Nonequilibrium sorption and transport of volatile petroleum hydrocarbons in surfactant-modified zeolite.

We characterized the nonequilibrium sorption and transport of benzene, toluene, ethylbenzene, and xylenes (BTEX) by surfactant-modified zeolite (SMZ) in batch and column tests. The SMZ was shown in previous studies to be an effective sorbent for removal of BTEX from oilfield wastewaters prior to disposal or reuse. A two-site, first-order chemical nonequilibrium model was used to determine sorption parameters from the batch results. Individual BTEX linear sorption coefficients, K(d), ranged from 7.5 to 37 L kg(-1) and were independent of BTEX concentration or competing solutes, suggesting that partitioning was the mechanism of sorption. The K(d) values were the same whether the zeolite was covered by a monolayer or bilayer of the surfactant hexadecyltrimethylammonium (HDTMA). Batch rate coefficients and the fraction of "instantaneous" sorption sites decreased with BTEX hydrophobicity and with total BTEX concentration. The fraction of "instantaneous" sites was 3-11 times greater for the monolayer as compared to the bilayer SMZ. These observations are consistent with a conceptual model in which BTEX are rapidly partitioned into hydrophobic monolayer surfaces and more slowly partitioned to hydrophilic bilayer surfaces. Results from the batch experiments were used to predict BTEX transport through columns of SMZ. Batch-derived rate and site-distribution parameters accurately described the transport dynamics, but the batch-derived K(d)s significantly underestimated BTEX retardation. Excess dissolved HDTMA in the batch experiments likely led to anomalously low K(d) values for those determinations.