Biochar‐Reinforced Alginate‐Guar Gum Hydrogels: Composition‐Dependent Swelling and Sorption of Cadmium Ions and Diuron

This study focuses on the development of bio‐based composite hydrogels (HGs) from an alginate/guar gum matrix reinforced with biochar (BC) and cross‐linked with Ca ²⁺ ions for the removal of inorganic and organic pollutants. The effects of polymer ratio, cross‐linker concentration, and BC incorporation on swelling and sorption toward Cd(II) ions and diuron herbicide were systematically investigated. The Fourier transform infrared (FTIR) spectroscopy confirmed successful network formation and cross‐linking. The swelling ratio of BC‐filled HGs varied from 20 to 177 g g ⁻¹ , depending on composition. Increasing guar content, Ca ²⁺ concentration, and BC loading reduced swelling and Cd(II) uptake. The HGs exhibited pH‐responsive behavior, with swelling increasing at higher pH values. The effect of electrolyte concentration on swelling shifted from a monotonic decrease at low cross‐linker concentrations to a nonmonotonic swelling behavior at high concentrations. Increasing guar fraction extended the time needed to reach swelling equilibrium. The highest Cd(II) sorption capacity (150 mg g ⁻¹ ) was observed for pure alginate HG, while BC incorporation enhanced diuron uptake (from 3.1 to 4.1 mg g ⁻¹ ). The opposite pH‐dependent sorption trends for Cd(II) and diuron suggested distinct binding mechanisms. Low desorption rates confirmed strong affinity, highlighting the potential of these tunable, sustainable composites for environmental remediation.