Interactions of Polycyclic Aromatic Hydrocarbons with Mineral Surfaces

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Abstract

The toxicity and ubiquitousness of PAHs within different terrestrial environments has been an increasing cause for concern amongst environmental scientists in the last decades, in particular regarding their transport within the soil. In an attempt to understand the role of pure inorganic phases in PAH-mobility; experiments exposing mineral soil components with low organic matter content to a PAH-representative were carried out. The systems consisted of four different mineral phases (quartz sand, hematite, iron coated quartz sand and montmorillonite) which were individually exposed to naphthalene in electrolyte solutions prepared at increasing ionic strengths (NaNO3: 0.001 M; 0.01 M; 0.1 M) and pH (4.0 and 5.5). All experiments were conducted over at 24 reaction intervals and at ambient temperature conditions.Mineral geosorbents are traditionally known to be poor PAH-scavengers; in particular when compared to organic, high surface area materials such as activated carbons. On this basis, a preliminary validation experiment (Proof of Concept Experiment) was conducted to test the sensitivity of the selected extraction method (SPME) under complete uptake (activated carbon) and very low uptake (quartz sand) conditions. By extracting and analysing the supernatant after 24 hr of exposure of both sorbents to naphthalene under identical conditions it was concluded that SPME was a feasible extraction technique, yielding good reproducibility (n=3, inter-day RSD%= 11.18% ) even at very low PAH concentrations (0.2 µg / L). The final concentration of naphthalene in the sample supernatant after 24 hours was determined by GC-FID. All samples were extracted using the Solid Phase Microextraction method developed during the Proof of Concept which allowed the rapid extraction of naphthalene in the headspace HS-SPME (extraction time = 3 minutes) using temperature control and ultrasonication as means of agitation. Each sample set included triplicates of blanks and samples as well as calibration standards (in duplicate where possible)Out of the four minerals, only quartz sand and hematite showed a slight tendency towards naphthalene removal from solution; a finding which correlated well with increasing ionic strength. The other two minerals did not show any such trend and the results were deemed inconclusive. In regards to the results for quartz and hematite; the detected uptake was found to be below the sensitivity of the current SPME extraction method according to the error analysis carried out by comparing the sample and blank means whilst accounting for error equal to 1. The overlapping of both means in the majority of the samples indicated that both averages were too close to be accurately resolved (due to very low naphthalene uptake). Modifications to the SPME method could improve the reproducibility and decrease the spread of the data; however, this measure would only guarantee higher statistical confidence (95 %) and not higher naphthalene uptake by these minerals. These observations lead to the conclusion that naphthalene was being salted out of solution rather than being removed by sorption; and under these experimental conditions it would not have been possible to detect any real PAH-mineral interaction. In view of this outcome, a different approach was attempted in order to detect surface reactions between the minerals and naphthalene. A series of preliminary (qualitative) surface analysis (AFM, XPS and ATR-FTIR) on pre-loaded mineral specimens were carried out in air at ambient temperature conditions. No naphthalene was positively identified on the surfaces of the studied sorbents. Factors such as molecular size, sorbents characteristics (i.e. roughness, surface charge) and loading conditions impeded the detection of the target molecules. Innovative sample preparation protocols as well as controlled analytical conditions would need to be implemented and evaluated before this kind of analytical tool can be used. The main outcome of this research work was the successful adaptation of SPME to the rapid extraction of naphthalene in electrolyte solutions at optimal and sub-optimal concentration levels; as the proof of concept preliminary experiment showed.

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PAH; SPME; minerals

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