Waste treat waste: Alginate calcium versus alginate acid gels in upcycling waste cotton linter as composite biosorbent

Transforming waste materials into biosorbent offers a valuable solution for cost-effective water decontamination while simultaneously promoting waste upcycling. This study focused on the gelation of alginate calcium (AC) and alginate acid (AA) in recycling waste cotton linters (WCL) to create composite hydrogels for dye removal. Both hydrogel systems effectively incorporate WCL, displaying an impressive 80 wt% WCL entrapping capacity in the composite hydrogels. The presence of WCL leads to a fascinating volume expansion phenomenon, particularly noticeable in AC composite systems after freeze-drying. Additionally, the composite hydrogels demonstrate efficient adsorption of methylene blue (MB), ranging from 5 to 22 mg/g in MB dye solution (30 mg L−1), and exhibit good compatibility with various adsorption kinetic models. However, AC composite systems outperform AA composite systems and display higher MB adsorption capacity, faster adsorption rate, and more consistent performance. Mechanistic studies reveal that electrostatic forces contribute to volume expansion and dye adsorption, while the denser crosslinking structure and more abundant carboxylate groups in the AC composite system contribute to its superior performance. Our current study not only establishes the criteria for selecting proper alginate system as bio-composite hydrogel matrices, but also provides a sustainable “waste treat waste” upcycling strategy.