Phenanthrene binding by humic acid-protein complexes as studied by passive dosing technique

Influence of water quality factors on cake layer 3D structures and water channels during ultrafiltration process

The three-dimensional (3D) structure of the cake layer, which could be influenced by water quality factors, plays a significant role in the ultrafiltration (UF) efficiency of water purification. However, it remains challenging to precisely reveal the variation of cake layer 3D structures and water channel characteristics. Herein, we systematically report the variation in the cake layer 3D structure at the nanoscale induced by key water quality factors and reveal its influence on water transport, in particular the abundance of water channels within the cake layer. In comparison with pH and Na+, Ca2+ played more significant role in determining cake layer structures. The sandwich-like cake layer, which was induced by the asynchronous deposition of humic acids and sodium alginate (SA), shifted to an isotropic structure when Ca2+ was present due to the Ca2+ bridging. In comparison with the sandwich-like structure, the isotropic cake layer has higher fractions of free volume (voids) and more water channels, leading to a 147% improvement in the water transport coefficient, 60% reduction in the cake layer resistance, and 21% increase in the final membrane specific flux. Our work elucidates a structure-property relationship where improving the isotropy of the cake layer 3D structure is conducive to the optimization of water channels and water transport within cake layers. This could inspire tailored regulation strategies for cake layers to enhance the UF efficiency of water purification.

Calibration of an acute toxicity model for the marine crustacean, Artemia franciscana, nauplii to support oil spill effect assessments

Oil spill risk and impact assessments rely on time-dependent toxicity models to predict the hazard of the constituents that comprise crude oils and petroleum substances. Dissolved aromatic compounds (ACs) are recognized as a primary driver of aquatic toxicity in surface spill exposure scenarios. However, limited time-dependent toxicity data are available for different classes of ACs to calibrate such models. This study examined the acute toxicity of 14 ACs and 3 binary AC mixtures on Artemia franciscana nauplii at 25 °C. Toxicity tests for 3 ACs were also conducted at 15 °C to evaluate the role of temperature on toxicity. The ACs investigated represented parent and alkylated homocyclic and nitrogen-, sulfur- and oxygen-containing heterocyclic structures with octanol-water partition coefficients (log K_ow) ranging from 3.2 to 6.6. Passive dosing was used to expose and maintain concentrations in toxicity tests which were confirmed using fluorometry, and independently validated for 6 ACs using GC-MS analysis. Mortality was assessed at 6, 24, and 48 h to characterize the time course of toxicity. No mortality was observed for the most hydrophobic AC tested, 7,12-dimethylbenz[a]anthracene, due to apparent water solubility constraints. Empirical log LC₅₀ s for the remaining ACs were fit to a linear regression with log K_ow to derive a critical target lipid body burden (CTLBB) based on the target lipid model. The calculated 48 h CTLBB of 47.1 ± 8.1 μmol/g octanol indicates that Artemia nauplii exhibited comparable sensitivity to other crustaceans. A steep concentration-response was found across all compounds as evidenced by a narrow range (1.0-3.1) in the observed LC₅₀ /LC₁₀ ratio. Differences in toxicokinetics were noted, and no impacts of temperature-dependence of AC toxicity were found. Toxicity data obtained for individual ACs yielded acceptable predictions of observed binary AC mixture toxicity. Results from this study advance toxicity models used in oil spill assessments.

Insight into the Soil Dissolved Organic Matter Ligand-Phenanthrene-Binding Properties Based on Parallel Faction Analysis Combined with Two-Dimensional Correlation Spectroscopy

Dissolved organic matter (DOM) can strongly bind to organic contaminants and control phenanthrene in soil. Herein, four individual parallel factor analysis (PARAFAC) components were found in soil DOM. Component C1 was the humic-like component ligand T, and component C2 was a combination of humic fluorophore ligands M1 and M2. Furthermore, components C3 and C4 were characterized as terrestrial and ubiquitous humic substances. Then, the modified Stern-Volmer complexation model was used to reveal soil DOM component-phenanthrene-binding properties. The overall binding characteristics of a PARAFAC component could not express the phenanthrene-binding properties. Therefore, two-dimensional correlation spectroscopy was used to reveal DOM ligand-phenanthrene-binding properties. After binding with phenanthrene, DOM ligands T, M2, A2, and C1 were quenched but DOM ligands M1, A1, and C2 were excited. The ligands with higher humification presented higher phenanthrene-binding ability. With these promising results, the DOM ligand-phenanthrene-binding characteristics offered theoretical support for soil pollution control.

