Retardation coefficients of nonionic organic compounds determined by batch and column techniques

Batch and column techniques were utilized to determine retardation coefficients of two nonionic organic compounds: benzene and dimethylphthalate. Three sandy soil materials with medium to high organic C content were used as sorbents. Batch data consistently overestimated retardation coefficients of the two compounds. Several previously reported factors that may cause discrepancy between the two techniques were experimentally investigated. Sorption nonsingularity, sorption nonequilibrium, presence of immobile water regions in the column, and reduction in particle spacing in the column were eliminated as the cause of this discrepancy. Loss of sorbent from the column is unlikely to be the cause of the discrepancy. Although rapidly mixed batch tubes are subjected to soil abrasion, it causes no apparent impact on the value of sorption distribution coefficients. Batch isotherms at two solids concentrations were identical, indicating that differences in solids concentration between batch and column setup is not a significant factor. This was also confirmed by a column study. Sorption nonlinearity was found to have a minor impact. Column residence time had a major impact on one of the cases studied, but no effect on the other cases. When column residence time was accounted for, some differences in sorption distribution coefficients, which seemed to be independent of the organic C content, were noticed. In this study, all previously suggested causes for the discrepancy between batch-and column-determined retardation coefficients were investigated and rejected. It remains unclear why the values determined by the two techniques were different.