Desorption dissolution process

The method of soil suspensions extracts is based on metal desorption/dissolution processes, which primarily depend on the physico-chemical characteristics of the metals, selected soil properties and environmental conditions. Metal adsorption/ desorption and solubility studies are important in the characterization of metal mobility and availability in soils. Metals are, in fact, present within the soil system in different pools and can follow either adsorption and precipitation reactions or desorption and dissolution reactions (Selim and Sparks, 2001). The main factors affecting the relationship between the soluble/mobile and immobile metal pools are soil pH, redox potential, adsorption and exchange capacity, the ionic strength of soil pore water, competing ions and kinetic effects (e.g. contact time) (Evans, 1989 Impelhtteri et al., 2001 McBride, 1994 Sparks, 1995). 

Brununer G.W., Tiller K.G., Herms U., Clayton P.M. Adsorption-desorption and/or precipitation-dissolution processes of zinc in soils. Geoderma 1983 31 337-354. 

The principle we have applied here is called microscopic reversibility or principle of detailed balancing. It shows that there is a link between kinetic rate constants and thermodynamic equilibrium constants. Obviously, equilibrium is not characterized by the cessation of processes at equilibrium the rates of forward and reverse microscopic processes are equal for every elementary reaction step. The microscopic reversibility (which is routinely used in homogeneous solution kinetics) applies also to heterogeneous reactions (adsorption, desorption dissolution, precipitation). 

Radionuclide transport in natural waters is strongly dependent on sorption, desorption, dissolution, and precipitation processes. The first two sections discuss laboratory investigations of these processes. Descriptions of sorption and desorption behavior of important radionuclides under a wide range of environmental conditions are presented in the first section. Among the sorbents studied are basalt interbed solids, granites, clays, sediments, hydrous oxides, and pure minerals. Effects of redox conditions, groundwater composition and pH on sorption reactions are described. 

Equation 4.3 is formally similar to a complexation reaction between SR(s) and the aqueous solution species on the left side. Indeed, the solid-phase product on the right side can be interpreted on the molecular level as either an outer-sphere or an inner-sphere surface complex. The latter type of adsorbed species was invoked in connection with the generic adsorption-desorption reactions in Eqs. 3.46 and 3.61, which were applied to interpret mineral dissolution processes. In general, adsorbed species can be either diffuse-layer ions or surface complexes,7 and both species are likely to be included in macroscopic composition measurements based on Eq. 4.2. Equation 4.3, being an overall reaction, does not imply any particular adsorbed species product, aside from its stoichiometry and the electroneutrality condition in Eq. 4.4. 

Schultze and Dickertmann [3.117-3.120] measured current and potential transients under potentiostatic and galvanostatic conditions, respectively, for the adsorption and desorption processes of Bi UPD on Au(lll). All transients were non-monotonous. A typical example is given in Fig. 3.47. The results are interpreted in terms of nucleative 2D Meads phase formation and dissolution processes. 

The basic element in the flow system is the perfectly-mixed reactor. In the multi-reactor system heat and mass transfer operation (absorption, desorption, dissolution of solids, heat generation or absorption as well as heat interaction between the reactor and the surroundings etc.) as well as chemical reactions may occur simultaneously, or not. The processes are governed by Eqs.(5-8), (5-12), (5-16), (5-19), (5-23) and (5-25) in the following, on the basis of which transition probabilities are derived as well as the single step transition matrix. 

Macpherson and Unwin (43) developed the theory for dissolution processes at the substrate induced by depleting of electroactive species at the SECM tip. The UME tip can oxidize or reduce the species of interest in solution at the crystal surface. If this species is one of the crystal components, the depletion of its concentration in the solution gap between the tip and substrate induces crystal dissolution. This process produces additional flux of electroactive species to the tip similarly to positive feedback situation discussed in previous sections. Unlike the desorption reaction, where only a small amount of adsorbed species can contribute to the tip current, the dissolution of a macroscopic crystal is not limited by surface diffusion. Accordingly, the developed theory is somewhat similar to that for finite heterogeneous kinetics at the substrate. Several models developed in Ref. 43a-d use different forms of the dissolution rate law applicable to different experimental systems. In general, the rate of the substrate process is (43a) ... 

It has been stressed by Salomons (1985) that from an Impact point of view it import to know whether the concentrations in the pore waters are determined by adsorption/desorption processes or by precipitation/-dissolution processes. If the latter is the case the concentrations in the pore waters of pollutants are independent of the concentrations in the solid phase. The is strong direct (Luther et al., 1980 Lee Kitt-rick, 1984) and indirect (Lu Chen, 1977) evidence that the concentrations of copper, cadmium and zinc in sulfidic pore waters are determined by precipitation-dissolution processes the concentrations of arsenic and chromium in pore waters are probably controlled by adsorption-desorption processes, and mainly depend on the concentrations in the solid phase (Salomons, 1985). 

