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Absorption, or more generally "sorption," is the process by which one material (the sorbent) takes up and retains another (the sorbate) to form a homogenous concentration at equilibrium.

The general term is "sorption," which is defined as adhesion of gas molecules, dissolved substances, or liquids to the surface of solids with which they are in contact. In soils, three types of mechanisms, often working together, constitute sorption . They can be grouped into physical sorption, chemiosorption, and penetration into the solid mineral phase. Physical sorption (also known as adsorption ) involves the attachment of the sorbent and sorbate through weak atomic and molecular forces. Chemiosorption involves chemical bonds similar to holding atoms in a molecule. Electrostatic forces operate to bond minerals via ion exchange , such as the replacement of sodium, magnesium, potassium, and aluminum cations (+) as exchangeable bases with acid (-) soils. While cation (positive ion ) exchange is the dominant exchange process occurring in soils, some soils have the ability to retain anions (negative ions) such as nitrates , chlorine and, to a larger extent, oxides of sulfur.

Absorption and Wastewater Treatment

In on-site wastewater treatment, the soil absorption field is the land area where the wastewater from the septic tank is spread into the soil. One of the most common types of soil absorption field has porous plastic pipe extending away from the distribution box in a series of two or more parallel trenches, usually 1.52 ft (30.561 cm) wide. In conventional, below-ground systems, the trenches are 1.52 ft deep. Some absorption fields must be placed at a shallower depth than this to compensate for some limiting soil condition, such as a hardpan or high water table . In some cases they may even be placed partially or entirely in fill material that has been brought to the lot from elsewhere.

The porous pipe that carries wastewater from the distribution box into the absorption field is surrounded by gravel that fills the trench to within a foot or so of the ground surface. The gravel is covered by fabric material or building paper to prevent plugging. Another type of drainfield consists of pipes that extend away from the distribution box, not in trenches but in a single, gravel-filled bed that has several such porous pipes in it. As with trenches, the gravel in a bed is covered by fabric or other porous material.

Usually the wastewater flows gradually downward into the gravel-filled trenches or bed. In some instances, such as when the septic tank is lower than the drainfield, the wastewater must be pumped into the drainfield. Whether gravity flow or pumping is used, wastewater must be evenly distributed throughout the drainfield. It is important to ensure that the drainfield is installed with care to keep the porous pipe level, or at a very gradual downward slope away from the distribution box or pump chamber, according to specifications stipulated by public health officials. Soil beneath the gravel-filled trenches or bed must be permeable so that wastewater and air can move through it and come in contact with each other. Good aeration is necessary to ensure that the proper chemical and microbiological processes will be occurring in the soil to cleanse the percolating wastewater of contaminants. A well-aerated soil also ensures slow travel and good contact between wastewater and soil.

How Common Are Septic Systems with Soil Absorption Systems?

According to the 1990 U.S. Census, there are about 24.7 million households in the United States that use septic tank systems or cesspools (holes or pits for receiving sewage) for wastewater treatment. This figure represents roughly 24% of the total households included in the census.

According to a review of local health department information by the National Small Flows Clearinghouse, 94% of participating health departments allow or permit the use of septic tank and soil absorption systems. Those that do not allow septic systems have sewer lines available to all residents. The total volume of waste disposed of through septic systems is more than one trillion gallons (3.8 trillion l) per year, according to a study conducted by the U.S. Environmental Protection Agency's Office of Technology Assessment, and virtually all of that waste is discharged directly to the subsurface, which affects groundwater quality.

[Carol Steinfeld ]



Elliott, L. F., and F. J. Stevenson, Soils for the Management of Wastes and Waste Waters. Madison, WI: Soil Science Society of America, 1977.


Fact Sheet SL-59, a series of the Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. February 1993.

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The process by which substances are taken into the tissues of organisms is called absorption. It is essential to functions such as digestion, circulation, and respiration.

During digestion, valuable nutrients are absorbed across the epithelial lining of the digestive tract. Absorption occurs largely in the small intestine, which has developed a large surface area for this purpose. The walls of the small intestine contain numerous finger-like projections called villi, which are in turn covered by countless microvilli. Different nutrients are absorbed across the gut epithelium in different ways.

The methods of absorption include active transport , facilitated diffusion , and passive diffusion . Active transport requires energy in the form of adenosine triphosphate (ATP ), as well as special carrier molecules that ferry nutrients, (their substrates), across the gut lining. Active transport is involved in the absorption of proteins, which have usually been processed into amino acids or other small peptides. Most ions are also absorbed through active transport, as are most carbohydrates.

Some carbohydrates, however, are absorbed in a process known as facilitated diffusion. Facilitated diffusion describes a situation in which special carrier molecules are necessary, but energy (ATP) is not. Fructose is an example of a carbohydrate that is absorbed through facilitated diffusion.

