Heavy metal ion contamination in aquatic systems presents substantial environmental and public health challenges, demanding innovative remediation solutions. This study reports the synthesis of phosphorylated cellulose (PC) monoliths via thermally induced phase separation (TIPS) as a sustainable solution for heavy metal removal. Through systematic optimization of phosphorylation degree (ranging from 16.3% to 49.5%) and pore architecture, we developed materials with exceptional adsorption capacity and flow characteristics. Comprehensive characterization (SEM, FTIR, and elemental analysis) confirmed the successful incorporation of the phosphate group while revealing tunable three-dimensional porous structures controlled by cellulose acetate concentration (80–120 mg mL⁻¹) and phosphorylation parameters. Optimal Cu²⁺ adsorption occurs at 43.9% phosphorylation, coupled with stable permeability, under continuous flow conditions. The monolith effectively removes heavy metal ions (e.g., Cu²⁺, Ni²⁺, Cd²⁺) across a wide pH range, driven by electrostatic and hard-soft acid-base interactions. Additionally, the material maintains an absorption capacity of over 90% after multiple regeneration cycles. These findings highlight the potential of PC monoliths as cost-effective, scalable, and environmentally friendly adsorbents for heavy metal ion removal in water treatment applications.