In this study, the adsorption properties of a composite material consisting of polyacrylamide, an inert polymer, and an extract obtained from the water-soluble part of a green walnut shell were investigated for Hg(II) ions. SEM, EDX, FTIR, and PZC analyses were performed to characterize the newly synthesized material. SEM and EDX analyses confirmed that the surface of the synthesized adsorbent became softer and smoother after adsorption, indicating the presence of Hg in its elemental composition. FTIR analysis showed that mercury enters the structure through chemical interactions, and there are changes in bond vibration frequencies in the presence of Hg(II). According to the PZC point analysis, the point at which the surface charge was zero was found to be pH 4. The Langmuir model was used to calculate the adsorption capacity after investigating the effect of concentration on adsorption. The adsorption capacity was found to be 1.808 molkg−1 (362,67 mgg−1) from the Langmuir model, which is very high compared to similar adsorbents. PFO model was used to explain the adsorption kinetics and very fast adsorption kinetics were observed. The adsorption entropy increased, free enthalpy of adsorption was negative, and heat of adsorption was in the energy-consuming direction.