Reducing the inhibitive effect of fluorine and heavy metals on nitrate reduction by hydroxyapatite substrate in constructed wetlands.

Bio-toxic inorganic pollutants, e.g., fluorine (F) and heavy metals (HMs), in wastewaters are the potential threats to nitrate (NO-N) reduction by microorganisms in constructed wetlands (CWs). Selection of suitable substrate with high F and HMs adsorption efficiency and capacity is a potential alternative for simultaneous removal of these pollutants in CWs. Herein, this study investigated the feasibility of applying hydroxyapatite (HA)-gravel media for F and HMs adsorption and its effect on NO-N reduction in CWs (HA CWs) by comparing the CWs filled with gravel substrate (CK CWs). The results indicated that the removal efficiency of F, Cr, As, and NO-N in HA CWs increased by 113.6-, 3.3-, 2.7-, and 0.6-folds, respectively, compared to CK CWs. The NO-N reduction rate decreased by 11-46% in CK CWs after the presence of F and HMs in influent, while for HA CWs, it was only 13-22%. Excellent F and HMs adsorption capacity of HA substrate availed for wetland plants resisting F/HMs toxicity and making catalase activity lower. The HA substrate in CWs resulted in the certain succession of nitrogen-transforming bacteria, e.g., nitrifiers (Nitrospira) and denitrifiers (Thiobacillus and Desulfobacterium). More importantly, key functional genes, including nirK/nirS, korA/korB, ChrA/ChrD, arsA/arsB, catalyzing the processes of nitrogen biotransformation, energy metabolism, NO-N and metal ions reduction were also enriched in HA CWs. This study highlights HA substrate reduce the inhibitive effect of F and HMs on NO-N reduction, and provides new insights into how microbiota structurally and functionally respond to different substrates in CWs.