THE EFFECT OF CHLORIDE VERSUS NITRATE ON THE SOLUBILITY OF PYROLUSITE

Manganese oxides are widespread in the environment and are highly reactive. Manganese oxides play an important role in reactions such as the biogeochemical cycling of heavy metals, toxic trace elements and soil nutrients. Such reactions are mediated by the high solubility of manganese oxide phases. This presentation will describe a study examining the solubility of pyrolusite (β-MnO₂) in sodium chloride and sodium nitrate media. Pyrolusite was selected for study as it is the most stable and abundant MnO₂ polymorph.

A commercial pyrolusite sample was used in all experiments. Prior to use, the sample was washed and characterized extensively. Characterization included XRD, TGA, and SEM imaging. The XRD confirmed that the sample was pure, crystalline pyrolusite. The SEM images showed that the sample has a blocky morphology.

Parallel solubility experiments were performed in NaCl and NaNO₃ media, at 0.3 m ionic strength. Each suite of experiments (NaCl or NaNO₃ media) comprised 7 solutions at varying pH, ranging from pH 3 to 9.4. All experiments were conducted in duplicate. The solubility experiments were performed at room temperature (23 °C) by reacting 0.8 g of pyrolusite with 30 g of aqueous solutions, in air-tight syringes that were placed on a rotating rack. The syringes were sampled at discrete intervals, with the initial sample taken after one hour. At each sampling 1 g of solution was filtered through a 0.2 μm membrane filter, the sample was acidified with HNO₃ for later ICP analysis. At the fourth and fifth sampling an additional l.5 g sample was withdrawn for pH measurements. The concentration of dissolved manganese was determined by ICP analysis. The experimental results show that pyrolusite dissolves at similar rates in the NaCl and NaNO₃ solutions. Additionally, it was observed that with increasing time the concentration of manganese in solution increased. Conversely, the concentration of manganese decreased as pH increased. At pH values above 5 the concentration of manganese remained nearly constant.