Enhanced Sorption of Polycyclic Aromatic Hydrocarbons to Tetra-Alkyl Ammonium Modified Smectites via Cation−π Interactions

The objective of this study was to characterize molecular sorptive interactions of polycyclic aromatic hydrocarbons (PAHs) by organoclays modified with quaternary ammonium cations. Three PAHs, naphthalene (NAPH), phenanthrene (PHEN), and pyrene (PYR), and three chlorobenzenes, 1,2-dichlorobenzene (DCB), 1,2,4,5-tetrachlorobenzene (TeCB), and pentachlorobenzene (PtCB), were sorbed from aqueous solution to reference montmorillonite clays (SWy-2) exchanged respectively with tetramethyl ammonium (TMA), tetraethyl ammonium (TEA), tetra-n-butyl ammonium (TBA), and hexadecyltrimethyl ammonium (HDTMA) cations. Solute hydrophobicities are compared between PAHs and chlorobenzenes using the solute n-octanol–water partition coefficient, n-hexadecane−water partition coefficient, and polyethylene−water distribution coefficient. The PAHs show several- to more than 10-fold greater sorption than the chlorobenzenes having close hydrophobicities but fewer delocalized π electrons (NAPH/DCB, PHEN/TeCB, and PYR/PtCB) by TEA−, TBA−, and HDTMA−clays. Furthermore, the PAHs show greater trends of solubility enhancement than the compared chlorobenzenes by TMA, TEA, and TBA in aqueous solution. The enhanced sorption and aqueous solubility of PAHs are best described by cation−π interactions between ammonium cations and PAHs relative to chlorobenzenes that are incapable of such interactions. Cation−π complexation between PAHs and tetra-alkyl ammonium cations in chloroform was verified by ring-current-induced upfield chemical shifts of the alkyl groups of cations in the 1H NMR spectrum

The Partitioning of PAHs to Egg Phospholipids Facilitated by Copper and Proton Binding via Cation-π Interactions

The partitioning to lipid-containing solids (cell membranes, natural organic matters) plays an important role in the fate of organic pollutants. We herein studied sorption of a series of aromatic compounds from aqueous solution to gel-phase egg phospholipids. The regression line describing the free-energy relationship between lipid−water distribution coefficient (Kd) and n-octanol–water partition coefficient (KOW) for the high-polar compounds (phenolics, dinitrobenzene, trinitrobenzene) is displaced upward relative to the low-polar compounds (chlorobenzenes, polycyclic aromatic hydrocarbons (PAHs), nitrobenzene, dichlorobenzonitrile), suggesting additive polar extra-interactions besides hydrophobic effects in sorption. Binding of Cu2+ or decreasing pH increases sorption of the three and four-ring PAHs but not the rest compounds. These results led us to propose a specific sorption mechanism, cation-π bonding between PAHs and complexed metal ions or protonated amine groups of phospholipids. The Cu2+-PAH complexation in solution was supported by the observation that PAHs enhance the saturated solubility of CuSO4 in chloroform, and the enhancement correlates with π-donor strength of PAH (pyrene > phenanthrene > naphthalene). The electron coupling between the protonated amine groups of phospholipids and PAHs in chloroform was verified by the electronic deshielding-induced downfield chemical shifts of phenanthrene at low pH in the 1H NMR spectrum

Biosorption of Nonpolar Hydrophobic Organic Compounds to <i>Escherichia Coli</i> Facilitated by Metal and Proton Surface Binding

We observed that the presence of transition metal ion, Ag+, Cu2+, or Fe3+, at a concentration of 3 mg L-1 increases sorption of two nonpolar hydrophobic organic compounds (HOCs), phenanthrene (PHEN), and 1,2,4,5-tetrachlorobenzene (TeCB) by 1.5−4 times to Gram-negative bacteria Escherichia coli. Complexation of transition metals with the deprotonated functional groups (mainly carboxyl) of bacterial cell walls neutralizes the negative charge, making the bacterial surface less hydrophilic and enhancing hydrophobic partition of HOCs. This is evidenced by the fact that the zeta potential (ζ) value of bacteria becomes less negative when a transition metal is present. Furthermore, the observed higher sorption of PHEN than TeCB at low pH (3.8) cannot be fully explained by the pH-dependent hydrophobic effects. The results led us to propose two specific sorption mechanisms for π-donor compounds:  cation−π interactions with protonated amines and π H-bonding with protonated carboxyls. The biosorption of PHEN was best described as π-donor compared to the biosorption of TeCB considered non-π-donor. Results of the present study highlight that the presence of coexisting transition metals and changes on pH have a major effect on the biosorption of nonpolar HOCs

