Dissolution into soil water

The iron cycle shown in Fig. 10.14 illustrates some redox processes typically observed in soils, sediments and waters, especially at oxic-anoxic boundaries. The cycle includes the reductive dissolution of iron(lll) hydr)oxides by organic ligands, which may also be photocatalyzed in surface waters, and the oxidation of Fe(II) by oxygen, which is catalyzed by surfaces. The oxidation of Fe(II) to Fe(III)(hydr)-oxides is accompanied by the binding of reactive compounds (heavy metals, phosphate, or organic compounds) to the surface, and the reduction of the ferric (hydr) oxides is accompanied by the release of these substances into the water column. 

In the previous sections of this book, we focused on the nature of contaminants and the geochemical reactions that can occur in the subsurface environment. Chemical compounds introduced into infiltrating water or in contact with soil or rock surfaces are subject to chemically and biologically induced transformations. Other compounds are retained by the soil constituents as sorbed or bound residues. Thus, in terms of geochemical interactions and reactions among dissolved chemical species, interphase transfer occurs in the form of dissolution, precipitation, volatilization, and various forms of physicochemical retention on the solid surfaces. 

The reductive dissolution of solid compounds in anaerobic soils, sediments, and waters begins with the reduction of prominent cations within the compounds. Many Fe(III) (oxy)(hydr)oxide compounds are especially susceptible to reductive dissolution. The reduction process converts Fe(III) into more water-soluble Fe(II). The formation of Fe(II) causes the (oxy)(hydr)oxides to decompose in water. In some cases, the Fe(II) rapidly precipitates as new solid compounds, such as siderite (FeCCT) or magnetite (FesCL). 

The availability of the analytes for uptake by plants, for transport through the soil, and for dissolution into water can be estimated from a well-studied speciation scheme. Risk assessment for disposal of wastes in landfills or for land disposal of dredge spoils or sewage sludges requires knowledge not only of the total metal content but also of the content in each separate fraction to begin to understand how the metals will act in the environment. Table 5.7 summarizes the methods available for speciation of metals in samples. 

The fate of chemicals in the environment depends not only on processes taking place within compartments, but also by chemical partitioning between compartments. For example, there may be exchange of chemicals between air and water or soil. Movement from the water or soil into the air is accomplished by volatilization and evaporation of volatile or semivolatile compounds. Movement of chemicals from the air to water or soil is accomplished by deposition or diffusion into the water. Chemicals can also move from water to soil or sediment and vice versa. If a solid chemical in the soil or sediment dissolves into the water, this is called dissolution , while the opposite is called precipitation . If a chemical dissolved in water attaches to a soil or sediment particle, this is called adsorption , while the opposite is called desorption . The fugacity of a chemical, that is, its tendency to remain within a compartment, is affected by the properties of that chemical, as well as the chemical and physical properties of the environments such as temperature, pFF, and amount of oxygen in water and soil. Wind or water currents, wave action, water turbulence, or disturbance of soil or sediment (through the action of air or water currents, animals, or human activities) may also affect partitioning of chemicals. 

Laboratory studies provide some insight into the possible role of DOM onstituents in metal mobilization in soils. These studies suggest that LMW organic ligands, specifically bidentate ligands, are more likely to promote min-rral dissolution than are humic substances. LMW organic acids have been identified in soil waters and in leachates of forest litter at concentrations sufficient to effect oxide dissolution. 

Soil and groundwater contamination from packing and storage facilities would be a distinctly different problem. Here the contaminant source is a relatively well-defined mixture of microcrystalline explosives that are sparingly soluble in water at environmental temperatures. The source material is spread locally across a soil surface by unintentional release. The composition of material subject to dissolution and percolation into soils is likely to be that of the ordnance, primarily RDX and TNT. little data are available concerning environmental alteration of solid phase explosives. Although aqueous solubility data are available for many explosives, dissolution kinetics data for pure or mixed solids are not available. 

Some minerals remain virtually unweathered despite their inherent instability, because them dissolution rate in water is exceedingly slow. Quartz particles larger than several micrometers in size (fine silt) remain in soils for so long that quartz appears to be the most stable state for soil silicon, When finely divided into clay-sized particles, however, quartz persists only slightly longer in soils than does clay-sized feldspar. Feldspar disappears from the sand and silt fractions relatively rapidly. 

When rainfall intensity exceeds infiltration rate and surface-storage capacity has been reached, overland flow begins. The transfer of dissolved pesticides from the soil matrix to overland flow consists of several mechanisms desorption from soil organic matter, mineral surfaces and plant residues dissolution of insecticide crystals or granules and diffusive and turbulent transport of dissolved insecticide from soil water into overland flow (2, 62). The relative importance of each process depends on the physico-chemical properties of the chemical, formulation, initial placement, soil properties, recent hydraulic history and vegetation (62). 

For desinfection of drinking water, the ozone-containing air is mixed with water in the water pipe, after which the bubbled water is supplied into a special designed chamber filled by porous ceramics charge for intensification of ozone dissolution into water. Some problems concerning the optimization of the UV stimulated oxidation of organic contaminants in soil water with ozone and hydroperoxide is discussed [44]. 

The adsorption of ions on iron oxides regulates the mobility of species in various parts of the ecosystem (biota, soils, rivers, lakes, oceans) and thereby their transport betv een these parts. Examples are the uptake of plant nutrients from soil and the movement of pesticides and other pollutants from soils into aquatic systems. In such environments various ions often compete with each other for adsorption sites. Adsorption is the essential precursor of metal substitution (see Chap. 3), dissolution reactions (see Chap. 12) and many interconversions (see Chap. 14). It also has a role in the synthesis of iron oxides and in crystal growth. In industry, adsorption on iron oxides is of relevance to flotation processes, water pollution control and waste and anticorrosion treatments. 

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