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Permeability barrier chemical nature

Apatite, a natural calcium fluoride phosphate, can adsorb low to moderate levels of dissolved metals from soils, groundwater, and waste streams. Metals naturally chemically bind to the apatite, forming extremely stable phosphate phases of metal-substituted apatite minerals. This natural process is used by UFA Ventures, Inc., and is called phosphate-induced metals stabilization (PIMS). The PIMS material can by used in a packed bed, mixed with the contaminated media, or used as a permeable barrier. The material may be left in place, disposed of, or reused. It requires no further treatment or stabilization. Research is currently being conducted on using apatite to remediate soil and groundwater contaminated with heavy metals, and the technology may also be applicable to radionuclides. The technology is not yet commercially available. [Pg.1088]

Cuticle. The cuticle of adult nematodes is relatively impermeable and is composed primarily of a complex of several proteins. These proteins include collagens, fibroids, elastoids, and keratoids, possibly hardened by tanning with pol3rphenols or quinones. Sometimes lipide materials are also present (8, 10y 52). When such lipide materials are present on the adult cuticle, they probably take the form of hydroxy fatty acids or esters of fatty acids with monohydroxy alcohols. Such materials may form ring compounds. Regardless of the exact chemical nature, the lipides are unquestionably one of the chief barriers to permeability. These materials are commonly difficult to demon-... [Pg.92]

Zeolite surface chemistry resembles that of smectite clays. In contrast to clays, however, natural zeolites can occur as millimeter- or greater-sized particles and are free of shrink-swell behavior. As a result, zeolites exhibit superior hydraulic characteristics and are suitable for use in filtration systems (Breck 1974) and as permeable barriers to dissolved chemical migration. Internal and external surface areas up to 800 m2 g have been measured. Total cation exchange capacities in natural zeolites vary from 250 to 3000 meq kg 1 (Ming and Mumpton 1989). External cation exchange capacities have been determined for a few natural zeolites and typically range from 10 to 50 percent of the total cation exchange capacity (Bowman et al. 1995). [Pg.163]

Membranes are semi-permeable barriers that are used to isolate and separate constituents from a fluid stream. The separatirai process can be accomplished through a number of physical and chemical properties of the membrane as well as the material being separated. Separation can occur through processes such as size, ionic char, solulnlity, and combinations of several processes. Membranes can remove materials ranging from large visible particles to molecular and ionic chemical species. Membrane materials are diverse and can consist of synthetic polymers, natural fabrics, porous metals, porous ceramics, or liquids. The surface of the membrane can be chemically or biologically altered to perform separations on specific chemical... [Pg.169]

Human skin has a multifunctional role, primary among which is its role as a barrier against both the egress of endogenous substances such as water and the ingress of xenobiotic material (chemicals and drugs). This barrier function of the skin is reflected by its multilayered structure (Fig. 5.1). The top or uppermost layer of the skin known as the stratum comeum (SC) represents the end product of the differentiation process initially started in the basal layer of the epidermis with the formation of keratinocytes by mitotic division. The SC, therefore, is composed of dead cells (comeocytes) interdispersed within a lipid rich matrix. It is the brick and mortar architecture and lipophilic nature of the SC, which primarily accounts for the barrier properties of the skin [1,2]. The SC is also known to exhibit selective permeability and allows only relatively lipophilic compounds to diffuse into the lower layers. As a result of the dead nature of the SC, solute transport across this layer is primarily by passive diffusion [3] in accordance with Pick s Law [4] and no active transport processes have been identified. [Pg.120]

Thin films with a broad spectrum of properties can be deposited in plasma chemical processes [30, 31]. Hard coatings such as diamond films and TiN films, soft plasma polymer films, insulating SiO films, highly conducting Si films, anti-reflection coatings, semi-permeable membranes and very effective diffusion barriers can be deposited. Important parameters for the film deposition are (i) the nature of the precursor, (ii) the gas mixture and... [Pg.2806]


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