Insights into the highly selective and efficient adsorption of Pb2+ by fish skin collagen-enabled sodium alginate-based composite gel spheres: adsorption and interference mechanisms

Abstract

Excessive lead content in wastewater and seaweed products is an urgent scientific problem to be solved. Sodium alginate (SA) is often used as a synthetic gel material to adsorb Pb²⁺ due to its good biocompatibility and affinity for metal ions. However, simple SA gels suffer from poor mechanical stability and limited adsorption capacity for Pb²⁺. Therefore, there is a need to develop a stable composite SA gel material with high adsorption capacity for Pb²⁺. Fish skin collagen (FC), as a by-product, is not highly utilized at present, and its own rich amino and carboxyl groups, is a natural and environmentally friendly food-grade adsorbent alternative material. Sodium carboxymethyl cellulose (CMC), a polysaccharide rich in carboxyl groups, is also a good raw material for the preparation of new and safe composite SA gels. Therefore, in this study, a green and non-toxic CMC-FC-SA composite gel sphere was developed to adsorb Pb²⁺ efficiently by physically mixing SA/CMC/FC in the framework of a one-pot method with Ca²⁺ cross-linking. The prepared porous composite gel spheres not only had good swelling and thermal stability, but also showed excellent specific adsorption capacity for Pb²⁺. The results revealed that the adsorption process of CMC-FC-SA on Pb²⁺ was more consistent with the proposed secondary kinetic model and Langmuir isothermal adsorption model, suggesting that the adsorption process was governed by monolayer chemisorption. The maximum theoretical value of Qm for Pb²⁺ adsorption was determined to be 586.56 mg/g, and the thermodynamic results indicated that the adsorption of Pb²⁺ was an unprompted exothermal reaction. However, the adsorption mechanism of the composite spheres is not yet clear. Therefore, the adsorption mechanism of Pb²⁺ on CMC-FC-SA was revealed to be dominated by ion-exchange reactions and ligand interactions, supplemented by physical adsorption by characterization such as FTIR and XPS. Notably, the CMC-FC-SA spheres displayed good removal efficiency (>88 %, under the Pb²⁺ residue limit of GB.) for Pb²⁺ in the five seaweed concentrates and could maintain the quality of seaweed concentrates. The effect of coexisting organic matter fractions in seaweed concentrates on adsorption has not been evaluated. Therefore, interference tests revealed that SA, phlorotannins and fucoxanthin antagonized Pb²⁺ adsorption through hydrogel diffusion blocking, hydrophobic cluster competition for coordination and micelle adsorption, respectively. While mannitol, glutamate and aspartate enhanced the removal rate by solubilization via -OH/-COOH complexation, synergistic mass transfer via bidentate chelation and hydrophilic optimization. The green strategy proposed in this study provides an efficient adsorbent for treating water and regulating heavy metal content in seaweed products.