Lead , solubility

Lead(fl) ethanoate, Pb(02CCH3)2,3H20, sugar of lead. Soluble in water (ethanoic acid and ethanoic anhydride plus Pb304). 

Jonioh, V., Obbard, J.P., and Stanforth, R.R., Impact of treatment additives used to reduce lead solubility and microbial toxicity in contaminated soils, in Bioremediation of Metals and Inorganic Compounds, Leeson, A. and Alleman, B.C., Eds, Battelle Press, Columbus, OH, 1999, pp. 7-12. 

Heubach has developed a process for the alternate precipitation of metal oxides and silicates [3.141 ], [3.142]. A homogenizer is used to disperse the pigment particles during stabilization. Products obtained have a very good temperature resistance and very low lead solubility in acid (< 1 % Pb by DIN 55770, 1986 or DIN/ISO 6713, 1985). 

Harada, K., and Tsunogai, S. (1988) Is lead soluble at the surface of sediments in biologically productive seas Cont. Shelf Res. 8, 387-396. 

James and Healy s ( ) expression for AG olv was further refined (prior to publication) by Levine ( ). This, according to Wiese aJ-. ( ), constitutes a more accurate and rigorous theoretical analysis of the changes in solvation energy that accompany adsorption. Accordingly, the Levine ( 5) expression was used here in calculations which were used to attempt to predict lead solubility. 

Properties Ductile shiny silver-white metal. D 7.31 (20C), mp 156C, bp 2075C, Mohs hardness 1.2. Softer than lead. Soluble in acids insoluble in alkalies. Corrosion-resistant at room temperature. Oxidizes readily at higher temperatures. 

In the case of lead pipes, simple oxidative corrosion of the metal forms a coating of lead carbonate on the inside wall of the pipe. As shown in theoretical and empirical studies, the lead solubility (maximum lead concentration) is a function of water characteristics, mainly pH, alkalinity and temperature, as well as, eventually, orthophosphate concentration (Sheiham and Jackson, 1981 Kuch and Wagner, 1983 Van den Hoven, 1986 Schock, 1989,1990,1994 Wagner, 1992 Leroy, 1993). Fora given alkalinity, lead solubility decreases when pH increases. Theoretical lead solubility varies between several mg/1 for very soft waters with low pH (alkalinity < 30 mg/1 CaCOs and pH < 6.5) to less than 100 pg/1 in waters with alkalinity between 50 and 150 mg/1 CaCOs and pH above 8. 

Schock, M. R. (1994), Response of lead solubility to dissolved carbonate in drinking water,Joumal of the American Water Works Association 72(12), 122-127. 

Initial laboratory tests aimed at stabilizing the arsenic at pH 5 and 8 showed a lead solubility of 30 an 32 ppm. These tests were carried out without excess lime addition. During the TCLP test, acetic acid is added so that the pH of the leach solution is not controlled at any particular value. This explains the relatively high Pb solubility. Phosphate was then added during the arsenic precipitation stage, again without excess alkaline material. The results of the TCLP tests with phosphate additions showed a Pb leachate concentration of 40 to 60 mg/1. These data indicate that phosphate alone is not able to control the leaching of the lead. 

In terms of lead solubility, the most important water quality parameters are pH, alkalinity, dissolved inorganic carbonate and orthophosphate levels. In general, low pH levels have been associated with higher lead levels at the tap. Soft waters that are low in pH and alkalinity are often corrosive toward lead and other metals. Water treat-... 

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. 

Fig. 3.6 Equilibrium lead solubility as a function of pH and total carbonate concentration...

Lead Silicate. A material obtained by fritting lead oxide with silica in various ratios. The usual ratio approximates to Pb0.2Si02 and a frit of this composition is known as lead bisilicate a specification (Trans. Brit. Ceram. Soc., 50,255,1951) is 63-66% PbO, 2-3.5% AI2O3, and PbO + Si02 + AI2O3 to exceed 98%. A lead solubility test (q.v.) is also specified. The bisilicate is used in pottery glazes. 

