1,4-Dioxane Chemistry, Uses, and Occurrence - Environmental Investigation and Remediation - page 83

particles is transferred to
solvent molecules, which then achieve an excited state and transfer excess energy to adjacent solvent
molecules. The solvents selected for liquid scintillation cocktails remain in an excited state and
return to the zero state without emitting light. Energy from excited solvent molecules is captured by
the l uorescent solutes, and excited l uors dissipate the energy by emitting light photons, which
permits radiation counting in the visible light range. Ideally, each

which includes l uorescent compounds or l uors. Energy from emitted β

β emission results in a pulse of

light, though β -counting efi ciencies are generally below 100% and vary by the strength of the β

emitter. The liquid scintillation counter instrument consists of two photomultiplier tubes in a circuit.
Only those light pulses reaching both tubes are counted. Each l uorescence event is proportional to
a radioactive decay event, and the frequency of these events is directly proportional to the number
of 14 C, 3 H, 32 P, 35 S, or other radionuclides present in the sample.
1,4-Dioxane was favored for liquid scintillation cocktails because it is fully miscible and holds a
large amount of water in a homogeneous solution. Therefore, 1,4-dioxane is an ideal solvent for
samples containing various amounts of water. Impurities in 1,4-dioxane, such as peroxides formed
during storage, will quench the energy transmitted from

particles and must be avoided. The i rst
widely used liquid scintillation formulation, today known as Bray's solution, was a mixture of naph-
thalene and 1,4-dioxane, with ethylene glycol as an antifreeze to allow long-term storage of count-
ing vials in freezers (Bray, 1960). Bray's solution consisted of the following mixture dissolved in
1,4-dioxane; the 1,4-dioxane comprised more than 80% of the solution (Perkin Elmer, 2006):

β

Naphthalene 60 g/L

Methanol 100 mL/L

Ethylene glycol 20 mL/L

PPO = diphenyloxazole 4 g/L

POPOP = 1,4-bis-(5-phenyloxazolyl)benzene
(note that 5-phenyloxazolyl = phenyloxazolyl-
phenyl-oxazolyl-phenyl)

0.2 g/L

MSDSs indicate that scintillation-grade 1,4-dioxane used to make the cocktail has a composition
of 95-99% 1,4-dioxane (National Diagnostics, Inc., 1987a, 1987b; Fisher Scientii c, 2006). There
are many other formulations for liquid scintillation cocktails; toluene and xylene are the common
components. Larger laboratories have been known to recycle their scintillation wastes by distillation
in a rotary evaporator and desiccation with excess potassium hydroxide. These steps were also
followed to reduce waste volumes prior to disposal (Miyatake and Saito, 1984). Because the half-
lives of the radioactive components of scintillation wastes are relatively short, wastes are often
stored long enough for sufi cient decay to occur until radioactivity is no longer measurable.
Scintillation wastes have also been managed by incineration and by incorporating l ammable liquid
scintillation l uids into fuels; however, this practice releases the remaining radioactive components
to the environment. The University of Illinois once burned liquid scintillation cocktails as a fuel
supplement at its campus power plant with approval by the Nuclear Regulatory Commission, which
conducted inspections to coni rm that the levels of radioisotopes at the point of release were within
regulatory limits. Because only small amounts were burned, the practice was exempt from RCRA
(Resource Conservation and Recovery Act) Part B permit requirements. A i rm in Gainesville,
Florida, has processed liquid scintillation wastes by removing enough radiative components to
allow burning of the residual liquid in cement kilns (USEPA, 1996b).
A 1990 survey of more than 1000 facilities producing liquid scintillation l uid wastes showed that
the leading producers were industrial facilities, followed by academic facilities, government facilities,
and medical facilities. The total quantity of all liquid scintillation l uids produced at the facilities
surveyed was approximately 100,000 ft 3 (approximately 748,000 gallons) (USEPA, 1996b).