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Application of various methods for removal of polycyclic aromatic hydrocarbons from synthetic solid matrices a
a
Gizem Karaca & Yücel Tasdemir a
Department of Environmental Engineering, Faculty of Engineering, Uludag University, Nilüfer/Bursa 16059, Turkey Published online: 27 Feb 2014.
Click for updates To cite this article: Gizem Karaca & Yücel Tasdemir (2014) Application of various methods for removal of polycyclic aromatic hydrocarbons from synthetic solid matrices, Environmental Technology, 35:14, 1840-1850, DOI: 10.1080/09593330.2014.884634 To link to this article: http://dx.doi.org/10.1080/09593330.2014.884634
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Environmental Technology, 2014 Vol. 35, No. 14, 1840–1850, http://dx.doi.org/10.1080/09593330.2014.884634
Application of various methods for removal of polycyclic aromatic hydrocarbons from synthetic solid matrices Gizem Karaca and Yücel Tasdemir∗ Department of Environmental Engineering, Faculty of Engineering, Uludag University, Nilüfer/Bursa 16059, Turkey
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(Received 18 July 2013; accepted 13 January 2014 ) In the present study, removal of polycyclic aromatic hydrocarbons (PAHs) from synthetic solid matrices with various methods was investigated. PAH removal experiments were conducted in a specifically designed UV apparatus for this study. Polyurethane foams (PUF) cartridges were used to remove PAHs from the incoming air and to capture PAHs from the evaporated gases. Sodium sulphate (Na2 SO4 ) was used as a synthetic solid matrices. The effects of temperature, UV radiation, titanium dioxide (TiO2 ) and diethylamine (DEA) dose on the PAH removal were determined. TiO2 and DEA were added to the Na2 SO4 sample at the rate of 5% and 20% of dry weight of samples. PAHs’ removal from the Na2 SO4 enhanced with increasing temperature. 12 PAH content in the Na2 SO4 reduced up to 95% during UV light application. Moreover, the 12 PAH removal ratio was calculated as 95% with using 5% of TiO2 , and increasing of TiO2 dose negatively affected PAH removal. PAH concentration in the samples decreased by 93% and 99% with addition of 5% and 20% DEA, respectively. Especially, 3- and 4-ring PAH compounds evaporated during the PAH removal applications. As expected, evaporation mechanism became more effective at high temperature for light PAH compounds. It was concluded that PAHs can successfully be removed from synthetic solid matrices such as Na2 SO4 with the applications of UV light and UV-photocatalysts. Keywords: PAH removal; solid matrix; advanced oxidation; titanium dioxide; diethylamine
Introduction Polycyclic aromatic hydrocarbons (PAHs) are among the major semi-volatile organic compounds (SVOCs) originating from natural and anthropogenic activities. PAHs are organic pollutants and some of these compounds can have potential mutagenic/carcinogenic effects when they eventually reach the food chain. Most PAHs cause environmental pollution and significantly affect the biological balance due to their long residence time and bio-accumulation.[1] Since PAHs can have mutagenic/carcinogenic effects, they need to be removed from all environments.[2] Therefore, 16 of the PAHs are in the priority list prepared by the USEPA.[3] The destruction of ecological balance caused by the movement and accumulation of PAHs in the environment has become an important environmental problem requiring a solution. PAH removal studies in different matrices are important to understand PAH movement among environmental media. Various studies on the amount and removal of PAHs in solid matrices such as treatment sludge and soils have been carried out.[4–8] PAH compounds can be degraded through chemical methods such as the Fenton process, biological and thermal methods.[9–12] UV light technology is one of the PAH removal methods. UV rays cause photodegradation by breaking the benzene ring in PAH’s structure in the presence of radicals.[13] Especially, UV-C rays with shorter ∗ Corresponding
author. Email: [email protected]
© 2014 Taylor & Francis
wavelengths in particular have a high level of energy that is required to degrade PAHs.[14,15] Several researchers showed that PAHs in solid matrices such as soil, sludge, etc. can be treated by UV light technology.[13,14,16,17] Besides, it was observed that the complex nature of sludge highly influenced the photodegradation of PAHs.[16] Studies investigating PAH removal from completely synthetic solid matrices, on the other hand, are limited in number. Various additives can be used in UV light applications to enhance removal of PAHs.[14,18,19] TiO2 , methyl orange, ammonium chloride and DEA classified as photo-catalysts are the major additives for the UV light application.[20] Several researchers observed that polychlorinated biphenyl (PCB),[21,22] PAH [14,16] and pesticide [23] can be removed from solid matrices such as soil and sludge with using TiO2 as a photocatalyst during UV light application. TiO2 enhances the photodegradation reactions by producing OH· Radicals.[18,24] DEA contributes to photodegradation by acting as an electron source during the UV light applications.[25] It was reported that DEA increased the photodegradation ratio of pollutants such as fluorophenyl azides, PCBs and PAHs.[17,20,26,27] In the literature, there are several studies about the removal of PAHs from different solid matrices and interconvertion between PAH compounds was observed during photodegradation
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applications. It was thought that impurity constituents in the sludge might have effect on the photodegradation process. In the present study, Na2 SO4 with an inert structure was used as a synthetic solid matrix to prevent interactions between impurities and PAH compounds. In the scope of this study, a special UV apparatus was designed and the removal of PAH compounds from sodium sulphate (Na2 SO4 ) was investigated. In this content, effects of temperature, UV light, titanium dioxide (TiO2 ) addition and diethylamine (DEA) as photo-catalysts were investigated. Furthermore, evaporated PAH amounts from Na2 SO4 to air were determined and presented.
Materials and methods UV light apparatus The apparatus, designed by our group, is provided in Figure 1.[17] The indoor ambient temperature was controlled with a heater installed on the exterior surface of the set-up. The air taken into the apparatus was purified from PAHs by employing a polyurethane foam (PUF) cartridge and this was one of the main differences between the apparatuses used for UV light applications on solid matrices as provided in the literature.[14,19] The PUF column placed at the outlet was used to collect the evaporated PAHs. The volatilized PAH compounds during PAH removal applications were captured by vacuuming the air with a flow of 0.6–0.8 m3 /h. The incoming air stayed in the apparatus for around 6 s. This way it was ensured that fresh and clean air contacts with the sludge at all times. Materials without organic content were used in the apparatus to prevent interference with PAH. Three UVC lamps with 24 W of power at 254 nm (Philips TUV G8T5) were installed on the top of the set-up at 2 cm intervals. The UV-C light intensity applied to the samples was
Figure 1.
UV apparatus.[11]
Figure 2.
Experimental flow chart.
0.6 mW/cm2 . Na2 SO4 samples were laid on petri dishes with a diameter of 8 cm and placed on the rack with a grid design. The sample height was about 5 mm. The distance between the rack and the UV light source was 18 cm. The moisture and temperature inside the set-up were monitored by a HOBO-S-Thb M002 series sensor and the data were collected at a H21-002 HOBO data logger. The Experimental apparatus used in this study can be seen elsewhere in detail.[16,28] The Experimental flow chart can be seen in Figure 2.
