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Bioavailability Science and Risk Assessment of Organic Chemicals
Kirk T. Semple
Professor of Environmental MicrobiologyDirector of International Engagement
Lancaster Environment Centre, Lancaster University, Lancaster, United Kingdom
k.semple@Lancaster.ac.uk
Pepper et al., 1996
Fate and behavior of organic chemicals in soil
Non-extractable resides formation?
Fate and behaviour of organic chemicals in soil
OH
OH
Semple et al., 2001.
Organic chemical
Degradationproduct
Mineralisationto CO2
+Assimilation into
biomass
Uptake
Volatilisation
Leaching
Intra-soil processing
Available residuesv
Non-available/Non-extractable/persistent residues
Fate and behaviour of organic chemicals in soil
Time
Co
nta
min
ant
con
cen
trat
ion
Non-extractable:persistent residues
Adapted from Semple et al., 2003
Recalcitrant fraction: harsh extraction
Readily accessible/available fraction
Degradable/removable fraction/readily extractable
Non-available fraction?
Do these residues represents a risk?
~ 4 kJ/mol
Physico-chemical interactions and availabilityMain binding mechanisms
Covalent binding
Ionic binding /ligand exchange
Hydrogen bonds
Charge transfer complexes
Van der Waals interaction
Hydrophobicpartioning
Sequestration / EntrapmentTime and/ or sizedependent process
Binding energy
250-500 kJ/mol
50-150 kJ/mol
2-150 kJ/mol
1-50 kJ/mol
1-2 kJ/mol
?
Ageing processes
Sequestration and biotic interaction
Mineral
fraction
Surface
sorption
Surface
sorption
Diffusion into glassy
organic matter
Diffusion
into pores Diffusion into rubbery
organic matter
Water soluble
fraction
www.canada.ca/en/health-canada/services/environmental-workplace-health/environmental-contaminants/human-biomonitoring-environmental-chemicals.html
Routes of exposure to organic chemicals
Retrospective and Prospective Risk Assessment
Total concentration
Total extractable
concentration
Internal effect concentration
Concentration gradient
Figure modified from Hammel and Herrchen (1999)
• Risk assessment is site specific, based on total extractable concentrations
• Bioavailability assumed to be 100%
• Protective (overly?)
Measured effect
Organism
Soil
Problem Formulation
Risk Prioritisation Hazard Identification
Exposure Assessment
Risk Estimation
Risk Characterisation
* Stages with each tier of Risk Assessment
Economics Technology
Social Issues Management
Risk Management
Collect data, iterate processes & monitor
Tier 1 Risk Screening *
Tier 2 Generic QuantitativeRisk Assessment *
Tier 3 Detailed QuantitativeRisk Assessment *
Options Appraisal
(after, DETR et al, 2000)
Framework for environmental risk assessment and management
The Framework provides a
Tiered approach to
Environmental risk
assessment and management.
Where the level of effort put
into assessing each risk is
proportionate to:
- its priority in relation to other risks
- its complexity in relation to understanding of the likely impacts.
EFSA Scientific assessment
• Chemical identification
• Hazard identification
• Physico-chemical properties
• Environmental fate
Classification (CLP)Hazard approval criteria
Risk basedapproval criteria
(Eco)Toxicological profile:
CMR, Endocrine effects, PBT, POP
• Hazard characterisation/assessment• Exposure assessment• Risk estimation• Risk characterisation
Definitions:
Bioavailability and Bioaccessibility
Semple, K.T., Doick, K.J., Jones, K.C., Craven, A., Burauel, P. and Harms, H. 2004. Environmental Science and Technology 38, 228A-231A.
The bioavailable compound as that which is freely
available to cross an organism’s (cellular) membrane from
the medium the organism inhabits at a given point in
time,
i.e. available now (no constraints)
The bioaccessible compound is that which is available to
cross an organism’s (cellular) membrane from the
environment it inhabits, if the organism has access to it;
however, it may be either physically removed from the
organism, or only bioavailable after a period of time,
i.e. what is actually bioavailable now plus what is
‘potentially bioavailable’ (spatial and temporal
constraints).
