Sorption potential of Moringa oleifera pods for the removal of organic pollutants from aqueous solutions

Moringa oleifera pods Lamarck (Drumstick or Horseradish) is a multipurpose medium or small size tree from sub-Himalayan regions of north-west India and indigenous to many parts of Asia, Africa, South America, and in the Pacific and Caribbean Islands. Its pods (MOP) have been employed as an inexpensive and effective sorbent for the removal of organics, i.e., benzene, toluene, ethylbenzene and cumene (BTEC) from aqueous solutions using HPLC method. Effect of different parameters, i.e., sorbent dose 0.05-0.8 g, 25 cm-3 agitation time 5-120 min, pH 1-10, temperature 283-308 K and concentration of sorbate (1.3-13) x 10-3, (1.1-11) x 10-3, (0.9-9) x 10-3, (0.8-8) x 10-3 mol dm-3, on the sorption potential of MOP for BTEC have been investigated. The pore area and average pore diameter of the MOP by BET method using nitrogen as a standard are calculated to be 28.06 ± 0.8 m2 g-1 and 86.2 ± 1.3 nm respectively. Freundlich, Langumir and Dubinin-Radushkevich (D-R) sorption isotherms were employed to evaluate the sorption capacity of MOP. Sorption capacities of BTEC onto MOP have been found to be 46 ± 10, 84 ± 9, 101 ± 4, 106 ± 32 mmol g-1 by Freundlich, 8 ± 0.1, 9 ± 0.1, 10 ± 0.3, 9 ± 0.1 mmol g-1 by Langumir and 15 ± 1, 21 ± 1, 23 ± 2, 22 ± 3 mmol g-1 by D-R isotherms respectively, from BTEC solutions at 303 K. While the mean energy of sorption process 9.6 ± 0.3, 9.2 ± 0.2, 9.3 ± 0.3, 9.5 ± 0.4 kJ mol-1 for BTEC is calculated by D-R isotherm only. Rate constant of BTEC onto MOP 0.033 ± 0.003, 0.030 ± 0.002, 0.029 ± 0.002, 0.027 ± 0.002 min-1 at solution concentration of 1.3 x 10-3, 1.1 x 10-3, 0.9 x 10-3 and 0.8 x 10-3 mol dm-3 and at 303 K have been calculated by employing Lagergren equation. Thermodynamic parameters ΔH -8 ± 0.4, -10 ± 0.6, -11 ± 0.7, -11 ± 0.7 kJ mol-1, ΔS -22 ± 2, -26 ± 2, -27 ± 2, -29 ± 3 J mol-1 K-1 and ΔG 303K -0.9 ± 0.2, -1.9 ± 0.2, -2.3 ± 0.1 and -2.6 ± 0.2 kJ mol-1 were also estimated for BTEC respectively at temperatures 283-308 K. The negative values of ΔH, ΔS and ΔG suggest exothermic, stable (with no structural changes at solid-liquid interface) and spontaneous nature of sorption process under optimized conditions. MOP has been used extensively to accrue and then to preconcentrate benzene, toluene and ethylbenzene in wastewater sample