where, qe (mg/g) is the amount of adsorbed metal ions per unit weight of adsorbent at equilibrium concentration, Ce (mg/L). The Q (mg/g) and b (L/mg) are the Langmuir constants related to the maximum monolayer capacity and energy of adsorption, respectively. The K and 1/n are Freundlich constants related to adsorption capacity and intensity of adsorption respectively.
Table 8 shows that the Langmuir isotherm fits the data better than the Freundlich isotherm. This result is also confirmed by the high R2 of the Langmuir model (0.991 and 0.990) compared to that of the Freundlich model (0.894 and 0.905), indicating that Ni2+ adsorption on MIOS and THOS occurs as monolayer adsorption on a surface that is homogeneous in adsorption affinity. Thus, the adsorption isotherm data were best described by the Langmuir isotherm and the adsorption capacity was determined to be 12.0 and 8.42 mg/g for MIOS and THOS, respectively, indicating that MIOS has a higher adsorption capacity compared to THOS.
The adsorption capacities of MIOS and THOS for other heavy metal ions which studied elsewhere[38, 39] were in the order Fe2+ > Pb2+ > Cu2+ > Zn2+ > Ni2+ > Cd2+. The results obtained agreed with those of the study of Li et al., in which the following order for the sorption of metals onto sawdust and modified peanut husk was reported: Pb2+ > Cu2+ > Cr6+. The next, by Xie et al., found a similar order of affinity: Pb2+ > Cu2+ > Cd2+ onto sewage sludge activated carbon. Further, by Brown et al. examined peanut hull pellets and found a similar order of affinity: Pb2+ > Zn2+ > Cu2+ > Cd2+. Spinti et al. reported a similar order of affinity for immobilized biomass (peat) beads: Fe2+ > Al3+ > Cu2+ > Cd2+, Zn2+. Another study by Tuzen et al. reported that, the affinity order of the metal ions toward carbon nanotubes was Cu2+ > Pb2+ > Zn2+ > Co2+ > Ni2+ > Cd2+ > Mn2+. Another study by Reddad et al. investigated sugar beet pulp and found a similar order of affinity: Pb2+ > Cu2+ > Zn2+ > Cd2+ > Ni2+.
According to Lim et al., the heavy metal's electronegativity and its ionic radius along with other factors such as solution pH might affect the metal's affinity to be adsorbed by activated carbon. Electronegativity represents the metal ion attraction to the negatively charged sites. The electronegativity of Pb2+, Fe2+, Cu2+, Zn2+, Ni2+, and Cd2+ are 2.33, 1.83, 1.9, 1.65, 1.91, and 1.69, respectively. As a general trend, higher electronegativity corresponds to a higher sorption level of the metal ion. It can be noted that Pb2+ has the highest electronegativity compared to other metal ions meanwhile Zn2+ and Cd2+ have the lowest electronegativity. It is also known that the adsorption of any heavy metal with a smaller ionic radius is greater than that with a larger ionic radius. This is due to smaller radii metal ions have more accessibility to the surface and pores of the adsorbents than the bigger ones. The ionic radii of the metals, namely, Fe2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+ are 0.64, 0.69, 0.73, 0.74, 0.97, and 1.19 A°, respectively. It can be noted that Pb2+ and Cd2+ have the largest ionic radius compared to other metal ions meanwhile Fe2+ has the smallest ionic radius. Based on research results, Fe2+ has the highest adsorption affinity compared to other metals due to combination of small ionic radius and high electronegativity. Meanwhile, Cd2+ has the lowest adsorption affinity compared to other heavy metals due to its large ionic radius and low electronegativity. As mentioned in effect of initial solution pH, the pH of the aqueous solution is an important parameter controlling the adsorption process. The increase in solution pH leads to the increase in the presence of negative charge on the activated carbon surface which responsible for the heavy metals binding. Aforementioned, the adsorption selectivity is a combined action of electronegativity, ionic radius, pH, and other factors. Thus, the sorption depends not only on specific surface area, pH, and surface functional groups but also metal ions to be adsorbed.