Mechanisms of heavy metal immobilization and resistance genes reduction following phosphate-modified biochar application during anaerobic digestion of swine manure

This study investigated the effects of biochar (BC), H₃PO₄-modified biochar (BP) and KH₂PO₄-modified biochar (BK) on heavy metals (HMs) passivation and the distribution of antibiotic resistance genes (ARGs) and metal resistance genes (MRGs) during anaerobic digestion (AD). Characterization results revealed that the application of BK significantly decreased the mobility of Cr, Cu, and Pb, attributing to metal phosphate precipitation formation. EEM-PARAFAC and 2D-FTIR-COS analysis showed that the C-O stretching, polysaccharide-like structures, N-contain substance or carboxylic reacted faster after adding biochar. Biochar addition promoted the synthesis of fluorescent and humic-like components, and accelerated humification process, thus showing a greater complexation ability for HMs. In BK treatments, most detected ARGs abundance were reduced with efficiencies of 85%–96%, and MRGs abundance, such as phrT, yieF, chrR, etc., were significantly declined by more than 90%. Biochar addition directly reduced MRGs abundance by improving the immobilization degree of HMs. Biochar addition inhibited horizontal gene transfer (HGT) facilitated by mobile genetic elements (MGEs) and reduced co-selective pressure by HMs, thereby decreasing ARGs abundance. Biochar stimulated growth of HMs tolerant bacteria, such as Bacillus, Romboutsia, Clostridiales, etc., which functioned well in HMs immobilization. P-loaded biochar as additive is recommended in AD to mitigate ARGs distribution and reduce HMs risks. (Figure presented.)