Adsorption Isotherms, Kinetics and Thermodynamic Studies of the removal of Manganese from Aqueous Solutions using Activated Carbon Produced from Lantana camara stem

Abstract

This study investigated the equilibrium adsorption mechanism and thermodynamics of Manganese (II) ion (Mn+2) present in wastewater onto the activated carbon produced from Lantana camara stem as a low cost adsorbent. The Lantana camara stem was carbonized at 300 ˚C for 2 h, ground and chemical-activated. The Chemical-Activated Lantana camara (CALC) stem carbon was characterized using Scanning Electron Microscope (SEM) and Fourier Transform Infrared (FTIR) Spectrophotometry before and after adsorption of Mn2+. Batch experiments were conducted at different temperature (35 - 65 ˚C) to study the effects of pH, contact time, adsorbent dosage, initial concentration and temperature of Manganese ion. The FTIR bands at 3425, 2366, 1635, 1446, 1288, and 1116 cm−1 were shifted to 3903, 3331, 2378, 2339, 2090, 1845, 1398 and 1342 cm−1 after Mn2+ adsorption. Similarly, the Scanning Electron Microscopy (SEM) analysis indicated that the average pore size on the activated carbon was 50 μm this shows that H3PO4 was effective in creating well-developed pores on the surface morphology of the precursor. The Manganese (II) ion (Mn+2) uptake increased with increase in pH from 2 to 6. However the uptake of Manganese (II) ion decreased slightly from pH of 8 to 12. The adsorption of Manganese (II) ion having the maximum percentage removal at pH 6. Similarly, there was an increase in the uptake of Mn2+ as the dosage increases from 0.5 to 1.0 g∙L−1 and shows slight decreased from 1.5 to 2.5 g∙L−1, at different metal ion concentrations (50 – 200 mg∙L−1) The percentage removal decreased with increasing initial Mn2+ concentration for CALC. The Mn2+ adsorption increased as contact time increased, and reached equilibrium at 120 minutes. The adsorption capacity decreases more as temperature of the reaction medium increases from 35 to 65 °C, which indicates the exothermic adsorption process of Mn2+ onto CALC. The equilibrium adsorption data were analyzed using different model equations such as Langmuir, Freundlich, Temkin and Dubinin- Radushkevich (D-R) isotherms. The experimental results were reasonably correlated by Langmuir (R2 = 0.9915) than other three isotherm models. The maximum adsorption capacity (q_m), intensity of adsorption (b) and separation factor (R_L) were calculated from Langmuir plot and activation energy of adsorption (E_a) was determined from D-R isotherm. The adsorption kinetic data were analyzed using pseudo-first-order, pseudo-second-order, intraparticle diffusion and Elovich models. It was found that the pseudo-second order kinetic model was the most appropriate model, describing the adsorption kinetics model (R2 = 0.99). Thermodynamic parameters such as changes in the free energy of adsorption (ΔGo), enthalpy (ΔHo) and entropy (ΔSo) were calculated. The negative values of ΔGo indicate that the Manganese (II) ion adsorption process is spontaneous in nature and the negative value of ΔHo shows the exothermic nature of the process. The adsorption capacity was evaluated as 147.65 mg/g at 35 °C showing that Lantana camara stem is a promising adsorbent and can be used effectively for the adsorption of metal ions in wastewater.