European Centre for Ecotoxicology and Toxicology of Chemicals (ECETOC) Technical Report No 117. (2013) Understanding the Relationship between Extraction Technique and Bioavailability
Circulation within organism, accumulation in target organ, toxicokinetics, toxic effects, biodegradation
B&CContaminant-soil/
Sediment interactionsTransport
Passage across cell membrane
Soil/sediment-associatedcontaminant(s)
Released/mobilecontaminant(s)
Equilibrium?
Cell membrane
Absorbed contaminantin organism
Site of biological response
(Biological tests)
Definitions and processes…….
Ehlers and Luthy (2003)
Bioavailability processes
Semple et al. (2004; 2007)
Bioavailability
Bioaccessibility Desorption extraction
Adapted from Semple et al., 2004, 2007; Ortega-Calvo et al. 2015
A, B and C also occur internally – e.g. in the gut lumenPhysiology – organism specific; gastric pH, rate of absorption, pre-exposurePhysico-chemical – contaminant properties, mineralogy, soil particle size, soil properties
Bioavailable compound
Sorbed compound (rapidly reversible)[bioavailable or bioaccessible: temporally constrained]
Occluded compound[non-bioaccessible]
Bioaccessible compound[physically constrained]
Sorbed compound(slowly/very slowly reversible)[bioaccessible: temporally constrained]
Plant root
Earthworm
Microbes
Semple et al., 2004
Inclusion of bioaccessibility/bioavailability into RA?
Total concentration
Total extractable
concentration
Bioaccessible concentration
Bioavailable concentration
Internal effect concentration
Concentration gradient
Figure modified from Hammel and Herrchen (1999)
• Risk assessment is based on total extractable concentrations➢ Bioavailability assumed to be 100%
• We need to know the total extractable concentration
• Bioavailability is organism specific
• Bioaccessibility provides a closer estimation of exposure, uptake, toxicity/ biodegradation
Measured effect
Soil
Organism
• Partition-based, non-depletive extractions that measure freely dissolved concentrations (Cfree) in pore/interstitional water:
• Measuring what is “actually available” for uptake and causing effects
➢ a. Semi-permeable membrane devices
➢ b. Solid phase microextraction fibers
➢ c. Polymer sheets
➢ d. Polymer films in vials
Passive sampling methods
Desorption methods aiming to assess the:
• Water soluble
• Loosely-bound/rapidly-desorbing fraction
• Potential availability (what is and can become available in time)
➢ Bioaccessibility: Tenax and Cyclodextrin
Desorption methods
Dutch Bioavailability workshop (2008) and advisory group report (2009/2011): Selection of promising/ viable methods
PAH & phenol concentration remaining
after degradation by Indegenous degraders (mg kg-1)
1 10 100 1000
PA
H &
ph
en
ol co
nce
ntr
atio
n r
em
ain
ing
aft
er
de
gra
tio
n b
y b
acte
ria
l in
ocu
la (
mg
kg-1
)
1
10
100
1000
Soil A
Soil B
Soil C
St/S0 = frap ∙ exp(-krap · t) + fslow ∙ exp(-kslow · t)
Mineral fraction
Surface sorption
Surface sorption
Diffusion into glassy organic matter
Diffusion into pores
Diffusion into rubbery organic matter
Water solublefraction
Uptake
Uptake and/or diffusion
Uptake and/or diffusion
Organism
Chemical mimic
Soil-associated HOC
DesorbedHOC
Adapted from Semple et al., 2003
Chemical mimicry?
TenaxCyclodextrins
• Suite of tests:
➢ Earthworm reproduction
➢ Collembolan reproduction
➢ Predatory mite reproduction
➢ Seedling emergence➢ Plant vegetative vigor
• Tests consider the different putative exposure pathways via
➢ pore water and soil (earthworms and Collembola),
➢ via food (mites)➢ pore water and air (plants)
• Biodegradation?