Interactions between dissolved organic matter and perfluoroalkyl acids in natural rivers and lakes: A case study of the northwest of Taihu Lake Basin, China

Understanding the interactions between dissolved organic matter (DOM) and perfluoroalkyl acids (PFAAs) is essential for predicting the distribution, transport, and fate of PFAAs in aquatic environments. Based on field investigations in the northwest of Taihu Lake Basin combined with laboratory experiments, we obtained DOM and PFAA concentrations as well as compositions and investigated key factors of DOM affecting PFAA variability and capture of PFAAs by DOM. Results indicated that the total concentrations of PFAAs were 73.4-689 ng/L in surface water and that PFAAs were dominated by C3-7 perfluoroalkyl carboxylic acids and perfluorooctane sulfonic acid. The main components of DOM included tyrosine-, fulvic-, and tryptophan-like substances. The Mantel test revealed a significant positive correlation between DOM and PFAAs (P = 0.0001). Fulvic-like substances were identified as the most crucial factors affecting PFAA variability. The laboratory experiments revealed that DOM can spontaneously aggregate into a microgel. Furthermore, 19.1-50.9% of PFAAs, DOM characteristic peaks, and several metals (Ca, Mg, Cu, and Fe) can be removed during aggregation, indicating the capacity of DOM binding organic/inorganic substances. The fulvic-like substances were more effectively removed than the protein-like substances. The distribution coefficients of all PFAAs except perfluorohexanoic acid significantly correlated with their perfluorinated carbon numbers (r = 0.975, p<0.001). Our results provided insights into the interactions between DOM and PFAAs, improving the understanding of the distribution, transport, and fate of PFAAs in aquatic environments.

Heteroaggregation between graphene oxide and titanium dioxide particles of different shapes in aqueous phase

Graphene oxide (GO) inevitably interacts with engineered and/or natural nanoparticles (NPs) after releasing into aquatic environments. It is observed that GO could form heteroaggregates with differently shaped TiO₂ NP_S at pHs 4.0 and 7.0 rather than pH 10.0. Optimal pHs for the maximal heteroaggregation were around 6.63 and 4.92 for TiO₂ spheres and fibers, respectively, which was dominated by electrostatic attraction. The optimal concentration ratio for TiO₂ spheres was 10 times higher than TiO₂ fibers, due to cross-linking configuration and stronger van der Waals force of TiO₂ fibers with GO. Pre-homoaggregation of TiO₂ NPs could promote subsequent heteroaggregation while the promotion was negligible at high ionic strength (e.g., > critical coagulant concentrations), and a longer pre-homoaggregation time showed a stronger promotion (750 s > 150 s > 0 s). Natural organic matter (NOM) obviously inhibited GO-TiO₂ heteroaggregation at pH 4.0 compared with pH 7.0 due to stronger steric hindrance. Moreover, strong disturbance (e.g., sonication) could break down the heteroaggregates, and the disaggregation and re-aggregation processes were shape-independent. This work provides new insights into the role of NPs shapes and homoaggregation on the interaction of GO with both engineered and natural particles/minerals, and the fate of GO in aquatic environments.

Adsorption and catalytic degradation of preservative parabens by graphene-family nanomaterials

Parabens pose increasing threats to human health due to endocrine disruption activity. Adsorption and degradation of parabens by three types of graphene-family nanomaterials (GFNs) were therefore investigated. For a given paraben, the maximum adsorption capacities (Q⁰) followed the order of reduced graphene oxide (RGO) > multilayered graphene (MG) > graphene oxide (GO); for a given GFN, Q⁰ followed the order of butylparaben (BuP) > propylparaben (PrP) > ethylparaben (EtP) > methylparaben (MeP), dominated by hydrophobic interaction. MeP removal by all the three GFNs was highly enhanced (0.55-4.37 times) with the assistance of H₂O₂ due to additional catalytic degradation process, and MG showed the highest removal enhancement. ∙OH was confirmed as the dominant radicals responsible for parabens degradation. For MG and RGO, the metal impurities (Fe, Cu, Mn, and Co) initiated Fenton-like reaction with H₂O₂ to generate ∙OH. GO contained oxygen-centered free radicals, which were responsible for ∙OH formation via transferring electron to H₂O₂. Four degradation byproducts of MeP were identified, including oxalic, propanedioic, fumaric, and 2,5-dihydroxybenzoic acids. Combined with density function theory calculations, the degradation sites and pathways were identified and confirmed. These findings provide useful information on mechanistic understanding towards the adsorption and degradation of parabens by GFNs.

