. 2009), thermal treatment (Saito et al. 1998; Renoldi et al. 2003), and biological treatment (Mora et al. 2014) have also been reported in the literature. Other researchers have focused on fate of the PAHs in soils (Pernot et al. 2014; Wang et al. 2015; Tansel et al. 2013). Furthermore, PAH photodegradation in fly

Environ Sci Pollut Res

ash (Niu et al. 2007) and fuel oil (Plata et al. 2008) was investigated. Bentonite is a natural, inorganic material that is created through the devitrification of rocks of volcanic origin and the hydrothermal decomposition of acidic volcanic rocks (Chimeddorj 2007). Turkey has 370 million tons of bentonite reserves, making up 19 % of the total world bentonite reserve (TRMD). Bentonite, which primarily consists of montmorillonite clay minerals, can be widely used in borehole applications, pharmaceutical applications, ceramics, and foundry industries because it has a high water absorption capacity, large surface area, and low cost (Diaz-Nava et al. 2012; Holtzer et al. 2012; Shah et al. 2013a, b). Due to its cation exchange capacity, bentonite is also used as an adsorbent in environmental applications (Chai et al. 2007; Şide 2013; Grisdanurak et al. 2012; Santi et al. 2008). The cation exchange capacity of bentonite ranges between 80 and 150 meq/100 g. Copper (Grisdanurak et al. 2012) and textile dye (Şide 2013) were removed from aqueous solution through adsorption onto a bentonite surface. This special property also makes bentonite the perfect material to carry out any combination of chemical, physical, and biological remediation. The bio-reactive properties of bentonite have recently attracted great interest in the field of bioremediation (Sarkar et al. 2012). By replacing its hydrated cations with organic cations, bentonite can be modified into organobentonite, which has certain advantages in organic pollutant abatement (Zhu et al. 1997; Tian et al. 2004). Organically modified bentonites have been evaluated for their suitability as components of compacted earthen liners for waste-disposal sites (Smith and Jaffe 1994). The application of clays for sorption in Europe was approximately 2 million ton/year in the 1990s, up from slightly over 1 million ton/year a decade earlier. Turkey has large sorbent clay deposits, especially of bentonite, but these deposits are poorly defined or explored (Demirel et al. 1995). The relationships between organic matter and PAH compounds have been investigated by many researchers. PAHs are strongly bound by soil with a high organic matter content (Oleszczuk and Pranagal 2007; Nam et al. 2008; Bucheli et al. 2004). High molecular weight PAHs are characterized by a higher hydrophobicity than 2- and 3-ring PAH compounds, resulting in an increased adsorption to organic matter (Pignatello and Xing 1996). PAH sorption by clay minerals such as bentonite has also been investigated by some researchers (Huang et al. 2013; Zhou et al. 2013; Changchaivong and Khaodhiar 2009). Zhou et al. (2013) employed selective sorption with organobentonite to remove PAHs from an aqueous surfactant solution. Huang et al. (2013) investigated the application potential of bentonites in phenanthrene bioremediation. The removal of PAHs sorbed onto clay minerals can be examined under different conditions, such as thermal application, UVexposure, and catalyst addition. UV technology is one of the most frequently studied PAH removal methods. Several researchers have shown that PAHs

