A Review of Mechanistic Models for Predicting Adverse Effects in Sediment Toxicity Testing.

Since recognizing the importance of bioavailability for understanding the toxicity of chemicals in sediments, mechanistic modeling has advanced over the last 40 years by building better tools for estimating exposure and making predictions of probable adverse effects. Our review provides an up-to-date survey of the status of mechanistic modeling in contaminated sediment toxicity assessments. Relative to exposure, advances have been most substantial for non-ionic organic contaminants (NOCs) and divalent cationic metals, with several equilibrium partitioning-based (Eq-P) models having been developed. This has included the use of Abraham equations to estimate partition coefficients for environmental media. As a result of the complexity of their partitioning behavior, progress has been less substantial for ionic/polar organic contaminants. When the EqP-based estimates of exposure and bioavailability are combined with water-only effects measurements, predictions of sediment toxicity can be successfully made for NOCs and selected metals. Both species sensitivity distributions and toxicokinetic and toxicodynamic models are increasingly being applied to better predict contaminated sediment toxicity. Furthermore, for some classes of contaminants, such as polycyclic aromatic hydrocarbons, adverse effects can be modeled as mixtures, making the models useful in real-world applications, where contaminants seldomly occur individually. Despite the impressive advances in the development and application of mechanistic models to predict sediment toxicity, several critical research needs remain to be addressed. These needs and others represent the next frontier in the continuing development and application of mechanistic models for informing environmental scientists, managers, and decisions makers of the risks associated with contaminated sediments. Environ Toxicol Chem 2023;00:1-17. © 2023 SETAC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

Repeatability and Reproducibility of the RTgill-W1 Cell Line Assay for Predicting Fish Acute Toxicity.

Predicting fish acute toxicity of chemicals in vitro is an attractive alternative method to the conventional approach using juvenile and adult fish. The rainbow trout (Oncorhynchus mykiss) cell line assay with RTgill-W1 cells has been designed for this purpose. It quantifies cell viability using fluorescent measurements for metabolic activity, cell- and lysosomal-membrane integrity on the same set of cells. Results from over 70 organic chemicals attest to the high predictive capacity of this test. We here report on the repeatability (intralaboratory variability) and reproducibility (interlaboratory variability) of the RTgill-W1 cell line assay in a round-robin study focusing on 6 test chemicals involving 6 laboratories from the industrial and academic sector. All participating laboratories were able to establish the assay according to preset quality criteria even though, apart from the lead laboratory, none had previously worked with the RTgill-W1 cell line. Concentration-response modeling, based on either nominal or geometric mean-derived measured concentrations, yielded effect concentrations (EC50) that spanned approximately 4 orders of magnitude over the chemical range, covering all fish acute toxicity categories. Coefficients of variation for intralaboratory and interlaboratory variability for the average of the 3 fluorescent cell viability measurements were 15.5% and 30.8%, respectively, which is comparable to other fish-derived, small-scale bioassays. This study therefore underlines the robustness of the RTgill-W1 cell line assay and its accurate performance when carried out by operators in different laboratory settings.

Evaluating solid phase (micro-) extraction tools to analyze freely ionizable and permanently charged cationic surfactants.

Working with and analysis of cationic surfactants can be problematic since aqueous concentrations are difficult to control, both when taking environmental aqueous samples as well as performing laboratory work with spiked concentrations. For a selection of 32 amine based cationic surfactants (including C8- to C18-alkylamines, C14-dialkyldimethylammonium, C8-tetraalkylammonium, benzalkonium and pyridinium compounds), the extraction from aqueous samples was studied in detail. Aqueous concentrations were determined using solid phase extraction (SPE; 3 mL/60 mg Oasis WCX-SPE cartridges) with recoveries of ≥80% for 30 compounds, and ≥90% for 16 compounds. Sorption to glassware was evaluated in 120 mL flasks, 40 mL vials and 1.5 mL autosampler vials, using 15 mM NaCl, where the glass binding of simple primary amines and quaternary ammonium compounds increased with alkyl chain length. Sorption to the outside of pipette tips (≤20% of total amount in solution) when sampling aqueous solutions may interfere with accurate measurements. Polyacrylate solid phase microextraction (PA-SPME) fibers with two coating thicknesses (7 and 35 µm) were tested as potential extraction devices. The uptake kinetics, pH-dependence and influence of ionic strength on sorption to PA fibers were studied. Changing medium from 100 mM Na+ to 10 mM Ca2+ decreases Kfw with one order of magnitude. Results indicate that for PA-SPME neutral amines are absorbed rather than adsorbed, although the exact sorption mechanism remains to be elucidated. Further research remains necessary to establish a definitive applicability domain for PA-SPME. However, results indicate that alkyl chain lengths ≥14 carbon atoms and multiple alkyl chains become problematic. A calibration curve should always be measured together with the samples. In conclusion, it seems that for amine based surfactants PA-SPME does not provide the reliability and reproducibility necessary for precise sorption experiments, specifically for alkyl chain lengths beyond 12 carbon atoms.

