Natural pyrite improves nitrate removal in constructed wetlands and makes wetland a sink for phosphorus in cold climates

Pyrite, a natural ore was introduced into constructed wetlands (CWs) to intensify the denitrification and phosphorus deposition in cold climate conditions. Here, CWs with matrices of pyrite/volcanic rock ratios (m/m) of 0:1(CW–V), 1:10 (CW–P1), 1:5 (CW–P2), 1:1 (CW–P3) and 2:1 (CW–P4) were set up. Compared with the control group (CW–V), CW-P1-4 produced a gradual increase of nitrate-nitrogen (NO3−-N) and total nitrogen (TN) removal by 6.4–19.9% and 6.2–17.5%, respectively, under a 24 h-hydraulic retention time (HRT). Accordingly, the pyrite based autotrophic denitrification (PAD) contributed to 6.7–24.7% of the TN removal in CW-P1-4. For phosphate-phosphorus (PO43--P) and total phosphorus (TP) removal, CW-P3 and CW-P4 shared the highest efficiency, which was approximately 60% higher than that in CW-V. The main deposition form of phosphorus in CW-P1-4 was iron and aluminium bound phosphorus (Fe/Al–P), which accounted for 43.2–74.2%. By contrast, calcium and magnesium bound phosphorus (Ca/Mg–P) was the major form in CW-V. In addition, sequencing data implied that Simplicispira was the most abundant heterotrophic denitrifiers in CWs in cold climate. Thiobacillus, which is involved in PAD, was highly enriched only in the rhizosphere (6.9%) and matrix (7.3%) of CW with pyrite.