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PAH removal applications in sodium sulphate Na2 SO4 with a pH of 6 and 100% dry matter content was used in the PAH removal studies as a synthetic solid matrices. GC grade Na2 SO4 (Merck 106649) with density: 2.7 g/cm3 , LOI: 99% was purchased from Merck Millipore-Turkey. Before first usage, Na2 SO4 was kept at 450◦ C for 24 h for the removal of organics. Twenty grams of Na2 SO4 was laid on a glass petri dish and 1 mL of 1000 ng/μL PAH Mix 63 standard containing Nap, Acy, Ace, Flu, Phe, Ant, Fl, Pyr, BaA, Chr, BbF, BkF, BaP, InP, DahA and BghiP was added to the sample. PAH-dosed samples were subjected to different temperatures (25◦ C, 40◦ C and 54◦ C), UV light conditions (UV light Turned on-Turned off), UV-TiO2 and UV-DEA applications. In the UV-TiO2 applications, TiO2 (Degussa P25) was dosed to the sample at the rate of 5% and 20% of dry weight of NaSO4 , respectively. The sample was then mixed for more than 5 min in order to have homogeneous distribution. Nano-crystalline powder form of TiO2 (Aeroxide TiO2 P 25, Evonik Industries, Germany) with a pH: 4, specific surface area (BET): 50 m2 /g, density: 0.1 g/cm3 and loss on ignition (LOI): 2% was used. The average nano particle size diameter of TiO2 was 21 nm. The powder contained anatase (70%) and rutile-amosphous (30%) phases. Another additive used in this study was diethyl amine (Merck 8.03010.2500), with the molecular formula CH3 CH2 NHCH2 CH3 (Merck, M803010). In the UV-DEA applications, DEA was added at the rate of 5% and 20% of the dry Na2 SO4 . In all cases, TiO2 and DEA addition was conducted as follows: The reagent (TiO2 /DEA) was added to the Na2 SO4 sample. The mixture was homogenized by stirring them with glass rods for 10 min and then 20 g of this mixture was weighted and placed in a petri dish. At the end of the 24 h applications, Na2 SO4 and PUF samples were subjected to PAH analysis and their PAH concentrations were measured by employing
Table 1.
a gas chromatography-mass spectrometry (GC-MS). The variables and experimental conditions are provided in Table 1. PAH removal efficiencies were calculated as below (Equation (1)): Initial PAHNa2 SO4 (ng/g DM) −Residual PAHNa2 SO4 (ng/g DM) . %PAH Removal = Initial PAHNa2 SO4 (ng/g DM) (1) The following equation (Equation (2)) was used to calculate the evaporated PAH ratios at the end of the PAH removal applications: (Evaporated conc.(ng/m3 )∗ Vacuumed air volume(m3 ))∗ 100 %PAH Evaporation = . InputNa2 SO4 (ng) (2) Furthermore, evaporated PAH amounts (ng) were calculated for each of the PAH removal experiment period (24 h) with the equation (Equation (3)): Evaporated PAH amounts (ng) per day ng = Evaporated conc. m3 3 m × Vacuumed air flow rate day
Extraction of Na2 SO4 samples After experiments, the samples were removed from the UV light apparatus. They were weighed and placed in a glass vial after the addition of 50 mL of acetone/hexane (ACE/HEX) (1:1, v/v) and 1 mL of surrogate standard including PAH Mix 31. Only GC-Grade chemicals were
Experimental conditions. Parameters
Group no. 1 2 3 4 5 6 7 8 9 10 11 12
UV − − + + + + + + + + + +
Temperature 25 40 25 54 25 54 25 54 25 54 25 54
(◦ C)a
(3)
TiO2 dose (%)
DEA dose (%)
– – – – 5 5 20 20 – – – –
– – – – – – – – 5 5 20 20
a Average temperature values over a 24-h period measurements in the apparatus.
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used in this study. The samples were shaken at 280 rpm in an orbital shaker for 12 h and afterwards they were extracted for 30 min at a temperature of 15◦ C in an Elma S 80H brand ultrasonic bath. The extracted samples were filtered and 50 mL of ACE/HEX (1/1) was added again to the residual sample in the bottle and it was extracted for 30 min and filtered. Fifty millilitre of ACE/HEX was added to the sample for a third time and it was waited in the freezer for 12 h. Then, the sample was extracted for 30 min. with an ultrasonic bath. The same methods used in the study carried out by Karaca and Tasdemir [29] and Karaca [28] were used to treat the samples from this stage onwards. Extraction of PUF samples The PUF samples were subjected to Soxhlet extraction for 24 h after the addition of 250 mL ACE/HEX (1/1:v/v) and 1 mL of surrogate standard. Dewatering, volume reduction and other procedures that were performed on the PUF extracts were identical to the procedures for Na2 SO4 samples. Quality assurance/quality control Field blanks at 10% of the number of samples were taken to determine any contamination during sample handling and analyses.[30] Twenty grams of Na2 SO4 were used in each field blank.[17,28] The blanks were extracted and cleaned in the same manner as the other samples. PAH surrogate standard (4 ng/mL) at the amount of 1 mL was added to each sample to determine recovery efficiencies of targeted PAH compounds.[31,32] Phenanthrene-d10 was used as a surrogate for Phe, Ant, Fl, Pyr and BaA. Chrysene-d12 was used, which served as a surrogate for Chr, BbF, BkF and BaP. Perylene-d12 served as a surrogate for InP, DahA and BghiP. PAH concentrations measured in Agilent 5975C inert XL mass selective with triple axis detector (MSD) of Agilent 7890 Model GC were calibrated in accordance with the surrogate standards. Limits of detections (LODs) were defined for each PAH compound. LOD was calculated by adding 3 standard deviations to the average of the field blanks.[6,28,33] Amounts in the samples less than the LOD were not used in the calculations. Prior to analysis of the samples, GC-MS was calibrated for seven concentration levels (0.01, 0.1, 0.5, 1.25, 2.5, 5 and 10 μg/mL) to determine the linearity of the responses. The r 2 value of the calibration curve was ≥0.99. System performance was verified by analysis of the midpoint calibration standard every 24 h during the analysis period. Instrument detection limits (IDLs) were determined using the area of a peak with a signal/noise ratio of 3 at the lowest standard calibration level. All reported values were much above IDL values. The quantifiable PAH amount for a 1-μL injection was 0.1pg.
