ABSTRACT

The need to source for environmentally friendly adsorbents for the removal of Cd2+, Pb2+, Hg2+ and As3+ from aqueous solution was investigated in this study. Okra and kenafstems were tested as adsorbents for the removal of Cd2+, Pb2+, Hg2+ and As3+. Modification of the adsorbents was carried out using TolueneDiisocyanate and the effectiveness of the Modified Okra Stem and Modified Kenaf Stem and Unmodified Okra stem and Unmodified Kenafstem were investigated. At varying concentrations of metal ions (10, 20, 30 and 40 mg/L), the amount of Cd2+ adsorbed by Modified Okra Stem , Unmodified Okra Stem , Modified Kenaf Stem and Unmodified Kenaf stem were in the concentration ranges, (mg/L) 68.73 to 107.51, 72.99 to 81.73, 59.27 to 94.38 and 69.48 to 73.51 respectively. Corresponding ranges for adsorption of Pb2+ were 52.56 to 109.15, 60.83.37 to 83.37, 43.10 to 96.01 and 56.88 to 75.14 respectively. Hg2+ adsorption covered the ranges 56.01 to 89.28, 63.52 to 78.37, 41.81 to 72.18 and 3.31 to 59.26 mg/L. while that of As3+ (mg/L) were 58. 16, 34.36 to 71.58, 46.97 to 78.87 and from 11.03 to 66.89 respectively. Generally, removal efficiency was observed to increase with increase in concentration for the modified adsorbents (Modified Okra Stem and Modified Kenaf Stem) but a sharp decrease in adsorption before subsequent increase were observed for the unmodified adsorbents. Variation of equilibrium amount of metal ions adsorbed with pH, temperature and contact time was observed to follow similar trend, that is, the adsorption increases up to a maximum before a slight decrease. A linear relationship was observed between equilibrium amount of metal ion adsorbed and adsorbent dosage. Analysis of the Fourier Transform Infra-Red spectra of the adsorbents before and after adsorption gave evidences of interaction and involvement of functional groups in the adsorption process showed some functional groups were missing, some were shifted, while other new ones were formed. The adsorption characteristics obeyed the Langmuir, Temkin, Freundlich and Harkins-Jura adsorption models. Thermodynamic data reveals that adsorption mechanism is physisorption. However, pseudo first and pseudo second order kinetics were excellently obeyed while the adsorption process was proposed to be diffusion controlled through the observed fitness ofthe adsorption data to intra particle diffusion. The Scanning Electron Microscope profiles ofthe adsorbents before and after adsorption evidently supported surface modification after adsorption. From the desorption studies, the percentage desorption values showed that greater percentage of the adsorbed metal ions on the adsorbents were held by weak forces supporting physisorption as the mode of bonding ofthe metal ions on the surfaces. These results indicate that the adsorbents are suitable for the removal ofCd2+, Pb2+, Hg2+, and As3+ from aqueous solution.