The extraction of aged polycyclic aromatic hydrocarbon (PAH) residues froma clay soil using sonication and a Soxhlet procedure: a comparative study

A sonication method was compared with Soxhlet extraction for recovering pol ycyclic aromatic hydrocarbons (PAH) from a clay soil that had been contamin ated with tar materials for several decades. Using sonication over an 8 h e xtraction period, maximum extraction of the 16 US EPA priority PAH was obta ined with dichloromethane (DCM)-acetone (1 + 1). The same procedure using h exane-acetone (1 + 1) recovered 86% of that obtained using DCM-acetone (1 1). PAH recovery was dependent on time of extraction up to a period of 8 h . The sonication procedure showed that individual PAH are extracted at diff ering rates depending on the number of fused rings in the molecule. Soxhlet extraction [with DCM-acetone (1 + 1)] over an 8 h period recovered 95% of the PAH removed by the sonication procedure using DCM-acetone (1 + 1), indi cating that rigorous sonication can achieve PAH recoveries similar to those obtained by Soxhlet extraction. The lower recovery with the Soxhlet extrac tion was explained by the observed losses of the volatile PAH components af ter 1 - 4 h of extraction. The type of solvent used, the length of time of extraction and extraction method influenced the quantification of PAH in th e soil. Therefore, the study has implications for PAH analyses in soils and sediments, and particularly for contaminated site assessments where the da ta from commercial laboratories are being used. The study emphasizes the im portance of establishing land being consistent in the application of a vigo rous extraction, particularly for commercial laboratories that handle sampl es of soil in batches (at different times) from a single site investigation or remediation process. The strong binding of PAH to soil, forming aged re sidues, has significant implications for extraction efficiency. This paper illustrates the problem of the underestimation of PAH using the US EPA meth od 3550, specifically where a surrogate spike is routinely employed and the efficiency of the extraction procedure for aged residues is unknown. The i mplications of this study for environmental monitoring, particularly where numerous batches of samples from a single site assessment or remediation pr ogram are submitted to commercial laboratories, is that it would be advisab le for these laboratories to check their existing method's extraction effic iencies by conducting a time course sonication extraction on their particul ar soil to determine the optimum extraction time.