Desorption from soil

Brady et al. [52] have discussed pressure-temperature phase diagrams for carbon dioxide polychlorobiphenyls and examined the rate process of desorption from soils. Supercritical carbon dioxide was used to extract polychlorobiphenyls and DDT and Toxaphene from contaminated soils. 

Fig. 5.10 Effect of measurement technique on sulfate desorption from soils (modified after Hodges and Johnson 1987)...

Irrespective of the sources of phenolic compounds in soil, adsorption and desorption from soil colloids will determine their solution-phase concentration. Both processes are described by the same mathematical models, but they are not necessarily completely reversible. Complete reversibility refers to singular adsorption-desorption, an equilibrium in which the adsorbate is fully desorbed, with release as easy as retention. In non-singular adsorption-desorption equilibria, the release of the adsorbate may involve a different mechanism requiring a higher activation energy, resulting in different reaction kinetics and desorption coefficients. This phenomenon is commonly observed with pesticides (41, 42). An acute need exists for experimental data on the adsorption, desorption, and equilibria for phenolic compounds to properly assess their environmental chemistry in soil. 

Freese, D. Lookman, R. Merckx, R. van Riemsdijk,W.H. (1995) New method for assessment of long-term phosphate desorption from soils. Soil Sci. Soc. Am. J. 59 1295-1300... 

Aerobic or anaerobic Enhances desorption from soil Short incubation periods Requires soil pretreatment... 

The issue of bioavailability is further clouded by the physical characteristics of soil and the role of a possible mass transfer limitation. Soil constituents are not simply flat surfaces with free and equal access to all bacterial species. The formation of aggregates from sand-, silt-, and clay-sized particles results in stable structures which control microbial contact with the substrate (Figure 2.7). Discussion of sorption mechanisms and binding affinities must include the possible impact of intra-aggregate transport of the substrate. If the substrate is physically inaccessible to the microorganism then both desorption from soil constituents and diffusion to an accessible site are necessary. The impact of intra-aggregate diffusion on degradation kinetics has been modeled for y-hexachlorocyclohexane (Rijnaarts et al., 1990) and naphthalene (Mihelcic Luthy, 1991). 

Deitsch, J.J., and Smith, J.A. (1995). Effect of Triton X-100 on the rate of trichloroethene desorption from soil to water. Environ. Sci. Technol., 29, 1069-1080. 

To quantify the rate of solute desorption from soil, the following mathematical expression is often used ... 

In previous studies, the solubilization of hydrophobic organic contaminants using surfactants has been shown to increase the rate of contaminant desorption from soil to water (Deitsch and Smith 1995 Yeom et al. 1995 Tiehm et al. 1997). A 3,000 mg/L solution of Triton X-100 (CMC = 140 mg/L) increased the rate of desorption of laboratory-contaminated TCE from a peat soil (Deitsch and Smith 1995). However, the solubilization effect was secondary compared to the surfactant s effect on the desorption rate coefficient. Yeom et al (1995) developed a model that satisfactorily predicted the extent of polycyclic aromatic hydrocarbon solubilization from a coal tar-contaminated soil. Only at high surfactant dosages did the model fail to accurately predict the ability of different surfactants to solubilize polycyclic aromatic hydrocarbons. It was hypothesized that mass-transfer limitations encountered by the polycyclic aromatic hydrocarbons in the soil caused the observed differences between the data and the model simulations. In another study (Tiehm et al. 1997), two nonionic surfactants, Arkopal N-300 and Saogenat T-300, increased the rate of polycyclic aromatic hydrocarbon desorption from a field-contaminated soil. The primary mechanism for the enhanced desorption of polycyclic aromatic hydrocarbons was attributed to surfactant solubilization of the polycyclic aromatic hydrocarbons. 

Table 21.8 References on the quantification of triazine desorption from soil...

Clay, S.A. and W.C. Koskinen (1990a). Characterization of alachlor and atrazine desorption from soils. Weed Sci., 38 74—80. 

To verify that Eq. (2.4) is indeed elementary, one can employ experimental conditions that are dissimilar from those used to ascertain the rate law. For example, if the k values change with flow rate, one is determining nonmechanistic or apparent rate coefficents. This was the case in a study by Sparks et al. (1980b), who studied the rate of potassium desorption from soils using a continuous flow method (Chapter 3). They found the apparent desorption rate coefficients ( d) increased in magnitude with flow rate (Table 2.1). Apparent rate laws are still useful to the experimentalist and can provide useful time-dependent information. 

