Optimization and life cycle assessment of cotton straw hydrothermal carbonization: a sustainable approach for enhanced saline-alkali soil remediation

The low utilization of agricultural and livestock waste, such as cotton straw and dung, results in significant environmental challenges, including pollution and resource wastage. This study examines the optimized conversion of cotton straw through hydrothermal carbonization (HTC) to enhance saline-alkali soil, along with an environmental impact assessment of this HTC compared to pyrolysis (Py). A comprehensive experimental procedure was designed using a central composite design, and the resulting hydrochar and liquid products were evaluated based on their potential to improve saline-alkali soil. The optimal conditions achieved a 42.6 wt% yield of hydrochar with a pH of 5.20 and a 57.4 wt% yield of the fertile liquid phase with a pH of 5 at reaction temperatures of 180–200 °C over 3.5 h, with a liquid–solid ratio of 15 mL/g and a model validation accuracy of 98 %. The environmental impacts of the two pathways heavily depend on their energy inputs and outputs. HTC is more environmentally friendly (−4.9E-05 kg CO₂-Eq of total environmental impact per kg of cotton straw) than Py. HTC exhibited a lower environmental burden, with effect drop values of 29, 14, 24, and 11 % for global warming, ecotoxicity, acidification, and eutrophication potential, respectively. The potted evaluation indicated that alfalfa plants in the solid–liquid combined phase (SLP) achieved the highest survival rate (91.67 %) and average growth height (1.86 cm). All experimental groups outperformed the control group under saline-alkaline conditions, demonstrating that hydrochar derived from optimized HTC of cotton straw improves saline-alkaline soil, offering a sustainable pathway for environmentally friendly agricultural practices.