Mechanically Stable Algal Biochar‐Alginate Hydrogels as Bio‐Based Adsorbents for Resource‐Efficient Heavy Metal Removal

Bio‐based materials that valorize waste streams while enabling resource conservation are central to advancing circular and sustainable environmental solutions. This study reports the development of ionically crosslinked biochar‐encapsulated hydrogels (Bio‐H) as sustainable adsorbents for the removal of heavy metals from contaminated water systems. Cyanidioschyzon merolae ‐derived biochar, produced by hydrothermal liquefaction of biomass was incorporated into calcium‐crosslinked sodium alginate using a two‐step gelation process, creating a multifunctional, biodegradable composite. Swelling behavior demonstrated that biochar incorporation significantly reduced the initial contraction observed in pristine alginate hydrogels and enabled continuous, monotonic swelling, achieving equilibrium swelling ratios of 1.16 within 7 h, substantially faster than pristine systems. The mechanical integrity of the Bio‐H was assessed by dynamic mechanical analysis (DMA). The storage modulus (G′) exceeded the loss modulus (G″) across the entire 0.1–100 Hz range with no crossover, reaching a plateau of about 372 kPa, confirming a stable, elastically dominant crosslinked network. Scanning electron microscopy revealed an open macroporous network that became textured with particulate deposits after adsorption, with EDS confirming Cu and Fe uptake. Batch adsorption experiments showed that maximum adsorption capacities were achieved at pH 4 and 298 K. The maximum equilibrium adsorption capacity for Cu 2+ was 136 mg/g, whereas for Fe 3+ , the adsorption capacity was observed to be 1.25 mg/g. The M 5 model suggests that the adsorption of both metals proceeds through a physical multilayer mechanism, which involves heterogeneous binding sites and sequential stacking of metal ions. The Bio‐H still removed the metal ions after the 3rd cycle of adsorption‐regeneration.