Kinetic and Thermodynamics of Adsorption of Heavy Metals from Spent Automobile Engine Oil onto Activated Carbon from Thevetia peruviana

Abstract

The presence of heavy metals was analysed from oil samples employing the dry ashing method. The specific densities and viscosities of the spent and virgin AZ oil samples were determined and found to be 0.9230, 0.9501 and 70.27, 73.27cP respectively. Kinetic and thermodynamic studies of the adsorption of lead (Pb) and chromium (Cr) from the spent oil was also investigated. The spent oil samples were digested and mixed with methyl isobutyl ketone (MIBK), agitated with activated carbon produced from Thevetia peruviana (bush milk) shells at various contact times of 2, 4, 5, 10, 15, 20, 30, 40, 50, 60, 90 and 120 minutes. Kinetic studies revealed that for lead (Pb), the rate constants were found to be 0.044, 0.060 and 0.067 ppm/min at 27oC, 35 oC and 45 oC, respectively, while higher values or rate constants of 0.126, 0.127 and 0,168 ppm/min were observed at 27 oC, 35 oC and 45 oC, respectively, for chromium (Cr). For the thermodynamic studies the Gibb's free energy (ΔG) values were; –1.29, -1.61 and –1.82 KJmol-1at 27 oC, 35 oC and 45 oC, respectively, indicating spontaneity. However, the Gibb's free energy (ΔG) for chromium showed +0.050, -2.13 and -2.14 KJmol-1at 27 oC, 35 oC and 45 oC, indicating non-spontaneity at 27 oC. The positive values of ΔS for both metals with Pb (21 - 30 Jmol-1) and Cr (695 – 1867 Jmol-1 ) indicated that the metals were dissociatively adsorbed onto the adsorbent while for the E values, Cr (8 – 61 KJmol-1) had a higher than with Pb (6 – 35 KJmol-1) further explaining the non-spontaneity of the process for Cr at some reaction condition.