Solutions geochemistry

Most of the chemical processes discussed before (acid-base equilibria, precipitation-dissolution, neutralization, complexation, and oxidation-reduction) are interrelated that is, reactions of one type may influence other types of reactions, and consequently must be integrated into aqueous- and solution-geochemistry computer codes. 

Thrailkill, J., Solution geochemistry of the water of limestone terrains, Univ- Kentucky Water ReSour. Inst. Res. Rept. 19, 125 p (197oy "... 

Wood W. W. and Low W. H. (1988) Solute geochemistry of the Snake River Plain Regional Aquifer System, Idaho and Eastern Oregon. US Geol. Surv. Prof. Pap. 1408-D, 79pp. 

In the geochemistry of fluorine, the close match in the ionic radii of fluoride (0.136 nm), hydroxide (0.140 nm), and oxide ion (0.140 nm) allows a sequential replacement of oxygen by fluorine in a wide variety of minerals. This accounts for the wide dissemination of the element in nature. The ready formation of volatile silicon tetrafluoride, the pyrohydrolysis of fluorides to hydrogen fluoride, and the low solubility of calcium fluoride and of calcium fluorophosphates, have provided a geochemical cycle in which fluorine may be stripped from solution by limestone and by apatite to form the deposits of fluorspar and of phosphate rock (fluoroapatite [1306-01 -0]) approximately CaF2 3Ca2(P0 2 which ate the world s main resources of fluorine (1). 

Aoki, M., Comsti, E.C., Florin, B.L. and Matsuhisa, Y. (1993) Evolution of the hydrothermal system with special reference to the geochemistry of alunite solid-solution. Baguio Report of Research and Development Cooperation 1717 Project, 8741, 42-63. 

Seward, T.M. (1981) Metal complex formation in aqueous solutions at elevated temperatures and pressures. In Rickard, D.T. and Wickman, F.E. (eds.). Chemistry and Geochemistry of Solutions at High Temperatures and Pressures, Phys. Chem. Earth, 13 and 14, 113-132. 

Sakai, H. and Matsubaya, O. (1974) Isotopic geochemistry of the thermal waters of Japan and its bearing on Kuroko ore solutions. Econ. Geol, 69, 974-991. 

In aqueous geochemistry, the important distinguishing property of metals is that, in general, they have a positive oxidation state (donate electrons to form cations in solution), but nonmetals have a negative oxidation state (receive electrons to form anions in solution). In reality, there is no clear dividing line between metals and nonmetals. For example, arsenic, which is classified as a nonmetal, behaves like a metal in its commonest valence states and is commonly listed as such. Other nonmetals, such as selenium, behave more like nonmetals. 

An autocatalytic reaction is one promoted by its own reaction products. A good example in geochemistry is the oxidation and precipitation of dissolved Mn11 by C>2(aq). The reaction is slow in solution, but is catalyzed by the precipitated surface and so proceeds increasingly rapidly as the oxidation product accumulates. Morgan (1967) studied in the laboratory the kinetics of this reaction at 25 °C and pH > 9. 

Arsenic geochemistry in Chesapeake Bay, Maryland, depends on anthropogenic inputs and phytoplankton species composition (Sanders 1985). Inputs of anthropogenic arsenic into Chesapeake Bay are estimated at 100 kg daily, or 39 tons/year — probably from sources such as unreported industrial discharges, use of arsenical herbicides, and from wood preservatives (Sanders 1985). The chemical form of the arsenic in solution varies seasonally and along the axis of the bay. Arsenic is present only as arsenate in winter, but substantial quantities of reduced and methylated forms are present in summer in different areas. The forms and distribution patterns of arsenic... 

One of the most basic requirements in analytical chemistry is the ability to make up solutions to the required strength, and to be able to interpret the various ways of defining concentration in solution and solids. For solution-based methods, it is vital to be able to accurately prepare known-strength solutions in order to calibrate analytical instruments. By way of background to this, we introduce some elementary chemical thermodynamics - the equilibrium constant of a reversible reaction, and the solubility and solubility product of compounds. More information, and considerably more detail, on this topic can be found in Garrels and Christ (1965), as well as many more recent geochemistry texts. We then give some worked examples to show how... 

In all physical and chemical processes, and in particular those of relevance to geochemistry, that involve the oxide/aqueous solution interface, it is important to understand the general, non-specific characteristics of that interface before focussing on those specific processes or mechanisms of interest. Due to the structure of mineral surfaces, the mineral oxide/aqueous solution interface will invariably acquire a net charge or electrostatic potential relative to the bulk solution. The electrical state of the interface will depend in part on the chemical reactions that can take place on the mineral surface, and in part on the electrolytic composition of the aqueous environment. 

O Neil JR, Truesdell AH (1991) Oxygen isotope fractionation studies of solute-water interactions. In Stable Isotope Geochemistry A Tribute to Samuel Epstein. Taylor Jr. HP, O Neil JR, Kaplan IR (eds), Geochem Soc Spec Pub 3 17-25... 

Gas-phase molecules play a relatively minor role in the geochemistry of most elements other than FI, C, N, O, and S, so it is important to consider extensions of the theory outlined in the preceding section to other types of materials, particularly aqueous solutions and crystals. In general, the same energetic concepts (especially zero-point energy) apply, but it is necessary to make additional assumptions to deal with the complexities and uncertainties that arise in dealing with condensed phases. 

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