Measuring the bioavailability of two hydrophobic organic compounds in the presence of dissolved organic matter

Bioavailability of benzo[a]pyrene (BaP) and 3,3',4,4'-tetrachlorobiphenyl (TCB) was studied in natural lake water containing dissolved organic matter (DOM). Lake water was diluted to give a dissolved organic carbon (DOC) range of 1 to 20 mg/L. Partition coefficients for the model compounds were assessed at different DOM concentrations and over time with three different methods, namely equilibrium dialysis and reverse-phase and liquid-liquid extraction. In addition, biological partition coefficients were estimated from the difference in the bioconcentration of the model compounds in Daphnia magna in the presence and absence of DOM. Results showed that bioavailability of the model compounds was reduced by the presence of DOM. The equilibrium dialysis method gave the best estimates for bioavailability of the model compounds when compared with biologically determined values. Both the reverse-phase and the liquid-liquid extraction overestimated the bioavailable fraction. The more pronounced overestimation of bioavailable fraction of TCB suggested that the sorption of TCB was not only lower but the interaction was also weaker than that of BaP. Increasing DOM concentration produced lower partition coefficients and the effect seemed to be more pronounced when measured by the reverse-phase and the extraction methods.     

Enhanced kinetics of solid-phase microextraction and biodegradation of polycyclic aromatic hydrocarbons in the presence of dissolved organic matter

The uptake kinetics of fluorene, phenanthrene, fluoranthene, pyrene, and benzo[e]pyrene by solid-phase microextraction fibers was studied in the presence of dissolved organic matter (DOM) obtained from sediment pore water and resulted in increased fiber absorption and desorption rate coefficients. Compared to the control without DOM, these rate coefficients were increased at a DOM concentration of 36.62 mg/L by a factor of 1.27 to 2.21 and 1.31 to 2.10 for fluorene and benzo[e]pyrene, respectively. The calculated values for the fiber absorption and desorption rate coefficients show that diffusion through an unstirred boundary layer (UBL) surrounding the fiber probably forms the rate-limiting step of the process. The mineralization of aqueous-phase phenanthrene and pyrene by a representative polycyclic aromatic hydrocarbon (PAH)-degrading bacterium (Mycobacterium gilvum VM552) also was found to be enhanced by DOM. The initial degradation rates of phenanthrene (9.03 (microg/L) and pyrene (1.96 microg/L) were significantly higher compared to the control values and were enhanced by a factor of 1.32 and 1.26 at a DOM concentration of 43.14 and 42.15 mg/L, respectively. We suggest that such an enhancement results from the combination of faster uptake kinetics of the water-dissolved compounds in the UBL surrounding microbial cells and direct access of the bacteria to DOM-associated PAHs. These enhanced kinetic effects of DOM may have strong implications in sediment processes like desorption, nonequilibrium exposure, and biodegradation.     

Characterization,DBPs formation,and mutagenicity of different organic matter fractions in two source waters

Dissolved organic matters (DOM) are critical in the formation of the mutagenic disinfection byproducts (DBPs). In this study, five DOM fractions were isolated and investigated from a contaminated river and a clean reservoir source waters using resin adsorption. The DOM fractions were characterized with excitation–emission matrix, and several typical DBPs formation potentials and the mutagenicity of each DOM fraction were measured. Among these fractions, hydrophobic neutrals (HON), hydrophilic acids (HIA) and hydrophilic bases (HIB) generated the highest carbon-containing DBPs in the river source water, as did HIB and hydrophobic bases (HOB) in reservoir water. Hydrophobic acids (HOA), HON, and HIA were the three most important fractions forming nitrogen-containing DBPs. Following chlorination, the mutagenicity of HON, HIA, HOA and HIB was 1503, 626, 422 and 116 ng 4-NQO/mg DOC in river water, respectively. Only HIA and HOA were mutagenic with 85 and 10 ng 4-NQO/mg DOC in reservoir water, respectively. The soluble microbial products like substances and aromatic proteins contributed significantly to the mutagenicity of river water; whereas the humic acid-like and fulvic acid-like substances were the primary contributors to the mutagenicity of reservoir water.     

