Removal of Lead Contaminants

Aqueous hydrofluoric acid can be freed from lead by adding 1ml of 10% strontium chloride per 100ml of acid, lead being co-precipitated as lead fluoride with the strontium fluoride. If the hydrofluoric acid is decanted from the precipitate and the process repeated, the final lead content in the acid is less than 0.003ppm. Similarly, lead can be precipitated from a nearly saturated sodimn carbonate solution by adding 10% strontium chloride dropwise (l-2ml per 100ml) followed by filtration. (If the sodimn carbonate is required as a solid, the solution can be evaporated to dryness in a platinum dish.) Removal of lead from potassium chloride uses precipitation as lead sulfide by bubbling H2S, followed, after filtration, by evaporation and reciystallisation of the potassium chloride. 

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Fig. 4. Examples of emission spectrometry as a diagnostic monitoring tool for plasma processing, (a) The removal of chlorine contamination from copper diode leads using a hydrogen—nitrogen plasma. Emissions are added together from several wavelengths, (b) The etching and eventual removal of a 50-p.m thick polyimide layer from an aluminum substrate, where (x ) and (° ) correspond to wavelengths (519.82 and 561.02 nm, respectively) for molecular CO2...

Reversible inhibition caused by materials that can function as ligand. Many compounds will bind to a metal this might be the solvent or impurities in the substrate or the solvent. It can also be a functional group in the substrate or the product, such as a nitrile. Too many ligands bound to the metal complex may lead to inhibition of one of the steps in the catalytic cycle. Likely candidates are formation of the substrate-catalyst complex or the oxidative addition of hydrogen. Removal of the contaminant will usually restore the catalytic activity. 

The decline in acceptable childhood blood levels was a function of research and improved control of lead contamination, such as the removal of lead from gasoline. A blood lead level of 10 lg/dl does not represent a safe level, only one where it is prudent to take action to reduce exposure. But it must be noted that a level of 10 (Xg/dl is considered an action level and does not provide any margin of safety for a child s developing nervous system. Currently, there appears to be no safe level of lead exposure for the developing child. 

As an alternative to acid washing, soils can also be flushed with chelating agents. Examples of effective chelating agents include ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA), both of which readily bind and solubilize metals. Using this approach, Peters Shem (1992) have recently reported on the removal of lead from a contaminated soil. In this study, 0.1M EDTA removed 60% of the lead in a soil containing 10 000 mg lead/kg. 

Emission studies show that lead is only a small part of the automotive pollution problem. Prior to control, hydrocarbon emissions were more than 40 times and the oxides of nitrogen emissions more than 15 times the emission of the lead compounds. Obviously, however, legislation will result in the eventual elimination of lead from gasoline. The removal of lead, besides eliminating a possible toxic pollutant, simplifies the problem of handling other automotive exhaust pollutants in that catalytic exhaust chambers perform much better in the absence of lead contaminant. All emission standards become particularly severe in 1975 and 1980. The particulate standards are equivalent to 1 gram Pb/gal in 1975 and 0.3 gram Pb/gal in 1980. Since the particulates include all solid materials, tolerable lead levels will be less than indicated above. 

One can compare the biomarker-based risk derived for lead with population-based biomonitoring results. Data from NHANES 2000-2001 can be used to estimate the number of children in the United States who have increased blood lead (CDC 2005). Blood lead in U.S. children declined dramatically after the removal of lead from gasoline—from a median of 15 pg/dL in 1978 to 2 pg/dL in 1999 (Rogan and Ware 2003). Nonetheless, an estimated 1.6% of U.S. children 1-5 years old had blood lead greater than 10 pg/dL in 1999-2002, according to NHANES data (CDC 2005). The major exposure sources of lead for U.S. children are deterioration of lead-based paint and the resulting dust and soil contamination (CDC 2005). 

Additional purification steps lead not only to additional yield losses, but also to an increase in processing time and cost. For research purposes, the process scales are usually small and the cost of an additional step may not be important. On the other hand, at an industrial scale, the introduction of an additional step may make a purification process uneconomic. Removal of residual contaminants is, in most of the cases, significantly more difficult than the earlier purification steps and involves several additional steps, reducing the final yield and increasing the processing cost and time. 

Chapter 2 considers the removal of inorganic water contaminants using photocatalysis. Metal cations react via one-electron steps first leading to unstable chemical intermediates, and later to stable species. Three possible mechanisms are identified (a) direct reduction via photo-generated conduction band electrons, (b) indirect reduction by intermediates generated from electron donors, and (c) oxidative removal by electron holes or hydroxyl radicals. The provided examples show the significance of these mechanisms for the removal of water contaminants such as chromium, mercury, lead, uranium, and arsenic. 

The effect of varying the EDTA (X)iteentration for the removal of lead from contaminated soil is shown in Figure 4. The removal efficiency is shown for four separate lead concentrations on the soil, ranging from 500 to 10,000 mg/kg soil. The removal efficiency was greatest for the case of a lead contamination level of 5000 mg/kg, although all the removal efficiencies were fairly constant, ranging from 54 to 68%. 

According to the 1992 NCHRP data, nearly 80% of the bridges have been coated with lead-based paints. About 4100- 130 million is spent on painting annually. Lead paint removal generates an estimated 181 million kg (200,000 ton) of lead-contaminated abrasives. 

A) the removal of lead-based paint and lead-contaminated dust, the permanent containment or encapsulation of lead-based paint, the replacement of lead-painted surfaces or fixtures, and the removal or covering of lead-contaminated soil and... 

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