Microbial Enhancement of Selenium Removal in Chemically Modified Zeolite Columns

Abstract

Selenium (Se) is an essential micro-nutrient for living organisms, but elevated concentrations in water can adversely affect health. In this research, we investigate the removal of selenium oxyanions (selenate and selenite) in aqueous systems by integration of adsorption on modified zeolites and microbial reduction. Dynamic sorption-reduction experiments were conducted using two sets of zeolite columns for the removal of selenite and selenate oxyanions, respectively. In each case, one column was fully packed with natural, unmodified zeolites, while the other column was composed of 80% natural and 20% iron-coated zeolites, by mass. The initial selenium concentration, selenite (SeIV) or selenate (SeVI), was 790 μg/L, the pH was 7.5, and the flow rate was 3 mL/min. Initially, as expected, the higher selenate removal (34%) was observed with coated zeolite, twice as high compared to the results with unmodified zeolite. Maximum selenite removal was 89% in the column with modified zeolite. Within approximately 14 days, as the biofilm developed inside the columns, selenium reduction in all four columns reached approximately 99%. Biofilm microbial community composition, assessed by 16S rRNA sequencing, is consistent with the presence of mainly selenium-reducing bacteria (Veillonella, Bacteroides, and Escherichia). Selenium oxyanions were reduced to elemental selenium, visible within the bioreactors as red-color aggregates.