in solid matrices can be treated by UV light (Zhang et al. 2006b, 2008; Salihoglu et al. 2012; Dong et al. 2010). Various additives are used in UV applications to enhance the removal of PAHs. Titanium dioxide (TiO2) is one of the most frequently used photocatalysts. It has a large surface area and is stable in acidic and basic environments (Rababah and Matsuzawa 2002). Diethylamine (DEA) is a volatile and flammable alkali liquid with a molecular formula of CH3CH2NHCH2CH3. DEA contributes to photodegradation by acting as an electron source during the UV degradation of SVOCs. PAH removal applications from wastewaters, sludges, and soils containing both organic and inorganic constituents have been studied by our group and several researchers (Karaca and Tasdemir 2011, 2013c; Salihoglu et al. 2012; Lima et al. 2012; Trably and Patureau 2006; Zhang et al. 2008; Sponza and Gok 2011). However, there are a limited number of studies in the literature on PAH removal from completely inorganic matrices. Thus, these types of studies will make a significant contribution to the literature. In the present study, the removal and evaporation of PAHs from inorganic clay minerals were investigated. Temperature and UV applications were performed on bentonite samples, and the effects of using TiO2 and DEA for PAH removal processes were evaluated. In the apparatus used in this study, the quantity of PAHs that migrated into the air (i.e., evaporated PAHs) was also determined.

Materials and methods UV apparatus The apparatus used to study PAH removal is shown in Fig. 1. This apparatus was specially designed by our group (Karaca and Tasdemir 2013b). The air taken into the apparatus is passed from the polyurethane foam (PUF) column in the inlet. A PUF column was placed at the entrance of the apparatus to purify air from the PAHs. This is the main difference from the apparatus used for UVapplications on solid matrices provided in the literature (Zhao et al. 2004; Zhang et al. 2008). The apparatus was made of stainless steel to minimize the surface accumulation of PAHs. Three lamps with a total 24 W of power (254 nm UV-C, Philips TUV G8T5) were installed on the roof of the apparatus at 2-cm intervals, and the apparatus’ dimensions were 45×30×55 cm (width×length×height). A metal fan was placed on the right side surface to homogenize the indoor air. The samples were laid on glass Petri dishes that were 8 cm in diameter and placed on a shelf. The distance between the shelf and the UV source was 18 cm. The indoor temperature was controlled with a heater. The relative humidity and the temperature inside the apparatus were monitored using a HOBO-S-Thb M002 brand sensor, and the data were collected in an H21-002 HOBO data logger.

Environ Sci Pollut Res Fig. 1 UV apparatus. a Exterior appearance. b Internal appearance (Karaca and Tasdemir 2013b)

a) Temperature and Humidity Sensor Data Logger

Outlet PUF

Heater

Pump

Air Flow Meter Inlet PUF

b) Temperature and Humidity Sensor Data Logger

UV Lights Indoor Fan

Outlet PUF

Petri dish

Pump

Air Flow Meter Inlet PUF

PAH removal applications in bentonite Bentonite with a pH of 7.5 was used as the inorganic clay mineral. The order of the procedures in the experiments carried out using bentonite samples can be summarized as follows. A total of 20 g of the bentonite sample was laid on a glass Petri dish with an 8-mm diameter. Pure water was added to the bentonite to obtain a dry matter content of 50 %. Then, a 1000 ng/μL PAH Mix 63 standard was added to the sample, which was then shaken for 5 min. The PAH addition ratio was selected according to our previous studies on solid matrices, and the levels of ∑12 PAH were between 500 and 9500 ng/g DM (Salihoglu et al. 2012; Karaca and Tasdemir 2013b, 2012; Karaca 2013). Samples were separately subjected to temperature, UV, UV-TiO2, and UV-DEA applications at 25 °C. The temperature experiment was conducted at 25 °C without using UV or photocatalysts. The UV experiments (with and without using photocatalysts) were performed to simulate

exposure to high energy. The dose of both TiO2 and DEA was 5 % of the dry weight of the bentonite sample. Extraction and other procedures In the present study, 12 PAH compounds prioritized by the United States Environmental Protection Agency (U.S. EPA) were targeted, phenanthrene (Phe), anthracene (Ant), fluoranthene, (Fl), pyrene (Pyr), benzo[a]anthracene (BaA), chrysene (Chr), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), indeno[1,2,3-c,d]pyrene (InP), dibenzo[a,h]anthracene (DahA), and benzo[g,h,i]perylene (BghiP). Extraction and other procedures for the samples are described in the following subsections. Bentonite and internal wall samples At the end of the PAH removal applications (t=24 h), bentonite samples taken from the apparatus were extracted and


