Lead in the soil ecosystem-

Lead exists naturally at 'background' levels in all soils, originating from the weathering and decomposition of the parent rock material, igneous, metamorphic or sedimentary in origin (e). The concentrations are approximately equal to the average concentrations of the earth's lithosphere (7-’). The world-wide average lead concentration of 4,970 soils has been calculated at 29.2 mg/kg with a range of <1-888 mg/kg C220). Harrison and Laxen- Duncan <7®), suggest typical concentrations for natural soils at between 10 mg/kg and 200 mg/kg., with polluted or mineralised soils between 100 mg/kg and 10,000 mg/kg.

Anthropogenic lead is made available to the soil by a variety of environmental processes (A .8 -79.8o)| primarily by the atmospheric deposition of vehicular particulate lead, smelter emissions and remobilisation by wind of contaminated dusts. Many workers have established that the highest concentrations of lead in soil profiles generally occur at the surface horizons (*2 ,0 1,6 2,8 3)

owing to enrichment from the atmosphere and by biological processes.

Lead may exist in the soil in a variety of chemical forms

which govern the type of analysis which can be performed on the soil. When tightly bound in complex molecules lead is very difficult to extract from soil, consequently very strong chemical

reagents may be required in order to determine the total lead content of the soil. Such species of lead may not be readily available to plants for uptake pi.eA.ee.ee) an£ therefore it is often important to know the extractable or available lead content of the soil, if plant uptake of lead is being investigated (e7).

1.3.1. Available lead.

Little is known about the mobility and availability of lead in soils, but it has been observed that lead is lost from soils only very slowly by leaching. Therefore a soil is likely to remain polluted for a long period of time C7S). It tends to accumulate in the topsoil and litter horizons (43'ee), held with other plant available nutrients in the soil-clay-humus complex (e,6 i,7 9 1ee))

although lead itself is not an essential nutrient (®). Plant availability to lead is dependent on a number of factors (67,6i)7i.79,eBIe9(9ol9i(92)) including soil texture (65,64,85,32,93)^ cation exchange capacity organic matter (7 9,ee,9 4)| an(i particular pH (79.es189,9o 192)) latter

factor is important as it has been noted that raising of pH by the application of lime or phosphate reduces the availability of lead to plants (eiS), therefore pH can be an important factor in experimental design. It also affects the extractability of lead from samples and its value should be stated where possible to permit comparative interpretations of results. Crump and Barlow (®3>, discuss factors governing availability and the problems associated with its assessment. Extracting available lead is problematic, not least since the use of extractants is generally

not underpinned by any significant theoretical framework, though it has been of use in agronomy and environmental research (3&).

The extraction of available lead has been achieved through the use of a variety of extractants and the efficiencies of various methods have been investigated -96-97-90). Khan (eA) identified four groups of lead compounds and suggested techniques suitable for extraction of each type. The first group includes ionic and molecular forms of the metal, removable from samples by water (G7). Readily exchangeable metal ions from inorganic clay or organic material can be removed by ion exchange with ammonium acetate or other neutral salts p9.8B,92,96)i More firmly bound ions in exchange complexes can be displaced using dilute acetic acid (3818B,9il95(98l99)) or other dilute acids, such as hydrochloric acid (A3>. Predictions of total lead have been made using the acetic acid/acetate method by Nicklow, et al. (7S). Organically complexed lead has been extracted by ethylene diamine tetra-acetic acid (EDTA) (6 1,7 3,9 9,1 0 0) or other chelating agents by liquid/liquid extraction. The use of some of these reagents and techniques by various authors is discussed below and is summarised in Appendix l.a.

Acetic acid extracts.*.

Acetic acid is widely used as an extractant of available lead (e*>, as it is said to stimulate plant uptake and gives a guide to plant availability (101). The general procedure is to extract an air dried sample with 0.5M acetic acid (3 5,4 6,-1 7,9 8,1 0 2) for a given period of time, usually overnight, filtering the residue and

evaporating to dryness over a steam bath, before uptake in a suitable analytical medium. The H+ ions in the acid displace bound ions from the exchange complexes in the soil, but as it acts below normal pH ranges, must be considered to only give an estimate of available lead (SA). tfeuhauser and Hartenstein C-n ) and Clayton and Tiller (9,3> record the relative efficiency of this extraction method for various soils. tficklow, et al. (76) describes its use in Morgan Solution (100 g of sodium acetate, 50 ml water and 30 ml glacial acetic acid at pH 4.8), modified with EDTA, to predict total soil lead.

Ac&tate extracts.

Although the amount of lead that can be extracted by neutral ammonium acetate is generally quite small (ei4) it has been used by several investigators P - 67'85'103' 10A*10S',06), particularly in early studies. Samples are usually shaken with 0.5M acetate solution overnight. The residue may then be leached for a further period prior to analysis. Petrov, et al. C92) describe tests involving preconcentration by liquid/liquid extraction, and claim to improve detection limits up to 1 0 times by this method, although contaminated samples can give erroneous results.