Novel Ca–Ti-modified alkali-activated metakaolin adsorbent: multimetal adsorption and regeneration mechanisms

Abstract

Adsorbents such as activated carbon and ion exchange resins have several limitations, including high operational and regeneration costs. These drawbacks have prompted the search for alternative adsorbent materials that offer benefits such as cost-effectiveness, chemical stability, safe regenerability, and minimal waste generation. Alkali-activated materials (AAMs) have emerged as a promising solution, especially when engineered into larger forms—such as casted columns—via alkali-activation manufacturing. This approach not only broadens their applicability across various processes but also enhances surface area and porosity, thereby improving adsorption performance. In this study, titanate-modified metakaolin was cast into a column, achieving multimetal adsorption capacities of 13.4, 32.3, 43.3, 49.0, 52.8, 54.0, 61.8, and 66.6 mg g⁻¹ for Li, Ni, Co, Zn, Mn, Cu, Cd and Pb, respectively. The regeneration ability of AAM adsorbent was demonstrated through 31 consecutive adsorption–desorption cycles. A novel regeneration chemical, 0.5 M citric acid (pH 6.6), exhibited exceptional regeneration potential without compromising the mechanical strength of the AAM—an issue commonly encountered with other regeneration chemicals. The removal efficiency remained above 95% throughout all cycles, indicating only a 4% reduction in adsorption performance. Both adsorption and regeneration mechanisms were proposed in this study. The AAM was characterized using X-ray diffraction (XRD), X-ray 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). The sustainability and economic viewpoint of the process was studied through a life-cycle assessment (LCA) method.