subjected to PAH analysis procedures. Fifty milliliters of an acetone/hexane (Ace/Hex; v/v, 1/1) mixture and 1 mL of a surrogate standard were added to the samples, which were then shaken at 280 rpm in an orbital shaker for 8 h. They were subsequently extracted for 30 min at 15 °C in an Elma S 80H brand ultrasonic bath. The extracted samples were filtered on a Sartorius Stedim (FT-3-1103-047) glass microfiber filter. This extraction procedure was repeated three times. The water content of the extracts was removed by sodium sulfate (Na2SO4). Following this phase, solvent exchange was performed in a rotary evaporator. The sample volume was reduced to 2 mL under a gentle nitrogen stream and was then it was cleaned up with a cleaning column. A gentle nitrogen stream was again used to decrease the volume of the sample to 1 mL. A detailed description of the PAH analysis procedures can be found in Karaca (2013) and Salihoglu et al. (2012). A paper towel dampened with 5 mL of Ace/Hex was used to sample the accumulated PAHs on the internal walls. The paper towel (internal wall sample) was analyzed in a manner similar to the bentonite samples.

PUF samples Before their first usage, the PUFs were extracted with distilled water, methanol (MeOH), Ace/Hex (v/v, 1:1), and dichloromethane (DCM) in a Soxhlet extractor for 24 h and then dried under 60 °C. The PUF cartridges were kept in glass jars with Teflon lids until use. To determine the quantities of PAHs evaporating from the bentonite during the PAH removal applications, the air inside the apparatus was removed via vacuum at a flow rate of 0.8 m3/h and passed through the PUF. The PUF samples were subjected to Soxhlet extraction for 24 h after addition of 250 mL of Ace/Hex (1/1) and 1 mL of the surrogate standard. After this phase, the procedures performed on the PUF extracts (dewatering, volume reduction, etc.) were identical to the procedures for the bentonite extracts. The % PAH evaporation ratios were calculated and compared with the % PAH removal ratios. This method was used to evaluate the migration of PAHs from the bentonite to the air and possible transformations of PAHs. The evaporation experiment was conducted for each PAH removal experiment. The following equation (Eq. 1) was used to calculate the PAH evaporation ratios: %PAH Evaporation ¼

ðevaporated conc:ðng=m3 Þ*vacuumed air volumeðm3 ÞÞ*100 initial PAHbentonite ðngÞ

ð1Þ

Mass balance calculations Mass balances were calculated by Eq. 2 for each PAH compound. In the first step, the quantities of the 12 PAH species in the bentonite (ng) were determined prior to the PAH removal applications. In the second step, the PAH masses (ng) remaining in the bentonite and evaporating to the air were determined at the end of each experiment. Incoming indoor air was cleaned from PAHs using an inlet PUF column; thus, it could be assumed that the bentonite was the only PAH source in the apparatus. The evaporated PAHs were collected by an outlet PUF column. At the end of each 24-h experimental period, the PAHs were expected to disappear, remain in the bentonite, evaporate to the air, or accumulate on the surface of the apparatus. For this reason, the sum of the PAH contents cited above yielded the total PAH content (∑Output PUF) at the end of 24 h. The total PAH content in the ambient air prior to the PAH removal application is shown as (∑Input PAH) in Eq. 2. X X PAH ¼ PAH Input Output ð2Þ P1 þ P2 ¼ P3 þ P4 þ P5 þ P6

(P1) PAHs in the inside air at the beginning=0 % (P2) PAHs in the bentonite at the beginning (%) (P3) PAHs remaining in the bentonite after the 24-h experimental period (%) (P4) PAH evaporated to the air during the 24-h experimental period (%) (P5) PAHs accumulated on the apparatus’ surface after the 24-h experimental period (%) (P6) PAHs disappeared from the bentonite after the 24-h experimental period (%)

