Environ. Sci. Technol. 2008, 42, 3773–3779
Secondary Effects of Catalytic Diesel Particulate Filters: Conversion of PAHs versus Formation of Nitro-PAHs N O R B E R T V . H E E B , * ,† P E T E R S C H M I D , ‡ MARTIN KOHLER,‡ ERIKA GUJER,‡ MARKUS ZENNEGG,‡ DANIELA WENGER,‡ ADRIAN WICHSER,‡ ANDREA ULRICH,‡ URS GFELLER,† PETER HONEGGER,§ KERSTIN ZEYER,§ LUKAS EMMENEGGER,§ JEAN-LUC PETERMANN,| JAN CZERWINSKI,| THOMAS MOSIMANN,⊥ MARKUS KASPER,⊥ AND ANDREAS MAYER# Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Solid State Chemistry and Catalysis, Laboratory for Analytical Chemistry, Laboratory for Air ¨ berlandstrasse 129, Pollution/Environmental Technology, U CH-8600 Dübendorf, Switzerland, UASB, University of Applied Sciences Biel, Laboratory for Exhaust Emission Control, Gwerdtstrasse 5, CH-2560 Nidau, Switzerland, Matter Engineering AG, Bremgarterstrasse 62, CH-5610 Wohlen, Switzerland, and TTM, Technik Thermischer Maschinen, Fohrhölzlistr. 14b, CH-5443 Niederrohrdorf, Switzerland
Received October 25, 2007. Accepted February 26, 2008.
Diesel particulate filters (DPFs) are a promising technology to detoxify diesel exhaust. However, the secondary combustion of diesel soot and associated compounds may also induce theformationofnewpollutants.Dieselsootisratedascarcinogenic to humans and also acts as a carrier for a variety of genotoxic compounds such as certain polycyclic aromatic hydrocarbons (PAHs) or nitrated PAHs (nitro-PAHs). Furthermore, diesel exhaust contains considerable amounts of nitric oxide (NO), which can be converted to more powerful nitrating species like nitrogen dioxide (NO2), nitric acid (HNO3), and others. This mix of compounds may support nitration reactions in DPFs. Herein we report effects of two cordierite-based, monolithic, wall-flow DPFs on emissions of genotoxic PAHs and nitro-PAHs and compare these findings with those of a reporter gene bioassay sensitive to aryl hydrocarbons (AHs). Soot combustion was either catalyzed with an iron- or a copper/iron-based fuel additive (fuel-borne catalysts). A heavy duty diesel engine, operated according to the 8-stage ISO 8178/4 C1 cycle, was used as test platform. Emissions of all investigated 4- to 6-ring PAHs were reduced by about 40–90%, including those rated as carcinogenic. Emissions of 1- and 2-nitronaphthalene increased by about 20–100%. Among the 3-ring nitro-PAHs, emissions of 3-nitrophenanthrene decreased by about 30%, whereas * Corresponding author phone: +41 44 823 4257; fax: +41 44 823 4041; e-mail:
[email protected]. † Laboratory for Solid State Chemistry and Catalysis. ‡ Laboratory for Analytical Chemistry. § Laboratory for Air Pollution/Environmental Technology. | University of Applied Sciences Biel. ⊥ Matter Engineering AG. # TTM, Technik Thermischer Maschinen. 10.1021/es7026949 CCC: $40.75
Published on Web 04/10/2008
2008 American Chemical Society
9-nitrophenanthrene and 9-nitroanthracene were found only after DPFs. In case of 4-ring nitro-PAHs, emissions of 3-nitrofluoranthene, 1-nitropyrene, and 4-nitropyrene decreased by about 40–60% with DPFs. Total AH-receptor (AHR) agonist concentrations of diesel exhaust were lowered by 80–90%, when using the iron- and copper-based DPFs. The tested PAHs accounted for 18 kW) for workplace application. In brief, these procedures include (i) testing of filtration efficiency and penetration of nanoparticles (10–400 nm), (ii) on-site evaluation of durability and failure rates, and (iii) an assessment of toxic secondary emissions such as polychlorinated dibenzodioxins/furans (PCDD/Fs) and genotoxic polycyclic aromatic hydrocarbons (PAHs) and nitro-PAHs. The overall effects of DPFs on aryl hydrocarbons (AHs) were tested with a reporter gene assay sensitive to AH-receptor (AHR) agonists such as PCDD/Fs, PAHs, and nitro-PAHs (4). PAHs and nitro-PAHs will be addressed in this contribution and the risks of a DPF-induced PCDD/F formation have been assessed before (5). Among other anthropogenic activities, evaporation and incomplete combustion of gasoline and diesel fuels are important PAH sources. Substantial amounts are found in fuels (6–8), vehicle exhaust (9), and road-tunnel air (7, 10, 11). Diesel exhaust in particular, is a