Effects of ionic strength, pH value and humic acid on the settlement of graphitic carbon nitride

Graphitic Carbon Nitride (g-C3N4) has widely been used on the photocatalytic degradation of agricultural organic pollutants (e.g. pesticides, polycyclic aromatic hydrocarbons, and antibiotics). Therefore, it is inevitably entering into the agricultural environment soil, leading to potential risks. This study aims to better evaluate and then predict the photocatalytic efficiency and environmental risk of g-C3N4 application. Experiment, simulation, and theoretical calculation were conducted to investigate the settlement and dispersion stability of g-C3N4 in the aqueous environment. Three typical water factors were also considered, including ionic strength, pH values, and humic acid concentration. Experimental data showed that the water ionic strength was the most important factor in the settlement and dispersion stability of g-C3N4. A one-site kinetic settlement model was also well established to fit the settlement rate data of g-C3N4 from the experimental measurement. The extended DLVO theory was selected to explain the energy distribution between g-C3N4 particles in the aqueous environment. The settlement of g-C3N4 was remarkably enhanced with the increasing ionic strength, thereby reducing the dispersion stability. Specifically, the final standardized concentration (after 360 min) of g-C3N4 suspension decreased from 0.86 to 0.58, while the fitted settlement rate increased from 0.014 4 to 0.019 1 cm/min, as well as the Zeta potential of g-C3N4 particles increased from -37.1 to -12.7 mV, with the ionic strength increased from 0 to 50 mmol/L. The variation of Zeta potential indicated that the charge shielding of g-C3N4 particles increased with the increasing ionic strength, leading to the compressed electrical double layer and enhanced aggregation of g-C3N4 for better settlement and reduced dispersion stability. Additionally, the water pH showed a relatively low impact on the settlement and dispersion stability of g-C3N4. The settlement of g-C3N4 was firstly enhanced and then reduced with the increasing pH values. Specifically, the final standardized concentration (after 360 min) of g-C3N4 suspension firstly decreased from 0.63 to 0.57, as the pH values increased from 2 to 4, and then increased from 0.57 to 0.78 with the pH values further increased from 4 to 10. Particularly, the tendency of fitted settlement rate was well consistent with the experimental measurement. By contrast, the hydrodynamic radius of g-C3N4 also firstly increased from 1 116 to 1 271 nm, as the pH values increased from 2 to 4, whereas, then decreased from 1 271 to 862 nm with the pH values further increased from 4 to 10. The Zeta potential of g-C3N4 particles decreased from 18.6 to -57.0 mV with the increasing pH values. Correspondingly, the highest settlement and lowest dispersion stability of g-C3N4 were achieved in the aqueous environment, when the pH value approached the isoelectric point (pH was 4). Furthermore, the electrostatic repulsion and steric hindrance between g-C3N4 particles increased in the presence of humic acid, leading to the reduced settlement and enhanced dispersion stability of g-C3N4 with the increasing concentrations. However, there was a critical point for the enhanced efficiency. More importantly, the final standardized concentration (after 360 min) of g-C3N4 suspension significantly increased from 0.60 to 0.87, as the humic acid concentrations increased from 0 to 5 mg/L, but remained at 0.91 with the humic acid concentrations further increased to 10 mg/L. Anyway, the finding can be expected to well elucidate the settlement and dispersion stability of g-C3N4 in the aqueous environment under typical conditions, particularly for the better understanding of potential behaviors of graphitic carbon nitride in modern agriculture.