SURFACE CHARGE AND DISSOLUTION AT THE α-MN2O3 – WATER INTERFACE IN NACL MEDIA

Manganese oxides are widespread in the environment and are found in nearly all soils. They are highly reactive and participate in a variety of acid-base, sorption and redox reactions. Their adsorption capacity for heavy metals and toxic contaminants is of particular environmental importance.For example, manganese oxides are used for wastewater treatment and metal remediation. To evaluate the adsorption capacity of these phases, it is necessary to understand the solid-water interface properties of manganese-oxides.

This presentation will outline an experimental study investigating the surface charging and dissolution behavior of α-Mn₂O₃ in NaCl media. A synthetic α-Mn₂O₃, end-member bixbyite, is being used. Prior to use in experiments, the bixbyite sample was washed extensively. Washing comprised rinsing in HCl, nanopure water and hydrothermal pre-treatment. The sample was characterized by XRD and SEM, which verified the presence of crystalline bixbyite and no other phases were identified. The surface area of the sample was determined by BET measurement. The sample has a low surface area of < 1m²/g.

Potentiometric titrations are being performed using a Mettlerautotitrator and Ross combination pH electrode. All titrations are completed in NaCl media at ionic strengths of 0.03, 0.06 and 0.3 m, with the temperature controlled at 25 ºC. Titrations performed from acid-base showed considerable variation, arising from dissolution of the bixbyite. Therefore, titrations are completed from base to acid, and samples are withdrawn at acidic pH values. The withdrawn samples are filtered then analyzed by ICP to determine Mn concentrations. The bixbyitehas a pH_znpc value between 7 and 8, and a high pH-dependent surface charge.

Research sponsored by: NSF EAR–0842526.