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Results and discussion Heat and UV light applications The average temperature inside the apparatus while the UV and the thermostat were turned off was 25◦ C. The average temperature increased up to 40◦ C and 54◦ C when the apparatus was UV-closed, thermostat-open andUV-open, thermostat-open, respectively. Twelve PAH ( 12 PAH) concentration values (ng/g dry matter-DM) in Na2 SO4 samples after heat and UV light applications are shown in Figure 3. The removal efficiency for 12 PAH of Na2 SO4 samples which have been kept under 25◦ C (UV light-closed, thermostat-closed) was 53%. This value increased to 58% by increasing the temperature up to 40◦ C. On the other ◦ hand, in the UV light applications performed at 25 C and ◦ 54 C, 12 PAH removal efficiencies were 93% and 95%, respectively. Approximately 30◦ C temperature increase has provided 2% contribution into the PAH degradation process. Similarly, it was observed that the PAH removal efficiency at 40◦ C was higher than the PAH removal efficiency at25◦ C. It was a reliable result that the removal rate of 12 PAHs increased based on evaporation and acceleration of photodegradation reactions with increasing temperature. It is a known fact that the PAH removal efficiencies increase by increasing the temperature.[34,35] Especially, concentrations of 3- and 4-ring PAH compounds decreased by increasing the temperature during UV light applications. It can be expressed that these compounds which have more volatile characteristics compared with the 5 and 6-ring compounds, were evaporated and removed from the environment by means of high-vapour pressures [16,17,28,36] In the study conducted by Salihoglu et al.[16] on PAH removal from the municipal treatment sludge, the removal efficiency for 12 PAH during UV light applications did not exceed 21% at 54◦ C. The working conditions and UV light apparatus were the same as the ones used in this study and the only difference between these two studies was the applied matrices. It was observed that performing UV light applications in the treatment sludge was difficult. Sludge is a more complex matrix than synthetic matrices (Na2 SO4 ) and also contains various pollutants. In the present study, it was determined that the UV-C light penetrated sufficiently into the PAHs in Na2 SO4 . It was concluded that the characteristics and composition of the studied matrices were important in the UV light applications. Likewise, other studies performed within the scope of the literature emphasized that the matrices’ structure [37,38] and the organic composition [39] affect the photodegradation process of PAHs. When the PAH concentrations at 25◦ C were taken into consideration, it was observed that the UV light was effective on 4-, 5- and 6-ring PAH compounds rather than the 3-ring ones. While the UV light was closed, the removal efficiencies of 3-, 4-, 5- and 6-ring compounds were 97%, 12%, 31% and 23%, respectively. These values were calculated as
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(a)
(b)
Figure 3.
The PAH concentrations in Na2 SO4 samples after PAH removal applications. (a) Heat applications and (b) UV light applications.
70%, 92%, 91% and 89%, respectively while the UV light was open. Similarly, there was an increase in removal of 4-, 5- and 6-ring compounds during the UV light applications at 54◦ C. Several researchers found that the absorption characteristics of UV light by PAH compounds varied in liquid and solid matrices. The 3-ring light compounds absorbed the UV light easily in the liquid matrices while 5and 6-ring heavy compounds absorbed them easily in the solid matrices.[14–16,35,40] It was observed that the data of the present study agree with the literature and it was shown that the removal efficiency was increasing as the number of
PAH rings increased in the UV light applications performed on solid matrices. UV-TiO2 applications PAH concentrations in Na2 SO4 samples at the end of the UV-TiO2 applications are presented in Figure 4. The maximum PAH removal efficiency in the samples was around 95% during UV-TiO2 applications. When TiO2 absorbs light with wavelengths shorter than 385 nm, a paired of negative charged electrons and positive charged
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(a)
(b)
Figure 4.
The PAH concentrations of Na2 SO4 at the end of UV-TiO2 applications (a) 5% TiO2 and (b) 20% TiO2 .
dual cells (electrons (e− ) and cells (h+ )) emerge.[41] It is known that the hydroxyl radical (OH· ) which is formed as a result of a series of reactions [18,41–43] disintegrates the organic compounds in the different environmental media.[18,44] In the present study, it can be stated that TiO2 enhanced the photodegradation of PAHs by means of the OH· radical. Furthermore, O·− 2 might have formed on the surface of Na2 SO4 by means of the reactions between O2 absorbed from the atmosphere and the electrons in the environment.[18] These radicals with high tendency for the
reaction might have contributed to the degradation of PAH compounds. performed at 25◦ C, the In the UV-TiO2 applications removal efficiencies for 12 PAH compounds in the samples containing 5% and 20% TiO2 were 91% and 90%, respectively while these values were 95% and 89% at 40◦ C, respectively. Increased PAH removal ratios were obtained in the samples containing 5% TiO2 with increasing temperature. It was an expected result that higher catalyst efficiency was observed at high temperatures.[45] Likewise,
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within the scope of the UV-TiO2 studies performed by Zhang et al.[34] and Nadal et al.,[35] it was emphasized that the PAH removal efficiencies increased by increasing the temperature. The photodegradation process of PAHs was negatively influenced by increasing the TiO2 dose from 5% to 20%. The increase in TiO2 amount might have caused scattering of the UV light and decreasing the absorption of light in the reaction environment.[14] Moreover, it was possible for surface poisoning to occur in the catalysts.[46] It should not be ignored that the reaction products formed on the TiO2 surface compete with the reactant and cause surface poisoning by diffusing the capillary pores of the catalyst.[46] In the present study, it was thought that PAH removal efficiencies decreased with increasing of TiO2 dose because of scattering and poisoning effects of TiO2. [13,14,16] In the UV-TiO2 applications, removal ratios of 4-, 5and 6-ring PAH compounds were higher than the ratios of 3-ring PAHs. In other words, TiO2 was more effective on the compounds which can absorb the UV light more easily. Similarly, Salihoglu et al.[16] showed that removal rates of the heavy PAH compounds especially increased by the usage of TiO2 as a photocatalyst. The PAH removal efficiencies of the samples containing 5% TiO2 increased by increasing the temperature for all ring groups. At 25◦ C, the removal efficiencies of the 3-, 4-, 5- and 6-ring PAH compounds were calculated as 72%, 97%, 98% and 98%, respectively, these values rose to 82%, 99%, 99% and 99%, respectively at 54◦ C. However, this increase was partially observed on the samples (4- and 5-ring compounds) containing 20% TiO2 . It can be stated that high doses of TiO2 might have caused such differences due to its negative effects on the PAH removal process. UV-DEA applications Na2 SO4 samples were mixed with DEA at an amount of 5% and 20% of the dry weight and the samples were exposed to UV-C light for 24 h in the apparatus at 25◦ C and 54◦ C. The alteration of the PAH concentrations of these samples are given in Figure 5. In the UV-DEA applications performed at 54◦ C, 12 PAH removal efficiency increased up to 99% and it was determined that DEA was an effective photo-sensitizer at that temperature. DEA initiates the photodegradation reactions by electron transfer and provides contribution into the PAH removal process.[25] Bunce et al.[47] emphasized that non-bonding electrons in DEA were taken by PCBs and formed the (−) charged PCB radical and (+) charged DEA radical. It was thought that the effect of DEA on photodegradation of PAHs was similar to the effect of PCBs. 12 PAH removal efficiencies of the samples containing 5% and 20% DEA at 25◦ C were 91% and 95%, respectively. At 54◦ C, 93% of PAHs were removed with 5% DEA addition while 99% of PAHs were removed with 20% DEA addition. In other words, PAHs were removed more
