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 kobs 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 4 d, suggesting that much of the observed decrease resulted from reversible inhibition by non-target constituents (e.g., scavenging of photocatalyst-generated OH). 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.

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► Benchtop continuous-flow reactor with thin film TiO2 photocatalysts developed.
► Treatment of four model pharmaceutical micropollutants simultaneously monitored.
► Catalysts inhibited and deactivated by non-target wastewater effluent constituents.
► A large fraction of effluent-promoted catalyst inhibition reversible after 4 d.