Graphitization-engineered biochar for solar-driven nutrient recovery and sustainable biogas slurry valorization

Abstract

Solar-driven interfacial evaporation using low-cost biochar evaporators presents a sustainable and highly promising strategy for valorizing nutrient-rich biogas slurry. However, its progress is hampered by an incomplete mechanistic understanding of how the tunable physicochemical attributes of biochar govern the intricate interplay of photothermal conversion, water transport, and nutrient crystallization dynamics within this complex wastewater matrix. This study addresses this knowledge gap by integrating advanced experiments with first-principles simulations (DFT and MD) to systematically clarify these structure–performance relationships. The high-temperature biochar evaporator (HBE) achieves a superior evaporation rate of 3.58 kg m⁻² h⁻¹, underpinned by a synergistic optimization of its electronic and physical structures stemming from enhanced graphitization. DFT calculations reveal that this graphitization narrows the HOMO–LUMO gap, fundamentally boosting broadband solar absorption (92.4%) and the efficiency of photothermal conversion. Concurrently, MD simulations demonstrate that the optimized multiscale pore network and hydrophilic surface functional groups synergistically facilitate rapid water transport while lowering the evaporation enthalpy. This energy-efficient process enables a 57.9% reduction in slurry volume and achieves precise nutrient valorization, concentrating liquid NO₃⁻–N (55.11% increase) while recovering solid-phase P (31.0%) and K (38.3%). This work establishes a multiscale mechanistic framework for the rational design of high-performance photothermal evaporators, offering a viable path for sustainably valorizing nutrient-rich wastewater.