Sediment organism equilibrium

Efforts have been undertaken mainly be the United States Environmental Protection Agency to develop standard procedures and criteria for the assessment of environmental impact of sediment-associated pollutants. Initial discussions (Anon., 1984, 1985) suggested five methodological approaches which merit closer consideration (i) "background approach", (ii) "water quality/pore water approach", (iii) sediment/water equilibrium partitioning approach , (iv) sediment/organism equilibrium" approach, and (v) "bioassay" approach. Of these possibilities, applications of "bioassays" and "background approach" have been outlined in sections 6.1 and 6.2, respectively. 

Sorption. Capture of neutral organics by non-living particulates depends on the organic carbon content of the solids (9). Equilibrium sorption of such "hydrophobic" compounds can be described by a carbon-normalized partition coefficient on both a whole-sediment basis and by particle size classes. The success of the whole-sediment approach derives from the fact that most natural sediment organic matter falls in the "silt" or "fine" particle size fractions. So long as dissolved concentrations do not exceed 0.01 mM, linear isotherms (partition coefficients) can be used. At higher concentrations, the sorptive capacity of the solid can be exceeded, and a nonlinear Freundlich or Langmuir isotherm must be invoked. 

De Seze, G., Valsaraj, K.T., Reible, D.D., Thibodeaux, L.J. (2000) Sediment-air equilibrium partitioning of semi-volatile hydrophobic organic compounds. Part 2. Saturated vapor pressures, and the effects of sediment moisture content and temperature on the partitioning of polyaromatic hydrocarbons. Sci. Total Environ. 253, 27-44. 

In the sediment, the equilibrium concentration of an organic compound adsorbed to the sediment is dependent on the fraction by weight of organic carbon. 

By analogy to the case in the water column, we can define an equilibrium constant reflecting PCB partitioning between the organism lipid pool and the sediment organic carbon pool ... 

Suspended and bottom sediments are widely regarded as a sink for PCB s and other hydrophobic organic pollutants released Into aquatic systems. The mechanism commonly proposed to explain the affinity of PCB s for sediments Is equilibrium sorption or partitioning, which Is a function of the aqueous solubility of PCB Isomers and the attractiveness of the sedimentary matrix to PCB s (1-4). Field and laboratory studies Indicate that sedimentary organic matter plays a fundamental role In PCB-sedlment associations (5-12) and that humic substances are Important components of the sedimentary organic matrix for such associations (6,13-16). For example, Choi and Chen (O found that sedimentary PCB and DDT concentrations were linearly related to the humic acid content of sediments from Los Angeles Harbor. Fierce et al. (15) determined that humic acid could account... 

Adsorption of purely hydrophobic compounds onto soils and sediments has been successfully modeled and predicted by treating the organic matter fraction as a separate phase in equilibrium with the aqueous phase (see Karick-hoff et al., 1979). Morel (1983) succinctly summarizes findings from the considerable amount of work that has been completed in this area (i) within a typical range of concentrations, sorption isotherms are linear, (ii) the extent of partitioning is proportional to the sediment organic C content, and... 

Owing to the stability of the uranyl carbonate complex, uranium is universally present in seawater at an average concentration of ca. 3.2/rgL with a daughter/parent activity ratio U) of 1.14. " In particulate matter and bottom sediments that are roughly 1 x 10 " years old, the ratio should approach unity (secular equilibrium). The principal source of dissolved uranium to the ocean is from physicochemical weathering on the continents and subsequent transport by rivers. Potentially significant oceanic U sinks include anoxic basins, organic rich sediments, phosphorites and oceanic basalts, metalliferous sediments, carbonate sediments, and saltwater marshes. " ... 

Adsorption — An important physico-chemical phenomenon used in treatment of hazardous wastes or in predicting the behavior of hazardous materials in natural systems is adsorption. Adsorption is the concentration or accumulation of substances at a surface or interface between media. Hazardous materials are often removed from water or air by adsorption onto activated carbon. Adsorption of organic hazardous materials onto soils or sediments is an important factor affecting their mobility in the environment. Adsorption may be predicted by use of a number of equations most commonly relating the concentration of a chemical at the surface or interface to the concentration in air or in solution, at equilibrium. These equations may be solved graphically using laboratory data to plot "isotherms." The most common application of adsorption is for the removal of organic compounds from water by activated carbon. 

Table I summarizes some typical distribution coefficients. Sediments become enriched in plutonium with respect to water, usually with a factor of vlO5. Also living organisms enrich plutonium from natural waters, but usually less than sediments a factor of 103 - 101 is common. This indicates that the Kd-value for sediment (and soil) is probably governed by surface sorption phenomena. From the simplest organisms (plankton and plants) to man there is clear evidence of metabolic discrimination against transfer of plutonium. In general, the higher the species is on the trophic level, the smaller is the Kd-value. One may deduce from the Table that the concentration of plutonium accumulated in man in equilibrium with the environment, will not exceed the concentration of plutonium in the ground water, independent of the mode of ingestion.

Research into the aquatic chemistry of plutonium has produced information showing how this radioelement is mobilized and transported in the environment. Field studies revealed that the sorption of plutonium onto sediments is an equilibrium process which influences the concentration in natural waters. This equilibrium process is modified by the oxidation state of the soluble plutonium and by the presence of dissolved organic carbon (DOC). Higher concentrations of fallout plutonium in natural waters are associated with higher DOC. Laboratory experiments confirm the correlation. In waters low in DOC oxidized plutonium, Pu(V), is the dominant oxidation state while reduced plutonium, Pu(III+IV), is more prevalent where high concentrations of DOC exist. Laboratory and field experiments have provided some information on the possible chemical processes which lead to changes in the oxidation state of plutonium and to its complexation by natural ligands. 

We cover each of these types of examples in separate chapters of this book, but there is a clear connection as well. In all of these examples, the main factor that maintains thermodynamic disequilibrium is the living biosphere. Without the biosphere, some abiotic photochemical reactions would proceed, as would reactions associated with volcanism. But without the continuous production of oxygen in photosynthesis, various oxidation processes (e.g., with reduced organic matter at the Earth s surface, reduced sulfur or iron compounds in rocks and sediments) would consume free O2 and move the atmosphere towards thermodynamic equilibrium. The present-day chemical functioning of the planet is thus intimately tied to the biosphere. 

Adsorption Coefficient (K c)—The ratio of the amount of a chemical adsorbed per unit weight of organic carbon in the soil or sediment to the concentration of the chemical in solution at equilibrium. 

As shown in Fig. 3, CHEMGL considers 10 major well-mixed compartments air boundary layer, free troposphere, stratosphere, surface water, surface soil, vadose soil, sediment, ground water zone, plant foliage and plant route. In each compartment, several phases are included, for example, air, water and solids (organic matter, mineral matter). A volume fraction is used to express the ratio of the phase volume to the bulk compartment volume. Furthermore, each compartment is assumed to be a completely mixed box, which means all environmental properties and the chemical concentrations are uniform in a compartment. In addition, the environmental properties are assumed to not change with time. Other assumptions made in the model include continuous emissions to the compartments, equilibrium between different phases within each compartment and first-order irreversible loss rate within each compartment [38]. 

Dewulf, J., van Langenhove, H., Grare, S. (1999) Sediment/water and octanol/water equilibrium partitioning of volatile organic compounds temperature dependence in the 2-25°C range. Water Res. 33, 2424—2436. 

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