The removal of toxic lead ions from aqueous solution by a combined nano-membrane separation technique was investigated. Biodegradable poly-gamma-glutamic acid (gamma-PGA), a linear biopolymer, and its cross-linked nanoparticles were used to capture the metal ions by forming nanosized particles. The polymer-metal ion particles, with sizes in the range of 80-350 nm, were then removed by membrane separation. Two ultrafiltration techniques were studied with the aim of developing a nanoparticle-enhanced separation process for the efficient removal of lead ions from aqueous solution. The influence of parameters such as the feed lead ion and gamma-PGA concentrations and their proportions, the gamma-PGA cross-linking ratio and the pH of the solution on the lead-removal efficiency and permeate flux was studied. It was found that gamma-PGA could bind and remove more than 99.8% of the lead ions from water through a convenient, low-pressure ultrafiltration technique, resulting in a permeate that satisfied the standard for drinking water recommended by the WHO. (C) 2012 Elsevier B.V. All rights reserved.