pH-dependent K provides new insights in understanding the adsorption mechanism of ionizable organic chemicals on carbonaceous materials.

The dominant adsorption mechanism of ionizable organic chemicals (IOCs) on carbonaceous materials is still unclear. This study used benzoic acid (BA), o-chlorobenzoic acid (2-CBA) and p-chlorobenzoic acid (4-CBA) as representatives of IOCs to investigate the dominant adsorption mechanism on graphene oxide (GO) and graphite (GP), with a specific emphasis on the impact of pH-dependent K. The order of apparent adsorption of the investigated chemicals changed with pH, which could not be explained by their constant K. According to our measurement, K varied greatly with pH. The pH-dependent adsorption of BA, 2-CBA and 4-CBA were significantly correlated with pH-dependent K for both GO and GP. Our observations indicated that the hydrophobic effects may dominate the apparent adsorption of BA, 2-CBA and 4-CBA on GO and GP. Electrostatic repulsion played a minor role in their reduced adsorption at high pH. Negative charge-assisted H-bond and π-π interactions may explain the adsorption of negatively charged BA, 2-CBA and 4-CBA on negatively charged GO. This study implied that pH-dependent K rather than constant K should be incorporated to understand IOC adsorption as affected by pH.