<p>A novel magnetic diatomite–carboxymethyl cellulose (MDM–CMC) nanocomposite was synthesized using co-precipitation followed by solution blending methods with carboxymethyl cellulose (CMC). The synthesized material was thoroughly characterized using XRF, FTIR, XRD, BET, and SEM. The MDM–CMC nanocomposite was evaluated in Cd(II) remediation from wastewater. The adsorption parameters namely pH, adsorbent dose, pollutant concentration, and contact duration have been optimized using Box–Behnken design. MDM–CMC showed a maximum monolayer adsorption capacity of 86.12&#xa0;mg/g for Cd(II) in the aqueous solutions under optimal conditions. MDM–CMC has demonstrated selectivity for Cd(II) than other competing ions Na⁺, Ca²⁺, and Mg²⁺, with significant inhibitions by Mg²⁺. Adsorption kinetic studies revealed that pseudo-second-order provided the best fit to the experimental data. An equilibrium adsorption study confirmed that the Freundlich isotherm best represented the equilibrium data with the highest <i>R</i>² of 0.981 and smallest <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\chi^{2}\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi>χ</mi> <mn>2</mn> </msup> </math></EquationSource> </InlineEquation> of 1.45, whereas the thermodynamic analysis of the experimental data indicated that the adsorption process was spontaneous and exothermic. The regenerated MDM–CMC adsorbent demonstrated a 78.87% adsorption removal efficiency, with a magnetic saturation of 1.2&#xa0;emu/g after six adsorption–desorption cycles, indicating its reusability and ease of magnetic separation. These findings highlight the potential of MDM–CMC as economically feasible and ecologically friendly composite offers a practical solution for handling wastewater while maintaining access to clean water.</p>