Real-time changes of the adsorption process and conformation of marine dissolved organic matters on the solid-liquid interface

In this study, the adsorption characteristics of marine dissolved organic matters (MDOMs) on the solid-liquid interface in the coastal waters was investigated. The results showed that the organic macromolecules with adsorption ability in MDOMs are not rigid molecules. However, the macromolecules have viscoelasticity properties. At different dilution ratios, the MDOMs adsorption process includes rapid (0-200 s) and slow adsorption (200 s later) periods. MDOMs adsorption in the solid-liquid interface is a dynamic process in which adsorption and hydration occur simultaneously. MDOMs concentration is an important driving force for adsorption. The three macromolecules of acid polysaccharides, protein-like, and polycarboxylate-type humic acids in MDOMs are rich in functional groups and they have the ability to absorb to solid surface. Acidic polysaccharides exhibit a sustained adsorption ability, while the adsorption of other macromolecules occurred only in the initial rapid adsorption period. In addition, the acid polysaccharides show weak thixotropy during the adsorption process. It would cause the stretching of macromolecular structure of the adsorption layer, enhancing the hydration of the adsorption layer. The study shows the adsorption process of MDOMs at the solid-liquid interface and the structural characteristics of the adsorption layer. It can provide helpful information for the inhibition and removal of MDOMs pollution during the actual development of marine resources.

Insights into the impacts of dissolved organic matter of different origins on bioaccumulation and translocation of per- and polyfluoroalkyl substances (PFASs) in wheat

Per- and polyfluoroalkyl substances (PFASs) have been found to be widely present in soil. Dissolved organic matter (DOM) in soil are supposed to greatly affect the bioavailability of PFASs in soil. Herein, hydroponic experiments were conducted to understand the impacts of two kinds of typical DOM, bovine serum albumin (BSA) and humic acid (HA), on the uptake and translocation of legacy PFASs and their emerging alternatives, perfluorooctane sulfonic acid (PFOS), perfluorooctane acid (PFOA), perfluorohexane sulfonic (PFHxS) and 6:2 chlorinated polyfluoroalkyl ether sulfonate (6:2 Cl-PFESA) in wheat (Triticum aestivum L.). The results indicated that both HA and BSA significantly inhibited the bioaccumulation and translocation of PFASs in the roots and shoots of wheat, and the impacts of BSA were greater than HA. This difference was explained by the greater binding affinities of the four PFASs with BSA than with HA, as evidenced by the equilibrium dialysis and isothermal titration calorimetry (ITC) analyses. It was noting that inhibition impacts of the BSA-HA mixture (1:1) were lower than BSA alone. The results of Fourier transform infrared (FT-IR) spectroscopy and excitation-emission matrix (EEM) fluorescence spectroscopy suggested that HA could bind with the fluorescent tryptophan residues in BSA greatly, competing the binding sites with PFASs and forming a cover on the surface of BSA. As a result, the binding of PFASs with BSA-HA complex was much lower than that with BSA, but close to HA. The results of this study shed light on the impacts of DOM in soil on the bioaccumulation and translocation of PFASs in plants.

Critical assessment of an equilibrium-based method to study the binding of waterborne organic contaminants to natural dissolved organic matter (DOM)

Dissolved organic matter (DOM) can play a major role in determining availability of pollutants to aquatic biota. Equilibrium dialysis is the most commonly used method to assess the interaction between DOM and organic contaminants. However, results obtained through this method can be affected by confounding factors linked to the diffusion of DOM through the membrane or the interaction of DOM and/or the compounds with the membrane itself. In this study, we propose an improved experimental approach, where highly hydrophilic cellulose-ester membranes with small molecular cut-off (100-500 Da) were used to overcome some of these hindrances. The performance of the method to determine the binding of a commonly used moderately hydrophobic herbicide (Isoproturon - ISU) with natural DOM was critically evaluated through a set of quality assurance criteria, across a range of DOM concentrations and pH conditions. DOM trans-membrane diffusion was prevented by the smaller pore size of the dialysis membrane. Good measurement reproducibility, mass balance closure, and successful trans-membrane equilibrium of ISU were obtained. ISU showed relatively low affinity with DOM (log K_DOC 1-2 L g^-1), which was significantly influenced by varying pH and DOM concentration. An alternative membrane may be needed for higher pH conditions as the greater adsorption effect blurred the observation of trans-membrane equilibrium and confounding mass balance closure. The paper makes recommendations on how to avoid measurement artefacts, while considering criteria for the expected mass distribution of compounds at equilibrium and for sorption onto the membrane and surfaces of the experimental units.