effectively from the Na2 SO4 samples when the temperature and DEA dose increased. In the UV-DEA applications, it was supposed that the high PAH removal efficiencies were obtained by means of the evaporation and accelerated photodegradation reactions at 54◦ C. Similarly, Karaca and Tasdemir,[17] showed that the 12 PAH removal efficiency increased from 46% to 51% by increasing the temperature from 38◦ C to 53◦ C during the UV-DEA applications. It was observed that 20% DEA dose was more effective than 5% DEA dose for the removal of PAHs from the Na2 SO4 samples. The SVOCs removal applications in the literature were proven that the optimum DEA dose varied according to the matrices type.[17,25] In the UV light applications performed on municipal treatment sludge, it was emphasized that the PAH removal efficiencies of the samples containing 0.5% DEA were higher than the efficiencies of the samples containing 5% DEA.[17] With respect to the automotive sludge, PAH removal efficiencies increased by increasing the DEA dose from 5% to 20%.[29,48] Evaporated PAH amounts The air incoming into the apparatus was cleaned by employing a PUF cartridge in front of the apparatus. Therefore, the vacuumed air was free from the organics. Then, the air inside the apparatus was vacuumed and passed through the PUF columns in order to determine the amount of PAHs evaporated from Na2 SO4 samples during the PAH removal applications. Evaporated PAH concentrations at the end of the 24 h are given in Figure 6. None of the PAH compounds were encountered in the evaporated samples after the UVDEA applications. High removal efficiencies were obtained in these experiments. It was believed that PAHs were converted to the PAH compounds other than the investigated compounds. Therefore, PAHs were not detected in the PUF samples of Na2 SO4 containing DEA. It was determined that 99% of the evaporated PAHs were consisting of 3- and 4-ring compounds. As the molecular weight of PAHs increase, their volatile characteristics also increase.[49] While the compounds with higher molecular weight (5- and 6-ring compounds) have tendency for bonding solid matrices, the compounds with lighter molecular weight (3- and 4-ring compounds) have tendency to evaporate.[21,50] Consequently, it was expected that the 3- and 4-ring PAH compounds migrate more into air than the 5- and 6-ring compounds. The PAH amount that evaporated from Na2 SO4 increased with increasing temperature in the UV light and UV-TiO2 applications (Figure 6). In the UV light applications, while 305 ng PAH was evaporating at 25◦ C, this value increased to 2785 ng at 54◦ C. It is known that evaporation of the SVOCs from sludge increases with increasing of temperature.[51]. The PAH amount evaporated from the samples containing 5% TiO2 was measured as 675 ng at 25◦ C and 1254 ng at 54◦ C. These values
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(a)
(b)
Figure 5.
The PAH concentrations of Na2 SO4 at the end of UV-DEA applications (a) 5% DEA and (b) 20% DEA.
for the samples containing 20% TiO2 were 680 ng and 1620 ng, respectively. It was observed that the temperature increase had more impact on the evaporation of PAHs than the TiO2 dose. At 54 ◦ C, PAH migration ratios decreased with using TiO2 during the UV applications. In the UV-TiO2 applications, it was thought that the intermediate compounds were formed as a result of the photodegradation reactions of PAHs.[13,52] Otherwise, it would be expected that the heavy PAH compounds were
converted into the light PAH compounds after the photodegradation and evaporation under the heat and then they migrated into the air. As a result of the UV-TiO2 study performed by Wen et al.,[53] it was determined that Phe and Pyr were transformed to phenanthrenedialdehyde and 1,6-pyrenedione compounds. Moreover, Woo et al.[54] found that the 3-, 4- and 5-ring PAH compounds were turned into PAH-dione compounds as a result of UV-TiO2 applications.
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(a)
Figure 6.
(b)
Evaporated PAH concentrations (a) UV light applications and (b) UV-TiO2 applications.
Conclusions It is possible to encounter PAH removal applications performed with advanced oxidation methods in solid matrices such as the soil, sediment and treatment sludge. In the experimental studies in waste matrices, some pollutants in the wastes attempt photodegradation reactions with PAHs [55] and/or compete with PAHs in order to absorb UV light. Therefore, the studies about synthetic liquid matrices and synthetic solid matrices are important for correctly evaluating the PAH removal applications. While there are several researches on the removal of PAHs from synthetic liquid matrices,[40,56,57] it is difficult to find similar studies performed on synthetic solid matrices. The main results obtained from the experimental studies are summarized below: • While maximum 58% decrease on 12 PAH amount was obtained in the heat applications (UV light is closed), this value increased up to 95% by means of UV light applications. In the experimental studies performed with and without using UV light, it was proven that 12 PAH removal efficiency increased by increasing the temperature. Several researches also showed that PAH degradation in different materials (i.e. air, soil and grass) increased with the temperature increase.[2,34] • UV lights were more effective on removal of 4-, 5- and 6-ring PAH compounds rather than the 3ring (Phe and Ant) compounds in the PAH removal applications which was performed at 25◦ C and 54◦ C. As it was indicated in the literature, UV lights were absorbed more easily by the heavy PAH compounds.[14–16] • 12 PAH removal efficiency was 95% in the sample including 5% TiO2 by dry weight. This value decreased by increasing the TiO2 ratio. It was observed due to the catalyst poisoning effect and/or
scattering of UV lights (scattering effect). These results were in line with the literature.[13,14] • During the UV-TiO2 and UV-DEA applications, the PAH removal efficiencies at 54◦ C were higher than the efficiencies obtained at 25◦ C (except 20% TiO2 ). It was concluded that evaporation and acceleration of degradation reactions might have caused to obtain increased PAH removal efficiencies at a high temperature. • It was observed that DEA was a more effective additive than TiO2 and 99% of PAHs was removed with 20% DEA addition. • 99% of PAHs that migrated to the air from Na2 SO4 comprised 3- and 4-ring compounds. On the other hand, PAH amount that migrated into the air by evaporation from Na2 SO4 increased with increasing temperature during the UV light and UV-TiO2 applications Funding This work was supported by The Commission of Scientific Research Projects of 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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[41] Fujishima A, Hashimoto K, Watanabe T. TiO2 photocatalysis, fundamentals and applications. Tokyo: BKC, Inc Publishers; 1999. [42] Kawahara FK, Davila B, Alabed SR, Vesper SJ, Ireland JC, Rock S. Polynuclear aromatic hydrocarbon (PAH) release from soil during treatment with Fentons reagent. Chemosphere. 1995;31:4131–4142. [43] Dhol AS. An investigation of a photochemical approach fort the remediation of PCB-contaminated soils. University of Calgary; 2005. [44] Kurt U, Apaydin O, Gonullu MT. Reduction of COD in wastewater from an organized tannery industrial region by Electro-Fenton process. J Hazard Mater. 2007;143:33–40. [45] March J. Advanced organic chemistry, reactions, mechanisms and structures. Photochemistry. New York: Wiley; 1985. [46] Cebe M. Physicochemistry volume II: fundamental concepts in reaction kinetics. Turkey: Uludag University Press; 1995. [47] Bunce K, N.J. Y, Ravanal L, Safe S. Photochemistry of chlorinated biphenyls in isooctane solution. J Chem Soc. 1978;2:880–884. [48] Karaca G, Tasdemir Y. Removal of polycyclic aromatic hydrocarbons (PAHs) from industrial sludges in the ambient air conditions:automotive industry. J Environ Sci Health A. 2013;48:855–861. [49] Wang DG, Chen JW, Xu Z, Qiao XL, Huang LP. Disappearance of polycyclic aromatic hydrocarbons sorbed on surfaces of pine [Pinua thunbergii] needles under irradiation of sunlight: volatilization and photolysis. Atmos Environ. 2005;39:4583–4591.