Other nutrients, such as lipids , are absorbed through passive diffusion. In passive diffusion, neither energy expenditure nor a special carrier molecule is required. Lipids interact with bile salts from the liver, combining with them to form structures known as micelles. Micelles are able to diffuse freely through cell membranes and so can pass directly across the gut lining. Water is another substance that diffuses passively across the gut walls.

The circulatory system transfers nutrients and other products throughout the body. Tissues absorb the products they need from tiny blood vessels called capillaries. Capillaries are characterized by very high surface areas and very low blood-flow rates, both of which facilitate absorption. The walls of capillaries are also very thin, consisting of only one or a few layers of flattened endothelial cells. Capillaries also possess small pores through which transport and absorption can occur.

The absorption of materials from the capillaries occurs in one of several ways. Lipid-soluble substances are able to diffuse directly across the cell membranes of capillary cells into the tissues. Water diffuses directly as well, although it makes use of special pores in the cell membranes of capillary cells. Exchange via diffusion is comparatively rapid.

The absorption of other nutrients from the blood requires transportation through the capillary walls inside special vesicles. This process is called transcytosis. The vesicles are membrane-bound and are believed to be constructed by a cellular organelle known as the Golgi apparatus. Vesicles shuttle products repeatedly between the inner and outer walls of capillary cells. Because capillary beds in the brain are characterized by fewer transport vesicles, many substances cannot be absorbed into brain tissue, and the absorption of those that can be is slowed. This is often referred to as the blood-brain barrier.

In the process of respiration, oxygen is absorbed by the integument , lungs , gills , or trachea from the air or water. As with the circulatory and digestive systems, large respiratory surface areas allow for efficient absorption.

Oxygen is absorbed from the environment by the red blood cells, or erythrocytes . Erythrocytes contain respiratory pigments , which bind oxygen and works to transport it to tissues. These specialized oxygen-binding molecules are called pigments because they are often brightly colored when carrying bound oxygen. Respiratory pigments have a high affinity for oxygen and are also able to dramatically increase the oxygen-carrying capacity of blood.

Hemoglobin is the respiratory pigment in vertebrate erythrocytes and is also common throughout the animal kingdom. Hemoglobin is a large molecule consisting of four polypeptide chains, each of which is capable of binding an oxygen molecule. The oxygen-binding part of the chain is called the heme group and includes an iron atom. Hemoglobin binds oxygen cooperatively, meaning that once it has bound a single oxygen molecule, it is more likely to bind additional oxygen molecules. Hemoglobin's oxygen affinity, or the degree to which oxygen binds to it, varies according to such external factors as pH. This plasticity (flexibility) of oxygen affinity allows hemoglobin simultaneously to bind oxygen in the oxygen-rich environment of the lungs and to release it in the oxygen-poor environments of the tissues.

Another respiratory pigment, myoglobin, is present in the muscles and is responsible for pulling oxygen molecules from the blood into the tissues. Myoglobin resembles hemoglobin but consists of only a single polypeptide chain.

see also Digestion; Transport.

Jennifer Yeh


Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W. W. Norton, 1996.

Withers, Philip C. Comparative Animal Physiology. Fort Worth, TX: Saunders College Publishing, 1992.

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ab·sorp·tion / əbˈzôrpshən; -ˈsôrp-/ • n. 1. the process or action by which one thing absorbs or is absorbed by another. ∎ Physics the process or action by which neutrons are absorbed by the nucleus. 2. the fact or state of being engrossed in something. DERIVATIVES: ab·sorp·tive / -tiv/ adj.

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absorption The movement of fluid or a dissolved substance across a plasma membrane. In many animals, for example, soluble food material is absorbed into cells lining the alimentary canal and thence into the blood. In plants, water and mineral salts are absorbed from the soil by the roots. See osmosis; transport protein.

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absorption The amount of seismic energy lost during transmission, by conversion to heat. The absorption coefficient is the fractional loss of energy over a distance of one wavelength; hence higher-frequency signals are attenuated more readily than those of lower frequencies over the same path. Typical values for rocks range from 0.25 to 0.75 dB per wavelength.

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absorption Taking up chemically or physically of molecules of one substance into another.

This includes a gas taken in by a liquid and a liquid or gas absorbed by a solid. The process is often utilized commercially, such as the purification of natural gas by the absorption of hydrogen sulphide in aqueous ethanolamine. Absorption is also the term used when light and other electromagnetic waves lose some energy as they pass through a medium. See also adsorption

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absorption. Transition between a wall-sur-face or a pier and the springing of a vault or vault-ribs in Gothic architecture, where the vault seems to flow into (or be absorbed by) the wall or pier.

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absorption (ăb-sorp-shŏn) n. the uptake of digested food from the intestine into the blood and lymphatic systems. See also assimilation, digestion.

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absorption The uptake of substances, usually nutrients, water, or light, by cells or tissues.

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