FATE (SORPTION AND DESORPTION) AND TRANSPORT OF CARBAMAZEPINE IN A PALYGORSKITE- MONTMORILLONITE FILTER MEDIUM

Contamination of rivers and associated groundwater systems by pharmaceuticals and endocrine-disrupting compounds is a widespread problem due mainly to discharge from sewage treatment plants (STPs). One compound with potentially harmful ecosystem and human health impacts is carbamazepine (CMZ), an anticonvulsant and analgesic. Palygorskite-Montmorillonite (PM) clay is known for its high sorption capacity for a wide variety of organic compounds. In this study, PM was investigated as a potential STP filter material for CMZ.

Physicochemistry and mineralogy of the PM clay were characterized with SEM, TEM, XRD and BET-surface area. To optimize flow and retention capacity, batch sorption and laboratory-transport experiments were performed on three PM granule sizes: 0.3-0.6, 1.7-2.0 and ≈ 2.8 mm. Sodium azide and aluminum foil were used to suppress bio- and photo-degradation of CMZ. High performance liquid chromatography and an ultraviolent detector were used to analyze aqueous concentrations of CMZ. Batch experiments were used to develop sorption isotherms under a variety of pHs and ionic strengths commonly found in sewage effluent. Desorption experiments were run to determine if there was hysteresis indicating irreversible sorption. For transport experiments, CMZ and bromide solutions were passed through columns of the PM and column effluent was collected over time. Observed breakthrough curves were simulated with a 1-D advection-dispersion-reaction model, and results were compared with batch sorption results.

Due to the presence of some smectite clay minerals, the medium has a mix of fibrous and plate-like morphologies. Mean 1-point BET surface areas were 123, 114 and 92 m²/g for the large, medium and small granule sizes, respectively. All sorption isotherms were fit to Freundlich equation. Highest and lowest sorption capacities corresponded to the small and large granule sizes, respectively. Higher sorption was observed at elevated concentrations with the Freundlich exponent greater than one, indicating cooperative sorption where sorbed CMZ stimulated further sorption. Sorption was somewhat pH dependent, with less sorption at pH of 9.0 for dissolved concentrations up to 5.0 mg/L. There was a sorption/desorption hysteresis effect indicating irreversible sorption was occurring.