Sorption and partitioning of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) onto sediments of Diep and Plankenburg river systems Western Cape, South Africa

Abstract

Partitioning behaviour of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) in sediments obtained from river systems were investigated in this study. The partitioning coefficient (log K OC ) and distribution coefficient (log K D ) of PFOA and PFOS between sediments and the aqueous solution were estimated from the sorption data. Levels of PFOA and PFOS in the sediment samples ranged between 30.01–191.96 ng/g dw and 0.50–248.14 ng/g dw, respectively. The kinetic study showed that the time required for equilibrium was <400 min for the compounds. Sediment samples SS-C provided the highest sorption capacities (0.11 and 0.13 mg/g) for both PFOA and PFOS. Sorption studies revealed that the removal kinetics fitted well into the Elovich rate equation (R 2 = 0 . 98 and 0.92) followed by the pseudo-second-order kinetic model (R 2 = 0 . 91 and 0.89) for PFOA and PFOS, respectively. Mean values for the partitioning coefficient (log K OC ) in the river systems were 1.04 ± 0.26 and 1.85 ± 0.65 (cm 3 /g) for PFOA and PFOS respectively. Partitioning coefficient (log K OC ) significantly (p < 0.05) increased over time. This study represents the first report on partitioning and distribution of PFOA and PFOS in sub-Sahara Africa, which provide data and a scientific base to understand the fate and sorption of PFOA and PFOS in an aquatic system. The findings showed that lower pH values and increasing ionic strength favours sorption and partitioning of perfluoroalkyl substances (PFAS) in aqueous solution onto sediments; hence, plays an important role in their availability in water. Presence of PFAS in the aquatic system sediments is a potential risk to organisms. • Maximum concentration of PFOA and PFOS in the sediments from the river systems were 191.9 and 248.1 ng/g dw, respectively. • Mean values for the log K OC in the river systems were 1.04 ± 0.2 and 1.85 ± 0.6 (cm 3 /g) for PFOA and PFOS respectively. • Sorption kinetics for PFOA and PFOS fitted well into the elovich rate equation for the investigated sediment samples. • Lower solution pH and increase in Ca 2 + concentration favours the sorption and partitioning behaviour of PFOA and PFOS.