Competitive adsorption of nitrate, phosphate, and sulfate on amine-modified wheat straw: In-situ infrared...

Amine-modified wheat straw (AMWS) has already been reported as a promising adsorbent for nitrate (NO₃) removal due to its cost-effectiveness and high efficiency. However, the NO₃ removal mechanism has not been well understood, especially in the presence of co-existing ions. Here, the effect of co-existing anions on NO₃ removal by AMWS was investigated and the underlying mechanisms were revealed using a combination of in-situ infrared (IR) spectroscopy and computational modeling. The in-situ IR results indicated that NO₃, sulfate (SO₄), and phosphate (PO₄) are all adsorbed as outer-sphere complexes on AMWS. The two-dimensional-correlation spectroscopy analysis implied the adsorption sequence of SO₄ > PO₄ > NO₃. The adsorption energies obtained from density functional theory calculation range from -0.24 to 0.51 eV (-23.2 to 49.2 kJ/mol), confirming that these anions adsorb on AMWS as outer-sphere complexes. For the first time, this study provides direct spectroscopic evidence of the outer-sphere adsorption of NO₃ on AMWS, as well as identifies the adsorption sequence, confirmed by computational modeling. The competitive mechanism of NO_3, SO₄, and PO₄ revealed in this study is helpful to understand and predict the applications of AMWS.