Iron-exchanged Y-type zeolites for phosphorus recovery from eutrophic waters: Kinetics and performance in real lake matrix

Zeolites have shown strong potential in various applications, including removing phosphorus from eutrophic lake waters. This study investigated the adsorption of inorganic phosphate ions from aqueous solutions using Y-type zeolites (Hydrogen form of Y HY and Ultra-Stable Y USY), both in their original form and modified with ferric ions (Fe³) as adsorbents. Equilibrium and kinetic experiments were conducted to better understand the underlying adsorption mechanisms. For all tested adsorbents, the adsorption kinetics followed a pseudo-second-order model, with qe,calc varying from 3.59±0.04 to 3.71±0.03 mgP/g. Phosphorus removal by iron-modified zeolites ranged from 76.8% with Fe1HY to 100% with Fe2USY (corresponding to 26.25±0.92 mgP/g/gFe and 15.21±0.09 mgP/g/gFe for Fe1HY and Fe2USY, respectively). The iron content influenced the adsorption isotherms of the Y zeolite: samples with lower iron concentrations adhered to the Langmuir isotherm model, indicating monolayer adsorption, while those with higher iron concentrations followed the Freundlich isotherm, suggesting multilayer adsorption on heterogeneous surfaces. The performance and potential environmental impact of the selected adsorbents were further evaluated through toxicity tests using germination seeds and by treating a real water sample from the eutrophic Furnas Lake on São Miguel Island, Azores, Portugal, with phosphorus removal rates between 86.3±2.7% and 97.6±2.7% (1.49 mgP/L for Fe1HY to 1.69 mgP/L for Fe1USY). The results confirmed the effectiveness and low toxicity of these adsorbents. An adsorption mechanism was proposed, involving both chemical interactions and electrostatic forces.