<p>This study presents the development and application of a novel eco-friendly composite, RSNC-CS@Fe₃O₄ (rice straw nanocellulose-chitosan crosslinked magnetic core–shell composite), for the efficient removal of heavy metals and the biocide glutaraldehyde from treated produced water in oil and gas fields. Unlike previous Fe₃O₄-cellulose-based adsorbents, this composite uniquely integrates rice straw-derived nanocellulose (RSNC), an abundant agricultural waste, with chitosan through crosslinking, enhancing adsorption capacity via abundant amine and hydroxy functional groups. The composite was synthesized by crosslinking rice straw-derived nanocellulose (RSNC) with chitosan (CS), followed by coating with magnetic Fe₃O₄ nanoparticles to facilitate magnetic separation and enhance reusability. The magnetic Fe₃O₄ nanoparticle coating not only facilitates rapid magnetic separation but also improves the composite’s stability and reusability, addressing common limitations in existing materials<b>.</b> The composite was deeply characterized using SEM, TEM, FTIR, XRD, and VSM, confirming its core–shell morphology, successful functionalization, and magnetic properties. Batch adsorption experiments were conducted under varying conditions, including pH (2–10), contact time (10–180 min), adsorbent dosage (0.1–1.5 g/L), and initial pollutant concentrations. The composite demonstrated high adsorption efficiencies of 95.2% for Pb²⁺, 92.8% for Cd²⁺, 90.5% for Cu²⁺, and 88% for glutaraldehyde. Adsorption behavior followed the Langmuir isotherm and pseudo-second-order kinetics, indicating monolayer chemisorption. Regeneration studies confirmed the material’s reusability over five cycles with minimal efficiency loss. These findings&#xa0;highlight the RSNC-CS@Fe₃O₄ composite as a sustainable, cost-effective, and superior alternative to conventional Fe₃O₄-cellulose adsorbents, offering promising potential for large-scale produced water treatment in the oil and gas industry.</p>