Mineral Surface Modification of d-MnO2 Decreases Bisphenol a Oxidation Rate.

Bisphenol A is an endocrine-disrupting compound that is widely used in plastic products and is found in natural waters at concentrations that are considered harmful for aquatic life. BPA is often introduced into the environment by wastewater treatment plant effluent and landfill leachate. Current approaches for BPA removal are focused on drinking water treatment systems and involve advanced oxidation processes. Mn(III/IV) oxides are commonly found in near-surface environments and their ability to degrade phenolic contaminants (e.g., BPA) is widely acknowledged. However, in subsurface environments, mineral surface modification and product accumulation may limit BPA degradation. Here we quantify changes in the degradation rate of BPA by d-MnO₂, a synthetic analog of naturally-occurring Mn(IV) oxides, during sequential additions of 20 µM BPA to 1.3 g/L MnO₂. We find that BPA degradation rate can be modeled with pseudo-first order kinetics and decreases by a factor of 20 from the first addition to the twelfth BPA addition. Characterization of solids sampled throughout the sequential additions using extended X-ray absorption fine structure (EXAFS) spectroscopy reveals an increase in the fractional occupancy of the Mn-layers, indicative of Mn(III) accumulation. Interestingly, relatively little dissolved Mn(II) is observed, even after 12 sequential BPA additions. Our results demonstrate that degradation of BPA by environmentally relevant Mn(IV) oxides initially proceeds rapidly and the buildup of inorganic and possibly organic products may ultimately limit BPA oxidation. Nevertheless, Mn(IV) oxides may provide a means for limiting BPA migration in subsurface environmental systems.