Fragment-based approach to calculate hydrophobicity of anionic and nonionic surfactants derived from chromatographic retention on a C18 stationary phase.

To predict the fate and potential effects of organic contaminants, information about their hydrophobicity is required. However, common parameters to describe the hydrophobicity of organic compounds (e.g., octanol-water partition constant [KOW ]) proved to be inadequate for ionic and nonionic surfactants because of their surface-active properties. As an alternative approach to determine their hydrophobicity, the aim of the present study was therefore to measure the retention of a wide range of surfactants on a C18 stationary phase. Capacity factors in pure water (k'0 ) increased linearly with increasing number of carbon atoms in the surfactant structure. Fragment contribution values were determined for each structural unit with multilinear regression, and the results were consistent with the expected influence of these fragments on the hydrophobicity of surfactants. Capacity factors of reference compounds and log KOW values from the literature were used to estimate log KOW values for surfactants (log KOWHPLC). These log KOWHPLC values were also compared to log KOW values calculated with 4 computational programs: KOWWIN, Marvin calculator, SPARC, and COSMOThermX. In conclusion, capacity factors from a C18 stationary phase are found to better reflect hydrophobicity of surfactants than their KOW values. Environ Toxicol Chem 2017;36:329-336. © 2016 The Authors. Environmental Toxicology and Chemistry Published by Wiley Periodicals, Inc. on behalf of SETAC.

Which Molecular Features Affect the Intrinsic Hepatic Clearance Rate of Ionizable Organic Chemicals in Fish?

Greater knowledge of biotransformation rates for ionizable organic compounds (IOCs) in fish is required to properly assess the bioaccumulation potential of many environmentally relevant contaminants. In this study, we measured in vitro hepatic clearance rates for 50 IOCs using a pooled batch of liver S9 fractions isolated from rainbow trout (Oncorhynchus mykiss). The IOCs included four types of strongly ionized acids (carboxylates, phenolates, sulfonates, and sulfates), three types of strongly ionized bases (primary, secondary, tertiary amines), and a pair of quaternary ammonium compounds (QACs). Included in this test set were several surfactants and a series of beta-blockers. For linear alkyl chain IOC analogues, biotransformation enzymes appeared to act directly on the charged terminal group, with the highest clearance rates for tertiary amines and sulfates and no clearance of QACs. Clearance rates for C12-IOCs were higher than those for C8-IOC analogues. Several analogue series with multiple alkyl chains, branched alkyl chains, aromatic rings, and nonaromatic rings were evaluated. The likelihood of multiple reaction pathways made it difficult to relate all differences in clearance to specific molecular features the tested IOCs. Future analysis of primary metabolites in the S9 assay is recommended to further elucidate biotransformation pathways for IOCs in fish.

Sorption of structurally different ionized pharmaceutical and illicit drugs to a mixed-mode coated microsampler.

