Effect of zero-valent iron (ZVI) on nitrogen and Fe(II) metabolism in anammox-hydroxyapatite (HAP) system under low-temperature stress

Low temperatures could inhibit the activity of anaerobic ammonium oxidizing bacteria (AnAOB). This study investigated the effect of exogenous zero-valent iron (ZVI) on Anammox-hydroxyapatite (HAP) system at 17 +/- 1 degrees C. The results showed that long-term addition of ZVI increased the nitrogen removal efficiency by 8.59 %, and alleviated the inhibition of AnAOB under the condition of low temperature. After addition of ZVI, the activity of AnAOB and the heme c content increased by 58.20 % and 4.83 %, respectively. Moreover, the functional enzyme activities of NIR, HZS, and HZO increased by 94.12 %, 36.36 %, and 16.67 %, respectively. The concentrations of quorum sensing molecules, i.e., C6-HSL and C8-HSL, increased by 191.17 % and 41.54 %, respectively, after ZVI addition. Metagenomic analysis revealed that the addition of 5 g/L ZVI stimulated the relative abundance of Fe (II) metabolism-related genes in AnAOB at 17 +/- 1 degrees C, thereby contributing to the uptake, transport, utilization, and storage of Fe(II). In summary, the addition of ZVI could improve the nitrogen removal performance of the anammox-HAP system at the low temperature.