Weixiao Chen

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.

Association of 16 priority polycyclic aromatic hydrocarbons with humic acid and humin fractions in a peat soil and implications for their long-term retention

To elucidate the environmental fate of polycyclic aromatic hydrocarbons (PAHs) once released into soil, sixteen humic acids (HAs) and one humin (HM) fractions were sequentially extracted from a peat soil, and sixteen priority PAHs in these humic substances (HSs) were analyzed. It was found that the total concentration of 16 PAHs (∑16PAHs) increased evidently from HA1 to HA16, and then dramatically reached the highest value in HM. The trend of ∑16PAHs in HAs relates to surface carbon and C-H/C-C contents, the bulk aliphatic carbon content and aliphaticity, as well as the condensation enhancement of carbon domains, which were derived from elemental composition, XPS, 13C NMR, as well as thermal analyses. HM was identified to be the dominant sink of 16 PAHs retention in soil, due to its aliphatic carbon-rich chemical composition and the highly condensed physical makeup of its carbon domains. This study highlights the joint roles of the physical and chemical properties of HSs in retention of PAHs in soil and the associated mechanisms; the results are of significance for PAH-polluted soil risk assessment and remediation.

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.

A mechanistic study of stable dispersion of titanium oxide nanoparticles by humic acid

Stable dispersion of nanoparticles with environmentally-friendly materials is important for their various applications including environmental remediation. In this study, we systematically examined the mechanisms of stable dispersion of two types of TiO2 nanoparticles (TNPs) with anatase and rutile crystalline structures by naturally occurring dissolved organic matter (humic acid) at different pHs, including at, below and above the Point of Zero Charge (PZC). The results showed that stable dispersion of TNPs by humic acid (HA) at all pHs tested can only be achieved with the assistance of ultra-sonication. The dispersion of TNPs by HA differed at the three pHs tested. Generally, HA greatly decreased the hydrodynamic diameters of TNPs at a very low concentration. The dispersion of TNPs became relatively stable when the HA concentration exceeded 5 mg/L, indicating that this HA concentration is required for stable dispersion of TNPs. The mechanisms involved in dispersion of TNPs by HA included electrostatic repulsion, steric hindrance and hydrophobic interaction. Electrostatic repulsion was identified to be the dominant mechanism. The dispersion of TNPs was enhanced when HA was added before ultra-sonication to avoid the partly irreversible re-aggregation of TNPs after sonication. The crystalline phases and concentrations of TNPs were also found to influence their stable dispersion. The findings from this work enhance understanding of the combined effects of HA, pH, ultra-sonication and crystalline structures of TNPs on their stable dispersion. The mechanisms identified can improve applications of TNPs in environmental water pollution control.