Screening and characterization of thermally modified biowaste-derived hydrochars for slow-release fertilizer for alkaline soil reclamation

Global biowaste generation is increasing, posing environmental and health risks, while offering opportunities for resource recovery, particularly through bioenergy and soil enhancement. This study evaluates the physicochemical properties, structural modifications, and urea sorption capacity of hydrochars derived from wheat straw (WS), fruit peels and pulps (PP), and sewage sludge (SS) under hydrothermal carbonization (HTC) at 180°C, 220°C, and 260°C with a 1-hour retention time. The goal is to identify the optimal feedstock and HTC temperature for slow-release fertilizers in alkaline soil reclamation. Proximate analysis showed that WS and PP hydrochars at 260°C were most suitable for soil applications due to their high fixed carbon (FC) and ash content (AC), enhancing carbon sequestration and stability. Hydrochars with an O/C ratio below 0.4, particularly WS-260 and PP-260, demonstrated enhanced stability. Fourier-transform infrared (FTIR) analysis revealed functional group changes like dehydration and decarboxylation, improving soil properties such as cation exchange capacity (CEC). Scanning electron microscopy (SEM) indicated increased porosity and surface deformation with temperature, boosting nutrient retention and microbial activity. X-ray diffraction (XRD) confirmed reduced crystallinity, enhancing hydrochar effectiveness in contaminant stabilization and soil health. Freundlich isotherm modeling showed high urea sorption, with WS-260 achieving the highest adsorption efficiency (KF = 9.35; n = 2.37). WS-260 hydrochar holds promise for soil reclamation, carbon sequestration, and fertility improvement in nutrient-poor soils.