Competitive sorption experiments reveal new regression models to predict PhACs sorption on carbonaceous materials

Sorption of hydrophobic organic contaminants onto thermally altered carbonaceous materials (TACM) constitutes a widely used technology for remediation of polluted waters. This process is typically described by sorption isotherms, with one of the most used models, the Polanyi-Dubinin-Manes (PDM) equation, including water solubility (S_w) as a normalizing factor. In case of pharmaceutical active compounds (PhACs), S_w depends on the pH of the environment due to the ionic/ionizable behavior of these chemicals, a fact frequently ignored in sorption studies of PhACs. In this work, we set the theoretical framework to include the variation of S_w with pH in the definition of the PDM model, and we applied this approach to describe the effect of ambient pH in the competitive sorption of three commonly detected PhACs (carbamazepine, ibuprofen, and sulfamethoxazole) onto three carbonaceous sorbents (biochar, powder activated carbon, and colloidal activated carbon). Changes in the ambient pH and hence in the hydrophobicity of the compounds could explain the strong variations observed in single-solute sorption and also in competitive sorption. Furthermore, S_w was used as a parameter for the linear regression model of sorption coefficients of our experiments, suggesting the incorporation of this variable as an improvement to existing approaches for prediction of PhACs sorption onto TACM.