Carbonate speciation

While the TFC-ULP membrane is the membrane with the higher charge, but more open pores (see Dextran experiments), it can be concluded that the sodium rejection occurs due to charge, not ion size. This is also confirmed by the pH effect. A higher rejection of cations at pH 10 may be due to solution speciation (carbonate dominates at pH>10.3 as divalent COj- (over HCO3) which will act as a co-ion... 

Fig. 6. Speciation diagram of plutonium as function of Eb and pH in aqueous solution at 25°C (a) carbonate-free (89) (b) 0.004 M total carbonate (90). Eb...

In Figure 2 the solubility and speciation of plutonium have been calculated, using stability data for the hydroxy and carbonate complexes in Table III and standard potentials from Table IV, for the waters indicted in Figure 2. Here, the various carbonate concentrations would correspond to an open system in equilibrium with air (b) and closed systems with a total carbonate concentration of 30 mg/liter (c,e) and 485 mg/liter (d,f), respectively. The two redox potentials would roughly correspond to water in equilibrium wit air (a-d cf 50) and systems buffered by an Fe(III)(s)/Fe(II)(s)-equilibrium (e,f), respectively. Thus, the natural span of carbonate concentrations and redox conditions is illustrated. 

Equilibrium complexation constants for Cu reactions with natural organic matter and the details of Cu speciation are bound to remain somewhat uncertain, since the composition of the complexing molecules varies from site to site. What is not in dispute is that the fraction of dissolved copper present as free aquo Cu is probably very small in any natural water. In extremely pristine waters, hydroxide and carbonate complexes may dominate, but organic complexes usually dominate in waters containing more than a few tenths of a mg/L organic carbon. 

TBTO is a colorless liquid of low water solubility and low polarity. Its water solubility varies between <1.0 and >100 mg/L, depending on the pH, temperature, and presence of other anions. These other anions determine the speciation of tributyltin in natural waters. Thus, in sea water, TBT exists largely as hydroxide, chloride, and carbonate, the structures of which are given in Figure 8.5. At pH values below 7.0, the predominant forms are the chloride and the protonated hydroxide at pH8 they are the chloride, hydroxide, and carbonate and at pH values above 10 they are the hydroxide and the carbonate (EHC 116). 

When TBTO is released into ambient water, a considerable proportion becomes adsorbed to sediments, as might be expected from its lipophilicity. Studies have shown that between 10 and 95% of TBTO added to surface waters becomes bound to sediment. In the water column it exists in several different forms, principally the hydroxide, the chloride, and the carbonate (Figure 8.5). Once TBT has been adsorbed, loss is almost entirely due to slow degradation, leading to desorption of diphenyl-tin (DPT). The distribution and state of speciation of TBT can vary considerably between aquatic systems, depending on pH, temperature, salinity, and other factors. 

Presently, the precise determination of the true dissolved Th fraction in water samples remains a challenge. Results from ultrafiltration experiments on organic-rich water samples from the Mengong river tend to demonstrate that Th concentration is less than 15 ng/L in absence of DOC (Table 2 and Viers et al. 1997), and that Th is still controlled by organic carbon in the final filtrate of the ultrafiltration experiments. The latter conclusion is also supported by the results obtained for the Kalix river (Porcelli et al. 2001). These results therefore not only raised the question of the determination of the amount of dissolved Th in water but also of the nature of Th chemical speciation. 

In addition, dissolved organic carbon (DOC) is also an important soil solution solute affecting speciation and bioavailability of many trace elements in soil solution. Many trace elements and heavy metals complex with dissolved organic carbon. This is especially important in arid and semi-arid environments since high soil pH increases the solubility of organic molecules and accordingly increases concentrations of dissolved organic carbon in soil solution. 

Since carbonate and high pH are unique characteristics of arid and semi-arid soils, we will first examine solution speciation and the equilibrium reactions of the C02-H20 system. We will then examine the solution speciation of Ca and Mg, Zn, Cu, Ni, Cd, Pb, Cr(III) and Cr(IV), Hg, and Se. 

In addition to soil solution, speciation of trace elements in water of the Nahr-Ibrahim river valley of Lebanon was studied with the AQUACHEM model. The results indicate that a high percentage of Pb and Zn is present as carbonate species, but in low percentages in free hydrated ion species. Cadmium exhibits as a high percentage of a free hydrated Cd2+. 