Quality assurance/quality control Field blanks amounting to 10 % of the sample were prepared, and any occurrence of contamination during the experiments was determined (Wang et al. 2007). Twenty grams of Na2SO4 were weighed and placed in amber vials to prepare the field blanks (Karaca 2013). The PAH procedures performed on the bentonite samples taken from the apparatus were also performed on the blanks. A 4 ng/mL Standard Mix A PAH surrogate standard (1 mL) was added to each sample (Esen et al. 2008; Vardar and Noll 2003). PAH concentrations measured by gas chromatography-mass spectrometry (GC-MS) were calibrated in accordance with the surrogate standards. PAH concentrations were measured using an Agilent 5975C inert XL mass selective with triple axis detector (MSD) interfaced to an Agilent 7890 Model gas chromatograph (GC). The limits of detection (LODs) were defined for each PAH


compound. The LODs were calculated by adding three standard deviations to the average of the field blanks (Stevens et al. 2003; Tasdemir et al. 2004). Samples less than the LOD were not used. Prior to the PAH analysis, calibrations at seven concentration levels (0.01, 0.1, 0.5, 1.25, 2.5, 5, and 10 g/mL) were carried out to determine the linearity of the GC-MS results. The r2 value of the calibration curve was ≥0.99. The performance of the device was verified by analysis of the midpoint calibration standard every 24 h. The quantifiable amount of PAHs for a 1-μL injection was 0.1 pg.

Results and discussion Effect of UV light on the removal of PAHs from bentonite

PAH Removal Efficiencies (%)

The PAH removal efficiencies from the bentonite samples are provided in Fig. 2. When the UV light was turned off (temperature application), a 44 % Σ12 PAH reduction was obtained in the bentonite samples, whereas this value reached 75 % when the UV light was turned on (UV application). During the temperature application, the removal of PAHs might have occurred by evaporation and thermal degradation mechanisms. Photodegradation did not occur during this application because of the absence of UV radiation. One of the other reason for the reduction in the amounts of PAHs at the end of the experiments could be the water molecules in the bentonite samples. KarimiLotfabad et al. (1996) emphasized

that water molecules successfully competed with PAHs for the active sites on the bentonite surface. These researchers showed that the amounts of PAHs in the bentonite decreased in the presence of water at the end of 48 h (KarimiLotfabad et al. 1996). Bentonite, as required by the basic chemical interaction mechanism with charged kinetic particles, acted as a cation trapper of inorganic and organic cations at pH values in excess of the isoelectric point of the medium because of the (−) charges on its surface. However, at pH values below the isoelectric point, it was able to trap the (−)-charged particles on its surface (Hunter 1986). Therefore, chargeless molecules, such as PAHs, were not expected to hold on to the bentonite surface by means of electrical attraction. Another interaction of the bentonite with particles was through physical means. Bentonite expands with small or large amounts of water due to its crystalline structure (IPCS 2005). The gaps between the layers of the bentonite become saturated with water and water-soluble substances. It is also thought that waterinsoluble substances such as PAHs can also be physically (without any chemical bonding) trapped inside this structure and between the bentonite particles (Huang et al. 2013; KarimiLotfabad et al. 1996). During the temperature applications, the removal efficiencies for 3-, 4-, 5-, and 6-ring PAH compounds were calculated to be 56, 35, 50, and 48 %, respectively. In the presence of UV light, these values increased to 86, 78, 74, and 72 %. For all ring groups, it was determined that the PAH removal efficiencies increased with the use of UV rays. The differences

a)

b)

100 90 80 70 60 50 40 30 20 10 0

100 90 80 70 60 50 40 30 20 10 0 3

4

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6

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100 90 80 70 60 50 40 30 20 10 0

