This paper presents the results of laboratory column tests aimed at defining the optimum weight ratio of Zero-Valent Iron (ZVI)/Pumice granular mixtures to be used in Permeable Reactive Barriers (PRBs) for the removal of nickel from contaminated groundwater. The tests were carried out feeding the columns with aqueous solutions of nickel nitrate at concentrations of 5 and 50 mg/l using three ZVI/Pumice granular mixtures at various weight ratios (10/90, 30/70 and 50/50), for a total of six column tests; two additional tests were carried out using ZVI alone. The most successful compromise between reactivity (higher ZVI content) and long-term hydraulic performance (higher Pumice content) seems to be given by the ZVI/Pumice granular mixture with a 30/70 weight ratio. (C) 2011 Elsevier B.V. All rights reserved.