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Comparison of the release of constituents from granular materials under batch and column testing
(2008)
Column leaching testing can be considered a better basis for assessing field impact data than any other available batch test method and thus provides a fundamental basis from which to estimate constituent release under a variety of field conditions. However, column testing is time-intensive compared to the more simplified batch testing, and may not always be a viable option when making decisions for material reuse. Batch tests are used most frequently as a simple tool for compliance or quality control reasons. Therefore, it is important to compare the release that occurs under batch and column testing, and establish conservative interpretation protocols for extrapolation from batch data when column data are not available. Five different materials (concrete, construction debris, aluminum recycling residue, coal fly ash and bottom ash) were evaluated via batch and column testing, including different column flow regimes (continuously saturated and intermittent unsaturated flow). Constituent release data from batch and column tests were compared. Results showed no significant difference between the column flow regimes when constituent release data from batch and column tests were compared. In most cases batch and column testing agreed when presented in the form of cumulative release. For arsenic in carbonated materials, however, batch testing underestimates the column constituent release for most LS ratios and also on a cumulative basis. For cases when As is a constituent of concern, column testing may be required.
When used in highway applications, granular materials are submitted to changing environmental conditions. While column testing can provide a closer approximation to field percolation conditions than batch testing, there is still a need to develop column testing procedures that more closely approximate realistic conditions. Current column studies are conducted under continuously saturated conditions, without considering unsaturated intermittent flow conditions that exist in the field in response to precipitation events. This study evaluates the effect of different types of column flow on the release of constituents from a granular material. Two different types of intermittent unsaturated flow were studied and compared to continuously saturated flow. Leaching data, including pH, conductivity, and constituent release was compared from the three types of columns. Results showed that there is no difference between the two types of intermittent unsaturated flow, nor is there a significant difference between these two flows and continuously saturated flow, other than higher initial concentrations of salts in intermittent unsaturated flow column testing at low LS ratios. However, this difference is not significant after a LS ratio of 2mL/g. Release of constituents was similar in all three cases. Also, all three cases show that after a LS ratio of 5mL/g there is no significant change in the pH, conductivity or release of constituents.