<p>The fuel additive and pesticide 1,2-dibromoethane (DBA) is a problematic organic pollutant found in soil and aquifers. Remediation methods face drawbacks because of undesirable by-products and slow rates of removal. Biochars (BCs) are catalysts of reductive dehalogenation in anoxic environments by the layered iron(II)-iron(III) hydroxide green rust (GR). However, so far, such catalytic properties have generally only been observed for BCs produced from animal substrates, while BCs from easily available plant substrates, seem to lack these properties. In this study, the potential of CO₂ activation to enhance the catalytic properties of plant-substrate BCs was tested, specifically for BCs from barley straw (BARST), soybean meal (SOYM), wheat straw (WHST) and eelgrass (Zostera marina, ZOST). All BCs were tested for catalyzing reductive debromination of DBA with GR as the reductant. CO₂ activation led to clear increases in the extent of DBA debromination (after 7&#xa0;days) by a factor of 1.5, 8.8, 3.3, and 2.1 for GR reactions with BARST, SOYM, WHST, and ZOST BCs respectively. Changes in BC properties were observed upon CO₂ activation, and these varied between substrates, making it difficult to define the key properties that led to the observed reactivity increase. However, surface polarity, structural defects and the presence of surface O and N functional groups were identified as key BC properties regulating the extent of DBA sorption and degradation. The findings of this study contribute to the optimization of BC mediators for reduction of halogenated pollutants.</p> Graphical abstract <p></p>