New Technologies for Chlorine-Containing Solvents

More than 280,000 landfills and 340,000 surface impoundments, including both operating and closed facilities, may currently exist in the United States, according to estimates released by the U.S. Congress's Office of Technology Assessment (OTA) in 1985. A significant number of these sites are known to contain appreciable amounts of hazardous materials, including solvents.

Halogenated solvents, which include chlorine-containing chemicals that have been widely used in applications such as dry cleaning, degreasing, aerosols, and as a feedstock in the manufacture of other chemicals, figure prominently among this group of materials. In recent years, increasingly stringent state and federal regulation of solvent production and disposal has markedly decreased the quantity of halogenated solvents that are disposed of at public sites. Therefore, rectifying past mistakes appears to be the major challenge in dealing with these hazardous materials.

Six halogenated solvents—chloroform, carbon tetrachloride, trichloroethylene, tetrachloroethylene, methyl chloroform, and methylene chloride—are among those chemicals most frequently found at waste sites. Environmental concerns began to affect the production, use, and disposal of these solvents during the late 1960s and early 1970s. For example, before about 1975, trichloroethylene (used as a degreaser) was typically removed from industrial premises by a waste hauler and disposed of at a waste site, often a municipal dump. Substantial amounts of oil, grease, and dirt, and often kerosene and other hydrocarbons accompanied the trichloroethylene.

Since that time, new federal regulations have greatly changed customary practices. Waste fluids are now segregated. The dirty hydrocarbons can be burned as fuels and are of economic value for their heat content. In fact, most of the halogenated hydrocarbons now go to recycling plants where they are redistilled and later sold.

During use in dry-cleaning establishments and manufacturing plants, major portions of the solvents evaporate and subsequently are destroyed by reactions with active radicals in the atmosphere. Thus, even during the period predating the advent of environmental concerns, only a relatively minor fraction of the solvent was destined to become a liquid waste.

Once the liquid was brought to the disposal site, additional volatilization often occurred. In fact, the results of extensive studies have led geophysicists to estimate that 95 percent of all carbon tetrachloride and methyl chloroform distributed commercially has gone into the atmosphere. In general, concentrations of the halogenated hydrocarbons in the atmosphere are tiny in comparison with those of such toxic substances as ozone, which causes extensive damage to plants. The use of the atmosphere as a sink and destructive agent for trichloroethylene, tetrachloroethylene, chloroform, and methylene chloride is much to be preferred to the alternative of their incorporation in the ground and subsequent migration to aquifers.

Groundwater contamination

Nevertheless, even the minor fractions of halogenated solvents that have been buried constitute a serious source of pollution in aquifers. Much of this contaminating material comes from three sources—leaking underground tanks, waste sites, and spills.

Under the Superfund program, the federal government has levied funds to clean up hazardous chemicals at selected abandoned sites, including landfills, lagoons, and waste storage sites. In 1985 a total of 845 sites had been placed on the Superfund National Priority List. In addition to these abandoned sites, however, many company-owned locations (also including lagoons and landfills that take in pollutants from leaky underground storage tanks and pipes) are contributing to groundwater contamination.

Although there are no comprehensive statistics on the means by which halogenated solvents have been introduced into soil, anecdotal evidence suggests that about one-third of these solvents originally were contained in drums at landfills, another one-third originated as fluids dumped at landfills, and the remainder entered the soil by means of leaky tanks and plumbing at users' sites.

Underground storage tanks constitute a major source of potential pollution. According to a recent Environmental Protection Agency (EPA) fact book, it has been estimated that there are a million underground tanks, some of which are abandoned, and that 10 percent or more of the total are leaking.

Most of these tanks have held gasoline or other petroleum products, but a substantial number have been devoted to halogenated solvents, some of which have leaked out of the tanks. These solvents tend to migrate downward in the soil, unnoticed and undetected until they appear in groundwater wells. A considerable volume of soil and plumes of water in an aquifer may be contaminated before the leakage becomes evident.

When leaking solvents have reached groundwater, the generally preferred technique for removing them is "air-stripping." In this procedure water is pumped to the top of a tower and is broken up into small droplets. These droplets fall through a rising current of air, allowing the solvent molecules to volatilize and either to escape directly into the atmosphere or else to be captured by an adsorber.

