HAZARDOUS WASTES IN THE HYDROSPHERE

21.14 HAZARDOUS WASTES IN THE HYDROSPHERE

Hazardous-waste substances can enter the hydrosphere as leachate from waste landfills, drainage from waste ponds, seepage from sewer lines, or runoff from soil. Deliberate release into waterways also occurs, and is a particular problem in countries with lax environmental enforcement. There are, therefore, numerous ways by which hazardous materials may enter the hydrosphere.

For the most part, the hydrosphere is a dynamic, moving system, so that it provides perhaps the most important variety of pathways for moving hazardous- waste species in the environment. Once in the hydrosphere, hazardous-waste species can undergo a number of processes by which they are degraded, retained, and transformed. These include the common chemical processes of precipitation- dissolution, acid-base reactions, hydrolysis, and oxidation-reduction reactions. Also included is a wide variety of biochemical processes which, in most cases, reduce hazards, but in some cases, such as the biomethylation of mercury, greatly increase the risks posed by hazardous wastes.

The unique properties of water have a strong influence on the environmental chemistry of hazardous wastes in the hydrosphere. Aquatic systems are subject to constant change. Water moves with groundwater flow, stream flow, and convection currents. Bodies of water become stratified so that low-oxygen reducing conditions may prevail in the bottom regions of a body of water, and there is a constant interaction of the hydrosphere with the other environmental spheres. There is a continuing exchange of materials between water and the other environmental spheres. Organisms in water may have a strong influence on even poorly biode- gradable hazardous-waste species through bioaccumulation mechanisms.

Figure 21.5 shows some of the pertinent aspects of hazardous-waste materials in bodies of water, with emphasis upon the strong role played by sediments. An interesting kind of hazardous-waste material that may accumulate in sediments consists of dense, water-immiscible liquids that can sink to the bottom of bodies of water or aquifers and remain there as “blobs” of liquid. Hundreds of tons of PCB wastes have accumulated in sediments in the Hudson River in New York State and are the subject of a heated debate regarding how to remediate the problem.

Hazardous-waste species undergo a number of physical, chemical, and biochem- ical processes in the hydrosphere that strongly influence their effects and fates. The major ones are listed below:

• Hydrolysis reactions are those in which a molecule is cleaved with addition of a molecule of H 2 O. An example of a hydrolysis reaction is the hydrolysis of dibutyl phthalate, Hazardous Waste Number U069:

C O C 4 H 9 C OH

+H 2 2 O

+ 2HOC 4 H 9

C O C 4 H 9 C OH O

Another example is the hydrolysis of bis(chloromethyl)ether to produce HCl and formaldehyde:

Cl C O C Cl + H 2 O

2 HCH + 2 HCl

H H Compounds that hydrolyze are normally those, such as esters and acid anhy-

drides, originally formed by joining two other molecules with the loss of

H 2 O. • Precipitation reactions, such as the formation of insoluble lead sulfide

from soluble lead(II) ion in the anaerobic regions of a body of water: Pb 2+ + HS- → PbS(s) + H + An important part of the precipitation process is normally aggregation of

the colloidal particles first formed to produce a cohesive mass. Precipitates are often relatively complicated species, such as the basic salt of lead

carbonate, 2PbCO 3 • Pb(OH) 2 . Heavy metals, a common ingredient of hazardous-waste species precipitated in the hydrosphere, tend to form

hydroxides, carbonates, and sulfates with the OH-, HCO 3 -, and SO 2 4 - ions

Dissolved hazardous organics and inorganics

Photosynthetic generation of biomass

Shallow sediment Release of hazardous- stirred by waves

waste species from sediments

Settling microparticles of bio- mass and mineral matter

CO 2 CH 4

Sorption of hazardous

Deep unstirred sediments

substances in sediment

Figure 21.5 Aspects of hazardous wastes in surface water in the hydrosphere. The deep unstirred sediments are anaerobic and the site of hydrolysis reactions and reductive processes that may act on hazardous-waste constituents sorbed to the sediment.

that commonly are present in water, and sulfides are likely to be formed in bottom regions of bodies of water where sulfide is generated by anaerobic bacteria. Heavy metals are often coprecipitated as a minor constituent of some other compound, or are sorbed by the surface of another solid.

• Oxidation-reduction reactions commonly occur with hazardous-waste

materials in the hydrosphere, generally mediated by microorganisms. An example of such a process is the oxidation of ammonia to toxic nitrite ion mediated by Nitrosomonas bacteria:

NH 3 + 3 / 2 O 2 → H + + NO 2 -(s) + H 2 O • Biochemical processes, which often involve hydrolysis and oxidation-

reduction reactions. Organic acids and chelating agents, such as citrate, produced by bacterial action may solubilize heavy metal ions. Bacteria also produce methylated forms of metals, particularly mercury and arsenic.

• Photolysis reactions and miscellaneous chemical phenomena. Photolysis of hazardous-waste compounds in the hydrosphere commonly occurs on surface films exposed to sunlight on the top of water.

Hazardous-waste compounds have a number of effects on the hydrosphere. Perhaps the most serious of these is the contamination of groundwater, which in some cases can be almost irreversible. Waste compounds accumulate in sediments, such as river or estuary sediments. Hazardous-waste compounds dissolved in, suspended in, or floating as surface films on the surface of water can render it unfit for use and for sustenance of aquatic organisms.

Many factors determine the fate of a hazardous-waste substance in water. Among these are the substance’s solubility, density, biodegradability, and chemical reactivity. As discussed above and in Section 21.16, biodegradation largely deter- mines the fates of hazardous-waste substances in the hydrosphere. In addition to bio- degradation, some substances are concentrated in organisms by bioaccumulation processes and may become deposited in sediments as a result. Organophilic mater- ials may be sorbed by organic matter in sediments. Cation-exchanging sediments have the ability to bind cationic species, including cationic metal ions and organics that form cations.