Green synthesis of network nanostructured calcium alginate hydrogel and its removal performance of Cd2+ and Cu2+ ions

In this work, we present a simple and green preparation method for the synthesis of three-dimensional (3D) network nanostructured sodium alginate (NNSA) hydrogel. Such special structure derives from the self-weaving of sodium alginate nanowires (SA NWs) with diameters of <30 nm rather than the introduction of divalent cations, binders, and skeleton supports. By the appropriate cross-linking of the NNSA hydrogel and Ca2+ ions, calcium alginate (CA) hydrogel with the same morphology and structure can be easily prepared in large quantities and the stability of the 3D network nanostructures in aqueous solution is evidently improved. To evaluate the adsorption performances of the 3D network nanostructured calcium alginate (NNCA) hydrogel, we select heavy metal ions Cd2+ and Cu2+ as the model inorganic contaminants in aqueous solutions. The effects of dosage, pH, initial concentration, contact time, and temperature on the adsorption performances are investigated. The testing results show relatively high removal efficiency, rapid adsorption rate, and small absorbent dosage. The adsorption behavior of the NNCA hydrogel matches well with the Langmuir isotherm model and pseudo-second-order kinetics model. Besides, adsorption thermodynamics and adsorption mechanism are also studied, and the results show diffusion-controlled process and bidentate bridging (BB)/bidentate chelating (BC) coordination modes, respectively.

Automated summary

In this study, a simple and green method for synthesizing three-dimensional network nanostructured sodium alginate hydrogel was presented, which could be easily cross-linked with Ca2+ ions to prepare calcium alginate hydrogel with the same morphology and structure. The NNCA hydrogel exhibited high removal efficiency, rapid adsorption rate, and required a small amount of absorbent. Adsorption behavior of NNCA hydrogel followed the Langmuir isotherm model and pseudo-second-order kinetics model, with diffusion-controlled process and bidentate bridging/bidentate chelating coordination modes as the adsorption mechanism.