Solubility Control

Typically three lead compounds limit the solubility of lead in water, via the following equilibria 

Lead ions (Pb , however, have a strong tendency to form ion pairs, principally PbHCOa and PbCOa at the pH of most waters. The formation of these species reduces the Pb concentration and drives the above equilibria to the right, enhancing the lead solubility. A similar enhancement of lead solubility occurs when organic compounds complex with the Pb ions. The calculated equilibrium solubility of lead, allowing for the formation of inorganic ion pairs, is illustrated as a function of pH in Fig. 3.6. 

The identification of lead in association with carbonates in the sediment phase of natural waters implies that lead solubility equilibria can indeed determine the particulate lead species in certain waters. Precipitated lead salts will almost certainly be present in the particulate phase of effluents treated by pH control (Section 6.4.1). Clearly, if lead is present in waters as carbonate/hydroxy precipitates, then any reduction in the water pH will increase the concentration of soluble lead. In most natural waters, however, the concentration of soluble lead is less than that predicted by equilibrium solubiHty considerations and other mechanisms will account for the presence of particulate lead. 

This paper highlights two of the four chemical treatment methods which have been used quite extensively in the foundry industry. 

Evidence for a redox mechanism is found in EP Toxicity Test results. The addition of iron filings resulted in a final pH in the EP Toxicity Test slightly higher than untreated samples, indicating that the hydrogen ion may have been neutralized by chemical reduction. 

Some success has been achieved using metallic iron to treat wastewater directly. An advantage of using metallic iron instead of iron hydroxide is that less sludge volume is generated. The presence of oxidizing agents in the wastewater can limit the effectiveness of metallic iron treatment. 

Another method of chemical treatment involves controlling the pH of the waste solution so metals will not be soluble when exposed to acidic conditions. Since 

FIGURE 7 LEAD AND CADMIUM LEACHATE CONCENTRATIONS FROM CUPOLA EMISSION CONTROL SLUDGE TREATED WITH METALLIC IRON 

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Table 3.8. Possible soil solution solubility controls for selected trace elements3...

Dang et al. (1994) observed that the experimentally determined solubility lines for Zn2+ in 14 soil solutions from southern Queensland with soil pH from 7.45-8.98 and 0.08-2.07% CaC03 were not undersaturated with respect to the solubility of any known mineral form of Zn. Therefore, they suggested that Zn2+ activity was mainly controlled by adsorption-desorption reactions in these soils. Similar observation on solubility of Cr(VI) in arid soils was reported by Rai et al. (1989). In the absence of a solubility controlling solid phase, Cr(VI) aqueous concentrations under slightly alkaline conditions may be primarily controlled by adsorption/desorption reactions (Rai et al., 1989). Chromuim(VI) is adsorbed by iron and aluminum oxides, and kaolinite and its adsorption decreases with increasing pH. 

Table 8.29 Trace element solubility control at T Turekian, 1969). = 25 °C and P = 1 bar (from...

Hart, K. P., Glassley, W. E. McGlinn, P. J. 1992. Solubility control of actinide elements leached from Synroc in pH-buffered solutions. Radiochimica Acta, 58/59, 33-35. 

JOHANNESSON, K. H., LYONS, W. B., STETZENBACH, K. J. Byrne, R. H. 1995. The solubility control of rare earth elements in natural terrestrial waters and the significance of P043- and C032-in limiting dissolved concentrations A review of recent information. Aquatic Geochemistry, 1, 157-173. 

Heavy metal cations precipitate readily as hydroxides or carbonates in alkaline media. Dissolved carbonate content will be limited by calcite precipitation or by conversion of hydroxyl AFm to carbonate AFm. Hydroxide ions, on the other hand, are abundant. Here only the solids that may be present under oxic conditions will be discussed. Figure 5a shows the total dissolved heavy metal cation concentrations that would prevail if hydroxide precipitation were to be the dominant solubility-controlling process. Figure 5b shows the solubility of Ca metallate species, as these are likely to act as solubility-controlling phases for oxyanionic species. 

Fig. 9. Possible solubility controlling phases for dissolved concentrations of Ca(II), Al(III), SO4- and Si(IV) in Landfill Lostorf leachate. The symbols represent measured concentrations for different rain events and the curves represent model calculations for the conditions given in Table 1.

Fruchter, J. S., Rai, D. Zachara, J. M. 1990. Identification of solubility-controlling solid phases... 

Theis Richter (1979) showed that adsorption onto hydrous Fe-oxide was the major solubility control for Cd, Ni, and Zn in soils surrounding two ash disposal ponds, and stated that certain fly ashes are capable of causing the deposition of secondary Fe-oxides. The ash pond leachate conditions favoured the precipitation of Pb and Cr hydroxides, Pb carbonate, and precipitation of Cu as malachite. The leaching of As (as arsenate) and Se (as selenite) was found to be controlled by adsorption onto amorphous Fe-oxide by van der Hoek Comans (1994). 

Garavaglia, R. Caramuscio, P. 1994. Coal fly-ash leaching behaviour and solubility controlling solids. In Goumans, J. J. J. M., van der Sloot, H. A. Aalbers, Th. G. (eds) Environmental Aspects of Construction with Waste Materials. Elsevier Science BV, Amsterdam, 87-102. 

Bioreactions. The use of supercritical fluids, and in particular C02, as a reaction media for enzymatic catalysis is growing. High diffusivities, low surface tensions, solubility control, low toxicity, and minimal problems with solvent residues all make SCFs attractive. In addition, other advantages for using enzymes in SCFs instead of water include reactions where water is a product, which can be driven to completion increased solubilities of hydrophobic materials increased biomolecular thermostability and the potential to integrate both the reaction and separation bioprocesses into one step (98). There have been a number of biocatalysis reactions in SCFs reported (99—101). The use of lipases shows perhaps the most commercial promise, but there are a number of issues remaining unresolved, such as solvent—enzyme interactions and the influence of the reaction environment. A potential area for increased research is the synthesis of monodisperse biopolymers in supercritical fluids (102). 

The separation mechanisms, which were operative in the above experiments, will be discussed below. Carbon tetrachloride is a good solvent for all types of polybutadienes, and the separation with this solvent should have proceeded according to the adsorption mechanism. On the other hand, cis-1,4 and 1,2-vinyl poly butadiene were soluble in amyl chloride even below —5 °C but tram-1,4 polymer was insoluble in this solvent below ca. 40 °C. This suggests that the separation with this solvent would have proceeded according to the solubility-controlled mechanism. 

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