Synthesis and multi-objective optimization of fly ash-slag geopolymer sorbents for heavy metal removal using a hybrid Taguchi-TOPSIS approach

This study develops and evaluates a highly effective geopolymer sorbent specifically designed for the removal of heavy metals from wastewater. Single and binary blends of fly ash and slag were utilized as the binding material. The geopolymer composite was activated using either sodium hy solely or in combination with sodium silicate. The Taguchi method was employed to design the geopolymer mixes, having four factors, each with four levels of variation. These factors included the fly ash-to-slag ratio (FA), binder content (BC), the molarity of the sodium hydroxide solution (M), and the sodium silicate-to-sodium hydroxide ratio (SS/SH). The performance of the geopolymer sorbent was rigorously assessed against a comprehensive set of criteria, including metal sorption capacity, setting time, flowability, density, compressive strength, abrasion resistance, water absorption, permeable voids, sorptivity, carbon footprint, and cost. The TOPSIS methodology was applied to aggregate the response criteria and determine the optimal mix for superior performance. The results showed that the optimum mix consisted of an FA of 33 %, BC of 1050 kg/m³, M of 10, and SS/SH of 3. A sensitivity analysis confirmed that changes in the weighting of the performance criteria did not significantly impact the proportioning of the optimum mix. Furthermore, the analysis of variance (ANOVA) revealed that the FA contributed the most to the sorbent's performance, followed by the SS/SH, BC, and M. The development of this geopolymer sorbent not only addresses a critical environmental concern but also offers great potential for practical applications in industries and municipalities worldwide.