100 90 80 70 60 50 40 30 20 10 0 3

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6

PAH Rings Fig. 2 PAH removal efficiencies for bentonite samples. a Temperature application. b UV application. c UV-TiO2 application. d UV-DEA application


between the removal efficiencies in the experiments with and without UV showed the photodegraded PAH amounts for each number of PAH rings (%PAH removal with UV −%PAH removal without UV). In the present study, PAHs might not have strongly physically sorbed on the surface of bentonite due to water molecules. It was thought that the PAHs were freely physically distributed between the layers and particles. Therefore, UV light with a high energy easily degraded the PAH compounds, as indicated in the literature (Zhang et al. 2006a, 2008; Guieysse et al. 2004). UV light contributes to photodegradation by degrading the benzene ring PAHs (Kubat et al. 2000). Salihoglu et al. (2012) showed that only 21 % of the ∑12 PAH was removed from municipal treatment sludge by UV light. In the present study, the same apparatus as in Salihoglu et al. (2012) was used under similar conditions (UV intensity, distance from the UV source, sample thickness, etc.), and the ∑12 PAH removal efficiencies reached 75 %. This result was also significant because it revealed that the removal ratios of PAHs vary depending on the matrix type and waste compounds present in the matrix. Effect of photocatalysts on the removal of PAHs from bentonite The variation in PAH removal efficiencies during UV-TiO2 and UV-DEA experiments is observed in Fig. 2. TiO2 and DEA were added at a ratio of 5 % of the dry bentonite weight. The Σ12 PAH removal efficiencies in the UV-TiO2 and UVDEA experiments were 75 and 88 % at 25 °C, respectively. During the UV-TiO2 applications, the removal efficiencies for 3-, 4-, 5-, and 6-ring PAH compounds were calculated to be 43, 63, 72, and 58 %, respectively. However, during the UVDEA applications, these values were 92, 88, 88, and 85 %, respectively. TiO2 contributes to the PAH removal process by forming a hydroxyl (OH·) radical (Quan et al. 2005). When TiO2 absorbs UV light, a paired negatively charged electron (e−) and positively charged hole (h+) emerges (Hoffmann et al. 1995). Water molecules are oxidized by the photogenerated holes because the effect of UV light and the presence of TiO2 created OH. radicals (Quan et al. 2005) and this radical cause the photodegradation of PAHs. However, in the present study, the PAH removal ratios were not higher in the UV-TiO2 applications than in the UV applications. Therefore, it was concluded that TiO2 was not an effective photocatalyst at these ratios (5 and 20 %) for the removal of PAHs. It is known from the literature that the appropriate TiO2 dose can vary depending on the matrix type. Karaca and Tasdemir (2013a) showed that the highest ∑12 PAH removal ratio for treatment sludge was obtained with 20 % TiO2 addition. On the contrary, Zhang et al. (2008) emphasized that 0.5 % TiO2 was sufficient for the removal of PAHs from soil samples. An excessive amount of titanium particles might have caused the UV light to scatter,

decreasing the absorption of light in the reaction environment (Zhang et al. 2008). In light of these data, it was thought that a dose lower than 5 % TiO2 might be effective in removing PAHs from bentonite samples. DEA acts as an electron source (Freeman et al. 1986; Lin et al. 1995) during PAH photodegradation reactions. DEA initiates a series of photodegradation reactions by electron transfer (Kubat et al. 2000). Karaca and Tasdemir (2011) showed that DEA was an effective additive to remove PAHs from municipal treatment sludge. Furthermore, Lin et al. (2004) proposed that DEA contributed to the removal of polychlorinated biphenyl (PCBs) from transformer oil. Bunce et al. (1978) emphasized that PCBs used the nonbonding electrons in DEA to form a (−)-charged PCB radical. It was thought that the effect of DEA on the photodegradation of PAHs was similar to the effect of PCBs. It was estimated that the PAHs stimulated with photons attracted one of the nonbonding electrons of the nitrogen in DEA into its own structure and became (PAH·−) (Lin et al. 1995). Because high-energy, unstable (PAH·−) was formed, the PAH removal process was complete after a series of reactions (Kubat et al. 2000). In this way, the highest ∑12 PAH removal ratio (88 %) was obtained using DEA. In conclusion, DEA was an effective photocatalyst at a dose of 5 %.

