Mechanochemically Enhanced Degradation of Pyrene and Phenanthrene Loaded on Magnetite

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.

Comment on “Mechanochemically Enhanced Degradation of Pyrene and Phenanthrene Loaded on Magnetite”

“…It is also possible that the "magnetite" grinding may generate new Fe III -sites, which would be responsible for PAH oxidation as claimed by the authors. 1 If the latter was the sole mechanism, similar results should be obtained with Fe III -oxides like maghemite or hematite. Without this type of data, the specific contribution of magnetite as a Fe II −Fe III mixed valence oxide in the observed oxidation of PAH cannot be evidenced.…”

“…It is worthwhile to note that the mechanochemical degradation of pyrene was observed here 1 only with divalentcation bearing minerals (i.e., Cu-mont and magnetite), and not with the Mn VI −Mn III -oxide (i.e., birnessite) that is usually shown to induce this type of degradation, as reported in numerous works cited by the authors. 1 Third, the authors found that the water addition to the PAHspiked soil/magnetite mixture followed by drying before milling resulted in 10−16% of PAH degradation. This has been attributed by the authors to desorption of PAH from soil components, following by a diffusion of molecules to reach the added magnetite surface.…”

“…20,21 In addition, the highest extent of degradation was observed for the less soluble PAH (the most hydrophobic one) according to the data of the authors (16% for pyrene vs 10% for phenanthrene). 1 The lack of data on the composition of the investigated soil 1 makes difficult a relevant interpretation of the observed behavior. It is, however, reasonable to suppose that adding distilled water (and especially the sudden wetting), following by air drying for 72h and milling may disturb the soil structure and/or cause a rearrangement of soil aggregates (as reported in ref 22).…”

“…I read this paper with great interest and would like to raise a few scientific and practical questions on the interpretation of the experimental data and the proposed pathway of PAH degradation in “magnetite” and “soil” systems, and I hope that helps toward a better understanding of the main mechanisms.…”

“…From a chemical point of view, correlation for variations in particle size with variations in surface reactivity is valid only if the investigated particle size fractions are derived from the same material. In this study, however, three different samples were tested (one natural (I presume?) and two commercials) of the so-called “magnetite”.…”