The mixed-mode (C18/strong cation exchange-SCX) solid-phase microextraction (SPME) fiber has recently been shown to have increased sensitivity for ionic compounds compared to more conventional sampler coatings such as polyacrylate and polydimethylsiloxane (PDMS). However, data for structurally diverse compounds to this (prototype) sampler coating are too limited to define its structural limitations. We determined C18/SCX fiber partitioning coefficients of nineteen cationic structures without hydrogen bonding capacity besides the charged group, stretching over a wide hydrophobicity range (including amphetamine, amitriptyline, promazine, chlorpromazine, triflupromazine, difenzoquat), and eight basic pharmaceutical and illicit drugs (pKa>8.86) with additional hydrogen bonding moieties (MDMA, atenolol, alprenolol, metoprolol, morphine, nicotine, tramadol, verapamil). In addition, sorption data for three neutral benzodiazepines (diazepam, temazepam, and oxazepam) and the anionic NSAID diclofenac were collected to determine the efficiency to sample non-basic drugs. All tested compounds showed nonlinear isotherms above 1mmol/L coating, and linear isotherms below 1mmol/L. The affinity for C18/SCX-SPME for tested organic cations without Hbond capacities increased with longer alkyl chains, ranging from logarithmic fiber-water distribution coefficients (log Dfw) of 1.8 (benzylamine) to 5.8 (triflupromazine). Amines smaller than benzylamine may thus have limited detection levels, while cationic surfactants with alkyl chain lengths >12 carbon atoms may sorb too strong to the C18/SCX sampler which hampers calibration of the fiber-water relationship in the linear range. The log Dfw for these simple cation structures closely correlates with the octanol-water partition coefficient of the neutral form (Kow,N), and decreases with increased branching and presence of multiple aromatic rings. Oxygen moieties in organic cations decreased the affinity for C18/SCX-SPME. Log Dfw values of neutral benzodiazepines were an order of magnitude higher than their log Kow,N. Results for anionic diclofenac species (logKow,N 4.5, pKa 4.0, log Dfw 2.9) indicate that the C18-SCX fiber might also be useful for sampling of organic anions. This data supports our theory that C18-based coatings are able to sorb ionized compounds through adsorption and demonstrates the applicability of C18-based SPME in the measurement of freely dissolved concentrations of a wide range of ionizable compounds.

Critical micelle concentration values for different surfactants measured with solid-phase microextraction fibers.

The amphiphilic nature of surfactants drives the formation of micelles at the critical micelle concentration (CMC). Solid-phase microextraction (SPME) fibers were used in the present study to measure CMC values of 12 nonionic, anionic, cationic, and zwitterionic surfactants. The SPME-derived CMC values were compared to values determined using a traditional surface tension method. At the CMC of a surfactant, a break in the relationship between the concentration in SPME fibers and the concentration in water is observed. The CMC values determined with SPME fibers deviated by less than a factor of 3 from values determined with a surface tension method for 7 out of 12 compounds. In addition, the fiber-water sorption isotherms gave information about the sorption mechanism to polyacrylate-coated SPME fibers. A limitation of the SPME method is that CMCs for very hydrophobic cationic surfactants cannot be determined when the cation exchange capacity of the SPME fibers is lower than the CMC value. The advantage of the SPME method over other methods is that CMC values of individual compounds in a mixture can be determined with this method. However, CMC values may be affected by the presence of compounds with other chain lengths in the mixture because of possible mixed micelle formation. Environ Toxicol Chem 2016;35:2173-2181. © 2016 SETAC.

Mechanisms of Neutral and Anionic Surfactant Sorption to Solid-Phase Microextraction Fibers.

Octanol-water partitioning (Kow) is considered a key parameter for hydrophobicity and is often applied in the prediction of the environmental fate and exposure of neutral organic compounds. However, surfactants can create difficulties in the determination of Kow because of emulsification of both water and octanol phases. Moreover, not only is sorption behavior of ionic surfactants related to hydrophobicity, but also other interactions are relevant in sorption processes. A different approach to develop parameters that can be applied in predictive modeling of the fate of surfactants in the environment is therefore required. Distribution between solid-phase microextraction (SPME) fibers and water was used in this study to measure the affinity of surfactants to a hydrophobic phase. Fiber-water sorption coefficients of alcohol ethoxylates, alkyl carboxylates, alkyl sulfates, and alkyl sulfonates were determined at pH 7 by equilibration of the test analytes between fiber and water. Distribution between fiber and water of anionic compounds with pKa ∼ 5 (i.e., alkyl carboxylates) was dominated by the neutral fraction. Anionic surfactants with pKa ≤ 2 (i.e., alkyl sulfates and alkyl sulfonates) showed strong nonlinear distribution to the fiber. The fiber-water sorption coefficients for alcohol ethoxylates and alkyl sulfates showed a linear trend with bioconcentration factors from the literature. Fiber-water sorption coefficients are promising as a parameter to study the effects of hydrophobicity and other potential interactions on sorption behavior of neutral and anionic surfactants.

