Dissolution and Sorption

The dynamics of the process of dissolution and sorption of biogenic silica from siliceous rocks, diatomite and opoka, has been studied. 

Both precipitation-dissolution and sorption-desorption can be strongly influenced by microbial processes. Microbial redox transformations of constituent X may (especially for metals) significantly alter solubility, and microbial production of ligands can promote the release of X from immobile phases by dissolution and desorption and stabilize X in solution. 

A conceptual and mechanistic model of particle interactions in silica-iron binary oxide suspensions is described. The model is consistent with a process involving partial Si02 dissolution and sorption of silicate onto Fe(OH)3. The constant capacitance model is used to test the mechanistic model and estimate the effect of particle interactions on adsorbate distribution. The model results, in agreement with experimental results, indicate that the presence of soluble silica interferes with the adsorption of anionic adsorbates but has little effect on cationic adsorbates. 

The first attempt to model sorption in these suspensions was based on the adsorptive additivity approach. The model parameters which best fit the sorption data for pure Fe(OH)3 and Si02 suspensions were identified (Table II) and using these values the binary systems were modeled by including surface sites from both pure solids in the calculations. However, because this approach overestimated the overall removal of ions from solution in almost all cases, dissolution and sorption of silicate were then incorporated into the model. 

The reaction rate Rj in these equations is a catch-all for the many types of reactions by which a component can be added to or removed from solution in a geochemical model. It is the sum of the effects of equilibrium reactions, such as dissolution and precipitation of buffer minerals and the sorption and desorption of species on mineral surfaces, as well as the kinetics of mineral dissolution and precipitation reactions, redox reactions, and microbial activity. 

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. 

The main physical processes leading to the above reactions that occur with PRB treatment are sorption, dissolution, and precipitation (USEPA, 1998). 

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. 

Plummer L.N., Parkhurst D.L. and Wigley T.M.L. (1979) Critical review of the kinetics of calcite dissolution and precipitation. In Chemical Modelling--Speciation, Sorption, Solubility and Kinetics in Aqueous Systems (ed. E. Jenne), pp. 537-573. American Chemical Society, Washington, D.C. 

These membranes are similar to the simple sorption-diffusion membranes, but involve some additional phenomena as well as simple penetrant dissolution and diffusion. Two types can be identified (i) facilitated transport for various gas types, and (ii) palladium and related alloys for hydrogen. 

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 ... 

In catalytic systems morphological changes of the pore structure, brought upon by the reaction and sorption processes, typically result in a reduction of the available pore volume. In some instances the internal pore structure is eventually blocked and becomes completely inaccessible to transport and/or reaction. In the field of noncatalytic fluid-solid reactions and acid rock dissolution, on the other hand, the chemical reaction consumes the solid matrix of the porous medium leading eventually to fragmentation and... 

The entry of strongly reduced landfill leachate into a pristine, often oxidized, aquifer, leads to the creation of very complex redox environments. Important processes include organic matter biodegradation, biotic and abiotic redox processes, dissolution/precipitation of minerals, complexa-tion, ion exchange, and sorption. The resulting... 

Some examples for PAH compounds suggest that decreased solubility and non-aqueous-phase partitioning and sorption processes are restrictive toward microbial degradation. Wodzinski and Bertolini (16) and Wodzinski and Coyle (17) concluded that bacteria utilize naphthalene, biphenyl, and phenanthrene as dissolved solutes, with the rate of biodegradation independent of the total amount of solid-phase hydrocarbon. Stucki and Alexander (18) found that the dissolution rate of phenanthrene may limit the biodegradation rate. 

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