Retention-release of ciprofloxacin and azithromycin in biosolids and biosolids-amended soils

Ciprofloxacin (CIP) and azithromycin (AZ) are commonly prescribed antibiotics, often found at elevated concentrations in treated sewage sludge (biosolids), and could pose human and ecological risks when land applied. Limited retention-release data preclude assessing potential risks from the target antibiotics in biosolids and biosolids-amended soils. The present work assessed sorption-desorption of CIP and AZ in biosolids and biosolids-amended soils using the "traditional" batch equilibration method. The batch equilibration method also included un-amended soils for comparison. Release potentials of the biosolids-borne antibiotics were assessed via multiple desorption equilibrations in the presence of CaCl2, soils, PbCl2, or competing antibiotic (CIP versus AZ) solutions. Desorption kinetics of CIP from biosolids were also evaluated by the diffusive gradient in thin films technique (DGT), coupled with a diffusion transport-exchange model available in 2D-DIFs. Sorption of both antibiotics followed linear models with partitioning coefficient (Kd) values for CIP ranging between 40 and 334 L kg-1 in soils and 357 L kg-1 in biosolids, and values for AZ ranging between 11 and 202 L kg-1 in soils and 428 L kg-1 in biosolids. Antibiotic desorption from the biosolids was highly hysteretic (hysteresis coefficients < 0.003) and desorption of the biosolids-borne chemicals was extremely small (<3%) using any of the various desorption equilibration approaches. Desorption was hysteric in soils too; where desorption percentages were 4, 5, and 26% for CIP and 6, 32, and 50% for AZ in the silt loam soil, manured sand, and sand, respectively. CIP release from biosolids determined by DGT was also small (<1%), ascribed to low dissolved and labile concentrations in the solid phase and a small effective diffusion coefficient. Results obtained using equilibrium and dynamic approaches suggest that the target antibiotic bioaccessibilities from biosolids and finer-textured (typical agricultural) soils would be minimal and that biosolids (not soils) control desorption of the two biosolids-borne chemicals.