Soil, lead solid phases

Badawy S.H., Helal M.I.D., Chaudri A.M., Lawlor K., McGrath S.P. Soil solid-phase controls lead activity in soil solution. J Environ Qual 2002 31 162-167. 

Contaminant volatilization from subsurface solid and aqueous phases may lead, on the one hand, to pollution of the atmosphere and, on the other hand, to contamination (by vapor transport) of the vadose zone and groundwater. Potential volatihty of a contaminant is related to its inherent vapor pressure, but actual vaporization rates depend on the environmental conditions and other factors that control behavior of chemicals at the solid-gas-water interface. For surface deposits, the actual rate of loss, or the pro-portionahty constant relating vapor pressure to volatilization rates, depends on external conditions (such as turbulence, surface roughness, and wind speed) that affect movement away from the evaporating surface. Close to the evaporating surface, there is relatively little movement of air and the vaporized substance is transported from the surface through the stagnant air layer only by molecular diffusion. The rate of contaminant volatilization from the subsurface is a function of the equilibrium distribution between the gas, water, and solid phases, as related to vapor pressure solubility and adsorption, as well as of the rate of contaminant movement to the soil surface. 

Santillan-Medrano, J. Jurinak, J. J. 1975. The chemistry of lead and cadmium in soil solid phase formation. Soil Science Society of America Proceedings, 39, 851-856. 

Values of 12 can be monitored as time passes to determine whether equilibrium between a solid phase and a soil solution exists. If 12 < 1, the soil solution is termed undersaturated with respect to the solid phase if 12 > 1, the soil solution is supersaturated. Measurements of 12 thus lead to one or more of the following conclusions about the reaction of Eq. 3.1 whenever 12 1 ... 

Bioaccessibility, and therefore oral bioavailability of soil contaminants, depends on soil type and contaminant (Davis et al., 1997 Gr0n and Anderson, 2003 Hamel et al, 1998 Ruby et al., 1999). PTMs occur in soil as a complex mixture of solid-phase chemical compounds of varying particle size and morphology, characterised by variable metal bioavailability. Mineral phases that form under acidic conditions (e.g. lead sulphate, iron-lead sulphate) will tend to be more stable in the acidic conditions of the stomach and hence less bioaccessible. By contrary, mineral phases... 

As mentioned earlier, the composition of natural groundwaters depends on the composition of the geological formations where they originate from they contain dissolved rock and soil components that were soluble under the conditions (such as temperature and pressure) of their formation. Their dissolution is governed by the law of thermodynamics that is, dissolution occurs when the solution is undersaturated with respect to components such as rocks and soils. Provided that the solid components are present in sufficient quantity and there is no kinetic barrier, this process may lead to a thermodynamic equilibrium. The reversed process of dissolution is precipitation, that is, the formation of a solid phase from the dissolved components of a supersaturated solution. The composition of the... 

In soils, F can be found in four major fractions (1) dissolved in soil solution (2) sorbed to Al, Fe, and Mn oxides and hydroxides and carbonates (3) solid phases, such as fluorite and fluorophlogopite and (4) associated with organic compounds. The solubility of F in soil solution is variable and is affected by pH, speciation, adsorption and desorption reactions, and dissolution and precipitation reactions (Luther et al., 1996). Acidic conditions and low calcium carbonate content are favorable to F solubility and can therefore enhance both root uptake (Weinstein and Alscher-Herman, 1982) and migration to surface and ground water (Smith, 1983). These conditions can lead to human, plant, and animal health issues. Soils that do contain appreciable amounts of calcium carbonate and are neutral to slightly alkaline conditions can fix F as insoluble calcium fluoride (CaF2), and reduce its bioavailability and mobility (Kubota et al., 1982 Tracy et al., 1984 Reddy et al., 1993 Poulsen and Dudas, 1998). 

Chloropyromorphite is a common constituent of lead-bearing solid phase in lead-mining areas of the UK, and it was considered the end product of lead weathering (Cotter-Howells, 1994). The presence of lead phosphate in metal-contaminated soils was also confirmed by Ruby et al. (1994). Furthermore,... 

The most frequently discussed topic in washing is the role of solubilisation processes. Many investigators [76] attract attention to the fact that the surfactant concentration in a washing solution is much lower than CMC, and in this connection, solubilisation of oils is principally excluded due to absence of surfactant micelles. At the same time, the review of recent works [85, 86] show that solubilisation can play a dominant role both in washing fabrics and in the removal of soils from solid surfaces. These views are based on the following mechanisms. Surfactants adsorb at w/o interfaces under formation of densely packed adsorption layers which leads to a high local surfactant concentration as compared with the rather low concentration in the washing solution. After that, noticeable penetration of water into the oily soil is possible, under formation of liquid-crystal phases. Then, mesomorphic phases are swelled and destroyed under the formation of emulsion droplets. 

However, one should be aware of the following three important issues when geochemical models are applied to determine the solid-phase control of soil solutions (1) Solid-phase chemistry is based on the assumption of equilibrium. Therefore, soil solutions to be tested should be close to a steady-state condition (i.e., the condition in which little change has occurred in the major ions involved). (2) The mass-balance equation for each dissolved species should contain all possible solution species to ensure accurate calculation of the free concentration of the dissolved species. Omission of any significant solution species from the mass-balance equation will cause overestimation of the free concentration of the dissolved species. (3) Variations occur in the equilibrium constants for solution species and solid phases. All these factors could lead to misinterpretation of solid-phase equilibria in soil solutions. 

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