Extraction of heavy metal from contaminated soils with chelating agents is reported in the literature. This paper explores the feasibility of nanofiltration for separating chelating agents from heavy metals, which would make it possible to reuse chelating agents. The influence of membrane characteristics and operation variables on process efficiency is checked and analyzed by use of Cu2+ and citrate as target reagents. The results obtained with the most suitable membrane show that copper rejection decreases when citrate is added. Furthermore, in spite of their similar sizes, Cu2+ and citric acid can be separated at low pH. Key to understanding this process is that electrostatic interactions between membrane and solutes have a more important function than steric hindrance. (C) 2009 Elsevier B.V. All rights reserved.