Environment

Chemical and physical reactions in water can be

• Homogeneous — occurring entirely among dissolved species.

• Heterogeneous — occurring at the liquid-solid-gas interfaces.

Most environmental water reactions are heterogeneous. Purely homogeneous reactions are relatively rare in natural waters and wastewaters. Among the most important reactions occurring at the liquid-solid-gas interfaces are those that move pollutants from one phase to another.

The following are processes by which a pollutant becomes distributed (or is partitioned) into all the phases it comes in contact with.

• Volatilization: At the liquid-air and solid-air interfaces, volatilization transfers volatile contaminants from water and solid surfaces into the atmosphere, and into air in soil pore spaces. Volatilization is most important for compounds with high vapor pressures. Contaminants in the vapor phase are the most mobile in the environment.

• Dissolution: At the solid-liquid and air-liquid interfaces, dissolution transfers contaminants from air and solids to water. It is most important for contaminants of high water solubility. The environmental mobility of contaminants dissolved in water is generally intermediate between volatilized and sorbed contaminants.

• Sorption*: At the liquid-solid and air-solid interfaces, sorption transfers contaminants from water and air to soils and sediments. It is most important for compounds of low

* Sorption is a general term including both adsorption and absorption. Adsorption means binding to a particle surface. Absorption means becoming bound in pores and passages within a particle.

FIGURE 2.1 Partitioning of a pollutant among air, water, soil, and free product phases.

solubility and low volatility. Sorbed compounds undergo chemical and biological transformations at different rates and by different pathways than dissolved compounds. The binding strength with which different contaminants become sorbed depends on the nature of the solid surface (sand, clays, organic particles, etc.), and on the properties of the contaminant. Contaminants sorbed to solids are the least mobile in the environment.

2.6 PARTITIONING BEHAVIOR OF POLLUTANTS

A pollutant in contact with water, soil, and air will partially dissolve into the water, partially volatilize into the air, and partially sorb to the soil surfaces, as illustrated in Figure 2.1. The relative amounts of pollutant that are found in each phase with which it is in contact, depends on intermolecular attractive forces existing between pollutant, water, and soil molecules. The most important factor for predicting the partitioning behavior of contaminants in the environment is an understanding of the intermolecular attractive forces between contaminants and the water and soil materials in which they are found.

Partitioning from a Diesel Oil Spill

Consider, for example, what happens when diesel oil is spilled at the soil's surface. Some of the liquid diesel oil (commonly called free product) flows downward under gravity through the soil toward the groundwater table. Before the spill, the soil pore spaces above the water table (called the soil unsaturated zone) were filled with air and water, and the soil surfaces were partially covered with adsorbed water. As diesel oil, which is a mixture of many different compounds, passes downward through the soil, its different components become partitioned among the pore space air and water, the soil particle surfaces, and the oil free product. After the spill, the pore spaces are filled with air containing diesel vapors, water carrying dissolved diesel components, and diesel free product that has changed in composition by losing some of its components to other phases. The soil surfaces are partially covered with diesel free product and adsorbed water containing dissolved diesel components.

Diesel oil is a mixture of hundreds of different compounds each having a unique partitioning, or distribution pattern. The pore space air will contain mainly the most volatile components, the pore space water will contain mainly the most soluble components, and the soil particles will sorb mainly the least volatile and soluble components. The quantity of the free product diminishes continually as it moves downward through the soil because a significant portion is lost to other phases. The composition of the free product also changes continually because the most volatile, soluble, and strongly sorbed compounds are lost preferentially. The chemical distributions attain quasi-equilibrium, with compounds continually passing back and forth across each phase interface, as indicated in Figure 2.1. As the remaining free product continues to change by losing components to other phases (part of the "weathering process"), it increasingly resists further change. Since the lightest weight components tend to be the most volatile and soluble, they are the first to be lost to other phases, and the remaining free product becomes increasingly more viscous and less mobile. Severely weathered free product is very resistant to further change, and can persist in the soil for decades. It only disappears by biodegradation or by actively engineered removal.

Depending on the amount of diesel oil spilled, it is possible that all of the diesel free product becomes "immobilized" in the soil before it can reach the water table. This occurs when the mass of free product diminishes and its viscosity increases to the point where capillary forces in the soil pore spaces can hold the remaining free product in place against the force of gravity. There is still pollutant movement, however, mainly in the non-free product phases. The volatile components in the vapor state usually diffuse rapidly through the soil, moving mostly upward toward the soil surface and along any high permeability pathways through the soil, such as a sewer line backfill. New water percolating downward, from precipitation or other sources, can dissolve additional diesel compounds from the sorbed phase and carry it downward. Percolating water can also displace some soil pore water already carrying dissolved pollutants, as well as free product held by capillary forces, forcing them to move farther downward. Although the diesel free product is not truly immobilized, its downward movement can become imperceptible.

However, if the spill is large enough, diesel free product may reach the water table before becoming immobilized. If this occurs, liquid free product being lighter than water, cannot enter the water-saturated zone but remains above it, effectively floating on top of the water table. There, the free product spreads horizontally on the groundwater surface, continuing to partition into ground-water, soil pore space air, and to the surfaces of soil particles. In other words, a portion of the free product will always become distributed among all the solid, liquid and gas phases that it comes in contact with. This behavior is governed by intermolecular forces that exist between molecules.

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