Weeds
The study on effectiveness of lead adsorption by sodium hydroxide treated lalang (one of the ten worst weed of the world) or Imperata cylindrica leaf powder was carried out by Hanafiah et al. (2006a). Maximum adsorption of lead occurred at pH range 4–5 and an adsorbent dose of 4 g l−1. Temperature study indicates that chemisorption was the main rate limiting step and pseudo-second-order kinetic model was fitted well. The value of maximum adsorption capacity obtained by NaOH treated Imperata cylindrica (13.50 mg g−1) is much higher than untreated adsorbent (5.89 mg g−1) (Hanafiah et al., 2007). A faster equilibrium time (20 min) was observed for treated Imperata cylindrica compared to untreated (120 min). The effect of metal concentration and thermodynamics on the sorption of Pb(II) and Cd(II) ions by nitric acid treated wild cocoyam (Caladium bicolor) biomass was investigated by Horsfall and Spiff (2005). The biomass was first reacted with nitric acid before washed with distilled water, then suspended in hydroxylamine to remove all O-acetyl groups. It was noticed that the amount of Pb(II) sorbed was higher than Cd(II) ions. The smaller ionic radii of Cd(II) (0.97 Å) compared to Pb(II) (1.20 Å) means greater tendency of Cd(II) to be hydrolyzed, leading to reduced sorption. Sorption capacity increased with increasing metal concentrations for both cations. The maximum sorption capacities were 49.53 and 48.20 mM g−1 for Pb(II) and Cd(II), respectively. Freundlich and Langmuir models were found to be fitted well. The sorption was reported to occur by ion-exchange mechanism involving hydroxyl groups as represented by the following equation:
equation(2)
M2++2BOH↔M(BO)2+2H+
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Spontaneous sorption of Pb(II) and Cd(II) was noticed as the negative values of ΔG° were obtained in all concentrations studied.