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Abstract:
Polycyclic aromatic hydrocarbons (PAH) are known as omnipresent contaminants in the environment. Over the last 40 years, PAH contaminated soils from industrial sites are monitored according to a list of 16 United States Environmental Protection Agency (EPA) PAH. However, high molecular weight (alkylated) PAH along with polycyclic aromatic heterocycles (PAXH, X = N, S, O) can occur in the contaminated soil as well. Questions have been arisen, such as: “How complex are PAH contaminated soils?” or “How do contaminants other than 16 EPA PAH behave under different remediation conditions?” In order to answer these questions, a new analytical strategy has to be applied. This can be realized by applying the state-of-the-art analytical instrumentation, for instance Fourier transform mass spectrometry (FTMS), by a non-targeted approach.
As the first step, the extraction efficiency of different extraction methods, including Soxhlet extraction with various extraction solvents as well as supercritical fluid extraction (SFE) using CO2, were compared for the non-targeted analysis of PAXH in a model sample (sand with crude oil spiked). Dichloromethane turned out to be the most suitable solvent when using Soxhlet extraction for the non-targeted analysis of PAXH in contaminated soils.
Subsequently, a highly PAXH contaminated soil (with 64,500 ± 9,500 mg kg-1 solvent extractable organics (SEO)) was characterized using FTMS with three atmospheric pressure ionization (API) methods in both polarities. In total, 21.958 distinct elemental compositions could be assigned for this single sample. Results revealed that highly aromatized PAH with double bond equivalent (DBE) over 70 and PAXH, especially azaarenes, co-occurred in the contaminated soil. The pyrogenic origin of this contaminated soil could be proven by the unique DBE vs. carbon count distributions of PAXH.
After the characterization of the contaminated soil, it was subjected to different remediation techniques, including physical remediation via density separation and solvent extraction as well as thermal remediation via pyrolysis. By applying integrated remediation techniques the SEO in the contaminate soil was reduced to 860 ± 280 mg kg-1, which represent a remediation efficiency of 98.7%. Additionally, with the help of API-FTMS and gas chromatography (GC)-FTMS a detailed analysis of contaminated soil before and after remediation on a molecular level was achieved. This enabled a deeper understanding of selected remediation processes and, more importantly, provided valuable information about the how different PAXH behave under different remediation techniques.
