Article pubs.acs.org/est
Mechanochemically Enhanced Degradation of Pyrene and Phenanthrene Loaded on Magnetite Hadas Joseph-Ezra,† Ahmed Nasser,*,† Julius Ben-Ari,‡ and Uri Mingelgrin† †
Institute of Soils, Water and Environmental Sciences, Agricultural Research Organization, Volcani Center, P.O. Box 6, Bet Dagan 50-250, Israel ‡ The Interdepartmental Equipment Unit, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76100, Israel S Supporting Information *
ABSTRACT: The enhancement of the degradation of polycyclic aromatic hydrocarbons (PAHs), exemplified by pyrene and phenanthrene, using mild grinding in the presence of common minerals was investigated. Magnetite, birnessite, and Na- and Cu-montmorillonite samples were loaded with pyrene or phenanthrene and ground manually or in a ball mill for short periods of time. The ground samples were analyzed for PAHs and for their metabolites, using high-performance liquid chromatography and liquid chromatography−mass spectrometry. No degradation of pyrene occurred when it was in contact with Na-montmorillonite or birnessite. Sorption of pyrene on Cu-montmorillonite enhanced its degradation, but grinding of the loaded clay actually inhibited pyrene’s degradation. Phenanthrene hardly degraded on Cu-montmorillonite. Grinding magnetite loaded with either PAH resulted in a significant degradation of both (∼50% after grinding for 5 min), while in the nonground samples, negligible degradation was detected. The extent of degradation increased with the duration of grinding. The degradation of either PAH loaded on magnetite yielded oxidized products. In soil samples contaminated with PAHs and mixed with magnetite, a similar grinding-induced degradation pattern was observed, but with a lower rate. A liquid phase was required to initiate degradation in the soil. The liquid phase apparently served as the medium through which the pollutants reached the surface of the degradation-enhancing mineral.
1. INTRODUCTION
A mechanochemical process is defined here as a chemical reaction induced by mechanical means. Such means include, for example, compression and milling and grinding (where milling refers to the application of a mechanical force using a mill, such as a ball mill, and grinding refers to the manual application of such a force, e.g., using a mortar and pestle). Application of a mechanical force may break down solid particles and thus increase their surface area and create freshly exposed surfaces that contain chemically active sites. Applying a mechanical force may also allow the mass transfer required to bring into contact with each other the species participating in a solid phase chemical reaction (e.g., refs 11−13). Minerals such as montmorillonite, manganese oxides, and iron oxides are common in soils and are known to induce a variety of abiotic transformations of organic pollutants, including hydrolysis, oxidation−reduction, or rearrangement (e.g., refs 14−18). Grinding of minerals when they are loaded with organic pollutants was shown to enhance many abiotic
Pyrene and phenanthrene (PHE) are polycyclic aromatic hydrocarbons (PAHs). Contamination of soils with these mutagenic and carcinogenic compounds1 has been a major environmental concern for many years (e.g., ref 2), and as a result, many studies focused on developing remediation procedures, both biotic and abiotic, for soils contaminated with PAHs (e.g., refs 3−6). Most methods used for breaking down organic pollutants in the soil are based on biological degradation (e.g., ref 7). However, biological procedures have a number of shortcomings; for example, they are often slow, or they cease being effective when unacceptably high concentrations of residues are still left in the soil (e.g., ref 8). The biotic degradation of PAHs poses a particular challenge because these compounds are resistant to natural, biological breakdown processes (e.g., ref 9). Because of their hydrophobic nature, namely their low aqueous solubility, PAHs are adsorbed readily to the soil’s organic fraction, making them less accessible to an attack by microorganisms.8,10 Consequently, it is necessary to develop complementary, e.g., abiotic, soil remediation methods, and the mechanochemical procedure described here is an example of such a method. © 2014 American Chemical Society
Received: Revised: Accepted: Published: 5876
October 29, 2013 February 25, 2014 April 14, 2014 April 14, 2014 dx.doi.org/10.1021/es404679y | Environ. Sci. Technol. 2014, 48, 5876−5882
Environmental Science & Technology
Article
All analytical grade reagents were purchased from Merck (Darmstadt, Germany). All high-performance liquid chromatography (HPLC) grade solvents were purchased from Bio Lab Ltd. (Jerusalem, Israel). 2.2. Methods. All runs were performed in triplicate and the standard deviations calculated. 2.2.1. Preparation of Cu-montmorillonite. Cu-mont was prepared by washing the original Na-mont three times with a 1 N solution of Cu2+ as a chloride and then four times with distilled water. In each wash, the mineral:solution ratio was 1:10 (w/w). 