Site Energy Distribution Coupled with Statistical Physics Modeling for Cr(VI) Adsorption onto Coordination Polymer Gel

In this work, a novel pristine coordination polymer gel composed of zirconium and 2-amino-5-mercapto-1,3,4-thiadiazole is unveiled and explored to remove Cr(VI) from aqueous systems. A Box-Behnken design, coupled with a genetic algorithm and desirability function, was used for optimizing the controllable factors for maximum removal efficiency. Under optimized conditions (A = 50 mg L^-1, B = 40 mg, C = 90 min, and D = 4), 99% of Cr(VI) was removed, and the saturation adsorption capacity recorded was 132.37 mg g^-1. The adsorption data were investigated through statistical physics modeling. The most suited statistical physics model (monolayer with three energies; <i>R</i>² = 0.994-0.997, χ² = 0.008-0.024), combined with site energy distribution analysis, XPS, and FTIR, unraveled the uptake mechanism. At the first and third active sites, Cr(VI) uptake was multimolecular (<i>n</i> > 1), while at the second active site, it was a mixed multimolecular (298 K, <i>n</i> > 1) and multidocking (308 and 318 K, <i>n</i> < 1). The adsorption energy values indicated the involvement of coordination exchange (<i>E</i>₁ = 53.40-58.47 kJ mol^-1), electrostatic interaction (<i>E</i>₂ = 29.56-32.29 kJ mol^-1), and hydrogen bonding (<i>E</i>₃ = 24.68-28.35 kJ mol^-1) in Cr(VI) adsorption. The BSf(1.5, α) model fitted best (<i>R</i>² = 0.976-0.994, χ² = 0.023-0.377) to the kinetic data under all conditions. Common coexisting ions had no significant impact on removal efficiency (%<i>R</i> > 97% at 1:3), and the sorbent could be reutilized up to 5 uptake-elution cycles (>96% efficiency). The practical utility of ZrAMTD was investigated by remediating Cr(VI) contaminated real water samples (%<i>R</i> ≥ 92.91%).