Groundwater solutions precipitation

Microbes are ubiquitous in the subsurface environment and as such may play an important role in groundwater solute behavior. Microbes in the subsurface can influence pollutants by solubility enhancement, precipitation, or transformation (biodegradation) of the pollutant species. Microbes in the groundwater can act as colloids or participate in the processes of colloid formation. Bacterial attachment to granular media can be reversible or irreversible and it has been suggested that extracellular enzymes are present in the system. Extracellular exudates (slimes) can be sloughed-off and act to transport sorbed materials [122]. The stimulation of bacterial growth in the subsurface maybe considered as in situ formation of colloids. 

Groundwater-inflow rates as calculated by the solute and isotope mass-balance methods for several northern Wisconsin lakes are listed in Table I. Dissolved calcium was used as the solute tracer because it is the constituent whose concentration differs the most between groundwater and precipitation, the two input components to be separated by the method. In addition, calcium is nearly conservative in the soft-water, moderately acidic to cir-cum-neutral lakes in northern Wisconsin. Results from the two methods agree relatively well, except for Crystal Lake, where groundwater-flow reversals are frequent. 

Stalactites and stalagmites are formed by the precipitation of calcium carbonate, CaCOj, when gaseous COj escapes from groundwater solutions. 

Inorganic reactions in the soil interstitial waters also influence dissolved P concentrations. These reactions include the dissolution or precipitation of P-containing minerals or the adsorption and desorption of P onto and from mineral surfaces. As discussed above, the inorganic reactivity of phosphate is strongly dependent on pH. In alkaline systems, apatite solubility should limit groundwater phosphate whereas in acidic soils, aluminum phosphates should dominate. Adsorption of phosphate onto mineral surfaces, such as iron or aluminum oxyhydroxides and clays, is favored by low solution pH and may influence soil interstitial water concentrations. Phosphorus will be exchanged between organic materials, soil inter-... 

Dissolution and precipitation in the subsurface are controlled by the properties of the solid phases, by the chemistry of infiltrating water, by the presence of a gas phase, and by environmental conditions (e.g., temperature, pressure, microbiological activity). Rainwater, for example, may affect mineral dissolution paths differently than groundwater, due to different solution chemistry. When water comes in contact with a solid surface, a simultaneous process of weathering and dissolution may occur under favorable conditions. Dissolution of a mineral continues until equilibrium concentrations are reached in the solution (between solid and liquid phases) or until all the minerals are consumed. 

The Unipnre Environmental, Unipnre process technology is a unique iron co-precipitation method for removal of heavy metals from waste streams or groundwater. It can act as a primary metal-removal system or as a polishing step to an existing treatment system. The reactor mod-nle replaces the nentrahzation tank in a conventional wastewater treatment system. The process prodnces solids that are extremely insolnble in water and mild acid solutions. 

U-bearing minerals and adsorption processes (Salah et al. 2000 Perez del Villar et al. 2000). The vertical and lateral flow of groundwater is responsible for the oxidation and dissolution of primary sulphides, leading to acidic solutions that facilitated the oxidation and dissolution of uraninite. The resulting uranyl cations migrated and precipitated as uranyl minerals, mainly phosphates, silicates, silico-phosphates. In certain local conditions, reduction of these uranyl cations allowed precipitation of coffinite with a high content of P and LREE. Adsorption of uranium, together with P, mainly occurs on Fe-oxyhydroxides, but this kind of uranium retention seems less efficient than the precipitation, at least in the close vicinity to the... 

Environmental pH is the most important factor affecting CP adsorption and mobility (Choi Aomine, 1972, 1974a,b Christodoulatosetal., 1994 Stapleton etal., 1994). Since the dissociation constants (p Ka) of CPs are in the same range as the pH in groundwater, both protonated and deprotonated CPs may exist under natural conditions. Lower chlorinated phenols are more protonated in neutral environments than their polychlorinated congeners. With PCP, for example, the sorption to clay decreases threefold between pH 4 and 8.5 (Stapleton et al., 1994). Low soil pH might also cause CP precipitation, especially from alkaline solution. 

Metal reclamation from acid mine drainage and contaminated surface- and groundwater and wastewaters has been extensively studied. Technologies for metal removal from solution are based on the microbial—metal interactions discussed earlier the binding of metal ions to microbial cell surfaces the intracellular uptake of metals the volatilization of metals and the precipitation of metals via complexation with microbially produced ligands. 

As a contaminant moves through soil and groundwater, chemical processes will affect both contaminant concentration and overall hydrogeochemistry (Schoonen, 1998) of the system. Different adsorption mechanisms cause pollutants to adsorb onto the soil, volatilize, precipitate, and be part of the oxidation-reduction processes. Adsorption is loosely described as a process in which chemicals partition from a solution phase into or onto the surfaces of solid-phase materials. Adsorption at particle surfaces tends to retard contaminant movement in soil and groundwater. 

Eh-pH diagrams are sometimes used to predict or describe the major dissolved species and precipitates that should exist at equilibrium in aqueous solutions, including groundwaters, surface waters, laboratory solutions, and porewaters from soils, sediments, or rocks. However, as previously described, many natural aqueous systems are not at equilibrium and they often contain metastable species that are not predicted by Eh-pH diagrams. Metastable species refer to compounds, other substances, or ions that are present under redox, pH, pressure, temperature, or other conditions where chemical equilibrium indicates that they should be unstable and absent. Many metastable species (such as As(III) in oxygenated seawater) result from biological activity. 

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