At the Savannah River Site (SRS), the In-Tank Precipitation (ITP) process uses sodium tetraphenylborate to decontaminate soluble waste by precipitating cesium-137. The first batch operations in October 1995 successfully decontaminated one-half million gallons of high-level waste producing 346,000 gallons of filtrate containing only 3 nCi cesium-137 per gram. During the precipitation and filtration, measured benzene generation fell well below safe limits. During a pause in the test sequence, the excess sodium tetraphenylborate decomposed. The decomposition occurred in highly alkaline solution in which tetraphenylborate was previously shown to be stable. The reaction al high pH appears catalyzed by trace amounts of transition metals in the waste. The reaction proceeded by successive removal of phenyl groups, ending in borate ion with major products benzene (87% yield), phenol (7%), and biphenyl (4.5%). The benzene remained in the slurry and was released under a nitrogen overblanket during carefully controlled operations of the slurry pumps in the tank. Subsequent experiments demonstrated transition metals and excess sodium tetraphenylborate required for the rapid decomposition reaction. The rate of benzene formation increases with temperature and under anoxic conditions.
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