The Adsorption Characteristics of Uranium(VI) from Aqueous Solution on Leonardite and Leonardite-Derived Humic Acid: A Comparative Study

The humic substance is a low-cost and effective adsorbent with abundant functional groups in remediating uranium (U) (VI)-contaminated water. In this research study, leonardite together with leonardite-derived humic acid (L-HA) was used to eliminate U(VI) from water under diverse temperatures (298, 308, and 318 K). L-HA showed a higher adsorption volume for U(VI) than leonardite. U adsorption was varied with pH and increased with temperature. The adsorption kinetics of L-HA had a higher determination coefficient (R-2) for pseudo-second-order (R-2 > 0.993) and Elovich (R-2 > 0.987) models, indicating possible chemisorption-assisted adsorption. This was further supported with the activation energies (15.9 and 13.2 kJ/mol for leonardite and L-HA, respectively). Moreover, U(VI) equilibrium adsorption on leonardite was better depicted with the Freundlich model (R-2 > 0.970), suggesting heterogeneous U(VI) adsorption onto the leonardite surface. However, U(VI) adsorption onto L-HA followed the Langmuir equation (R-2 > 0.971), which implied the dominant role of monolayer adsorption in controlling the adsorption process. Thermodynamic parameters, including standard entropy change (Delta S-0 > 0), Gibbs free energy (Delta G(0) < 0), and standard enthalpy change (Delta H-0 > 0), suggested a spontaneous and endothermal adsorption process. In addition, ionic species negatively affected U(VI) adsorption by leonardite and L-HA.

Automated summary

The study successfully utilized leonardite and leonardite-derived humic acid (L-HA) to remove U(VI) from water, showing that L-HA had a higher adsorption capacity for U(VI) compared to leonardite. The adsorption process may be influenced by chemisorption-assisted mechanisms, as indicated by the higher determination coefficients for the pseudo-second-order and Elovich models of L-HA.