Facile fabrication of amphoteric SA-S@PDA-H hydrogel microspheres with acid-substitution for enhanced adsorption of cationic and anionic dyes

The coexistence of cationic and anionic dyes in wastewater necessitates the development of amphoteric adsorbents. Herein, amphoteric hydrogel microspheres SA-S@PDA-H was successfully fabricated for their simultaneous removal. The microspheres were synthesized by crosslinking sodium alginate (SA) with Ca²⁺, followed by coating with polydopamine (PDA) and stearyl trimethyl ammonium chloride (STAC). The microspheres prepared through acid treatment to replace STAC was labeled SA-S@PDA-H, endowing the microspheres with enhanced adsorption capacity. FT-IR, SEM, zeta potential was utilized to characterize the structure and morphology of SA-S@PDA-H. The adsorption behavior and mechanism of SA-S@PDA-H for cationic and anionic dyes was investigated in depth. The results show SA-S@PDA-H exhibit 127.3 % of swelling ratio. The adsorption of SA-S@PDA-H for MB and MO reaches equilibrium at about 300 min followed by PSO model. The adsorption process of SA-S@PDA-H for MB and MO followed Temkin and Dubinin-Radushkevich model, respectively, and the maximum adsorption capacity displayed 869.1 mg/g at 45°C and 730.9 mg/g at 15°C, for MB and MO, respectively. MB adsorption increased with higher temperature and pH, while MO adsorption increased with lower temperature and pH. Adsorption of both dyes decreased at high ionic strength, and Cu 2 + and Fe 3+ had a significant inhibitory for MB than K + , while the inhibitory effects of SO 4 2- or CO 3 2- is more obvious for MO than Cl - . SA-S@PDA-H exhibited excellent adsorption performance for mixed cationic and anionic dyes through synergistic effects including electrostatic interactions, hydrogen bonds, π-π stacking and micropore filling. The microspheres maintained over 80 % removal efficiency after five consecutive adsorption-desorption cycles.