Combined effects of aqueous suspensions of fullerene and humic acid on the availability of polycyclic aromatic hydrocarbons: Evaluated with negligible depletion solid-phase microextraction

- HA accelerated the rate constants (k2) of PAH uptake to nd-SPME fibers.
- The mixture promoted or retarded the k2 depending on PAH hydrophobicity.
- Coexistence with HA reduced PAH availability in nC60 solutions.
- Higher PAH sorption coefficients were obtained in aqu/nC60 than in son/nC60.
- PAH hydrophobicity played an important role in determining availability.

The wide application of engineered carbon nanomaterials, such as fullerene (C60), will inevitably result in their introduction into the aqueous environment. Interactions of C60 with abundant natural organic matter (NOM) will likely alter the bioavailability of organic compounds to aquatic organisms. The availability of 12 types of polycyclic aromatic hydrocarbons (PAHs) in various aqueous suspensions of fullerene (nC60) prepared by different methods, e.g., in humic acid (HA) and particularly in combined systems of nC60 and HA, was investigated by negligible depletion solid-phase microextraction (nd-SPME). The results showed that HA accelerated the rate constants (k2) of almost all PAH uptakes to the nd-SPME fibers compared with the solutions without the matrix; the combined matrices of nC60 and HA significantly promoted the k2 of highly hydrophobic PAHs (logKOW 5.81-6.20), whereas they retarded that of less hydrophobic ones (logKOW 3.82-4.63) (p < 0.05). Remarkable or minor reduction of free concentration of PAH was observed in the combined system of nC60 and HA depending on the properties of individual PAHs. Sorption coefficients (KHA, KC60 and KC60 + HA) of various PAHs in different matrices were provided, and matrix concentrations showed no significant effects. For highly hydrophobic PAHs (logKOW 5.16-6.20), the logKC60 + HA >= logKHA > logKC60, whereas for less hydrophobic PAHs (logKOW 3.8-4.63), the Kmatrix values in various matrices showed no noticeable trend. In addition, higher KC60 values were obtained for aqu/nC60 than for son/nC60 for most highly hydrophobic PAHs. The above results suggest that hydrophobicity plays an important role in determining Kmatrix in addition to the matrix effects. The interactions between nC60 and HA have critical or minor impacts on availability, and thus bioavailability, of PAHs. This paper contributes to the understanding of the bioavailability mechanisms of organic pollutants in the aquatic environment with both nC60 and NOM.

Relationship between logk2 and logKOW of 12 types of PAHs in different matrices. (×) aqueous solutions without any matrix; (∆) 5 mg L− 1nC60; (∇) 5 mg L− 1 HA; (□) 5 mg L− 1nC60 + 5 mg L− 1 HA. k2: rate constant of PAH uptake on the negligible depletion solid-phase microextraction (nd-SPME) fibers; nC60: aqueous suspensions of fullerene; HA: humic acid.HA accelerated the rate constants (k2) of almost all PAH uptakes to the nd-SPME fibers; the combined matrices of nC60 and HA promoted the k2 of highly hydrophobic PAHs whereas it retarded the k2 of less hydrophobic PAHs.