The two-constant rate equation was also used to describe P04 desorption from soil (Dalai, 1974), K-Ca exchange on soils (Sparks etal., 1980a), and recently, by Jopony and Young (1987) to study the kinetics of copper desorption from soil and clay minerals. 

An example of a two-site model for pesticide desorption kinetics from soils was presented by McCall and Agin (1985). Using a reversible first-order equation to describe picloram desorption from soil, the authors plotted ln(CB - CBeq) versus t, where CB is the bound form of picloram and CBeq is the bound form at equilibrium, in Fig. 9.4. It is obvious that desorption conforms to a two-step process where a fast step occurs for about a 5-h period, and then a slow process as shown from the linear portion of Fig. 9.4 occurs from 5 to 300 h. McCall and Agin (1985) used the following two-step model to describe the data shown in Fig. 9.4 ... 

Griffin, R. A., and Burau, R. G. (1974). Kinetic and equilibrium studies of boron desorption from soil. Soil Sci. Soc. Am. Proc. 38, 892-897. 

Jopony, M., and Young, S. D. (1987). A constant potential titration method for studying the kinetics of Cu2+ desorption from soil and clay minerals. J. Soil Sci. 38, 219-228. 

Thomas, G, W. (1963). Kinetics of chloride desorption from soils. J. Agric. Food Chem. 110, 201-203. 

Bioavailability from Environmental Media. Available absorption kinetics of dinitrophenols following ingestion and dermal contact are discussed in Section 2.3. No experimental or estimated data were located that provide information about the bioavailability of dinitrophenols from natural air, water, and soil. The observation that dinitrophenols are found at least partly in the particulate-sorbed state in the air (Nojima et al. 1983) indicates that their bioavailability from air is <100%. The adsorption of dinitrophenols to soil and sediment depends on the nature of soil and sediment (e.g., organic matter and clay content) and the pH of the medium (Callahan et al. 1979 Kaufman 1976). Therefore, the bioavailability of particle-sorbed dinitrophenols due to desorption from soil and sediment containing a high percent of organic matter and clay may be less than that of the free form (unadsorbed) of dinitrophenol. The bioavailability of dinitrophenols from inhaled air particulates with small particle diameters remains unknown. 

TABLE 5.6. Kinetics of Cd Desorption from Soils by mol L NH4C1 and Phosphate... 

Infinite sinks have been used primarily to study P and K desorption kinetics from soils. Ion-exchange resins have been the more popular choice. Recent examples include the studies of P desorption kinetics by Pavlatou and Polyzopoulos (1988) and K desorption kinetics by Sadusky et al. (1987). Van der Zee et al. (1987) used Fe-oxide impregnated filter paper as an infinite sink to study P desorption kinetics from soils. The affinity and capacity of this sink for P was large enough to maintain negligible P concentrations in solution, and thus it served as an infinite sink. Griffin and Burau (1974) used mannitol as a precipitation sink for B to study B desorption from soils. I wo separate pseudo-first-order reactions and a slow reaction were found. 

The electrolysis reaction produces both H+ and OH ions in the anode and cathode, respectively. The movement of H+ ions (acid front) advancing through soil toward the cathode can result in the release of Fe°/Fe ions to the EO flow via mineral dissolution (corrosion of the ZVI). At the same time, the OH ion move toward the anode creates a favorable alkaline zone for Cr(VI) desorption from soil. Thus, the chromates are reduced either at the Fe°-water interface in the barrier or by the Fe°/Fe ions in the EO flow path. The existence of Cr(VI) in the cathode reservoir due to the diffusion of soluble Cr(VI) caused by the concentration gradient near the cathode will also be reduced to Cr(III) by Fe ions. In addition, the occurrence of a secondary electrolysis reaction could reduce Cr(VI) to Cr(III)... 

When rainfall intensity exceeds infiltration rate and surface-storage capacity has been reached, overland flow begins. The transfer of dissolved pesticides from the soil matrix to overland flow consists of several mechanisms desorption from soil organic matter, mineral surfaces and plant residues dissolution of insecticide crystals or granules and diffusive and turbulent transport of dissolved insecticide from soil water into overland flow (2, 62). The relative importance of each process depends on the physico-chemical properties of the chemical, formulation, initial placement, soil properties, recent hydraulic history and vegetation (62). 

In addition, many remediation processes require information concerning the diffusion of various gases through the soil. Diffusion may possibly be a rate-limiting step in aerobic biodegradation as these processes require sufficient amounts of oxygen in the soil. Brusseau (1991) noted that diffusion is often the rate-limiting step in contaminant sorption and desorption from soil. 

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