Effects of mixing granular iron with sand on the efficiency of methylene blue discoloration

► Mixing iron metal (Fe0) and sand is a conventional approach in designing Fe0 filtration systems. ► Reliable concepts to optimize Fe0/sand systems are still lacking. ► Methylene blue (MB) discoloration is used to characterize the efficiency of Fe0/sand systems. ► Results demonstrate that sand impairs MB discoloration by Fe0 as a rule. ► The urgent need of systematic investigations for efficient system design is delineated. The influence of granular sand on the efficiency of metallic iron (Fe0) for the discoloration of a methylene blue (MB) solution was investigated in the current work. The initial MB concentration was 10mgL−1 and mass loadings within the range of 0–90gL−1 for sand and 0–45gL−1 for Fe0 were applied. The batch reaction vessel used was a graduated essay tube containing 22.0mL of the MB solution. Shaking intensities of 0 and 75.0rpm were applied for experimental durations of 7, 21 and 45days. Results provide clear evidence that both Fe0 and sand were independently effective for the discoloration of MB. However, the latter material was significantly less effective, recording 54.0% compared to 82.0% recorded for the Fe0 after 45days in experiment with 45.0gL−1 of each material. Similarly, mixing 90gL−1 sand with 45.0gL−1 of Fe0 depicted a MB discoloration efficacy of 72.0% demonstrating that the discoloration capability of the Fe0 was significantly ‘masked’ by the presence of sand. This observation provides clear evidence to question the common approach of using adsorbents for contaminant accumulation in the vicinity of Fe0 materials in order to facilitate chemical reduction by Fe0. Further research is required to determine the relative affinity of different materials that can be used in Fe0 mixtures for maximum contaminant removal efficacies.