Sediment sorption

In conclusion, the processes dominating the environmental fate of non-ionic surfactants in estuaries are biodegradation in the water column, sorption/sedimentation and volatilisation, together with the dynamics of the estuary, in particular water residence times. 

It appears that pesticides with solubiHties greater than 10 mg/L are mainly transported in the aqueous phase (48) as a result of the interaction of solution/sediment ratio in the mnoff and the pesticide sorption coefficient. For instance, on a silt loam soil with a steep slope (>12%), >80% of atra2ine transport occurs in the aqueous phase (49). In contrast, it has been found that total metolachlor losses in mnoff from plots with medium ground slopes (2—9%) were <1% of appHed chemical (50). Of the metolachlor in the mnoff, sediment carried 20 to 46% of the total transported pesticide over the monitoring period. 

Pollutant Distribution. Of particular importance for the aquatic ecosystem is the distribution of volatile substances, eg, gases and volatile organic compounds, between the atmosphere and water, and the sorption of compounds at soHd surfaces, eg, settling suspended matter, biological particles, sediments, and soils (41,42). 

The biogeochemical processes that generally describe the interaction of elements with particles are quite well known dissolution, flocculation, ion exchange, sorption, (co)precipitation, electron transfer, and biological uptake. In aquatic environments these reactions often occur simultaneously and competitively. In order to utilize marine tracers effectively, we must understand how elements are associated with particles and sediments. 

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. 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

Berg M, Arnold CG, Muller SR, Muhlemann J, Schwarzenbach RP (2001) Sorption and desorption behavior of organotin compounds in sediment-pore water systems. Environmental Science and Technology, 35(15) 3151-3157. 

Karickhoff SW. 1981. Semi-empirical estimation of sorption of hydrophobic pollutants on natural sediments and soils. Chemosphere 10 833-846. 

Rates of hydrolysis may be influenced by the presence of dissolved organic carbon, or organic components of soil and sediment. The magnitude of the effect is determined by the structure of the compound and by the kinetics of its association with these components. For example, whereas the neutral hydrolysis of chlorpyrifos was unaffected by sorption to sediments, the rate of alkaline hydrolysis was considerably slower (Macalady and Wolf 1985) humic acid also reduced the rate of alkaline hydrolysis of 1-octyl 2,4-dichlo-rophenoxyacetate (Perdue and Wolfe 1982). Conversely, sediment sorption had no effect on the neutral hydrolysis of 4-chlorostilbene oxide, although the rate below pH 5 where acid hydrolysis dominates was reduced (Metwally and Wolfe 1990). 

Macalady DL, NL Wolfe (1985) Effects of sediment sorption and abiotic hydrolyses. 1. Organophosphorothio-ate esters. J Agric Food Chem 33 167-173. 

Kan AT, G Fu, M Hunter, MB Tomson (1998) Irreversible sorption of nentral hydrocarbons to sediments experimental observations and model predictions. Environ Sci Technol 32 892-902. 

Doucette, W.J. (2003) Quantitative Structure-Activity Relationships for Predicting SoU-Sediment Sorption Coefficients for Organic Chemicals. Environmental Toxicology and Chemistry, 22(8), 1771-1788. 

An important consideration prior to sample collection is transportation and storage. Samples should be treated so as to retain the integrity of the sample from the moment of collection to the time of analysis. The physico-chemical characteristics of a sediment sample change during drying, with effects on the sorption-desorption behavior of chemicals. 

Osthols E (1995) Thorium sorption on amorphous silica. Geochim Cosmochim Acta 59(7) 1235-1249 Palmer MR, Edmond JM (1993) Uranium in river water. Geochim Cosmochim Acta 57 4947-4955 Paytan A, Moore WS, Kastner M (1996) Sedimentation rate as determined by Tla activity in marine barite. Geochim Cosmochim Acta 60(22) 4313-4320... 

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. 

Soil Sorption Constant - Soil/Water (Knp.). The distribution of a chemical between soil and water can be described with an equilibrium expression that relates the amount of chemical sorbed to soil or sediment to the amount in the water at equilibrium. 

The evaluative lake environment is similar to the "unit world" described by Mackay and Paterson (2), consisting of a 1 km square area with an atmosphere 6000 m high, a water column 80 m deep (the approximate depth of Lake Michigan) containing suspended solids (5 parts per million by volume) and biota (considered to be fish) of 1 ppm by volume, and underlain by a sediment 3 cm deep. The bottom sediment contains 4% organic carbon and the value for suspended sediment was arbitrarily selected as ten times these bottom sediment values reflecting the enhanced sorption discussed by O Connor and Connally (14). 

The data presented here indicates that the extent of binding for a particular compound is related to the octanol/water partition coefficient for that compound. This is very similar to the sorption of compounds from water to sediment. Compounds with log Kow values less than four (such as Lindane) will probably not be bound to an appreciable extent in the environment. Compounds with very high log Kow values (DDT and DEHP) may be bound to a significant extent. The extent of binding will depend on both the concentration of humic material and on the nature of the humic material. The humic materials used in this research showed dramatically different affinities for DDT. The reasons for this are poorly understood and deserve further study. 

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