The negative ΔG0 values indicate thermodynamic feasibility and spontaneity of the biosorption process. The increase in the magnitude of ΔG0 values for Pb (II) biosorption with an increase in temperature shows an increase in its feasibility. The slight decrease in the value of ΔG0 in the biosorption of Pb (II) ions by WSH at 343 K may be due to gradual damage of active binding sites in the biomass (Hannachi 2012) or the tendency to desorb metal ions from the interface to the solution (Saltali et al. 2007). The ΔH0 value for Pb (II) ions was calculated to be 25.35 kJ mol−1, indicating that the binding of Pb (II) ions onto WSH was endothermic. The enthalpy change agrees with previous works in the literature (Tewari et al. 2005; Senthilkumar et al. 2007). The magnitude of ΔH0 value gives an indication of the type of adsorption, which can either be physical or chemical. The heat of adsorption, ranging from 2.1–20.9 kJ mol−1 is said to be physical adsorption, and the activation energy for chemical adsorption is of the same magnitude as the heat of chemical reactions, 20.9–418.4 kJ mol−1 (Yalçin 2014). The ΔH0 value obtained in the biosorption experiment indicates that the binding of Pb (II) ions to the WSH is by chemical adsorption. Moreover, the ΔS0 was observed to be positive (86.1 kJ mol−1 K−1), indicating the increasing randomness at the solid/liquid interface during biosorption (Fat'hi et al. 2014). A comparison of the maximum adsorption capacities for Pb (II) ions adsorption to different adsorbents is presented in Table 4.

Table 4

Caulerpa lentlifera 28.72 5–6 Pavasant et al. (2006)
Modified peanut sawdust 29.1 Li et al. (2007)
Grape stalks 49.7 5.5 Martinez et al. (2006)
Barley straw 15.2 5.5 Conrad et al. (2007)
Coir 48.8 4.5 Quek et al. (1998)
Syzygium cumini L. 32.47 6.0 King et al. (2007)
C. inophyllum 34.51 4.0 Lawal et al. (2010)
Tea leaves 2.1 — Ahluwalia et al. (2005)
Rice husk 4.0 5.5 Conrad et al. (2007)
Olive pomace 7.0 5.0 Pagnanelli et al. (2003)
Walnut seed husk 3.0 4.0 This study