Molecular simulation of a model of dissolved organic matter

A series of atomistic simulations was performed to assess the ability of the Schulten dissolved organic matter (DOM) molecule, a well-established model humic molecule, to reproduce the physical and chemical behavior of natural humic substances. The unhydrated DOM molecule had a bulk density value appropriate to humic matter, but its Hildebrand solubility parameter was lower than the range of current experimental estimates. Under hydrated conditions, the DOM molecule went through conformational adjustments that resulted in disruption of intramolecular hydrogen bonds (H-bonds), although few water molecules penetrated the organic interior. The radius of gyration of the hydrated DOM molecule was similar to those measured for aquatic humic substances. To simulate humic materials under aqueous conditions with varying pH levels, carboxyl groups were deprotonated, and hydrated Na+ or Ca2+ were added to balance the resulting negative charge. Because of intrusion of the cation hydrates, the model metal-humic structures were more porous, had greater solvent-accessible surface areas, and formed more H-bonds with water than the protonated, hydrated DOM molecule. Relative to Na+, Ca2+ was both more strongly bound to carboxylate groups and more fully hydrated. This difference was attributed to the higher charge of the divalent cation. The Ca-DOM hydrate, however, featured fewer H-bonds than the Na-DOM hydrate, perhaps because of the reduced orientational freedom of organic moieties and water molecules imposed by Ca2+. The present work is, to our knowledge, the first rigorous computational exploration regarding the behavior of a model humic molecule under a range of physical conditions typical of soil and water systems.     

Effects of XAD-8 fractions of dissolved organic carbon on the sorption and bioavailability of organic micropollutants

The dissolved organic carbon (DOC) from a stream water near a peat deposit was fractionated into hydrophobic-acid (HbA), hydrophobic-neutral (HbN), and hydrophilic (Hl) subcomponents by XAD-8 chromatography. The capacity of these fractions and the total (unfractionated) water to bind hydrophobic organic contaminants was measured by equilibrium dialysis, and the effect of binding on contaminant bioavailability was measured inDaphnia magna. Model contaminants were the poly cyclic aromatic hydrocarbons, naphthalene (NPH) and benzo(a)pyrene (BaP), and the polychlorinated biphenyl, 2,2,5,5-tetrachlorobiphenyl (TCB).Both BaP and TCB exhibited high partition coefficients (Kp's) for binding to both the total DOC and the hydrophobic components of the DOC. BaP had a higher affinity for binding to the HbA fraction, while TCB (and 3 other PCB's had a higher affinity for the HbN fraction. The K_p's for binding to the Hl fraction were twofold to tenfold lower than for binding to the hydrophobic fractions. The less hydrophobic compound, NPH, had a much lower K_p, and little difference was seen between the fractions. The total water and the different DOC fractions reduced the uptake and accumulation of BaP and TCB byD. magna in proportion to the capacity of the DOC for binding the contaminants. Data were consistent with the hypothesis that a contaminant bound to DOC (total water or any of the fractions) is unavailable for uptake by biota. Uptake of NPH was not substantially affected by the DOC, consistent with its lower K_p.     

Influence of dissolved organic matter on the complexation of mercury under sulfidic conditions

The complexation of Hg under sulfidic conditions influences its bioavailability for microbial methylation. Neutral dissolved Hg-sulfide complexes are readily available to Hg-methylating bacteria in culture, and thermodynamic models predict that inorganic Hg-sulfide complexes dominate dissolved Hg speciation under natural sulfidic conditions. However, these models have not been validated in the field. To examine the complexation of Hg in natural sulfidic waters, octanol/water partitioning methods were modified for use under environmentally relevant conditions, and a centrifuge ultrafiltration technique was developed. These techniques demonstrated much lower concentrations of dissolved Hg-sulfide complexes than predicted. Furthermore, the study revealed an interaction between Hg, dissolved organic matter (DOM), and sulfide that is not captured by current thermodynamic models. Whereas Hg forms strong complexes with DOM under oxic conditions, these complexes had not been expected to form in the presence of sulfide because of the stronger affinity of Hg for sulfide relative to its affinity for DOM. The observed interaction between Hg and DOM in the presence of sulfide likely involves the formation of a DOM-Hg-sulfide complex or results from the hydrophobic partitioning of neutral Hg-sulfide complexes into the higher-molecular-weight DOM. An understanding of the mechanism of this interaction and determination of complexation coefficients for the Hg-sulfide-DOM complex are needed to adequately assess how our new finding affects Hg bioavailability, sorption, and flux.     