Sensitive determination of plasma protein binding of cationic drugs using mixed-mode solid-phase microextraction.

Freely dissolved concentrations are considered to be the most relevant concentration in pharmacology and toxicology, as they represent the active concentration available for interaction with its surroundings. Here, a solid-phase microextraction (SPME) coating that combines octadecyl and propylsulfonic acid groups as strong cation exchange sites, known as C18/SCX or "mixed-mode" SPME, is used to measure freely dissolved concentrations of amitriptyline, amphetamine, diazepam and tramadol to different binding matrices, including bovine serum albumin (BSA), human serum albumin (HSA), human plasma and human whole blood. A potential confounding factor in binding studies is that proteins may sorb to the fiber coating leading to incorrect measurement of protein sorption or changes in uptake kinetics to the fiber coating. Sorption of bovine serum albumin (BSA) was observed and quantified using a Lowry assay. BSA binds to the C18/SCX fiber in small amounts, but large changes in uptake kinetics were not observed. All experiments were performed at equilibrium. In addition, however, the effect of depletion and non-equilibrium extraction on the estimation of protein binding affinities was also studied. Binding affinities to BSA and human serum albumin (HSA) were calculated as log KBSA or log KHSA. These values were very similar to reported literature values. Sampling at either equilibrium or non-equilibrium resulted in similar binding affinities. Furthermore, SPME fibers were used to measure freely dissolved concentrations in undiluted human plasma and whole blood. Analysis of SPME extracts could be performed using HPLC-UV or HPLC with fluorescence detection without prior clean-up of the samples. Measured bound fractions in plasma using this SPME approach were comparable to literature reference values. Bound fractions in whole blood were always higher than in plasma, due to red blood cell partitioning. This work shows the potential of SPME as sampling tool for freely dissolved concentrations, especially for highly protein-bound compounds. Conventional SPME coatings such as polyacrylate (PA) or polydimethylsiloxane (PDMS) might be lacking sensitivity when sampling the small neutral fraction of highly protein-bound positively charged compounds, but the C18/SCX fiber is able to sorb the charged species of organic cations, thereby improving sensitivity for these types of compounds.

Sorption of amitriptyline and amphetamine to mixed-mode solid-phase microextraction in different test conditions.

A solid-phase microextraction (SPME) method based on a sampler coating that includes strong cation groups (C18/SCX) is explored as a rapid direct sampling tool to detect and quantify freely dissolved basic drugs. Sampling kinetics, sorption isotherms and competitive effects on extraction yields in mixtures were tested for amphetamine and the relatively large/hydrophobic tricyclic antidepressant amitriptyline. Both compounds are >99% ionized at pH 7.4 but their affinity for the C18/SCX fiber is markedly different with distribution coefficients (Dfw values) of 2.49±0.02 for amphetamine and 4.72±0.10 for amitriptyline. Typical changes in electrolyte homeostasis that may occur in biomedical samples were simulated by altering pH and ionic composition (Na(+) and K(+) concentrations). These changes were shown to affect C18/SCX sorption affinities of the tested drugs with less than 0.2log units. At relatively low fiber loadings (<10mmol/L coating) and at all tested exposure times, linear sorption isotherms were obtained for both compounds but at aqueous concentrations of the individual drugs corresponding to concentrations in blood that are lethal, sorption isotherms became strongly nonlinear. Competition effects within binary mixtures occurred only if combinations of aqueous concentrations resulted in total fiber loadings that were in the nonlinear range of the SPME sorption isotherm for the individual compounds. We also compared sorption to the (prototype) C18/SCX SPME coating with analogue (biocompatible) C18 coated SPME fibers. C18/SCX fibers show increased sorption affinity for cationic compounds compared to C18 fibers, as tested using amitriptyline, amphetamine and trimethoprim. Surprisingly, sorption affinity of these ionized compounds for the C18 SPME fibers were within 1log unit of the C18/SCX SPME fibers. This shows that the strong cation exchange groups within the C18/SCX coating only has a relatively small contribution to the total sorption affinity of cationic compounds. Also the role of negatively charged silanol groups in both the C18 and C18/SCX coating seems small, as anionic diclofenac species sorbed strongly to the C18 fiber. Ionized organic species seem to be substantially adsorbed to the high surface area of C18 in SPME types using porous silica based coatings.

