Clay minerals, sorption reactions

The main components of soils are sand, clay and humus. Whereas interaction between radionuclides and sand is rather weak, as in the case of sediments, sorption by clay minerals and reactions with the organic compounds in humus are most important for the migration behaviour of radionuclides. Cs+ ions are quite strongly bound in clay particles, as already mentioned. Ra + and 2i0pjj2+... 

Carrizosa MJ, Hermosin MC, Koskinen WC, Cornejo J (2004) Interactions of two sulfonylurea herbicides with organoclays. Clays Clay Miner 52 643-649 Celis R, Hermosin MC, Cornejo J (2000) Heavy metal adsorption by functionalized clays. Environ Sci Technol 34 4593-4599 Chappell MA, Laird DA, Thompson ML, Li H, Teppen BJ, Johnston CT, Boyd SA (2005) Influence of smectite hydration and swelling on atrazine sorption behavior. Environ. Sci Technol 39 3150-3156 Chiou CT (1989) Theoretical considerations of the partition uptake of nonionic organic compounds by soil organic matter. In Sawhney BL, Brown K (eds) Reactions and movement of organic chemicals in soils. Soil Science Society of America, Madison, WI, pp 1-29... 

Hassett IJ, Banwart WL (1989) The sorption of nonpolar organics by soils and sediments In Sawhney BL, Brown K. (eds) Reactions and movement of organic chemicals in soils Soil Science Society of America, Madison, Wl, pp 31 5p Hayes MHB, Malcom RL (2001) Considerations of compositions and aspects of the structure of humic substances. In Clapp CE, Hayes MHB, Senesi N, Bloom PR Jardine PM, Humic substances and chemical contaminants. Soil Science Society of America, Madison, Wl, pp 1-39 Herbillon AJ, Erankart R, Vielvoye L (1981) An occurrence of interstratified kaoUnite-smectite minerals in a red-black soil top sequence. Clay Miner 16 195-201 Horne RA (1969) Marine chemistry. Wiley, New York... 

Sparks et al. (1980b) introduced a continuous flow method (next subsection) that is quite similar in principle to liquid-phase column chromatography. This method was used to study potassium adsorption dynamics on soils and clay minerals (Sparks and Jardine, 1981 Sparks and Rechcigl, 1982 Jardine and Sparks, 1984 Ogwada and Sparks, 1986a,b,c), silicate sorption on soils (Miller et al., 1986), S04 sorption and desorption on soils (Hodges and Johnson, 1987), and Al reactions on clay minerals and peat (Jardine etal., 1985a). 

The sorption and desorption of pesticides by soils and soil constituents such as clay minerals and humic substances has generally been characterized by an initial rapid rate folllowed by a much slower approach to an apparent equilibrium (Haque et al., 1968 Leenheer and Ahlrichs, 1971 Khan, 1973 McCall and Agin, 1985). The initial reaction(s) have been associated with diffusion of the pesticides to and from the surface of the sorbent, while the slower reaction(s) have been related to PD of the pesticides into and out of micropores of the sorbent. 

Radionuclide transport in natural waters is strongly dependent on sorption, desorption, dissolution, and precipitation processes. The first two sections discuss laboratory investigations of these processes. Descriptions of sorption and desorption behavior of important radionuclides under a wide range of environmental conditions are presented in the first section. Among the sorbents studied are basalt interbed solids, granites, clays, sediments, hydrous oxides, and pure minerals. Effects of redox conditions, groundwater composition and pH on sorption reactions are described. 

Intercalation reactions were referred to by Chatakondu et al. [94], who observed the deposition of n-hexylamine in TaS2. Mechanically activated Al(OH)3 shows enhanced dissolution rates in acids, and can also be used as a host material for foreign molecules. The use of the layers of hydrargillite as a selective sorption medium for small ions such as Li can be enhanced by a mechanical pretreatment [2], and this can also be used in the electrolysis of bauxite. The use and preparation of clay minerals and kaolinite has been reviewed by Yariv and Lapides [95]. 