Copper complexation by dissolved organic matter from surface water and wastewater effluent

Organic matter from wastewater treatment plants (wastewater organic matter, WWOM) has not been extensively studied with respect to complexation with copper, unlike natural organic matter (NOM). Acid-base and copper titrations were conducted on both types of organic matter. Experimental copper complexation data were compared to predictions from the Windermere Humic Aqueous Model (WHAM) Version VI. We found that NOM and WWOM have ligands with similar proton binding, but the copper binding of WWOM is not well predicted by WHAM especially at low copper concentrations because the concentrations of ligands that are most important at the low copper concentrations (below 10(-6) M) were found to be about 15 times higher in the WWOM. Consideration of sulfide present in the wastewater effluent does not fully explain this deviation. Due to the possibility that there exist nonhumics like biological macromolecules in WWOM, it may need to be considered as an alternative ligand to humics in toxicity and speciation predicting models like the biotic ligand model (BLM).     

Influence of dissolved organic matter source on silver toxicity to Pimephales promelas

In the environment, the formation of organic and inorganic silver complexes can decrease Ag bioavailability (toxicity) to aquatic organisms. However, current water quality regulations do not consider the protective effects of water quality parameters such as dissolved organic carbon (DOC) concentration. To determine the effect of DOC concentration and source on silver toxicity, nine different natural organic matter isolates were used in 96-h static-renewal toxicity tests with fathead minnow (Pimephales promelas). The 96-h dissolved silver median lethal concentrations (LC50) among different sources of dissolved organic matter varied by up to fivefold (4.5-23.3 microg/L). Further, toxicity tests with organic matter from the site with the lowest 96-h LC50 value suggested only limited additional attenuation of silver toxicity when DOC concentration was increased from 5.1 to 14.0 mg/L. With this site excluded, we found little more than a twofold difference among 96-h dissolved Ag LC50s for the remaining sources (10.1-23.3 microg/L). However, significant toxicological differences among sites remained. It was apparent that organic matter from different sources varied both chemically and toxicologically, but no conclusions could be drawn that related compositional variation to observed Ag toxicity for these isolates.     

Effects of dissolved organic matter and reduced sulphur on copper bioavailability in coastal marine environments

Copper-induced toxicity in aqueous systems depends on its speciation and bioavailability. Natural organic matter (NOM) and reduced sulphur species can complex copper, influencing speciation and decreasing bioavailability. NOM composition in estuaries can vary, depending on inputs of terrigenous, autochthonous, or wastewater source material. At a molecular level, variability in NOM quality potentially results in different extents of copper binding. The aims of this study were to measure acute copper EC₅₀ values in coastal marine and estuarine waters, and identify the relationships between total dissolved copper EC₅₀ values and measured water chemistry parameters proportional to NOM and reduced sulphur composition. This has implications on the development of marine-specific toxicity prediction models. NOM was characterised using dissolved organic carbon (DOC) concentration and fluorescence measurements, combined with spectral resolution techniques, to quantify humic-, fulvic-, tryptophan-, and tyrosine-like fractions. Reduced sulphur was measured by the chromium-reducible sulphide (CRS) technique. Acute copper toxicity tests were performed on samples expressing extreme DOC, fluorescent terrigenous, autochthonous, and CRS concentrations. The results show significant differences in NOM quality, independent of DOC concentration. CRS is variable among the samples; concentrations ranging from 4 to 40 nM. The toxicity results suggest DOC as a very good predictive measure of copper EC₅₀ in estuaries (r²=0.87) independent of NOM quality. Furthermore, for filtered samples, CRS exists at concentrations that would be saturated with copper at measured EC₅₀, suggesting that while CRS might bind Cu and decrease bioavailability, it does not control copper speciation at toxicologically relevant concentrations and therefore is not a good predictive measure of copper toxicity in filtered samples.     