Three-Dimensional Analysis of the Natural-Organic-Matter Distribution in the Cake Layer to Precisely Reveal Ultrafiltration Fouling Mechanisms

Cake layer formation is the dominant ultrafiltration membrane fouling mechanism after long-term operation. However, precisely analyzing the cake-layer structure still remains a challenge due to its thinness (micro/nano scale). Herein, based on the excellent depth-resolution and foulant-discrimination of time-of-flight secondary ion mass spectrometry, a three-dimensional analysis of the cake-layer structure caused by natural organic matter was achieved at lower nanoscale for the first time. When humic substances or polysaccharides coexisted with proteins separately, a homogeneous cake layer was formed due to their interactions. Consequently, membrane fouling resistances induced by proteins were reduced by humic substances or polysaccharides, leading to a high flux. However, when humic substances and polysaccharides coexisted, a sandwich-like cake layer was formed owing to the asynchronous deposition based on molecular dynamics simulations. As a result, membrane fouling resistances were superimposed, and the flux was low. Furthermore, it is interesting that cake-layer structures were relatively stable under common UF operating conditions (i.e., concentration and stirring). These findings better elucidate membrane fouling mechanisms of different natural-organic-matter mixtures. Moreover, it is demonstrated that membrane fouling seems lower with a more homogeneous cake layer, and humic substances or polysaccharides play a critical role. Therefore, regulating the cake-layer structure by feed pretreatment scientifically based on proven mechanisms should be an efficient membrane-fouling-control strategy.

The transformation of phosphorus fractions in high-solid sludge by anaerobic digestion combined with the high temperature thermal hydrolysis process

Transforming inactive phosphorus (P) to active P to recover it from waste activated sludge is important. The transformation of P fractions from high-solid sludge by the anaerobic digestion (AD) and acidification phase of AD (AAD) combined with a high temperature thermal hydrolysis process (HTTHP) was investigated. The results showed that the sequence of P release effects by three processes was HTTHP + AAD > AD + HTTHP > HTTHP + AD. The PO₄^3--P release from high-solid sludge was directly affected by the temperature of HTTHP. At 140 °C, each process had more PO₄^3--P release than that at 160 °C. The total amount of PO₄^3--P release in AD + HTTHP was approximately 6 times that of HTTHP + AD. Based on the experimental results, a new process of mesophilic AD - post HTTHP was recommended, in which, enhancement of P release by sulfide ions was also proposed.

Effects of humic matter on the anaerobic digestion of sewage sludge: New insights from sludge structure

A sludge structure perspective of the effects of humic matter (HM) on the anaerobic digestion (AD) of sewage sludge was proposed and investigated in this study. It was observed that the net cumulative methane production of sludge was significantly lower in the presence of HM. The fractal dimension values of sludge samples were increased by the addition of HM, whilst their total surface energy values were decreased, indicating that HM reinforced the stability of the sludge structure. Analyses of the physicochemical properties of different sludge samples revealed that HM enhanced the structural stability of sludge organic substances and restricted its organic solubility by interacting with the biopolymers to form a dense granular aggregate, thereby reducing the random-coil degree of extracellular organic substances and its active sites for enzymes, which suggested that HM could inhibit AD by changing the sludge structure. It was further confirmed by the biochemical methane potential (BMP) assay process. The BMP tests and corresponding kinetic analyses revealed that HM can restrict the potentials of sludge organic solubilisation, hydrolysis and acidification processes, but not necessarily the rates of these processes by enhancing the structural stability of high molecular weight readily biodegradable organic matter (RBOM) and restricting the mobility of low molecular weight RBOM (i.e., hydrolysis products). These findings can enrich understanding of the AD of sludge and may thus aid in the development of more effective methods to improve sludge treatment.

