Water-dominated chemistry

Optical properties of liquids are similar in many ways to those of solids. Electrically, there are metallic liquids such as mercury and molten iron, but the majority of common liquids are nonmetallic. As an illustration of a liquid we have chosen H20, a ubiquitous substance on our planet water dominates not only atmospheric processes but the chemistry of life. 

Elevated pH values occur in waters whose chemistry is dominated by minerals, most of which, as noted above, are salts of strong bases and weak acids. Thus, in the absence of sources of acidity, mineral carbonate and silicate and alumino-silicate minerals tend to raise the pH to values of 9 to 10 or even higher. Such high pH s are found in arid soils and in some deep groundwaters that are not exposed to fresh recharge and so can be said to be rock dominated as opposed to water dominated. In such systems the minerals present can exist stably, without weathering. Water pH values above 10 are exceptional and may reflect contamination by strong bases, such as NaOH or Ca(OH)2. 

Shown in Fig. 9.9 are water-composition ranges for some humid-climate streams (in New Jersey), a dilute, freshwater lake (Lake Huron) and lake-bottom muds from the Great Lakes (Sutherland 1970), and deep-soil moisture from Pennsylvania (Sears 1976 Sears and Langmuir 1982). Lake Huron and the Delaware River are dilute, humid-climate waters. They both plot near the kaolinite-gibbsite boundary. Their composition can be described as water dominated. In other words, their chemistries are controlled chiefly by dilution with fresh rainfall and runoff, not by reactions with geological materials. In a study of acid rain (water-dominated) control of soil moisture and ground-water chemistry of a sandy aquifer in Denmark, Hansen and Postma (1995) found that pore waters were close to equilibrium with gibbsite and supersaturated with kaolinite (Fig. 9.9). Precipitation pH = 4.34 at the site, and log([K+]/lH+]) = -0.95. 

A comparison of the resistance in air and water for different gases shows that the resistance in the water dominates for gases with low solubility that are unreactive in the aqueous phase (e.g. O2, N2, CO2, CH4). For gases of high solubility or rapid aqueous chemistry (e.g. H2O, SO2, NH3), processes in the air control the interfacial transfer. 

Note that the pH and alkalinity of the Grand River are lower in January when the temperature is low. In natural waters, carbonate chemistry is the dominating factor in determining pH and alkalinity. When water temperature is low the equilibrium between carbon dioxide in the air and water shifts toward higher concentrations in the water. In water there is an equilibrium reaction between four forms of the carbonate species, dissolved carbon dioxide gas (C02aq), carborric acid (H2CO3), bicarbonate ion (HCOJ) and carbonate ion (COJ ), as irtdicated by... 

Considering the anion concentration ranges in natural waters (Table II) and the magnitude of the corresponding plutonium stability constants (Table III), the chemistry of plutonium, as well as that of uranium and neptunium, is almost entirely dominated by hydroxide and carbonate complexation, considering inorganic complexes only (41, 48, 49). ... 

Research into the aquatic chemistry of plutonium has produced information showing how this radioelement is mobilized and transported in the environment. Field studies revealed that the sorption of plutonium onto sediments is an equilibrium process which influences the concentration in natural waters. This equilibrium process is modified by the oxidation state of the soluble plutonium and by the presence of dissolved organic carbon (DOC). Higher concentrations of fallout plutonium in natural waters are associated with higher DOC. Laboratory experiments confirm the correlation. In waters low in DOC oxidized plutonium, Pu(V), is the dominant oxidation state while reduced plutonium, Pu(III+IV), is more prevalent where high concentrations of DOC exist. Laboratory and field experiments have provided some information on the possible chemical processes which lead to changes in the oxidation state of plutonium and to its complexation by natural ligands. 

River water chemistry is determined by the relative concentrations of major dissolved components (bicarbonate, calcium ion, silica, and sulfate), which are in turn controlled by the environment. Rivers in precipitation-dominated... 

The failure to identify the necessary authigenic silicate phases in sufficient quantities in marine sediments has led oceanographers to consider different approaches. The current models for seawater composition emphasize the dominant role played by the balance between the various inputs and outputs from the ocean. Mass balance calculations have become more important than solubility relationships in explaining oceanic chemistry. The difference between the equilibrium and mass balance points of view is not just a matter of mathematical and chemical formalism. In the equilibrium case, one would expect a very constant composition of the ocean and its sediments over geological time. In the other case, historical variations in the rates of input and removal should be reflected by changes in ocean composition and may be preserved in the sedimentary record. Models that emphasize the role of kinetic and material balance considerations are called kinetic models of seawater. This reasoning was pulled together by Broecker (1971) in a paper called "A kinetic model for the chemical composition of sea water."... 

Oxidation-reduction reactions in water are dominated by the biological processes of photosynthesis and organic matter oxidation. A very different set of oxidation reactions occurs within the gas phase of the atmosphere, often a consequence of photochemical production and destruction of ozone (O3). While such reactions are of great importance to chemistry of the atmosphere - e.g., they limit the lifetime in the atmosphere of species like CO and CH4 - the global amount of these reactions is trivial compared to the global O2 production and consumption by photosynthesis and respiration. 

Chemists use many different liquid solvents, but we focus most of our attention on water. When water Is the solvent, the solution Is said to be aqueous. A rich array of chemistry occurs in aqueous solution, including many geological and biochemical processes. Aqueous solutions dominate the chemistry of the Earth and the biosphere. The oceans, for instance, are rich broths of various cations and anions, sodium and chloride being the most abundant. The oceans can be thought of as huge aqueous solvent vessels for the remarkably complex chemistry of our world. Blood is an... 

Wolery (1978) and Reed (1982, 1983) have indicated based on a computer calculation of the change in chemistry of aqueous solution and mineralogy during seawater-rock interactions that epidote is formed under the low water/rock ratio less than ca. 50 by mass. Humphris and Thompson (1978), Stakes and O Nell (1982) and Mottl (1983) have also suggested on the basis of their chemical and oxygen isotopic data of the altered ridge basalts that epidote is formed by seawater-basalt interaction at elevated temperatures (ca. 200-350°C) under the rock-dominated conditions. If epidote can be formed preferentially under such low water/rock ratio, the composition of epidote should be influenced by compositions of the original fresh rocks. 

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