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Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Application of various methods for removal of polycyclic aromatic hydrocarbons from synthetic solid matrices a
a
Gizem Karaca & Yücel Tasdemir a
Department of Environmental Engineering, Faculty of Engineering, Uludag University, Nilüfer/Bursa 16059, Turkey Published online: 27 Feb 2014.
Click for updates To cite this article: Gizem Karaca & Yücel Tasdemir (2014) Application of various methods for removal of polycyclic aromatic hydrocarbons from synthetic solid matrices, Environmental Technology, 35:14, 1840-1850, DOI: 10.1080/09593330.2014.884634 To link to this article: http://dx.doi.org/10.1080/09593330.2014.884634
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Environmental Technology, 2014 Vol. 35, No. 14, 1840–1850, http://dx.doi.org/10.1080/09593330.2014.884634
Application of various methods for removal of polycyclic aromatic hydrocarbons from synthetic solid matrices Gizem Karaca and Yücel Tasdemir∗ Department of Environmental Engineering, Faculty of Engineering, Uludag University, Nilüfer/Bursa 16059, Turkey
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(Received 18 July 2013; accepted 13 January 2014 ) In the present study, removal of polycyclic aromatic hydrocarbons (PAHs) from synthetic solid matrices with various methods was investigated. PAH removal experiments were conducted in a specifically designed UV apparatus for this study. Polyurethane foams (PUF) cartridges were used to remove PAHs from the incoming air and to capture PAHs from the evaporated gases. Sodium sulphate (Na2 SO4 ) was used as a synthetic solid matrices. The effects of temperature, UV radiation, titanium dioxide (TiO2 ) and diethylamine (DEA) dose on the PAH removal were determined. TiO2 and DEA were added to the Na2 SO4 sample at the rate of 5% and 20% of dry weight of samples. PAHs’ removal from the Na2 SO4 enhanced with increasing temperature. 12 PAH content in the Na2 SO4 reduced up to 95% during UV light application. Moreover, the 12 PAH removal ratio was calculated as 95% with using 5% of TiO2 , and increasing of TiO2 dose negatively affected PAH removal. PAH concentration in the samples decreased by 93% and 99% with addition of 5% and 20% DEA, respectively. Especially, 3- and 4-ring PAH compounds evaporated during the PAH removal applications. As expected, evaporation mechanism became more effective at high temperature for light PAH compounds. It was concluded that PAHs can successfully be removed from synthetic solid matrices such as Na2 SO4 with the applications of UV light and UV-photocatalysts. Keywords: PAH removal; solid matrix; advanced oxidation; titanium dioxide; diethylamine
Introduction Polycyclic aromatic hydrocarbons (PAHs) are among the major semi-volatile organic compounds (SVOCs) originating from natural and anthropogenic activities. PAHs are organic pollutants and some of these compounds can have potential mutagenic/carcinogenic effects when they eventually reach the food chain. Most PAHs cause environmental pollution and significantly affect the biological balance due to their long residence time and bio-accumulation.[1] Since PAHs can have mutagenic/carcinogenic effects, they need to be removed from all environments.[2] Therefore, 16 of the PAHs are in the priority list prepared by the USEPA.[3] The destruction of ecological balance caused by the movement and accumulation of PAHs in the environment has become an important environmental problem requiring a solution. PAH removal studies in different matrices are important to understand PAH movement among environmental media. Various studies on the amount and removal of PAHs in solid matrices such as treatment sludge and soils have been carried out.[4–8] PAH compounds can be degraded through chemical methods such as the Fenton process, biological and thermal methods.[9–12] UV light technology is one of the PAH removal methods. UV rays cause photodegradation by breaking the benzene ring in PAH’s structure in the presence of radicals.[13] Especially, UV-C rays with shorter ∗ Corresponding
author. Email: [email protected]
© 2014 Taylor & Francis
wavelengths in particular have a high level of energy that is required to degrade PAHs.[14,15] Several researchers showed that PAHs in solid matrices such as soil, sludge, etc. can be treated by UV light technology.[13,14,16,17] Besides, it was observed that the complex nature of sludge highly influenced the photodegradation of PAHs.[16] Studies investigating PAH removal from completely synthetic solid matrices, on the other hand, are limited in number. Various additives can be used in UV light applications to enhance removal of PAHs.[14,18,19] TiO2 , methyl orange, ammonium chloride and DEA classified as photo-catalysts are the major additives for the UV light application.[20] Several researchers observed that polychlorinated biphenyl (PCB),[21,22] PAH [14,16] and pesticide [23] can be removed from solid matrices such as soil and sludge with using TiO2 as a photocatalyst during UV light application. TiO2 enhances the photodegradation reactions by producing OH· Radicals.[18,24] DEA contributes to photodegradation by acting as an electron source during the UV light applications.[25] It was reported that DEA increased the photodegradation ratio of pollutants such as fluorophenyl azides, PCBs and PAHs.[17,20,26,27] In the literature, there are several studies about the removal of PAHs from different solid matrices and interconvertion between PAH compounds was observed during photodegradation
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applications. It was thought that impurity constituents in the sludge might have effect on the photodegradation process. In the present study, Na2 SO4 with an inert structure was used as a synthetic solid matrix to prevent interactions between impurities and PAH compounds. In the scope of this study, a special UV apparatus was designed and the removal of PAH compounds from sodium sulphate (Na2 SO4 ) was investigated. In this content, effects of temperature, UV light, titanium dioxide (TiO2 ) addition and diethylamine (DEA) as photo-catalysts were investigated. Furthermore, evaporated PAH amounts from Na2 SO4 to air were determined and presented.
Materials and methods UV light apparatus The apparatus, designed by our group, is provided in Figure 1.[17] The indoor ambient temperature was controlled with a heater installed on the exterior surface of the set-up. The air taken into the apparatus was purified from PAHs by employing a polyurethane foam (PUF) cartridge and this was one of the main differences between the apparatuses used for UV light applications on solid matrices as provided in the literature.[14,19] The PUF column placed at the outlet was used to collect the evaporated PAHs. The volatilized PAH compounds during PAH removal applications were captured by vacuuming the air with a flow of 0.6–0.8 m3 /h. The incoming air stayed in the apparatus for around 6 s. This way it was ensured that fresh and clean air contacts with the sludge at all times. Materials without organic content were used in the apparatus to prevent interference with PAH. Three UVC lamps with 24 W of power at 254 nm (Philips TUV G8T5) were installed on the top of the set-up at 2 cm intervals. The UV-C light intensity applied to the samples was
Figure 1.