Carbonates, organic matter, Fe and Mn oxides, and clay minerals play important roles in controlling overall reactivity of trace elements in soils and sediments. This chapter addresses the interaction of trace elements with carbonates, organic matter, Fe and Mn oxides and clay minerals. Analytical techniques for trace element speciation in solid-phase and their distribution among various solid-phase components in arid and semi-arid soils are reviewed. Solubilities of trace elements in solid phases and their mineralogical characteristics in arid and semi-arid soils also are discussed. 

Greater adsorption of trace metals is found at higher pH and C02(g) concentrations. Sites available for Zn2+ sorption are less than 10% of the Ca2+ sites on the calcite surface, and Zn adsorption is independent of surface charge. This indicates a surface complex with a covalent character (Zachara et al., 1991). Furthermore, the surface complex remains hydrated and labile because Zn2+ is rapidly exchangeable with Ca2+, Zn2+ and ZnOH. At the dolomite-solution interface, the carbonate(C03)-metal (Ca/Mg) complex dominates surface speciation at pH > 8, but at pH 4-8, hydroxide (OH) -metal (Ca/Mg) dominates surface speciation (Pokrovsky et al., 1999). Calcite has an observed selectivity sequence Cd > Zn > Mn > Co > Ni > Ba = Sr, but their sorption reversibility is correlated with the hydration energies of the metal sorbates. Cadmium and Mn dehydrate soon after adsorption to calcite and form a precipitate, while Zn, Co and Ni form surface complexes, remaining hydrated until the ions are incorporated into the structure by recystallization (Zachara et al., 1991). 

Little is known of the oceanic distribution or speciation of cobalt, because very low concentrations (< 200 pM) make its determination difficult. Laboratory studies indicate that cobalt exists in seawater primarily as the cobalt (II) ion and as the carbonate complex. Organic complexes are not considered important. 

Garcia-Monco Carra et al. [296] have described a hybrid mercury film electrode for the voltammetric analysis of copper (and lead) in acidified seawater. Mercury plating conditions for preparing a consistently reproducible mercury film electrode on a glassy carbon substrate in acid media are evaluated. It is found that a hybrid electrode , i.e., one preplated with mercury and then replated with mercury in situ with the sample, gives very reproducible results in the analysis of copper in seawater. Consistently reproducible electrode performance allows for the calculation of a cell constant and prediction of the slopes of standard addition plots, useful parameters in the study of copper speciation in seawater. 

Batley [28] examined the techniques available for the in situ electrodeposition of lead and cadmium in estuary water. These included anodic stripping voltammetry at a glass carbon thin film electrode and the hanging drop mercury electrode in the presence of oxygen and in situ electrodeposition on mercury coated graphite tubes. Batley [28] found that in situ deposition of lead and cadmium on a mercury coated tube was the more versatile technique. The mercury film, deposited in the laboratory, is stable on the dried tubes which are used later for field electrodeposition. The deposited metals were then determined by electrothermal atomic absorption spectrometry, Hasle and Abdullah [29] used differential pulse anodic stripping voltammetry in speciation studies on dissolved copper, lead, and cadmium in coastal sea water. 

Prause et al. 1985). At pH 6.5 and water alkalinity of 25 mg CaC03/L, elemental Pb+2 is soluble to 330 pg/L however, Pb+2 under the same conditions is soluble to 1000 pg/L (Demayo et al. 1982). In acidic waters, the common forms of dissolved lead are salts of PbS04 and PbCl4, ionic lead, cationic forms of lead hydroxide, and (to a lesser extent) the ordinary hydroxide Pb(OH)2. In alkaline waters, common species include the anionic forms of lead carbonate and hydroxide, and the hydroxide species present in acidic waters (NRCC 1973). Unfortunately, the little direct information available about the speciation of lead in natural aqueous solutions has seriously limited our understanding of lead transport and removal mechanisms (Nriagu 1978a). 

Geochemical speciation modelling indicated saturation with respect to gypsum and several carbonates, slight under-saturation with respect to calcium arsenate (Ca3[As04]2) and ferrihydrite. 

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