Evaporated PAH concentrations from bentonite The PAH concentrations that evaporated from the bentonite samples during the experiments are shown in Fig. 3. At the end of the temperature, UV, UV-TiO2, and UV-DEA applications, and the evaporated ∑12 PAH amount from the bentonite samples were 545, 47, 66, and 10 ng/m3, respectively. In the other words, the PAH evaporation ratios were 2, 1.9, 2.5, and 0.3 %, respectively. The evaporation ratios were much lower than removal ratios, which were 44, 75, 75, and 88 %, respectively. Only 3- and 4-ring compounds were detected in the PUF samples. During the temperature and UV applications, the evaporated 3-ring compounds composed 63 and 81 % of the evaporated ∑12 PAH, respectively. At the end of the UV-TiO2 and UV-DEA applications, 100 and 94 % of the evaporated PAHs consisted of 3-ring compounds, respectively. It was concluded that 3-ring compounds have a higher evaporation tendency compared to the heavier compounds (Wang et al. 2005; Huang et al. 2004). PAHs with lower molecular weight evaporated more easily from the matrix due to their higher vapor pressures. From the literature, the tendency of light PAH compounds to evaporate is higher than that of the heavier compounds (Hawthorne and Grabanski 2000; Huang et al. 2004; Wang et al. 2005; Karaca and Tasdemir 2014). Similarly, Salihoglu et al. (2012) and Karaca and Tasdemir (2011) showed that the amount of 3-ring compounds that

Environ Sci Pollut Res Temperature application

Fig. 3 Evaporated PAH concentrations from bentonite to air

Evaporated PAH Concentration (ng/m3)

Evaporated PAH Concentration (ng/m3)

10

Initial (Bentonite)

400 200 0

0 Ant

Residual (Bentonite)

Fl

Pyr

BaA Chr BbF BkF PAH Compounds

BaP

InP DahA BghiP

applications were higher than in the other applications. The increased amount of PAHs evaporating into the air with the increasing the removal efficiency after UV application demonstrates that some part of the 5- and 6-ring PAH compounds c o n v e r t e d i n t o 3 - r i n g PA H c o m p o u n d s t h r o u g h photodegradation, which is in line with the literature (Salihoglu et al. 2012; Guieysse et al. 2004; Karaca et al. 2014). Contrary to this situation, the evaporated PAH ratios decreased during the UV-TiO2 and the UV-DEA applications.

b)

PUF

Internal wall

Initial (Bentonite)


10000

10000

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1000

PAH Mass (ng)

600

PAH Rings

The mass balance data obtained at the end of the PAH removal applications are shown in Fig. 4. The PAH distribution during the PAH removal applications are also given in Table 1. It was observed that migrated PAH amounts during the UV

Internal wall

800

3 Rings 4 Rings 5 Rings 6 Rings

PAH mass balance in bentonite samples

100

100

10

10

1

1 0

0 Phe

c)

UV-DEA application

1000

1

evaporated from the sludge was higher than the amount of 4-, 5-, and 6-ring PAH compounds.

