Clay moisture content

Figure 4. Degradation of parathion on kaolinite as a function of the clay moisture content. (Reproduced from Ref. 71. Copyright 1976 American Chemical Society.)...

Many national programs plan to surround containers of their nuclear waste in a geologic repository, with a backfill of compacted bentonite clay (Fig. 13.33). A chief function of the clay backfill is to adsorb radionuclides and so retard their release from the engineered barrier system. Conca (1992) measured the apparent diffusion coefficient (D ) and apparent distribution coefficient (K [ml/g]) of some radionuclides in bentonite clay as a function of clay moisture content and compaction density. Measurements were made for clay densities from 0.2 to 2.0 g/cm, which correspond to porosities of 93 to 25%, respectively. With decreasing porosity, values declined by roughly 10 to 10 -fold. However, for the same porosity reduction, values were usually lowered by 10-fold and more, indicating less adsorption with compaction (Fig. 13.38). 

FIGURE 8.32 Temperature dependence of relative permittivity of a clay (Rideau clay, moisture content by volume 40%). Data from Annan and Davis (1978) and Scott et al. (1990). 

Many factors affect the mechanisms and kinetics of sorption and transport processes. For instance, differences in the chemical stmcture and properties, ie, ionizahility, solubiUty in water, vapor pressure, and polarity, between pesticides affect their behavior in the environment through effects on sorption and transport processes. Differences in soil properties, ie, pH and percentage of organic carbon and clay contents, and soil conditions, ie, moisture content and landscape position climatic conditions, ie, temperature, precipitation, and radiation and cultural practices, ie, crop and tillage, can all modify the behavior of the pesticide in soils. Persistence of a pesticide in soil is a consequence of a complex interaction of processes. Because the persistence of a pesticide can govern its availabiUty and efficacy for pest control, as weU as its potential for adverse environmental impacts, knowledge of the basic processes is necessary if the benefits of the pesticide ate to be maximized. 

Capillary Flow Moisture which is held in the interstices of solids, as liquid on the surface, or as free moisture in cell cavities, moves by gravity and capiUarity, provided that passageways for continuous flow are present. In diying, liquid flow resulting from capiUarity appUes to liquids not held in solution and to aU moisture above the fiber-saturation point, as in textiles, paper, and leather, and to all moisture above the equiUbrium moisture content at atmospheric saturations, as in fine powders and granular solids, such as paint pigments, minerals, clays, soU, and sand. 

Liquid Diffusion The movement of liquids by diffusion in soUds is restricted to the equihbrium moisture content below the point of atmospheric saturation and to systems in which moisture and solid are mutually soluble. The first class apphes to the last stages in the diying of clays, starches, flour, textiles, paper, and wood the second class includes the diying of soaps, glues, gelatins, and pastes. 

Mined clay with 22 percent moisture is broken up into pieces of less than 5 cm (2 in) in a rotary impact mill without screen, and fed to a rotary gas-fired lain for drying (see Fig. 20-58). The moisture content is then 8 to lOpercent, and this material is fed to a miU, usually a Raymond ring-roll mill with an internal whizzer classifier or a pan mill. Hot gases introduced to the mill complete the diying while the material is being pulverized to the required fineness. 

Clays have plate-like molecules with charges on their surfaces (Chapter 16). The charges draw water into the clay as a thin lubricating layer between the plates. With the right moisture content, clays are plastie they can be moulded, extruded, turned or carved. But when they are dried, they have sufficient strength to be handled and stacked in kilns for firing. 

Corrosion in soil is aqueous, and the mechanism is electrochemical (see Section 1.4), but the conditions in the soil can range from atmospheric to completely immersed (Sections 2.2 and 2.3). Which conditions prevail depends on the compactness of the soil and the water or moisture content. Moisture retained within a soil under field dry conditions is largely held within the capillaries and pores of the soil. Soil moisture is extremely significant in this connection, and a dry sandy soil will, in general, be less corrosive than a wet clay. 

The persistence of the N-nitrosamine that may be formed in soil will depend on a host of conditions, such as soil type, organic matter content, clay content, pH, the microflora present in the soil, moisture content and temperature, etc. Superimposed on all these factors will be the chemical nature of the pesticide. The N-nitrosoatrazine ( ) formed in soil from the herbicide atrazine ( ) was shown to be rapidly disappeared (1). Thus, in soil W-nitrosoatrazine was observed after one week, but was absent 4 and 10 weeks later (Table IV). In contrast, N-nitroso-butralin (11 ) persisted much longer than N-nitrosoatrazine (9) under the same conditions (Table V) and was still detectable after 6 months (3). Our studies demonstrated that N-nitrosoglyphosate is persistent in the soil. Fox soil treated with 20 ppm of nitrite nitrogen and 740 ppm glyphosate contained about 7 ppm of N-nitrosoglyphosate even after 140 days (6). 

Physical properties involve tests of the physical index parameters of the materials. For spent foundry sand, these parameters include particle gradation, unit weight, specific density, moisture content, adsorption, hydraulic conductivity, clay content, plastic limit, and plastic index. These parameters determine the suitability of spent foundry sand for uses in potential applications. Typical physical properties of spent green foundry sand are listed in Table 4.5. 

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. 

Abiotic organic reactions, such as hydrolysis, elimination, substitution, redox, and polymerization reactions, can be influenced by surfaces of clay and primary minerals, and of metal oxides. This influence is due to adsorption of the reactants to surface Lewis and Br nsted sites. Temperature and moisture content are the most important environmental variables. Under ambient environmental temperatures, some reactions are extremely slow. However, even extremely slow transformation reactions may be important from environmental and geochemical viewpoints. 

