Ultrafast removal of humic acid by amine-modified silica aerogel: Insights from experiments and density functional theory calculation

The efficient and rapid treatment of natural organic matter is a difficult task in wastewater treatment, especially for highly concentrated humic acid (HA). In this work, using agricultural waste rice husk ash (RHA) as silica source, a facile one-pot sol-gel synthesis was introduced to prepare an novel adsorbent amine-modified silica aerogel (AMSA) for HA removal. The characterization demonstrated the amine groups were successfully grafted on the surface of AMSA. Even at the high concentration (2000 mg/L) low adsorbent dosage (1 g/L) and short reaction time (15 min), the AMSA exhibited an excellent adsorption capacity (1749.7 mg/g) on adsorption of HA, increased 184.2 times compared with the pristine silica aerogel (SA). The removal efficiency of HA increased from 84.0% to 92.0% when the pH of the solution increased from 3.0 to 5.0, and it remained at a high level when the pH increased from 5.0 to 9.0. The removal of HA on AMSA shows the high stability in the exist of different interfering cations (Na+, NH4+, Mg2+) and anions (Cl-, CO32-, and SO42-) with different concentrations. Kinetic experiments showed that the adsorption process fit the pseudo-second-order kinetic model and the isotherms of the adsorption process matched the Freundlich model, indicating the non-homogeneity of the adsorption process. According to density functional theory (DFT) calculation results, the AMSA has the most negative adsorption energy -273.15 kCal/mol, indicating that this configuration is the most stable. The electrostatic interaction, O-H...N hydrogen bonding, and self-aggregation were the main adsorption mechanism for the rapid and high capacity adsorption of HA by AMSA.

Automated summary

A facile one-pot sol-gel synthesis was used to prepare an amine-modified silica aerogel (AMSA) using agricultural waste rice husk ash as a silica source. The AMSA exhibited excellent adsorption capacity for highly concentrated humic acid (HA). The adsorption process followed the pseudo-second-order kinetic model and the Freundlich model, and the main mechanisms were electrostatic interaction, O-H...N hydrogen bonding, and self-aggregation.