Xiaofang Shen

Sorption mechanisms of organic compounds by carbonaceous materials: site energy distribution consideration.

Sorption of naphthalene, lindane, and atrazine on 10 kinds of carbonaceous materials which included four kinds of graphene, three kinds of graphite, two kinds of carbon nanotubes and one kind of mesoporous carbon was investigated. The approximate sorption site energy distributions were calculated based on Dubinin-Ashtakhov (DA) model. The average sorption site energy and standard deviation of the site energy distribution were deduced and applied to analyze the interaction between sorbents and sorbates, and the sorption site heterogeneity. The introduction of oxygen-containing functional groups to the sorbents caused a decrease in their average sorption energy for the studied compounds. However, relative to the decrease in average site energy, the reduction in number of sorption sites as indicated by surface area more strongly reduced their sorption capacity to the tested carbonaceous materials based on the result of the linear regression analysis. Sorption site heterogeneity of the sorbents decreased as their oxygen contents increased, which is attributed to the better dispersion of the oxygen-containing materials as indicated by their TEM images. The method proposed in this study to quantify the average sorption site energy and heterogeneity is helpful for a better understanding of the sorption mechanisms of organic pollutants to carbonaceous materials.

Displacement and competitive sorption of organic pollutants on multiwalled carbon nanotubes

Displacement of lindane presorbed on the pristine and OH-functionalized multiwalled carbon nanotubes (MWCNTs) by phenanthrene, naphthalene, and atrazine, and competition of these compounds with lindane on the aforementioned sorbents were investigated. Displacement of lindane presorbed on MWCNTs by atrazine, naphthalene, and phenanthrene, and competitive sorption effect of these chemicals with lindane on MWCNTs followed the same order: atrazine > naphthalene > phenanthrene. The lowest competition and displacement of lindane by phenanthrene were mainly because of the strong interactions between these two chemicals, whereas interaction of lindane with atrazine and naphthalene was quite low. The more pronounced displacement of lindane by atrazine than naphthalene and higher competitive sorption of lindane with atrazine than with naphthalene can be ascribed to the larger molecular volume of atrazine; thus, the steric hindrance effect is higher relative to naphthalene. This study is valuable for evaluating influence of the coexisting organic compounds on sorption of primary solute towards MWCNTs in the environment.

Sorption mechanisms of sulfamethazine to soil humin and its subfractions after sequential treatments

Sorption mechanisms of an antibiotic sulfamethazine (SMT) to humin (HM) isolated from a peat soil and its subfractions after sequential treatments were examined. The treatments of HM included removal of ash, O-alkyl carbon, lipid, and lignin components. The HF/HCl de-ashing treatment removed a large amount of minerals (mainly silicates), releasing a fraction of hydrophobic carbon sorption domains that previously were blocked, increasing the sorption of SMT by 33.3%. The de-O-alkyl carbon treatment through acid hydrolysis greatly reduced polarity of HM samples, thus weakening the interaction between sorbents with water at the interfaces via H-bonding, leaving more effective sorption sites. Sorption of SMT via mechanisms such as van der Waals forces and π-π interactions was enhanced by factors of 2.04-2.50. After removing the lipid/lignin component with the improved Soxhlet extraction/acid hydrolysis, the organic carbon content-normalized sorption enhancement index Eoc was calculated. The results demonstrated that the Eoc-lipid for SMT (16.9%) was higher than Eoc-lignin (10.1%), implying that removal of unit organic carbon mass of lipid led to a higher increase in sorption strength than that of lignin. As each component was progressively removed from HM, the sorption strength and isotherm nonlinearity of the residual HM samples for SMT were gradually enhanced. The Koc values of SMT by HM samples were positively correlated with their aromatic carbon contents, implying that π-π electron donor-acceptor interactions between the benzene ring of sorbate and the aromatic domains in HM played a significant role in their interactions.

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

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.

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

The impact of multi-walled carbon nanotubes (outer diameter 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.