Upgrading soybean dreg to caproate via intermediate of lactate and mediator of biochar

High-yield soybean dreg (SD) poses environmental hazards, demanding eco-friendly and resource-efficient treatment urgently. Hence, this study firstly proposes a high-value upcycling strategy, specifically, caproate synthesis via chain elongation (CE). Optimized conditions for lactate-rich broth as an intermediate, utilizing 50% inoculum ratio, 40 g/L substrate concentration, and pH 5, which reluctantly achieved 2.05 g/L caproate output from fermenting directly. Leveraging a lactate-rich broth supplemented with ethanol, caproate yield was optimized to 2.76 g/L under a refined electron donor to acceptor of 2:1. To further enhance caproate production, incorporating 20 g/L biochar elevated the yield to 3.05 g/L and significantly shortened the lag phase. Mechanistic insights revealed that biochar's surface-existed quinone and hydroquinone groups exhibit potent redox characteristics, thereby facilitating electron transfer. Furthermore, biochar up-regulated the key genes abundance involved in CE process (especially fatty acids biosynthesis pathway), which also enriched Lysinibacillus and Pseudomonas as an unrecognized cooperation to CE process.