Removal of Crystal Violet and Reactive Green 19 from aqueous solutions using the brown macroalga <i>Halopteris scoparia</i> as a natural biosorbent

Industrial discharges of toxic synthetic dyes, such as anionic Reactive Green 19 (RG-19) and cationic Crystal Violet (CV), threaten aquatic ecosystems. This study investigates the marine brown macroalga Halopteris scoparia as an untreated, mechanically ground biosorbent for removing RG-19 and CV from wastewater. Characterization confirmed a rough surface with a negative charge (−17.8 ± 4.08 mV), favoring cationic dye adsorption. Batch experiments optimized pH and dosage. Optimum removal occurred at pH 5.0 for RG-19 and pH 7.0 for CV. Using a biosorbent dosage of 20.0 g L−1, 95.2% of CV and 31.8% of RG-19 were removed from a 50.0 mg L−1 solution within 120.0 min. The maxımum experımental adsorption capacities were 33.5 and 5.5 mg g−1, respectively. Kinetics followed the pseudo-second-order model, while isotherms fit the Langmuir model for RG-19 and the Freundlich model for CV. Hydroxyl, carbonyl, and polysaccharide groups participated in dye binding. Due to electrostatic attraction, H. scoparia prefers cationic CV over anionic RG-19, proving to be a low-cost, sustainable biosorbent.