Soil, lead half-life estimations

There is a global literature on the topic of lead contamination of soils, particularly for industrialized countries. A notable feature of Pb in soils is its persistence there, with studies showing a residence time or half-life estimated to range from decades to centuries (U.S. EPA, 2006). For example, Freitas et al. (2004) reported a mean soil Pb of 2,694 ppm for samples gathered at a long-defunct Portuguese copper mining site worked in pre-Roman and Roman times. 

An extensive pesticide properties database was compiled, which includes six physical properties, ie, solubiUty, half-life, soil sorption, vapor pressure, acid pR and base pR for about 240 compounds (4). Because not all of the properties have been measured for all pesticides, some values had to be estimated. By early 1995, the Agricultural Research Service (ARS) had developed a computerized pesticide property database containing 17 physical properties for 330 pesticide compounds. The primary user of these data has been the USDA s Natural Resources Conservation Service (formerly the Soil Conservation Service) for leaching models to advise farmers on any combination of soil and pesticide properties that could potentially lead to substantial groundwater contamination. 

Soils amended with arsenic-contaminated plant tissues were not measurably affected in C02 evolution and nitrification, suggesting that the effects of adding arsenic to soils does not influence the decomposition rate of plant tissues by soil microorganisms (Wang et al. 1984). The half-life of cacodylic acid is about 20 days in untreated soils and 31 days in arsenic-amended soils (Hood 1985). Estimates of the half-time of inorganic arsenicals in soils are much longer, ranging from 6.5 years for arsenic trioxide to 16 years for lead arsenate (NRCC 1978). 

It has been reported that low concentrations of tributyltin fluoride were readily transformed to tributyltin chloride in sea water (Strand 1983). Tributyltin has been shown to undergo slow photolysis (Maguire et al. 1983). The half-life of the photolysis reaction was estimated to be greater than 89 days. The direct photolysis of tributyltin in water initiates a sequential removal of the butyl groups, leading to inorganic tin as a residual. The reaction was much faster in the presence of fulvic acid (a major component of soil organic matter). 

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