Effect of nitrogen species on an activated carbon surface on the adsorption of Cu(II) ions from aqueous solution

Abstract

Various activated carbons (ACs) of different oxygen contents were treated with NH3 gas at 700°C (7AGs) to introduce nitrogen species on the graphene layers for metal ion removal in aqueous solution. The nitrogen content increased with increasing oxygen content of the precursor. The AC was also outgassed in He flow at 1000°C (OG) to remove surface functional groups, and was compared with 7AGs. The amount of Cu(II) adsorption increased with increasing nitrile, pyridine and secondary or tertiary amine groups, which have a lone electron pair. Equilibrium pH decreased with increase in total nitrogen content. It is known that Cu(II) makes a very stable complex regardless of the ligand species and that the solution pH decreases as the complex stability increases. Therefore free basic nitrogen such as nitrile-N can affect the Cu(II) adsorption property of the ligand. Because positively charged quaternary-N has no lone pair, it adsorbs neither Cu(II) nor the proton. Thus the solution pH was influenced by quaternary-N in addition to free base nitrogen. Adsorption affinities and amounts of Cu(II) adsorbed on 7AGs were greater than those of OG, while amounts of Cu(II) desorbed from 7AGs were smaller than OG. Consequently Cu(II)-N binding was considered to be stronger than the Cu(II)-Cπ interaction.