Bioaccessibility of p,p'-DDT and p,p'-DDE in tropical soil ecosystems: a model based on pollutant speciation coefficients and the desorption free energy of adsorbed speciation forms.

The bioaccessibility of organic pollutants in the environment depends on the nature and speciation of the contaminants and is determined using in vitro methods that simulate gastro-intestinal digestion. The objectives of the present study were to study the bioaccessibility coefficients of p,p'-DDT and p,p'-DDE in tropical soils based on the physico-chemical properties of the contaminant. The behavior of organic contaminants in soil ecosystems is characterized by their speciation and persistence, both of which depend on the extent to which the contaminant adsorbs to particulates in the soil matrix, as revealed in the characteristic persistence curve of the contaminant. Data are presented showing that the bioaccessibility coefficient of soil contaminants can be represented by a model based on the contaminant's speciation coefficients in the sample matrix, the desorption free energy of contaminant adsorbed speciation forms and temperature, provided its dissipation conforms to the multi-phase pseudo-zero order rate law. When the model was applied to determine the bio-accessibility coefficients of p,p'-DDT and p,p'-DDE in tropical soils based on previously published data from the literature, mean bioaccessibility coefficient values of 0.30 ± 0.21 (n = 8) and 0.35 ± 0.13 (n = 9) (p,p'-DDT), and 0.43 ± 0.05 (n = 4) and 0.20 ± 0.01 (n = 2) (p,p'-DDE), were obtained depending on whether determinations were carried out using chromatographic or radiotracer methods. The results fall within the range of literature values obtained using in vitro methods, thus attesting to the potential of the model presented for predicting the bioaccessibility coefficients of persistent organic pollutants in soil ecosystems.