Alkali-activated materials from steel industry slags: optimization of prewashing and its effect on zinc adsorption and regeneration

Abstract

Interest in alkali-activated materials (AAMs) has increased because of their effectiveness as adsorbents and their low cost. An additional advantage of AAMs is that raw materials obtained from industrial side streams can support a circular economy. In this study, slags from the steel industry were used as raw materials for an AAM. As the material was highly alkaline, prewashing was mandatory before adsorption studies to avoid precipitation of hydroxides. As washing agents, different concentrations of several chemicals (strong and weak acids) were used. Among them, oxalic acid performed the best by stabilizing the pH near neutral during adsorption and minimizing mass loss. Thus, using oxalic acid, prewashing was optimized in relation to the time and concentration. In adsorption experiments, the adsorption capacity was 78 mg g⁻¹ for zinc, which was 18 times higher than that without optimization. The AAM adsorbent was also regenerable with oxalic acid because its adsorption efficiency remained stable in the next adsorption cycle. The AAM was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray fluorescence (XRF), field emission scanning electron microscopy with energy dispersive X-ray spectrometry (FESEM-EDS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Analyses showed how prewashing affected the surface structure and functional groups in different process stages. The adsorption mechanism was determined to be complex formation. This study not only revealed more environmentally friendly options for material preparation but also showed how to improve environment quality with more advanced material preparation and process optimization methods.