Sorption and dialysis experiments to assess the binding of phenolic xenobiotics to dissolved organic matter in soil

The binding of two phenolic compounds, nonylphenol (NP) and bisphenol A (BPA), and the herbicide metabolite hydroxydesethyl terbuthylazine (HDT) to dissolved organic matter (DOM) as well as its impact on the environmental fate of the compounds in soil have been investigated. In sorption studies with an acidic silty loam soil and a neutral loam soil, no impact of added DOM on adsorption and desorption coefficients was observed. Adsorption coefficients normalized to soil organic carbon (Koc) of the compounds on the loamy silt soil spanned three orders of magnitude: 90, 890, and 9,140 L/kg for HDT, BPA, and NP, respectively. Significantly lower Koc values for HDT on the neutral loam soil as compared to the acidic loamy silt soil indicated additional polar interactions between the latter and the basic herbicide metabolite. In dialysis experiments with fulvic acid from silty loam soil (FA) and peat humic acid (HA), a formation of analyte-DOM associates was observed only for HA. The degree of association was characterized by distribution coefficients normalized to the dissolved organic carbon concentration (KDOC). Values for NP (8,970 L/kg) and BPA (860 L/kg) corresponded with the respective soil sorption coefficients, suggesting similar binding mechanisms in both cases.     

Variable toxicity of ionic liquid-forming chemicals to Lemna minor and the influence of dissolved organic matter

Ionic liquids (ILs) are nonvolatile organic salts that remain liquid over a wide range of temperatures. Ionic liquids are promoted as environmentally friendly alternatives to the volatile organic solvents that are currently in widespread industrial usage. Although ILs are unlikely to contribute to air pollution, their potential effects on aquatic ecosystems are largely unknown. Furthermore, information is lacking on how ILs will interact with common features of aquatic environments, such as dissolved organic matter (DOM). We assessed the effect of five IL-forming chemicals on the growth of duckweed, Lemna minor, a common aquatic vascular plant. In general, 1-alkyl-3-methylimidazolium chemicals with longer alkyl chains were more toxic to L. minor than those with short alkyl chain lengths. The concentration that produced a 50% reduction (the EC50) in root growth was 8.56 ppm when a butyl chain was present but was 0.25 ppm (i.e., much more toxic) when an octyl chain was substituted. Butyl-substituted 3-methylpyridinium (root growth EC50 of 7.49 ppm) and 3-methylimidazolium cations had similar toxicity, whereas a tetrabutyl ammonium cation was considerably less toxic (root growth EC50 of 32.71 ppm). When we tested whether DOM reduced the toxicity of these cations, we saw no effect of a low-molecular-weight organic acid or commercial humic matter. In contrast, natural DOM reduced the toxicity of imidazolium, but only at low concentrations. Design and use of ILs and other new chemicals should incorporate not only standard toxicity tests but also information on how such chemicals will interact with other components of aquatic ecosystems.     

Wescodyne: lack of activity against poliovirus in the presence of organic matter.

The National Institute of Health Guidelines for Recombinant DNA Research recommends 2% aqueous Wescodyne, an iodophore that is used in many hospitals and laboratories as a disinfectant, as a decontaminant for biological safety cabinets and 5% for a spill outside a cabinet. A contact time of 10 to 15 minutes was given for the 2% solution and 20 minutes was considered adequate for the 5% concentration. The results indicate: 1. Aqueous Wescodyne (5%) is ineffective when used for 80 minutes against poliovirus in a test mixture containing 8.5% bovine serum albumin (a mixture equivalent in protein concentration to the higher range in serum). 2. Wescodyne (10%) employed under the same conditions for 40 minutes is also ineffective. 3. Wescodyne (10% v/v) in 50% ethanol (w/w) was effective and this mixture, originally recommended for hand washing, should be considered for use in biohazard situations, particularly for decontamination of work surfaces and biological safety cabinets. These results are of significance for if a virucide cannot inactivate poliovirus one would be concerned about using the virucide against hepatitis B or SV40 viruses.     

