Speciation changes

The amount of gold removed from solution was a function of pH, with near neutral pH resulting in more complete gold removal from solution in comparison to lower pH values. This behavior is caused by speciation changes associated with the successive replacement of a CL ligand with a OH- ion upon pH increase. 

XANES to ensure the quality of the synthates. Three batches of ferrihydrite were synthesized and precipitates were washed 5-6 times to ensure a chloride-free synthate. Ferrihydrite precipitates were redispersed in 200 mL of double deionized (DDI) water at (1) room temperature (25°C), as well as preheated in water baths to temperatures of (2) 50°C and (3) 75°C. For all of these slurries, pH was kept constant at 10 using 1M KOH. 40 mL samples were pipetted from each reaction vessel after 0, 1,2, 3, and 7 days. Slurries were centrifuged, washed three times with DDI water and air dried for analyses (BET, XRD, and XANES). BET analyses were used to evaluate the decrease in surface areas with increasing crystallinity, and XRD and XANES were used to detail the structural and speciation changes in iron. 

Drying causes instant and major speciation changes in anoxic but also in oxic sediments (Thomson et al., 1980 Kersten and Forstner, 1987 Bordas and Bourg,... 

Nuclear magnetic resonance (NMR) has been used to characterize chemical speciation changes occurring in both soluble and insoluble fractions of silicone polymers for decades (d-9). One of the most common strategies for... 

Figure 5. Illustration of speciation changes at polymer-filler interface that would be expected to alter the interaction strength and effect reinforcement. (See page 7 of color insert.)...

The strong affinity of iron oxides for Se(IV) has been well documented (Dzombak and Morel, 1990) and calculations based on the Dzombak and Morel (1990) diffuse double-layer model and default HFO database show the principal response to pH and redox speciation changes (Figure 11). The selenate species is less strongly adsorbed by iron oxides at near-neutral pH than the selenite species (Figure 11). Clay minerals (Bar-Yosef and Meek, 1987) also adsorb Se(IV). 

Figure 27. Copper XANES spectra for a single fluid inclusion (30 pm) in the Mole granite showing the reversible speciation change from Cu(II) at ambient conditions to Cu(I) at elevated temperature (adapted from Mavrogenes et al. 2002)...

There are two main issues. First, are the extracted chemical fractions specific and free from redistribution Second, can the methods be applied to buried sediment. The second question involves three different points. Does speciation change on sampling Does it change during diagenesis Most importantly, is any useful palaeolimnological information recorded in the extracted fractions ... 

The same techniques have been applied to sediment cores (e.g., Reuter et al., 1981 Tsai et al., 1998), but it is not possible to verify that the data can be interpreted in the same way as for surface sediments. White Gubala (1990) found that speciation changes rapidly on exposure to the air, requiring very careful handling of the sediment material, and argue that the initial particulate speciation is unlikely to be preserved. They conclude, as did Engstrom... 

Over the years there have been many anecdotes and much controversy over the wide variations in silica properties reported in the literature and the mechanisms of the processes by which silica and silicate species exert their influence. Much of the discussion results from the fact that both the exact nature of the speciation of silica in diluted solution and at the interface between sdicas and silicates and water and how this speciation changes in mixtures with other substances is unclear (25,26). Also, from a very practical standpoint sdica in some form is already present in many natural systems and in vitro experiments must seek to replicate the speciation to have any chance of learning from them. Complicating the situation, the extensive literature contains many observations drawn from experiments carried out under conditions where soluble silica might have been present in an uncontrolled manner and at concentrations sufficient to exert significant effects or conclusions drawn from insufficient mathematical models. The high potential for artifacts, which can influence the behavior of silica, even in the simplest systems, was well characterized by Hazel (my first college chemistry professor) [27], Debye [28] and Her [29] who made it clear that one had to be aware of the purity and/or preparation history of the silica. 

Notwithstanding the above constrains, computational methods are useful for a first approximation for a speciation problem or to predict speciation changes caused by changes in a parameter of the environmental conditions. Published practical examples of speciation by using a computational approach are the aluminum speciation in serum and distribution of trace elements in fresh waters. 

Whilst numerous literature methods exist to separate chlorine and iodine, many of these are not ideal for a commercial laboratory. They often rely on the use of relatively hazardous chemicals that require particular means of disposal, such as carbon tetrachloride in solvent extraction methods . In addition, the processes can be labour intensive, such as methods employing a speciation change of iodine as well as solvent extraction washes with chemicals such as sodium metabisulfite (Na2S205) °. In addition the dissolution of Agl in order to provide a more suitable medium for liquid scintillation counting can present difficulties as many of the techniques for dissolving Agl involve the use of relatively hazardous chemicals such as alkali cyanides, or the chemicals (and/or their required concentrations) are not compatible with liquid scintillation cocktails. 

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