UV apparatus.[11]
Figure 2.
Experimental flow chart.
0.6 mW/cm2 . Na2 SO4 samples were laid on petri dishes with a diameter of 8 cm and placed on the rack with a grid design. The sample height was about 5 mm. The distance between the rack and the UV light source was 18 cm. The moisture and temperature inside the set-up were monitored by a HOBO-S-Thb M002 series sensor and the data were collected at a H21-002 HOBO data logger. The Experimental apparatus used in this study can be seen elsewhere in detail.[16,28] The Experimental flow chart can be seen in Figure 2.
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PAH removal applications in sodium sulphate Na2 SO4 with a pH of 6 and 100% dry matter content was used in the PAH removal studies as a synthetic solid matrices. GC grade Na2 SO4 (Merck 106649) with density: 2.7 g/cm3 , LOI: 99% was purchased from Merck Millipore-Turkey. Before first usage, Na2 SO4 was kept at 450◦ C for 24 h for the removal of organics. Twenty grams of Na2 SO4 was laid on a glass petri dish and 1 mL of 1000 ng/μL PAH Mix 63 standard containing Nap, Acy, Ace, Flu, Phe, Ant, Fl, Pyr, BaA, Chr, BbF, BkF, BaP, InP, DahA and BghiP was added to the sample. PAH-dosed samples were subjected to different temperatures (25◦ C, 40◦ C and 54◦ C), UV light conditions (UV light Turned on-Turned off), UV-TiO2 and UV-DEA applications. In the UV-TiO2 applications, TiO2 (Degussa P25) was dosed to the sample at the rate of 5% and 20% of dry weight of NaSO4 , respectively. The sample was then mixed for more than 5 min in order to have homogeneous distribution. Nano-crystalline powder form of TiO2 (Aeroxide TiO2 P 25, Evonik Industries, Germany) with a pH: 4, specific surface area (BET): 50 m2 /g, density: 0.1 g/cm3 and loss on ignition (LOI): 2% was used. The average nano particle size diameter of TiO2 was 21 nm. The powder contained anatase (70%) and rutile-amosphous (30%) phases. Another additive used in this study was diethyl amine (Merck 8.03010.2500), with the molecular formula CH3 CH2 NHCH2 CH3 (Merck, M803010). In the UV-DEA applications, DEA was added at the rate of 5% and 20% of the dry Na2 SO4 . In all cases, TiO2 and DEA addition was conducted as follows: The reagent (TiO2 /DEA) was added to the Na2 SO4 sample. The mixture was homogenized by stirring them with glass rods for 10 min and then 20 g of this mixture was weighted and placed in a petri dish. At the end of the 24 h applications, Na2 SO4 and PUF samples were subjected to PAH analysis and their PAH concentrations were measured by employing
Table 1.
a gas chromatography-mass spectrometry (GC-MS). The variables and experimental conditions are provided in Table 1. PAH removal efficiencies were calculated as below (Equation (1)): Initial PAHNa2 SO4 (ng/g DM) −Residual PAHNa2 SO4 (ng/g DM) . %PAH Removal = Initial PAHNa2 SO4 (ng/g DM) (1) The following equation (Equation (2)) was used to calculate the evaporated PAH ratios at the end of the PAH removal applications: (Evaporated conc.(ng/m3 )∗ Vacuumed air volume(m3 ))∗ 100 %PAH Evaporation = . InputNa2 SO4 (ng) (2) Furthermore, evaporated PAH amounts (ng) were calculated for each of the PAH removal experiment period (24 h) with the equation (Equation (3)): Evaporated PAH amounts (ng) per day ng = Evaporated conc. m3 3 m × Vacuumed air flow rate day
Extraction of Na2 SO4 samples After experiments, the samples were removed from the UV light apparatus. They were weighed and placed in a glass vial after the addition of 50 mL of acetone/hexane (ACE/HEX) (1:1, v/v) and 1 mL of surrogate standard including PAH Mix 31. Only GC-Grade chemicals were
Experimental conditions. Parameters
Group no. 1 2 3 4 5 6 7 8 9 10 11 12
UV − − + + + + + + + + + +
Temperature 25 40 25 54 25 54 25 54 25 54 25 54
(◦ C)a
(3)
TiO2 dose (%)
DEA dose (%)
– – – – 5 5 20 20 – – – –
– – – – – – – – 5 5 20 20
a Average temperature values over a 24-h period measurements in the apparatus.
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used in this study. The samples were shaken at 280 rpm in an orbital shaker for 12 h and afterwards they were extracted for 30 min at a temperature of 15◦ C in an Elma S 80H brand ultrasonic bath. The extracted samples were filtered and 50 mL of ACE/HEX (1/1) was added again to the residual sample in the bottle and it was extracted for 30 min and filtered. Fifty millilitre of ACE/HEX was added to the sample for a third time and it was waited in the freezer for 12 h. Then, the sample was extracted for 30 min. with an ultrasonic bath. The same methods used in the study carried out by Karaca and Tasdemir [29] and Karaca [28] were used to treat the samples from this stage onwards. Extraction of PUF samples The PUF samples were subjected to Soxhlet extraction for 24 h after the addition of 250 mL ACE/HEX (1/1:v/v) and 1 mL of surrogate standard. Dewatering, volume reduction and other procedures that were performed on the PUF extracts were identical to the procedures for Na2 SO4 samples. Quality assurance/quality control Field blanks at 10% of the number of samples were taken to determine any contamination during sample handling and analyses.[30] Twenty grams of Na2 SO4 were used in each field blank.[17,28] The blanks were extracted and cleaned in the same manner as the other samples. PAH surrogate standard (4 ng/mL) at the amount of 1 mL was added to each sample to determine recovery efficiencies of targeted PAH compounds.[31,32] Phenanthrene-d10 was used as a surrogate for Phe, Ant, Fl, Pyr and BaA. Chrysene-d12 was used, which served as a surrogate for Chr, BbF, BkF and BaP. Perylene-d12 served as a surrogate for InP, DahA and BghiP. PAH concentrations measured in Agilent 5975C inert XL mass selective with triple axis detector (MSD) of Agilent 7890 Model GC were calibrated in accordance with the surrogate standards. Limits of detections (LODs) were defined for each PAH compound. LOD was calculated by adding 3 standard deviations to the average of the field blanks.[6,28,33] Amounts in the samples less than the LOD were not used in the calculations. Prior to analysis of the samples, GC-MS was calibrated for seven concentration levels (0.01, 0.1, 0.5, 1.25, 2.5, 5 and 10 μg/mL) to determine the linearity of the responses. The r 2 value of the calibration curve was ≥0.99. System performance was verified by analysis of the midpoint calibration standard every 24 h during the analysis period. Instrument detection limits (IDLs) were determined using the area of a peak with a signal/noise ratio of 3 at the lowest standard calibration level. All reported values were much above IDL values. The quantifiable PAH amount for a 1-μL injection was 0.1pg.
