Uptake of perfluorobutane sulfonate and perfluoroctane sulfonate in rice (Oryza sativa)

The wastewater reuse in agriculture can pose serious environmental and health problems related to the accumulation of the highly soluble emerging pollutants in the irrigated vegetables. Laboratory studies were carried out to assess the uptake mechanisms, translocation and distribution of perfluorooctanesulphonic acid (PFOS) and perfluorobutanesulphonic acid (PFBS), which is a common substitute of PFOS, within rice plant (Oryza sativa). Short-term uptake studies (from 4 to 48 h), at high contaminant concentration (1 mg/L) were performed at different pH values (4.5, 5.5 and 6.5), temperature (4° and 20°C) and illumination (darkness and light). In a life-cycle experiment rice plants were grown in hydroponic greenhouse from the seeds to the maturity exposed at 10 µg/L and 100 µg/L of PFBS and PFOS. At maturity the plants were harvested, and roots, stems, leaves, shoot, caryopsis and edible grains were analysed separately. Moreover growth (in term of biomass) and flower fertility (as fraction of fertile flowers) were evaluated. Experiments carried out at different temperature under darkness allowed to assess the transport mechanism of uptake when the transpiration is stopped. Data suggested that the passive diffusion (simple or facilitated) represented 63% and 74 % of the total flow for PFBS e PFOS respectively. Under normal growth conditions, root concentration factor of PFOS (RCF) is 65 L/kg and the uptake constant ku is 3.98 L/kg/h which are values very similar to those measured for maize. PFBS uptake is one hundred-fold lower than PFOS one, being RCF 0.69 L/kg and Ku 0.115 L/kg/h. The accumulation in foliage and roots is equivalent for PFBS (the foliage/root concentration factor, FRCF is 0.81), while PFOS preferably accumulates in roots (FRCF = 0.083). Differences in uptake mechanism between PFBS and PFOS have been evidenced also in tests at different pH, from 4.5 to 7: PFBS uptake is independent of the pH value, while uptake of PFOS increases with pH decreasing. Life cycle experiments showed that plants exposed to PFOS and PFBS had a better growth than plants in control experiments and this fact can be explained by considering that perfluorinated sulfonic acids can act as bacteriostatic in the Hoagland solution used as culture medium. Nevertheless even if the biomass growths at 10 and 100 µg/L of PFOS and PFBS are comparable, at higher concentration of exposition the flower fertility was significantly reduced.