Selenium removal by activated alumina in batch and continuous-flow reactors

Selenium removal by activated alumina (AA) in batch and continuous-flow reactors was investigated in this study. The adsorption kinetics fitted pseudo-first as well as pseudo-second order models with equilibrium time for Se(VI) and Se(IV) adsorption of 8 and 12 hr, respectively. A significant higher adsorption capacity for Se(IV) than Se(VI) was observed in isothermal adsorption experiments. The adsorption isotherms of Se(VI) and Se(IV) agreed well with both Langmuir and Freundlich adsorption models. The results also showed that Se(VI) adsorption was adversely affected by NaHCO₃ concentrations, while Se(IV) adsorption was not due to the selectivity of activated alumina for anions. In the continuous-flow reactor packed with AA and inoculated with Shigella fergusonii strain TB42616 under a hydraulic detention time of 3 days, approximately 74% and 70% Se(VI) were removed after 40 days at influent concentrations of 10 and 50 mg/L, respectively. Effluent concentrations of Se(IV) from the bioreactor were insignificant due to the combination of bioactivities and adsorption. The AA-packed bioreactor is promising in both Se(VI) and Se(IV) removal as better removal efficiency may be accomplished by increasing the liquid retention time. PRACTITIONER POINTS: The adsorption capacity for Se(IV) of activated alumina was significantly higher than that for Se(VI). Se(VI) adsorption was inhibited by NaHCO₃ , while Se(IV) adsorption was not. Se(IV) formed by biological Se(VI) reduction was adsorbed by activated alumina in continuous-flow reactors. Both Se(VI) and Se(IV) were significantly removed by activated alumina-packed continuous-flow reactors.