At pH > 7, the rate-controlling step was OH- adsorption followed by Mg2+ and OH- desorption. These processes are part of the overall dissolution reaction (8.4) ... 

The purpose of this chapter is to review our present knowledge of the mineralogy and chemical processes controlling dissolved Mo in soils. This chapter emphasizes speciation, adsorption and desorption, and the precipitation and dissolution processes of dissolved Mo. In addition, the importance of dissolved organic carbon in these processes is discussed. 

In earlier sections we have discussed the speciation, adsorption, and desorption processes of dissolved Mo in soil solutions. In this section we review the principles of precipitation and dissolution processes and discuss the potential Mo solid phases that may control dissolved Mo in alkaline soil solutions. 

Another way to improve the remediation efficiency is to combine the EK principle with other methods, or to manipulate the electric field in such a way that the efficiency is increased. The main reasons for this combination/manipulation are (a) to reduce the concentration gradients generated during the process with a constant electric field, (b) to speed up the desorption/dissolution of the heavy metals, or (c) to decrease the heavy metal migration distance. 

The diversity of reactions that are considered to be surface mediated has also increased over the past decade. It is not only strict sorption/desorption and precipitation/dissolution processes that are important but also the surface mediation of reactions such as electron transfer (eg. 14-17), hydrolysis 18) and various photochemical transformations. In addition certain solid phases, in particular metallic iron, iron oxides and smectitic clays, are capable of transferring electrons in and out of their bulk structure (eg. 19-23), When viewed in this context, minerals should not be considered as passive solids, or even as simple sources of a reactive surface but must be considered as bulk reactants. 

The first section deals specifically with the spectroscopic/ microscopic tools that can be used in concert with macroscopic techniques. The second section emphasizes computer models that are used to elucidate surface mediated reaction mechanisms. The remainder of the volume is organized around reaction type. Sections are included on sorption/desorption of inorganic species sorption/desorption of organic species precipitation/dissolution processes heterogeneous electron transfer reactions photochemically driven reactions and microbially mediated reactions. What follows are a few highlights taken from the work presented in this volume. 

Absorption/desorption (dissolution) Easy method Options for data analysis UV-detectable groups are abundant in proteins Protein adsorption onto hydrogel is possible Pol5Tner swelling complicates diffusion process... 

The transport, mobilization and precipitation of an element is a balance between processes which release the element from its precursor through dissolution or desorption to processes which scavenge or fix the element through mineral precipitation or adsorption (Appelo Postma 1993 Stumm Morgan 1995). The actual mechanisms involved are complex and reviews have been published elsewhere (Lowson 1982 Nordstrom 1982 Morse 1983 Trescases 1992 Bigham 1994 Bowell et al. 1996). 

In principle, the oxidation of proceeds at an electrode potential that is more negative by about 0.7 V than the anodic decomposition paths in the above cases however, because of the adsorption shift, it is readily seen that practically there is no energetic advantage compared to CdX dissolution in competing for photogenerated holes. Similar effects are observed with Se and Te electrolytes. As a consequence of specific adsorption and the fact that the X /X couples involve a two-electron transfer, the overall redox process (adsorption/electron trans-fer/desorption) is also slow, which limits the degree of stabilization that can be attained in such systems. In addition, the type of interaction of the X ions with the electrode surface which produces the shifts in the decomposition potentials also favors anion substitution in the lattice and the concomitant degradation of the photoresponse. 

The major processes affecting the geochemical fate of hazardous inorganics are acid-base adsorption-desorption, precipitation-dissolution, complexation, hydrolysis, oxidation-reduction, and catalytic reactions. The significance of these processes to inorganic wastes is discussed only briefly here additional information on individual elements is given in Table 20.16. 

Concentrations of trace elements in soil solution may be controlled by the solubility of certain solid phases via dissolution/(co-)precipitation or by other physicochemical and biological processes such as adsorption-desorption, complexation, and redox reactions. 

Dissolved arsenic is correlated with ammonia (Fig. 4), consistent with a release mechanism associated with the oxidation of organic carbon. Other chemical data not shown here provide clear evidence of iron, manganese and sulfate reduction and abundant methane in some samples indicates that methanogenesis is also occurring. It is not clear however if arsenic is released primarily by a desorption process associated with reduction of sorbed arsenic or by release after the reductive dissolution of the iron oxide sorbent. Phreeqc analysis shows PC02 between 10"12 and 10"° bars and that high arsenic waters are supersaturated with both siderite and vivianite. 

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