RHP 488/588 Radioecology

1
RHP 488/588Radioecology

• Radionuclide behavior in ecosystems
  Supplementary Reading in Ch 5 of Whicker
  Shultz, vol I

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Major processes impacting radionuclide transport
see W S, Vol 1, p 131
SOURCE TERM
AIR
SOIL SEDIMENT
WATER
PLANTS
HERBIVORES
SINK
DETRITUS FOOD CHAIN
DETRITUS
CARNIVORES
3
Compartment Systems

• Relevance
• Tracer kinetics
• Prediction of radionuclide transport and
  accumulation
• Reference
• Whicker Schultz, Vol II pps 64-80

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Terminology System

• Aggregate of interacting parts where sum is
  greater than components
• Occurs at cellular, organism, or ecologic level
  in biology
• Other examples industrial, social mechanical,
  electric
• Components air, water, soil, plants, consumer
  organisms.
• Focus transfer/accumulation of radionuclides
  between such components

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Terminology - Compartment

• Space, with defined boundaries
• Materials are free to move and mix to homogeneous
  state
• Open compartment exchanges material with
  outside space
• Closed compartment no exchange
• Sink receives material from outside but does
  not release it

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Terminology Steady State

• Compartment is in steady state when materials are
  entering the compartment at the same rate they
  are leaving
• Content of material within compartment is steady
  over time
• Reality in biological systems few compartments
  are in actual steady state but close enough

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Terminology - Tracee

• Kinetics of compartment are governed by flow of
  fluid medium, or substances within the fluid.
• e.g., water and natural elements in a pond.
• Water is the tracee

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Terminolgy - Tracer

• e.g., pollutant
• Substance in small quantities relative to tracee
• Behavior mimics that of the tracee

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Terminology Transport Pathway

• Route by which material passes into, from, or
  between compartments
• Represents specific processes
• Involves specific processes
• Examples include deposition, resuspension,
  sorption, desorption, ingestion, molting,
  decomposition

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Cycle of Mass Transfers
Atmosphere
02, , Pollutant Gases
H2O , Solutes
PLANTS
Dust
H2O CO2 N2
Dead Biomass
H2O , Solutes
SOIL Inorganic Organic Colloids
SOIL Solution
SOIL Air
H2O Solutes
UNSATURATED ZONE
GROUNDWATER
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Chemical Cycles

• Soil Chemistry is essential part of cycling of
  elements
• All weathered ions eventually circulate back to
  land
• C, N, S cycle relatively quickly among
  atmosphere, oceans, and soil
• Others cycle more slowly, but movement is rapid
  on geologic time-scale

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Soil-ion Interactions

• Removal rate of elements slowed by adsorption.
  precipitation, pH buffering plant uptake
• Soil clay content largely controls
• Ion entry into soil includes
• mineral weathering
• organic matter decay
• rain
• irrigation/fertilization
• release of ions from colloid/clays

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Interactions, contd

• Soil-retained ions are largest fraction of
  nutrients available for plants
• Mineral weathering slow relative to plant needs
• Organic decay rapid, but most ions which are
  released then react with soils solid phase
  before plants can use
• Ion retention in soils strong -- minerals recycle
  many times

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Ion/molecule Retention on Soils

• Ions retained in soils by
• cation/anion exchange,
• precipitation,
• weak electrostatic attraction,
• complex formation,
• retention within microbial cells.
• Each ion reacts by several mechanism, with
  different solid phases
• Reactions exceedingly complex

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Uptake from Soils

• Most elements can be found in soils
• Only sixteen are essential for growth/reproduction
  (?)
• C, H, O, N, P, K, Ca, Mg, Fe, Mn, Zn, Cu, Mo, B,
  Cl
• All are obtained from soil, except C, H, O which
  can also come from the atmosphere
• If an element is in soil, it is probably in
  plants

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Soil Plant Compositions

• Plants derive mineral components from soil
• While soil contents vary/ plant composition is
  less so (soil development limits availability of
  elements)
• Soil is an O-Si-Al-Fe matrix with trace amounts
  of essential elements

