Retention/Release of Biosolids-Borne Ciprofloxacin (CIP) and Azithromycin (AZ).

Predicting the fate of trace organic compounds (TOrCs) in the environment requires understanding the extent of retention and various mechanisms involved in retention/release. Most retention/release studies on the targeted TOrCs have been limited to the sorption behavior of CIP in soils, and studies on biosolids-borne CIP or AZ are scarce or absent. Further, data on desorption behavior (which is equally or perhaps more important) are largely lacking. We focused on the environmentally relevant aspects of retention/release behavior of biosolids-borne CIP and AZ and conducted non-mechanistic sorption/desorption studies using ³H-labeled compounds. We employed batch equilibration methods to assess sorption/desorption isotherms and partitioning coefficient (Kd) values of CIP and AZ in a class A biosolids material. In a separate study, the same biosolids material was pre-equilibrated with the target TOrCs for a week before CIP and AZ desorption was assessed using 4 approaches – (i) “traditional” sorption/desorption batch experiments (using CaCl₂), (ii) addition of PbCl₂ (Pb²⁺being a specifically sorbing cation), (iii) addition of “sister compounds” (i.e., competitive desorption of CIP in the presence of AZ, and vice-versa), and (iv) application of the contaminated biosolids to three different soil media. We found moderately high partitioning coefficient values of 357 L/kg for CIP and 428 L/kg for AZ and extremely small hysteresis coefficients < 0.003. Desorption of biosolids-borne CIP and AZ was negligible using any of the 4 desorption approaches. The results strongly suggest that the bioaccessibility of biosolids-borne CIP and AZ is minimal and biosolids (not soils) control desorption when CIP and AZ are truly biosolids-borne.