Linear and nonlinear partition of nonionic organic compounds into resin ADS-21 from water

The predominance of natural organic matter (NOM) in nonlinear sorption of nonionic organic compounds (NOCs) is a fundamental behavior that controlling the fate, transfer and bioavailability of NOCs in natural environment. There is a debate, i.e., whether the nonlinear sorption is captured by nonlinear partition mechanism or adsorption mechanism. The debate has been going on for decades because characteristics of nonlinear partition are still unknown due to the lack of an adsorbent that can partition NOCs nonlinearly. We find a resin ADS-21, with specific surface area undetectable (<0.5 m² g^-1) but high sorption capacity for NOCs (up to 1000 mg g^-1 for phenol as an example), is an ideal adsorbent for examining characteristics of nonlinear partitioning. This resin has nonlinear isotherms for phenols and anilines but linear isotherms for polycyclic aromatic hydrocarbons and nitrobenzenes. The observed positively linear relationship of sorption capacities of NOCs with NOCs solubility in water or octanol, could be one of the characteristics of nonlinear partition. Moreover, competitive sorption and no desorption hysteresis could be observed for the nonlinear partition. Hydrogen-bonding of phenols and anilines with ADS-21 is responsible for nonlinear partition, competitive sorption and isotherm nonlinearity. These evidences would be supportive for understanding nonlinear partition and the nonlinear sorption of NOCs by NOM.

Effects of natural dissolved organic matter on the complexation and biodegradation of 17α-ethinylestradiol in freshwater lakes

Natural dissolved organic matter (DOM) produced in algal blooms and overgrowths of macrophyte changes the elimination and ecotoxicity of estrogens in freshwater lakes. The complexation of 17α-ethinylestradiol (EE2) and various DOMs, including the water- and sediment-derived DOMs from the algal-dominant zone in Lake Taihu (TW and TS, respectively) and the macrophyte-dominant zone in Poyang Lake (PW and PS, respectively), and the humic acid (HA), was investigated along with the subsequent effects on EE2 biodegradation. Dialysis equilibrium experiments showed that binding to DOM significantly decreased the freely soluble concentrations of EE2. The binding capacity of the five DOMs followed the order of PW < TW < PS ≈ TS < HA. A negative correlation was found between the organic-carbon-normalized sorption coefficient (logK_DOC) and the absorption ratio (E₂/E₃) of DOM, indicating that the large sized, aromatic molecules were involved in the complexation. The reduced freely soluble concentrations of EE2 did not inhibit its biodegradation by an EE2-degrading strain, Rhodobacter blasticus. Conversely, the autochthonous-dominated water-derived DOMs stimulated a more extensive biodegradation of EE2 than the sediment-derived DOMs, and the existence of HA resulted in the smallest increase in EE2 biodegradation. The promoting effect was associated with the increased concentration, activity, and transforming rate of R. blasticus by the bioavailable components in DOM. The present study suggests that the significant impact of natural DOM should be fully considered when assessing the fate and ecological risks of estrogens in eutrophic waters.

Interaction between humic acid and protein in membrane fouling process: A spectroscopic insight

Membrane fouling remains a major challenge for applying membrane technology to water treatment and, therefore, new tools to recognize the key foulants are essential for characterizing and evaluating the membrane fouling process. In this work, fluorescence excitation emission matrix coupled with parallel factor framework-clustering analysis was used to investigate the membrane fouling during the filtration process of humic acid (HA) and bovine serum albumin (BSA) solution by polyvinylidene fluoride membrane. Interestingly, the interaction between BSA and HA in the membrane fouling process was observed, and was further confirmed by infrared microspectroscopy and two-dimensional correlation spectroscopic analysis. In addition, the HA-induced membrane fouling was observed to be initially relieved, but became aggravated when a certain amount of BSA was added. Furthermore, with such an integrated approach, the OH groups in HA and amide bands in BSA were found to be mainly responsible for the membrane fouling and the HA-BSA interaction was mainly caused by the encapsulation of BSA with HA. This work develops a new method for probing membrane fouling and demonstrates the interaction between membrane foulants and its roles in membrane fouling process. Furthermore, the integrated approach developed in this work has a potential to explore other types of interfacial interactions.

