Sorption kinetic experiments for Cd and Pb were conducted with each sorbent, as described in a previous study [46]. First, 20 g of each sorbent was added to a 250 mL flask containing 100 mL of 200 mg/L Cd (CdCl2·2.5H2O and Pb (Pb(NO3)2) solution that was prepared individually. The solution pH was maintained at the range of 5.19−5.25 during the experiment. Each Cd and Pb flask was then shaken at 150 rpm for 24 h until sorption attained an equilibrium. Furthermore, the supernatants were collected at a time interval of 0, 1, 2, 5, 10, 20, 30, 60, 360, 720, and 1440 min and filtered through a 0.45 μm membrane filter. The heavy metal concentration of the filtrate was measured using an inductively coupled plasma optical emission spectrometer (8300 DV Perkin Elmer, Waltham, MA, USA).

The sorption isotherm experiment was conducted with various initial concentrations (50−1000 mg/L) of Cd and Pb. Briefly, 10 g of each sorbent was added to a 250 mL flask containing 100 mL of varying initial concentrations of Cd and Pb. The flask was shaken at 150 rpm for 24 h, filtered through a 0.45 μm membrane filter, and the filtrate was used to measure Cd and Pb concentrations.

The sorption isotherms were modeled using the Langmuir and Freundlich isotherm models. Both isotherm models express the relationship between the mass of sorbed Cd and Pb at a constant temperature per unit mass of each sorbent and the heavy metal concentration in the solution [47]. The Langmuir and Freundlich isotherm models can be expressed as follows.

where qe is the total amount of Cd or Pb sorbed on the sorbent at equilibrium (mg/g); Ce is the concentration of the solution at equilibrium (mg/L); b is the Langmuir constant (L/mg; qm is the total number of binding sites (alternatively, the maximum amount of Cd or Pb per unit mass of sorbent)); and C0 is the initial concentration (mg/L).

where qe, C0, Ce, V, and W represent the metal ion sorption amount per unit mass of sorbent (mg/g), initial metal ion concentration (mg/L), equilibrium metal ion concentration (mg/L), solution volume (L), and mass of sorbent (g), respectively.

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