Biodegradation of Biosolids-Borne Triclosan (TCS).

Triclosan (TCS) is an active ingredient in antibacterial liquid soaps, personal care products, and a common constituent of domestic wastewater. Activated sludge treatment readily removes TCS from the liquid waste stream and concentrates the antimicrobial in the solid fraction, which is often processed to produce biosolids. Greater than half of the biosolids generated in the US are land-applied, resulting in a systematic release of biosolids-borne TCS into the terrestrial environments. Little is known about the environmental persistence of TCS following land application of biosolids. The objective of our study was to determine the half-life of TCS, and to identify the metabolites in biosolids-amended systems. A laboratory biodegradation study was conducted using a biosolids spiked with ¹⁴C-TCS (concentration = 41 mg kg^-1) amended to Immokalee fine sand and Ashkum silty clay loam at agronomic rates (22 Mg ha^-1). The 18-week study revealed minimal (<0.5%) mineralization (conversion to ¹⁴CO₂) of ¹⁴C-TCS from biosolids-amended soils, but a degradation metabolite (Methyl TCS) was detected as early as week 4. The primary degradation half-life (TCS to Methyl TCS) was estimated as 77 d in the Ashkum silty clay loam and >126 d in the Immokalee fine sand. Half-life estimates are similar to the 56-107 d reported in the literature. Thus, a range of 100-125 d can be used as a first approximation of degradation half-life in biosolids-amended soil systems. Half-lives measured in biosolids-amended systems are greater than in unamended soils (18-58 d) due to the greater adsorption of TCS to the biosolids. Future work will include the investigation of possible TCS plant uptake, and effects of biosolids-borne TCS on soil organisms. The results will contribute to an improved TCS risk assessment for the routine land application of biosolids to soils.