The key role of contact time in elucidating the mechanisms of enhanced decontamination by Fe(0)/MnO(2)/sand systems

Metallic iron (Fe(0)) has shown outstanding performances for water decontamination and its efficiency has been improved by the presence of sand (Fe(0)/sand) and manganese oxide (Fe(0)/MnO(x)). In this study, a ternary Fe(0)/MnO(x)/sand system is characterized for its discoloration efficiency of methylene blue (MB) in quiescent batch studies for 7, 18, 25 and 47 days. The objective was to understand the fundamental mechanisms of water treatment in Fe(0)/H(2)O systems using MB as an operational tracer of reactivity. The premise was that, in the short term, both MnO(2) and sand delay MB discoloration by avoiding the availability of free iron corrosion products (FeCPs). Results clearly demonstrate no monotonous increase in MB discoloration with increasing contact time. As a rule, the extent of MB discoloration is influenced by the diffusive transport of MB from the solution to the aggregates at the bottom of the vessels (test-tubes). The presence of MnO(x) and sand enabled the long-term generation of iron hydroxides for MB discoloration by adsorption and co-precipitation. Results clearly reveal the complexity of the Fe(0)/MnO(x)/sand system, while establishing that both MnO(x) and sand improve the efficiency of Fe(0)/H(2)O systems in the long-term. This study establishes the mechanisms of the promotion of water decontamination by amending Fe(0)-based systems with reactive MnO(x).