Accumulation of perfluorinated compounds in radish: a hierarchical model approach.

Biogas technology is a competitive process for managing biodegradable wastes and by-products and a green method for energy production. Biogas plants digestates (BGD) can be used as fertilizers or soil improvers. This is of great importance, since phosphorus is a depleting nutrient and nitrogen fertilizers are manufactured in an energy-intensive process. However, BGDs may contain hazardous organic chemicals. When BGDs are used in agriculture, these compounds may accumulate in the soil and eventually in food of plant and animal origin.

Perfluoroalkyl compounds (PFC) comprise a large group of compounds that have been widely used in a myriad of applications such as electronic parts, firefighting foams, photo imaging, hydraulic fluids and stain-resistant coatings. PFCs are persistent and have harmful effects on humans and the environment. PFCs can be taken up by plants via the roots and they can accumulate in food of animal origin. The most widely used compounds have been perfluorooctane sulphonic acid (PFOS) and perfluorooctanoic acid (PFOA).

The aim of this study was to assess the accumulation of PFCs in radish as a result of the use of biogas plant digestate as an agriculture fertilizer. We measured the concentrations of four PFCs (perfluorohexanoic acid PFHxA, perfluoroheptanoic acid PFHpA, PFOA, PFOS) in 19 digestates from ten biogas plant production lines in Finland. A Bayesian hierarchical model was used to predict the concentration of these compounds in agricultural soil after a single addition of digestate (15 t fresh matter (f.m.)/ha). Bioconcentration factors for the selected PFCs in radish (Raphanus sativus var. sativus) were determined in a pot experiment, in which a known amount of PFC was mixed in soil and the concentration of the compounds was determined in the soil and plants. Finally, we estimated the concentration of the compounds in radish grown on agricultural land using the predicted concentration of PFCs in the soil and the posterior distribution of the bioconcentration factors (BCFs) for radish.                           

The predicted median increase in concentration of PFOS in radish after a single addition of digestate to agricultural land described in this paper (0.02 ng/kg f.w.) was 1/200 of the concentration of PFOS in vegetables in Sweden (4.1 ng/kg), while the predicted concentration of PFOA (0.01 ng/kg f.w.) was 1/2200 of the concentrations in vegetables in Sweden (22 ng/kg). The 97.5th percentiles of the estimates reported in this paper for PFOS and PFOA, respectively, were 1/9 and 1/200 of the concentrations in vegetables in Sweden.