Pb2+ adsorption from aqueous solutions on activated carbons obtained from lignocellulosic residues

Abstract

Activated carbons obtained from cane sugar bagasse (ACB), African palm pit (ACP) and sawdust (ACS) were prepared through an impregnated with HNO3 and thermal treatment in an atmosphere in N2/steam water at 1173 K. Adsorption isotherms of N2 at 77 K and of CO2 at 273 K were determined for the activated carbons for which surface area and pore volume values were from 868 to 1100 m²g-1 and from 0.27 to 0.55cm³ g-1, respectively. These results were correlated, with the ones obtained for adsorption the adsorption isotherms of Pb2+ in aqueous solutions. Impregnation of the lignocellulosic materials with nitric acid produced acid-type activated carbons with total acid site contents between 4.13 and 6.93 mmol g-1 and pH at the point of zero charge values between 2.7 and 4.1, which were within range of the adsorption, at different pH values, since they determined, the surface charge of the activated carbons. Adsorption isotherms of Pb2+ at different pH values (2-8) at 298 K were determined. The ion adsorption capacity on ACB, ACP and ACS were 13.7, 15.2 and 17.5 mg.g-1, respectively. Experimental data were fitted to the Langmuir and Freundlich models and all cases the former fit better. The highest values for the quantity adsorbed on the monolayer, qm, were at pH 4, whereas the surface, charge of activated carbons was negative and the lead species mainly present was Pb2+. For higher pHs, the quantity of Pb2+ adsorbed decreased, and this had an important effect on adsorption, the surface characteristics of the solids and the hydroxilated lead species that were formed in the system.