Metals, adsorptive separation

J. W. Novak, Jr., R. R. Burr, andR. Bednarik, "Mechanisms of Metal Ion Adsorption of Activated Alumina," Vol. 35, Proc. Int. Symp. on Metals Speciation, Separation, and Recorey, Chicago, lU., July 27—Aug. 1, 1986, Industrial Waste Elimination Research Center of the Illinois Institute of Technology, Chicago, lU. 

Prange et al. [809,810] carried out multielement determinations of the stated dissolved heavy metals in Baltic seawater by total reflection X-ray fluorescence (TXRF) spectrometry. The metals were separated by chelation adsorption of the metal complexes on lipophilised silica-gel carrier and subsequent elution of the chelates by a chloroform/methanol mixture. Trace element loss or contamination could be controlled because of the relatively simple sample preparation. Aliquots of the eluate were then dispersed in highly polished quartz sample carriers and evaporated to thin films for spectrometric measurements. Recoveries (see Table 5.10), detection limits, and reproducibilities of the method for several metals were satisfactory. 

In the case of trace metals, adsorption is typically much faster than the time intervals for which it is practically possible to separate the cells. Therefore, in practice, values of kf and kr are most often estimated by assuming that water loss from the hydrated cation is rate-limiting (Eigen-Wilkins mechanism, see Section 4.3.1 above). In some cases, uptake transients can be observed at the start of a short-term uptake experiment or by using pulse-chase experiments for which a metal solution containing a radioactive tracer is replaced by a solution... 

Figure 1. Log concentration-pH diagram showing the effect of the macroscopic proton coefficient on metal adsorption onto two hypothetical solids in separate, single adsorbent systems. Both adsorbents are present at the same site concentration top,...

The use of centrifugation to separate the liquid from solid phases in traditional batch or tube techniques has several disadvantages. Centrifugation could create electrokinetic effects close to soil constituent surfaces that would alter the ion distribution (van Olphen, 1977). Additionally, unless filtration is used, centrifugation may require up to 5 min to separate the solid from the liquid phases. Many reactions on soil constituents are complete by this time or less (Harter and Lehmann, 1983 Jardine and Sparks, 1984 Sparks, 1985). For example, many ion exchange reactions on organic matter and clay minerals are complete after a few minutes, or even seconds (Sparks, 1986). Moreover, some reactions involving metal adsorption on oxides are too rapid to be observed with any batch or, for that matter, flow technique. For these reactions, one must employ one of the rapid kinetic techniques discussed in Chapter 4. 

The chemical behavior of monolayer coverages of one metal on the surface of another, i.e., Cu/Ru, Ni/Ru, Ni/W, Fe/W, Pd/W, has recently been shown to be dramatically different from that seen for either of the metallic components separately. These chemical alterations, which modify the chemisorption and catalytic properties of the overlayers, have been correlated with changes in the structural and electronic properties of the bimetallic system. The films are found to grow in a manner which causes them to be strained with respect to their bulk lattice configuration. In addition, unique electronic interface states have been identified with these overlayers. These studies, which include the adsorption of CO and H2 on these overlayers as well as the measurement of the elevated pressure kinetics of the methanation, ethane hydrogenolysis, cyclohexane dehydrogenation reactions, are reviewed. 

Ordered macroporous materials (OMMs) are a new family of porous materials that can be synthesized by using colloidal microspheies as the template. - The most unique characteristics of OMMs are their uniformly sized macropores arranged at micrometer length scale in three dimensions. Colloidal microspheres (latex polymer or silica) can self assemble into ordered arrays (synthetic opals) with a three-dimensional crystalline structure. The interstices in the colloidal crystals are infiltrated with a precursor material such as metal alkoxide. Upon removal of the template, a skeleton of the infiltrated material with a three-dimensionally ordered macroporous structure (inverse opals) is obtained. Because of the 30 periodicity of the materials, these structures have been extensively studied for photonic applications. In this paper, the synthesis and characterization of highly ordered macroporous materials with various compositions and functionalities (silica, organosilica, titana, titanosilicate, alumina) are presented. The application potential of OMMS in adsorption/separation is analyzed and discussed. 

As the existence of this review demonstrates, the various experimental problems, described above, have not discouraged everybody from attempting to perform structural determinations of adsorbates on metal oxide surfaces. The following sections of this paper detail such work. It is structured such that there is a separate section on each metal oxide for which quantitative structural determinations of adsorbates have been carried out. We have restricted ourselves to only those studies involving monolayer/submonolayer adsorbate coverages, but have included atoms, molecules and metal adsorption in this coverage regime. 

A very useful review of the subject of metal adsorption has recently appeared, which gives tables of the relative strengths of anionic and cationic adsorption for each of the metals in the periodic table, with separate tables for each of the different anion environments, i.e., F , Q", NO3, CbT, and sol". Table 2 shows some of the results from this reference that pertain to the catalytically important Group VIII and IB metals. There would seem to be ample scope for more detailed work along these lines for systems of more particular catalytic interest, e.g., those containing ammonia as a complexing ligand. Some more qualitative work has appeared in several papers, which will be discussed later. 

The rewards for being able to understand and control the process of carbonization to give a particular pore structure are potentially enormous, with applications which include catalysis, carbon-in-pulp metal adsorption and separation processing, molecular sieves and bioethical applications. 

Babarao R, Jiang J, Sandler SI (2009) Molecular simulations for adsorptive separation of CO2/CH4 mixture in metal-exposed, catenated, and charged meteil-oiganic frameworks. Langmuir 25 6590-6590... 

Many biopolymers arc known to bind metal ions strongly. Alginic acid used as the immobilizing agent also has a complexing ability with divalent metal ions. Many studies have been carried out on the application of AA to the aqueous phase separation of heavy metals, and the possibility of AA as the adsorbent material has been suggested [1-5]. HA-M comprises 20% AA on a dry weight basis. Therefore, it can be expected that AA plays an important role in metal adsorption by HA-M. 

This is another metal oxide used in thin-layer applications. The chemical formula is AI2O3. On aluminum oxide thin-layers sorption is based on partial positive and negative charges on the surface and any water sorbed thereon. In the manufacturing process aluminum oxide can be made to have a basic surface (pH 9-10), a neutral surface (pH 7-8), or an acidic surface (pH 4-4.5). This allows for a different type of adsorption separation based solely on surface pH. Other properties of the different aluminas available for TLC are shown in Table 2. The different pore size and surface area types will also impart different separation characteristics to these sorbent layers. Layers of this sorbent are available with and without binders both organic and inorganic. A whole range of... 

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