Clarithromycin and Tetracycline Binding to Soil Humic Acid in the Absence and Presence of Calcium. 
Numerous ionizable organic micropollutants contain positively charged moieties at pH values typical of environmental systems.  Describing organic cation and zwitterion interaction with dissolved natural organic matter requires explicit consideration of the pH-dependent speciation of both sorbate and sorbent.  We studied the pH-, ionic strength-, and concentration-dependent binding of relatively large, organic cations and zwitterions (viz.,  the antibiotics clarithromycin and tetracycline) to dissolved humic acid in the absence and presence of Ca(2+) and evaluated the ability of the NICA-Donnan model to describe the data.  Clarithromycin interaction with dissolved humic acid was well described by the model including the competitive effect of Ca(2+) on clarithromycin binding over a wide range of solution conditions by considering only the binding of the cationic species to low proton-affinity sites in humic acid.  Tetracycline possesses multiple ionizable moieties and forms complexes with Ca(2+).  An excellent fit to experimental data was achieved by considering tetracycline cation interaction with both low and high proton-affinity sites of humic acid and zwitterion interaction with high proton-affinity sites.  In contrast to clarithromycin, tetracycline binding to humic acid increased in the presence of Ca(2+), especially under alkaline conditions.  Model calculations indicate that this increase is due to electrostatic interaction of positively charged tetracycline-Ca complexes with humic acid rather than due to the formation of ternary complexes, except at very low TC concentrations.