pH-Dependent Sorption of Azithromycin By Soils.

The beneficial use of biosolids to enhance soil quality and soil fertility is constantly challenged by concerns about the chemical constituents that remain after the wastewater treatment process. Our ability to address these concerns is often hindered by the lack of data, or the inappropriate use of existing data, for many pH-dependent trace organic chemicals (TOrCs) which do not follow previously used paradigms for predicting risk. Among these is the second generation macrolide, azithromycin, which with combined use for humans, pets, and livestock led it to be the most prescribed antibiotic in 2010. The objective of this study is to quantify the pH- and cation-exchange capacity (CEC)- dependent behavior of azithromycin in the soil-water environment. Batch sorption studies at 5 concentrations in a weak electrolyte solution were conducted for azithromycin using 5 soils representing a relevant range in soil pH (4.3-8.6) and CEC (4 to 20 cmol_c/kg). Soils were characterized and pH-dependent CEC was be used or measured. In order to minimize biotransformation during equilibration with soils, all materials were sterilized prior to use. All studies were conducted in triplicate with blank solution and soil controls. Both aqueous and solid phases were analyzed separately on a Shimadzu 8040 LC/MS/MS. Sorption isotherms were modeled and correlated to soil properties. In quantifying the pH-dependent sorption of azithromycin by soils, this data will advance development of predictive tools and the overall risk assessment process.