Mn(VII)―Fe(II) pre-treatment for Microcystis aeruginosa removal by Al coagulation: Simultaneous enhanced cyanobacterium removal and residual coagulant control

A novel Mn(VII)-Fe(II) pre-treatment was proposed to simultaneously enhance the removal of Microcystis aeruginosa by aluminum chloride (AlCl3) coagulation and enabled lowering the dose of Al as effective coagulation can be achieved only by Al, however, at higher doses. In this process, permanganate [Mn(VII)] and ferrous sulfate [Fe(II)] were dosed sequentially prior to Al. The application of Fe(II) not only avoids the extensive oxidation of M. aeruginosa by Mn(VII) but also introduces Fe(III) formed in situ into the system. Results show that, at Al doses of 83.3-108.3 μM, Mn(VII)-Fe(II) pretreatment (Mn(VII) dose: 8.3-16.7 μM; Fe(II) dose: 39.5 μM) is capable of enhancing M. aeruginosa removal by 73.4-81.4%. In contrast, only 0-65.4% and 2.7-8.2% increase in M. aeruginosa removal is achieved by Mn(VII) and Fe(II) pre-treatment, respectively. The ESI-MS spectrum shows that the freshly formed Fe(III) hydrolyzes much more slowly than pre-formed Fe(III) does, and this effect results in its higher efficiency towards the removal of M. aeruginosa. Moreover, in the co-existing system, Fe tends to hydrolyze preferentially and the presence of Fe salts improves the precipitation of Al and vice versa. Thus, the use of Fe and Al as dual-coagulants is practically valuable to control the residual level of coagulant(s) besides its improvement on the removal of M. aeruginosa.