In this investigation, microcrystalline cellulose (MCC) was functionalized with metformin HCl using (3-chloropropyl)triethixysilane (CPTES) as a coupling agent. Polyethersulfone (PES) membranes were incorporated with different concentrations of modified MCC (MMCC) to enhance its affinity for heavy metals during filtration of aqueous solutions. The composite membranes were characterized via fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), atomic force microscopy (AFM), Brunauer–Emmett–Teller (BET) method, porosity and contact angle measurements and mechanical analysis. The presence of MMCC in the host matrix was confirmed by FTIR. Although composites decomposed at lower temperatures, their thermal stability was sufficient to meet their performance requirements. DSC showed enhanced glass transition temperature (Tg) due to the interfacial interactions between membrane constituents which restrict the mobility of polymer chains. Microscopic imaging revealed higher surface roughness of composites compared to neat PES. Inclusion of MMCC increased the porosity and hydrophilicity of the membrane which consequently, higher permeability can be achieved.