Acid/alkali-assisted hydrothermal valorization of brewer's spent grain: Insights from element distribution and product potential

Brewer's spent grain (BSG), a high-moisture, protein-rich lignocellulosic biowaste, offers significant renewable resource potential despite its challenging waste attributes. This study innovates an acid/alkali-assisted hydrothermal processing strategy for simultaneous solid-liquid valorization of BSG into energy-dense biofuels and nutrient-rich biofertilizer. Through a dual-phase experimental design, we decoupled the synergistic effects of process parameters and additives (H₂SO₄, Ca(OH)₂) on carbon-nitrogen phase partitioning and functional enhancement. At optimized conditions (180 °C, 45 min), the system achieved 57 wt% carbon in hydrochar alongside 4 wt% nitrogen fixation, with the liquid phase concentrating 1419 ± 50.92 mg L⁻¹ dissolved carbon and 233 ± 9.67 mg L⁻¹ bioavailable nitrogen. Additive dosage critically governed molecular restructuring: stabilizing C=C bonds and pyridinic-N/pyrrolic-N moieties in solids while preserving non-phytotoxic nitrogenous organics in liquids. Consequently, additive-enhanced hydrochar exhibited higher combustion stability, improved thermal reactivity, and suppressed emissions of inert and nitrogen-containing gases, whereas the hydrolysate accelerated seed gemination rate and biomass accumulation in rice seedlings, rivaling urea efficacy. Notably, Ca(OH)₂ emerged as the superior additive, achieving dual enhancements of biofuel and biofertilizer metrics. This work establishes a carbon-nitrogen decoupling-reconfiguration paradigm, offering a scalable blueprint for nitrogen-rich biomass valorization in circular bioeconomy.