Precipitation dominance

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... 

Watras CJ, Morrison KA, Hudson RJM, Frost TM, Kratz TK. 2000. Decreasing mercury in northern Wisconsin temporal patterns in bulk precipitation and a precipitation-dominated lake. Environ Sci Technol 34 4051 057. 

Under 10 the deposition is slow, while above 10.2 it is too fast and bulk precipitation dominates. 

In dilute solutions and at neutral pH, dissolved fluorides are usually present as the fluoride ion (F ). As pH decreases, the proportion of F decreases, while hydrogen fluoride (HF ) and nondissociated hydrogen fluoride increase. Levels of nondissociated hydrogen fluoride also increase in concentrated solutions. In seawater, fluorides exist in equilibrium. Calcium carbonate precipitation dominates the removal of dissolved fluoride from seawater. The next most important removal mechanism is incorporation into calcium phosphates. Undissolved fluoride... 

In a survey of U.S. stream sediments, Kennedy (1965) concluded that the makeup and properties of the stream sediments essentially equaled that of local soils. In the eastern states (50 to 150 cm precipitation), dominant clays in the <4 jum (0.004 mm) fraction were illite, kaolinite, ver-miculite, and interlayered clays, with a CEC of 14 to 28 meq/lOOg. In central and west-central states (25 to 100 cm precipitation) Kennedy found dominant smectite, vermiculite, mixed-layer illite, kaolinite, quartz, and feldspar in the <4 /zm fraction, with a CEC range of 25 to 65 meq/100 g. In California and Oregon, because of the wide range of wet and dry conditions (<25 to >200 cm precipitation), clays were highly variable, and had a range of CEC s from 18 to 65 meq/l(X) g for the <4 /um fraction. 

Precipitation. Precipitation is the heart of the Bayer plant where recovery of the Al(OH)3 from process Hquors occurs in high yield and product quahty is controUed. The dominant use for Bayer Al(OH)2 is to calcine it into smelting grade alumina. 

Solution hardening is not confined to 5000 series aluminium alloys. The other alloy series all have elements dissolved in solid solution and they are all solution strengthened to some degree. But most aluminium alloys owe their strength to fine precipitates of intermetallic compounds, and solution strengthening is not dominant... 

Fig. 17.1. (a) Dislocation motion is intrinsically easy in pure metals - though alloying to give solid solutions or precipitates con moke it more difficult. (b) Dislocation motion in covalent solids is intrinsically difficult because the interatomic bonds must be broken and reformed. ( ) Dislocation motion in ionic crystals is easy on some planes, but hard on others. The hard systems usually dominate. 

In order to account for both micromixing and mesomixing effects, a mixing model for precipitation based on the SFM has been developed and applied to continuous and semibatch precipitation. Establishing a network of ideally macromixed reactors if macromixing plays a dominant role can extend the model. The methodology of how to scale up a precipitation process is depicted in Figure 8.8. 

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