Site Energy\nDistribution Coupled with Statistical\nPhysics Modeling for Cr(VI) Adsorption onto Coordination Polymer Gel

In\nthis work, a novel pristine coordination polymer gel\ncomposed\nof zirconium and 2-amino-5-mercapto-1,3,4-thiadiazole is unveiled\nand explored to remove Cr(VI) from aqueous systems. A Box–Behnken\ndesign, coupled with a genetic algorithm and desirability function,\nwas used for optimizing the controllable factors for maximum removal\nefficiency. Under optimized conditions (A = 50 mg L<sup>–1</sup>, B = 40 mg, C = 90 min, and D = 4), 99% of Cr(VI) was removed, and\nthe saturation adsorption capacity recorded was 132.37 mg g<sup>–1</sup>. The adsorption data were investigated through statistical physics\nmodeling. The most suited statistical physics model (monolayer with\nthree energies; <i>R</i><sup>2</sup> = 0.994–0.997,\nχ<sup>2</sup> = 0.008–0.024), combined with site energy\ndistribution analysis, XPS, and FTIR, unraveled the uptake mechanism.\nAt the first and third active sites, Cr(VI) uptake was multimolecular\n(<i>n</i> > 1), while at the second active site, it was\na mixed multimolecular (298 K, <i>n</i> > 1) and multidocking\n(308 and 318 K, <i>n</i> < 1). The adsorption energy\nvalues indicated the involvement of coordination exchange (<i>E</i><sub>1</sub> = 53.40–58.47 kJ mol<sup>–1</sup>), electrostatic interaction (<i>E</i><sub>2</sub> = 29.56–32.29\nkJ mol<sup>–1</sup>), and hydrogen bonding (<i>E</i><sub>3</sub> = 24.68–28.35 kJ mol<sup>–1</sup>) in\nCr(VI) adsorption. The BSf(1.5, α) model fitted best (<i>R</i><sup>2</sup> = 0.976–0.994, χ<sup>2</sup> =\n0.023–0.377) to the kinetic data under all conditions. Common\ncoexisting ions had no significant impact on removal efficiency (%<i>R</i> > 97% at 1:3), and the sorbent could be reutilized\n up\nto 5 uptake-elution cycles (>96% efficiency). The practical utility\nof ZrAMTD was investigated by remediating Cr(VI) contaminated real\nwater samples (%<i>R</i> ≥ 92.91%).