PROCEDURE FOR WASTE MINIMIZATION
A. Methods for Treating Hazardous Wastes in the Laboratory
Scientific and engineering research and teaching activities in academic institutions can result in the generation of relatively small quantities of a wide variety of waste and surplus chemicals. The small-scale treatment and deactivation of these sorts of chemical products and by-products as part of the experimental plan (i.e., as part of the routine procedure) is one approach that can be used to address the problem of waste minimization at the laboratory level.
B. Reagent Substitutions can Result in Less Hazardous and/or Less Costly Waste
To enhance safety and minimize the environmental consequences of an experiment, careful thought should be given to the materials to be used and the scale of the experiment. Traditionally, chemists have chosen reagents and materials for experiments to meet scientific criteria without always giving careful consideration to waste minimization or environmental objectives. In synthetic procedures, overall yield and purity of the desired product were usually regarded as the most important factors. Material substitution emerged as an important consideration in manufacturing process design because of the large quantities of chemicals involved.
The following questions should now be considered when choosing a material to be used as a reagent or solvent in an experimental procedure:
- Can this material be replaced by a substance that will expose the user to a lower order of potential hazard?
- Can this material be replaced by one that will reduce the generation of hazardous waste and disposal cost?
The following illustrate applications of these principles to common laboratory procedures:
- Mercury Apparatus
Mercury thermometers are widely used and easily broken, which results in worker exposure to mercury, release of the vapors to the environment, and increased waste disposal costs as all of the cleanup material must be disposed of as mercury contaminated waste. Substitution of alcohol thermometers for mercury thermometers eliminates these problems. Thermometers containing alcohol are as accurate and have as wide a temperature range as mercury thermometers, and the waste from the cleanup of broken alcohol thermometers can be thrown in the regular trash. Moreover, the breakage of alcohol thermometers does not expose the lab personnel to poisonous vapors. (see NMSU Policy: Mercury Mandate)
Organic Solvent Replacement
Organic solvents for liquid-liquid extraction or chromatography can often be replaced by other solvents with significant benefit. Benzene, once a widely used solvent, is now recognized as a human carcinogen and must be handled accordingly. Toluene can often serve as a satisfactory substitute. Diethyl ether is a flammable solvent whose handling must take into account its tendency to form explosive peroxides. Methyl t-butyl ether offers only slight advantages over diethyl ether with respect to flammability, but its greatly reduced tendency to form peroxides eliminates the need to monitor peroxide formation during handling and storage.
Organic Solvents & Supercritical fluids
The technology for handling supercritical fluids has developed rapidly in recent years. Supercritical carbon dioxide can replace organic solvents for high-performance chromatography and is beginning to find use as a reaction medium. While supercritical solvents require specialized equipment for handling, they offer the potential benefit of large reductions in organic solvent waste.
- Phosgene in Organic Transformations
Phosgene is a highly toxic gas used as a reagent in many organic transformations. Its use requires proper precautions to deal with the containment of the gas and the handling and disposal of cylinders. Commercially available substitutes such as diphosgene (trichloro-methyl)chloroformate, a liquid, or triphosgene bis(trichloromethyl)carbonate, a low-melting solid, can often be substituted for phosgene by appropriate adjustment of experimental conditions or can be used to generate phosgene only on demand. Both chemicals are highly toxic themselves, but they offer a means to avoid the problems associated with handling a toxic gas.
- Heavy Metal Substitutions
Many widely used reagents contain toxic heavy metals, such as chromium and mercury. Waste containing these materials cannot be incinerated and must be handled separately for disposal. Thus, substitution of other reagents for heavy metal reagents will almost always be beneficial with respect to hazard and waste minimization. Chromic acid cleaning solutions for glassware can be replaced by proprietary detergents used in conjunction with ultrasonic baths. Various chromium (VI) oxidants have been important in synthetic organic chemistry, but their use can often be avoided through the substitution of organic oxidants. The Swern oxidation of alcohols (oxalyl chloride/DMSO) produces relatively innocuous byproducts, which can be handled with other organic waste. Other oxidation reagents tailored to the specific needs of a given transformation are available.
Fluorine and fluorinating reagents such as perchloryl fluoride are among the most demanding of reagents to handle because of their high reactivity and toxicity. Accordingly, there has been considerable incentive to develop substitutes for these materials. One example is F-TEDA-BF4, or 1-chloromethyl-4-fluoro-1,4-diazonia[2.2.2]bicycloctane bis(tetrafluoroborate). This reagent can be substituted for more hazardous reagents in many fluorination procedures.