Sorption and release of the pesticides oxyfluorfen and boscalid in digestate from olive pomace and in digestate-amended soil

PurposeThe decline in organic matter content in many agricultural soils results in a dramatic decrease in their ability to retain xenobiotics. Due to its carbon-rich nature and sorptive behaviour, digestate used as soil amendment can counteract this trend. This study investigated the sorption efficiency of the herbicide oxyfluorfen and the fungicide boscalid by a digestate from olive pomace only, and by a loamy calcareous agricultural soil before and after its amendment with 2 and 5% (w/w) digestate.MethodsTo investigate the surface micromorphology and the functional groups of the digestate, scanning electron microscopy (SEM-EDX) and Fourier-transform infrared (FTIR) spectroscopy were employed, respectively. Adsorption kinetics and adsorption/desorption isotherms of the compounds on the digestate and the soil were performed. Adsorption data were described using the Henry, Freundlich, Langmuir and Temkin equations.ResultsBoth oxyfluorfen and boscalid reached the steady state on both substrates in approximately 2 h according to a pseudo-second order model, thus denoting a prevalent chemisorptive interaction. The Freundlich model was generally the best fit for both molecules on any substrate. The KFads values for oxyfluorfen on the digestate, soil, soil + 2% digestate, and soil + 5% digestate were, respectively, 7158, 19, 60 and 170 L kg-1, while for boscalid, in the same order, they were 3700, 11, 37 and 31 L kg-1, at a temperature of 20 degrees C. The desorption of both compounds from the non-amended and amended soil and, especially, from the digestate was quite slow and incomplete, indicating the occurrence of a hysteretic process. Highly significant correlations were found for both molecules between the adsorption and desorption parameters of all adsorbents and their organic carbon content.ConclusionThis study confirms the prominent role of organic matter in the retention/release of pesticides in soil. It is expected that the addition of digestate to soil can reduce the risk of transport of toxic compounds in natural waters and/or limit their uptake in edible plant organs.