Biochar–Hydrogel composites as multifunctional materials for environmental remediation and circular resource recovery

• BC–HG composites studied as multifunctional materials. • Links feedstock, polymer chemistry, and synthesis to performance outcomes. • Covers pollutant removal, nutrient recovery, and climate resilience roles. • Integrates kinetics, isotherms, and thermodynamics with application insights. • Positions BC–HGs within the circular bioeconomy for waste and nutrient cycling. • Identifies gaps in regeneration, scalability, and waste-derived feedstock risks. Soil degradation, water contamination, nutrient depletion, and climate change are interlinked global challenges that demand multifunctional, sustainable solutions. Conventional remediation methods such as activated carbon adsorption, coagulation–flocculation, and advanced oxidation are often costly, energy-intensive, and limited in scalability. Biochar–hydrogel (BC–HG) composites have recently emerged as promising materials that integrate the high porosity, surface functionality, and carbon sequestration potential of biochar with the water-retention and controlled-release properties of hydrogels. This review critically synthesizes advances in BC–HG research by linking substrate choice, synthesis techniques, and physicochemical characterization with performance outcomes across environmental and agricultural applications. This review further highlights how synthesis-driven composite architecture governs dominant adsorption pathways, enabling design-oriented optimization beyond model-based interpretation. Mechanistic insights from adsorption kinetics, isotherm modeling, and thermodynamics are integrated to explain the composites’ ability to remove heavy metals, dyes, pharmaceuticals, and emerging contaminants, while simultaneously supporting nutrient recovery, soil fertility, drought resilience, and greenhouse gas reduction. Compared to standalone biochar or hydrogels, BC–HGs exhibit enhanced mechanical stability, swelling behavior, and pollutant affinity, positioning them as multifunctional platforms for remediation and sustainable resource use. However, challenges remain in scaling up synthesis, improving regeneration efficiency, ensuring stability under real-matrix conditions with competing ions and natural organic matter, and addressing risks from waste-derived feedstocks. Framing BC–HGs within the circular bioeconomy this review highlights their potential to valorize waste resources, recycle nutrients, and contribute to carbon management.