Impact of Sunlight and Humic Acid on the Deposition Kinetics of Aqueous Fullerene Nanoparticles (nC₆₀)

Nanoparticle transport in natural settings is complex due to interactions with the surrounding environment. In this study, the impact of UVA irradiation and humic acid (HA) on deposition of aqueous fullerene nanoparticles (nC₆₀) on a silica surface as a surrogate for natural sediments was studied using packed column experiments and quartz crystal microbalance with dissipation monitoring under various solution conditions. Surface oxidation of nC₆₀ induced by UVA irradiation greatly retarded its deposition due to the increased negative surface charge and hydrophilicity. Dissolved HA, once adsorbed onto the nC₆₀ surface, also hindered its deposition mainly through steric hindrance forces. The extent of this effect depended on the properties and the amount of HA adsorbed, which is a function of ionic strength and HA concentration. HA has limited adsorption on UVA-irradiated nC₆₀ and is expected to play a less important role in its stability. HA immobilized onto the silica surface had a variable effect on nC₆₀ deposition, depending on the complex interplay of Derjaguin–Landau–Verwey–Overbeek (DLVO) and non-DLVO interactions such as electrostatic interaction, steric hindrance, and hydrogen bonding as well as HA molecular conformation. These results highlight the importance of environment-induced changes in nC₆₀ surface chemistry in its fate and transport in aquatic environments

Photochemical Transformation of Carboxylated Multiwalled Carbon Nanotubes: Role of Reactive Oxygen Species

The study investigated the photochemical transformation of carboxylated multiwalled carbon nanotubes (COOH-MWCNTs), an important environmental process affecting their physicochemical characteristics and hence fate and transport. UVA irradiation removed carboxyl groups from COOH-MWCNT surface while creating other oxygen-containing functional groups with an overall decrease in total surface oxygen content. This was attributed to reactions with photogenerated reactive oxygen species (ROS). COOH-MWCNTs generated singlet oxygen (¹O₂) and hydroxyl radical (^•OH) under UVA light, which exhibited different reactivity toward the COOH-MWCNT surface. Inhibition experiments that isolate the effects of ^•OH and ¹O₂ as well as experiments using externally generated ^•OH and ¹O₂ separately revealed that ^•OH played an important role in the photochemical transformation of COOH-MWCNTs under UVA irradiation. The Raman spectroscopy and surface functional group analysis results suggested that ^•OH initially reacted with the surface carboxylated carbonaceous fragments, resulting in their degradation or exfoliation. Further reaction between ^•OH and the graphitic sidewall led to formation of defects including functional groups and vacancies. These reactions reduced the surface potential and colloidal stability of COOH-MWCNTs, and are expected to reduce their mobility in aquatic systems

Prediction of Apolar Compound Sorption to Aquatic Natural Organic Matter Accounting for Natural Organic Matter Hydrophobicity Using Aqueous Two-Phase Systems

The equilibrium partitioning of organic compounds to natural organic matter (NOM) plays a key role in their environmental fate as well as bioavailability. In this study, a prediction model for organic compound sorption to NOM was theoretically derived based on two-phase systems. In this model, the hydrophobicity of NOM was scaled by their partition coefficients in an aqueous two-phase system (KATPS) and that of organics was scaled by their octanol–water partition coefficients (KOW). The model uses only KATPS and KOW as variables. Coefficients in the model were determined using a data set including the organic carbon–water partition coefficient (KOC) of four polycyclic aromatic hydrocarbons (PAHs) sorption to 10 NOM samples collected from surface waters. The resulting model was validated using additional NOM samples and reference NOM, which suggested good prediction power for PAH sorption to aquatic NOM. The model performance was compared with commonly used linear free energy relationship models, and its applicability was discussed. Sorption behavior unexpected by this model is attributed to additional sorption mechanisms other than partitioning. Overall, this approach allows prediction of KOC for apolar organic compound sorption to aquatic NOM simply using their respective partition coefficients in two-phase systems based on a specific model

