The Suitability of Methylene Blue Discoloration (MB Method) to Investigate the Fe0/MnO2 System

Abstract

The typical time-dependent decrease of the iron corrosion rate is often difficult to consider while designing Fe0-based remediation systems. One of the most promising approaches is the amendment with manganese dioxide (Fe0/MnO2 system). The resulting system is a very complex one where characterization is challenging. The present communication uses methylene blue discoloration (MB method) to characterize the Fe0/MnO2 system. Shaken batch experiments (75 rpm) for 7 days were used. The initial MB concentration was 10 mg L−1 with the following mass loading: [MnO2] = 2.3 g L−1, [sand] = 45 g L−1, and 0 < [Fe0] (g L−1) ≤ 45. The following systems where investigated: Fe0, MnO2, sand, Fe0/MnO2, Fe0/sand, and Fe0/MnO2/sand. Results demonstrated that MB discoloration is influenced by the diffusive transport of MB from the solution to the aggregates at the bottom of the test-tubes. Results confirm the complexity of the Fe0/MnO2/sand system, while establishing that both MnO2 and sand improve the efficiency of Fe0/H2O systems in the long-term. The mechanisms of water decontamination by amending Fe0-based systems with MnO2 is demonstrated by the MB method.

The key role of contact time in elucidating the mechanisms of enhanced decontamination by Fe0/MnO2/sand systems

“…Iron corrosion determined the extent of MB discoloration and the intuitive increase of MB discoloration with increasing Fe 0 loading is observed in all systems only after a pseudo-steady state is established in the systems or the capacity of MnO 2 is exhausted. Alyoussef 59 , 70 , 71 tested a parallel system with 4.5 g L –1 of MnO 2 and observed a larger decrease of MB discoloration for 18 days. Similar observations were made by Noubactep et al 82 in their experiments for uranium removal.…”

“…This is achieved herein by investigating the systems for 7, 18, 25 and 47 days. This corresponds to following the fate of aqueous MB (discoloration) as the contact time increases from 7 to 47 days 59 , 70 , 71 . In particular the variation of the pH value in the systems will be discussed in detail.…”

“…This experimental section is adapted from Cao et al 70 using the same experimental design and two more MnO 2 minerals.…”

“…The use of tap water rather than deionised water was motivated by the fact that tap water is closer to natural water in its chemical composition. The MB molecular formula is C 16 H 18 N 3 SCl corresponding to a molecular weight of 319.85 g. MB was chosen in this study because of its well-known strong adsorption onto solids 70 .…”

“…The material was used without any further pre-treatment. Fe 0 was proven as a powerful discoloration agent for MB given that discoloration agents in the form of FeCPs are progressively generated in-situ 70 .…”