AMOchar: an amorphous MnOx functionalized biochar to stabilize metal(loid)s in soil and optimize phytostabilization

Biochar (BC) is widely used to immobilize metal(loid)s but its efficiency is not ubiquitous and can be improved via various surface modifications, such as impregnating amorphous Mn oxides (called AMOchars), which showed benefits for metal(loid) immobilization, but also induced considerable Mn leaching. To reduce those drawbacks, we tested two AMOchars, synthesized using molasses (BCM) or sucrose (BCS) as reducing agents, to evaluate the effectiveness of Mn oxide fixation on biochar to reduce metal(loid) transfer to plants and select the most appropriate reducing agent. Soil solution monitoring showed that all materials increased pH and immobilized Cd, Pb, and Zn. The AMOchars further improved Cd and Zn immobilization efficiency while releasing dissolved organic carbon, As, Cu, and Mn. In plants, metal(loid) shoot transfer was reduced by BC (As, Cd, Cu, Mn, and Zn) and BCS (Cd, Pb, and As). To conclude, we demonstrated the effectiveness of Mn oxide fixation on biochar using sucrose to improve phytostabilization. Sucrose as a reducing agent led to a more stable (less organic carbon leaching) material and thus less toxic to the plant. We believe that our study will help raise up the knowledge on biochar modification and make aided phytostabilization process more practiced on the field.