Sorption processes organic content

An example of first-order plots is shown in Fig. 1 for benzo[a]pyrene (i.e., B[a]P) sorption on three different soils (in terms of organic matter content) and two sediment samples (marine and fresh water) at two different concentrations [1]. It can be noted that the plots are linear at both concentrations, which would indicate that the sorption process is first order. The findings that the rate constants are not significantly changed with concentration is a good indication that the reaction is first order under the experimental conditions that were imposed. 

The duration of the remediation is dependent on the soil type, water content, and the nature of the contaminants. The HRUBOUT process cannot remove metals from soils. Polychlorinated biphenyls (PCBs) cannot be totally removed. The in situ HRUBOUT process is designed for removing contaminants from the vadose zone, (i.e., the zone between the surface aud the water table). Low permeability lowers system effectiveness and raises remediation costs. Soils with variable permeabilities may cause uneven delivery of air to contaminants. VOC removal rates may be reduced by high organic content in the soil because soil orgauics have a high VOC-sorption capacity. 

Soil sorption of most hydrophobic organic compounds (e.g., nonpolar pesticides) is directly related to SOM content. HS are the major SOM components (Ferreira et al., 2002). FTIR spectroscopy makes possible the observation of how some chemical functions, present in humic structures, are involved in the sorption process. 

Soil Organic Matter Organic compounds may exist in soil either in solution or the vapor phase and can be absorbed through the roots in either state, absorption from solution would be the most likely process. From the discussion of the sorption process in soil (see Sorption, Chapter 3) the concentration of the compound in aqueous solution, Caq, would be a function of the concentration in the soil (Csoii). and the soil distribution ratio, K, which in turn is dependent primarily on SOM content. One would predict that uptake would be inversely related to the level of SOM. Observations of the uptake of diel-drin by carrots raised in different soils provide an opportunity to evaluate this relation and demonstrate the dependence on the concentration of the compound in soil solution. If the uptake efficiency is defined by the ratio of the concentration of dieldrin in carrots to that in the soil it is clear that higher levels of soil organic matter reduce uptake by carrots (Table 5.6). The ATom for dieldrin is 6980 mL g from which values were calculated for dieldrin in each soil. Since = Cjoii/Caq the concentration of dieldrin in soil solution can be determined. If uptake is defined as Ccarrot/Caq consistent value... 

For soils and sediments of differing organic matter content, the useful concept of Kqc has been introduced this form of the partition coefficient makes the simplifying assumption that only the organic carbon is active in the sorption process, and the partition coefficient expression can be rewritten... 

Johnson et al. [25] tested the sorption behavior by equilibrating five materials with solutions of PFOS (high initial crmcentrations were between 0.12 and 8 mg/L). The Kd values ranged from 2.8 to 8.9 L/kg (see Table 1). They also found a decrease in adsorption with increasing pH and also suggested that both, the inorganic and the organic content of the sediment play an important role in the sorption process. 

The variables that affect SBSE extraction performance are similar to the ones affecting SPME, namely, extraction time, extraction temperature (mainly for headspace mode, it should be noted however that the sorption process on the PDMS coating is not favored at high temperatures), selection of modifiers to increase the extraction efficiency (sueh as small amounts of organic solvents or high contents of salts), pH, stirring, and sample volume. 

Volatilization. The susceptibility of a herbicide to loss through volatilization has received much attention, due in part to the realization that herbicides in the vapor phase may be transported large distances from the point of application. Volatilization losses can be as high as 80—90% of the total applied herbicide within several days of application. The processes that control the amount of herbicide volatilized are the evaporation of the herbicide from the solution or soHd phase into the air, and dispersal and dilution of the resulting vapor into the atmosphere (250). These processes are influenced by many factors including herbicide application rate, wind velocity, temperature, soil moisture content, and the compound s sorption to soil organic and mineral surfaces. Properties of the herbicide that influence volatility include vapor pressure, water solubility, and chemical stmcture (251). 

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. 

Retention of organic contaminants on subsurface solid phase constituents in general is not completely reversible, so that release isotherms differ from retention isotherms. As a consequence, the extent of sorption depends on the nature of the sorbent. Subsurface constituents as well as the types of bonding mechanisms between contaminants and the sohd phase are factors that control the release of adsorbed organic contaminants. Saltzman et al. (1972) demonstrated the influence of soil organic matter on the extent of hysteresis. Adsorption isotherms of parathion showed hysteresis (or apparent hysteresis) in its adsorption and desorption in a water solution. In contrast, smaller differences between the two processes were observed when the soils were pretreated with hydrogen peroxide (oxidized subsamples) to reduce initial organic matter content. The parathion content of the natural... 

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