Insights into Adsorption of Humic Substances on Graphitic Carbon Nitride

Graphitic carbon nitride (CN) has been widely used in environmental pollution remediation. However, the adsorption of organic compounds on CNs, which has practical significance for the environmental application of CNs, is poorly understood. For the first time, this study systematically investigated the adsorption behaviors and mechanisms of humic substances (HSs), i.e., humic acid (HA) and fulvic acid (FA), on CNs derived from four typical precursors. Intriguingly, CN derived from urea (CN-U) showed a great capacity for HS adsorption due to its porous structure and large surface area, with maximum adsorption amounts of 73.24 and 51.62 mgC/g for HA and FA, respectively. The formation, influencing factors, and relative contributions of multiple interactions to HS adsorption on CNs were thoroughly elucidated. HS adsorption on CNs was mainly mediated by electrostatic interactions, π-π interactions, and H-bonding. The dominance of electrostatic interactions resulted in HS adsorption being highly dependent on pH and ionic strength. HS components with high aromaticity and high molecular weight were preferentially adsorbed due to π-π interactions. These multiple interactions were largely affected by amino groups and tri-s-triazine units of CNs, as well as the moieties of aromatic rings and oxygen-containing groups of HSs.