In recent years, research on the widespread use of biodegradable polymers, such as sodium alginate (SA), in removing pollutants from water sources has shown significant growth. The present study aims to evaluate the efficiency of SA hydrogels cross-linked with ferric chloride (for mechanical strength) in optimizing the removal of nickel (Ni) from aqueous solutions. Various factors, including Ni initial concentration, solution pH, amount of adsorbent, and contact time, were considered using response surface methodology (Box-Benken model). Based on the results of FTIR and SEM analyses, activated SA hydrogels were found to have hydroxyl and carboxyl functional groups, and their surface morphology was non-flat and prominent. The Ni removal values in the presence of SA hydrogels varied, with the minimum and maximum removal percentages being 53.39% and 85.43%, respectively. Increasing the initial Ni concentration resulted in a decreasing trend in Ni removal, while increasing contact time, solution pH, and amount of adsorbent led to an increasing trend in Ni removal. The evaluation of the data obtained from Ni removal using the Box-Benken model showed an accurate prediction (R² higher than 0.98%) in optimizing Ni removal from aqueous solutions in the presence of SA. The possible mechanisms of Ni removal by SA hydrogel were identified as the formation of Van der waals bonds (physical adsorption), interaction with carboxylate anions, chelate formation between Ni ions and hydroxyl functional groups, and electrostatic interactions (due to the negative surface charge of SA). Overall, the results of this study demonstrate that SA has a suitable ability to remove Ni from aqueous solutions. However, future studies should investigate, compare, and integrate other activation methods, such as physical activation (combination with inorganic substances) and chemical activation (surface grafting) on the SA polymer, to improve the removal efficiency of heavy elements using this polymer.