Sorption of 17β-estradiol to the dissolved organic matter from animal wastes: effects of composting and the role of fulvic acid-like aggregates

Steroid estrogens, such as 17β-estradiol (E2), in animal manure pose a potential threat to the aquatic environment. The transport and estrogenicity of estrogens influence the sorption of estrogens to dissolved organic matter (DOM) in animal manure, and composting treatment alters the structure and composition of the manure. The objectives of the present study were to identify the contribution of the molecular composition of DOM of composted manure to the sorption of E2 and then elucidate the dominant mechanisms involved in the interaction of E2 with manure-derived DOM. The excitation-emission matrix (EEM) spectra and atomic force microscopy (AFM) showed that composting significantly altered the chemical composition and structure of DOM. A decrease in the atomic ratios of oxygen (O)/carbon (C) occurred in conjunction with the formation of DOM aggregates in the composted manure, indicating that the hydrophilicity and polarity of the DOM decreased after composting. Composting increased the sorption coefficients (K_DOC-E2) for E2 to DOM, and K_DOC-E2 was positively correlated with the proportion of the fulvic acid (FA)-like fraction and molecular weight (MW) fractions of the DOM (range of 1.0 × 10³-7.0 × 10³ Da and 7.0 × 10³-1.4 × 10⁴ Da). Specifically, E2 showed a tendency for sorption to medium-sized FA-like molecules of DOM aggregates in composted manure. Hydrophobic forces and π-π binding appeared to be the main mechanisms underlying the aforementioned interaction.

Interactions between biogenic selenium nanoparticles and goethite colloids and consequence for remediation of elemental mercury contaminated groundwater

Ubiquitous colloidal minerals such as goethite can have a significant impact on the performance of nanoparticles-based groundwater remediation due to aggregation. Heteroaggregation and retention of Se nanoparticles (SeNPs) by goethite in groundwater, and its impact on Hg⁰ remediation by SeNPs were investigated in this study. In order to mitigate the adverse effects of aggregation, the effects of bacterial extracellular polymeric substances (EPS) on the stability of SeNPs and Hg⁰ sequestration using SeNPs were also evaluated. Heteroaggregation of SeNPs with goethite in groundwater was stronger than homoaggregation of SeNPs or goethite. Addition of EPS could slightly decrease homoaggregation of SeNPs and significantly reduce heteroaggregation. Column transport experiments showed that goethite coated quartz sand could retain 1.36 times a higher amount of SeNPs than uncoated quartz sand. Hg⁰ remediation by SeNPs was significantly inhibited by heteroaggregation of SeNPs with goethite and EPS could effectively mitigate this inhibitory effect. The Hg⁰ removal efficiency decreased to 71.6% and 66.9%, respectively in the presence of 20 and 100mgL^-1 goethite. When 200mgL^-1 EPS was added together with 100mgL^-1 goethite, 81.2% of the supplied Hg⁰ was removed from the groundwater. This study demonstrates that the widespread presence of goethite could significantly reduce the remediation efficiency of Hg⁰ contaminated groundwater and that EPS is a promising amendment for mitigating the adverse effects of heteroaggregation. This research also contributes to a further understanding of the environmental behaviour of nanoparticles.

Effect of extracellular polymeric substance components on the sorption behavior of 2,2',4,4'-tetrabromodiphenyl ether to soils: Kinetics and isotherms

Microbial extracellular polymeric substances (EPS) and persistent organic pollutants (POPs) commonly exist in the soil environment. Currently, there is a knowledge gap regarding the effect of EPS on the fate of POPs in soil. In the present study, 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) was used as a model compound to investigate the effects of bovine serum albumin (BSA) and sodium alginate (SA) - mimicking the main components of EPS - on sorption of POPs to soils, through batch experiments. Irrespective of the concentration of BSA: the addition of BSA did enhance the sorption capacity of BDE-47 to soils, due to generation of more sorption sites. For SA, it increased the sorption capacity of BDE-47 at low BDE-47 concentrations, while the presence of SA negatively affected sorption of BDE-47 at high BDE-47 concentrations. The partition effect dominates the sorption of BDE-47 to soils, but after adding either BSA or SA, the sorption of BDE-47 to soils is dominated by surface sorption. Film diffusion and intra-particle diffusion were also involved in the sorption process with and without BSA or SA, with the latter being the rate-limiting step. The heterogeneous surface and nonlinear sorption behavior of BDE-47 to soils increased in the presence of either BSA or SA. The FTIR spectra indicated that the aromatic CC, H-bonds and OH groups may be involved in the sorption process. Therefore, BSA enhanced the retention of BDE-47 to soil, while SA's influence on BDE-47 sorption to soil depended on the concentration of BDE-47.