Determining high-quality critical body residues for multiple species and chemicals by applying improved experimental design and data interpretation concepts.

Ecotoxicological effect data are generally expressed as effective concentrations in the external exposure medium and do thus not account for differences in chemical uptake, bioavailability, and metabolism, which can introduce substantial data variation. The Critical Body Residue (CBR) concept provides clear advantages, because it links effects directly to the internal exposure. Using CBRs instead of external concentrations should therefore reduce variability. For compounds that act via narcosis even a constant CBR has been proposed. Despite the expected uniformity, CBR values for these compounds still show large variability, possibly due to biased and inconsistent experimental testing. In the present study we tested whether variation in CBR data can be substantially reduced when using an improved experimental design and avoiding confounding factors. The aim was to develop and apply a well-defined test protocol for accurately and precisely measuring CBR data, involving improved (passive) dosing, sampling, and processing of organisms. The chemicals 1,2,4-trichlorobenzene, 1,2,3,4-tetrachlorobenzene, 2,3,4-trichloroaniline, 2,3,5,6-tetrachloroaniline, 4-chloro-3-methylphenol, pentylbenzene, pyrene, and bromophos-methyl were tested on Lumbriculus variegatus (California blackworm), Hyalella azteca (scud), and Poecilia reticulata (guppy), which yielded a high-quality database of 348 individual CBR values. Medians of CBR values ranged from 2.1 to 16.1 mmol/kg wet weight (ww) within all combinations of chemicals and species, except for the insecticide bromophos-methyl, for which the median was 1.3 mmol/kg ww. The new database thus covers about one log unit, which is considerably less than in existing databases. Medians differed maximally by a factor of 8.4 between the 7 chemicals but within one species, and by a factor of 2.6 between the three species but for individual chemicals. Accounting for the chemicals' internal distribution to different partitioning domains and relating effects to estimated concentrations in the target compartment (i.e., membrane lipids) was expected to but did not decrease the overall variability, likely because the surrogate partition coefficients for membrane lipid, storage lipid, protein, and carbohydrate that were used as input parameters did not sufficiently represent the actual partitioning processes. The results of this study demonstrate that a well-designed test setup can produce CBR data that are highly uniform beyond chemical and biological diversity.

The in vitro biokinetics of chlorpromazine and diazepam in aggregating rat brain cell cultures after repeated exposure.

Neurotoxic effects of compounds can be tested in vitro using cell systems. One example is aggregating rat brain cell cultures. For the extrapolation of in vitro data to the in vivo situation, it is important to take the biokinetics of the test compound into account. In addition, the exposure in vivo is often for a longer period of time; therefore, it is crucial to incorporate this into in vitro assays as well. In this study, aggregating rat brain cell cultures were exposed to chlorpromazine (CPZ) and diazepam (DZP) for 12-days with repeated exposure. Samples were taken from the stocks, test media, cell culture media and cells at specific time points on the first and last exposure day. These samples were analysed by HPLC-UV. The amount of CPZ in the medium decreased over time, whereas the amount in the cells showed an increase. Accumulation of CPZ in the cells was seen over the 12-day repeated exposure. The amount of DZP in the medium remained stable over time and only up to 2% of DZP added was found in the cells. Different biokinetic behaviour was found for CPZ and DZP. Possible explanations are differences in uptake into the cells or efflux out of the cells. The decrease of CPZ in the medium versus the stable amount of DZP results in differences in exposure concentrations over time, which should be taken into account when interpreting in vitro effect data.

