Valorization of algal biomass to synthesize visible-light driven gC3N4-biochar composite for dye degradation: Tuning of optical-electronic properties and persulfate-photocatalytic mechanistic insights

• An effective and efficient gCN@BC X photocatalyst was synthesized. • Improvement in visible-light absorption and slower charge recombination alongside reduced energy bandgaps (∼2.62 eV). • The integration of PS with gCN@BC 3 elevated the degradation efficacy (99.9 %) with a fast reaction kinetics of 0.041 min -1 , compare to control (0.029 min -1 ) system. • The S O 4 − • and O H • radicals are responsible to improve the degradation kinetics. • Optimized gCN@BC 3 is more stable and economical due to reusability up to five cycles. Photocatalysis, as an advanced oxidation process, is considered a green, simple, effective, and sustainable strategy to degrade organic pollutants of wastewater, i.e. wastewater from textile industries, which poses significant threats to the aquatic and public health. However, the implementation of photocatalyst, i.e. graphitic carbon nitride (gC 3 N 4 ), to treat dye-laden wastewater has proven to be ineffective, primarily owing to its restricted absorption of visible-light, rapid charge recombination, low absorption capacity, and inactive degradation efficacy. To improve these obstacle, gC 3 N 4 (gCN) was loaded on algal-biochar (BC) to enhance its optical properties and energy bandgap by synthesizing gCN@BC 3 and then integrated it with peroxydisulfate (PS) to improve the reaction kinetics for dye degradation . The synthesized gCN@BC 3 photocatalyst demonstrated better optical-electronic properties including light absorbance in visible region, slow charge recombination, and reduced energy bandgap (∼2.62 eV), as they improved the dye (methylene blue) degradation kinetics (degradation rate (min -1 )) and overall process efficacy. Upon the integration of PS with gCN@BC 3 photocatalyst, the process efficacy and degradation kinetics were significantly improved up to 99.94 % and 0.041 min -1 as compared to the control (96.82 % and 0.029 min -1 ) system without PS. Photogenerated radicals, including superoxide, hydroxyl, and sulfate species, play a key role in the degradation of organic dyes by enhancing process efficacy and kinetics. The reusability analyses demonstrated that the optimized gCN@BC 3 composite retains its stability and effectiveness over five successive cycles. The gCN@BC 3 photocatalyst exhibited a significantly higher adsorption efficiency of 70.92 %, surpassing that of algal-BC (62.31 %) and gCN (27.11 %). The adsorption process, as described by the well-fitted Pseudo-Second-Order and Freundlich models, endorses a favorable chemical interaction with a multilayer adsorption mechanism. Hence, it is suggested that the integration of PS with visible-light-driven gCN@BC 3 presents a rapid, efficient, and stable strategy to significantly boost the process kinetics and degradation efficacy of organic pollutants.