Evaluation of the effects of shaking intensity on the process of methylene blue discoloration by metallic iron.

The term mixing (shaking, stirring, agitating) is confusing because it is used to describe mass transfer in systems involving species dissolution, species dispersion and particle suspension. Each of these mechanisms requires different flow characteristics in order to take place with maximum efficiency. This work was performed to characterize the effects of shaking intensity on the process of aqueous discoloration of methylene blue (MB) by metallic iron (Fe(0)). The extent of MB discoloration by three different materials in five different systems and under shaking intensities varying from 0 to 300 min(-1) was directly compared. Investigated materials were scrap iron (Fe(0)), granular activated carbon (GAC), and deep sea manganese nodules (MnO(2)). The experiments were performed in essay tubes containing 22 mL of the MB solution (12 mg/L or 0.037 mM). The essay tubes contained either: (i) no reactive material (blank), (ii) 0-9.0 g/L of each reactive material (systems I, II and III), or (iii) 5g/L Fe(0) and 0 to 9.0g/L GAC or MnO(2) (systems IV and V). The essay tubes were immobilized on a support frame and shaken for 0.8-5 days. Non-shaken experiments lasted for duration up to 50 days. Results show increased MB discoloration with increasing shaking intensities below 50 min(-1), a plateau between 50 and 150 min(-1), and a sharp increase of MB discoloration at shaking intensities >or=200 min(-1). At 300 min(-1), increased MB discoloration was visibly accompanied by suspension of dissolution products of Fe(0)/MnO(2) and suspension of GAC fines. The results suggest that, shaking intensities aiming at facilitating contaminant mass transfer to the Fe(0) surface should not exceed 50 min(-1)