Eco-Friendly Fe ^III -Layered Double Oxide/Zeolite-P1 Nanocomposite with Low-Spin-State Ferric Iron Tuned for Pollutant Adsorption in Simulated Wastewater

Ferric oxides are promising energy-efficient adsorbents but face limitations in surface science due to their predominant high-spin state. Herein a Zeolite-P2 seed-assisted hydrothermal treatment is reported that synthesizes low-spin-state FeIII-LDO/Zeolite-P1. In terms of cost, this synthetic nanocomposite with a 311.19 m2 g–1 surface area amounts to ∼0.68 USD per gram. To assess its efficiently, we proposed the sorption of two typical azo dyes with different polarities, Congo red (CR) and methylene blue (MB). Furthermore, we systematically investigated the batch-to-column transferability, which was improved through the Box–Behnken Design (BBD) statistical analysis. In batch experiments, the results demonstrated that the pseudo-second-order (PSO) was in harmony with our sorption data, with removal efficiencies of 96.4% for CR (pH 5) and 92.1% for MB (pH 9) at 5 mg·L–1 after 210 min. Importantly, the strong host–guest chemisorption dominates the adsorption mechanism. This was clarified at the atomic level via the density functional theory (DFT). DFT revealed that the interaction involving FeIII–O–Si (electron transfer LDO-to-Zeolite) facilitated the uptake of CR by the low-spin FeIII sites, and thereby shows superior electrostatic multilayer adsorption performance; this observation was supported by the shortened FeIII–O bond length from 2.138 Å (high spin) to 1.819 Å (low spin) and increased d-band center from −2.93 eV to −1.96 eV in the presence of the CR molecules, consistent with the XPS results. For batch-to-column transferability, the breakthrough time (BT) was identified as a function of its key influential factors: capacity (qe) and kinetics (k2) fitted with PSO. A notable enhancement in qe and k2 was spotted after BBD analysis, with a desirability of 1.00, achieving >92% accuracy in BT estimation. With reuse over 3 cycles for CR and 5 cycles for MB, the cost of removing 1 g of dye from simulated wastewater is ∼9.88 USD and ∼11.70 USD, respectively. Finally, this study provides insights that may guide both future catalytic conversion of oxygen-containing molecules and preliminary fixed-bed column investigations.