The dissociation of water coordinated to exchangeable cations of clays results in Brtfnsted acidity. At low moisture content, the Brrfnsted sites may produce extreme acidities at the clay surface-As a result, acid-catalyzed reactions, such as hydrolysis, addition, elimination, and hydrogen exchange, are promoted. Base-catalyzed reactions are inhibited and neutral reactions are not influenced. Metal oxides and primary minerals can promote the oxidative polymerization of some substituted phenols to humic acid-like products, probably through OH radicals formed from the reaction between dissolved oxygen and Fe + sites in silicates. In general, clay minerals promote many of the reactions that also occur in homogenous acid or oxidant solutions. However, rates and selectivity may be different and difficult to predict under environmental conditions. This problem merits further study. 

The unique design of the tube press allows for this cycle to be amended, however, to include air pressing and/or cake washing. With air pressing, once the initial filtration is complete, air is introduced between the membrane and the cake. The pressure cycle is then repeated. Typically an air press will further reduce the moisture content of china clay by 2.5-8 per cent. The final moisture contents with other materials are shown in Figure 7.25. Water washing, which is used for the removal of soluble salts, is similar to air pressing, except that it is water that is introduced between membrane and cake. 

The moisture content of a material is usually expressed in terms of its water content as a percentage of the mass of the dry material, though moisture content is sometimes expressed on a wet basis, as in Example 16.3. If a material is exposed to air at a given temperature and humidity, the material will either lose or gain water until an equilibrium condition is established. This equilibrium moisture content varies widely with the moisture content and the temperature of the air, as shown in Figure 16.1. A non-porous insoluble solid, such as sand or china clay, has an equilibrium moisture content approaching zero... 

Figure 2.3 Relation between diffusion impedance factor, /l, and moisture content, 6>l, in a range of soils. Numbers shown are % clay contents. Mean of six soils volcanic ash soil. (After Tinker and Nye, 2000 Olesen et al., 2001). Reproduced by permission of Oxford University Press...

CASRN 3337-71-1 molecular formula C8H10N2O4S FW 230.24 Soil Asulum is not persistent in soils because its half-life is approximately 6-14 d (Hartley and Kidd, 1987). The short persistence time is affected by soil temperature and moisture content. The half-life of asulam in a heavy clay soil having a moisture content of 34% and maintained at 20 °C was 7 d (Smith and Walker, 1977). In soil, sulfanilamide was reported as a product of hydrolysis. In nonsterile soils, this compound degraded to unidentifiable products (Smith, 1988) which may include substituted anilines (Bartha, 1971). 

Soil The half-lives of captan in soil ranged from 3.5 d (pH 6.4, moisture content 17.5%) to >50 d (pH 6.2, moisture content 1.6%) (Burchfield, 1959). The half-lives of captan in a sandy loam, clay loam, and an organic amended soil under nonsterile conditions were 10-354, 3.4-587, and 7.0-213 d, respectively, while under sterile conditions the half-lives were 9.4-373, 2.8-303, and 4.3-191 d, respectively (Schoen and Winterlin, 1987). 

Zimdahl and Clark (1982) reported half-lives of 15-38 and 33-100 d for the herbicide in clay loam soil, and sandy loam soil, respectively. They also reported that soil moisture increased the dissipation rate. At 20 °C, the dissipation rates of metolachlor in the clay loam and sandy loam soils at 20, 50, and 80% soil moisture contents were 0.028, 0.053, 0.062, and 0.016, 0.028, and 0.037/day, respectively. The half-lives of metolachlor in soil maintained at temperatures of 30 and 40 °C were approximately 3.85 and 2.75 wk, respectively (Bravermann et al., 1986). The reported half-lives of metolachlor in soil is approximately 6 d (Worthing and Hance, 1991) and 3-4 wk (Bowman, 1988). 

Ismail and Lee (1995) studied the persistence of metsulfuron-methyl in a sandy loam (pH 5.1) and clay soil (3.1) under laboratory conditions. Degradation was more rapid in nonsterilized than in sterilized soil. In nonsterilized soil, the rate of degradation increased with increasing soil moisture content. When the moisture level in the sandy loam and clay soil was increased from 20 to 80% of field capacity at 35 °C, the half-lives were reduced from 9.0 to 5.7 and 11.2 to 4.6 d, respectively. The investigators concluded that the disappearance of metsulfuron-methyl in soil resulted from microbial degradation and chemical hydrolysis. 

The residual content of immiscible liquids can be defined by the amount of NAPL remaining in the subsurface when pore geometry permits NAPL flow greater than the retention capacity. In an outdoor pilot experiment. Fine and Yaron (1993) studied the effect of soil constituents and soil moisture contents on the retention of kerosene in the subsurface. This retention is termed the kerosene residual content (KRC). Ten soils were studied, with a broad spectrum of clay and organic matter contents, together with four soil moisture contents corresponding to oven-dried, air-dried. 

The organic matter content also contributes to the KRC. The relationship between clay, OM, and moisture contents and KRC is described by... 

Fig. 8.42 Kerosene residual content (KRC) of soils as a function of clay and moisture contents. Reprinted from Fine P, Yaron B (1993) Outdoor experiments on enhanced volatilization by venting of kerosene components from soil. J Contam Hydrology 12 335-374. Copyright 1994 with permission of Elsevier...

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