Advanced characterization of hydrochar derived from cow Manure: Insights into morphological structure, surface chemistry, and heavy metal speciation

The rapid expansion of the livestock industry has led to a substantial increase in manure production, posing environmental risks. Hydrothermal carbonization (HTC) offers a promising approach to convert livestock manure into hydrochar (HC), yet comprehensive evaluations of HC characteristics remain limited. This study investigated the fundamental properties, heavy metal element (HME) content, and HME speciation of HCs derived from cow manure (CM). The results showed that the HCs were weakly acidic (pH 5.13–5.38), and exhibited significantly reduced electrical conductivity (98.3–255 μs/cm). The decreased O/C ratio indicated enhanced carbonization and deoxygenation. HC samples also displayed increased specific surface area and pore volume due to the development of micropores and mesopores. Although HMEs were enriched in HCs compared to raw CM, the transformation in metal speciation led to a reduction in bioavailable fractions and an increase in stable fractions, thereby lowering environmental risk. In conclusion, CM-derived HCs are weakly acidic, stable, and environmentally safe materials with great potential for agricultural applications. This study provides a theoretical basis for their safe and sustainable use.