Co-hydrothermal carbonization of lignocellulosic biomass and swine manure: Optimal parameters for enhanced nutrient reclamation, carbon sequestration, and heavy metals passivation.

Hydrochar, the primary product of hydrothermal carbonization (HTC) of wet organic waste, is recognized as a versatile, carbon-abundant material with diverse applications. However, optimizing its performance for specific uses remains challenging. Therefore, this study introduced a co-HTC process involving carbon-rich lignocellulosic materials and ash-rich livestock manure [i.e., Zanthoxylum bungeanum branch residue (ZB) and swine manure (SM), respectively]. The impacts of HTC temperature (i.e., 180 °C, 220 °C, and 240 °C) and mass ratios (i.e., 1:0, 7:3, 5:5, 3:7, and 0:1) on hydrochar properties (e.g., pH, EC, nutrient contents, heavy metal content and availability, chemical stability, etc) and the characteristics of process water were evaluated. Results reveal that co-HTC dramatically improved the quality of hydrochars compared with that derived from a single feedstock. Notably, the ZB:SM ratio had a more substantial impact on total nutrient content, carbon stability, and heavy metal accumulation and mobility. Additionally, the synergistic effects of ZB and SM were greatly dependent on the HTC temperature. By adjusting the feedstock mass ratio and HTC temperature, a highly-functionalized hydrochar can be produced. For example, hydrochars produced at 240 °C with a 7:3 ZB to SM ratio (HC240-7) is optimal for degraded soil amendment, enhancing carbon sequestration and nutrient supplementation. Results from this study could provide valuable insights for improving waste management through HTC and expanding the environmental and agricultural application of hydrochar.