Influence of multi-walled carbon nanotubes and fullerenes on the bioaccumulation and elimination kinetics of phenanthrene in geophagous earthworms (Metaphire guillelmi)

Abstract

The impact of multi-walled carbon nanotubes (outer diameter <8 nm: MW8; >50 nm: MW50) and fullerene (C₆₀) at 300 or 3000 mg kg⁻¹ on phenanthrene bioaccumulation and elimination kinetics (L, 1.37 mg kg⁻¹; H, 16.14 mg kg⁻¹) in a geophagous earthworm (Metaphire guillelmi) was investigated. Prior to worm exposure, the residual phenanthrene concentrations in soil decreased by 56.4% (L) and 59.9% (H) after 12 h of equilibrium. Phenanthrene accumulation in earthworms exhibited a bell-shaped pattern for all treatments. However, both the rate and extent of bioaccumulation and elimination were significantly affected by the CNMs, dependent on both the type and amendment level. C₆₀ and MW8 at 300 mg kg⁻¹ in the L system significantly decreased the uptake rate of phenanthrene, resulting in delayed maximum accumulation at 2 d. All other treatments had little impact on the uptake rate, with the exception of a slight decrease induced by MW8 at 3000 mg kg⁻¹. The maximum phenanthrene accumulation followed an order of C₆₀ > MW8 > MW50 at each amendment level of these materials. All CNMs increased the uptake rate of phenanthrene by earthworms in the H system, with the exception of MW8 at 3000 mg kg⁻¹ where the maximum accumulation occurred earlier than the control. Furthermore, the uptake rate and maximum accumulation of phenanthrene in earthworms followed an order of C₆₀ > MW50 > MW8 at each level, which was the opposite of its strength of sorption to these materials. Interestingly, phenanthrene bioaccumulation in the H system amended with 3000 mg kg⁻¹ MW8 was 2.2–3.9 times that of all the other treatments at 35 d. For elimination, MW8 at this level increased phenanthrene amounts remaining in earthworms in both L (1.4–3.2 times) and H (1.1–15.2 times) systems. A higher burden of phenanthrene during the later bioaccumulation and elimination periods could result from ingestion and absorption of MW8; the accumulated phenanthrene may strongly bind to the internalized MW8, thereby reducing its elimination. Soil amendment with C₆₀ and MW50 at both levels and MW8 at 300 mg kg⁻¹ increased worm phenanthrene elimination in the L system, while that in the H system was not significantly affected. The results highlight the impact of CNMs on the bioaccumulation and elimination kinetics of phenanthrene in a typical geophagous earthworm, and provide important information for understanding of the potential risks of CNMs released into terrestrial systems.