Insights into kinetics, isotherms and thermodynamics of phosphorus sorption onto nanoscale zero-valent iron

The main aim of this study is to investigate the suitability of several adsorption isotherms and kinetic models for describing the removal of phosphorus (P) from aqueous solutions by nanoscale zero-valent iron (NZVI). Removal efficiency was assessed through batch experiments using initial P-concentration ranged from 5 to 100 mg/L. Statistical error analysis was conducted to examine the validity of the applied models to interpret the removal process. Results showed that NZVI dosage of 1 g/L could remove up to 76.8 mg-P with 100 mg/L of P-initial concentration. Moreover, Langmuir isotherm and pseudo 2nd order kinetic model were the best models to describe the experimental data of P-sorption onto NZVI, with the highest linear and non-linear correlation (R²). Results suggested that chemisorption is the main mechanism involved in P-removal by NZVI in addition to the possibility of physical-electrostatic deposition of P-species onto the surface of NZVI within the liquid-solid controlling step. Functional error analysis quantitatively confirmed the applicability of the proposed isotherm and kinetic models in describing the P-sorption data. Akaike Information Criterion (AIC) values confirmed the goodness-of-fit of the Langmuir isotherm model, linear pseudo 2nd and 1st order models, and non-linear pseudo 1st order model to the experimental data. Thermodynamic analysis, with high enthalpy value of 67.87 kJ/mol, confirmed endothermic nature in the NZVI/P system and the involvement of physio/chemisorption process. The present insights into sorption and reaction rate analysis of P by NZVI revealed that it could have subsequently occurred on two main steps of liquid-film and liquid-solid diffusions.