Impact of Fe0 amendment on methylene blue discoloration by sand columns

The influence of metallic iron (Fe0) 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 Fe0/sand ratio for efficient filtration systems. Columns contained 0, 100 or 200 g of a Fe0 material. The volumetric proportion of Fe0 in the reactive layer of the columns with 100 g of material varied from 10% to 100%. Results showed that, Fe0 amendment significantly impaired MB discoloration by sand for experiments lasting for up to 132 days. Early MB breakthrough in Fe0/sand columns delineated the paramount importance of particle cementation, which has caused preferential flow with a negative impact on discoloration efficiency. The most efficient Fe0/sand mixtures were the ones with 30–50% Fe0 (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 Fe0 and sand is needed before this observation can be generalized.

► Reliable methods for designing Fe0 filtration systems are still lacking. ► Inert admixtures are mostly to account for construction width requirements. ► A recent concept for Fe0 bed design is verified here for the first time. ► The suitability of sand to sustain Fe0 reactivity in dynamic systems is corroborated. ► The Fe0 volumetric proportion should be between 30% and 50%.