Developing understanding of the fate and behaviour of silver in fresh waters and waste waters

The Windermere Humic Aqueous Model (WHAM) is often used for risk assessment of metals; WHAM can be used to estimate the potential bioavailability of dissolved metals, where metals complexed to dissolved organic matter (DOM) are expected to be less toxic than ionic forms. Silver is a potential metal of concern but WHAM has not been rigorously tested against experimental measurements. This study compares WHAM predictions to measured ionic silver during fixed pH (4, 8 or 10) argentometric titrations of DOM from diverse origins. There were almost two orders of magnitude variation in free silver between sources but, within model uncertainty, WHAM captured this variability. This agreement, between measurements and models, suggests that WHAM is an appropriate tool for silver risk assessment in surface receiving waters when DOM is predominantly in the form of humic/fulvic acids. In sewage samples WHAM dramatically underestimated silver binding by approximately 3 orders of magnitude. Simulations with additional specific strong silver binding sulphide-like binding sites could explain Ag binding at low loadings, but not at higher loadings. This suggests the presence of additional intermediate strength binding sites. These additional ligands would represent components of the raw sewage largely absent in natural waters unimpacted by sewage effluents. A revised empirical model was proposed to account for these sewage-specific binding sites. Further, it is suspected that as sewage organic matter is degraded, either by natural attenuation or by engineered treatment, that sewage organic matter will degrade to a form more readily modelled by WHAM; i.e., humic-like substances. These ageing experiments were performed starting from raw sewage, and the material did in fact become more humic-like, but even after 30 days of aerobic incubation still showed greater Ag⁺ binding than WHAM predictions. In these incubation experiments it was found that silver (up to 1000 μg/L) had minimal impact on ammonia oxidation kinetics.

Vertical variation of mixing within porous sediment beds below turbulent flows

River ecosystems are influenced by contaminants in the water column, in the pore water and adsorbed to sediment particles. When exchange across the sediment-water interface (hyporheic exchange) is included in modeling, the mixing coefficient is often assumed to be constant with depth below the interface. Novel fiber-optic fluorometers have been developed and combined with a modified EROSIMESS system to quantify the vertical variation in mixing coefficient with depth below the sediment-water interface. The study considered a range of particle diameters and bed shear velocities, with the permeability Péclet number, PeK between 1000 and 77,000 and the shear Reynolds number, Re*, between 5 and 600. Different parameterization of both an interface exchange coefficient and a spatially variable in-sediment mixing coefficient are explored. The variation of in-sediment mixing is described by an exponential function applicable over the full range of parameter combinations tested. The empirical relationship enables estimates of the depth to which concentrations of pollutants will penetrate into the bed sediment, allowing the region where exchange will occur faster than molecular diffusion to be determined.

The effect of triclosan on microbial community structure in three soils

The application of sewage sludge to land can expose soils to a range of associated chemical toxicants. In this paper we explore the effects of the broad spectrum anti-microbial compound triclosan on the phenotypic composition of the microbial communities of three soils of contrasting texture (loamy sand, sandy loam and clay) using phospholipid fatty-acid (PLFA) analysis. Each soil type was dosed and subsequently re-dosed 6 weeks later with triclosan at five nominal concentrations in microcosms (10, 100, 500, 1000 mg kg(-1) and a zero-dose control). PLFA profiles were analysed using multivariate statistics focussing on changes in the soil phenotypic community structure. Additionally, ratios of fungal:bacterial PLFA indicators and cyclo:mono-unsaturated PLFAs (a common stress indicator) were calculated. It was hypothesised that triclosan addition would alter the community structure in each soil with a particular effect on the fungal:bacterial ratio, since bacteria are likely to be more susceptible to triclosan than fungi. It was also hypothesised that the PLFA response to re-dosing would be suppressed due to acclimation. Although the microbial community structure changed over the course of the experiment, the response was complex. Soil type and time emerged as the most important explanatory factors. Principal component analysis was used to detect phenotypic responses to different doses of triclosan in each soil. As expected, there was a significant increase in the fungal:bacterial ratio with triclosan dose especially in treatments with the highest nominal concentrations. Furthermore, the PLFA response to re-dosing was negligible in all soils confirming the acclimation hypothesis.

Fate of triclosan in field soils receiving sewage sludge

The anti-microbial substance triclosan can partition to sewage sludge during wastewater treatment and subsequently transfer to soil when applied to land. Here, we describe the fate of triclosan in a one-year plot experiment on three different soils receiving sludge. Triclosan and methyl-triclosan concentrations were measured in soil samples collected monthly from three depths. A large fraction of triclosan loss appeared to be explained by transformation to methyl-triclosan. After 12 months less than 20% of the initial triclosan was recovered from each soil. However, the majority was recovered as methyl-triclosan. Most of the chemical recovered at the end of the experiment (both triclosan and methyl-triclosan) was still in the top 10 cm layer, although there was translocation to lower soil horizons in all three soils. Between 16.5 and 50.6% of the applied triclosan was unaccounted for after 12 months either as a consequence of degradation or the formation of non-extractable residues.

