This study mechanistically investigated the influences of CO2 on syngas (H-2 and CO) production during thermo-chemical conversion of red seaweed, and further explored the possible utility of the produced biochar as a medium for adsorption of inorganic/organic contaminants in aqueous phase. In order to elucidate the key roles of CO2 in the thermo-chemical process, the composition analysis of syngas and the qualitative analysis of pyrolytic oil were conducted and compared with those in pyrolysis in N-2 condition. Pyrolysis of red seaweed in the presence of CO2 led to the enhanced generation of syngas at the entire experimental temperatures. For example, the ratio of CO to H-2 in the presence of CO2 at 620 degrees C was enhanced by similar to 400%, as compared to the case in N-2. This enhanced generation of syngas resulted in significant pyrolytic oil reduction by similar to 70% at 620 degrees C via the unknown reactions between VOCs and CO2. In addition, biochar generated in the CO2 environment exhibited comparatively higher surface area (61 m(2) g(-1)) and more porous structure. The morphological modification induced by CO2 provided the favorable condition for removal of methylene blue from the aqueous phase. Thus, this study experimentally demonstrated that exploiting CO2 as a reaction medium would provide an attractive option for the enhanced generation of syngas and the tuned adsorption capability of biochar. (c) 2016 Elsevier Ltd. All rights reserved.