2.2.2. Preparation of Loaded Mineral and Soil Samples. The loading procedure involved spiking 2 g samples of the mineral or 10 g samples of the soil with the PAH from the appropriate ACN stock solution to reach the desired final load (1000 μg/g for pyrene, 100 and 1000 μg/g for PHE, and 500 μg/g each for a mixture of pyrene and PHE). The spiked samples were mixed thoroughly and then kept in a hood until complete evaporation of the ACN (∼1 h). The point at which the mixture was judged to be free of ACN by inspection was defined as T0. For ground samples (section 2.2.3), T0 was defined as the instant at which the grinding was completed. 2.2.3. Manual Grinding and Ball Milling. Manual grinding was conducted for 5 min and performed using a mortar (5 cm radius) and pestle (1.5 cm radius). A ball mill (Retsch model MM2), equipped with a set of five agate grinding balls (7 mm in diameter), was used to grind samples at 2000 rpm for various periods of time. 2.2.4. Extraction and Analysis. Subsamples of the loaded minerals (0.01 g) or of the loaded soil (0.5 g) were extracted with 3 mL of ACN by sonication, using an ultrasonic bath (MRC, Ultrasonic cleaner) for 30 min. The samples were then centrifuged for 15 min at 1200g and filtered through a 0.4 μm syringe PTFE filter. The extracts were then analyzed by HPLC and liquid chromatography−mass spectroscopy (LC−MS) for the parent PAH and its transformation products. HPLC analysis was conducted on a Shimadzu LC10AT apparatus, equipped with a photodiode array (PDA) detector. A LiChrospher PAHs, 5 μm, 250-4 column (Merck, Darmstadt, Germany) was used. The mobile phase was 20% doubly distilled water and 80% ACN at a flow rate of 1 mL min−1, and the detector was set at 240 nm for pyrene and 251 nm for PHE. Mass spectrometric analysis was performed using a highresolution LC−MS system that consisted of the LTQ Orbitrap Discovery hybrid FT mass spectrometer coupled with an Accela High Speed LC system equipped with a PDA detector (Thermo Fisher Scientific Inc.). The mass spectrometer was equipped with an electrospray ionization (ESI) or atmosphericpressure chemical ionization (APCI) ion source and operated in a positive or negative ionization mode. Chromatographic separation of the compounds was achieved on a Gemini HexylPhenyl column (2 mm × 150 mm, particle size of 3 μm, Phenomenex), employing the following linear gradient: running 50% eluent A (ULCMS grade water with 0.05% acetic acid) and 50% eluent B (methanol) from 0 to 2 min, then gradually reducing the fraction of eluent A to 0% by 10 min, and running 100% eluent B until 20 min. The flow rate through the column and the temperature were 250 μL/min and 40 °C, respectively. The eluted compounds were identified using simultaneous detection by the MS and PDA detectors. The LC−MS system was controlled, and data were analyzed using Xcalibur (Thermo Fisher Scientific Inc.).
transformations, for example, the mechanochemical degradation of atrazine19 and of 2,4-D13 on the surface of manganese oxide. Both compounds degraded in a matter of a few hours following mild grinding at ambient temperature. The mechanochemical degradation of catechol and pentachlorophenol in the presence of birnessite20,21 and of γ-hexachlorocyclohexane on CaO22 was also reported recently. Nasser et al.23 studied the effect of grinding on the degradation of imazaquin in contact with soil minerals, including Cu-montmorillonite (Cu-mont) and manganese and iron oxides. The degradation rate of imazaquin was examined as a function of the duration of grinding, sorbate load, temperature, and moisture content. Loading imazaquin on Cu-mont followed by grinding was the most effective procedure for degrading imazaquin (more than 90% of the imazaquin degraded after it had been ground for 5 min). Nasser and Mingelgrin24 published a review of mechanochemically induced surface reactions and their environmental applications. Napola et al.25 investigated the use of milling by a ball mill for remediating a soil contaminated with phenanthrene. These authors also tested the effect on the degradation of the PAH of adding birnessite to the soil before grinding. Their results showed that grinding was more efficient in removing PHE when the pollutant was added as a solid phase (∼50% removal) than when it was spiked dissolved in acetone on the same soil (∼20% removal). The addition of birnessite to the soil did not change significantly the rate of disappearance of PHE. The overall objective of this work is to explore the potential use of mechanochemical procedures for enhancing the degradation of pyrene and PHE when they are in contact with soil minerals (magnetite, birnessite, Na-mont, and Cumont) or a whole soil. Both manual grinding and ball milling were tested, and magnetite was studied in considerable detail, because it displayed an exceptionally high capacity for breaking down pyrene and phenanthrene when mild grinding was applied for a short period of time.
2. EXPERIMENTAL SECTION 2.1. Materials. Pyrene (C16H10, 99% pure) and PHE (C14H10, 98% pure) were purchased from Sigma-Aldrich (Buchs, Switzerland). A pyrene stock solution (10000 μg/mL), a PHE stock solution (10000 μg/mL), and a pyrene/PHE mixture stock solution (5000 μg/mL each) were prepared in acetonitrile (ACN), stored in glass vials sealed with caps equipped with Teflon septa, and kept refrigerated at 4 °C. These stock solutions were used to spike mineral and soil samples with various concentrations of PAHs. Samples of magnetite (Fe3O4) from three different sources were investigated. Magnetite designated as “type a” with a particle diameter of