High TPMT enzyme activity does not explain drug resistance due to preferential 6-methylmercaptopurine production in patients on thiopurine treatment

BACKGROUND

Up to 20% of patients on thiopurine therapy fail to achieve adequate drug response. Many of these patients preferentially produce the toxic 6-methylmercaptopurine metabolites (6-MMP) rather than the active 6-thioguanine nucleotides (6-TGN) resulting in a high 6-MMP/6-TGN ratio (>20) and increased risk of hepatotoxicity.

AIM

To determine the prevalence of preferential 6-MMP producers and define the relationships between 6-TGN, 6-MMP and thiopurine methyltransferase (TPMT).

METHODS

The database of 6-TGN, 6-MMP and TPMT measurements from patients throughout New Zealand was used to calculate patients' 6-MMP/6-TGN ratios and identify those with high (>20) or normal ratio (≤20).The TPMT enzyme activity was compared amongst the groups.

RESULTS

Of 1879 patients with TPMT, 6-TGN and 6-MMP results, 349 (19%) had a 6-MMP/6-TGN ratio >20. The mean TPMT enzyme activity was slightly lower for those with a 6-MMP/6-TGN ratio ≤20 vs. >20, which achieved statistical significance (12.2 vs. 13.2; P < 0.001). However, the distributions of TPMT enzyme activity were similar, with 97% of TPMT results falling between 5.0 and 17.6 IU/mL for both groups. In all, 17% of those with 6-MMP/6-TGN ratio ≤20 were intermediate TPMT metabolisers (TPMT 5.0-9.2 IU/mL) vs. 7% in those with a ratio >20.

CONCLUSIONS

In this patient population with measured 6-MMP/6-TGN ratios, 19% of patients were preferential 6-MMP producers. The results show that high TPMT enzyme activity is not the major reason for preferential 6-MMP production in most patients with a high metabolite ratio. This suggests that there are one or more important alternative mechanisms for preferentially producing 6-MMP.

Dynamic multi-phase partitioning of decamethylcyclopentasiloxane (D5) in river water

The behaviour of decamethylcyclopentasiloxane (D5) in river water was evaluated by measuring concentration changes in open beakers. Effective values for the partition coefficient between organic carbon and water (K(OC)) were derived by least-squares optimisation of a dynamic model which accounted for partitioning between the sorbed and dissolved phases of D5, and for losses via volatilisation and hydrolysis. Partial mass transfer coefficients for volatilisation were derived from model fits to controls containing deionised water. Effective values of log (K(OC)) were between 5.8 and 6.33 (mean 6.16). These figures are higher than some other experimentally-derived values but much lower than those estimated from the octanol: water partition coefficient using single-parameter linear free energy relationships (LFERs). A poly-parameter LFER gave a predicted log (K(OC)) of 5.5. Differences in partitioning are believed to be due to the nature of the organic matter present. The new value for effective K(OC) was employed in a simple model of D5 behaviour in rivers to ascertain the extent to which a higher affinity for organic carbon would depress volatility. The results suggest that despite the revised K(OC) value, volatilisation of D5 remains a significant removal mechanism in surface waters.

Effect of Aldrich humic acid on water-atmosphere transfer of decamethylcyclopentasiloxane

The behaviour of the cyclic volatile methyl siloxane (cVMS) decamethylcyclopentasiloxane (D5) in surface waters is explored using a combination of laboratory experimentation and mathematical modelling. In the laboratory experiment, changes were observed in the concentration of radiolabelled D5 in open stirred beakers containing mineral medium with different concentrations of added Aldrich humic acid over 120 h. Although D5 is very volatile, its strong affinity for dissolved organic carbon (DOC) reduced the rate of water to air transfer significantly. The data were well described using a simple partitioning model which accounted for hydrolysis and for depth and DOC changes resulting from sampling and evaporation, although there was some evidence for the formation of a hypothetical bound residue with increasing time. The model was used to derive effective values for the partition coefficient between DOC and water [log(K(DOC))]. These values were relatively consistent across five treatments and varied between 5.04 and 5.40 log(L kg(-1)), with no systematic treatment bias. These values are significantly higher than previously published experimental estimates of K(OC) for D5 but more than two orders of magnitude lower than some K(OC) estimates based on the octanol:water partition coefficient (K(OW)). The data confirm that volatilisation will be an important loss mechanism from surface waters for D5, although the rate of loss will decrease with increasing DOC concentration.