Multi-spectroscopic investigation on the complexation of tetracycline with dissolved organic matter derived from algae and macrophyte

Interactions of antibiotics with algae-derived dissolved organic matter (ADOM) and macrophyte-derived dissolved organic matter (MDOM) are of vital importance to the transport and ecotoxicity of antibiotics in eutrophic freshwater lakes. Multi-spectroscopic techniques were used to investigate the complexation of tetracycline (TTC) with ADOM and MDOM collected from Lake Taihu (China). The 3 fluorescent components, tyrosine-, tryptophan-, and humic-like component, were identified by excitation emission matrix spectra with parallel factor analysis. Their fluorescence was quenched at different degree by TTC titration through static quenching. The complexation of TTC induced conformational changes in DOM fractions. Synchronous fluorescence spectra combined with two dimensional correlation spectroscopy further suggested that the formation of TTC-DOM complexes occurred on the sequential order of tryptophan-like→tyrosine-like→humic-like component. The effective quenching constants of tryptophan- and tyrosine-like component were similar, higher than those of humic-like component. The strong binding ability and abundant content of protein-like substances indicated their prominent role in the TTC-DOM complexation. Fourier transform infrared spectroscopy further revealed that the heterogeneous functional groups, including amide I and II, aromatics, and aliphatics, were responsible for the complexation. These results highlight the significant impact of the overgrowth of algae and macrophyte on the environmental behavior of antibiotics in waters.

Adsorption and bioaccessibility of phenanthrene on carbon nanotubes in the in vitro gastrointestinal system

Adsorption and bioaccessibility of phenanthrene on graphite and multiwalled carbon nanotubes (CNTs) were investigated in simulated gastrointestinal fluid using a passive dosing system. The saturated adsorption capacity of phenanthrene on different adsorbents follows an order of hydroxylated CNTs (H-CNTs)>carboxylated CNTs (C-CNTs)>graphitized CNTs (G-CNTs)>graphite, consistent with the order of their surface area and micropore volume. The change of phenanthrene adsorption on the adsorbents is different with the presence of pepsin (800mg/L) and bile salts (500mg/L and 5000mg/L, abbreviated as BS500 and BS5000). Both solubilization of phenanthrene by pepsin and bile salts and their competition with phenanthrene for the adsorption sites play a role. In addition, the large increase of the maximum adsorption capacity in BS5000 solution indicates an enhanced dispersion of CNTs or an exfoliation of graphite by bile salts, which consequently increases the exposed surface area. The bioaccessibility increases in pepsin and BS500 solution with a growing free phenanthrene concentration. Although the bioaccessibility of phenanthrene stalls or slightly decreases in the middle range of free phenanthrene concentration in BS5000 solution, the bioaccessibility overall is much higher than that in pepsin and BS500 solution at the same phenanthrene level. It is impossible to separate the effect of competition from dispersion (or exfoliation) at this stage, but the relative contribution of solubilization to phenanthrene desorption in pepsin and BS500 solutions was quantified, which improves our understanding of the mechanisms on bioaccessibility of adsorbed pollutants on CNTs.

Adsorption of sulfonamides on reduced graphene oxides as affected by pH and dissolved organic matter

With the significant increase in use and application of graphene and the frequent presence of sulfonamides (SAs) in water environments, their interactions have attracted extensive attention. In this study, adsorption of two selected SAs (sulfapyridine and sulfathiazole) by two reduced graphene oxides (rGO1 and rGO2) was examined as affected by pH and dissolved organic matter (DOM). Adsorption of SAs by rGOs was highly pH-dependent, and adsorption affinity of different SAs species followed the order of SA(0) > SA(+) > SA(-). The contribution of SA(0) to the overall adsorption was greater than its species fraction, implying the importance of the neutral species to adsorption. SAs adsorption isotherms at three selected pHs were in the order of pH 5.0 > pH 1.0 > pH 11.0, which was in accordance with the variation of site energy distribution analysis. Hydrophobic interaction, π-π EDA interaction and electrostatic interaction were the main mechanisms responsible for SAs adsorption by rGOs. Three representative natural DOMs including humic acid (HA), bovine serum albumin (BSA), and sodium alginate together with sodium dodecylbenzenesulfonate (SDBS) as a synthetic DOM were used to investigate their effect on SAs adsorption. The inhibition impact of DOM on SAs adsorption was lower for rGOs compared with carbon nanotubes and graphite, which might be attributed to the higher oxygen contents of rGOs. Also, the suppression effect of DOM generally followed an order of SDBS > HA ≥ BSA > alginate, indicating the importance role of DOM compositions. These results should be important for assessing the fate and transport of graphene and antibiotics in the environment.