source of different genotoxic compounds (12). Bioassay-directed chemical analyses lead to the identification of different carcinogenic and mutagenic PAHs, nitro-PAHs, and nitro-oxy-PAHs (13–15). Mainly 4- to 7-ring PAHs were identified to be most active. Besides these critical PAHs, other PAHs can act as precursors for genotoxic derivatives. For example, inactive pyrene can be converted to mutagenic nitro- or dinitropyrenes (16). Nitration of PAHs occurs during fuel combustion or catalytic exhaust gas treatment, as discussed herein, but VOL. 42, NO. 10, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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similar reactions also take place in the atmosphere (17). Airborne nitro-PAHs appear to be responsible for a substantial proportion of the total direct mutagenicity of respirable particles (17, 18). An impressive number of potential nitration products have to be expected, when considering the large variety of PAHs and alkylated PAHs found in diesel exhaust. Furthermore, several nitrating species support such transformation reactions (17, 18). Exposure of electron-rich PAHs to electrophilic oxides of nitrogen leads to nitro-PAH formation. Nitration reactions can occur in the gas-phase with electrophiles like NO2, NO3, and N2O5. This chemistry may require an additional activation step at ambient conditions, e.g., the attack of another radical like OH. But nitration reactions also proceed in solution or at particle surfaces in presence of nitrating species like nitrosyl- (NO+) or nitronium (NO2+) ions, or nitrous acid (HNO2), and nitric acid (HNO3). Thus, the lower troposphere is a complex nitration reactor, supporting gas-, liquid-, and solid-phase chemistry. DPFs have to be considered as chemical reactors as well. They are operated at significantly higher temperatures and reactant concentrations than those found in ambient air. Furthermore, DPFs trap compounds and elongate their exposure against reactive species. DPFs may also accumulate compounds acting as catalysts, e.g., metal ions from fuel additives, lubrication oils, and catalytic coatings, or nitrating species such as HNO2 and HNO3, and DPFs equally support gas-, liquid-, and solid-phase chemistry. With these similarities, we expect that DPFs must influence PAH- and nitro-PAH-profiles of diesel exhaust. We hypothesized that DPFs either (i) lower PAH and nitro-PAH emissions, assuming that both classes of compounds are adsorbed in DPFs, (ii) convert PAHs to nitro-PAHs, if nitration chemistry is supported in DPFs, or (iii) induce the de novo formation of both PAHs and nitro-PAHs via soot decomposition and subsequent nitration. Thus, depending on the relevance of these processes, DPFs may either reduce or increase the genotoxic potential of diesel exhaust.
Experimental Section Engine, Test Cycle, Fuel, Lubricant. All engine tests were carried out on the test stands of the University of Applied Sciences in Biel (UASB, Switzerland) using a heavy duty diesel engine with direct fuel injection (Liebherr, type 914 T, 6.11 L, 4 cylinders, 105 kW, Bulle, Switzerland). The engine was operated in the eight-stage ISO 8178/4 C1 test cycle, which consists of four load stages at maximum revolutions-perminute (RPM), three load stages at intermediate RPM (60% of max. RPM), an idling phase, and eight transients. Each stage is held for 10 or 15 min, resulting in a total cycle time of 100 min. The base fuel for all experiments was a commercial diesel (class D, SN 181190–1:2000) with a density of 824.3 kg/m3, a cetane number of 56.0, and a sulfur content of 16 mg/kg. The content of PAHs was 3%, as determined by HPLC. The fuel neither contained iron, copper, nor chlorine above detection limits of 0.1, 0.1, and 2 µg/g, respectively, as determined by wavelength dispersed X-ray fluorescence spectrometry (WD-XRF, PW 2400, Phillips, Netherlands) and by inductively coupled plasma optical emission spectrometry (ICP-OES, Vista Pro, Varian, Orlando, FL). A chlorine content of 120 µg/g was determined for the lubricant (Universal SAE, 15W40) by WD-XRF analysis, but iron- and copper levels were below 0.1 µg/g (ICP-OES). Further details are given in (5). Particulate Filters, Catalysts, Test Configurations. Two new, uncoated, cordierite-based, monolithic, wall-flow DPFs (Greentop, 100 cells per square inch, 22.8 L, Grävenwiesbach, Germany) were used in combination with an iron- and a copper/iron-based fuel additive (ITN, Krakow, Poland). Both additives were diluted with reference fuel to final iron- and 3774
