Heavy metal contamination is a critical environmental issue, often involving complex multicomponent systems. Swelling brittle micas, a family of designer sorbents, have demonstrated exceptional heavy metal removal capabilities, yet their behabior in competitive adsorption systems remains largely unexplored. This study systematically investigates the simultaneous uptake of Pb2*, Cd2*, and Hg2* on both as-synthesized brittle mica and its thiol-functionalized counterpart. Using X-ray diffraction (XRD) and nuclear magnetic resonance (NMR), we reveal critical structural transformations occurring at both short-and long-range scales during adsorption. Our findings demonstrate that competitive adsorption governs metal uptake, leading to a reduction in total adsorption capacity compared to single-metal systems. However, selectivity toward specific metal cations remains unchanged, irrespective of competing species or surface functionalization. Overall, this study not only improves our understanding of heavy metal adsorption but also paves the way for more effective and sustainable sorbent design in environmental remediation.