Characterizing the discoloration of methylene blue in Fe/H2O systems

Methylene blue (MB) was used as a model molecule to characterize the aqueous reactivity of metallic iron in Fe⁰/H₂O systems. Likely discoloration mechanisms under used experimental conditions are: (i) adsorption onto Fe⁰ and Fe⁰ corrosion products (CP), (ii) co-precipitation with in situ generated iron CP, (iii) reduction to colorless leukomethylene blue (LMB). MB mineralization (oxidation to CO₂) is not expected. The kinetics of MB discoloration by Fe⁰, Fe₂O₃, Fe₃O₄, MnO₂, and granular activated carbon were investigated in assay tubes under mechanically non-disturbed conditions. The evolution of MB discoloration was monitored spectrophotometrically. The effect of availability of CP, Fe⁰ source, shaking rate, initial pH value, and chemical properties of the solution were studied. The results present evidence supporting co-precipitation of MB with in situ generated iron CP as main discoloration mechanism. Under high shaking intensities (>150 min⁻¹), increased CP generation yields a brownish solution which disturbed MB determination, showing that a too high shear stress induced the suspension of in situ generated corrosion products. The present study clearly demonstrates that comparing results from various sources is difficult even when the results are achieved under seemingly similar conditions. The appeal for an unified experimental procedure for the investigation of processes in Fe⁰/H₂O systems is reiterated.