Inhibitory effects of carbon nanotubes on the degradation of 14C-2,4-dichlorophenol in soil.

Concerns on the potential risks of engineered nanoparticles to the environment are increasing; however, little is known about the effects of carbon nanotubes (CNTs) on the environmental fate of hydrophobic organic pollutants in soil. We incubated radioactive labeled 2,4-dichlorophenol ((14)C-2,4-DCP) in a soil in the presence of various concentrations (0, 2, 20, and 2000 mg kg(-1) dry soil) of single-walled (SWCNTs) and multi-walled (MWCNTs) carbon nanotubes, and determined the mineralization, degradation, and residue distribution of 2,4-DCP in the soil. CNTs were added to the soil either after the spiking of (14)C-2,4-DCP or together with (14)C-2,4-DCP as a mixture. CNTs at the concentration of 2000 mg kg(-1) significantly (P<0.05) inhibited the mineralization of (14)C-2,4-DCP and induced a 2.3- to 3.9-fold increase in the amounts of the non-degraded (14)C-2,4-DCP in the soil after 90 d of incubation. Pre-adsorption of (14)C-2,4-DCP on CNTs showed stronger inhibitory effects on the degradation of (14)C-2,4-DCP, already significant with CNTs at 20 mg kg(-1). In general, SWCNTs had a higher effect on the degradation and residue distribution of 2,4-DCP in the soil than MWCNTs. The inhibitory effects are supposed to be owing to limited activities of soil endogenous microorganisms, potential toxicities of CNTs to the microorganisms, and reduced bioavailability of 2,4-DCP in the presence of CNTs, even though a desorption hysteresis of 2,4-DCP on CNTs was not observed. Our results indicate that CNTs have more significant impacts on the environmental fate of the hydrophobic pollutants entering soil together with CNTs via strong sorption than the pollutants already present in soil.