HAZARDOUS WASTES IN THE GEOSPHERE

19.13. HAZARDOUS WASTES IN THE GEOSPHERE

The sources, transport, interactions, and fates of contaminant hazardous wastes in the geosphere involve a complex scheme, some aspects of which are illustrated in Figure 19.4 . As illustrated in the figure, there are numerous vectors by which hazardous wastes can get into groundwater. Leachate from a landfill can move as a waste plume carried along by groundwater, in severe cases draining into a stream or into an aquifer where it may contaminate well water. Sewers and pipelines may leak hazardous substances into the geosphere. Such substances seep from waste lagoons into geological strata, eventually contaminating groundwater. Wastes leaching from sites where they have been spread on land for disposal or as a means of treatment can contaminate the geosphere and groundwater. In some cases wastes are pumped into deep wells as a means of disposal.

The movement of hazardous waste constituents in the geosphere is largely by the action of flowing water in a waste plume as shown in Figure 19.4 . The speed and degree of waste flow depend upon numerous factors. Hydrologic factors such as The movement of hazardous waste constituents in the geosphere is largely by the action of flowing water in a waste plume as shown in Figure 19.4 . The speed and degree of waste flow depend upon numerous factors. Hydrologic factors such as

can be expressed by a distribution coefficient, K d ,

K d = Cs (9.13.1)

C w where C S and C W are the equilibrium concentrations of the constituent on solids and

in solution, respectively. This relationship assumes relatively ideal behavior of the hazardous substance that is partitioned between water and solids (the sorbate). A more empirical expression is based on the Freundlich equation,

C S = K F C eq 1/n (9.13.2) where and K F and 1/n are empirical constants.

Landfill Land

Waste lagoon

Waste plume Water table

Leakage

Leachate

Aquifer containing groundwater

Figure 19.4. Sources and movement of hazardous wastes in the geosphere. Several important properties of the solid determine the degree of sorption. One

obvious factor is surface area. The chemical nature of the surface is also important. Among the important chemical factors are presence of sorptive clays, hydrous metal oxides, and humus (particularly important for the sorption of organic substances).

In general, sorption of hazardous waste solutes is higher above the water table in the unsaturated zone of soil. This region tends to have a higher surface area and to favor aerobic biodegradation processes.

The chemical nature of the leachate is important in sorptive processes of hazardous substances in the geosphere. Organic solvents or detergents in leachates will solubilize organic materials, preventing their retention by solids. Acidic leach- ates tend to dissolve metal oxides,

M(OH) +

2 (s) + 2H → M 2+ + 2H 2 O

Heavy metals are among the most dangerous hazardous waste constituents that are transported through the geosphere. Many factors affect their movement and attenuation. The temperature, pH, and reducing nature (as expressed by the negative log of the electron activity, pE) of the solvent medium are important. The nature of the solids, especially the inorganic and organic chemical functional groups on the surface, the cation-exchange capacity, and the surface area of the solids largely determine the attenuation of heavy metal ions. In addition to being sorbed and undergoing ion exchange with geospheric solids, heavy metals may undergo oxidation-reduction processes, precipitate as slightly soluble solids (especially sulfides), and in some cases, such as occurs with mercury, undergo microbial methylation reactions that produce mobile organometallic species.

The importance of chelating agents interacting with metals and increasing their mobilities has been illustrated by the effects of chelating ethylenediaminetetraacetic acid (EDTA) on the mobility of radioactive heavy metals, especially 60 Co. 8 The EDTA and other chelating agents, such as diethylenetriaminepentaacetic acid (DTPA) and nitrilotriacetic acid (NTA), were used to dissolve metals in the decon- tamination of radioactive facilities and were codisposed with radioactive materials at Oak Ridge National Laboratory (Tennessee) during the period 1951–1965. Unexpectedly high rates of radioactive metal mobility were observed, which was

attributed to the formation of anionic species such as 60 CoT - (where T 3 - is the chelating NTA anion). Whereas unchelated cationic metal species are strongly retained on soil by precipitation reactions and cation exchange processes,

Co 2+ + 2OH- → Co(OH)

2 (s)

(9.13.5) anion bonding processes are very weak, so that the chelated anionic metal species

2Soil}-H + + Co 2+ → (Soil}-) 2 Co 2+ + 2H +

are not strongly bound. Naturally occurring humic acid chelating agents may also be involved in the subsurface movement of radioactive metals. It is now generally accepted that poorly biodegradable, strong chelating agents, of which EDTA is the prime example, will facilitate transport of metal radionuclides, whereas oxalate and citrate will not do so because they form relatively weak complexes and are readily

biodegraded. 9 Biodegradation of chelating agents tends to be a slow process under subsurface conditions. Soil can be severely damaged by hazardous waste substances. Such materials may alter the physical and chemical properties of soil and thus its ability to support plants. Some of the more catastrophic incidents in which soil has been damaged by exposure to hazardous materials have arisen from soil contamination from SO 2 emitted from copper or lead smelters, or from brines from petroleum production. Both of these contaminants stop the growth of plants and, without the binding effects of viable plant root systems, topsoil is rapidly lost by erosion.

Unfortunate cases of the improper disposal of hazardous wastes into the geosphere have occurred throughout the world. For example, in December 1998 it was alleged that Formosa Plastics had disposed of 3000 tons of mercury-containing wastes in Cambodia, the result of by-product sludge generated by the chloralkali electrolytic prcess for generating chlorine and sodium hydroxide. Subsequently, illegal dump sites containing mercury were found in many places in Taiwan, causing major environmental concerns. 10