Continuous-flow photocatalytic treatment of pharmaceutical micropollutants: Activity, inhibition, and deactivation of TiO2 photocatalysts in wastewater effluent

Titanium dioxide (TiO2) photocatalysts have been shown to be effective at degrading a wide range of organic micropollutants during short-term batch experiments conducted under ideal laboratory solution conditions (e.g., deionized water). However, little research has been performed regarding longer-term photocatalyst performance in more complex matrices representative of contaminated water sources (e.g., wastewater effluent, groundwater). Here, a benchtop continuous-flow reactor was developed for the purpose of studying the activity, inhibition, and deactivation of immobilized TiO2 photocatalysts during water treatment applications. As a demonstration, degradation of four pharmaceutical micropollutants (iopromide, acetaminophen, sulfamethoxazole, and carbamazepine) was monitored in both a pH-buffered electrolyte solution and a biologically treated wastewater effluent (WWE) to study the effects of non-target constituents enriched in the latter matrix. Reactor performance was shown to be stable over 7 d when treating micropollutants in buffered electrolyte, with 7-d averaged k(obs) values (acetaminophen = 0.97 +/- 0.10 h(-1): carbamazepine = 0.50 +/- 0.04 h(-1); iopromide = 0.49 +/- 0.03 h(-1); sulfamethoxazole = 0.79 +/- 0.06 h(-1)) agreeing closely with measurements from short-term circulating batch reactions. When reactor influent was switched to WWE, treatment efficiencies decreased to varying degrees (acetaminophen = 40% decrease; carbamazepine = 60%; iopromide = 78%; sulfamethoxazole = 54%). A large fraction of the catalyst activity was recovered upon switching back to the buffered electrolyte influent after 4d. suggesting that much of the observed decrease resulted from reversible inhibition by non-target constituents (e.g., scavenging of photocatalyst-generated (OH)-O-center dot). However, there was also a portion of the decrease in activity that was not recovered, indicating WWE constituents also contributed to photocatalyst deactivation (acetaminophen = 6% deactivation; carbamazepine = 24%; iopromide = 16%; sulfamethoxazole = 25%). Experiments conducted using pretreated WWE and synthetic WWE mimic solutions indicated that both effluent organic matter and inorganic constituents in WWE contributed to the observed photocatalyst inhibition/deactivation. Analysis of immobilized TiO2 thin films after 4 d of continuous treatment of the WWE matrix indicated minor deterioration of the porous film and formation of surface precipitates enriched in Al and Ca. Results demonstrated the marked influence of non-target constituents present in complex matrices on long-term photocatalyst activity and highlighted the need for further study of this important issue to advance the development of practical photocatalytic water treatment technologies. (C) 2012 Elsevier B.V. All rights reserved.