Plant toxicity and accumulation of biosolids-borne ciprofloxacin and azithromycin.

Trace organic chemicals (TOrCs) in land applied biosolids can cause phytotoxicities and contaminate human and animal food chains. Information on phytotoxicity and phytoaccumulation of environmentally relevant concentrations of two antibiotic TOrCs, ciprofloxacin (CIP) and azithromycin (AZ), from biosolids-amended soils is limited. Greenhouse studies were conducted to assess the plant toxicity and accumulation of a range of environmentally relevant concentrations of biosolids-borne CIP and AZ in biosolids-amended soils. Separate studies assessed phytotoxicity potential of soil-borne CIP and AZ (soils directly spiked with the target antibiotics without biosolids) at concentrations much greater than those of environmental relevance in biosolids-amended soils. Both the biosolids-borne and the soil-borne antibiotic studies involved three plants (radish (Raphanus sativus), lettuce (Lactuca sativa), and tall fescue grass (Festuca arundinacea)) of different morphologies, physiologies, and chemical exposure scenarios. Phytotoxicity and phytoaccumulation from the biosolids-borne antibiotics were minimal at environmentally relevant concentrations, even in sand. The separate phytotoxicity experiments involving the soil-borne antibiotics revealed no observed adverse effect concentration (NOAEC) of 3.2 mg kg-1 (AZ) and 36.1 mg kg-1 (CIP) for the three plants grown in soils mimicking typical agricultural soils. These NOAEC values are about 100-fold greater than the antibiotic concentrations expected in biosolids-amended soils. NOAEC values under an unrealistic worst-case where the antibiotics were directly spiked to sand (NOAEC = 3.2 mg kg-1 for AZ; and ≥0.36 mg kg-1 for CIP) were also greater than the environmentally relevant concentrations of the biosolids-borne antibiotics. The results suggest that land application of biosolids-borne CIP and AZ pose De minimis risks to plants. Point estimates of plant bioaccumulation factors (dry weight basis) were 0.01 (CIP) and 0.1 (AZ), suggesting minimal impacts of the target TOrCs on human and/or animal food chains.