Multiple removal mechanism of EDTA-Enteromorpha-Chitosan-based hydrogel for organic/inorganic cation-anion complex...

Organic/inorganic anion and cation composite pollution is prevalent in the environment, and clarifying the mechanism of functional hydrogels in their integrated removal process is crucial for environmental remediation. This study synthesized a novel EDTA-modified Enteromorpha-Chitosan-based hydrogel (EDTA-EP-HD) by suspension polymerization. It investigated the adsorption mechanisms and mutual interactions for the removal of Congo red (CR), hexavalent chromium (Cr(VI)), and divalent cadmium (Cd(II)). The compositional and structural properties were analyzed before and after reactions through characterization, revealing a porous structure with enhanced functional groups (-COOH, -NH₂) for efficient adsorption. The maximum adsorption capacities reached 189.35 mg/g for Congo red at pH 8, 23.40 mg/g for Cr(VI) at pH 1, and 56.58 mg/g for Cd(II) at pH 3. The thermodynamic parameter ∆G<0 indicated spontaneous reactions, while kinetics and isotherms aligned with pseudo-second-order and Langmuir models, indicating chemisorption-dominated monolayer adsorption. The adsorption mechanisms involved electrostatic attraction, hydrogen bonding, and van der Waals forces for Congo red; and electrostatic adsorption, oxidative reduction, ion exchange reduction, and complexation for Cr(VI) and Cd(II). In the composite system, Congo red and Cr(VI) exhibited competitive inhibition. In contrast, Congo red and Cd(II) showed no interaction, and Cr(VI) and Cd(II) demonstrated synergy, due to anions on electrostatic and coordination. This study provided a reference for selecting efficient gel materials and removing specific complex pollution, thereby promoting the efficient utilization of marine Enteromorpha.