Low-solubility Glaze. The Potteries etc (Modifications) Regulations 1990 (S.I. 305) define this as a glaze which does not yield to dilute HCl more than 5% of its dry weight of a soluble lead compound, when determined in accordance with an approved method see lead solubility test (q.V.). for the original approval method. 

For electrolytic refining, an electrolyte is required that has a reasonable lead solubility, is stable, has a high electrical conductivity and will yield a smooth compact deposit of lead. Various organic acids have good lead solubility and conductivity but tend to be unstable. It was found during the early development of the process that fluosilicic acid, fluoboric acid and sulfamic acid were most suitable and fluosilicic acid was the least costly. Sulfamic acid systems were also used, but showed instability at high current densities. Consequently, most electrolytic refining operations are based on a fluosilicate electrolyte. 

Lead solubility in waters tends to be under various control mechanisms. Two factors that control aquatic solubility are water pH and dissolved salt content. Davies and Everhart (1973) noted that, at pH above 5.4, solubility of lead is about 30 pg/1 in hard waters and 300 p.g/1 in soft waters. Overall, soluble lead in rivers is in proportionately lesser amounts than the insoluble (i.e., colloidal) forms. 

This is a propriety non-replacement approach for lead pipe solubility reduction. It consists of inserting a platinum coated anode through existing piping to implement a rapid method of control of lead solubility by an impressed DC potential. Both sides of the meter assembly (when it is sandwiched between two lead laterals) can be protected. 

In the late 1970s, trials in Eastern England indicated that pH elevation of high alkalinity waters could at best achieve a 30% reduction in lead solubility. There were concerns about calcite precipitation and the approach was abandoned in favour of ortho-phosphate dosing. However, few high alkaUnity suppUes were ortho-phosphate dosed until the mid-1990s, when compUance with the former 50 p.g/1 standard was enforced. 

This inhibitor is not used in the UK for corrosion control as work by WRc in the 1980s showed that it was less effective than ortho-phosphate for reducing lead solubility. It is used in some US supplies to reduce cuprosolvency and for lead corrosion control. The zinc has some noteworthy benefits in preventing scaling and controlling concrete corrosion. The published US research has generally not found it more effective in the case of lead piping. There is still debate and research to be done with respect to whether the cathodic... 

The most effective corrosion inhibitor for reducing lead solubility is orthophosphate which converts some of the lead carbonate, within the internal corrosion deposits of lead pipes, to lead phosphate which has a lower solubihty. This conversion depends on the alkaUnity of the water and the dose of orthophosphate. Lead solubihty can also be affected by natural organic matter, particularly the humic and fiilvic acids associated with coloured upland waters. Further details on corrective water treatment measures can be found in IWA s Best Practice Guide for the Control of Lead in Drinking Water (IWA, 2010). 

In order to control the plumbosolvency of the water supplies to consumers, it has been long-standing practice to elevate pH in order to suppress lead solubility. The pH within the system is typically between 9.5 and 10.0. Corrosion inhibitors such as orthophosphate are not used. This approach to plumbosolvency control has succeeded in general terms, with 80 to 90% of 6-1- hours stagnation samples being below the Action Limit of 15 pg/1 over the period 2007 to 2011. However, the system was marginally non-compliant with the criteria for lead set by the US Lead Copper Rule in 7 out of 9 surveys over this period. 

Lead driers have traditionally been the most common, although toxicity concerns now severely restriet their use. Lead is a secondary drier as its sole function is to catalyze oxygen uptake. This and its relatively low reactivity make it a good through drier. For this reason it was widely used in combinations with primary driers in the past. It is still used where toxicity and atmospheric sulfide staining do not rule it out. Caleium soaps have been used with lead driers to improve lead solubility and low temperature drying. 

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