Green Magnetic Adsorbent Functionalized with Humic Acid for Efficient Removal of Saxitoxin from Water

Water contamination by saxitoxin (STX), originating from harmful algal blooms intensified by anthropogenic eutrophication, poses a serious ecological and public health concern due to its potent neurotoxicity. This study developed a novel, eco-friendly magnetic humic acid-functionalized adsorbent (Fe3O4-HA) for STX removal from water. Fe3O4-HA was synthesized via co-precipitation, followed by humic acid (HA) functionalization. Successful synthesis was confirmed using Fourier-transform infrared spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy, and vibrating sample magnetometry, revealing the presence of key functional groups, heterogeneous morphology, elemental composition (C, O, Fe), and paramagnetic properties (23.385 emu/g). The point of zero charge (pH 2.5) indicated the surface neutrality threshold. Batch adsorption parameters—solution pH, adsorbent dosage, contact time, and STX concentration—were optimized. Maximum STX removal (71.04%) occurred at natural solution pH (7.94) using 30 mg adsorbent, within 8 min, at an initial STX concentration of 100 µg/L. Kinetic analyses demonstrated the adsorption followed a pseudo-second-order model (R² = 0.9983), indicating chemisorption as the dominant mechanism. Multiple interactions—including electrostatic attraction, hydrogen bonding, and π-π interactions between negatively charged functional groups in HA and STX—were proposed. An analytical eco-scale evaluation yielded a score of 81, classifying the synthesis as environmentally friendly. Comparison with existing methods confirmed that Fe3O4-HA offers superior adsorption efficiency, rapid action, and eco-sustainability. Thus, Fe3O4-HA represents a promising adsorbent for practical environmental remediation of STX-contaminated waters, warranting further studies for real-world scalability and applicability. Keywords adsorption, magnetized adsorbent, humic acid, paralytic shellfish toxin, saxitoxin