Evaluating ash and biochar mixed biomixtures for atrazine and fipronil degradation

Abstract Composition of biomixtures is crucial for the success of the bio-purification system. Therefore, compost in a rice straw-compost biomixture (BM) was partially replaced with rice husk ash (RHA, 5 & 10%), sugarcane bagasse ash (SBA, 5 & 10%) and wheat straw biochar (WBC, 1 & 5%). Degradation of atrazine and fipronil was studied in these biomixtures and effects of pesticide concentration (50 and 100 μ g g−1) and bio-augmentation with pesticide-degrading microbial cultures were evaluated. Ashes and biochar had varied effects on atrazine and fipronil degradation that followed first order kinetics, but, was biphasic for atrazine degradation. Atrazine degradation during the IInd phase was faster than the Ist phase. Compared to control (BM), RHA decreased the half-life ( t 1∕2 ) of atrazine at lower concentration (50 μ g g−1) while no significant effect of SBA and WBC (1%) was observed. However, WBC (5%) increased atrazine t 1∕2 by  200% (50 μ g g−1) and  300% (100 μ g g−1). No significant effect of RHA, SBA and WBC (1%) was evident on fipronil degradation and the t 1∕2 varied between 11.63–15.42 days, while, WBC (5%) slightly increased the t 1∕2 by 35%–40%. These results suggested that atrazine degradation in WBC-mixed biomixtures was limited by its availability due to higher adsorption, while higher fipronil adsorption in WBC-mixed biomixtures did not affect its degradation. Bio-augmentation of biomixtures with pesticide degrading cultures increased degradation of atrazine and fipronil and their metabolites, hydroxyatrazine and fipronil sulfone, respectively and the effect was significant on atrazine/hydroxyatrazine degradation. No significant effect of ashes/biochar was evident on microbial biomass carbon, dehydrogenase and fluorescein diacetate activity. This study suggested that bio-augmentation of ashes/biochar-mixed rice straw-compost biomixtures can be exploited for detoxification of these pesticides in bio-purification systems.