Soil, lead chemical forms

Xian X. 1989. Response of kidney bean to concentration and chemical form of cadmium, zinc, and lead in polluted soils. Environment Pollution 57 127-137. 

Morin, G., Ostergren, J.D., Juillot, F., Ildefonse, P., Calas, G., Brown, G.E. 1999. XAFS determination of the chemical form of lead in smelter-contaminated soils... 

Most substances are absorbed to a degree, but few are entirely absorbed. Lead absorption from food, for example, may be in the range of 50%, but is less when this heavy metal is in certain highly insoluble chemical forms, or when it is associated with certain media such as dried paint or soils. 

The impact to health has been mostly dependent on the concentration of the candidate metal. Some metals (e.g., mercury, lead, arsenic, cadmium, iron, copper) ultimately find their way into human systems via soil, minerals, and water. Studies have shown the presence of many metals in daily consumable products (e.g., food, fruits, milk, fabric materials, drinking water). Further, heavy metals associated with particle material can be accumulated in areas suitable for sedimentation or particle concentration (e.g., upstream from sills or dams, in estuary sludge clog, etc.). These accumulation areas are creating possible pollution sources, as particles pooled could be resuspended during punctual hydrologic periods (floods, drains). Bioavailability, and therefore toxicity of heavy metals, is strongly bound to the current chemical form. 

Admixes to the soil in stabilization process compound with the minerals and soil grains, and may lead to generation of new chemical mix with different properties in comparison with the primary properties of soil. Lime as an ordinary stabilizer compounds with soil in the form of quick-lime (CaO) or hydrated lime (Ca(OH)2) can improve soil physical properties [7]. 

Under each given set of circumstances, the process that allows for the maximal energy flow is selected. Many of the biogeochemical cycling reactions of individual elements are actually connected by factors such as redox potentials. Individual minerals are oxidized only at specific redox potentials. This leads to zonations in soil and aquatic environments where minerals accumulate in specific chemical forms and where specific microbial populations proliferate. 

Xian, X., and Shokohifard, G. (1989). Effect of pH on chemical forms and plant availability of cadmium, zinc and lead in polluted soils. Water Air Soil Pollut. 45, 265-273. 

From this outline of the factors that determine availability, we see that the soil controls elemental availability to the extent that it limits mobility (steps 1-3). The soil may also influence absorption by roots (step 4) because it has some control over the chemical forms (speciation) that elements take in solution. The important effect of speciation is discussed in more detail in the next section. The soil may even affect translocation of elements within the plant, since there is evidence that mobility in plants is sensitive to the specific chemical form absorbed from soil solution. For example, it appears that iron absorbed from soil solution as a bicarbonate salt is immobile within the plant root and not translocated to the top. Another example is the immobility of zinc and lead in plant roots that are well supplied with phosphate. It may be that chemical precipitation reactions within (or possibly on) roots are limiting translocation. The questions raised here about the chemical forms of elements within plants fall into the realm of plant physiology and will not be pursued further. 

Morin G, Juillot F, Ildefonse P, Calas G, Samama J-C, Chevallier P, Brown GE Jr (2001) Mineralogy of lead in a soil developed on a Pb-mineralized sandstone (Largentiere, France). Am Mineral 86 92-104 Morin G, Juillot F, Ostergren JD, Ildefonse P, Calas G, Brown GE Jr (1999) XAFS determination of the chemical form of lead in smelter-contaminated soils and mine tailings Importance of adsorption processes. Am Mineral 84 420-434... 

Metals applied to land are relatively immobile, particularly if the soil has a high pH and high organic content. Many complex physical-chemical processes affect metal mobility in soil. Lead and mercury, which form strong complexes, move very slowly. Zinc, cadmium, and copper move a little faster. Metal profiles in highly contaminated soil penetrate little below the 4-8 inch region even after 60 years. ... 

Soil pH appears to be the main factor affecting plant uptake of lead through the root system. Other parameters are the species of plant, the chemical form of lead, and diverse soil conditions (Dudka and Miller, 1999). Root and leafy vegetables appear to be the two types of food crop forms most prone to lead uptake (Dudka and Miller, 1999 Finster et al., 2004), while fruits and grains take up much less. 

The variety of sampling approaches for soil lead analyses is as complex as for lead paint. The type of sample taking for soils depends on the purpose of the analyses. Residential, industrial, and public area soils are often analyzed for regulatory, legal, risk assessment, or scientific research purposes. Soils may also be tested to quantify the amounts of lead present, to determine the source(s) of lead in the soils, and, in certain cases, to ascertain the chemical and physicochemical species of lead in the soils to ascertain both likely source and relative bioavaUability of lead forms present. 

There are a variety of field and laboratory analytical methods for soil lead measurement, depending on the type of analysis and its purposes in a given evaluation. Bulk soil lead measurement refers to measurement of the total lead content of the soil sample. Chemical speciation and micromineralogical studies in the context of human lead exposure variability refer to amounts of specific chemical forms of lead and their geochemical states. These studies are sometimes done in tandem with relative bioavailability testings, i.e., amounts of lead being absorbed under in vivo or in vitro simulation of in vivo conditions (Casteel et al., 2006) with respect to Pb source attribution. Stable isotopic analysis studies deal with the quantitative stratification of lead s stable isotopic composition into the four main stable isotopes lead-204, lead-206, lead-207, and lead-208 (Gulson et al., 1995, 1997). 

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