Analysis of biomixtures to determine the fate of pesticides

Worldwide, water contamination from agricultural use of pesticides has received increasing attention within the last decades. In general, sources of pesticide water pollution are categorized into diffuse (indirect) and point sources (direct). To reduce point pollution from farm yards, where the spray equipment is washed, biobed orbiofilter systems are conventionally used to treat the washing water. The organic material usually used in these systems is often not environmentally sustainable (e.g.peat) and incorporated organic material such as straw leads to a highly heterogeneous water flow, with negative effects on the retention and degradation behavior of the pesticides. Therefore, the objective of this present study was to substitute the classical materials (peat and straw) with bioenergy residues namely biochar and digestate to investigate their effects on fate of pesticides in soil at different mixing ratios. Prior to study the pesticides fate, the microbial respiration was measured over 3 months to gain information about the turnover rate of the organic biomixtures, which can be used as an indirect indicator of the soils/biomixture degradation potential for pesticides and provides information about the long-term stability of the material. Mixtures of biochar and digestate showed an intermediate CO$_{2}$ flux compared to the single addition of biochar or digestate, whereby the oxygen consumption in presence of biochar was generally significantly lower compared to the consumption after addition of digestate only. Additionally, to correlate the microbial respiration with the dissipation (or degradation) potential of pesticides a laboratory incubation study was performed over 135 days with three contrasting pesticides (bentazone, boscalid, and pyrimethanil). In general, biochar based mixtures resulted in stronger binding of all studied pesticides, and therefore, ensued higher dissipation. On the other hand, 5 % and 30 % digestate based mixtures enhanced mineralization and addition of 5 % biochar to these mixtures showed a desired balance between stronger sequestration and mineralization for all pesticides. A sorption-desorption study revealed that biochar and digestate based mixtures caused stronger sorption for all compounds compared to bare soil. K$_{d}$ and K$_{oc}$ values of the pesticides were different according to their physico-chemical properties and quality (nature) of organic matter. Desorption was hysteretic for all pesticides. Overall, this thesis elucidated and updated the knowledge of the mechanisms for Cturnover rates of novel biomixtures for biopurification (or biobed) systems along with the long term behavior of three different pesticides and their interaction with these biomixtures. However, future work is required to qualify these mixtures for long-term (>3 yrs) outdoor biofilter constructions under varying hydraulic and chemical conditions