The Individual and Simultaneous Adsorption of Co(II) and Ni(II) Onto Directly Alkaline-Activated Steel Slag

Abstract

Steel slag activated directly with alkaline treatment (AAS) was utilized for the individual and simultaneous removal of Co²⁺ and Ni²⁺ from the aqueous solutions. The characteristics of AAS were analyzed using x-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), SEM, and EDS techniques. The results revealed that AAS had a crystalline structure of calcium silicate hydrate (CSH). The optimal conditions for removing Co²⁺ and Ni²⁺ were found at pH 6, with a contact time of 120 min for individual adsorption and 150 min for simultaneous adsorption. The Langmuir model indicated that the maximum adsorption capacities for individual adsorption were 108.6 mg/g for Co²⁺ and 111.7 mg/g for Ni²⁺. However, in simultaneous adsorption, competition between the metal ions reduced the adsorption capacity, with maximum removal capacities of Co²⁺ and Ni²⁺ on AAS calculated as 58.2 and 72.3 mg/g, respectively. The adsorption of Co²⁺ and Ni²⁺, both individually and simultaneously, best conformed to the Freundlich isotherm model and pseudo-second-order kinetic model. In both the individual and simultaneous systems, the adsorption capacities followed the order of Ni²⁺ > Co²⁺. However, the adsorption process of Ni²⁺ onto AAS was more sensitive than that of Co²⁺, as evaluated by analyzing the effect of Co²⁺:Ni²⁺ concentration ratios on the adsorption process. The potential mechanisms of Co²⁺ and Ni²⁺ removal by AAS included cation exchange, complex formation with surface-active groups, precipitation of new insoluble substances on the adsorbent surface, and competition between metal ions during simultaneous adsorption. The high adsorption efficiency can be attributed to the release of Ca²⁺ and OH⁻ from the CSH crystals. These results suggest that AAS is a promising adsorbent for the removal of Co²⁺ and Ni²⁺ from water.