Current and Future Trends of Low and High Molecular Weight Polycyclic Aromatic Hydrocarbons in Surface Water and Sediments of China: Insights from Their Long-Term Relationships between Concentrations and Emissions

In this study, we analyzed the temporal trend of polycyclic aromatic hydrocarbons (PAHs) in China using data reported over the past 20 years. We found that the total concentrations of low molecular weight PAHs (CΣLPAHs) in surface water and sediments were positively correlated with their total emissions (EΣLPAHs), which increased between 2000 and 2008, then decreased until 2017. Additionally, the total concentrations of high molecular weight PAHs (C∑HPAHs) in surface water and sediments were positively correlated with their total emissions (EΣHPAHs), which increased significantly from 2000 to 2014 and then plateaued. Two future scenarios were assessed to explore C∑LPAHs and C∑HPAHs in surface water and sediments. PAH emissions were reduced by technological improvement in 2030 for coal consumption in Scenario 1 and for control of biomass burning in Scenario 2. Scenario 1 was more efficient than Scenario 2 in reducing C∑HPAHs in the surface water and sediments of China for the areas where CΣHPAHs in surface water exceeded the annual average standard (i.e., 30 ng L–1), with reductions of 38 and 24% in Scenarios 1 and 2, respectively. The observed relationships in this study can provide tools for emission reduction policies in the future

High-Throughput Method for Natural Organic Matter Hydrophobicity Assessment Using an Ionic Liquid-Based Aqueous Two-Phase System

Hydrophobicity of natural organic matter (NOM) is one of its fundamental properties that influence the environmental fate of pollutants and the performance of many water treatment unit processes. In this study, a high-throughput method was developed for NOM hydrophobicity measurement based on the phase separation technique in the 96-well format. It measures the partition coefficients of NOM (KATPS,IL) in an ionic liquid (IL)-based aqueous two-phase system (ATPS). The ATPS was made of 1-butyl-3-methylimidazole bromide solution and a salt solution containing potassium phosphate monobasic and potassium phosphate dibasic. The partition of NOM in IL-based ATPS is mainly affected by its hydrophobicity. log KATPS,IL linearly correlated with the commonly used NOM hydrophobicity scales, including (O + N)/C, O/C, and aromatic carbons. KATPS,IL provided a more accurate assessment of NOM hydrophobicity than spectroscopic indices. Furthermore, KATPS,IL can predict the organic carbon–water partition coefficients for hydrophobic organic chemical sorption to NOM based on the two-phase system model. The high-throughput KATPS,IL measurement and the two-phase system model can be applied to real surface water samples. Our results suggest that the proposed high-throughput method has great potential to be applied to monitor NOM hydrophobicity for environmental risk assessment and water treatment purposes

Nanoporous Pyrene-Based Metal–Organic Frameworks for Fluorescence Screening and Discrimination of Sulfonamide Antibiotics

The presence of antibiotics in aquatic environments has recently raised significant concerns due to their toxic effects as well as the propagation of antibiotic resistances. Thus, efforts are devoted to developing facile screening methods for sulfonamide antibiotics in waters. In this study, we designed a fluorescent array comprising three structurally different pyrene-based luminescent metal–organic framework (LMOF) sensors and applied it in the simultaneous screening of multiple sulfonamides. The pyrene-based LMOFs produced fluorescence quenching responses to sulfonamides via the static quenching mechanism and/or inner filter effect. The array was able to effectively sense and discriminate five structurally similar sulfonamides (sulfamethoxazole, sulfadiazine, sulfapyridine, sulfamerazine, and sulfamethazine), individual sulfonamide at different concentrations, and mixtures of two sulfonamides with various molar ratios. The detection limit of the pyrene-based MOF array for individual sulfonamide was lower than most previously reported fluorescent sensors. It also afforded a fast sulfonamide response (response equilibrium <2 min) owing to the open and uniform nanoporous structures of the LMOF array elements, which allowed a screening speed as fast as approximately 20 min/100 samples. The constructed array exhibited good performance in the discrimination of sulfonamides in real water samples, validating its practicability. This study provides a convenient strategy for the rapid screening of sulfonamide antibiotics in aquatic environments. It also validates the application potential of LMOF array in fluorescence sensing fields