Chemical sorption

Over the years efforts have been made to understand the various parameters to consider in the choice of plastics for packaging of drug products. Drug-plastic considerations have been divided into live separate categories permeation, leaching, sorption, chemical reaction, and alteration in the physical properties of plastics or products [89]. 

Total concentrations of chemicals in the compartment of exposure are considered of limited relevance because, due to sorption and sequestration processes, often only a small fraction of the total amount is available for uptake by organisms. Properties of the chemical and of the environment determine sorption, chemical speciation, and bioavailability. Information on chemical concentrations therefore is much more... 

Surface area Porosity (pore size, volume, and distribution) BET method (Brunauer, Emmett, and Teller method) Physical gas sorption Chemical gas sorption Helium picnometry Mercury intrusion porometry (MIP)... 

Nonlinear sorption of chem-bio agents can be based on the different kinetics of sorption. Chemical nonequilibrium must also be considered in real-world problems. One of the early kinetic sorption models used the CDE first-order reactions (Scott 2000). This is represented as follows ... 

Azizian, M.P. and Nelson, P.O., Lead sorption, chemically enhanced desorption, and equilibrium modeling in an iron-oxide-coated sand and synthetic groundwater system, in Adsorption of Metals by Geomedia, Jenne, E., ed.. Academic Press, New York, 1998, p. 165. 

The data required for input into the groundwater flow models to predict the hydrodynamic flow velocity include the porosity of the soil, the water table, rainfall, reversible absorption/desorption phenomena, irreversible sorption, chemical reactions, and microbial degradation kinetics 37). Mixing with seawater, air, or steam may also be considered. Based on these models, estimates of leaching and pollutant distribution can be made many years into the future although significant amounts of computer time are usually required (57). 

Voudrias, E.A. (2002). The concept of a sorption chemical barrier for improving effectiveness of landfill liners. Waste Management and Research, 20 251-258. Wilson, M J. (1986). Mineral weathering processes in podzolic soils on granitic materials and their implications for surface water acidification. Journal of the Geological Society of London, 143 691-697. 

FIGURE 22.24 There Isa fundamental difference between physisorption and chemisorption, shown here diagrammatically. (a) In physisorption, a molecule of hydrogen remains intact but is attracted to a surface due to van der Waals forces, London forces, or the like, (b) In chemi sorption, chemical species are virtually bonded chemically to the surface. In the case of hydrogen, it is necessary to break the H-H bond for it to be chemisorbed. This is not always required for carbon monoxide, the CO molecule might be as strongly bonded to the surface but still retain the C-O bond. 

A special section discusses changes taking place in phase of the functional polymers at continuous industrial use. This includes phenomena of semi-reversible and irreversible sorption, chemical and physical deterioration, aging, etc. 

These fibers may find use in controUed release of dmgs, bactericides, and corrosion prevention chemicals (103). Fibers with different active groups have been made for sorption of chemicals. These fibers are designed to replace granular sorbents for air purification, for example, in air filtration masks (104). 

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). 

Degradation of a herbicide by abiotic means may be divided into chemical and photochemical pathways. Herbicides are subject to a wide array of chemical hydrolysis reactions with sorption often playing a key role in the process. Chloro-j -triazines are readily degraded by hydrolysis (256). The degradation of many other herbicide classes has been reviewed (257,258). 

In air conditioning (qv) of closed spaces, a wider latitude in design features can be exercised (23,24). Blowers are used to pass room or cabin air through arrays of granules or plates. Efficiencies usuaHy are 95% or better. The primary limiting factor is the decreased rate of absorption of carbon dioxide. However, an auxHiary smaH CO2 sorption canister can be used. Control of moisture entering the KO2 canister extends the life of the chemical and helps maintain the RQ at 0.82. 

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. 

A variety of mechanisms or forces can attract organic chemicals to a soil surface and retain them there. For a given chemical, or family of chemicals, several of these mechanisms may operate in the bonding of the chemical to the soil. For any given chemical, an increase in polarity, number of functional groups, and ionic nature of the chemical can increase the number of potential sorption mechanisms for the chemical. 

Although most nonionic organic chemicals are subject to low energy bonding mechanisms, sorption of phenyl- and other substituted-urea pesticides such as diuron to sod or sod components has been attributed to a variety of mechanisms, depending on the sorbent. The mechanisms include hydrophobic interactions, cation bridging, van der Waals forces, and charge-transfer complexes. 

It appears that pesticides with solubiHties greater than 10 mg/L are mainly transported in the aqueous phase (48) as a result of the interaction of solution/sediment ratio in the mnoff and the pesticide sorption coefficient. For instance, on a silt loam soil with a steep slope (>12%), >80% of atra2ine transport occurs in the aqueous phase (49). In contrast, it has been found that total metolachlor losses in mnoff from plots with medium ground slopes (2—9%) were <1% of appHed chemical (50). Of the metolachlor in the mnoff, sediment carried 20 to 46% of the total transported pesticide over the monitoring period. 

Gaseous vent streams from the different unit operations may contain traces (or more) of HCl, CO, methane, ethylene, chlorine, and vinyl chloride. These can sometimes be treated chemically, or a specific chemical value can be recovered by scmbbing, sorption, or other method when economically justified. Eor objectionable components in the vent streams, however, the common treatment method is either incineration or catalytic combustion, followed by removal of HCl from the effluent gas. 

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

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