(687d) Fundamental Study on Treatment of Osmium-Containing Wastewater

Osmium tetroxide (OsO₄) is often used in microscopic analysis as a dyeing reagent and in some organic reactions as an oxidizer. Most of osmium compounds including metallic osmium are easily oxidized under ambient condition into OsO₄. Although OsO₄ is highly toxic substance, wastewater containing OsO₄ (highly soluble in water) is extremely hard to treat because of OsO₄â??s high fugacity into gas phase. Therefore, proper treatment method of osmium-containing wastewater has not been established yet. Considering that osmium is a precious metal, recovery of OsO₄ or metallic osmium from wastewater for subsequent recycling is preferable rather than just diluting the wastewater for dumping. In this study, various treatment methods were tested and the behavior of osmium compounds was studied.

Firstly, reduction of OsO₄ by simple reductants such as ethanol and acetaldehyde was investigated in various conditions of aqueous solutions. By using ethanol, OsO₄ was successfully reduced into water-insoluble species. For example, when 0.1 mL of ethanol was added into 3 mL of 100 ppm OsO₄ solution adjusted at pH = 13, OsO₄ concentration decreased rapidly in several minutes due to reduction by ethanol, and by subsequently adjusting its pH to 3 the osmium compound precipitated and residual osmium concentration in water phase became as low as 0.02 ppm. Considering treatment of real wastewater, effect of co-existing material such as phosphate buffer upon the behavior of OsO₄ was also studied.

Secondly, treatment of osmium-containing wastewater using both supercritical water oxidation (SCWO) and supercritical carbon dioxide extraction was investigated. SCWO oxidizes all the osmium compounds into OsO₄ and simultaneously gets rid of co-existing organic matter completely; and subsequent supercritical CO₂ process extracts OsO₄, which has high octanol/water partition coefficient, from water phase. Behavior of osmium compounds in this oxidation/extraction system will be reported.