Sono-chemical mechanism of simultaneous Green Malachite and Rhodamine B removal using blue crab shell biochar

This study investigates the simultaneous removal of Malachite Green (GM) and Rhodamine B (RB) using blue crab shell biomass and its derived biochar. Thermal and textural characterizations revealed the superior stability and hierarchical microporous structure of biochar compared with biomass. CCD optimization identified a contact time of 79 min, an initial dye concentration of 95 mg L^-1, and a temperature of 310 K as the optimal adsorption conditions, yielding removal efficiencies of 97.24 % and 94.31 % for GM and RB, respectively, using biochar, compared with 81.24 % and 76.31 % using biomass. Ultrasound-assisted adsorption was subsequently performed using a 40 kHz ultrasonic system at powers ranging from 50 to 300 W. Maximum cavitation activity was observed at 200 W, at which GM and RB removal reached 94.6 % and 92.3 % for biomass and 99.6 % and 99.0 % for biochar, respectively. Sono-chemiluminescence analysis confirmed enhanced acoustic cavitation at the optimum power. SEM observations further indicated improved mass transfer and more homogeneous dye distribution after ultrasonic treatment. A cavitation-based sonochemical mechanism involving microjets, shock waves, and microstreaming was proposed to explain the enhanced adsorption. These findings highlight the synergistic effect of acoustic cavitation and biochar for the efficient simultaneous removal of GM and RB from aqueous media.