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Results and discussion Heat and UV light applications The average temperature inside the apparatus while the UV and the thermostat were turned off was 25◦ C. The average temperature increased up to 40◦ C and 54◦ C when the apparatus was UV-closed, thermostat-open andUV-open, thermostat-open, respectively. Twelve PAH ( 12 PAH) concentration values (ng/g dry matter-DM) in Na2 SO4 samples after heat and UV light applications are shown in Figure 3. The removal efficiency for 12 PAH of Na2 SO4 samples which have been kept under 25◦ C (UV light-closed, thermostat-closed) was 53%. This value increased to 58% by increasing the temperature up to 40◦ C. On the other ◦ hand, in the UV light applications performed at 25 C and ◦ 54 C, 12 PAH removal efficiencies were 93% and 95%, respectively. Approximately 30◦ C temperature increase has provided 2% contribution into the PAH degradation process. Similarly, it was observed that the PAH removal efficiency at 40◦ C was higher than the PAH removal efficiency at25◦ C. It was a reliable result that the removal rate of 12 PAHs increased based on evaporation and acceleration of photodegradation reactions with increasing temperature. It is a known fact that the PAH removal efficiencies increase by increasing the temperature.[34,35] Especially, concentrations of 3- and 4-ring PAH compounds decreased by increasing the temperature during UV light applications. It can be expressed that these compounds which have more volatile characteristics compared with the 5 and 6-ring compounds, were evaporated and removed from the environment by means of high-vapour pressures [16,17,28,36] In the study conducted by Salihoglu et al.[16] on PAH removal from the municipal treatment sludge, the removal efficiency for 12 PAH during UV light applications did not exceed 21% at 54◦ C. The working conditions and UV light apparatus were the same as the ones used in this study and the only difference between these two studies was the applied matrices. It was observed that performing UV light applications in the treatment sludge was difficult. Sludge is a more complex matrix than synthetic matrices (Na2 SO4 ) and also contains various pollutants. In the present study, it was determined that the UV-C light penetrated sufficiently into the PAHs in Na2 SO4 . It was concluded that the characteristics and composition of the studied matrices were important in the UV light applications. Likewise, other studies performed within the scope of the literature emphasized that the matrices’ structure [37,38] and the organic composition [39] affect the photodegradation process of PAHs. When the PAH concentrations at 25◦ C were taken into consideration, it was observed that the UV light was effective on 4-, 5- and 6-ring PAH compounds rather than the 3-ring ones. While the UV light was closed, the removal efficiencies of 3-, 4-, 5- and 6-ring compounds were 97%, 12%, 31% and 23%, respectively. These values were calculated as
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(a)
(b)
Figure 3.
The PAH concentrations in Na2 SO4 samples after PAH removal applications. (a) Heat applications and (b) UV light applications.
70%, 92%, 91% and 89%, respectively while the UV light was open. Similarly, there was an increase in removal of 4-, 5- and 6-ring compounds during the UV light applications at 54◦ C. Several researchers found that the absorption characteristics of UV light by PAH compounds varied in liquid and solid matrices. The 3-ring light compounds absorbed the UV light easily in the liquid matrices while 5and 6-ring heavy compounds absorbed them easily in the solid matrices.[14–16,35,40] It was observed that the data of the present study agree with the literature and it was shown that the removal efficiency was increasing as the number of
PAH rings increased in the UV light applications performed on solid matrices. UV-TiO2 applications PAH concentrations in Na2 SO4 samples at the end of the UV-TiO2 applications are presented in Figure 4. The maximum PAH removal efficiency in the samples was around 95% during UV-TiO2 applications. When TiO2 absorbs light with wavelengths shorter than 385 nm, a paired of negative charged electrons and positive charged
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(a)
(b)
Figure 4.
The PAH concentrations of Na2 SO4 at the end of UV-TiO2 applications (a) 5% TiO2 and (b) 20% TiO2 .
dual cells (electrons (e− ) and cells (h+ )) emerge.[41] It is known that the hydroxyl radical (OH· ) which is formed as a result of a series of reactions [18,41–43] disintegrates the organic compounds in the different environmental media.[18,44] In the present study, it can be stated that TiO2 enhanced the photodegradation of PAHs by means of the OH· radical. Furthermore, O·− 2 might have formed on the surface of Na2 SO4 by means of the reactions between O2 absorbed from the atmosphere and the electrons in the environment.[18] These radicals with high tendency for the
reaction might have contributed to the degradation of PAH compounds. performed at 25◦ C, the In the UV-TiO2 applications removal efficiencies for 12 PAH compounds in the samples containing 5% and 20% TiO2 were 91% and 90%, respectively while these values were 95% and 89% at 40◦ C, respectively. Increased PAH removal ratios were obtained in the samples containing 5% TiO2 with increasing temperature. It was an expected result that higher catalyst efficiency was observed at high temperatures.[45] Likewise,
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within the scope of the UV-TiO2 studies performed by Zhang et al.[34] and Nadal et al.,[35] it was emphasized that the PAH removal efficiencies increased by increasing the temperature. The photodegradation process of PAHs was negatively influenced by increasing the TiO2 dose from 5% to 20%. The increase in TiO2 amount might have caused scattering of the UV light and decreasing the absorption of light in the reaction environment.[14] Moreover, it was possible for surface poisoning to occur in the catalysts.[46] It should not be ignored that the reaction products formed on the TiO2 surface compete with the reactant and cause surface poisoning by diffusing the capillary pores of the catalyst.[46] In the present study, it was thought that PAH removal efficiencies decreased with increasing of TiO2 dose because of scattering and poisoning effects of TiO2. [13,14,16] In the UV-TiO2 applications, removal ratios of 4-, 5and 6-ring PAH compounds were higher than the ratios of 3-ring PAHs. In other words, TiO2 was more effective on the compounds which can absorb the UV light more easily. Similarly, Salihoglu et al.[16] showed that removal rates of the heavy PAH compounds especially increased by the usage of TiO2 as a photocatalyst. The PAH removal efficiencies of the samples containing 5% TiO2 increased by increasing the temperature for all ring groups. At 25◦ C, the removal efficiencies of the 3-, 4-, 5- and 6-ring PAH compounds were calculated as 72%, 97%, 98% and 98%, respectively, these values rose to 82%, 99%, 99% and 99%, respectively at 54◦ C. However, this increase was partially observed on the samples (4- and 5-ring compounds) containing 20% TiO2 . It can be stated that high doses of TiO2 might have caused such differences due to its negative effects on the PAH removal process. UV-DEA applications Na2 SO4 samples were mixed with DEA at an amount of 5% and 20% of the dry weight and the samples were exposed to UV-C light for 24 h in the apparatus at 25◦ C and 54◦ C. The alteration of the PAH concentrations of these samples are given in Figure 5. In the UV-DEA applications performed at 54◦ C, 12 PAH removal efficiency increased up to 99% and it was determined that DEA was an effective photo-sensitizer at that temperature. DEA initiates the photodegradation reactions by electron transfer and provides contribution into the PAH removal process.[25] Bunce et al.[47] emphasized that non-bonding electrons in DEA were taken by PCBs and formed the (−) charged PCB radical and (+) charged DEA radical. It was thought that the effect of DEA on photodegradation of PAHs was similar to the effect of PCBs. 12 PAH removal efficiencies of the samples containing 5% and 20% DEA at 25◦ C were 91% and 95%, respectively. At 54◦ C, 93% of PAHs were removed with 5% DEA addition while 99% of PAHs were removed with 20% DEA addition. In other words, PAHs were removed more
