Solidification of hazardous materials

Solidification refers to a process in which waste materials are bound in a solid mass, often a monolithic block. The waste may or may not react chemically with the agents used to create the solid. Solidification is generally discussed in conjunction with stabilization as a means of reducing the mobility of a pollutant. Actually, stabilization is a broad term which includes solidification, as well as other chemical processes that result in the transformation of a toxic substance to a less or non-toxic form.

Experts often speak of the technologies collectively as solidification and stabilization (S/S) methods. Chemical fixation, where chemical bonding transforms the toxicant to non-toxic form, and encapsulation, in which toxic materials are coated with an additive are processes referred to in discussions of S/S methods. There are currently about 40 different vendors of S/S services in the United States, and though the details of some processes are privileged, many fundamental aspects are widely known and practiced by the companies.

S/S technologies try to decrease the solubility, the exposed surface area, and/or the toxicity of a hazardous material . While the methods also make wastes easier to handle, there are some disadvantages. Certain wastes are not good candidates for S/S. For example, a number of inorganic and organic substances interfere with the way that S/S additives will perform, resulting in weaker, less durable, more permeable solids or blocks. Another disadvantage is that S/S often double the volume and weight of a waste material, which may greatly affect transportation and final disposal costs (not considering potential costs associated with untreated materials contaminating the environment ). S/S additives, such as encapsulators, are available that will not increase the weight and volume of the wastes so dramatically, but these additives tend to be more expensive and difficult to use.

Methods for S/S are characterized by binders, reaction types, and processing schemes. Binders may be inorganic or organic substances. Examples of inorganic binders which are often used in various combinations include cements, lime, pozzolans, which react with lime and moisture to form a cement such as fly ash , and silicates. Among the organic types generally used are epoxies, polyesters, asphalt, and polyolefins (e.g. polyethylene). Organic binders have also been mixed with inorganic types, e.g., polyurethane and cement. The performance of a binder system for a given waste is evaluated on a case-by-case basis; however, much has been learned in recent years about the compatibility and performance of binders with certain wastes, which allows for some intelligent initial decisions related to binder selection and processing requirements.

Among the types of reactions used to characterize S/S are sorption , pozzolan, pozzolan-portland cement, and thermoplastic microencapsulation. Sorption refers to the addition of a solid to sorb free liquid in a waste. Activated carbon , gypsum, and clays have been used in this capacity. Pozzolan reactions typically involve adding fly ash, lime, and perhaps water to a waste. The mixture of fly ash, lime, and water form a low-strength cement that physically traps contaminants. This system is very alkaline and therefore may not be compatible with certain wastes. For example, a waste containing high amounts of ammonium ions would pose a problem because, under highly alkaline conditions, the toxic gas ammonia would be released. Also, sodium borate, carbohydrates, oil, and grease are known to interfere with the process.

The pozzolan-portland cement process consists of adding a pozzolan, often fly ash, and a portland cement to a waste. It may be necessary to add water if enough is not present in the waste. The resulting product is a high-strength matrix that primarily entraps contaminants. The performance of the system can be enhanced through the use of silicates to prevent interference by metals, clays to absorb excessive liquids and certain ions, surfactants to incorporate organic solvents, and a variety of sorbents that will hold on to toxicants as the solid matrix forms. Care is needed in selecting a sorbent because, for example, an acidic sorbent might dissolve a metal hydroxide, thereby increasing the mobility of the metal, or result in the release of toxic gases such as hydrogen sulfide or hydrogen cyanide. Borates, oil, and grease can also interfere with this process.

Thermoplastic encapsulation is accomplished by blending a waste with materials such as melted asphalt, polyethylene, or wax. The technique is more difficult and costly than the other methods introduced above because specialized equipment and higher temperatures are required. At these higher temperatures it is possible that certain hazardous materials will violently react. Additionally, it is known that high salt levels, certain organic solvents, and grease will interfere with the process.

There are basically four categories of processing schemes for S/S. For in-drum processing, S/S additives and waste are mixed and allowed to solidify in a drum. The drum and its contents are then disposed. In-plant processing is a second category which refers simply to performing S/S procedures at an established facility. The facility might have been designed by a company for their own wastes or as a S/S plant which serves a number of industries.

A third category is mobile-plant processing, in which S/S operations are moved from site to site. The fourth category is in-situ processing which involves adding S/S additives directly to a lagoon or contaminated soil .

As may be inferred from the above discussion, the goals of S/S operations are to remove free liquids from a waste, generate a solid matrix that will reliably contain hazardous materials, and/or create a waste that is no longer hazardous. The first goal is important because current regulations in the Resource Conservation and Recovery Act (RCRA) stipulate that free liquids are not to be disposed of in a landfill . The third goal is obviously important because disposing of a hazardous waste , called delisting, is much more costly and time-consuming than disposing of a regular waste.

Wastes are deemed to be hazardous on the basis of four characteristic tests and a series of listings. The tests are related to the ignitability, reactivity, corrosiveness, and extraction procedure (EP) toxicity of the waste. It is possible that S/S procedures delist a waste in any of the four test characteristics. The processes may also chemically transform a substance listed as hazardous waste by the Environmental Protection Agency (EPA) into a non-hazardous chemical. However, S/S techniques generally delist a waste through effecting a change in the results of the EP toxicity test, related to the goal of proper containment. If a waste can be contained well, it may pass the extraction test.

The extraction procedure has changed somewhat in recent years. Originally, the solid waste to be tested was stirred in a weakly acidic solution overnight, and then the supernatant was tested for certain inorganic and organic agents. Recently, the test has been replaced by what is known as the Toxicity Characteristic Leaching Procedure (TCLP). Shortcomings of the original extraction procedure were recognized some years ago, as Congress directed the EPA to develop a more reliable, second generation test through the Hazardous and Solid Waste Amendments of 1984. Work on the TCLP test actually began in 1981. In the TCLP, a solid waste is again suspended in an acidic solution, but the method of contacting the liquid with the solid has been changed. The suspension is now placed in a container that revolves about a horizontal axis, tumbling solids amidst the extracting solution. The extraction is allowed to proceed for 18 hours, after which time the solution is filtered and tested for a variety of organic and inorganic substances. The number of compounds tested under the TCLP is greater than the number measured for the original EP test. Failing the TCLP test dictates that a waste is hazardous and must be managed as such.

See also Hazardous Materials Transportation Act (1975); Storage and transport of hazardous materials

[Gregory D. Boardman ]

RESOURCES

BOOKS

Freeman, H. M. Standard Handbook of Hazardous Waste Treatment and Disposal. New York: McGraw-Hill, 1989.

Martin, E. J., and J. H. Johnson. Hazardous Waste Management Engineering. New York: Van Nostrand Reinhold, 1987.

Wentz, C. A. Hazardous Waste Management. New York: McGraw-Hill, 1989.

OTHER

U.S. Environmental Protection Agency. "Background Document: Toxicity Characteristic Leaching Procedure." Report No. PB87-154886, Washington, DC: 1986.

U.S. Environmental Protection Agency. "Guide to the Disposal of Chemically Stabilized and Solidified Waste." Report No. SW-872, Washington, DC: 1980.