Abstract
The role of manganese dioxide (MnO2) in the process of water treatment using metallic iron (Fe0/H2O) was investigated in quiescent batch experiments for t ≤ 60 d. MnO2 was used as an agent to control the availability of solid iron corrosion products (FeCPs) while methylene blue (MB) was an indicator of reactivity. The investigated systems were: (1) Fe0, (2) MnO2, (3) sand, (4) Fe0/sand, (5) Fe0/MnO2, and (6) Fe0/sand/MnO2. The experiments were performed in test tubes each containing 22.0 mL of MB (10 mg L−1) and the solid aggregates. The initial pH value was 8.2. Each system was characterized for the final concentration of H+, Fe, and MB. Results show no detectable level of dissolved iron after 47 days. Final pH values varied from 7.4 to 9.8. The MB discoloration efficiency varies from 40 to 80% as the MnO2 loading increases from 2.3 to 45 g L−1. MB discoloration is only quantitative when the operational fixation capacity of MnO2 for Fe2+ was exhausted. This corresponds to the event where adsorption and co-precipitation with FeCPs is intensive. Adsorption and co-precipitation are thus the fundamental mechanisms of decontamination in Fe0/H2O systems. Hybrid Fe0/MnO2 systems are potential candidates for the design of more sustainable Fe0 filters.
Highlights
The role of manganese dioxide (MnO2) in the process of water treatment using metallic iron (Fe0/H2O) was investigated in quiescent batch experiments for t ≤ 60 d. MnO2 was used as an agent to control the availability of solid iron corrosion products (FeCPs) while methylene blue (MB) was an indicator of reactivity
The objective of this study is to investigate the impact of various amounts of three different MnO2 on the efficiency of F e0/H2O systems for MB discoloration
The pH value was measured by combined glass electrodes (WTW Co., Germany)
Summary
The role of manganese dioxide (MnO2) in the process of water treatment using metallic iron (Fe0/H2O) was investigated in quiescent batch experiments for t ≤ 60 d. MnO2 was used as an agent to control the availability of solid iron corrosion products (FeCPs) while methylene blue (MB) was an indicator of reactivity. MB discoloration is only quantitative when the operational fixation capacity of MnO2 for Fe2+ was exhausted This corresponds to the event where adsorption and co-precipitation with FeCPs is intensive. Fe0 filters are a special case of "metal corrosion in porous media"[22] This process has two key characteristics[23,24]: (1) the time-dependent decrease of the reaction kinetics of iron corrosion (“reactivity loss”), and (2) the progressive decrease of the hydraulic conductivity (permeability loss) due to the initial porosity being filled by in-situ generated iron oxides and hydroxides. How fast the porosity decreases as a function of time is an open issue and has received very little attention[26,27,28] It is known from the broad corrosion literature that the corrosion kinetics of metals, including Fe0 under environmental conditions, is neither constant nor linear[22,29]. MnO2 is used to control the availability of ’free’ iron corrosion products (FeCPs)
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