Montmorillonite supported nanoscale zero-valent iron immobilized in sodium alginate (SA/Mt-NZVI) enhanced the nitrogen removal in vertical flow constructed wetlands (VFCWs).

•The novel material of SA/Mt-NZVI was used as electron donor-acceptor compounds.•Adding SA/Mt-NZVI in VFCWs increased the nitrogen removal efficiency.•The nitrate nitrogen removal efficiency was 10–20% higher in 3 d after adding SA/Mt-NZVI.•It is feasible to add SA/Mt-NZVI as an extra carbon source to treat polluted water.