Fixation clays

Nitrogen fixation, denitrification, soil weathering, phosphate fixation, clay mineral degradation, and potassium and transition metal fixation are problems for which the reaction rates are usually as, or more, important than equilibrium. Most soil chemical applications of kinetics have been in soil microbiology and soil biochemistry, where the lack of equilibrium is more obvious. The use of kinetics in inorganic soil chemistry will undoubtedly broaden in the future. It can even be argued that kinetics is basic to thermodynamics, because equilibrium is the condition where opposing reaction rates are equal. 

A variety of shale-protective muds are available which contain high levels of potassium ions (10). The reaction of potassium ions with clay, well known to soil scientists, results in potassium fixation and formation of a less water-sensitive clay. Potassium chloride, potassium hydroxide, potassium carbonate [584-08-7] (99), tetrapotassium pyrophosphate [7320-34-5] (100), and possibly the potassium salts of organic acids, such as potassium acetate [127-08-2] (101) and formate, have all been used as the potassium source. Potassium chloride is generally preferred because of its low cost and availabihty. 

It is assumed that the moisture content of the soil has been determined to be approximately 50% under worst-case conditions. Using this information and the results from vendor tests, it has been determined that a minimum dose of one part solidification reagent to two parts soil is required for the migration control of lead. Testing has shown that the optimum solidification reagent mixture would comprise ca. 50% fly ash and ca. 50% kiln dust. Thus, ca. 7000 t (6364 T) each of fly ash and cement kiln dust would be required. The reagents would be added in situ with a backhoe. As one area of the soil is fixed, the equipment could be moved onto the fixed soil to blend the next section. It may be anticipated that the soil volume would expand by ca. 20% as a result of the fixation process. This additional volume would be used to achieve the required slope for the cap. An RCRA soil/clay cap placed over the solidified material is necessary to prevent infiltration and additional hydraulic stress on the fixed soil. It is estimated that the fixation would reduce lead migration by 40% and that the fixed soil may pass the U.S. EPA levels for lead. 

Alternatives 4 and 5 would rely on a soil/clay cap to control infiltration for Area 1 (lead-contaminated) as well as treatment or fixation. Upon completion, some long-term maintenance of the cap and groundwater monitoring would be required until each alternative has met the health-based cleanup goals for groundwater. These alternatives would have almost no long-term reliance on institutional controls. 

Figure 3. The general nitrogen model for illustrating the bio geochemical cycling in Forest ecosystems. Explanations for the fluxes 1, ammonia volatilization 2, forest fertilization 3, N2-fixation 4, denitrification 5, nitrate respiration 6, nitrification 7, immobilization 8, mineralization 9, assimilatory and dissimilatory nitrate reduction to ammonium 10, leaching 11, plant uptake 12, deposition N input 13, residue composition, exudation 14, soil erosion 15, ammonium fixation and release by clay minerals 16, biomass combustion 17, forest harvesting 18, litterfall (Bashkin, 2002).

Extremely high selectivities are frequently interpreted as "ion fixation", which suggests an irreversible phenomenon. This is the case for exchanges of Cs, Rb and K in illite clay minerals (95-96) as well as for Cu(NHj) exchange in fluorhectorite (66). However, reversibility was verified from the Hess law for adsorption of Cs, Rb and K on the high affinity sites in illite (91) and modified montmorillonites (101) as well as for the exchange of transition metal complexes (29, 75). 

Another type of reaction that responds to WD cycles is the fixation of K and NH4 ions by smectite (3-7). The fixation of K in smectite has been studied extensively by soil scientists because of its effect on the availability of plant nutrients. The reaction also decreases smectite s ability to swell, decreases its cation exchange capacity (CEC), and modifies its BrjSnsted acidity. Therefore, an understanding of this phenomenon is applicable to many fields of study that are concerned with swelling clays, fields such as soil fertility, soil mechanics, waste disposal, clay catalysis, and the geochemistry of ground and surface waters. 

Anderson MA, Bertsch PM, Miller WP (1989) Exchange and apparent fixation of lithium in selected soils and clay minerals. Soil Sci 148 46-52... 

After delivery to the ocean, clay minerals react with seawater. The processes that alter the chemical composition of the terrigenous clay minerals during the first few months of exposure are termed halmyrolysis. These include (1) cation exchange, (2) fixation of ions into inaccessible sites, and (3) some isomorphic substitutions. Another important transfiarmation is flocculation of very small (colloidal-size) clay particles into larger ones. 

