The impact of carbon nanotubes on bioaccumulation and translocation of phenanthrene, 3-CH3-phenanthrene and 9-NO2-phenanthrene in maize (Zea mays) seedlings

Abstract

The impact of soil amendment with carbon nanotubes (CNTs) including single-walled CNTs (SW), and two multiwalled ones, MW50 and MW8, on the bioaccumulation and translocation of phenanthrene, 3-CH₃-phenanthrene and 9-NO₂-phenanthrene in maize seedlings in single- (F1), bi- (F2), and tri-compound (F3) systems was examined. The CNT concentration in various systems was 50, 500 or 3000 mg kg⁻¹. The initial soil concentrations were 296.57 ± 27.61 μg kg⁻¹ of phenanthrene in F1, 287.92 ± 51.24 μg kg⁻¹ of phenanthrene and 186.96 ± 26.78 μg kg⁻¹ of 3-CH₃-phenanthrene in F2; and 292.11 ± 28.73 μg kg⁻¹ of phenanthrene, 181.06 ± 37.92 μg kg⁻¹ of 3-CH₃-phenanthrene, and 167.33 ± 31.73 μg kg⁻¹ of 9-NO₂-phenanthrene in F3. All CNTs were detected in plant roots, mostly taken up by secondary roots and accumulated in the Casparian strip, but they were hardly translocated to the shoots. As F1, F2, and F3 were amended with a given CNT, the mean phenanthrene concentrations in plant roots gradually decreased by 36.8, 27.8, and 43.7% for MW50, by 28.0, 23.6, and 46.3% for MW8, and by 24.2, 39.2, and 30.8% for SW in F1, F2, and F3, respectively, as the CNT amendment level was increased from 50 to 3000 mg kg⁻¹, attributable to the increased amount of pollutants retained in the soil. As the systems were amended with a specific CNT at the same level, the mean phenanthrene concentration in plant roots in F1, F2 and F3, and 3-CH₃-phenanthrene in F2 and F3, generally increased with an increasing co-exposed compound number, because competitive sorption on CNTs and soil particles reduced pollutant retention in the soil: for phenanthrene by 27.5, 39.7, and 26.6% for MW50, MW8 and SW at 50 mg kg⁻¹, by 28.5, 21.7, and −0.9% at 500 mg kg⁻¹, and by 13.6, 4.1, and 15.6% at 3000 mg kg⁻¹; and for 3-CH₃-phenanthrene by 10.1, 70.0, and 21.7% for MW50, MW8 and SW at 50 mg kg⁻¹, by 16.6, −18.5, and 2.2% at 500 mg kg⁻¹, and by 30.7, 37.3, and 30.5% at 3000 mg kg⁻¹. Contrarily, the mean translocation factor of phenanthrene and 3-CH₃-phenanthrene from roots to shoots in the corresponding systems decreased by 59.0, 60.0, and 53.6% for MW50, MW8 and SW at 50 mg kg⁻¹, by 74.3, 85.7, and 71.4% at 500 mg kg⁻¹, and by 61.6, 0, and 0% at 3000 mg kg⁻¹ for phenanthrene; and by 11.1, 42.9, and 40.0% for MW50, MW8 and SW at 50 mg kg⁻¹ for 3-CH₃-phenanthrene, reflecting that their translocation to the shoots tended to be weaker as more chemicals were co-exposed. This was due to their water solubility reduction in the transpiration stream flux and greater steric hindrance in the translocation process.