Sorption of phenanthrene on single-walled carbon nanotubes modified by DOM: effects of DOM molecular weight and contact time

Roles of dissolved humic acid and tannic acid in sorption of benzotriazole to a sandy loam soil

To clarify the role of dissolved organic matter (DOM) in sorption of organic pollutants, batch experiments on effects of two representatives of DOM (dissolved humic acid (HA) and tannic acid (TA)) on sorption of benzotriazole (BTA) to a sandy loam soil were conducted. Both HA and TA promoted BTA sorption to soil. Strong positive correlation between sorbed amount of BTA and DOM confirmed the contribution of cumulative sorption by HA or TA in enhancing BTA binding. TA promoted BTA sorption more obviously than HA by providing more sites. For HA with complex structure composed of heterogeneous fractions, its high molecular weight (>3200 Da) fraction could be preferentially sorbed by soil, and it can enhance BTA sorption more obviously than the low molecular weight fraction. The promoting effect of HA on BTA sorption decreased with pH increasing from 6.5 to 10.5 due to reduced sites and electrostatic repulsion between anionic BTA, HA and soil. Sorption of neutral BTA to soil affected by DOM could be well predicted by a modified Freundlich model with K_d (L kg^-1) deviations less than 0.2 log unit.

Competitive Adsorption of Polycyclic Aromatic Hydrocarbons to Carbon Nanotubes and the Impact on Bioavailability to Fathead Minnow (Pimephales promelas)

Recent studies investigating the influence of carbon nanotubes (CNTs) on the bioavailability of organic contaminants have mostly focused on single-solute systems; however, a more likely scenario in the natural environment is a multisolute system where chemical interactions at the surface of the CNT may alter the bioavailability of these chemicals. In the present study bisolute adsorption isotherms of pairs of chemically similar polycyclic aromatic hydrocarbons (PAHs) by multiwalled carbon nanotubes (MWCNTs) were established, in conjunction with quantifying the bioavailability of the 2 competing MWCNT-adsorbed PAHs to Pimephales promelas using bile analysis by high-performance liquid chromatography with fluorescence detection. The results showed that whereas adsorption and bioavailability of chemically similar PAHs (anthracene and phenanthrene, and fluoranthene and pyrene) were the same in a single-solute system, in bisolute systems, PAHs that could better align or flex with the MWCNT surface due to morphological characteristics would outcompete the more rigid or planar PAHs. The bioavailability of individual PAHs in bisolute solutions increased by as much as 50% compared with single-solute solutions. However, the relationship between adsorption (i.e., K_d ) and concentration of PAH in the fish bile was similar in single and bisolute systems. This finding indicates that competitive interactions at the surface of MWCNTs influence bioavailability by way of altering adsorption affinity in a moderately predictable manner. Environ Toxicol Chem 2020;39:1702-1711. © 2020 SETAC.

High Pressure Size Exclusion Chromatography (HPSEC) Determination of Dissolved Organic Matter Molecular Weight Revisited: Accounting for Changes in Stationary Phases, Analytical Standards, and Isolation Methods

We reassessed the molecular weight of dissolved organic matter (DOM) determined by high pressure size exclusion chromatography (HPSEC) using measurements made with different columns and various generations of polystyrenesulfonate (PSS) molecular weight standards. Molecular weight measurements made with a newer generation HPSEC column and PSS standards from more recent lots are roughly 200 to 400 Da lower than initial measurements made in the early 1990s. These updated numbers match DOM molecular weights measured by colligative methods and fall within a range of values calculated from hydroxyl radical kinetics. These changes suggest improved accuracy of HPSEC molecular weight measurements that we attribute to improved accuracy of PSS standards and changes in the column packing. We also isolated DOM from wetlands in the Prairie Pothole Region (PPR) using XAD-8, a cation exchange resin, and PPL, a styrene-divinylbenzene media, and observed little difference in molecular weight and specific UV absorbance at 280 nm (SUVA₂₈₀) between the two solid phase extraction resins, suggesting they capture similar DOM moieties. PPR DOM also showed lower SUVA₂₈₀ at similar weights compared to DOM isolates from a global range of environments, which we attribute to oxidized sulfur in PPR DOM that would increase molecular weight without affecting SUVA₂₈₀.

Influence of humic acids on sorption of alkanes by carbon nanotubes--implications for the dominant sorption mode

The presence of humic substances (HS) has previously been shown to alter sorption properties of multi-walled carbon nanotubes (MWCNTs). To systematically study this process, three alkane pairs were selected as molecular probe sorbates. The influence of HS on sorption affinity, sorption linearity, and the dominant sorption mode (i.e., ad- or absorption) by MWCNTs was investigated. The addition of HS led to a continuous decrease in sorption affinity and an increase in sorption linearity with increasing HS addition. Furthermore, the comparison of distribution coefficients of n- and cycloalkanes showed that the dominant sorption mode remains to be adsorption regardless of the presence of HS on MWCNT surface. From this, it can be concluded that instead of a change in sorption mode to absorption of sorbates into HS-coated MWCNT, HS blocks high-energy sorption sites for subsequently added sorbates and that sorbates continuously sorb on the MWCNT surface.