Porous alginate hydrogel beads with spatially separated acidic and basic moieties for selective adsorption of dyes

Currently, obtaining an efficient dual-functional adsorbent for the treatment of complex wastewater containing multiple coexisting anionic and cationic pollutants remains a challenge. In this study, sulfonated waste tire rubber powder (S-WTR) was employed as the cationic binding site, while polyethyleneimine crosslinked with sodium alginate (PS) served as the anionic binding site. Finally, nanoporous CaCO 3 was introduced for pore formation, resulting in the development of S-WTR @ PS sodium alginate composite porous hydrogel beads (S-WTR@PS/SA) with distinct adsorption site spatial separation characteristics. S-WTR@PS/SA enable rapid surface charge conversion based on pH, facilitating the selective removal of dyes, and exhibit outstanding adsorption performance. At pH = 9, it achieves maximum adsorption capacities of 480.7 mg/g for Methylene Blue (MB) and 1256.28 mg/g for Crystal Violet (CV), while at pH = 2, it reaches respective maximum adsorption capacities of 840.34 mg/g for Acid Red-73 (AR-73) and 869.86 mg/g for Acid Yellow-17 (AY-17). Recycling experiments indicated that S-WTR@PS/SA could be reused more than five times, confirming its robust reusability and structural stability. Combining characterization methods such as FT-IR and XPS, the adsorption mechanisms of S-WTR@PS/SA were explored. The primary mechanism involves electrostatic interactions, complemented by hydrogen bonding and π-π stacking interactions. Ultimately, fixed-bed adsorption experiments were carried out to evaluate the practical application capability of S-WTR@PS/SA. It was demonstrated that S-WTR@PS/SA maintained its structural integrity and excellent adsorption capacity during the continuous adsorption of various dyes, further confirming its potential for industrial applications.