In this study, the mechanochemical treatment with and without reagent (citric acid) was presented as a new modification method to improve the adsorptive capacity of corncob in the removal of Cu2+, Pb2+ and Zn2+ heavy metal ions from aqueous solutions. A high-energy planetary ball mill was used for modification. The parameters affecting both reagent-assisted (CA-MCM)/non-reagent (MCM) modifications and adsorption effectiveness were investigated via the response surface methodology approach. As a result of optimisation, over 95% heavy metal removals were achieved for both modified materials. To explain the adsorption mechanism; isothermal, kinetic, thermodynamic and activation parameters were examined. The pseudo-second-order and Langmuir isotherm models were found to be appropriate for all adsorbents and heavy metals. According to the thermodynamic parameters, the adsorption processes were spontaneous and endothermic. Desorption studies were conducted to determine the reusability of the adsorbents. Ternary adsorption system of metals was also investigated. Characterisation studies were carried out using Fourier Transform Infrared Spectrometer (FTIR), Scanning Electron Microscopy (SEM), Brunauer-Emmett-Teller (BET), and surface analysis (point of zero charge (pH(pzc)), Boehm titration) to investigate the effects of modifications on the adsorbents' surface and chemical structure. In addition, the financial advantages of the modification methods were demonstrated by alternative selection and cash flow analyses. All the results revealed that this method as a low-cost and eco-friendly alternative modification treatment to increase the adsorption efficiency of agricultural waste of similar nature.