Adsorption of Pb(II) from aqueous solution by Azadirachta indica (Neem) leaf powder

An adsorbent was developed from the mature leaves of the Neem (Azadirachta indica) tree for removing Pb(II) from water. Adsorption was carried out in a batch process with several different concentrations of Pb(II) by varying amount of adsorbent, pH, agitation time and temperature. The uptake of the metal was very fast initially, but gradually slowed down indicating penetration into the interior of the adsorbent particles. Both first-order and second-order kinetics were tested and it was found that the latter gave a better explanation. The experimental data closely followed both Langmuir and Freundlich isotherms. The adsorbent had a considerably high Langmuir monolayer capacity of 300 mg/g. A small amount of the adsorbent (1.2 g/L) could remove as much as 93% of Pb(II) in 300 min from a solution of concentration 100mg/L at 300 K. The adsorption continuously increased in the pH range of 2.0-7.0, beyond which the adsorption could not be carried out due to the precipitation of the metal. The adsorption was exothermic at ambient temperature and the computation of the parameters, DeltaH, DeltaS and DeltaG, indicated the interactions to be thermodynamically favourable.