Competitive sorption behaviors and mechanisms of Pb2+, Zn2+, Cd2+, and Ni2+ onto metakaolin- and fly ash-based geopolymeric sorbents

Efficient removal of multiple heavy metals (HMs) from industrial wastewater remains challenging due to the coexistence and competition of ions. This study comparatively evaluates metakaolin-based (MK-GP) and fly ash-based (FA-GP) geopolymers synthesized at a constant Si-modulus (1.4), aiming to clarify their sorption performance and competitive mechanisms toward Pb 2+ , Zn 2+ , Cd 2+ , and Ni 2+ . Comprehensive characterization revealed that MK-GP sorbents developed a more reactive aluminosilicate framework and a four-fold larger surface area than FA-GP sorbents, enabling markedly higher sorption capacities. Batch experiments across mono-, binary-, ternary-, and quaternary-metal systems demonstrated a consistent affinity sequence of Pb 2+  > Zn 2+  > Cd 2+  > Ni 2+ , with pronounced competitive suppression in multi-ion environments. Column transport experiments further confirmed the preferential retention of Pb 2+ and the accelerated breakthrough of Zn 2+ , Cd 2+ , and Ni 2+ under flow conditions. Mechanistic analysis indicated that sorption is governed by synergistic surface complexation and pore-controlled diffusion, influenced by hydrated ionic radii and hydrolysis behavior. Overall, this work provides a systematic comparison of two widely used GP sorbents and offers mechanistic insights that can inform the rational design of GP-based technologies for complex industrial wastewater treatment.