A modelling assessment of the atmospheric fate of volatile methyl siloxanes and their reaction products

Volatile methyl siloxanes break down in the atmosphere by reacting with OH radicals to form OH-substituted silanols. As the silanols become increasingly OH substituted they are increasingly likely to be removed from the atmosphere by wet and dry deposition. A simple equilibrium partitioning model was constructed to explore the relative rates of removal by different mechanisms (reaction vs. deposition) for siloxanes and their resultant silanols. A mass balance is calculated for the parent siloxane molecule and for each silanol, characterised by the number of OH substitutions. The model includes the effect of incomplete equilibrium between the vapour, adsorbed and dissolved phases of silanols in the atmosphere using a non-equilibrium factor (epsilon) expressing relative departure from equilibrium. Model results show: (1) maximum vapour-phase concentrations for non-substituted siloxanes and single-OH-substituted silanols; (2) maximum dissolved-phase and adsorbed-phase concentrations for two-OH-substituted silanols; (3) >99% of the original material will be removed in wet deposition and <1% in dry deposition as silanols. For increasing OH-substitutions, the decreasing concentration of precursor molecules (as a consequence of combined removal processes) means that concentrations are negligible, in all phases, beyond three or four substitutions. Predictions were relatively insensitive to assumed departures from phase equilibrium. Predictions of silanol hydrolysis in liquid water droplets suggest that the mix of diol chain lengths in precipitation may not be in thermodynamic equilibrium and will depend on atmospheric residence time and pH.

Assessment of bioconcentration and secondary poisoning of surfactants

The relevance of the bioconcentration behaviour of surfactants for the secondary poisoning assessment and for the risk characterisation in the bird and mammalian food chain has been investigated. The approach used is described in the recently revised EU Technical Guidance Document for the Risk Assessment of Substances. The results demonstrate that, based on experimentally derived bioconcentration factors, environmental concentrations and effects in animals, there is a clear level of safety for both linear alkylbenzene sulphonate (LAS) and alcohol ethoxylates (AE), the most important surfactants by volume. To assess other surfactants used in detergents, a bioconcentration factor that would need to be attained for secondary poisoning to be of concern has been estimated from predicted environmental concentrations and known long-term effects data in animals. Based on the known structural similarity of these surfactants to LAS and AE and the ubiquitous nature of the enzymatic systems that are present in biotransformation processes in organisms, it is concluded that bioconcentration of these surfactants to these levels is highly unlikely. Therefore the potential for secondary poisoning effects of these surfactants is extremely low.

Ecological risk assessment of endocrine disruptors

The European Centre for Ecotoxicology and Toxicology of Chemicals proposes a tiered approach for the ecological risk assessment of endocrine disruptors, integrating exposure and hazard (effects) characterization. Exposure assessment for endocrine disruptors should direct specific tests for wildlife species, placing hazard data into a risk assessment context. Supplementing the suite of mammalian screens now under Organization for Economic Cooperation and Development (OECD) validation, high priority should be given to developing a fish screening assay for detecting endocrine activity in oviparous species. Taking into account both exposure characterization and alerts from endocrine screening, higher tier tests are also a priority for defining adverse effects. We propose that in vivo mammalian and fish assays provide a comprehensive screening battery for diverse hormonal functions (including androgen, estrogen, and thyroid hormone), whereas Amphibia should be considered at higher tiers if there are exposure concerns. Higher tier endocrine-disruptor testing should include fish development and fish reproduction tests, whereas a full life-cycle test could be subsequently used to refine aquatic risk assessments when necessary. For avian risk assessment, the new OECD Japanese quail reproduction test guideline provides a valuable basis for developing a test to detecting endocrine-mediated reproductive effects; this species could be used, where necessary, for an avian life-cycle test. For aquatic and terrestrial invertebrates, data from existing developmental and reproductive tests remain of high value for ecological risk assessment. High priority should be given to research into comparative endocrine physiology of invertebrates to support data extrapolation to this diverse fauna.

The influence of height and key on the perceptual similarity of transposed melodies

In two experiments, the perceptual similarity between a strong tonal melody and various transpositions was investigated using a paradigm in which listeners compared the perceptual similarity of a melody and its transposition with that of the same melody and another transposition. The paradigm has the advantage that it provides a direct judgment regarding the similarity of transposed melodies. The experimental results indicate that the perceptual similarity of a strong tonal melody and its transposition is mainly determined by two factors: (1) the distance on the height dimension between the original melody and its transposition (pitch distance), and (2) the distance between keys as inferred from the circle of fifths (key distance). The major part of the variance is explained by the factor pitch distance, whereas key distance explains only a small part.

Perceived similarity of exact and inexact transpositions

The purpose of this study was to investigate the perceived similarity between a melody and the exact and inexact transpositions of that melody. Exact transpositions, which preserve the interval structure of the original melody, were formed by manipulating the variables key-distance and pitch-distance. Inexact transpositions, having a different interval structure than the original melody, were created by altering one tone of the exact transpositions. Two types of alteration, retaining the contour of the original melody, were used: (1) a chromatic alteration of one tone fitting the key of the original melody; (2) a diatonic alteration of one tone fitting the key of the transposed melody. The coding model of Deutsch and Feroe (1981) was used as a qualitative predictor of the perceived similarity. The resulting predictions were tested using a paired-comparison paradigm. Results indicate that both pitch-distance and alteration explain a significant part of the variance, whereas key-distance does not contribute significantly. It was also found that exact transpositions are perceptually more similar to the original melody than inexact transpositions, and that chromatically altered transpositions are perceptually more similar to the original melody than diatonically altered transpositions. These results are broadly in accordance with the applied coding model.