Sorption–desorption behaviour of bentazone, boscalid and pyrimethanil in biochar and digestate based soil mixtures for biopurification systems

Abstract Pesticide pollution caused by point or diffuse sources may lead to the contamination of ground and surface water. Biobed or biofilter systems, which are filled with a variety of organic materials (soil, peat and straw), are commonly used on farm to treat pesticide remnants. The objective of this study was to assess the sorption–desorption potential of three pesticides with contrasting physico-chemical properties (bentazone, boscalid, and pyrimethanil) on novel biofilter materials based on bioenergy residues (mixtures of soil with digestate and/or biochar) in laboratory batch equilibrium experiments. The results show that the biomixtures of digestate and biochar with soil increased pesticides sorption potential and the 1 / n des values were lower than the 1 / n ads values indicating that the desorption was hysteretic for all pesticides on these materials. The adsorption and desorption of all the chemicals conformed to linear and Freundlich isotherms. Higher values of distribution [K d (> 78 L kg − 1 )] and Freundlich sorption coefficient [ K f (> 900 μg 1–1/n  L 1/n  kg − 1 )] were observed for all pesticides for the digestate and biochar based mixtures compared with the blank soil, which was attributed to the lower organic carbon content of the latter. However, unlike for sorption there was no statistically significant difference between the biochar and the digestate mixtures for pesticides desorption ( p  > 0.05; t -test). Specific UV-absorbances at 254 nm (SUVA 254 ) indicated the aromatic character of digestate (5 and 30%) and biochar (5%) biomixture, which showed the highest organic-carbon-partition coefficients (K oc ) and/or K d values amongst all biomixtures for all pesticides. Therefore, these biomixtures were found to be the most promising substrates amongst the tested ones for a novel biobed setup and can be used as effective and alternative adsorbents for removing pesticides from percolating water in biofilters.