Enhanced biodegradation of PAHs by biochar and a TiO2@biochar composite under light irradiation: Photocatalytic mechanism, toxicity evaluation and ecological response

Biochar (BC) and its composites have been widely used for pollutant treatment upon their adsorption and photocatalytic efficiency, but their toxicity and ecological effects have received little attention. In this study, BC was prepared and modified with nano-titanium dioxide to obtain BC composite (TiO2@BC), then the biotoxicity and ecological effects were investigated from two dimensions (single bacteria and microflora). In detailed, the effects of different dosage of BC and TiO2@BC on the degradation function, growth characteristics and toxicity mechanism of the strain Achromobacter sp. LH-1 were comprehensively studied. It was found that when the dosage of BC and TiO2@BC was less than or equal to 0.5 g/L, the degradation function of LH-1 was enhanced, but the effect on growth was negligible. However, when the dose exceeded 0.5 g/L, a large amount of ROS accumulated, which inhibited the cell respiratory chain and destroyed the cell wall. In another word, ·OH and ·O2– generated by BC and TiO2@BC played important roles in the inactivation of LH-1. Furthermore, a laboratory scale bioreactor (SBR) was built and applied to study the ecological effects of BC and TiO2@BC, and it was found that proper amount (0.5 g/L) of BC and TiO2@BC can not only effectively remove polycyclic aromatic hydrocarbons (PAHs) pollutants, but also increase the diversity of microbial communities, and promote the carbon and nitrogen cycle in the sewage system. This study analyzes the ecotoxicity risk of BC and TiO2@BC under photocatalysis in detail, which has important reference value for future practical applications in ecosystems.