Green and facile recycling of bauxite residue to biochar-supported iron-based composite material for hydrothermal liquefaction of municipal solid waste

Industrial and municipal wastes remain significant sources of air, soil, and water pollution, thus causing adverse climate and health impacts. EU faces challenges in developing green recycling processes and reducing GHG emissions. Innovation in green catalysis is a key driver toward the fulfilment of these goals. This study demonstrated a single-step “Green Recycling” route by which different wastes e.g., industrial and bioorganic wastes are treated to produce biochar/Fe(0) (BC-Fe(0)) material. Typically, three different biomass namely organic fraction of municipal solid waste (biopulp), wheat straw (WS), and microalgae (MA) were used as green reducing agents for reducing bauxite residue (BR). Among all biomass, the high reduction potential of amino acids present in biopulp facilitated the synthesis of BC-Fe(0). BC-Fe(0) material acted as an effective catalyst for HTL of biopulp as the results showed the highest bio-crude yield (44 wt%) at 300 °C for 30 min with 10 wt% BC-Fe(0) loading (containing 2.5 wt% Fe). Furthermore, BC-Fe(0) also assisted in-situ hydrogenation and deoxygenation of chemical compounds present in the bio-liquid product, therefore bio-crude exhibited a higher H/C ratio (1.73) and lower oxygen contents (9.78 wt%) in comparison to bio-crude obtained without catalyst. However, Raw BR and reduced BR (RED) as catalysts showed no significant effect on the yield and oxygen content of bio-crude, which confirms the high catalytic activity of Fe(0) containing BC-Fe(0). Therefore, this study demonstrates the greener path for the one-step valorization of industrial and organic wastes, as an alternative to existing chemical and high temperature-based waste recycling and catalyst synthesis technologies.