Ziziphus nuts are abundant in Khuzestan province, Iran, and are considered as an unwanted natural biomass waste. The present study aimed to develop low-cost activated carbon from Ziziphus nuts as a new precursor for phosphate removal from the water environment. Iron oxide modification was performed to simultaneously facilitate adsorbent separation via a simple magnetic process and increase phosphate removal capacity. The iron oxide/activated carbon composite (IOAC) was characterized using XRD, EDX, SEM, and BET methods. The specific surface area for IOAC reached 569.41 m2/g, comparable to that of commercial activated carbon. While other similar biomass-derived activated carbons reached phosphate removal capacity around 15 mg/g, IOAC demonstrated excellent phosphate removal performance as high as 27 mg/g. Also, IOAC showed fast adsorption kinetics, achieving equilibrium in only 60 minutes. On the basis of the results, the pseudo-second-order kinetic model was more consistent with the phosphate adsorption data onto the adsorbent than the pseudo-first-order model. The adsorption results were interpreted using Langmuir, Freundlich, and Webber-Morris diffusion models. The maximum Langmuir adsorption capacity was calculated to be 27 mg/L. The adsorbent was removed from the aqueous solution via a simple magnetic process.