PUF

UV-Titanium application

100

Phe

a)

UV application

Ant

PUF

Fl

Pyr

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BaP

InP DahA BghiP

Phe

Ant

Fl

Pyr

BaA

Chr

BbF

BkF

PAH Compounds Fig. 4 Mass balance for PAHs. a Temperature application. b UV application. c UV-TiO2 application. d UV-DEA application

BaP

InP DahA BghiP

Environ Sci Pollut Res Table 1

PAH distribution during PAH removal applications

Experiment

Temperature UV UV-TiO2 UV-DEA

%PAH distribution (P1)

(P2)

(P3)

(P4)

(P5)

(P6)

0 0 0

100 100 100

56 25 25

2 17 2

5 3 13

37 55 60

0

100

12

1

0.1

86.9

It was thought that, in the presence of TiO2/DEA, the PAH compounds were convert to PAH-dione, PAH-quinone, or other intermediate compounds due to radical reactions with the PAH molecules (Kot-Wasik et al. 2004; Woo et al. 2009; Wen et al. 2003). In this case, because the PAHs in the bentonite converted to intermediate compounds, the amount of PAHs that evaporated to the air would be limited. When the disappeared PAH ratios were investigated, it was revealed that UV-DEA was the most appropriate application to reduce the PAH amounts in all environments (in bentonite and in air). The PAH concentrations measured on the internal walls (P5) at the end of 24 h are given in Fig. 4. Low percentages of PAHs in the bentonite were accumulated in the apparatus during the PAH removal applications. The ∑12 PAH accumulation ratios were lower than 5 % (except for the UV-TiO2 application). The lowest accumulation ratio (0.1 %) was obtained during the UV-DEA application. The amount of accumulated ∑12PAH reached 13 % with TiO2 addition (Table 1). It was observed that primarily, 3-ring light compounds accumulated on the apparatus’ internal walls during all the experiments. The dampening forming on the internal walls of the apparatus creates a surface for the PAHs to hold onto the internal walls. The average relative humidity value measured inside the apparatus was greater than 20 % during the PAH removal experiments. This value reached 68 % during the UVTiO2 application. High levels of humidity in the apparatus enhanced the possible accumulation of PAHs. The PAHs were thought to accumulate onto the apparatus in two steps: the 3and 4-ring light compounds with high vapor pressure evaporated from the bentonite, and then some part of the evaporated PAHs accumulated on the wet surface of the internal walls.

Conclusions &

There is a gap in the literature regarding the behavior of PAHs in clay minerals such as bentonite. In the present study, the removal of the PAHs from bentonite was investigated. The degree to which PAHs remained or not within the bentonite was examined under different conditions (ambient conditions, exposure to UV rays, addition of TiO2 and DEA). A considerable amount of PAHs was

&

&

&

&

&

removed from the bentonite during the UV applications. The results obtained in this study are summarized as follows: During the UV application, 75 % of the ∑12PAH were removed from the bentonite. The PAHs were thought to be freely distributed physically between the bentonite layers and the particles, instead of holding on to the bentonite surface through electrical attraction. The 5 % DEA addition to bentonite provided an 88 % decrease in the ∑12 PAH amount, and DEA was a more effective photocatalyst than TiO2 for the removal of PAHs from bentonite. The accumulated PAHs primarily consisted of 3-ring PAH compounds. The dampening forming on the internal walls of the apparatus created a surface for the PAHs to hold on to the internal walls. During the PAH removal applications with and without UV, the amount of evaporated 3-ring compounds was calculated to be 63 and 81 % of the evaporated ∑12 PAH amount, respectively. Similar results were obtained at the end of the UV-TiO2 and UV-DEA applications. It was concluded that most of the evaporated PAH compounds consisted of lighter compounds due to their higher vapor pressures. When the mass balance calculations were evaluated, the content of PAHs evaporating to the air increased as the PAH content in the bentonite decreased. This result indicates that the pollution altered the environment (from bentonite to air) during the UVapplication. The conversion of 5- and 6-ring PAH compounds into 3-ring PAH compounds through photodegradation might have occurred. UV-DEA was the most appropriate application to reduce the PAH load in all environments (in bentonite and air).

Acknowledgments This work was supported by The Commission of Scientific Research Projects of the Uludag University with Project Number: UAP (M) 2009/20. The authors would like to thank to Emel Yıldırım for her tiresome efforts during the laboratory studies.

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