Mechanistic Study of Selective Adsorption of As(V) by a Surface Ion-Imprinted Diatomite-Based Adsorbent

In this study, a novel As(V) ion-imprinted and 4-picolinic acid (4-PA)-grafted porous diatomite (DE)-based adsorbent (As(V)-IID) was prepared by using the surface ion imprinting technique. The results of the selective adsorption experiment show that the adsorption capacity of As(V) on As(V)-IID reaches 62.78 mg/g in a solution with an initial As(V) concentration of 25 mg/L and a pH value that is near pH 4.0 of the imprinting. The selectivity coefficients (K) of As(V) with respect to Cr(VI) and Mo(VI) were up to 12.14 and 27.79, respectively. The binding process was described well by the pseudo-second-order kinetics and Weber–Morris and Langmuir models. Fourier transform infrared spectra (FTIR), scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDS), and X-ray photoelectron spectroscopy (XPS) analyses suggested that the nitrogen of secondary amide and pyridine was the main imprinting site and played a crucial role in the selectivity of binding of As(V) to As(V)-IID. To further validate the sorption mechanisms, density functional theory (DFT) was employed to study the properties of binding of As(V), Cr(VI), and Mo(VI) with As(V)-IID. The computational results showed that the As(V) was selectively recognized by complexing with N-containing functional groups of the As(V)-IID surface into a tridentate mononuclear configuration with a minimal binding energy (Eb) of −18.16 eV, which was completely consistent with characterization analyses. This work revealed the selective adsorption mechanisms of As(V)-IID for As(V) and provided new insight into the selective recognition of oxyanions from water by ion-imprinted adsorbents.