Metals, adsorption from solution

The information on inhibitor adsorption, derived from direct measurements and from inhibitive efficiency measurements, considered in conjunction with general knowledge of adsorption from solution indicates that inhibitor adsorption on metals is influenced by the following main factors. 

Hydrogen adsorption from solution Oxygen adsorption from solution Underpotential deposition of metals Adsorption of probe molecules from solution ... 

Adsorption of ions from the solution. There are two types of ionic adsorption from solutions onto electrode surfaces an electrostatic (physical) adsorption under the effect of the charge on the metal surface, and a specific adsorption (chemisorption) under the effect of chemical (nonelectrostatic) forces. Specifically adsorbing ions are called surface active. Specific adsorption is more pronounced with anions. 

As shown in Fig. 1 biosorption comprises a variety of processes including ion exchange, chelation, adsorption, and diffusion through cell walls and membranes all of which are dependent on the species used, the biomass origin and, and solution chemistry (Gavrilescu 2004). Biosorption is a fast and reversible process for removing toxic metal ions from solution. 

Considering all we know up to now, the specific properties of zeolites can be summarized as follows. Zeolites are aluminosilicates with defined microporous channels or cages. They have excellent ion-exchange properties and can thus be used as water softeners and to remove heavy metal cations from solutions. Furthermore, zeolites have molecular sieve properties, making them very useful for gas separation and adsorption processes, e.g., they can be used as desiccants or for separation of product gas streams in chemical processes. Protonated zeolites are efficient solid-state acids, which are used in catalysis and metal-impregnated zeolites are useful catalysts as well. 

In their description of metal ion adsorption, Benjamin and Leckie used an apparent adsorption reaction which included a generic relationship between the removal of a metal ion from solution and the release of protons. The macroscopic proton coefficient was given a constant value, suggesting that x was uniform for all site types and all intensities of metal ion/oxide surface site interaction. Because the numerical value of x is a fundamental part of the determination of K, discussions of surface site heterogeneity, which are formulated in terms similar to Equation 4, cannot be decoupled from observations of the response of x to pH and adsorption density. As will be discussed later, It is not the general concept of surface-site heterogeneity which is affected by what is known of x> instead, it is the specific details of the relationship between K, pH and T which is altered. 

C02(CO)8 SiOj Adsorption from solution and thermal treatment under hydrogen High metal dispersion, small Co clusters favor alcohol formation in the CO hydrogenation [138]... 

Among the different adsorptive materials that have conventionally been used to capture metal ions from solution are activated charcoal (2), zeolites (3, 4), and clays (5). 

The foregoing description assumes adsorption of colloidal metal hydroxide from solution onto the substrate as the primary nucleation step. However, hydroxides can also adsorb on solid surfaces at pH values below that of bulk hy-... 

No attempt has been made to discuss the voluminous literature on molecular sieves or supported metals and the reader is referred to papers by Nacchace and Uytterhoeven (31) and Sheppard (32) respectively. IR studies on adsorption from solution are particularly relevant to polymer adsorption and have been reviewed by Rochester (33). 

Elements 108 - 116 are homologues of Os through Po and are expected to be partially very noble metals. Thus it is obvious that their electrochemical deposition could be an attractive method for their separation from aqueous solutions. It is known that the potential associated with the electrochemical deposition of radionuclides in metallic form from solutions of extremely small concentration is strongly influenced by the electrode material. This is reproduced in a macroscopic model [70], in which the interaction between the microcomponent A and the electrode material B is described by the partial molar adsorption enthalpy and adsorption entropy. By combination with the thermodynamic description of the electrode process, a potential is calculated that characterizes the process at 50% deposition ... 

As late as 1987, McDougall and Fleming [27], whose interests rested primarily in the extraction of gold by activated carbons, commented that the adsorption of organic and inorganic species onto carbon may occur by several mechanisms, the unraveling of which, especially in the extraction of certain metal complexes from. solution, is extremely difficult. ... 

Evidence was obtained that this catalysis by the nickel ions was due to the adsorption from solution of enough nickel ions for a complex to be formed in the surface. One molecule of a-hydroxystearic acid combines with one nickel ion, so that very small amounts of metal ions are concentrated at the surface through reaction with the molecules in the monolayer. 

Adsorption is an important process in many industrial, biological, and environmental systems. One compelling reason to study adsorption phenomena is because an understanding of colloid stability depends on the availability of adequate theories of adsorption from solution and of the structure and behavior of adsorbed layers. Another example is the adsorption of pollutants, such as metals, toxic organic compounds, and nutrients, onto line particles and their consequent transport and fate, which has great environmental implications. Often, these systems are quite complex and it is often favorable to separate these into specific size for subsequent study. 

