The influence of metallic iron (Fe-0) amendment on the efficiency of sand to discolor a 2.0 mg L-1 methylene blue (MB) solution was investigated in column studies. MB was used as an indicator to identify the optimum Fe-0/sand ratio for efficient filtration systems. Columns contained 0, 100 or 200 g of a Fe-0 material. The volumetric proportion of Fe-0 in the reactive layer of the columns with 100 g of material varied from 10% to 100%. Results showed that, Fe-0 amendment significantly impaired MB discoloration by sand for experiments lasting for up to 132 days. Early MB breakthrough in Fe-0/sand columns delineated the paramount importance of particle cementation, which has caused preferential flow with a negative impact on discoloration efficiency. The most efficient Fe-0/sand mixtures were the ones with 30-50% Fe-0 (v/v). These volumetric ratios correspond 33-41% weight ratios showing that the commonly used 1:1 weight ratio (50%) may not be optimal. Further research with compounds exhibiting different affinities to both Fe-0 and sand is needed before this observation can be generalized. (C) 2012 Elsevier B.V. All rights reserved.