Lichens as biomonitors for atmospheric polycyclic aromatic hydrocarbons

Abstract

For the first time in Africa, studies investigating the polycyclic aromatic hydrocarbons (PAHs) levels in lichens (species Parmotrema austrosinense (Zalhbr.) Hale) was performed successfully using various sample preparation techniques. Sampling campaigns were conducted at 5 different sites across South Africa. Different sample preparation techniques were explored, and it was found that a quick, easy, cheap, effective, rugged and safe (QuEChERS) technique using n-­‐ hexane:acetone (1:1, v/v), never before applied to lichens, provided the best recoveries of internal standards (deuterated phenanthrene and pyrene), the highest total peak area for all PAHs of interest and relative standard deviations (%RSDs) comparable with the other preparation techniques explored (ultrasound assisted extraction, Soxhlet and microwave assisted extraction). Statistical analysis confirmed that QuEChERS performed significantly better than the other sample preparation techniques. The optimized sample preparation technique met the study’s requirements, since it was a fast method, with good recoveries, using less solvents and minimal energy. It was determined that the internal standard needs a prolonged equilibration time (12 hr) on the lichen matrix for optimal recoveries, a new finding. Matrix matched standards were prepared and strong matrix effects were found; both strong enhancement (for the lighter PAHs) and strong suppression (for the heavier PAHs) were observed. The use of matrix matched standards were found to be imperative for the accurate determination of PAH concentrations in the samples, described for the first time in this study, since the matrix effects were found to be PAH and lichen specific and are crucial for accurate quantitation. Varying chlorophyll contents of the extracts was observed using fluorescence spectroscopy, despite being prepared in an identical manner and being of the same lichen species. Therefore, the chlorophyll content needs to be given along with determined PAH concentrations in lichens, based on a dried weight basis to allow for comparison between studies. All the lichen samples contained naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene and benzo[a]pyrene. It was seen that a full range of PAHs in terms of mass was extracted and identified, thus Parmotrema austrosinense (Zalhbr.) Hale was an appropriate choice of lichen species. The order of most impacted sites to least impacted was found to be Pretoria Industrial Area (total PAH concentration 1001 ng.g-­‐1) > Daspoort 2 (737 ng.g-­‐1) > Daspoort 3 (693 ng.g-­‐1) > Bulk urban site (633 ng.g-­‐1) > Cathedral Peak (547 ng.g-­‐1) > Cape Point Nature Reserve 1 (525 ng.g-­‐1) > Cape Point Nature Reserve 2 (514 ng.g-­‐1) > Cape Point Nature Reserve 3 (425 ng.g-­‐1). The PAH profiles and PAH ring-­‐size profile were used for source diagnostics and was seen to vary according to the types of pollution sources.