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Soil and Plant Compositions

• Plants derive mineral content from soil (minor
  exceptions NOx, NH3, SO2)
• H, C, O from air
• Major factors of ion availability
• concentration in soil solution
• release rate from solid phase
• activity of soil microorganism
• discrimination by plant roots in ion uptake

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Typical Concentrations Plants Soils
Element Soil wt Plant ash/soil Plant need kg/ha/y Soil life, yrs
O 49 - -
H - - - -
Si 33 0.3 20 21,000
Al 7 0.03 0.5 180,000
Fe 4 0.1 0.5 100,000
C 1 - - -
Ca 1 25 50 260
K 1 15 30 430
Na 0.7 1 2 4,600
Mg 0.6 3 4 2,000
N 0.1 15 30 40
P 0.08 4 7 150
Se 0.000001 500 0.0003 40
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Elements Essential for Plants Animals

• Macronutrients
• Required in large amounts
• H, C, N, O, Mg, P, S, K, Ca
• Animals require Na, Cl
• Micronutrients
• Required in small amounts
• B, Cl, V, Mn, Fe, Cu, Zn, Mo
• Animals require F, Si, Cr, Ni, Co, As, Se, Sn, I
• Animals derive almost all essential nutrients
  from plants

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Essential Ions
Periodic Table of the elements differentiated on
the basis of soil chemistry. Essential elements
are in white common toxic elements are in
crosshatched areas elements normally present in
inconsequential amounts are in dotted areas.
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Plants Ability to Provide Nutrients

• Retention of ions in soil solution
• Plant selectivity
• Limited ion translocation from root to plant top.
• Plants can tolerate higher range of mineral
  concentrations than animals
• Grazing animals can suffer from deficiencies or
  excesses from plants in natural environments

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Mechanisms of Plant Uptake
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Uptake from Soils

• Lyotropic series (relative ion replaceability, or
  ease of removal from specific soil colloids)
• Li Na gt K NH4 gtRbgtCs Mg2 gtCa2 gt Sr2
  Ba2 gt La3 H(Al3) gt Th4
• (this is for montmorillonite clays)
• Transfers from soils to plants
• Sr gtgt I gt Ba gt Cs, Ru gt Ce gt Y, Pm, Zr, Nb gt Pu

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Plant-to-Soil Concentration Ratio

• Radionuclide uptake from soil described by
  empirical concentration ratio
• Reported for dry soil and wet (or dry)
  vegetation
• Water content of crops 90

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Food Chain - Root Uptake From Soils
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Plant Species and Translocation

• Leguminous plants
• (soybeans, peas, alfalfa, clover)
• have symbiotic relationship with nitrogen fixing
  bacteria
• higher uptake than non-legumes
• Elements not soluble in plant fluids
• remain at site of deposition
• Translocation occurs
• to rapidly growing parts
• leaves / grains / fruits

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Relative Mobility of Elements in Plants
Intermediate Li Ba Fe Mn Z Co Cu Mo Ra

• Mobile
• K
• Na
• Rb
• Cs
• Mg
• Ca
• Sr
• P
• S
• Cl

Immobile B Pb Po Th Pu
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Foliar deposition

• Direct deposition from passing plume (deposition
  velocity) onto plant surfaces
• Rainsplash
• particles lt 100 Fm
• heights up to 40 cm
• May be dominant mechanism of animal / human
  exposure where root uptake minimal
• Dependent on growing season
• Plant shape/size

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Removal of Foliar-Deposited Contaminants

• Radioactive decay
• Volatilization
• Rain
• Weathering
• Senescence
• Washing prior to consumption
• Loss rate in field 14 days

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Radionuclide Transfer to Animal Products

• Uptake Retention by Animals
• function of metabolism
• function of solubility/need
• intake rate

EXCRETION
ORGAN OR FOOD PRODUCT
INTAKE
BLOOD
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Trophic Level Effect

• Organisms with same number of steps between
  themselves primary producers (plants) are at
  same trophic level
• Radionuclides are poorly assimilated (in general)
• Concentration of radionuclides decrease at higher
  trophic levels
• There are notable exceptions
• 137Cs (due in part to biological half-time)