The principal cost of remedying groundwater contamination at any particular location will depend on the volume of groundwater that has been contaminated. The volume that is regarded as being contaminated will depend on the tolerance that the regional EPA office sets. One target that has been proposed is one part per billion. In some instances, very large volumes of water will be involved, with corresponding costs for treatment.

Waste site cleanup proceeding slowly

Waste sites usually present a more complex cleanup problem than do leaky underground tanks. Most Superfund sites contain many different chemicals and large volumes of contaminated soil.

Remedial measures have included the removal of drums containing liquid from the surface of some sites, but metal detectors have revealed the presence of other, buried containers. These containers may be corroded and leaking, but only rarely have efforts been made to remove them.

After cursory studies of a targeted site, the usual practice is to build a fence around it. This is sometimes followed by enclosing the area with bentonite slurry walls that are designed to have low permeability and to reduce leakage substantially. The enclosed area is then often capped by a clay layer or a synthetic membrane, or both. Drums of contaminating chemicals have been removed from the surface at some Superfund sites, but many leaking drums remain buried.

An additional remedial measure often employed is to locate wells down-gradient from a site and to pump leachate from the wells; the leachate is then processed to remove the contaminants.

Only a fraction of the contaminants at a waste site is usually recovered during initial pumping. Some of the remaining material is treated by being dissolved in immobile fluids, some adsorbs to particles of soil, and some is still contained in corroded drums. Effective treatment of waste chemicals from a particular site could thus require many years of pumping.

At a limited number of sites EPA has paid for a cleanup process that involves excavating soil and moving it to a Super-fund disposal site. However, the agency has been criticized for this kind of remediation, which can cost up to $200 per ton. (Some sites contain millions of tons of contaminated soil.) Critics also point out that removal of soil only transfers the problem elsewhere.

A more promising solution for cleaning up waste sites and contaminated groundwater may lie in modifying existing conditions so that naturally occurring microbial activity can be stepped up.

Results of successful large-scale microbial action at municipal dumps, sewage treatment plants, and oil refineries support the growing belief that, ultimately, bio-degradation will systematically be applied on a large scale at hazardous waste sites.

Though it has long been known that anaerobes can be used to dechlorinate chlorine-containing substances, the process has only recently taken on a new significance in light of its potential for remediation work with halogenated solvents. Researchers at a number of institutions have been studying these microorganisms. Experiments at Stanford University have shown that rapid degradation of halogenated solvents can occur under favorable anaerobic conditions. One experiment resulted in nearly quantitative conversion of tetrachloroethylene to vinyl chloride, and a series of experiments with 14C-tagged tetrachloroethylene resulted in variable fractions of the material being converted into carbon dioxide.

Knowledge derived from these and other experiments with anaerobes provides a basis for interpreting some of the observations that have been made on leachates at sites on the Superfund National Priority List. For instance, the trans-dichloroethylene that is present at ninety-seven sites could only have been produced by microbial activity, presumably from trichloro-ethylene.

Based on this and other evidence, there is strong reason to believe that substantial biotransformations of halogenated solvents are already occurring at waste sites. In general, these transformations have proceeded without constructive human intervention. In most cases, there probably have been restraining factors such as a lack of inorganic nutrients or suboptimal pH.

With constructive human intervention, reductive dechlorination by anaerobes at waste sites seems applicable to a large fraction of chlorine-containing chemicals that have more than trivial solubility in water. If anaerobic activity were fostered, at least partial dechlorination of wastes other than solvents would also occur. Partial dechlorination would make the wastes more susceptible to aerobic bacteria and further degradation at a later phase. Ultimately it may be useful to take advantage of new genetic engineering techniques to obtain superior organisms.

The OTA has estimated that hazardous waste cleanup costs could be as high as several hundred billion dollars. Actual costs, which may be less than estimated, will depend on standards yet to be adopted. Present standards (based on animal experiments of increasingly doubtful applicability for humans) are tentative, and requirements for cleanup vary from site to site. In any event, the systematic use of microbes could provide a cost-effective alternative to methods such as spending thirty or more years of pumping leachates and considered likely.