5 research outputs found
Critical Review and Interpretation of Environmental Data for Volatile Methylsiloxanes: Partition Properties
Volatile methylsiloxanes (VMS) enter the environment through industrial
activities and the use of various consumer products. Reliable measurements
of environmental partition properties for these compounds are critical
for accurate prediction of their environmental fate, distribution,
transport, exposure and potential effects. In this study, the measured
partition properties including air/water (<i>K</i><sub>AW</sub>), octanol/water (<i>K</i><sub>OW</sub>), and octanol/air
partitioning coefficients (<i>K</i><sub>OA</sub>), soil
organic carbon/water distribution coefficient (<i>K</i><sub>OC</sub>), and biological medium/fluid partition coefficients, and
their temperature dependence were critically reviewed. Based on these
results, organosilicon compounds such as methylsiloxanes are expected
to behave differently in the environment compared to conventional
hydrophobic environmental contaminants, as a result of their inherent
characteristics related to molecular size and capacity for different
types of molecular interactions that control partitioning. The differences
are critical and need to be taken into consideration in environmental
exposure and risk analyses of these compounds
Bioconcentration and Aquatic Toxicity of Superhydrophobic Chemicals: A Modeling Case Study of Cyclic Volatile Methyl Siloxanes
Many
chemicals in commerce are classified as âsuperhydrophobicâ,
having log octanolâwater partition coefficients (log <i>K</i><sub>OW</sub>) approaching or exceeding 7. Examples include
long-chain alkanes, halogenated aromatics, and cyclic volatile methylsiloxanes
(cVMS). We show that superhydrophobic chemicals present unique assessment
challenges because of their sparing solubility in water and difficulties
in empirical determinations of bioconcentration factors (BCFs) and
aquatic toxicity. Using cVMS as an example, BCFs are considerably
lower than expected due to biotransformation. Reviewed aquatic toxicity
test data for cVMS in a range of aquatic organisms show little or
no toxic effects up to solubility limits in water and sediment. Explanations
for this apparent lack of toxicity of cVMS, and by extension to other
superhydrophobic chemicals, are explored using a conventional one-compartment
uptake model to simulate bioconcentration and toxicity tests using
an assumed baseline narcotic critical body residue (CBR) and a range
of organism sizes. Because of the low aqueous concentrations, equilibration
times are very long and BCFs are sensitive to even very slow rates
of biotransformation. Most organisms fail to achieve the assumed CBR
during feasible test durations even at the solubility limit. Regulatory
evaluation of superhydrophobic substances requires specially designed
test protocols addressing biotransformation and dietary uptake
Modeling Human Exposure to Indoor Contaminants: External Source to Body Tissues
Information
on human indoor exposure is necessary to assess the
potential risk to individuals from many chemicals of interest. Dynamic
indoor and human physicologically based pharmacokinetic (PBPK) models
of the distribution of nonionizing, organic chemical concentrations
in indoor environments resulting in delivered tissue doses are developed,
described and tested. The Indoor model successfully reproduced independently
measured, reported time-dependent air concentrations of chloroform
released during showering and of 2-butyoxyethanol following use of
a volatile surface cleaner. The Indoor model predictions were also
comparable to those from a higher tier consumer model (ConsExpo 4.1)
for the surface cleaner scenario. The PBPK model successful reproduced
observed chloroform exhaled air concentrations resulting from an inhalation
exposure. Fugacity based modeling provided a seamless description
of the partitioning, fluxes, accumulation and release of the chemical
in indoor media and tissues of the exposed subject. This has the potential
to assist in health risk assessments, provided that appropriate physical/chemical
property, usage characteristics, and toxicological information are
available
Decabrominated Diphenyl Ethers (BDE-209) in Chinese and Global Air: Levels, Gas/Particle Partitioning, and Long-Range Transport: Is Long-Range Transport of BDE-209 Really Governed by the Movement of Particles?
In
this paper, we report air concentrations of BDE-209 in both gas- and
particle-phases across China. The annual mean concentrations of BDE-209
were from below detection limit (BDL) to 77.0 pg·m<sup>â3</sup> in the gas-phase and 1.06â728 pg·m<sup>â3</sup> in the particle-phase. Among the nine PBDEs measured, BDE-209 is
the dominant congener in Chinese atmosphere in both gas and particle
phases. We predicted the partitioning behavior of BDE-209 in air using
our newly developed steady state equation, and the results matched
the monitoring data worldwide very well. It was found that the logarithm
of the partition quotient of BDE-209 is a constant, and equal to â1.53
under the global ambient temperature range (from â50 to +50
°C). The gaseous fractions of BDE-209 in air depends on the concentration
of total suspended particle (TSP). The most important conclusion derived
from this study is that, BDE-209, like other semivolatile organic
compounds (SVOCs), cannot be sorbed entirely to atmospheric particles;
and there is a significant amount of gaseous BDE-209 in global atmosphere,
which is subject to long-range atmospheric transport (LRAT). Therefore,
it is not surprising that BDE-209 can enter the Arctic through LRAT
mainly by air transport rather than by particle movement. This is
a significant advancement in understanding the global transport process
and the pathways entering the Arctic for chemicals with low volatility
and high octanolâair partition coefficients, such as BDE-209
Assessing the Fate of an Aromatic Hydrocarbon Fluid in Agricultural Spray Applications Using the Three-Stage ADVOCATE Model Framework
Components
of emulsifiable concentrates (ECs) used in pesticide
formulations may be emitted to air following application in agricultural
use and contribute to ozone formation. A key consideration is the
fraction of the ECs that is volatilized. This study is designed to
provide a mechanistic model framework for estimating emissions of
an aromatic hydrocarbon fluid used in ECs based on the results of
spray chamber experiments that simulate fate as the fluids become
subject to volatilization, sorption to soil, and biodegradation. The
results indicate the need to treat the volatilization losses in three
stages: (i) losses during spraying, (ii) losses up to 12 h after spraying
in which the soil is coated with the ECs, and (iii) subsequent longer
term losses in which the ECs become increasingly sorbed and subject
to biodegradation. A mass balance model, the agrochemical derived
volatile organic compound air transfer evaluation (ADVOCATE) tool,
is developed, treating the ECs as seven hydrocarbon component groups,
to estimate the volatilization and biodegradation losses using parameters
fitted to empirical data. This enables losses to be estimated for
each hydrocarbon component under field conditions, thereby providing
a basis for improved estimation of ozone formation potential and for
designing ECs that have lower emissions