Chemical carbonization of papaya seed originated charcoals for sorption of Pb(II) from aqueous solution

• Papaya seed charcoal was used as an alternative adsorbent precursor for removal of lead(II).
• Mechanism of adsorption process was reported based on FTIR analysis.
• A comprehensive discussion based on particle size and specific surface area is given.
• Langmuir, Freundlich, Dubinin–Radushkevich (D–R) and Temkin isotherm models were used for equilibrium modeling.
• Thermodynamic parameters (ΔG, ΔH, ΔS) were determined for predicting the nature of adsorption.

The potential of chemically carbonized papaya seed charcoals (PSCs) for the effective removal of Pb(II) ions from aqueous solution has been investigated in batch experiments. Lead adsorption significantly depends on the initial concentration of Pb(II), contact time, pH, adsorbent dosage and particle size. The adsorption behavior of Pb(II) was best described by the pseudo-second order model and it can be seen that the calculated qe (cal) values agree with the experimental values. Langmuir, Freundlich, Temkin and Dubinin–Radushkevich (D–R) adsorption isotherm models were applied to represent adsorption data. The equilibrium data were well fitted by the Langmuir isotherm model by revealing the maximum sorption capacity values 188.6 and 238.09 mg/g of PSC1 and PSC2, respectively. Thermodynamic parameters showed that the process is spontaneous, feasible and endothermic. The mechanism of adsorption was also studied using Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) analysis before and after the adsorption of lead(II) ions. PSC2 was regenerated using HCl (0.2 M) as eluent and reused for four adsorption–desorption cycles.