Competitive adsorption of nitrate, phosphate, and sulfate on amine-modified wheat straw: In-situ infrared spectroscopic and density functional theory study

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 NO3, SO4, and PO4 revealed in this study is helpful to understand and predict the applications of AMWS.