pH-dependent sorption of sulfonamide antibiotics onto biochars: Sorption mechanisms and modeling

It remains a challenge to precisely predict and control environmental behaviors of ionizable organic contaminants (IOCs) due to their species change relative to pH and because of the lack of appropriate models to illustrate the underlying pH-dependent mechanisms. We studied the pH-dependent sorption behavior of five sulfonamide antibiotics (SAs) as typical IOCs with different pK_a values towards a series of biochars as representative sorbents with well-characterized surface structures. After subtracting the contribution of the speciation effect using a classical speciation model, up to three unexpected enhanced sorption peaks could be found and regulated by the pK_a,SA of the SAs and the pK_{a, BC} of the biochars. The mono H-bond formation between the two pK_a,SA of the SAs (pK_a,SA1 is from NH₂, pK_a,SA2 is from SO₂NH), and the biochar surface functional groups with comparable pK_a values generated two peaks. Another peak around the middle between pK_a,SA1 and pK_a,SA2 appeared due to the aromatic π bonding-enhanced dual H-bond. All of these peaks were quantitatively separated by a novel two-compartment model, which was developed by capturing the characteristics of pH-dependent sorption. The quantified hydrogen bonding among different SAs elucidates the effectiveness and limits of the pK_a equalization principle to predict the strengthening of hydrogen bonding at the solid-aqueous interface. This work recognizes the quantitative relationship among the structure, sorption, and H-bond interaction of biochars and guides the prediction of the fate of IOCs in the environment and the development of remediation options.