STABILIZATION OF MERCURY IN HIGH pH TANK SLUDGES

The DOE complex contains many tank sludges contaminated with mercury. The high pH of these tank sludges typically fails to stabilize the mercury, resulting in these radioactive wastes also being characteristically hazardous or mixed waste. The traditional treatment for soluble inorganic mercury species is precipitation as insoluble mercuric sulfide. Sulfide treatment and a commercial mercury-stabilizing product were tested on surrogate sludges at various alkaline pH values. Neither the sulfide nor the commercial product stabilized the mercury sufficiently at the high pH of the tank sludges to pass the Toxicity Characteristic Leach Procedure (TCLP) treatment standards of the Resource Conservation and Recovery Act (RCRA). The commercial product also failed to stabilize the mercury in samples of the actual tank sludges. Tank sludge samples were sequentially extracted with deionized water, a weak hydrochloric acid solution (pH of 2), a 1 N potassium hydroxide solution, a 12 N nitric acid solution, and aqua regia. Only insoluble species such as mercuric sulfide typically survive to the aqua regia step. Surprisingly, significant amounts of mercury were extracted from the tank sludge at each step. One possible explanation is a distribution of various mercury compounds, soluble in different extractants. Such a suite of compounds is not expected for these sludges, especially a relatively insoluble specie(s) only extracted by aqua regia. Another possible explanation is that significant amounts, if not most, of the mercury is bound in the undissolved sludge solids and this mercury is released as more of this matrix is dissolved at each step of the sequential extraction. Only soluble inorganic mercury compounds or elemental mercury were disposed into these tank wastes based on process knowledge, not mercuric sulfide. In addition, soluble sulfides were not disposed into the tanks, although significant amounts of sulfates do exist in the tank waste. Thermodynamic evaluation indicates that mercuric oxide is the equilibrium specie at the tank waste conditions. Mercuric oxide has a small, but significant solubility at the tank pH. The mercuric oxide distributes between mercuric hydroxide in solution and solid mercuric oxide. It is surmised that the undissolved mercuric oxide becomes bound, perhaps co-precipitated, with other different undissolved species. Some clay is known to be in the tank wastes, which may even be resistant to the aqua regia. Perhaps, mercury bound to these clays is not released until extracted by aqua regia.