Sorption processes precipitation

We are not aware of any previous studies of the removal of plutonium or americium from (NH )2ZrF6-NHltF-NH N03 solutions. For ready plant-scale application, precipitation, sorption on inorganic materials, or batch solvent extraction processes may all be satisfactory. An inexpensive inorganic material with great selectivity and capacity for sorbing actinides, and with suitable hydraulic properties, would be especially attractive. 

Manganese (IV) oxide enjoys numerous applications in modem technologies. The most widely known areas of its usage are sorption processes one could recall that co-precipitation of contaminating cations with manganese oxide is still employed as part of the in-tank precipitation in processes of treatment of supernatant wastes at high concentration. Furthermore, co-precipitation data are usually used as benchmark results in studies of novel sorbents for strontium [4],... 

Relaxation studies have shown that the attachment of an ion to a surface is very fast, but the establishment of equilibrium in wel1-dispersed suspensions of colloidal particles is much slower. Adsorption of cations by hydrous oxides may approach equilibrium within a matter of minutes in some systems (39-40). However, cation and anion sorption processes often exhibit a rapid initial stage of adsorption that is followed by a much slower rate of uptake (24,41-43). Several studies of short-term isotopic exchange of phosphate ions between aqueous solutions and oxide surfaces have demonstrated that the kinetics of phosphate desorption are very slow (43-45). Numerous hypotheses have been suggested for this slow attainment of equilibrium including 1) the formation of binuclear complexes on the surface (44) 2) dynamic particle-particle interactions in which an adsorbing ion enhances contact adhesion between particles (43,45-46) 3) diffusion of ions into adsorbents (47) and 4) surface precipitation (48-50). 

Measurements of the chemical composition of an aqueous solution phase are interpreted commonly to provide experimental evidence for either adsorption or surface precipitation mechanisms in sorption processes. The conceptual aspects of these measurements vis-a-vis their usefulness in distinguishing adsorption from precipitation phenomena are reviewed critically. It is concluded that the inherently macroscopic, indirect nature of the data produced by such measurements limit their applicability to determine sorption mechanisms in a fundamental way. Surface spectroscopy (optical or magnetic resonance), although not a fully developed experimental technique for aqueous colloidal systems, appears to offer the best hope for a truly molecular-level probe of the interfacial region that can discriminate among the structures that arise there from diverse chemical conditions. 

When the kinetics of a sorption process do appear to separate according to very small and very large time scales, the almost universal inference made is that pure adsorption is reflected by the rapid kinetics (16,21,22,26). The slow kinetics are interpreted either in terms of surface precipitation (20) or diffusion of the adsorbate into the adsorbent (16,24). With respect to metal cation sorption, "rapid kinetics" refers to time scales of minutes (16,26), whereas for anion sorption it refers to time scales up to hours TT, 21). The interpretation of these time scales as characteristic of adsorption rests almost entirely on the premise that surface phenomena involve little in the way of molecular rearrangement and steric hindrance effects (16,21). 

The distribution of the major elements (Ca, Mg, Na, K,. ..) in soils is well known to be governed by ion-exchange processes (1). The behaviour of transition elements such as Co, Ni, Cd, Cu, etc. in natural systems (soils, sediments) often results from a combination of different effects such as precipitation, sorption in oxides, exchange in clay minerals and complexation with organic... 

The recent detailed investigations on Zn(II) and Pb(II) have advanced our understanding of heavy metal cation binding to C-S-H. However there are still a number of important questions that require attention. We need to understand the relationship, if any between Ca Si ratio and metal uptake. We also need to understand how to interpret the sorption process, whether as a solid solution (Tommaseo Kersten 2002) or as precipitation within the C-S-H particles or as a sorption process suggested by Glasser (1993) to the surfaces of the crystalline domains. 

We begin with a discussion of the most common minerals present in Earth s crust, soils, and troposphere, as well as some less common minerals that contain common environmental contaminants. Following this is (1) a discussion of the nature of environmentally important solid surfaces before and after reaction with aqueous solutions, including their charging behavior as a function of solution pH (2) the nature of the electrical double layer and how it is altered by changes in the type of solid present and the ionic strength and pH of the solution in contact with the solid and (3) dissolution, precipitation, and sorption processes relevant to environmental interfacial chemistry. We finish with a discussion of some of the factors affecting chemical reactivity at mineral/aqueous solution interfaces. 