Biokinetics of chlorpromazine in primary rat and human hepatocytes and human HepaRG cells after repeated exposure.

Since drug induced liver injury is difficult to predict in animal models, more representative tests are needed to better evaluate these effects in humans. Existing in vitro systems hold great potential to detect hepatotoxicity of pharmaceuticals. In this study, the in vitro biokinetics of the model hepatotoxicant chlorpromazine (CPZ) were evaluated in three different liver cell systems after repeated exposure in order to incorporate repeated-dose testing into an in vitro assay. Primary rat and human hepatocytes, cultured in sandwich configuration and the human HepaRG cell line were treated daily with CPZ for 14 days. Samples were taken from medium, cells and well plastic at specific time points after the first and last exposure. The samples were analysed by HPLC-UV to determine the amount of CPZ in these samples. Based on cytotoxicity assays, the three models were tested at 1-2 µM CPZ, while the primary rat hepatocytes and the HepaRG cell line were in addition exposed to a higher concentration of 15-20 µM. Overall, the mass balance of CPZ decreased in the course of 24 h, indicating the metabolism of the compound within the cells. The largest decrease in parent compound was seen in the primary cultures; in the HepaRG cell cultures the mass balance only decreased to 50%. CPZ accumulated in the cells during the 14-day repeated exposure. Possible explanations for the accumulation of CPZ are a decrease in metabolism over time, inhibition of efflux transporters or binding to phospholipids. The biokinetics of CPZ differed between the three liver cell models and were influenced by specific cell properties as well as culture conditions. These results support the conclusion that in vitro biokinetics data are necessary to better interpret chemical-induced cytotoxicity data.

Dose metric considerations in in vitro assays to improve quantitative in vitro-in vivo dose extrapolations.

Challenges to improve toxicological risk assessment to meet the demands of the EU chemical's legislation, REACH, and the EU 7th Amendment of the Cosmetics Directive have accelerated the development of non-animal based methods. Unfortunately, uncertainties remain surrounding the power of alternative methods such as in vitro assays to predict in vivo dose-response relationships, which impedes their use in regulatory toxicology. One issue reviewed here, is the lack of a well-defined dose metric for use in concentration-effect relationships obtained from in vitro cell assays. Traditionally, the nominal concentration has been used to define in vitro concentration-effect relationships. However, chemicals may differentially and non-specifically bind to medium constituents, well plate plastic and cells. They may also evaporate, degrade or be metabolized over the exposure period at different rates. Studies have shown that these processes may reduce the bioavailable and biologically effective dose of test chemicals in in vitro assays to levels far below their nominal concentration. This subsequently hampers the interpretation of in vitro data to predict and compare the true toxic potency of test chemicals. Therefore, this review discusses a number of dose metrics and their dependency on in vitro assay setup. Recommendations are given on when to consider alternative dose metrics instead of nominal concentrations, in order to reduce effect concentration variability between in vitro assays and between in vitro and in vivo assays in toxicology.

A decision support system for drinking water production integrating health risks assessment.

The issue of drinking water quality compliance in small and medium scale water services is of paramount importance in relation to the 98/83/CE European Drinking Water Directive (DWD). Additionally, concerns are being expressed over the implementation of the DWD with respect to possible impacts on water quality from forecast changes in European climate with global warming and further anticipated reductions in north European acid emissions. Consequently, we have developed a decision support system (DSS) named ARTEM-WQ (AwaReness Tool for the Evaluation and Mitigation of drinking Water Quality issues resulting from environmental changes) to support decision making by small and medium plant operators and other water stakeholders. ARTEM-WQ is based on a sequential risk analysis approach that includes consideration of catchment characteristics, climatic conditions and treatment operations. It provides a holistic evaluation of the water system, while also assessing human health risks of organic contaminants potentially present in treated waters (steroids, pharmaceuticals, pesticides, bisphenol-a, polychlorobiphenyls, polycyclic aromatic hydrocarbons, petrochemical hydrocarbons and disinfection by-products; n = 109). Moreover, the system provides recommendations for improvement while supporting decision making in its widest context. The tool has been tested on various European catchments and shows a promising potential to inform water managers of risks and appropriate mitigative actions. Further improvements should include toxicological knowledge advancement, environmental background pollutant concentrations and the assessment of the impact of distribution systems on water quality variation.

