Soil sorption from vapor phase

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). 

Pollutants with high VP tend to concentrate more in the vapor phase as compared to soil or water. Therefore, VP is a key physicochemical property essential for the assessment of chemical distribution in the environment. This property is also used in the design of various chemical engineering processes [49]. Additionally, VP can be used for the estimation of other important physicochemical properties. For example, one can calculate Henry s law constant, soil sorption coefficient, and partition coefficient from VP and aqueous solubility. We were therefore interested to model this important physicochemical property using quantitative structure-property relationships (QSPRs) based on calculated molecular descriptors [27]. 

Dichlorobenzene will exist predominantly in the vapor-phase in the atmosphere, and its detection in rainwater suggests that atmospheric removal via washout is possible (Ligocki et al. 1985). Depending on soil type, the compound is expected to be moderately mobile in soil and to volatilize from surface water and soil surfaces to the atmosphere. Volatilization, sorption, biodegradation, and bioaccumulation are likely to be competing processes, with the dominant fate being determined by local environmental conditions. 

It has been argued that the Kqa values can be used as a unifying property for describing volatilization of POPs from soils and sorption to aerosols. The limited experimentally obtained values typically are supplemented by estimates from octanol-water and air-water partition coefficients. The value of condensation temperature lies in its ability to estimate sorption of atmospheric contaminants to aerosols (Bidleman, 1988). At Tc, the chemical is equally partitioned between the gas phase and aerosols. Since POPs exist in the atmosphere both as gases (vapor phase), and in condensed form adsorbed to aerosol particles, the characteristic temperature of... 

PROBABLE FATE photolysis direct photolysis is slow, indirect photolysis may be important, vapor phase aldrin residues expected to react with photochemically produced hydroxyl radicals with a half-life of 35.46 min oxidation reacts to form dieldrin, photooxidation by ultraviolet light in aqueous medium 90-95°C forms 25% CO2 in 14.1 hr, 50% CO2 in 28.2 hr, 75% CO2 in 109.7 hr, photooxidation half-life in air 0.9-9.1 hrs hydrolysis too slow to be an important process volatilization an important process, evaporation rate from water 3.72x10 m4ir, will volatilize from soil surfaces sorption an important process, adsorption to sediment is... 

PROBABLE FATE photolysis, sensitized process may be important, reacts in the vapor phase with photochemically produced hydroxyl radicals at an estimated half-life of 6.2 hr, suggesting that this reaction is the predominate chemical removal process oxidation photooxidation half-life in air 5.2-51.7 hrs hydrolysis not an important process, first-order hydrolytic half-life >197,000 yrs volatilization probably an important process, can volatilize significantly from soil surfaces from which it is sprayed, particularly moist soil surfaces, volatilization half-life from a model pond, river, and lake is 18-26,3.6-5.2, and 14.4-20.6 days respectively sorption probably an important process biological processes bioaccumulation is an important process... 

PROBABLE FATE photolysis, photooxidation to chlorinated biphenyls and benzophe-nones probable, indirect photolysis may be significant based on the behavior of the related compound DDT, direct photolysis half-life in water >150 yrs, photooxidation half-life in air 13.3-133 hrs oxidation, not an important process, vapor phase half-life in the atmosphere 1.71 days from reaction with photochemically produced hydroxyl radicals hydrolysis, not an important process, will not hydrolyze in soil volatilization expected to be an important process, evaporation half-life 1.82 days from a river 1 m deep, flowing at Im/sec with a wind velocity of 3 m/sec sorption is an important process, expected to adsorb to sediment if released to water biological processes biotransformation and bioaccumulation are important processes biodegradation expected to be slow... 