In the case of vapour-phase processes for metal deposition on the support, only limited control of dispersion and distribution of the metal crystallites is possible. In the case of liquid-phase systems, they do not provide as wide a range of catalysts as is possible with techniques based on adsorption from solution. However, the technique does provide a means of preparing well characterized surface-impregnated supports. 

Complex Ionic Adsorption from Solution. - Much of the metal adsorption that normally is of importance in catalyst preparative chemistry concerns the interaction with complex ions. Obviously, for example, all metal anionic adsorption must of necessity involve the metal as a complex ion. This tendency to form complex ions can cause substantial deviations from what one might normally expect from considerations of simple ion-exchange equilibria. This is true in the case of ferric chloride, for example reactions (12)-(15). 

The same steps as discussed above for the case of isotope exchange (diffusion in liquid film, surface reaction, intraparticle diffusion) were considered in a kinetic model [771] of metal ion adsorption from solution. This model was presented in a book with diskettes (FORTRAN program, rate controlled by reaction, by transport or mixed control). 

In the case of the iodide ion with a radius of 206 pm, the value of pad for gas phase adsorption is 3.3x10 C m, or 10.0 debyes. For adsorption from solution, the dipole moment can be estimated when the charge density due to adsorbed ions Oad is equal and opposite to the charge density on the metal Om. On the basis of equation (10.8.8), the potential drop across the inner layer under these circumstances is... 

The support, zirconia (ISA), was supplied by the Norton Company. The oxide was grounded and sieved to a particle size ranged from 0.16 to 0.25 mm, and calcined at 773 K. Its surface properties, 63.3 m g of specific surface and average pore diameter of 8.60 nm, were determined from the nitrogen adsorption isotherms. The catalysts were prepared by adsorption from solution and/or impregnation of precursor(s), ruthenium nitrosyl nitrate (Alfa) and hexachloroplatinic acid (Aldrich), onto the support. Being zirconia isoelectric point 6.5 (determined by electrophoresis [17] using a Malvern Instrument Zetasizer 4) the precursors solution pH value was kept sufficiently low to enable the desired adsorption of complex metal anions. 

The tendency for easily hydrolyzed metals to appear to be strongly adsorbed on soil minerals may be caused in part by the manner in which adsorption is traditionally measured. Typically, the pH is adjusted over a range, and the amount of metal removed from solution is calculated from the change in solution concentration. As the pH is raised from an acid value, chemisorption is initially favored, but even before adsorption sites become saturated, metal ions cluster into metal oxide or... 

In actual experiments, because adsorption is operationally defined as the removal of metal ions from solution, complexation and precipitation are not separated, and adsorption seems to continue to increase at high pH. 

Adsorption from solution—Adsorption is defined as the selective removal of metal salts or metal ion species from their solution by a process of either physisorption or chemical bonding with active sites on the support. Depending upon the strength of adsorption of the adsorbing species, the concentration of the active material through the support may be varied and controlled. 

The study of ultrathin polymer layers on metals is relevant in understanding the behaviour of polymers on surfaces, as well as in the areas of adhesion and corrosion. Gold and copper surfaces can be covered with monolayers of polymers by adsorption from solution [227. 228, 229, 230, 231, 232, 233, 234 and 235]. 

Very straightforward effects of surface chemistry of carbons are seen on the adsorption from solutions of aromatics [10-17], dyes [18], heavy metals [19-24], pharmaceuticals [25-27], polar species such as alcohols [28-30], acids or aldehydes [31,32], and even small-molecule gases [33,34]. In those applications the species present on the carbon surface can enhance the specific interactions or even alter the porosity via blocking of pore entrances for molecules to be adsorbed. Specific interactions include hydrogen bonding, acid-base, and complexation. 

They suggested that metal ion adsorption is initiated at a pH value corresponding to surface nucleation, which seems to relate to the reduction of cation-solvent interactions, as a result of hydrolysis or ligand complex formation, leading to conditions favorable to the adsorption of hydrated metal ions from solution. Their model suggests that metal ion hydrolysis enhances adsorption on Si02, whereas Schindler proposes direct participation by unhydrated ions. 

Coprecipitation processes tend to be more effective than adsorption processes at removing aqueous heavy metal ions from solution. This could be due to a much larger effective surface area becoming available as various "layers" of the colloid form. Alternatively, it may result from the formation of mixed metal hydroxide complexes (either in solution or as a solid) that exhibit greater adsorption characteristics than those of the individual metal ions. 

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