RETARDATION COEFFICIENTS OF NONIONIC ORGANIC-COMPOUNDS DETERMINED BY BATCH AND COLUMN TECHNIQUES

Batch and column techniques were utilized to determine retardation coe fficients of two nonionic organic compounds: benzene and dimethylphtha late. Three sandy soil materials with medium to high organic C content were used as sorbents. Batch data consistently overestimated retardat ion coefficients of the two compounds. Several previously reported fac tors that may cause discrepancy between the two techniques were experi mentally investigated. Sorption nonsingularity, sorption nonequilibriu m, presence of immobile water regions in the column, and reduction in particle spacing in the column were eliminated as the cause of this di screpancy. 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 sorptio n distribution coefficients. Batch isotherms at two solids concentrati ons were identical, indicating that differences in solids concentratio n 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 o f the cases studied, but no effect on the other cases. When column res idence time was accounted for, some differences in sorption distributi on coefficients, which seemed to be independent of the organic C conte nt, were noticed. In this study, all previously suggested causes for t he discrepancy between batch-and column-determined retardation coeffic ients were investigated and rejected. It remains unclear why the value s determined by the two techniques were different.