A Study on the Adsorption of Methyl Orange in Aqueous Solution by Activated Carbon Prepared from Neem Oil Cakes: Kinetic and Thermodynamic Analyses

This worked is aimed at studying the thermodynamic and kinetic adsorption of methyl orange (MO) onto activated carbon (AC) obtained from Neem oil cakes (NOC). The ACs were synthesized by chemical activation of <i>Neem</i> oil cakes with H<sub>3</sub>PO<sub>4 </sub>of 2, 5 and 10 percent (respectively labeled AC-2, AC-5, and AC-10) followed by pyrolysis at 450°C for 1 hr. Various characterizations of the synthesized ACs include Fourier Transformed Infrared spectroscopy FTIR, microstructural and elemental analyses (SEM/TEM, EDS), pH<sub>PZC</sub>, moisture content, and iodine and methylene blue adsorption methods were used to determine the surface area. The ACs were employed to adsorb methyl orange (MO) from a synthetic aqueous solution. The results obtained show that: pH<sub>PZC</sub> was less than 7, indicating that the three activated carbons have predominantly acidic surface. The adsorbents AC-5 and AC-10 have microporous and mesoporous structures respectively, with respective specific surface area by iodine adsorption (S<sub>I2</sub>) method estimated to be around 688.45 and 689.70 m<sup>2</sup>/g. The adsorption of MO was pH dependent, with an optimal adsorption at pH =2. The EDS results confirm that these adsorbents are primarily composed of carbon. Results from kinetic studies showed that the adsorption process followed a pseudo second order kinetic model. The experimental data from the equilibrium adsorption of MO on the ACs showed the best fit with the Langmuir isotherm, suggesting monolayer adsorption. Maximum adsorption capacity of 232.558 mg.g<sup>-1</sup> was obtained for AC-10. These results show that the adsorption of MO is spontaneous and endothermic. Chemisorption is the predominant mechanism for MO removal on AC-2, AC-5, and AC-10.