Equilibrium between solution and adsorbed or sorbed phases is a condition commonly used to evaluate adsorption or sorption processes in soils or soil-clay minerals. As previously stated, equilibrium is defined as the point at which the rate of the forward reaction equals the rate of the reverse reaction. Two major techniques commonly used to model soil adsorption or sorption equilibrium processes are (1) the Freundlich approach and (2) the Langmuir approach. Both involve adsorption or sorption isotherms. A sorption isotherm describes the relationship between the dissolved concentration of a given chemical species (adsorbate) in units of micrograms per liter (pg L 1), milligrams per liter (mg L-1), microequivalents per liter (pequiv L-1), or millimoles per liter (mmol L-1), and the sorbed quantity of the same species by the solid phase (adsorbent) in units of adsorbate per unit mass of adsorbent (solid) (e.g., pg kg-1, mg kg-1, peq kg-1, or mmol kg 1) at equilibrium under constant pressure and temperature. Sorption isotherms have been classified into four types, depending on their general shape (Fig. 4.13) ... 

Sorption of As occurs simultaneously with the oxidation of As(III) to As(V) (Fig 8-1). With the exception of the Buffalo Pound sediment, over 90% of the added As is sorbed by the sediments within 72 h (Fig 8-lA, B). Arsenic is strongly sorbed onto Al and Fe oxides and the edge of clay minerals (Huang, 1975 Walsh and Keeney, 1975 Fordham and Norrish, 1979) through ligand exchange (Parfitt, 1978 Huang, 1980). The variation in the concentration of As(V) in solution at the end of the reaction periods (Fig. 8-IB) is attributed to the differences in the nature and surface properties of the sediments. 

PROBABLE FATE photolysis possible but actual significance is uncertain will also degrade by reactions with radicals oxidation oxidation by metal cations is very fast, reactions with oxygen and/or hyperoxy radicals are very important, photooxidation half-life in water 1.3-72.5 days, photooxidation half-life in air 0.312-3.12 hrs hydrolysis not an important process volatilization not an important process sorption very rapid adsorption by clay minerals, if spilled on soil, it will adsorb to it, especially if the soil is acidic biological processes no bioaccumulation, only slight biodegradation... 

There appears to be little point in a detailed evaluation of the more ambiguous hypotheses of heavy metal fixation. However, it may be noted that a number of investigators have considered the possibility of either the sorption of complex ions on clay mineral surfaces or reaction of heavy metal cations with clay surfaces in some manner other than simple electrostatic sorption (31, 66, 67, 156, 157, 217), However, the solubility products of Cu(OH)2 and Zn(OH)2 in aqueous suspensions of montmorillonite (19) have been found to be quite similar to those previously found for solutions in contact with only the pure hydroxides. This would indicate that metal ion-clay mineral surface complex formation is not important otherwise the apparent solubility would have been greater in the presence of montmorillonite. 

One of the factors which has confused mechanism analysis has been the failure to make a distinction between studies involving micro- and macro-concentrations of the heavy metals. Whereas, with micro-concentrations, sorption reactions may predominate, precipitation frequently occurs with macro-concentrations (68). Exchange of copper and zinc with hydrogen on clay mineral surfaces was proposed as a cause of zinc and copper fixation in soils by DeMumbrum and Jackson (59) who noted... 

On the other hand, a classification of a sorption process on the basis of kinetics data must be conditioned by other chemical properties of the phosphate-soil mixture. For example, if the soil solution is supersaturated initially with respect to some phosphate solid, precipitation is likely to influence the sorption reaction from the beginning. If the soil minerals have a low degree of crystallinity and/or a high degree of hydration, precipitation may be the dominant sorption mechanism even in the rapid stage.In general, low phosphate concentrations and well-crystallized, relatively unhydrated soil minerals tend to favor adsorption as the phosphate reaction mechanism. Other chemical properties, such as the pH value of the soil solution and the kinds of metals in soil clay minerals, exert a quantitative influence on the rapid stage of phosphate sorption, as do such physical properties as temperature. ... 

Time Scales of Metal Sorption Reactions. Metal sorption reactions can occur over time scales ranging from milliseconds to several weeks depending on the sorbate/sorbent system. The type of sorbent can drastically affect the reaction rate. For example, sorption reactions are often more rapid on clay minerals such as kaolinite than on vermiculitic and micaceous minerals. This is in large part due to the availability of sites for sorption. Kaolinite has readily available planar external sites and sorption is often complete in minutes (42). Vermiculite and micas have multiple sites for retention of metals including planar, edge, and interlayer sites, with some of the latter sites being partially to totally collapsed. Consequently, sorption and desorption reactions on these sites can be slow. 

Metal sorption reactions on clay minerals, (hydr)oxides, humic substances and soils is usually characterized by a rapid, followed by a slow, reaction (Figure... 

---