Abstract
Humic acid (HA) is well known as an inexpensive and effective adsorbent for heavy metal ions. However, the thermodynamics of uranium (U) adsorption onto HA is not fully understood. This study aimed to understand the kinetics and isotherms of U(VI) adsorption onto HA under different temperatures from acidic water. A leonardite-derived HA was characterized for its ash content, elemental compositions, and acidic functional groups, and used for the removal of U (VI) from acidic aqueous solutions via batch experiments at initial concentrations of 0–100 mg·L−1 at 298, 308 and 318 K. ICP-MS was used to determine the U(VI) concentrations in solutions before and after reacting with the HA. The rate and capacity of HA adsorbing U(VI) increased with the temperature. Adsorption kinetic data was best fitted to the pseudo second-order model. This, together with FTIR spectra, indicated a chemisorption of U(VI) by HA. Equilibrium adsorption data was best fitted to the Langmuir and Temkin models. Thermodynamic parameters such as equilibrium constant (K0), standard Gibbs free energy (ΔG0), standard enthalpy change (ΔH0), and standard entropy change (ΔS0), indicated that U(VI) adsorption onto HA was endothermic and spontaneous. The co-existence of cations (Cu2+, Co2+, Cd2+ and Pb2+) and anions (HPO42− and SO42−) reduced U(VI) adsorption. The high propensity and capacity of leonardite-derived HA adsorbing U(VI) suggests that it has the potential for cost-effective removal of U(VI) from acidic contaminated waters.
Highlights
Uranium (U) is widely but unevenly distributed in soils with an average concentration of 2.6 mg·kg−1 [1]
), and qe is the amount of adsorption at equilibrium state
Results and Discussion qe where R is the gas constant (8.314 J·mol−1 ·K−1 ), T is the temperature in K, Ce is the equilibrium concentration, and qe is the amount of adsorption at equilibrium state
Summary
Uranium (U) is widely but unevenly distributed in soils with an average concentration of 2.6 mg·kg−1 [1]. Acid mining drainage is a major source of U release into soil and water environments [2,3]. Occurring U consists of three isotopes: U-238 (99.2739–99.2752%), U-235 (0.7198–0.7202%) and U-234 (0.0050–0.0059%). In oxidizing environments U is usually found in hexavalent form. U accumulation moves up the food chain, and eventually, to human organs and tissues, causing severe damage to kidneys, liver and in extreme cases, death [4].
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