Insight into the synergetic, steric and energetic properties of zeolitization and cellulose fiber functionalization of diatomite during the adsorption of Cd(ii): advanced equilibrium studies

Abstract

The adsorption potentiality of zeolitized diatomite (ZD) frustules and their cellulose hybridized (C/ZD) product for Cd(II) ions was assessed in synergetic studies to investigate the impact of the modification processes. The adsorption properties were illustrated based on the steric and energetic parameters of the applied advanced equilibrium modeling (monolayer model of one energy). The cellulose hybridization process increased the adsorption properties of Cd(II) significantly to 229.4 mg g⁻¹ as compared to ZD (180.8 mg g⁻¹) and raw diatomite (DA) (127.8 mg g⁻¹) during the saturation state. The steric investigation suggested a notable increase in the quantities of the active sites after the zeolitization (N_m = 62.37 mg g⁻¹) and cellulose functionalization (N_m = 98.46 mg g⁻¹), which illustrates enhancement in the Cd(II) uptake capacity of C/ZD. Moreover, each active site of C/ZD can absorb about 4 ions of Cd(II) ZD, which occur in a vertical orientation. The energetic studies, including Gaussian energy (<8 kJ mol⁻¹) and retention energy (<8 kJ mol⁻¹), demonstrate the physical uptake of Cd(II), which might involve cooperating van der Waals forces (4–10 kJ mol⁻¹), hydrophobic bonds (5 kJ mol⁻¹), dipole forces (2–29 kJ mol⁻¹), and hydrogen bonding (<30 kJ mol⁻¹) in addition to zeolitic ion exchange mechanisms (0.6–25 kJ mol⁻¹). The behaviors and values of entropy, internal energy, and free enthalpy as the assessed thermodynamic functions validate the exothermic and spontaneous properties of the Cd(II) retention by ZD and the C/ZD composite.