PROBABLE FATE photolysis photoisomeraization occurs, rate undetermined, photooxidation half-life in air 59 minutes-9.8 hrs, vapor phase hepatchlor in air will react with photo-chemically produced hydroxy radicals, half-life 36 min., direct photolysis may occur oxidation information is not available hydrolysis rapid hydrolysis for heptachlor in solution, first-order hydrolytic half-life 23.1 hrs or 129.4 hrs, significant in moist soils volatilization expected to be an important process, evaporates slowly, release to soil will result in volatilization from the surface, especially in moist soils sorption probably an important process, but no reliable data is available, sticks strongly to soil particles biological processes will bioaccumulate if not hydrolyzed, biodegradation is significant... 

PROBABLE FATE photolysis , possible dechlorination of C=C bond, photooxidation half-life in air 6 hrs-2.5 days, vapor phase reactions with photochemically produced hydroxyl radicals have a half-life of 1.5 days oxidation not expected to be important hydrolysis very slow, not expected to be important volatilization not an important process, evaporates slowly to air, volatilization from a model river 60 hr sorption occurs to a moderate degree, sticks strongly to soil particles, if released to water, will adsorb strongly to suspended and bottom sediment biological processes biotransformation occurs very slowly, but could be important... 

PROBABLE FATE photolysis, too slow to be important, vapor-phase reaction with hydroxyl radicals has a half-life of 12.9 days to 3.1 months oxidation not important hydrolysis not important volatilization slow volatilization distributes PCB s globally, but is inhibited by adsorption, significant volatilization from soil surfaces rapid volatilization from water in the absence of adsorption, half-life of 2 months-1 yr in typical water bodies sorption PCB s are... 

Vapor-phase sorption onto the same dry Woodbum soil is illustrated in Figure 3.16 where soil uptake is expressed as a function of the relative pressure-equilibrium partial pressure/saturated vapor pressure. The data indicate a BET adsorption process. Increasing relative humidity decreases the adsorption (Fig. 3.17) of dichlorobenzene ultimately resulting in a linear isotherm. It is concluded that at low humidities adsorption on the mineral surfaces is involved. Increasing availability of water reduces the availability of these sites until sorption is due to partitioning into the SOM. It will be demonstrated that the converse of this relation is important in assessing the potential for evaporative loss from soils. 

Vapor-phase sorption of carbon tetrachloride and benzene onto a peat soil (fom = 0.864) give linear isotherms indicating partitioning into the organic matter (Fig. 3.18). The sorption of these two compounds on the same soil from water is only 40-50% less than that observed from the vapor phase. This effect is much less than that observed when the mineral fraction is the primary absorbent. 

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... 

Sorption/desorption is the key property for estimating the mobility of organic pollutants in solid phases. There is a real need to predict such mobility at different aqueous-solid phase interfaces. Solid phase sorption influences the extent of pollutant volatilization from the solid phase surface, its lateral or vertical transport, and biotic or abiotic processes (e.g., biodegradation, bioavailability, hydrolysis, and photolysis). For instance, transport through a soil phase includes several processes such as bulk flow, dispersive flow, diffusion through macropores, and molecular diffusion. The transport rate of an organic pollutant depends mainly on the partitioning between the vapor, liquid, and solid phase of an aqueous-solid phase system. 

The vapor pressure of a chemical is the pressure its vapor exerts in equilibrium with its liquid or solid phase. The vapor pressure s importance in environmental work results from its effects on the transport and partitioning of chemicals among the environmental compartments (air, water, and soil). The vapor pressure expresses and controls the chemical s volatility. The volatilization of a chemical from the water surface is determined by its Henry s law constant (see Chapter 4), which can be estimated from the ratio of a chemical s vapor pressure to its water solubility. The volatilization of a chemical from the soil surface is determined largely by its vapor pressure, although this is tempered by its sorption to soil solids and its Henry s law constant between soil, water, and air. A substance s vapor pres-... 

From an environmental impact standpoint, the four most important physical properties of PCBs are very low water solubUlty and vapor pressure, high octanol/water partition coefficients, and stability or persistence (NRC, 1979). The five mechanisms by which PCBs can be transported in the soil are as a dissolved material in the water by sorption as an emulsion with water as an immiscible oUy Uquid phase and as a discrete fluid. 

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