Concentration-dependent and simultaneous sorption and desorption of pyrene and fluorene on major soil minerals in sub-Saharan Africa

Abstract Polycyclic aromatic hydrocarbons (PAHs) sorption-desorption in soils, as influenced by soils' physicochemical conditions, control the extent of PAHs translocation, persistence and bioavailability. Though mono-PAHs sorption on soils reports abound, co-occurrence of PAHs in the environment is common and affects individual PAHs sorption characteristics competitively or cooperatively. Besides, most reports were not on sub-Saharan soils. Hence, the batch experiments to study simultaneous sorption–desorption of pyrene and fluorene on five soils, with varying minerals contents, from different sub-Saharan agro-ecological zones. Sorption was concentration dependent and increased as concentration. There was higher preference for pyrene, though lower sorption than in mono-PAHs solutions. Irrespective of competition, soil organic matter (≥ 4.5%) caused high pyrene sorption similar to mono-PAHs sorption. At 25 °C, pyrene had lower hysteresis in simultaneous desorption compared to mono-PAHs, while higher hysteresis was observed in fluorene for all soils. Simultaneous sorption was higher at 40 than 25 °C and higher hysteresis exhibited as temperature increased in contrast to desorption at 25 °C. Isotherm model studies showed that simultaneous sorption of both PAHs were on heterogeneous surfaces and are composites of several linear and non-linear isotherms. Ultimately, soil constituents, PAHs concentration, competition for sorption sites, size and structural relationship play vital roles in simultaneous sorption on these soils. Risk of environmental contamination is higher for fluorene than pyrene during co-occurrence and likely so in the warmer and low organic matter soils of northern sub-Sahara.