Functional group-modified biochar synergistically enhances nitrogen removal in iron-based constructed wetlands: The pivotal role of quinone groups in mediating iron redox and microbial electron transfer
Insufficient electron donor supply severely bottlenecks advanced nitrogen removal in iron-based constructed wetlands (ICWs) treating low carbon-to-nitrogen wastewater. To overcome this limitation, this study initially screened three functional chemical agents--anthraquinone-2,6-disulfonate (AQS), sodium humate (SH), and polypyrrole (PPy)--to modify reed biochar (BC) as core matrix-reinforcing materials (AQS/BC, SH/BC, PPy/BC). Performance evaluation showed that the SH/BC-ICW achieved the highest maximum total nitrogen removal rate of 57.82 +/- 2.48% (compared to 54.05 +/- 2.45% for AQS/BC-ICW and 50.33 +/- 1.25% for PPy/BC-ICW). Subsequent material characterization and chemical analysis revealed that, among various altered surface groups, the relative attachment increase of active quinone group (Q) was the pivotal determinant, with SH/BC exhibiting the highest Q enrichment (35.0%). This Q enrichment on BC significantly accelerated the Fe2+/TFe redox cycle within the ICWs. Metagenomic sequencing further elucidated that the Q-mediated electron transfer and iron cycling substantially enriched key functional denitrifiers and iron-metabolizing bacteria, specifically Rubrivivax, Hydrogenophaga, Gordonia, and Thermomonas. Overall, this study demonstrated a systematic selection of functionalized BC for ICW intensification and uncovered the underlying mechanism of Q in driving iron-microbe electron transfer, providing an innovative strategy for advanced wastewater purification.