Biochar-templated g-C₃N₄/Bi₂O₂CO₃/CoFe₂O₄ nano-assembly for visible and solar assisted photo-degradation of paraquat, nitrophenol reduction and CO₂ conversion

The development of novel visible powered nano-heterojunctions with multi-pronged capabilities for environmental and catalytic applications has been gaining importance for persistent pollutant degradation & clean energy production. A magnetically recoverable biochar supported ternary g-C₃N₄/Bi₂O₂CO₃/CoFe₂O₄ heterojunction (BCBF) was fabricated which shows a high visible photoactivity. The heterojunction was used for degradation of pesticide paraquat under visible radiation (Xe lamp), natural sunlight, photo-ozonation, peroxymonosulphate, and coupled conditions. A high degradation of 99.3% was achieved under visible radiation in 90 min and 92.1% under solar light in 120 min. Biochar supported ternary junction performs manifold faster than Bi₂O₂CO₃/CoFe₂O₄, g-C₃N₄/CoFe₂O₄, g-C₃N₄/CoFe₂O₄, Bi₂O₂CO₃ (BOC), CoFe₂O₄ (CF) and g-C₃N₄ (CN). The effect of operational parameters as effect of pH, H₂O₂, anions, ozone and peroxymonosulphate (PMS) was also studied. In BCBF + PMS + O₃ + Vis protocol a spectacular complete degradation was observed in less than 30 min. Ternary band structure efficiently reduces the charge recombination rate and promotes spectral activity which was confirmed by photoluminescence, photocurrent response and EIS analysis. By LC-MS, scavenging experiments and ESR studies a possible mechanism was also proposed. Cyto-toxicity studies of degraded products on human peripheral blood cells revealed almost 99% cell viability affirming the complete mineralization. The junction was also used for reduction of 4-nitrophenol into 4-aminophenol with complete conversion in less than 5 min with rate constant 13.05 × 10⁻³ s⁻¹. In addition the ternary junction shows a high visible powered conversion of CO₂ producing CH₄ (∼119 µmol g⁻¹), CO (∼131 µmol g⁻¹) and O₂ (∼242 µmol g⁻¹) with a high rate constant of 13.05 × 10⁻³ s⁻¹.