SURFACE CHARGE DEVELOPMENT AT THE PYROLUSITE–WATER INTERFACE IN NACL MEDIA

Ion adsorption phenomena at mineral-solution interfaces are important reactions in controlling the cycling and transport of toxic contaminants and heavy metals in surficial environments. The role played by manganese oxides in adsorbing metals is of special interest to environmental scientists, because they are ubiquitous in natural soils and waters, and they have high adsorption capacities. The high adsorption capacity of manganese oxides results in them being used widely in water treatment and remediation applications. To fully understand and predict the adsorption behavior of manganese oxides, and thereby more effectively utilize their adsorption capacity, fundamental knowledge of the solid-solution interface behavior of manganese oxides is needed.

In this work, pyrolusite (β-MnO₂) was selected for study as it is the most stable and abundant polymorph of MnO₂, is commonly found in natural deposits, and is utilized for numerous technological applications. Potentiometic titrations are being performed to evaluate the surface charging behavior of pyrolusite as a function of pH, temperature (15 – 50 ºC), and ionic strength (0.03 – 0.3 m). Commercially available and specifically synthesized pyrolusite samples are being studied. Prior to use, the pyrolusite samples are washed and characterized extensively. Washing comprises rinsing in HCl, nanopure water and hydrothermal pre-treatment. Sample characterization included SEM and TEM imaging, XRD, and BET surface area measurements. XRD confirmed that the samples are pure, crystalline pyrolusite. SEM and TEM imaging showed the commercial samples to be blocky; whereas, the synthesized samples are euhedral and elongated along the (110) crystal face. The samples have a low surface area of < 1 m²/g.

Potentiometic titrations show a pH_znpc value close to 7.5, and a high pH-dependent surface charge in NaCl media.

Research sponsored by: NSF EAR–0842526.