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