Effect of calcium and phosphorus on ammonium and nitrate nitrogen adsorption onto iron (hydr)oxides surfaces: CD-MUSIC model and DFT computation.

Calcium (Ca) and phosphorous (PO) significantly influence the form and effectiveness of nitrogen (N), however, the precise mechanisms governing the adsorption of ammonium nitrogen (NH-N) and nitrate nitrogen (NO-N) are still lacking. This study employed batch adsorption experiments, charge distribution and multi-site complexation (CD-MUSIC) models and density functional theory (DFT) calculations to elucidate the mechanism by which Ca and PO affect the adsorption of NH-N and NO-N on the goethite (GT) surface. The results showed that the adsorption of NH-N on the GT exhibited an initial increase followed by a decrease as pH increased, peaking at a pH of 8.5. Conversely, the adsorption of NO-N decreased with rising pH. According to the CD-MUSIC model, Ca minimally affected the NH-N adsorption on the GT but enhanced NO-N adsorption via electrostatic interaction, promoting the adsorption of ≡FeOH-NO and ≡FeO-NO species. Similarly, PO inhibited the adsorption of ≡FeOH-NO and ≡FeO-NO species. However, PO boosted NH-N adsorption by facilitating the formation of ≡FeO-NH via electrostatic interaction and site competition. DFT calculations indicates that although bidentate phosphate (BP) was beneficial to stabilize NH-N than monodentate phosphate (SP), SP-NH was the main adsorption configuration at pH 5.5-9.5 owing the prevalence of SP on the GT surface under site competition of NH-N. The results of CD-MUSIC model and DFT calculation were verified mutually, and provide novel insights into the mechanisms underlying N fixation and migration in soil.