Heteroaggregation of graphene oxide with minerals in aqueous phase

Upon release into waters, sediments, and soils, graphene oxide (GO) may interact with fine mineral particles. We investigated the heteroaggregation of GO with different minerals, including montmorillonite, kaolinite, and goethite, in aqueous phase. GO significantly enhanced the dispersion of positively charged goethite (>50%) via heteroaggregation, while there was no interaction between GO and negatively charged montmorillonite or kaolinite. Electrostatic attraction was the dominant force in the GO-goethite heteroaggregation (pH 4.0-8.5), and the dissolved Fe ions (<0.16 mg/L) from goethite were unable to destabilize GO suspension. The GO-goethite heteroaggregation was further quantitatively investigated through GO adsorption study. All adsorption isotherms of GO at different solution pH (4.0 and 6.5) followed the Linear model. The apparent intercept (1.0-6.9 mg/g) was observed for all the adsorption isotherms, indicating that this fraction of adsorbed GO was difficult to desorb from goethite (defined here as irreversible adsorption) under the tested conditions. Desorption hysteresis was observed, which could be explained by the formation of multilayered GO-goethite complex with high configurational stability. These findings are useful for understanding the interaction of GO with mineral surfaces, and potential fate and toxicity of GO under natural conditions in aquatic environments, as well as in soils and sediments.

Effects of solution chemistry on adsorption of selected pharmaceuticals and personal care products (PPCPs) by graphenes and carbon nanotubes

Adsorption of three selected pharmaceuticals and personal care products (PPCPs) (ketoprofen (KEP), carbamazepine (CBZ), and bisphenol A (BPA)) by two reduced graphene oxides (rGO1 and rGO2) and one commercial graphene was examined under different solution conditions. Single-walled carbon nanotubes (SWCNTs), multiwalled carbon nanotubes (MWCNTs), and powdered graphite were also investigated for comparison. All adsorption isotherms followed the order of SWCNTs > rGO1 > rGO2 > MWCNTs > graphene > graphite, consistent with the orders of their surface areas and micropore volumes. After surface area normalization, adsorption affinities of the three PPCPs onto graphenes were lower than onto graphite, suggesting incomplete occupation for adsorption sites because of the aggregation of graphene sheets and the presence of oxygen-containing functional groups. The observed pH effects on adsorption correlated well with the pH-regulated distribution of the protonated neutral species of the three PPCPs. Increasing ionic strength from 0 to 20 mM increased KEP adsorption due to the electrostatic screening by Na(+) and Ca(2+). Both humic acid (HA) and sodium dodecylbenzenesulfonate (SDBS) suppressed PPCPs adsorption to all adsorbents, but their impacts onto graphenes were lower than those onto CNTs because of their lower adsorption by graphenes. More severe HA (or SDBS) effect was found on negatively charged KEP at the tested solution pH 6.50 due to the electrostatic repulsion between the same charged KEP and HA (or SDBS). The findings of the present study may have significant implications for the environmental fate assessment of PPCPs and graphene.

Adsorption of phenanthrene on multilayer graphene as affected by surfactant and exfoliation

Surfactant mediated exfoliation of multilayer graphene and its effects on phenanthrene adsorption were investigated using a passive dosing technique. In the absence of surfactant (sodium cholate, NaC), multilayer graphene had higher adsorption capacity for phenanthrene than carbon nanotube and graphite due to the higher surface area and micropore volume. The observed desorption hysteresis is likely caused by the formation of closed interstitial spaces through folding and rearrangement of graphene sheets. In the presence of NaC (both 100 and 8000 mg/L), phenanthrene adsorption on graphene was decreased due to the direct competition of NaC molecules on the graphene surface. With the aid of sonication, multilayer graphene sheets were exfoliated by NaC, leading to better dispersion. The degree of dispersion depended on the graphene-NaC ratio in aqueous solution rather than critical micelle concentration of NaC, and the good dispersion occurred after reaching adsorption saturation of NaC molecules on graphene sheets. In addition, exfoliation weakened the competition between phenanthrene and NaC and enhanced the adsorption capacity of graphene for phenanthrene due to exposed new sites. The findings on exfoliation of graphene sheets and related adsorption properties highlight not only the potential applications of multilayer graphene as efficient adsorbent but also its possible environmental risk.