European (prospective) regulation on plant-protection products
• The 10th SETAC Europe Special Science Symposium, Hotel Marivaux, Brussels, Belgium. The was a two-day meeting on 14-15 October 2014.
➢ Bioavailability of organic chemicals: Linking science to risk assessment and regulation
➢ The main objective of this symposium is to identify and provide scientifically-based solutions to the challenges faced by regulators and industries in handling bioavailability issues during risk assessment and regulation of organic chemicals.
Out of Brussels……
Ortega-Calvo et al. 2015. 49, 10255-10264
Soil or sediment Water
BiodegradationEffects
Bioaccumulation(Biological tests)
Cell membrane
Organism
Adapted from Ortega-Calvo et al. 2015. 49, 10255-10264
Measurement of contaminants: over 2000 papers on bioavailability measurement
Non-extractable
Total concentration(not measurable)
Very slowly/slowly desorbing
Total extractable concentration(Organic solvent)
Rapidly desorbing
Bioaccessible concentration(Desorption extraction: Tenax/cyclodextrin)
DissolvedFree and
associated with DOM
Bioavailable concn
Dissolved:Passive samplingCfree extraction (aq)
Tiered Risk Assessment-Management Framework
Is it likely that the soil/sediment is polluted?
Tier 1:Generic quantitative
risk assessment
Bioaccessible/Bioavailable concentrationsLaboratory biodegradation
Higher-tier biological effects test
Tier 2:Detailed quantitative
risk assessment
Exit?
Exit?
Exit?
Decisio
n m
akin
g
Risk management Exit
Total extractable concentrationsEquilibrium partitioning (Koc)
Lower-tier biological effects testsSoil/sediment quality standards
Ris
k a
sses
smen
t
Chemical fate modelling including Bioaccessibility/bioavailability
Functional/structural biodiversity
Tier 3:Site-specific risk
assessment
Exit?
Ortega-Calvo et al. 2015. 49, 10255-10264. (Adapted from ISO (2014))
• Present risk assessment uses total extractable concentrations and has been the standard for over 30 years, despite being overly conservative and overly protective
• Chemical measurement of bioavailability is challenging➢ Bioavailability is organism-specific
• A clear explanation of the steps to be taken when including bioaccessibility/ bioavailability is required for RA
• For regulatory purposes, it is necessary to use a straightforward approach to retrospective or prospective risk assessment to determine the risks to human and environmental health
• As with chemical methods, there should be a restricted number of bioassays used, and where possible, these should be validated and preferably standardized in combination with proper quality assurance and control procedures.
Inclusion of bioaccessibility/bioavailability into RA?
• Key to inclusion of bioaccessibility/bioavailability-based guideline are:
➢ Science underpinning bioavailability
➢ Reliable tools for measurement
➢ Recognition and acceptance by regulators
➢ Careful consideration of liability inc. community acceptance
➢ Policies and bioavailability-based guidelines
Finally………….
• Dr Jose-Julio Ortega-Calvo
• Dr Joop Harmsen
• Prof John R. Parsons
• Prof Michael D. Aitken
• Dr Charmaine Ajao
• Dr Charles Eadsforth
• Dr Malyka Galay-Burgos
• Prof Ravi Naidu
• Dr Robin Oliver
• Prof Willie J.G.M. Peijnenburg
• Dr Jörg Römbke
• Dr Georg Streck
• Dr Bram Versonnen
Acknowledgements
• Prof Kevin C. Jones
• Prof Graeme I. Paton
• Dr Brian J. Reid
• Prof Hauke Harms
• Dr Lukas Wick
• Dr Peter Burauel
• My ‘long-suffering’ research group
• Funding from SETAC Europe, Defra, NERC, BBSRC
Thank you for listening
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