<p>The co-contamination of arsenite (As^III) and cadmium (Cd²⁺) poses a significant challenge in environmental remediation due to their divergent chemical speciation and transformation pathways. Here, a redox-active layered double oxide (MgMn-LDO) is designed, and achieves adsorption capacities of 821.7 mg g^‒1 for As^III and 1895.6 mg g^‒1 for Cd²⁺ in their coexisting system, presenting a state-of-the-art performance. Unexpectedly, the co-adsorption system not only enhances individual adsorption capacities but also accelerates the adsorption rate by 181‒fold compared to single-component systems. The LDO undergoes a four-stage spatiotemporally ordered topological transformation, which effectively decouples the oxidation of As^III from the adsorption of Cd²⁺ and reverses the conventional competitive sequence. This stepwise mechanism ensures preferential oxidation of As^III to As^V, followed by the alteration of Cd²⁺ adsorption pathway to isomorphous substitution, expanding diffusion pathways and accelerating As immobilization. Large-scale experiments demonstrate this material’s potential in synergistic remediation As/Cd in mining wastewater and contaminated soils.</p>