Sorption of Testosterone to Soil Colloids of Different Size Fractions: Development of a Centrifugation and Mass Balance-Based Methodology

Soil colloids play critical roles during the fate and transport of micropollutants (e.g., pesticides and hormones) due to their high sorption affinity and mobility. In this study, the authors developed a centrifugation and mass balance-based methodology for studying sorption of testosterone to soil colloids of different size fractions (DSFs). The method should allow one to conduct the sorption tests with the whole soil first and then separate soil colloids of DSFs to evaluate their contributions to sorption. Results indicate the colloids diameter ranges (i.e., 900-600, 600-400, and 400-200 nm) can be controlled by a group of series centrifuge conditions (i.e., speed and time). The sorption affinity of soil colloids for testosterone is three times higher than that of the bulk soil and increases significantly with decreasing diameter size because of the difference of specific surface area and organic carbon content. The authors propose that the centrifuge condition of 2,844 relative centrifugal force (RCF)+30 min is required to separate the dissolved (dnano<200 nm) and particle-associate phases to ensure the error of sorption results less than 2%. One may use the methodology to develop a framework to study the sorption of micropollutants on soil colloids of DSFs with whole soils or sediments as testing material media.