Insight into Organic Pollutant Adsorption Characteristics on a g‑C3N4 Surface by Attenuated Total Reflection Spectroscopy and Molecular Dynamics Simulation

Herein the adsorption characteristics of zwitterionic dye pollutant Rhodamine B (Rh+B–) on a g-C3N4 surface were investigated by both an attenuated total reflection spectroscopy (ATRS) experiment and a molecular dynamics simulation (MDS). For experimental investigation, g-C3N4 was coated on a silica optical fiber (SOF) surface to fabricate an adsorption film. According to the ATRS response, adsorption thermodynamics and thermodynamics results were in situ obtained and evaluated. The isothermal Langmuir model was used to calculate the adsorption equilibrium constants (K ads) and adsorption energies (ΔG ads) for Rh+B– as 27.25 × 104 M–1 and −31.01 kJ mol–1, respectively, which indicated the spontaneous adsorption behavior of Rh+B– at the g-C3N4 surface. Using dynamic Elovich modeling, the rate constants of Rh+B– were found to be k 1 = 0.0063 min–1 and k 2 = 0.0004 min–1, which indicated two-stage adsorption at the g-C3N4 surface. For theoretical simulation, adsorption configurations and adsorption energies were systematically calculated by a molecular dynamics simulation (MDS) . Rh+B– molecules were inclined to orient in a parallel position at the g-C3N4 surface during low concentration but a perpendicular position at the g-C3N4 surface during high concentration. Combined with experimental and calculation results, this work revealed the microscopic adsorption performance and elucidated the intermolecular interaction between localized interfaces of g-C3N4 and hazardous dye pollutant. We propose an adsorption model to explain the process of surface interaction, which is based on molecular orientation and a force-driven mechanism. Electrostatic attraction and π–π interaction dominated the adsorption interaction with an adsorption energy of ΔG low(ads) = −38.96 kJ mol–1 for low Rh+B– concentration, and electrostatic attraction dominated the adsorption interaction with an adsorption energy of ΔG high(ads) = −25.76 kJ mol–1 for high Rh+B– concentration. This work can provide a fundamental basis for a dye-pollutants removal application by g-C3N4 in both adsorption and photocatalyzation.