Sequential supercritical fluid extraction (SSFE) for estimating the availability of high molecular weight polycyclic aromatic hydrocarbons in historically polluted soils

Sequential supercritical fluid (CO2) extraction (SSFE) was applied to eight historically contaminated soils from diverse sources with the aim to elucidate the sorption-desorption behavior of high molecular weight polycyclic aromatic hydrocarbons (PAHs). The method involved five extraction phases applying successively harsher conditions by increasing fluid temperature and density mobilizing target compounds from different soil particle sites. Two groups of soils were identified based on readily desorbing (available) PAH fractions obtained under mildest extraction conditions (e.g., readily desorbing fractions of fluoranthene and pyrene significantly varied between the soils ranging from <10 to >90%). Moreover, extraction behavior strongly correlated with molecular weight revealing decreasing available PAH fractions with increasing weight. Physicochemical soil parameters such as particle size distribution and organic dry mass were found to have no distinct effect on the sorption-desorption behavior of PAHs in the different soils. However, PAH profiles significantly correlated with readily available pollutant fractions; soils with relatively less mobile PAHs had higher proportions of five- and six-ring PAHs and vice versa. Eventually, biodegradability corresponded well with PAH recoveries under the two mildest extraction phases. However, a quantitative relationship was only established for soils with biodegradable PAHs. Out of eight soils, five showed no biodegradation including the four soils with the lowest fraction of readily desorbing PAHs. Only one soil (which was found to be highly toxic to Vibrio fischeri) did not match the overall pattern showing no PAH biodegradability but large fractions of highly mobile PAHs, concluding that mass transfer limitations may only be one of many factors governing biodegradability of PAHs.     

Testing model feasibility and sufficiency through laboratory simulation: A case study

Complex computer simulations of ecological models often fail to have much impact on later stages of applied research and management. We believe that this reflects a methodological gap. We present a case study where we try to bridge this gap. A recently published general model for insect outbreak has not yet been tested adequately in the field because of the complexity, size, and time involved in real-world insect pest outbreaks. However, a modified version of the model can be used to predict complex dynamics for a laboratory managed population of vinegar flies (Drosophila melanogaster).A comparison of the dynamics of the laboratory system with the simulation predictions allowed us to conclude that the model is “feasible”, i.e. under certain circumstances the model can predict the dynamics of some real system. These results were obtained despite considerable environmental and biological variability, indicating the robustness of the model. The factors chosen for the model appear to be “sufficient” to describe the real system's essential dynamics. Testing for feasibility and sufficiency should, in our opinion, always precede the much more expensive stages of field testing.