Chemistry of lead in soils

This is an extremely complex subject, not yet fully understood. Any remarks here can serve only as a crude introduction. 

Lead added to soil may react with available soil anions such as S04 P04 or COa to form sparingly soluble salts [8]. Compounds such as basic lead carbonate, Pb3(0H)2(C03)2 (log — —46.8 at 25° C) and chloropyro-morphite, Pbs(P04)3Cl, (log = —84.4) provide the least soluble simple inorganic. lts at near-neutral pH. In fact, X-ray powder diffraction (XRD) studies by Olsen and Skogerboe [18] have shown the more soluble PbS04 to be the major crystalline lead compound in contaminated soils. The results of these workers are shown in Table 4.5. 

The actual mechanism of lead association with soils is no doubt a mixture of some or all of the above processes, the predominant mode probably depending upon the composition and pH of the specific soil in question. 

Highly elevated concentrations of lead may be found in deposited dust and in street dusts close to lead works. Concentrations of up to 300000 mg kg (30%) lead have been reported, but typical values are of the order of 1000-50 000 mg kg [11]. 

When viewing theie concentrations, one point worthy of note is that lead levels in deposited urban dusts sampled in 1928, before the widespread use of leaded petrol, showed concentrations very similar to those found today in street dusts [11]. Presumably the far greater emissions of smoke from coal combustion and the use of high-lead paints had a substantial influence upon lead deposition. 

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