2.7.1. Langmuir isotherm model

Langmuir isotherm is based on the assumptions that adsorption occurs on a homogeneous surface and each adsorption site can hold one metal ion at a time [20,21]. The linearized form of Langmuir isotherm is given in Eq. (3):

Where C_e (mg/L) is equilibrium concentration of the metal ion, qe is the mass of metal ions adsorbed per mass of adsorbent (mg/g), qmax is the maximum monolayer capacity of adsorption (mg/g) and KL is Langmuir constant. The experimental data were fitted into the above equation by plotting Ceqe against Ce. The values of qmax and KL were determined from the slope and intercept respectively.

To determine the viability of the sorption process, a separation factor, R_L (Eq. 4) was used as an indicator of the nature of the sorption process [21].

Where C1 is the initial metal concentration (mg/L); K_L is the Langmuir constant. The favourability of the Langmuir model is such that where RL <1, Langmuir model is unfavourable; if RL= 1, Langmuir model is linear; if RL <1 Langmuir model is favourable.