Deactivation and regeneration of immobilized titanium dioxide photocatalysts during treatment of pharmaceutical micropollutants in groundwater
As detection methods improve, a number of classes of emerging contaminants are being detected in natural waters, including many pharmaceuticals and personal care products (PPCPs). Many of these chemicals are recalcitrant to conventional treatment processes, so new treatment methods are being investigated. Titanium dioxide (TiO2)-based photocatalytic treatment has proven to be an effective method for degrading trace organic contaminants, including PPCPs. However, most studies on photocatalytic treatment of PPCPs to date have been conducted in short-term batch experiments using fresh catalysts in laboratory solutions devoid of non-target constituents that are often abundant in natural water matrices (e.g., Ca2+, HCO3-, natural organic matter). In this contribution, we describe the results of an investigation of the long-term stability and deactivation of immobilized TiO2 photocatalysts used to treat PPCPs in groundwater (GW). GW spiked with four model PPCPs (5 µg/L atenolol, sulfamethoxazole, carbamazepine, and 10 µg/L iopromide) was treated with immobilized thin films of TiO2 coated on glass slides under UV-A light in a serpentine plug-flow reactor. Initially, catalysts achieved 50-75% degradation of influent PPCPs using a 2-hr reactor residence time (higher removal can be achieved using longer residence times). Over one month of continuous operation, catalyst films developed visual discoloration and the extent of PPCP removal in the reactor diminished, eventually reaching complete catalyst deactivation for some of the target PPCPs. Calcite (CaCO3(s)) with smaller quantities of iron and copper were detected on the surface of deactivated catalyst films when analyzed by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Treatment of deactivated catalysts with 10 mM hydrochloric acid was able to restore catalyst activity to pre-GW levels, so introduction of recurrent acid washing stages during GW treatment were investigated to prolong catalyst activity. Pretreatment of GW using a sodium-loaded cation exchange softening resins was investigated to eliminate calcite precipitation in the reactor. Although calcite precipitation was eliminated, catalysts continued to experience a similar loss of activity observed for unsoftened GW. Analysis of catalyst surfaces after exposure to softened groundwater indicates deposits of zinc, copper, iron, and manganese. Results suggest that both physical blocking of active sites by calcite surface precipitates and adsorption of trace metals contribute to catalyst deactivation. Further work is needed to investigate other pretreatment methods such as pH modification to prevent surface deposition of catalyst-deactivating metal species on the TiO2 surface and prolong reactor activity between acid washing regeneration stages.
pharmaceuticals and personal care products (PPCPs), pharmaceutical micropollutants, titanium dioxide, photocatalysis, environmental engineering, advanced oxidation processes, deactivation
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign