Biocatalytic removal of perchlorate and nitrate in ion-exchange waste brine

Abstract

Biocatalytic technologies are characterized by targeted, rapid degradation of contaminants over a range of environmentally relevant conditions representative of groundwater, but have not yet been integrated into drinking water treatment processes. This work investigated the potential for a hybrid ion-exchange/biocatalytic process, where biocatalysis is used to treat ion-exchange waste brine, allowing reuse of the brine. The reduction rates and the fate of the regulated anions perchlorate and nitrate were tested in synthetic brines and a real-world waste brine. Biocatalysts were applied as soluble protein fractions from Azospira oryzae for perchlorate reduction and Paracoccus denitrificans and Haloferax denitrificans for nitrate reduction. In synthetic 12% brine, the biocatalysts retained activity, with rates of 32.3 ± 6.1 U (μg Mo)⁻¹ for perchlorate (A. oryzae) and 16.1 ± 7.1 U (μg Mo)⁻¹ for nitrate (P. denitrificans). In real-world waste brine, activities were slightly lower (20.3 ± 6.5 U (μg Mo)⁻¹ for perchlorate and 14.3 ± 3.8 U (μg Mo)⁻¹ for nitrate). The difference in perchlorate reduction was due to higher concentrations of nitrate, bicarbonate, and sulfate in the waste brine. The predominant end products of nitrate reduction were nitrous oxide or dinitrogen gas, depending on the source of the biocatalysts and the salt concentration. These results demonstrate biocatalytic reduction of regulated anions in a real-world waste brine, which could facilitate brine reuse for the regeneration of ion-exchange technologies and prevent reintroduction of these anions and their intermediates into the environment.