<p>In this study, biochar (BC) derived from pistachio pomace was loaded with nano zero-valent zinc (nZVZ). Subsequently, BC-nZVZ-SC and BC-nZVZ-CMC composites were synthesized using the complexing agent’s sodium citrate (SC) and sodium carboxymethyl cellulose (CMC) to enhance cadmium (Cd²⁺) removal from aqueous solutions. The composite morphological structures were characterized by FTIR, SEM–EDX, BET, and XRD analyses. The influence of key parameters, including pH (3–11), adsorbent dosage (0.5–2.5 g L⁻¹), contact time (20–100 min), and initial concentration Cd²⁺ (25–125 mg L⁻¹), on the adsorption performance of the synthesized composites was investigated and optimized using central composite design (CCD)-based response surface methodology (RSM). According to the results, SC and CMC can provide active sites and increase the removal percentage of Cd²⁺ ions. In addition, the order of the performance of the adsorbents was as follows: BC-nZVZ-CMC &gt; BC-nZVZ-SC &gt; BC-nZVZ &gt; BC. The proposed CCD-RSM model accurately predicted the experimental results for Cd, with high <i>R</i>² values (&gt; 0.99). The maximum adsorption capacity of BC-nZVZ-CMC toward Cd²⁺ was 166.7 mg g⁻¹ under the optimum values of Cd²⁺ concentration (60 mg L⁻¹), stirring time (60 min), pH (7), and adsorbent dosage (1.5 g L⁻¹). The adsorption of Cd²⁺ onto BC-nZVZ-CMC was well described by the pseudo-second-order kinetic (<i>R</i>² &gt; 0.94) and Langmuir isotherm models (<i>R</i>² &gt; 0.99). Also, the BC-nZVZ-CMC was reusable for at least four cycles. The results demonstrated that the integration of CMC and SC complexation with the BC-nZVZ composite offers a promising approach for the enhanced removal of Cd from aqueous solutions.</p>