CHARACTERIZATION OF NONEQUILIBRIUM SORPTION OF PRODUCED-WATER COMPONENTS BY A SURFACTANT-MODIFIED ZEOLITE

Surfactant-modified zeolite (SMZ) has been shown to effectively remove benzene, toluene, ethylbenzene, and xylenes (BTEX) from water generated during oil and gas production (produced water). The BTEX sorption isotherms are linear and noncompetitive, suggesting that the removal mechanism is partitioning into the surfactant's hydrophobic bilayer. Even though BTEX sorption in batch systems is rapid, chemical equilibrium models do not accurately describe BTEX transport through packed beds of SMZ. Comparison with transport of a nonreactive tracer (tritium) suggests that diffusive nonequilibrium sorption controls BTEX transport.

We conducted individual batch experiments with SMZ to determine the nonequilibrium sorption kinetics of each BTEX constituent. We also ran a combined batch experiment containing all BTEX compounds in order to quantify kinetic sorption variations due to competition among the multiple constituents.

A bicontinuum sorption model was fit to the batch experimental data. The model estimated three sorption parameters: the distribution coefficient, fraction of instantaneous sorption, and the sorption rate constant. The distribution coefficient was directly related to the octanol/water partition coefficient (Kow) of the BTEX compounds and was not affected by constituent competition. The rate constant and fraction of instantaneous sorption were inversely related to the Kow. The fraction of instantaneous sorption did not show constituent competition during the combined test; however, the rate constant decreased by 40% for the combined test. These kinetic sorption results are being used to parameterize a nonequilibrium transport model to predict BTEX removal under varying flow conditions. The accuracy of predictions is being tested in laboratory column experiments using produced water from the San Juan Basin, New Mexico.