Divergent Ni(II) Removal Mechanism of Geopolymer Microspheres from Industrial Solid Wastes and Calcined Mineral Precursors

Abstract

With the continued growth of the electroplating industry, effective treatment of Ni(II)-containing wastewater has become a major environmental concern. Geopolymer microspheres (GS) are considered promising and inexpensive adsorbents for Ni(II) removal; however, the absence of standardized evaluation among various industrial solid wastes and calcined mineral precursors makes material selection difficult for practical engineering use. In this study, GS were prepared via suspension solidification using metakaolin (MK), fly ash (FA), and ground blast furnace slag (BFS) as precursor materials, while a 39.1 wt % KOH solution (18 g KOH in 28 g deionized water) as the alkaline activator. The prepared slurry was cured in silicon oil at 80 °C for 10 h. After filtration, washing, calcination, and sieving, the resulting microspheres within the particle size range of 50–200 μm, were obtained and used as adsorbents. The prepared samples were named KOH-MKGS, KOH-FAGS, and KOH-BFSGS, respectively. And the BET-specific surface area followed the order of KOH-MKGS < KOH-FAGS < KOH-BFSGS. Morphological observations indicated that KOH-MKGS, KOH-FAGS, and KOH-BFSGS exhibited surface features dominated by particle aggregates, microsphere clusters, and a 3D wrinkled structure, respectively. Under all tested adsorption conditions, the adsorption capacity remained in the order of KOH-MKGS < KOH-FAGS < KOH-BFSGS. In particular, KOH-BFSGS achieved the greatest static adsorption capacity (371.44 mg/g) and the highest dynamic adsorption capacity (538.84 mg/g). Kinetic and thermodynamic analysis demonstrated that the adsorption behavior was spontaneous, endothermic, and well described by a pseudo-second-order model, with intraparticle diffusion serving as the rate-controlling step. The improved performance of KOH-BFSGS was primarily associated with five aspects: (i) abundant surface Si–OH/Al–OH groups, (ii) Ca(II) assisted generation of stable Ni2(OH)2CO3 phases, (iii) a well-developed hierarchical pore structure, (iv) stronger electrostatic attraction, and (v) a heavy-metal-induced increase in surface area enhancement effect. Validation with stoichiometrically simulated precursors ruled out the influence of natural impurities, thereby further verifying the inherent superiority of the slag-based precursor. Integrating considerations of cost-efficiency (75–240 RMB/ton), mechanical robustness, and remediation capability, this research establishes a mechanistic guideline for the rational design of industrial solid-waste-based materials, providing a theoretical basis for precursor selection in heavy metal wastewater treatment.