Adsorption of Cu and Pb on Goethite in the Presence of Low-Molecular Mass Aliphatic Acids

Abstract

The sorption of Cu and Pb added alone or in mixture in the absence or presence of low-molecular mass aliphatic acids (LMMAAs): oxalic, citric or glutamic acid onto a synthetic goethite at different pH values (from 3.0 to 5.5) was studied. Copper showed a higher affinity for goethite than lead both in absence or presence of LMMAAs. By increasing the initial organic ligand/metal molar ratio from 0 to 10, the adsorption of both the metals on the iron oxide initially increased and then decreased or remained constant. In the presence of oxalic acid the adsorption of Cu on the surfaces of goethite increased by increasing ratio up to 2–4 and then decreased. Citric acid showed a similar effect but the amounts of Cu fixed were lower. Glutamic acid had the lowest effect on the adsorption of this element onto goethite. Adsorption of Pb was strongly affected by citric acid. The amounts of Pb fixed onto the surfaces of goethite increased up to molar ratio 4 and then slightly decreased. On the contrary, oxalic and glutamic acid had a lower influence in Pb adsorption. These findings evidence that at certain ratio values the complexes formed by trace elements and organic ligands have a greater affinity for the sorbent than the uncomplexed ions with the formation of a ternary complexes among goethite, metals and organic ligands. Vice versa, high concentrations of the LMMAAs destabilize the ternary complex shifting the equilibrium in favor of soluble complexes. The adsorption of increasing concentrations of Cu (or Pb) at pH 5.0 both in the absence or presence of oxalic acid (molar ratio 1) showed that Pb (or Cu) strongly inhibited Cu (or Pb) adsorption, but its inhibition was affected by the initial Pb/Cu ratio, presence and sequence of addition of oxalic acid. The presence of oxalic acid enhanced Cu (mainly) and Pb adsorption more when it was added together than before the trace elements. In the systems containing equimolar amounts of Cu and Pb, the final Cu adsorbed/Pb adsorbed molar ratio was 1.58 in the absence of oxalic acid but greater than 2 in the presence of the organic ligand.