Unraveling the role of lattice incorporation in enhanced co-removal of trivalent arsenic and divalent cadmium: A comparative study of ternary and binary layered double oxides.

Binary layered double oxides (LDOs) are limited in the simultaneous removal of As(III) and Cd(II) due to weak precipitation and insufficient complexation. Herein, a ternary Ca-Mg-Al LDO (CMA-LDO) was synthesized via a facile precursor-calcination method and compared with binary Ca-Al (CA) and Mg-Al (MA) LDOs to elucidate its enhanced synergistic adsorption mechanisms. CMA-LDO exhibited superior physicochemical properties, including a higher specific surface area (84.27 m2/g), more abundant metal-containing groups, and a well-developed mesoporous flower-like morphology, which were attributed to the Mg2+-mediated structural bridging effect. In binary adsorption systems, CMA-LDO achieved faster equilibrium (20 min) and higher maximum adsorption capacities for As(III) (287.11 mg/g) and Cd(II) (195.75 mg/g) than CA-LDO and MA-LDO. In addition, CMA-LDO showed broader pH adaptability, greater thermodynamic stability, enhanced selectivity, and improved reusability. Notably, all LDOs exhibited higher adsorption in binary versus single systems due to the formation of As/Cd-bearing ternary complexes. The enhanced adsorption performance of ternary LDO is primarily ascribed to the lattice mineralization processes. Briefly, unlike binary LDOs which formed only amorphous or weakly crystalline As-Cd surface precipitates, CMA-LDO enabled a sequential "capture-and-lock" process. Strong surface adsorption and M-O-As/Cd inner-sphere coordination first concentrated As/Cd species on the surface, followed by lattice reconstruction, isomorphic substitution, and memory-effect-assisted mineralization. These processes induced the formation of stable Ca-Cd-As, Ca-Al-As, and Cd-Al-O crystalline phases, as supported by DFT, XRD, and TEM-EDS results. XPS further demonstrated that enhanced surface electron transfer in CMA-LDO promoted 78.23% of As(III) oxidation to As(V) and facilitated superior Cd coordination. This work highlights the ternary LDO as a promising candidate for deep immobilization of coexisting As and Cd through lattice incorporation mechanisms.