a researcher who accidentally combined a waste labeled as carbon-14 with 1 gallon of sulfuric acid-sodium dichromate solution. No disposal facility in the United States would accept the radioactive chromic acid waste. One option, the simple laboratory reduction of the dichromate, recovery of the chromium-containing precipitate, and neutralization of the acid, which would render the liquid waste only a radioactive hazard, may not be allowed without a permit in some states and EPA regions because it is considered to be treatment.
In large part, many of these regulatory dilemmas are unrelated to the real risks of laboratory mixed waste. Chief barriers to the safe and timely management of mixed waste include the following:
EPA regulations that inhibit laboratory and on-site minimization, storage, and treatment of mixed waste.
EPA regulations that discourage off-site minimization, treatment, storage, or disposal of mixed waste.
The U.S. NRC's reluctance at this time to establish a national policy that defines the de minimis level of contamination for all types of laboratory radioactive waste, below which the risk to health and the environment is not significant.
Community opposition to incinerators that could, with minimal risk, efficiently reduce hazard.
The low volume, unusual character, and great variety of this laboratory waste stream, which, together with the above barriers, discourage the development of commercial markets for mixed waste.
(For more complete information on regulations, see Chapter 9.)
Rigorous application of waste minimization principles can often solve the problems of managing mixed waste. Minimization of mixed waste can be achieved by modifying laboratory processes, improving operations, or using substitute materials. Such efforts are most successful when scientists and environmental health and safety staff work together to evaluate laboratory processes. Examples include the following:
Use of 2.5-mL scintillation vials ("minivials") rather than 10-mL vials. Adapters are available for scintillation counters with 10-mL vial racks.
Counting of phosphorus-32 (32p) without scintillation fluid by the Cerenkov method on the tritium (3H) setting of a liquid scintillation counter (approximately 40% efficiency); iodine-125 (125I) can be counted without scintillation fluid in a gamma counter.
Use of microscale chemistry techniques.
Elimination of the methanol/acetic acid (chemical) and radioactive mixed hazards in gel electrophoresis work by skipping the gel fixing step if it is not required.
Prevention of lead contamination by radioactivity by lining lead containers with disposable plastic or by using alternative shielding materials.
Reducing the volume of dry waste by compaction of items such as contaminated gloves, absorbent pads, and glassware.
Some simple operational improvements can also help minimize mixed waste. Surpluses can be minimized by limiting the acquisition of chemicals and radioactive materials to immediate needs. Contaminated equipment can sometimes be reused within restricted areas or decontaminated. Establishing procedures for noncontaminated materials can enable generators to keep normal trash separate from contaminated waste.
When possible, a substitute can be used for either the chemical or the radioactive source of the mixed waste. With radioactivity, the experimenter should use the minimum activity necessary and select the radionuclide with the most appropriate decay characteristics. Examples include the following:
Use of nonignitable scintillation fluid (e.g., phenylxylylethane, linear alkylbenzenes, and diisopropylnaphthalene) instead of flammable scintillation fluid (e.g., toluene, xylene, and pseudocumene). Liquid scintillation fluid that is sold as being "biodegradable" or "sewer disposable" is more appropriately labeled as "nonignitable" because biodegradability in the sanitary sewer can vary considerably with the local treatment facility.
Use of nonradioactive substitutes such as scintillation proximity assays for 32P or sulfur-35 (35S) sequencing studies or 3H cation assays, and enhanced chemiluminescence (ECL) as a substitute for 32P and 35S DNA probe labeling and southern blot analysis.
Substitution of enriched stable isotopes for radionuclides in some cases. Mass spectrometry techniques, such as ICP-MS, are beginning to rival the sensitivity of some counting methods. Examples include use of oxygen-18 (18O) and deuterium (2H) with mass spectrometry detection as substitutes for 19O and 3H.
Substitution of shorter-half-life radionuclides such as 32P (t1/2 = 14 days) for 33P (t1/2 = 25 days) in orthophosphate studies, or 33P or 32P for 35S (t1/2 = 87 days) in nucleotides and deoxynucleotides. In many uses, 131I (t1/2 = 8 days) can be substituted for 125I (t1/2 = 60 days). Additional exposure precautions may be required.