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The use of seawater as a substitute for pure water as supplemental moisture raises questions about its effect on the physicochemical properties of hydrochar. Therefore, this study aimed to investigate the feasibility of using seawater as supplemental moisture by comparing the physicochemical properties of products obtained through Co-hydrothermal carbonization of chicken manure and cornstalk under seawater and deionized water conditions. By varying the HTC temperature and blending ratios of CM and CS to investigate comprehensively the effect of seawater. Results indicated that the hydrochar yield experienced a variation from 54.54% to 57.40%, while the IC value changed from 7.69% to 8.46% as the ratio of CM:CS shifted from 3:1 to 1:3 under seawater conditions. The higher heating value of the hydrochars obtained under seawater conditions was lower than those obtained under deionized water conditions. This suggests that seawater conditions promote the hydrolysis reaction of organic solid waste. Furthermore, it was observed that when no lignin hydrolysis reaction occurred, seawater conditions had no discernible effect on the fuel quality of the hydrochar. However, at an HTC temperature of 250 °C, the fuel quality of the hydrochar obtained under seawater conditions was notably inferior to that of the hydrochar obtained under deionized water. Thus, an HTC temperature lower than 250 °C is necessary for the hydrothermal carbonization of organic solid waste under seawater conditions. Moreover, the relative content of surface -C-(C, H)/CC of the hydrochar obtained under seawater conditions was lower than that obtained under deionized water conditions, indicating that the hydrochar had a low degree of aromatization. Additionally, there was a significant increase in the immobilized Mg atoms in the hydrochar under seawater conditions, which affected the hydrochar yield and higher heating value of the hydrochar. This research presents a theoretical foundation for preparing solid fuels and materials using hydrothermal carbonization of saltwater as supplemental moisture., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)