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

Calcium (Ca ²⁺ ) and phosphorous (PO ₄ ^3- ) significantly influence the form and effectiveness of nitrogen (N), however, the precise mechanisms governing the adsorption of ammonium nitrogen (NH ₄ ⁺ -N) and nitrate nitrogen (NO ₃ ^- -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 ²⁺ and PO ₄ ^3- affect the adsorption of NH ₄ ⁺ -N and NO ₃ ^- -N on the goethite (GT) surface. The results showed that the adsorption of NH ₄ ⁺ -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 ₃ ^- -N decreased with rising pH. According to the CD-MUSIC model, Ca ²⁺ minimally affected the NH ₄ ⁺ -N adsorption on the GT but enhanced NO ₃ ^- -N adsorption via electrostatic interaction, promoting the adsorption of ≡FeOH-NO ₃ ^- and ≡Fe ₃ O-NO ₃ ^- species. Similarly, PO ₄ ^3- inhibited the adsorption of ≡FeOH-NO ₃ ^- and ≡Fe ₃ O-NO ₃ ^- species. However, PO ₄ ^3- boosted NH ₄ ⁺ -N adsorption by facilitating the formation of ≡Fe ₃ O-NH ₄ ⁺ via electrostatic interaction and site competition. DFT calculations indicates that although bidentate phosphate (BP) was beneficial to stabilize NH ₄ ⁺ -N than monodentate phosphate (SP), SP-NH ₄ ⁺ 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 ₄ ⁺ -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.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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