In this research, Pb (II), Co (II), and Ni (II) toxic heavy metal ions adsorption from synthetic aqueous system have been studied using the activated carbon prepared from Citrus limetta leaves. Therefore, the relationship between the adsorption parameters (solution pH, dosage of adsorbent, temperature, initial concentration of the ions, and adsorption time) and the removal percentage of the prepared adsorbent have been investigated. Additionally, the adsorbent was analyzed through BET, SEM, EDX, FTIR, and XRD analyses. According to the results, the maximal adsorption efficiencies for heavy metal ions were achieved in pH = 6, the adsorbent dosage of 1 g/L, temperature = 25 °C, the ion initial concentration of 5 mg/L, and contact time of 60 min, which were 99.53%, 98.63%, and 97.54% for Pb, Co, and Ni ions, respectively. Based on Kinetic studies, the performance of pseudo-second-order kinetic model was better than pseudo-first-order model for the description of time-dependent behavior of the process. Additionally, the equilibrium data were fitted by Langmuir and Freundlich isotherms, while the former performed better than the latter. The maximum adsorption capacity values for Pb, Co, and Ni ions were achieved equal to 69.82, 60.60, 58.139 mg/g, respectively. Considering the thermodynamic data, the studied processes were exothermic and spontaneous.

    Abstract
    1 Introduction
    2 Materials and methods
        2.1 Chemicals
        2.2 Preparation of stock solution
        2.3 Adsorbent preparation and characterization
        2.4 Adsorption experimental procedure
    3 Results and discussion
        3.1 Characterization of the activated carbon
        3.2 Effect of solution pH
        3.3 Effect of adsorbent dosage
        3.4 Temperature effect
        3.5 Metal ions initial concentration effect
        3.6 Contact time effect
        3.7 Adsorption isotherms
        3.8 Kinetic study of adsorption
        3.9 Thermodynamic studies
    4 Conclusion
    References

• Elham Aboli(Department of Chemical Engineering, School of Chemical Engineering, Kherad Institute of Higher Education, Bushehr, Iran)
• Dariush Jafari(Department of Chemical Engineering, Bushehr Branch, Islamic Azad University)
• Hossein Esmaeili(Department of Chemical Engineering, Bushehr Branch, Islamic Azad University)