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copper/iron-concentrations of 4.5 and 9.0 ( 0.5/7.5 ( 0.7 µg/g, respectively, as determined by ICP-OES. Four additional fuel blends were mixed with 1,6-dichlorohexane (Fluka, Buchs, Switzerland) as chlorine dopant (5). However, no differences were noticed with respect to PAH and nitro-PAH emissions with this dopant. Therefore, mean PAH and nitroPAH emission factors are discussed herein. Experimental conditions of all test configurations are given as Supporting Information (Table S1). All premixed fuel blends were delivered from barrels, minimizing cross contamination and allowing fast adaptation of engine and DPFs, both preconditioned for one hour at the first stage of the ISO 8178/4 cycle (max. torque, full load) after each fuel change. Exhaust Sampling, Chemical Analysis, AH-Receptor Bioassay. Analyses of the major combustion products carbon dioxide (CO2), carbon monoxide (CO), total hydrocarbons (THC), nitrogen monoxide (NO), and nitrogen oxides (NOx) have been described elsewhere (5) and results are given in Table S1. For the analysis of non- and semivolatile trace compounds such as PAHs, nitro-PAHs, and PCDD/Fs, 5–7 m3 of undiluted exhaust were collected through an all-glass sampling arrangement consisting of a sampling probe, a cooler, a condensate separator, a glass fiber filter, and a twostage adsorber unit (XAD-2). A water/isopropyl-alcohol cooling bath was used to keep the condensate below 4 °C during sampling, and exhaust temperatures remained below 30 °C all the time. Mass flow proportional aliquots of the exhaust were taken during two consecutive runs (200 min) covering both, steady-state and transient operation modes. Prior to sampling, the employed glass apparatus was intensively cleaned and heated to 450 °C. Aliquots of a mixture of perdeuterated-PAHs containing naphthalene, phenanthrene, pyrene, fluoranthene, chrysene, benz(a)anthracene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene, and D9-1nitropyrene (CIL, Andover, MA) were added to the samples as quantification standards. A mixture of unlabeled nitro-PAHs containing 1-nitronaphthalene, 2-nitronaphthalene, 3-nitrophenanthrene, 9-nitrophenanthrene, 9-nitroanthracene, 3-nitrofluoranthene, 1-nitropyrene, and 4-nitropyrene (Dr. Ehrenstorfer AG, Augsburg, Germany) were mixed with an aliquot of D9-1-nitropyrene and used as quantification standard. Separation of PAHs and nitro-PAHs was obtained by gas chromatography (Fisons Instruments HRGC Mega 2, Rodano, Italy) on a capillary column (PS086, 20 m × 0.28 mm, 0.15 µm) and detection and identification was achieved by high resolution mass spectrometry (Thermo Finnigan MAT 95, Bremen, Germany) in electron ionization mode (GC/EI-HRMS). Compounds inducing AHR-mediated gene expression were detected using the DR-CALUX assay, an in vitro reporter gene assay. This assay is based on stably transfected H4IIE rat hepatoma cells with an AHR-controlled luciferase reporter gene construct. Detailed descriptions of the procedures for cell cultivation, cell exposure with exhaust samples, and quantification of the luciferase activity are given elsewhere (4). Quality Assurance, Recovery Rates, Nitration Artifacts. To determine overall recovery rates and nitration artifacts, aliquots of 13C6-naphthalene, 13C6-phenanthrene, 13C3-pyrene, and 13C12-1,2,3,4,5,6-hexachlorodibenzodioxin (CIL) were spiked on quartz swab and placed in the condensate separator prior to sampling. Mean overall recoveries of 26 ( 9%, 34 ( 5%, 59 ( 5%, and 57 ( 11% were obtained, respectively, covering sampling, workup, cleanup, and analysis. As expected, recoveries for naphthalene, the most volatile PAH with a vapor pressure of 10 Pa at 25 °C, were low, but sufficient material could be obtained to study nitration artifacts. Recoveries for non- and semivolatile AHs with vapor pressures
