Abstract: Nanostructured ZnFe₂O₄ was synthesized by the heat treatment of a mechanically activated mixture of ZnO/α-Fe₂O₃. X-ray diffraction (XRD) and differential thermal analysis (DTA) results demonstrated that, after 5 h of the mechanical activation of the mixture, ZnFe₂O₄ was formed by heat treatment at 750°C for 2 h. To improve the characteristics of ZnFe₂O₄ for adsorption applications, the chemical activation process was performed. The 2 h chemical activation with 1 mol·L⁻¹ HNO₃ and co-precipitation of 52%−57% dissolved ZnFe₂O₄ led to an increase in the saturated magnetization from 2.0 to 7.5 emu·g⁻¹ and in the specific surface area from 5 to 198 m²·g⁻¹. In addition, the observed particle size reduction of chemically activated ZnFe₂O₄ in field emission scanning electron microscopy (FESEM) micrographs was in agreement with the specific surface area increase. These improvements in ZnFe₂O₄ characteristics considerably affected the adsorption performance of this adsorbent. Adsorption results revealed that mechano-thermally synthesized ZnFe₂O₄ had the maximum arsenic adsorption of 38% with the adsorption capacity of 0.995 mg·g⁻¹ in a 130 mg·L⁻¹ solution of As(V) after 30 min of agitation. However, chemically activated ZnFe₂O₄ showed the maximum arsenic adsorption of approximately 99% with the adsorption capacity of 21.460 mg·g⁻¹ under the same conditions. These results showed that the weak adsorption performance of mechano-thermally synthesized ZnFe₂O₄ was improved by the chemical activation process.