Fig. 1. Climate chart for SE Australia for the Cenozoic with dated weathering features from the Cobar region. Estimated precipitation from palynologioal reoords (Martin 1991), ocean water temperature (Zachos et al. 2001), hematite fixation and clay weathering ages (McQueen et al. 2002, 2007 Smith 2006).

Preparation of the PILC. As seen in Table 1, two factors determine the extent of A1 fixation (% Al O ) by the clay the final pH of the solution and the size of the clay particles. The influence of pH is readily explained by the equilibrium of formation of the polymer and by a competitive exchange w th the protons. The surface area increases from 42 to 180-360m /g upon intercalation, as reported on Table 1, and seems to be determined by the amount of A1 fixation. It appears that on sample G the extent of A1 fixation reaches a plateau at Al/clay=5. After this, diffusional limitations control the exchange on the large particles.The N2 adsorption gives a typical type IV isotherm, with 70% of the surface area localized in micropores smaller than 20A, after dehydration at 300°C. 

Potassium is determined in the acetic acid as well as the lactic acid extract. The potassium values for biologically managed fields lie between 100 and 200 mg K kg soil, and are less than those recommended for conventional agriculture. Light soils normally have low values, and clay soils, which bind more potassium, have higher K levels. Potash fixation is also assessed according to Schlichting and Blume (1951, p. 84). 

Fluoride is a natural component of most types of soil, in which it is mainly bound in complexes and not readily leached. The major source of free fluoride ion in soil is the weathering and dissolution of fluoride rich rock that depends on the natural solubility of the fluoride compound in question, pH, and the presence of other minerals and compounds and of water. The major parameters that control fluoride fixation in soil through adsorption, anion exchange, precipitation, formation of mixed solids and complexes are aluminium, calcium, iron, pH, organic matter and clay [19,20]. 

Banfif.ld, J. F. Eggleton, R. A. 1989. Apatite replacement and rare earth mobilization, fractionation, and fixation during weathering. Clays and Clay Minerals, 37, 113-127. 

Zevenbergen, C., Bradley, J. P van Reeuwijk, L. P., Shyam, A. K., Hjelmar, O. Comans, R. N. 19996. Clay formation and metal fixation during weathering of coal fly ash. Environmental Science and Technology, 33, 3405-3409. 

Sawhney, B. L., "Sorption and Fixation of Microquantities of Cesium by Clay Minerals Effect of Saturating Cations,"... 

Later studies by investigators (Alberts ei al 1979) have shown lhai 1 >7Cs introduced into a watershed is attached to soil panicles, which arc removed by erosion and runoff. Some of the eroded soil panicles comprise he sediments of the catchment basins in the watersheds and act as "sinks for, 7Cs. Other investigators have reponed an almost irreversible fixation of this clement in clay imerlattice sites in freshwater environments, and. that it is unlikely that this nuclide will he removed from these sediments under normal environmental conditions other than by exposure to solutions ol high ionic strength, such as may occur in estuarine environments. Studies of 15 Cs have been important because ihe element can be introduced into a water system from a leak in a nuclear fuel element. These findings are reported in some detail by Alberts ct al. in Science, 203. 649-651 (1979). 

Solinas, V., C. Gessa, P. Melis, M.A. Franco, and M. Sabbatini (1983). Fixation of OH-atrazine on homionic clay surface during hydrolysis of atrazine. Agrochim., 27 464—473. 

Beek and Frissel (1973) Growth of nitrifier and ammonifer bacteria by Michaelis-Menten kinetics NH4 oxidation by first-order kinetics with environmental variables mineralization of proteins, sugars, cellulose, lignin, and living biomass by first-order kinetics immobilization by first-order kinetics including considerations for microbial biomass and C/N ratio NH3 volatilization by diffusion NH4 clay fixation by equilibrium model. 

Stout, P.P., 1940. Alterations in the crystal structure of the clay minerals as a result of phosphate fixation. Proc. Soil Sci. Soc. Am, 4 177-182. 

Weaver, C. E., 1958. The effects and geologic significance of potassium fixation" by expandable clay minerals derived from muscovite, biotite, chlorite, and volcanic material. Am. Mineralogist, 43 839-861. 

Cavallaro, N., and McBride, M. B. (1984). Zinc and copper sorption and fixation by an acid soil clay Effect of selective dissolutions. Soil Sd. Soc. Am.J. 48, 1050—1054. 

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