effectively from the Na2 SO4 samples when the temperature and DEA dose increased. In the UV-DEA applications, it was supposed that the high PAH removal efficiencies were obtained by means of the evaporation and accelerated photodegradation reactions at 54◦ C. Similarly, Karaca and Tasdemir,[17] showed that the 12 PAH removal efficiency increased from 46% to 51% by increasing the temperature from 38◦ C to 53◦ C during the UV-DEA applications. It was observed that 20% DEA dose was more effective than 5% DEA dose for the removal of PAHs from the Na2 SO4 samples. The SVOCs removal applications in the literature were proven that the optimum DEA dose varied according to the matrices type.[17,25] In the UV light applications performed on municipal treatment sludge, it was emphasized that the PAH removal efficiencies of the samples containing 0.5% DEA were higher than the efficiencies of the samples containing 5% DEA.[17] With respect to the automotive sludge, PAH removal efficiencies increased by increasing the DEA dose from 5% to 20%.[29,48] Evaporated PAH amounts The air incoming into the apparatus was cleaned by employing a PUF cartridge in front of the apparatus. Therefore, the vacuumed air was free from the organics. Then, the air inside the apparatus was vacuumed and passed through the PUF columns in order to determine the amount of PAHs evaporated from Na2 SO4 samples during the PAH removal applications. Evaporated PAH concentrations at the end of the 24 h are given in Figure 6. None of the PAH compounds were encountered in the evaporated samples after the UVDEA applications. High removal efficiencies were obtained in these experiments. It was believed that PAHs were converted to the PAH compounds other than the investigated compounds. Therefore, PAHs were not detected in the PUF samples of Na2 SO4 containing DEA. It was determined that 99% of the evaporated PAHs were consisting of 3- and 4-ring compounds. As the molecular weight of PAHs increase, their volatile characteristics also increase.[49] While the compounds with higher molecular weight (5- and 6-ring compounds) have tendency for bonding solid matrices, the compounds with lighter molecular weight (3- and 4-ring compounds) have tendency to evaporate.[21,50] Consequently, it was expected that the 3- and 4-ring PAH compounds migrate more into air than the 5- and 6-ring compounds. The PAH amount that evaporated from Na2 SO4 increased with increasing temperature in the UV light and UV-TiO2 applications (Figure 6). In the UV light applications, while 305 ng PAH was evaporating at 25◦ C, this value increased to 2785 ng at 54◦ C. It is known that evaporation of the SVOCs from sludge increases with increasing of temperature.[51]. The PAH amount evaporated from the samples containing 5% TiO2 was measured as 675 ng at 25◦ C and 1254 ng at 54◦ C. These values
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(a)
(b)
Figure 5.
The PAH concentrations of Na2 SO4 at the end of UV-DEA applications (a) 5% DEA and (b) 20% DEA.
for the samples containing 20% TiO2 were 680 ng and 1620 ng, respectively. It was observed that the temperature increase had more impact on the evaporation of PAHs than the TiO2 dose. At 54 ◦ C, PAH migration ratios decreased with using TiO2 during the UV applications. In the UV-TiO2 applications, it was thought that the intermediate compounds were formed as a result of the photodegradation reactions of PAHs.[13,52] Otherwise, it would be expected that the heavy PAH compounds were
converted into the light PAH compounds after the photodegradation and evaporation under the heat and then they migrated into the air. As a result of the UV-TiO2 study performed by Wen et al.,[53] it was determined that Phe and Pyr were transformed to phenanthrenedialdehyde and 1,6-pyrenedione compounds. Moreover, Woo et al.[54] found that the 3-, 4- and 5-ring PAH compounds were turned into PAH-dione compounds as a result of UV-TiO2 applications.
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(a)
Figure 6.
(b)
Evaporated PAH concentrations (a) UV light applications and (b) UV-TiO2 applications.
Conclusions It is possible to encounter PAH removal applications performed with advanced oxidation methods in solid matrices such as the soil, sediment and treatment sludge. In the experimental studies in waste matrices, some pollutants in the wastes attempt photodegradation reactions with PAHs [55] and/or compete with PAHs in order to absorb UV light. Therefore, the studies about synthetic liquid matrices and synthetic solid matrices are important for correctly evaluating the PAH removal applications. While there are several researches on the removal of PAHs from synthetic liquid matrices,[40,56,57] it is difficult to find similar studies performed on synthetic solid matrices. The main results obtained from the experimental studies are summarized below: • While maximum 58% decrease on 12 PAH amount was obtained in the heat applications (UV light is closed), this value increased up to 95% by means of UV light applications. In the experimental studies performed with and without using UV light, it was proven that 12 PAH removal efficiency increased by increasing the temperature. Several researches also showed that PAH degradation in different materials (i.e. air, soil and grass) increased with the temperature increase.[2,34] • UV lights were more effective on removal of 4-, 5- and 6-ring PAH compounds rather than the 3ring (Phe and Ant) compounds in the PAH removal applications which was performed at 25◦ C and 54◦ C. As it was indicated in the literature, UV lights were absorbed more easily by the heavy PAH compounds.[14–16] • 12 PAH removal efficiency was 95% in the sample including 5% TiO2 by dry weight. This value decreased by increasing the TiO2 ratio. It was observed due to the catalyst poisoning effect and/or
scattering of UV lights (scattering effect). These results were in line with the literature.[13,14] • During the UV-TiO2 and UV-DEA applications, the PAH removal efficiencies at 54◦ C were higher than the efficiencies obtained at 25◦ C (except 20% TiO2 ). It was concluded that evaporation and acceleration of degradation reactions might have caused to obtain increased PAH removal efficiencies at a high temperature. • It was observed that DEA was a more effective additive than TiO2 and 99% of PAHs was removed with 20% DEA addition. • 99% of PAHs that migrated to the air from Na2 SO4 comprised 3- and 4-ring compounds. On the other hand, PAH amount that migrated into the air by evaporation from Na2 SO4 increased with increasing temperature during the UV light and UV-TiO2 applications Funding This work was supported by The Commission of Scientific Research Projects of 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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