Suspension of the pristine and COOH-substituted multi-walled carbon nanotubes (P- and C-MWCNTs) with different outer diameters (ODs) by humic acids (HAs) from a peat soil was examined. Under shaking condition, MWCNTs were not suspended within 5 d. Without HAs, C-MWCNTs were slightly suspended by sonication within 16 h, but no suspension was observed for the pristine ones (P-MWCNTs). HAs greatly enhanced suspension of both P- and C-MWCNTs. The suspension enhancement was attributed to HA sorption, which increased electrostatic repulsion and steric hindrance between individual MWCNTs. Introduction of O-containing hydrophilic moieties to MWCNTs via HA sorption enhanced the interactions of their surfaces with water through H-bonding. Suspending capability of various MWCNTs on suspended mass concentration basis by four HAs showed inconsistent orders with the increasing or decreasing trend of their ODs. However, the suspended surface area concentrations of both P- and C-MWCNTs by individual HAs consistently followed an order of P8 > P30 > P50, and C8 > C30 > C50 (P and C, respectively, refer to P- and C-MWCNTs, and the numbers represent their ODs). These data implied that MWCNTs with smaller OD could be more strongly suspended by a given HA relative to those with larger OD under sonication condition.

The sorption behavior of organic compounds (phenanthrene, lindane, and atrazine) to sequentially extracted humic acids and humin from a peat soil was examined. The elemental composition, XPS and 13C NMR data of sorbents combined with sorption isotherm data of the tested compounds show that nonspecific interactions govern sorption of phenanthrene and lindane by humic substances. Their sorption is dependent on surface and bulk alkyl carbon contents of the sorbents, rather than aromatic carbon. Sorption of atrazine by these sorbents, however, is regulated by polar interactions (e.g., hydrogen bonding). Carboxylic and phenolic moieties are key components for H-bonding formation. Thermal analysis reveals that sorption of apolar (i.e., phenanthrene and lindane) and polar (i.e., atrazine) compounds by humic substances exhibit dissimilar relationships with condensation and thermal stability of sorption domains, emphasizing the major influence of domain spatial arrangement on sorption of organic compounds with distinct polarity. Results of pH-dependent sorption indicate that reduction in sorption of atrazine by the tested sorbents is more evident than phenanthrene with increasing pH, supporting the dependence of organic compound sorption on its polarity and structure. This study highlights the different interaction mechanisms of apolar and polar organic compounds with humic substances.

Organic matter−mineral interactions greatly affect the fate of hydrophobic organic compounds (HOCs) in the environment. In the present study, the impact of organic matter−mineral interaction on sorption of phenanthrene (PHE) by the original and de-ashed humic acids (HAs) and humin (HM) was examined. After de-ashing treatment, the overall polarity of organic matter in HAs and HM consistently decreased. Differently, the surface polarity of HAs increased but that of HM decreased. No correlation between Koc values of PHE by all tested sorbents and their bulk polarity was observed due to inaccessibility of a portion of interior sorption domains. The inaccessibility of interior sorption domains in HAs and HM was partly due to the crystalline structure in organic matter as indicated by differential scanning calorimetric (DSC) and 13C NMR data and the interference from minerals. A good correlation between surface polarity of the original and de-ashed HAs and HMs and their Koc values for PHE indicated its importance in HOC sorption. Dissimilar changes in surface polarity of HAs and HM after de-ashing treatment can be ascribed to the distinct interactions between organic matter and minerals. The solid-state 13C NMR, XPS, and elemental composition data of all tested sorbents revealed that a larger fraction of O atoms in HAs were involved in organic matter−mineral interaction as compared to HM. Results of this work highlight the importance of soil organic matter (SOM)-mineral interactions, surface polarity, and microscaled domain arrangement of SOM in HOC sorption.

Sorption of humic acids (HAs) from a peat soil by multiwalled carbon nanotubes (MWCNTs) was examined in this work. Sorption rate of HAs to MWCNTs was dominantly controlled by their diffusion from liquid–MWCNT boundary to MWCNT surfaces. Size exclusion chromatography analysis did not detect preferential sorption of HA fractions to MWCNTs at equilibrium, whereas the components with lower molecular weight in some HA fractions (e.g., HA1) would more preferentially be sorbed to MWCNTs at the initial sorption stage. Equilibrium sorption intensity of HAs by MWCNTs was dependent on their surface area and a sum of meso- and macropore volume. The surface area and sum of meso- and macroporosity-normalized sorption coefficient (Kd) values of a given HA by MWCNTs increased with increasing outer diameter of MWCNTs, because MWCNTs with larger outer diameter were more strongly dispersed by HAs thereby making more sorption sites exposed for HA sorption. Van der Waals interaction between the alkyl components rather than the aromatic ones of HAs with MWCNTs was likely the key driving force for their sorption. This study highlights the sorption rate-controlling step of HAs from a same source to MWCNTs and the major factors affecting their sorption intensity at equilibrium.

Sorption of hydrophobic organic compounds (HOCs) (phenanthrene, lindane and atrazine) by original and OH-functionalized multiwalled carbon nanotubes (F-MWCNTs) was examined. Functionalization of MWCNTs with hydrophilic moieties greatly reduced their ability to sorb HOCs. The surface area (SA) and sum of meso- and macropore volumes of MWCNTs were governing characteristics that influenced their affinity and capacity for sorption of HOCs. Molecular size of HOCs markedly influenced their volume sorption capacity (Q0) by the original MWCNTs, and the differences in Q0 values between various HOCs were less for F-MWCNTs, than for the original MWCNTs. The introduced hydroxyl groups may have reduced accessibility of a large portion of sites in meso- and macropores of MWCNTs that were originally available for smaller HOCs, but not for those with larger molecular size. Sorption of HOCs by MWCNTs was dominantly controlled by hydrophobic interaction regardless of their chemical structure. Kow-normalized sorption coefficients (Kd/Kow) of the tested compounds for all MWCNTs followed the order: atrazine > phenanthrene > lindane, implying that π–π interactions enhanced sorption of aromatics by MWCNTs. The enhancement was more pronounced for chemicals with activating groups and larger molecular size relative to those with smaller molecular size and without any substituent.

Bioavailability of hydrophobic organic compounds (HOCs) is an important factor affecting their fate in the environment. Molecular-level HOC-soil organic matter (SOM) interactions and associated impacts on its bioavailability were investigated in this study. Our results showed that, phenanthrene (PHE) was mainly sequestrated in aromatic domains of lignin, as indicated by liquid-state 1H−13C heteronuclear multiple quantum coherence (HMQC) NMR along with solid-state 13C NMR data and information on surface domain distribution of this biopolymer as shown by its X-ray photoelectron spectroscopic data. Here, surface domain distribution was defined as the relative abundance of sorption domains at the surfaces versus that in the bulky biopolymer particles and their spatial positions. Wax had much higher sorption for PHE than cellulose, but no striking difference in degradability of wax- and cellulose-sorbed PHE was observed, which can be ascribed to much more hydrophobic surface of wax relative to that of cellulose, making it more favorable for bacteria PYR-1 attachment. This work highlighted the joint effects of functionalities and surface domain distribution of SOM on bioavailability of HOCs (e.g., PHE).