Sorption and Transport of Bisphenol A (BPA) in a Palygorskite-Montmorillonite: Effect of Granule Size

Bisphenol A (BPA), a basic monomer used for synthesis of epoxy and polycarbonates, is a frequently detected endocrine disrupting compound in the environment. Palygorskite-montmorillonite (PM) has high sorption capacity for a variety of organic compounds and was studied as a potential inexpensive and recyclable supplementary wastewater treatment technology for removal of BPA. The emphasis was on determining an optimum granule size (0.6, 1.7-2.0 and about 2.8 mm) for maximizing water flow while still achieving contaminant attenuation. Laboratory batch sorption and column-transport experiments were conducted. Continuous-source, 1-D, BPA transport experiments along with bromide as a conservative tracer were run. Contaminant breakthroughs were simulated and transport and all reaction parameter values under flow-through conditions were estimated using CXTFIT, a USGS 1D contaminant-transport model. Parameter estimation was compared to and constrained by results from batch sorption experiments.

Kinetic and equilibrium sorption experiment data fit a pseudo-second-order kinetic and Freundlich sorption models, respectively. In addition to a higher overall R-squared value, the pseudo-second-order kinetic sorption model estimated a more accurate maximum sorption capacity. The rate of BPA sorption was faster during the first 50 minutes with an intermediate rate followed by very slow sorption before reaching an apparent equilibrium. We hypothesize that during the first stage, sorption dynamics was fast due to faster BPA diffusion rate across a liquid-solid boundary layer followed by a gradually slower sorption rate limited by intra-particle diffusion. The final stage of very slow sorption may indicate diffusion across the boundary layer and the intra-particle pore spaces.

Generally, the BPA sorption capacity was highest for the smallest granule material followed by the medium and the largest granules. BPA removal percentages varied from 50-70, 27-38, and 15-20 for the smallest, the medium, and the largest granules, respectively. Sorption-desorption hysteresis was observed by the three granules, indicating irreversible sorption process taking place.