Lead Salts as Glazes and Pigments
One form of the oxide PbO is a yellow solid that has been used at least as far back in history as ancient Egypt to glaze pottery. In glazing, the material is fused as a thin film to the surface of the pottery in order to make it waterproof and to give it a brilliant high gloss. The oxide becomes a hazard if applied incorrectly: Some of it will dissolve over a period of hours and days if acidic foods and acidic liquids, such as cider, are stored in pottery containers, giving dissolved Pb2+, up to hundreds or even thousands of parts per million, in the food:
PbO(s) + 2 H+(aq)-> Pb2+(aq) + H20
Indeed, lead-glazed dishware is still a major source of dietary lead, especially, but not exclusively, in developing countries. The leaching of lead from glazed ceramics used to prepare food is one of the major sources of the element for children in Mexico, where lead contamination is a leading public health problem. Nowadays, lead silicate rather than oxide or sulfate is used for glazing in most countries since it is almost insoluble and thus much safer.
Various salts of lead have been used as pigments for millennia, since they give stable, brilliant colors. Lead chromate, PbCr04, is the yellow pigment used in paints applied to school buses and for the yellow stripes on roads. Red lead, Pb304, is used in corrosion-resistant paints and has a bright red color. It was used in great quantity in the past to produce a rust-resistant surface coating for iron and steel. Lead acetate is often used in preparations to cover gray hair, since the Pbz+ ion of this soluble salt will combine with the SH group of hair proteins to give a dark color.
Lead pigments have been used to produce the colors used in glossy magazines and food wrappers. In past centuries, lead salts were used as coloring agents in various foods. White lead, Pb3(C03)2(0H)2, was extensively used until the middle of the twentieth century as a major component of white indoor paint. Since it was more durable than unleaded paint, it was often used on surfaces subject to punishment such as kitchen cabinets and window trim. However, when the paint peels off, small children may eat the paint flecks since Pb2+ has a sweet taste. Persons who renovate old homes are urged to ensure that dust from layers of old paint is properly contained. Children in inner-city slums, in which old coats of paint continue to peel, are often found to have elevated blood levels of lead. In indoor paint, white lead has now been replaced by the pigment titanium dioxide, TiOz. Although now banned from use in indoor paints (since 1978 in the United States), lead pigments continue to be used in exterior paints, with the result that soil around houses may eventually become contaminated. Some of this lead-contaminated soil may be ingested by small children because of its sweet taste. Lead is still widely used in indoor paint sold in China, India, and some other Asian countries, sometimes at levels exceeding 180,000 ppm (as compared to the U.S. standard of 600 ppm maximum for new paints).
An additional source of sweet lead-containing dust was the surface of some types of PVC miniblinds that had lead incorporated as a stabilizer in the plastic and that underwent partial decomposition from exposure to UV in sunlight. Lead is used as a stabilizer in a variety of other PVC products as well, including children's toys.
Lead dust, which originates as soil containing tiny particles of lead compounds, is now t he biggest source of lead for children in U.S. inner cities. The lead collectively originates from individually small but numerous contributions from many sources already mentioned—paint flakes, ceramics, plastics, gasoline, recycling plants, and even lead salts used in hair coloring preparations for people with graying hair. The use of lead arsenate, Pb( As04)2, as a pesticide was another former source of Pb2^ in soil.
Green Chemistry: Replacement of Lead in Electrodeposition Coatings
Sheet metal surfaces made of steel undergo corrosion very rapidly unless they are covered with a protective coating. Since the 1960s a technique called electrodeposition has competed with spray painting for coating steel. In 1976 the first automobiles were treated by electrodeposition. In this technique, the surface to be treated is dipped in a bath, with the surface acting as a cathode or anode, and the coating is deposited electrophoretically. Electrodeposition has many advantages over spray painting, including:
• lower air pollution, due to decreased solvent emissions;
• better corrosion protection, due to better coverage of poorly accessible areas;
• reduced waste, due to high transfer efficiency; and
• more uniform coating thickness.
Virtually all primer coats for automobiles are done using this method. Red lead, mentioned previously, offers significant corrosion resistance, and primer coats use large amounts of this material. Although lead has been banned from house paints in the United States since 1972, the demand for corrosion resistance for motor vehicles has resulted in exemptions from environmental regulations regarding lead in automobile and truck paints.
PPG Industries has discovered that yttrium oxide serves as an excellent replacement for lead as a corrosion inhibitor and won a Presidential Green Chemistry Challenge Award in 2001. On a weight basis, yttrium is twice as effective in inhibiting corrosion as red lead but is only 1/120th as toxic. An additional consideration is the pretreatment process that is used to assist in adhesion and corrosion resistance prior to the application of the electrocoat. The use of yttrium eliminates chromium from metal pretreatments and reduces the amount of nickel compared to the lead process. It is estimated that employing yttrium in automobile electrodeposition will eliminate not only the use of 1 million pounds of lead but also 25,000 pounds of chromium and 50,000 pounds of nickel on an annual basis. As of September 2006, more than 38 million motor vehicles have been coated with the yittrium-containing product since its introduction in 2001. According to PPG Industries, no customers in either the United States or Europe purchase any lead-containing coating product for any application, including automotive.
Dissolution of Otherwise Insoluble Lead Salts
The presence of significant concentrations of lead in natural waters is seemingly paradoxical, given that both its sulfide, PbS, and its carbonate, PbCOj, are highly insoluble in water:
PbS(s) Pb2+ + S2~~ Ksp = 8.4 X 10"28
PbC03(s) Pb2+ + CO,2" Ksp = 1.5 X 10~13
However, the anions in both salts are fairly strong bases. Thus both of the above dissolution reactions are followed by the reaction of the anions with water:
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