Enhanced nitrate and cadmium removal performance at low carbon to nitrogen ratio through immobilized redox mediator granules and functional strains in a bioreactor

The coexistence of multiple pollutants and lack of carbon sources are challenges for the biological treatment of wastewater. To achieve simultaneous removal of nitrate (NO3−-N) and cadmium (Cd2+) at low carbon to nitrogen (C/N) ratios, 2-hydroxy-1,4-naphthoquinone (HNQ) was selected from three redox mediators as an accelerator for denitrification of heterotrophic strain Pseudomonas stutzeri sp. GF2 and autotrophic strain Zoogloea sp. FY6. Then, halloysite nanotubes immobilized with 2-hydroxy-1,4-naphthoquinone (HNTs-HNQ) were prepared and a bioreactor was constructed with immobilized redox mediator granules (IRMG) as the carrier, which was immobilized with HNTs-HNQ and inoculated with the two strains. The immobilized HNQ and the inoculated strains jointly improved the removal ability of NO3−-N and Cd2+ and the removal efficiency of NO3−-N (25.0 mg L−1) and Cd2+ (5.0 mg L−1) were 92.81% and 93.94% at C/N = 1.5 and hydraulic retention time (HRT) = 4 h. The Cd2+ was removed by adsorption of iron oxides (FeO(OH) and Fe3O4) and IRMG. The electron transport system activity (ETSA) of bacteria was improved and the composition of dissolved organic matter in the effluent was not affected by HNQ. The HNQ promoted the production of FeO(OH) and up-regulated the proportion of Zoogloea (54.75% in the microbial community), indicating that Zoogloea sp. FY6 was dominant in the microbial community. In addition, HNQ influenced the metabolic pathways and improved the relative abundance of some genes involved in nitrogen metabolism and the iron redox cycle.