Laboratory versus field soil aging: Impact on DDE bioavailability and sorption

Solid-phase microextraction (SPME), XAD, and the sequential supercritical fluid extraction (SFE) were used to assess the influence of aging of p,p'-DDE in a laboratory contaminated soil for up to 730 days. The end points determined were the freely dissolved concentration (C_free) using SPME, the potentially bioaccessible fraction (F_XAD, %) and the distribution of p,p'-DDE among fast, moderate, and slow desorbing soil sites determined by three sequentially stronger SFE conditions. C_free and F_XAD decreased during the first 35 days of aging by up to 40%. After this, no significant changes were observed up to the end of the aging experiment. The relative percentage of fast desorbing sites tended to exponentially decrease with aging, while the percentage of moderate and slow desorbing sites increased over time. These changes were most apparent within the first 90 days of aging, after which the relative distribution of p,p'-DDE among desorbing sites remained relatively constant. Significant correlations between SFE and XAD results demonstrated that the XAD method preferentially desorbed p,p'-DDE from fast and moderate desorbing sites and is capable of extracting the bioaccessible fraction. The distribution among desorbing sites, C_free and F_XAD values determined after different periods of laboratory aging were then compared to those measured for a field-contaminated soil where p,p'-DDE had resided for more than 40 years. C_free, F_XAD and SFE profiles measured for the field-aged p,p'-DDE were similar to those observed for p,p'-DDE aged in laboratory for between 35 and 90 days. These results suggest that aging in the laboratory must be carried out for periods of months if it is to approximate field aging.