Kinetic and Thermodynamic Features of Pb(II) Removal From Aqueous Solution by Leonardite-Derived Humic Acid

Though Pb(II) adsorption onto HA has been extensively studied, its kinetic and thermodynamic features are not fully understood. This work investigated the kinetic processes and isotherms of Pb(II) adsorption onto a humic acid (HA) derived from leonardite in an aqueous solution. The basic properties of the HA were determined by standard methods, and Fourier transform infrared spectroscopic (FTIR) technique. Adsorption kinetic experiments were conducted at 120 mg Pb/L at 288, 298, 308, and 318 K. The adsorption data were best fitted into the pseudo-second-order model, suggesting the chemisorption nature of Pb(II) adsorption. Batch adsorption experiments were conducted at 0–200 mg Pb/L, and the data fit the Freundlich and Temkin models well. Pb(II) adsorption onto HA initially increased, then decreased, with rising temperature. Thermodynamic parameters showed that Pb(II) adsorption was exothermic and spontaneous. Though Cu(II), Zn(II), or Cd(II) could compete with Pb(II) for adsorption, the low cost and high adsorption capacity of leonardite-derived HA determined that it was an excellent adsorbent to remove Pb(II) from an aqueous solution. The optimized experimental conditions derived from the central composite design (CCD) were 20 mg HA, pH 5.0, 4-h react time, and 80 mg Pb/L.