In this study, the ultrasound-assisted biosorption of Bemacid Red dye (BR) from an aqueous system using Casuarina Equisetifolia needles (CEP); an inexpensive, environmentally friendly, and efficient biosorbent, was evaluated. The biosorbent was characterized by the Fourier-transform infrared spectroscopy (FTIR), Scanning electronic microscopy (SEM), X-ray diffraction analysis (XRD), thermogravimetric analysis (TGA), and the point of zero charge (pHpzc). The effects of different analytical parameters; including contact time, initial dye concentration, biosorbent dosage, pH, and temperature, were investigated to optimize the biosorption process. Kinetic, thermodynamic, and isothermal biosorption studies were performed to elucidate the mechanisms and efficiency.

The elimination of BR was fast and efficient, with an equilibrium time of 7 minutes. The removal efficiency of BR increased with higher biosorbent dosage, initial dye concentration, and ionic strength but decreased with increasing pH, temperature, and particle size of the biosorbent.

The kinetic study was best represented by the pseudo-second-order model (R² = 0.99). Experimental data on dye biosorption onto CEP corresponded to the Sips model (R² = 0.98); which combines the Langmuir and Freundlich hypotheses. For thermodynamic parameters, the negative value of ΔH (∆H = -18.3 kJ/mol) suggests that the biosorption process is exothermic and that BR biosorption onto CEP has a pure physical nature. CEP is a suitable biosorbent for practical applications and holds potential for the development of purification and extraction technologies.