The effect of the chloride ions (Cl-) on the efficiency of Fe-0/sand systems for methylene blue (MB) discoloration was characterized in column studies for 4 months. Tested systems were: pure sand (0% Fe-0 reference) and Fe-0/sand (50% Fe-0-vol/vol). Tested Cl- concentrations were 0.0, 2.1, 21.1 and 42.2 mM. The used MB concentration was 16 mu M (5 mg L-1) and used Fe-0 mass was 100g. Discoloration experiments lasted for 89 days and was followed by a 26 days desorption experiment. Each system was characterized by the time-dependent changes of the pH value, the iron breakthrough, the MB breakthrough, and the hydraulic conductivity (permeability). During the discoloration experiments, a total of 17.4 L (184 pore volumes) of the MB solution flowed through each column (279.0 mu M dye or 87.2 mg). No significance changes in pH value and permeability could be documented. Discoloration and desorption results confirmed that sand is an excellent MB adsorbent (only 11% desorption in the reference system). In the absence of Cl-, 15 mg MB was discolored. Up to 18 mg of MB was discolored in the presence of Cl-. The same trend was observed in changes of the extent of iron release which was 7.3 mg at 0.0 mM Cl- and 16.2 mg at 42.2 mM Cl-. These results confirm that chloride ions enhance the kinetics of Fe-0 corrosion and thus the production of contaminant collectors (for MB co-precipitation). Results confirmed the suitability of MB as a powerful operative indicator for the characterization of processes in the Fe-0/H2O system. (C) 2015 Elsevier B.V. All rights reserved.