Enhanced continuous adsorption of silver ions using graphite oxide impregnated with zeolite LTA under high pressure and temperature conditions

Abstract

This study explores the potential of zeolite-impregnated graphite oxide (GOIZ) as a highly efficient adsorbent for the continuous removal of silver ions (Ag(I)) under elevated temperature and pressure conditions in a fixed-bed column. Adsorption performance significantly improved at higher temperatures (100–200 °C), with the best results obtained at 200 °C and 25 MPa. Under these conditions, the maximum stoichiometric adsorption capacity reached 407 mg/g, a substantial increase compared to 47 mg/g under normal conditions (25 °C and 0.101 MPa). This improvement is attributed to enhanced activation of adsorption sites, reduced mass transfer resistance, and progressive desolvation of Ag(I) ions under elevated temperatures, as well as liquid compression effects at high pressure. Interestingly, only at 200 °C and 25 MPa pressure did play a synergistic role, likely due to conditions approaching the subcritical state of water, altering the interaction between adsorbent and adsorbate. Mathematical modeling using the Dose-Response model demonstrated superior predictive accuracy (R² ≥ 0.954) compared to the Thomas model, accurately describing Ag(I) adsorption behavior under varying experimental conditions. Mechanistic analysis revealed a adsorption process governed by redox reactions, where Ag⁺ ions were reduced to Ag⁰ on the adsorbent surface, electrostatic interactions between Ag⁺ ions and the negatively charged functional groups of GOIZ and cationic-exchange. The formation of Ag⁰ on the adsorbent surface was confirmed, with further adsorption hindered by the resulting silver layer. These findings underscore the exceptional capability of GOIZ for Ag(I) removal in wastewater treatment, particularly under extreme temperature and pressure conditions.