Adsorption of polar, nonpolar, and substituted aromatics to colloidal graphene oxide nanoparticles.

We conducted batch adsorption experiments to understand the adsorptive properties of colloidal graphene oxide nanoparticles (GONPs) for a range of environmentally relevant aromatics and substituted aromatics, including model nonpolar compounds (pyrene, phenanthrene, naphthalene, and 1,3-dichlorobenzene) and model polar compounds (1-naphthol, 1-naphthylamine, 2,4-dichlorophenol, and 2,4-dinitrotoluene). GONPs exhibited strong adsorption affinities for all the test compounds, with distribution coefficients on the order of 10(3)-10(6) L/kg. Adsorption to GONPs is much more linear than to carbon nanotubes (CNTs) and C60, likely because GO nanoflakes are essentially individually dispersed (rendering adsorption sites of similar adsorption energy) whereas CNT/C60 are prone to bundling/aggregation. For a given compound GONPs and CNTs often exhibit different adsorption affinities, which is attributable to the differences in both the morphology and surface chemistry between the two nanomaterials. Particularly, the high surface O-content of GONPs enables strong H-bonding and Lewis acid-base interactions with hydroxyl- and amino-substituted aromatics.

Acute toxicity of the cationic surfactant C12-benzalkonium in different bioassays: how test design affects bioavailability and effect concentrations.

Using an ion-exchange-based solid-phase microextraction (SPME) method, the freely dissolved concentrations of C12-benzalkonium were measured in different toxicity assays, including 1) immobilization of Daphnia magna in the presence or absence of dissolved humic acid; 2) mortality of Lumbriculus variegatus in the presence or absence of a suspension of Organisation for Economic Co-Operation and Development (OECD) sediment; 3) photosystem II inhibition of green algae Chlorella vulgaris; and 4) viability of in vitro rainbow trout gill cell line (RTgill-W1) in the presence or absence of serum proteins. Furthermore, the loss from chemical adsorption to the different test vessels used in these tests was also determined. The C12-benzalkonium sorption isotherms to the different sorbent phases were established as well. Our results show that the freely dissolved concentration is a better indicator of the actual exposure concentration than the nominal or total concentration in most test assays. Daphnia was the most sensitive species to C12-benzalkonium. The acute Daphnia and Lumbriculus tests both showed no enhanced toxicity from possible ingestion of sorbed C12-benzalkonium in comparison with water-only exposure, which is in accordance with the equilibrium partitioning theory. Moreover, the present study demonstrates that commonly used sorbent phases can strongly affect bioavailability and observed effect concentrations for C12-benzalkonium. Even stronger effects of decreased actual exposure concentrations resulting from sorption to test vessels, cells, and sorbent phases can be expected for more hydrophobic cationic surfactants.

Evaluation of passive samplers with neutral or ion-exchange polymer coatings to determine freely dissolved concentrations of the basic surfactant lauryl diethanolamine: Measurements of acid dissociation constant and organic carbon-water sorption coefficient.

A passive sampler tool (solid-phase microextraction, SPME) was optimized to measure freely dissolved concentrations (Cw,free) of lauryl diethanolamine (C12-DEA). C12-DEA can be protonated and act as a cationic surfactant. From the pH-dependent sorption to neutral SPME coatings (polyacrylate and PDMS), a pKa of 8.7 was calculated, which differs more than two units from the value of 6.4 reported elsewhere. Polyacrylate coated SPME could not adequately sample largely protonated C12-DEA in humic acid solutions of pH 6. A new hydrophobic SPME coating with cation-exchange properties (C18/SCX) sorbed C12-DEA 100 fold stronger than polyacrylate, because it specifically sorbs protonated C12-DEA species. The C18/SCX-SPME fiber showed linear calibration isotherms in a concentration range of <1 nM-1 µM (well below the CMC). Using the C18/SCX-SPME fibers, linear sorption isotherms to Aldrich humic acid at pH 6 (ionic strength 0.015 M) were measured over a broad concentration range with a sorption coefficient of 10(5.3).