Adsorption of Indigo Carmine onto Zn–Cu–Fe layered triple hydroxides and oxides: a comparative investigation of removal mechanisms and matrix effects

Abstract

Layered triple hydroxides and their calcined oxides offer distinct adsorption pathways for anionic pollutants, yet how thermal transformation redirects these mechanisms, particularly for bulky dye molecules, remains poorly understood. This study addresses this gap by synthesizing a Zn–Cu–Fe layered triple hydroxide (LTH) via coprecipitation and its calcined oxide (LTO) at 500 °C, comparing their behavior toward Indigo Carmine (IC) through integrated characterization, batch adsorption studies, and mechanistic analysis. Both materials were comprehensively examined using FTIR, FESEM-EDS, HRTEM, BET, and zeta potential, revealing that LTH possesses a high surface area with a well-ordered layered structure, while LTO comprises intimately mixed ZnO, CuO, and ZnFe₂O₄ phases with reduced surface area but enlarged mesopores and magnetic properties. Adsorption studies demonstrate that LTH achieves superior removal through monolayer coverage on homogeneous external surfaces following pseudo-first-order kinetics, while LTO exhibits lower capacity with faster initial kinetics and heterogeneous binding across its multiphase surface. Thermodynamic analysis confirms endothermic physisorption for both. Crucially, mechanistic elucidation through post-adsorption FTIR and FESEM reveals that despite LTH's layered architecture, IC molecules are too large to access the interlayer galleries; uptake instead proceeds via electrostatic attraction to external surfaces and edge-localized carbonate displacement. For LTO, a synergistic mechanism operates, pore-filling within the macroporous network, surface complexation on coordinatively unsaturated metal centers, and memory effect reconstruction that partially reforms layered domains incorporating additional dye. Maximum Langmuir capacities of LTH and LTO for IC at 35 °C, were 71.34 and 69.15 mg g⁻¹. The PFO kinetic model best described IC adsorption on both materials with q_exp of 48.6 mg g⁻¹. The BET surface areas of LTH and LTO were 150.14 and 21.70 m² g⁻¹, with total pore volumes of 0.2645 and 0.1982 cm³ g⁻¹ at p/p⁰ = 0.990 and the mean pore diameters were 7.05 and 36.53 nm, for LTH and LTO, respectively. These values provide a clearer snapshot of the performance and mechanistic differences between LTH and LTO. Both materials maintain IC selectivity in binary dye systems with minimal interference from NaCl or humic acid. The contrasting pathways demonstrate that calcination fundamentally transforms the adsorption mechanism, offering complementary strategies for anionic dye remediation.