Effect of dissolved organic matter source on acute copper toxicity to Daphnia magna

The protective effect of dissolved organic matter (DOM) on metal toxicity to aquatic organisms has been reported by numerous authors. Bioavailability models such as the biotic ligand model (BLM) thus account for this factor to predict metal toxicity to aquatic organisms. Until now, however, few attempts have been made to assess the effect of the DOM source on metal speciation and toxicity and, accordingly, on BLM predictions. The aims of this study were to investigate to what extent DOMs differ in their ability to decrease acute copper toxicity to the cladoceran Daphnia magna and to evaluate if ultraviolet (UV) absorbance measurements may be a simple and effective method to incorporate DOM variability into the acute Cu-BLM for D. magna. Acute toxicity tests were carried out in artificial test water enriched with DOMs isolated from six locations in Europe and North America and in seven natural European surface waters. The acute Cu-BLM for D. magna was then used to estimate the copper complexing capacity of each DOM (expressed as % active fulvic acid, %AFA). A factor of 6 difference was observed between the lowest and the highest copper complexing capacity. A significant linear relationship was observed between the UV-absorbance coefficient at 350 nm (epsilon350) and the %AFA. Linking this relationship to the acute Cu-BLM resulted in a significant improvement of the predictive capacity of this BLM. Without accounting for this relationship, 90% of the predicted 48-h 50% effective concentrations (EC50) were within a factor of 2 of the observed EC50s; taking this relationship into account, 90% of the EC50s were predicted with an error of less than factor 1.3. The present study and other studies seem to indicate that UV absorbance may be a good measure of biologically and toxicologically relevant differences in copper binding behavior of DOM.     

Effect of dissolved organic matter of various origins and biodegradabilities on the bioaccumulation of polycyclic aromatic hydrocarbons in Daphnia magna

As a preliminary study of the influence of urban organic matter on the bioavailability of polycyclic aromatic hydrocarbons (PAHs), the effect of different types of dissolved organic matter (DOM) on the bioaccumulation of fluoranthene, pyrene, or benzo[a]pyrene in Daphnia magna was studied. Commercial humic substances, DOM from the aeration basin of a wastewater treatment plant, and highly biodegradable DOM (algae or animal extracts) were tested. The bioaccumulation of benzo[a]pyrene was reduced by each DOM (up to 80% reduction with humic substances). Pyrene bioaccumulation was also decreased by each DOM to a lesser extent. Fluoranthene bioaccumulation was affected by the presence of humic acids only. In each experiment, the solution containing humic DOM led to the lowest bioaccumulation. Supposing that only dissolved PAHs were bioavailable, the reduction of bioaccumulation allowed a biological estimate of the partition coefficients of DOM and PAH, K(DOC). The estimated coefficients were positively related to the aromaticity of DOM and negatively related to its biodegradability.     

Reduced adsorption of propanil to black carbon: effect of dissolved organic matter loading mode and molecule size

In the present study, the reduced adsorption of propanil on black carbon (BC) influenced by dissolved organic matter (DOM) was verified to be closely related to DOM molecule size and loading mode. Two congenetic carbons, a rice-residue-derived BC and the reduced product (RC), were characterized by similar specific surface area and different surface properties. Reduced product exhibits higher adsorption of propanil and DOM than BC. A series of model DOMs, including tannic acid (TA), pentagalloylglucose (PA), 3-O-galloylmucic acid (OA), and gallic acid (GA), characterized by different molecule sizes and molecular weights, were used to evaluate the different inhibitory effects. The DOM adsorption (mmol/g) on BC and RC follows the order of GA > PA > OA > TA, whereas the reduction of propanil adsorption influenced by the model DOM follows the order of PA ≈ TA > OA ≈ GA. The suppressive degree is connected to their molecule sizes rather than to molecular weights. Tannic acid and PA weakened propanil adsorption more effectively than OA and GA because the large DOM molecules may hinder propanil molecules into the micropore regions. Because of the similar molecule size, TA and PA present a similar suppressive effect on propanil adsorption. The influence of the DOMs was greater when preloaded than when in competition with propanil. The preloading of macromolecules (TA and PA) and OA on the carbons may lead to secondary and primary micropore blocks, respectively. The preloading of GA may cause partial GA molecule sequestration in the primary micropore, thus leading to strong attenuation of propanil adsorption on the carbons.     

