Mn(II) oxidation by HOCl in the presence of iron oxides: a catalyzing effect

The oxidation of soluble manganese (Mn(II)) to insoluble manganese dioxide (MnO_x(s)) is fairly well understood; however, the role of ferric hydroxide/oxides (Fe(OH)₃_(s)) in catalyzing the oxidation of Mn(II) by oxidants such as free chlorine (HOCI) is one specific aspect of manganese removal via oxidation that requires further investigation. Data collected in this study indicate that the rate of Mn(II) oxidation may be beneficially catalyzed by the presence of previously formed Fe(OH)₃_(s) particles. The mechanistic means by which this enhanced oxidation is accomplished was the focal point of this research. Specifically, the research objectives were as follows:

(1) To study all possible Mn(ID) removal mechanisms for a typical groundwater system to determine the necessary experimental conditions required to isolate the study of Mn(II) oxidation in the presence of ferric hydroxides/oxides (Fe(OH)₃_(s)).

(2) To investigate the means by which ferric hydroxides/oxides (Fe(OH)₃_(s)) may enhance the removal of Mn(II) during water treatment by interacting with HOCI; and

(3) To develop an engineered system that captures the observed catalyzing effect iron oxides have on Mn(II) oxidation by HOCI citing key system design parameters.

To complete these objectives a combination of batch and continuous flow bench-scale experiments were utilized. Batch study results indicated that the primary Mn(II) removal mechanism was a combination of adsorption and oxidation, specifically, adsorption of Mn(II) onto the iron oxide surface where Mn(II) is subsequently oxidized. A continuous flow system was developed to utilize this removal mechanism under water treatment plant conditions to improve the efficiency of iron and manganese removal. The results from experimentation with the continuous flow system indicated the following:

∙ Sufficient free chlorine residual in effluent insures consistent system performance,

∙ Initial iron oxide concentration within reactor system must have adequate adsorption capacity for initial adsorption-oxidation step to occur,

∙ Removal efficiency and reactor stability increase with the accumulation of manganese oxides, and

∙ Solution pH and reactor hydraulics affect system performance significantly.

The results suggest that this technology has the potential to change the look of conventional groundwater treatment systems that practice iron and manganese removal.