Effect of calcium and phosphorus on ammonium and nitrate nitrogen adsorption onto iron (hydr)oxides surfaces:...

Calcium (Ca²⁺) and phosphorous (PO₄^3-) significantly influence the form and effectiveness of nitrogen (N), however, the precise mechanisms governing the adsorption of ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃^--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 Ca²⁺ and PO₄^3- affect the adsorption of NH₄⁺-N and NO₃^--N on the goethite (GT) surface. The results showed that the adsorption of NH₄⁺-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 NO₃^--N decreased with rising pH. According to the CD-MUSIC model, Ca²⁺ minimally affected the NH₄⁺-N adsorption on the GT but enhanced NO₃^--N adsorption via electrostatic interaction, promoting the adsorption of ≡FeOH-NO₃^- and ≡Fe₃O-NO₃^- species. Similarly, PO₄^3- inhibited the adsorption of ≡FeOH-NO₃^- and ≡Fe₃O-NO₃^- species. However, PO₄^3- boosted NH₄⁺-N adsorption by facilitating the formation of ≡Fe₃O-NH₄⁺ via electrostatic interaction and site competition. DFT calculations indicates that although bidentate phosphate (BP) was beneficial to stabilize NH₄⁺-N than monodentate phosphate (SP), SP-NH₄⁺ was the main adsorption configuration at pH 5.5-9.5 owing the prevalence of SP on the GT surface under site competition of NH₄⁺-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.