Effect of dissolved organic matter on the growth of algae, Pseudokirchneriella subcapitata, in Korean lakes: the importance of complexation reactions

The objective of this study was to investigate the effect of dissolved organic ligands, such as EDTA, humic acids, hydrophobic and hydrophilic fractions of dissolved organic matter (DOM), on the growth of Pseudokirchneriella subcapitata in cultured media and natural lake waters. The growth of algae was gradually increased on the addition of dissolved organic ligands, but markedly declined at high concentrations, due to reduced bioavailable iron concentrations (e.g., Fe3+ and Fe'--inorganic ferric iron). The results demonstrated that the high growth rates of P. subcapitata were correlated with the hydrophobic, but not hydrophilic, DOM from five lakes, spiked under the nutrients-controlled conditions. This was attributed to the role of DOM in controlling the bioavailable iron due to complexation of Fe(III) with -COOH and -OH functional groups on the hydrophobic DOM. Therefore, the hydrophobic DOM, as chelating agent, is a biologically important component in the lake waters, which affects the algal growth by interacting with bioavailable iron in the lake water and can change the results of bioassay experiments.     

Bioaccumulation of waterborne Ni in Dreissena polymorpha: a stable isotope experiment to assess the effect of zinc, calcium, and dissolved organic matter

The effect of Ca, Zn, and dissolved natural organic matter (NOM) on waterborne Ni accumulation was investigated in a freshwater mussel. An enriched stable metal isotope tracer was required to measure the Ni uptake rate accurately. Zebra mussels were exposed to environmentally relevant concentrations of (62) Ni (from 0.5 to 8 μg/L) for 48 h in media spiked with Ca, Zn, or dissolved NOM. The (62)Ni uptake was inhibited by Ca (from 0.138?±?0.021 to 0.061?±?0.010?L/g/d for Ca concentrations ranging from 43 to 133 mg/L) and enhanced by Zn (from 0.051?±?0.006 to 0.109?±?0.007?L/g/d for Zn concentrations ranging from 6.6 to 38.3 μg/L). The mechanisms behind the synergistic effect of Zn remain unclear, yet it can be hypothesized that Ni uptake is facilitated by Zn-dependent transport sites. To formalize the effects of Ca and Zn, a model was proposed to express the Ni uptake rate as a function of the mussels' filtration rate and of Ca and Zn concentrations. The (62)Ni uptake increased at low NOM concentrations and decreased at higher concentrations. This could be explained by the influence of NOM on both the speciation of Ni and the filtration activity of mussels. At high NOM concentrations, a modification of the membrane's permeability might also have favored Ni uptake, although this was not clearly established in this study. Therefore, the effect of water composition on Ni bioavailability to zebra mussels cannot be predicted by competition and complexation models alone, because it also influences the animal's physiology.     

Occurrence of metals in various size fractions of particulate matter

Size distributions of trace metals of specific health concern (Pb, Cd, Cr, Ni, Cd, V, Zn and Fe) in the fine (PM2.5) and coarse (PM2.5-10) fractions of suspended particulate matter were evaluated. Samples were collected through a year in three sites (traffic oriented and urban background in the urban area of Rome, and remote, in the Regional Park of Simbruini Mountains). Mean concentrations never exceeded the values recommended by WHO and EU. During winter season, levels of Cr, Cd, Pb and Zn resulted higher than in summer, in both fine and coarse fractions. The series of Pb, Cd, and V concentrations clearly showed a predominant fine-particle distribution, whereas Cr, Mn and Ni were more equally distributed between the coarse and fine fractions. Fe prevailed in the coarse mode at the urban sites. At the urban sites mean concentrations of the different metals were from 4 to 22 times higher than at the remote site. For various metals the distribution mode, as well as the seasonal trends of the concentrations, showed significant differences between urban and remote site. In busy and densely populated urban areas, without relevant industrial emissions, anthropogenic sources of metallic species seem still to give a contribution to air pollution, though a trend downwards, compared to previous surveys, was observed, particularly for Pb.