<p>The escalating volumes of agricultural waste pose significant environmental challenges, while the demand for sustainable materials and water purification technologies continues to grow. This review synthesises recent advances in a promising circular economy approach: the use of alkaline treatment to transform lignocellulosic agricultural residues into versatile materials with dual functionality. The pathway of converting these treated residues into effective bioadsorbents for the removal of pollutants such as heavy metals and dyes from wastewater is critically evaluated. Simultaneously, the parallel pathway of using the same alkaline-treated biomass as a precursor for deriving biopolymers, such as cellulose nanofibers or regenerated cellulose films, is explored. A central theme of this review is the integrative analysis of how the chemical and structural modifications induced by alkaline treatment, primarily delignification and increased cellulose accessibility, unlock both adsorption capabilities and processability for biopolymer production. The efficacy of various adsorbents derived from different feedstocks, including rice husk, wheat straw, and sugarcane bagasse, is compared, and the potential of the extracted biopolymers for material applications is discussed. Finally, key knowledge gaps and future research priorities are identified, including techno-economic analysis, life-cycle assessment, and strategies for scaling integrated biorefineries that can simultaneously produce both bioadsorbents and biopolymers from a single waste stream.</p>