PROTON-INDUCED SURFACE CHARGE AND DISSOLUTION OF PYROLUSITE IN NACL MEDIA AT 25 OC
This work focuses on pyrolusite (β-MnO2), which is the most stable and abundant MnO2 polymorph. Two commercially available pyrolusite samples were studied. Prior to use, the samples were washed and characterized extensively. Characterization included SEM and TEM imaging, XRD, TGA and BET surface area calculations. SEM and TEM imaging showed that the samples have a blocky morphology. XRD confirmed the samples were pure, crystalline pyrolusite. The TGA analysis was performed over a temperature range of 20 to 500 ºC, with a small weight loss of 2%. The surface area of the samples was low, ˂ 1 m2 g-1. To increase surface area, a single commercial sample was crushed, which increased the surface area to 16.4 m2 g-1.
Potentiometric titrations were performed to determine the surface charging behavior of pyrolusite as a function of pH (2–10) and ionic strength (0.03–0.30 m), in NaCl media at 25 °C. Additionally, the dissolution behavior of pyrolusite at low pH was examined. Following completion of the titrations, a sample was withdrawn from the experimental solution. The withdrawn samples were later analyzed by ICP-AES to determine the dissolved Mn concentration. The potentiometric titrations show that the surface charging behavior of pyrolusite was highly pH-dependent; however, the effect of ionic strength was limited. The pHznpc value of one sample was ~ 4.7, with a higher pHznpc value for the second sample. Dissolution of the pyrolusite resulted in the titration curves steepening significantly at low pH.
Research sponsored by: NSF EAR–0842526.