Enhanced antimicrobial activities of nicotine-enriched hydrothermal liquids from tobacco wastes

Hydrothermal carbonization (HTC) offers a promising pathway to convert alkaloid-rich tobacco wastes into functional carbon materials and bioactive liquids. However, the release behavior of nicotine during HTC and the relationship between nicotine occurrence and the antimicrobial activity of the resulting hydrothermal liquids remain insufficiently understood. This study investigated the HTC of tobacco stalks (TS) and leaves (TL) at 150–200 °C with solid-to-liquid ratios of 1:5 and 1:10 and residence times of 6 and 12 h to evaluate nicotine partitioning, hydrochar properties, and antimicrobial performance. Increasing temperature promoted dehydration and aromatization of hydrochars and altered nicotine concentrations in both the solid and liquid products in a feedstock-dependent manner. At S/L = 1:5, the measured nicotine concentration in the hydrothermal liquids reached up to 2.29 mg/mL, largely reflecting the lower liquid volume and reduced dilution. Comparative kinetic modeling showed that nicotine-release profiles were best described by either the Weibull or two-compartment model, with model preference varying among hydrochars. The hydrothermal liquids exhibited concentration-dependent antimicrobial effects against the tested fungal and bacterial species. At 1% concentration, HLs inhibited Rhizoctonia solani mycelial growth by up to 21.23%, whereas lower concentrations promoted growth. Against Escherichia coli , 10% TLHL-5-200-6 delayed the exponential phase by 9 h and the stationary phase by 36 h. The antimicrobial responses were generally associated with hydrothermal-liquid concentration and measured nicotine content, although contributions from other dissolved HTC-derived compounds cannot be excluded. These findings demonstrate that HTC regulates nicotine partitioning and release from tobacco wastes while generating liquid products with measurable in vitro antimicrobial activity.