Normally, when such an experiment is carried out, it may not be possible to cover as wide a range of concentration of the nuclide shown in this hypothetical case Solubility or other constraints may limit the accessible range In the case of tri-valent ions like Eu(III), it is not possible to increase the concentration of the ion much above tracer levels at higher pH because of the possibility of precipitation and the formation of hydrolyzed and polynuclear species which would change the nature of the sorption process We have so far not been able to determine a reproducible sorption isotherm for Eu(III) on our samples of montmorillonite above about pH 5-6. 

Together with acid-base reactions, where a proton transfer occurs (pH-dependent dissolution/ precipitation, sorption, complexation) redox reactions play an important role for all interaction processes in aqueous systems. Redox reactions consist of two partial reactions, oxidation and reduction, and can be characterized by oxygen or electron transfer. Many redox reactions in natural aqueous systems can actually not be described by thermodynamic equilibrium equations, since they have slow kinetics. If a redox reaction is considered as a transfer of electrons, the following general reaction can be derived ... 

Under geological conditions the substances dissolved in aqueous solutions are sorbed on the surface of the mineral and humic substances. The sorption processes are influenced by the composition of the solution, the stable chemical species. At pH = 6-8, characteristic of the geological environment, some cations have hydrolytic tendencies, and hydroxides and oxides can precipitate. When studying the interfacial reaction of rocks and soils, hydrolysis has to be avoided. 

Application of ultrafiltration coupled with other processes such as precipitation, sorption, or complexation... 

An important requirement in the application of sorption processes in flow-through units where sludge is suspended or attached to a porous bed is a constant exchange capacity. The maximal aging time of AAH precipitates in the experiments was 48 h and during this time the exchange capacity of the AAH remained unchanged. 

Stage 8 column (c) on stand-by to become column (b) in next cycle PL = pregnant liquor W = water 1 = 1st eluant 2 = 2nd eluant P = to precipitation BD = barren discharge (Reproduced by permission from Ion Exchange In Uranium Extraction , in Ion Exchange Sorption Processes In Hydrometallurgy , Critical Reports on Applied Chemistry, Vol. 19, ed. M. Streat and D. Naden, Wiley, Chichester, 1987, Ch. 1)... 

FIGURE 4-39 The acid deposition process. Acid precursors, notably oxides of nitrogen and sulfur, are emitted to the atmosphere, primarily by fuel-burning equipment. Acid precursors are oxidized in the atmosphere to nitric and sulfuric acids by a variety of homogeneous and heterogeneous reactions. The acids are deposited by precipitation-related processes such as washout and rainout, by sorption of nitric acid vapor, and by dry deposition of acidic particulate material such as ammonium sulfate aerosol. (Stern et ah, 1984.)... 

Figure 3.10. Changes in sorption processes with time showing a continuum from adsoip-tion to precipitation to solid-phase transformations.

The surface precipitation model was implemented using MICROQL+ (Dzom-bak, 1989) using the fsp values shown in Table 1-A and assuming that approximately 2% of the aluminum oxide added to the system was able to participate in the formation of (he solid solution. Representative results of the surface precipitation modeling, shown in Fig, 7 9, suggest that the model can provide a reasonable representation ol the sorption process over a range of solution conditions. 

Sorption processes are very effective and include adsorption/desorption (reversible binding at the solid-water interface), absorption (diffusion of pollutants into the solid matrix), precipitation and coprecipitation (incorporation into a freshly formed solid), and occlusion (sequestration of adsorbed pollutants during mineral growth). The most important factors for retention processes are pollutant concentration, the composition of the solid matrix, solution composition (e.g., complexing agents) and E/pH conditions (Brady and Boms 1997). 

Chromium can exist in several oxidation states from Cr(0), the metallic form, to Cr(Vl). The most stable oxidation states of chromium in the environment are Cr(lll) and Cr(Vl). Besides the elemental metallic form, which is extensively used in alloys, chromium has three important valence forms. The trivalent chromic (Cr(lll)) and the tetravalent dichromate (Cr(Vl)) are the most important forms in the environmental chemistry of soils and waters. The presence of chromium (Cr(Vl)) is of particular importance because in this oxidation state Cr is water soluble and extremely toxic. The solubility and potential toxicity of chromium that enters wetlands and aquatic systems are governed to a large extent by the oxidation-reduction reactions. In addition to the oxidation status of the chromium ions, a variety of soil/sediment biogeochemical processes such as redox reactions, precipitation, sorption, and complexation to organic ligands can determine the fate of chromium entering a wetland environment. 

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