Graphene based ZnO nanoparticles to depolymerize lignin-rich residues via UV/iodide process

In this work, potassium iodide (KI) and graphene oxide (GO) were utilized to promote the selectivity of photocatalytic process for alkali lignin oxidation over ZnO. Different concertation of GO was added during the microwave synthesis procedure of ZnO, and the characterization results revealed that graphene can shift the conduction band to more reducing potential, resulting to higher production of superoxide anion radicals (O₂[rad]⁻) compared to [rad]OH. Response Surface Methodology revealed the most suitable interaction among loading of GO, KI and irradiation time on lignin and total phenolic compound (TPC) degradation. Specifically, the optimal conditions (i.e. maximum lignin (52%) and minimum TPC (55%) degradation) were at [KI] = 0.64 mM; GO content into ZnO 1.2 mg/mL; 240 min of irradiation time. The results showed that higher addition of graphene into structure of ZnO could preserve more phenolics from degradation due to less production of [rad]OH. Furthermore, the addition of KI at optimized conditions could enhance the selectivity of degradation of lignin and phenolics via producing I[rad] radicals and quenching the excess amount of generated [rad]OH, respectively. The lower generation of [rad]OH at optimized conditions was quantitatively confirmed by a photoluminescence simplified technique. In addition, the effect of the photocatalytic process on substrate's anaerobic degradability was examined in order to evaluate the suitability of the pretreated solution for energy recovery. Indeed, besides the higher TPC concentration, the biogas production of treated straw at optimized conditions was increased by 35% compared to the untreated sample.