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Factors Dictating Movement

• Movement and concentration of radionuclides in
  the environment are governed by many factors
• physical characteristic (liquid, solid, gas)
• chemical characteristic (valence state, compound,
  bond - ionic, covalent)
• half-life
• progeny or primary (progeny may stay with
  parent) 

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Movement, continued

• Other environmental aspects may influence
  movement as well
• ecosystem structure
• e.g., mineral cycling in bog versus stream
• High activity levels do not (necessarily)
  correlate to high atoms
• Behavior of contaminant may be dictated by
  tracee bulk movement

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Nutrient Analogs

• 131I -
• Fission product,
• Released in fallout,
• Fuel reprocessing
• Mimics stable iodine
• Accumulates in thyroid (herbivores)
• Concentration in thyroid gt Concentration in
  foliage by 104 or more
• 90Sr
• Long lived
• Ca analogue,
• Tbio long
• ...concentration in bone with time?

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Compartment Input Output Rates

• Input rates to ecological compartments
  characterized by
• abundance,
• high solubility,
• chemical and radiological activity,
• existence of nutrient analogs,
• particle size,
• and intermediate half-life
• Loss rates are enhanced by
• high solubility,
• existence of abundant nutrient analogs,
• and short half-lives

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Characteristics and Behavior of Organisms
Their Impact on Radionuclide Cycling

• Implications for radionuclide distribution and
  accumulation in ecosystems
• How can different species, with a common biotic
  community, vary in radionuclide content from
  common source?

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Characteristics and Behavior, continued

• Potential explanations
• Surface morphology
• Structure - flat, ridges, plates, branched
• Relative surface area
• Sphere vs plate,
• Small vs large
• Biological surface characteristics -
• hairy leaves,
• waxy substances,
• fur
• feathers
• stomata

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Characteristics and Behavior, continued

• grooming behavior
• metabolism -
• higher metabolism ? higher food consumption ?
  if radionuclides tracer is not homeostatically
  regulated ? net accumulation

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Characteristics and Behavior, continued

• Metabolism physiology function of age
• Growing animals have higher intake rate,
  assimilation may be enhanced
• Sex dietary needs vary for reproduction/lactation
  
• Season
• exposure varies (hibernation)
• Dietary
• what's eaten varies by species
• Lifespan
• longer lived nuclides may accumulate more in
  older individuals

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Ecosystem Attributes Their Impact on
Radionuclide Cycling

• Physical aspects
• Proximity to source (e.g., deep geologic
  repository versus waste pond)
• Climate -
• Impacts type of ecosystem developed as well as
  transport of nuclides within ecosystem (e.g.,
  wind dispersal - consider Columbia River Gorge
  versus Rain Forest)
• Topography - interacts with climate
• Volume of ecosystem (aquatic - pond volume)

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Ecosystem Attributes, continued

• Chemical Factors
• Availability of minerals
• High abundance of stable or analog
• Reduced concentration in organisms (dilution)
• Paucity of stable or analog
• Elevated concentration in organisms (nutrient
  conserving mechanisms)

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Ecosystem Attributes, continued

• Example CRs
• Concentration ratio
• for Sr and Cs in marine and freshwater systems -
  freshwater CRs higher,
• competing nutrients (K, Ca) are lt 10 ppm in
  freshwater,
• 400 ppm in marine
• Soil type (acidity/alkalinity, mineral
  composition)

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Ecosystems 5 Landscape Types

• Forest -
• Landscape dominated by trees close enough
  together for their crowns to touch
• Savanna
• Scattered trees in a grassy or shrubby area
• Thicket -
• Tall shrubs or small trees growing so thickly
  that they are difficult to walk through 

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Ecosystems landscape types

• Grassland
• Grass and other herbs dominating a landscape from
  which trees are scare or absent 
• Desert -
• Landscape in which plants are sparse and often
  scrubby here the sand and rock may be more
  important in the landscape than the vegetation

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Biological Aspects Impact on Cycling

• Biological aspects
• physiognomic aspects 
• growth form
• function
• deciduous or evergreen
• dominant species are trees, shrubs or herbs
• size
• leaf size - e.g., broad-leaved or needle-leaved
• leaf texture -e.g., succulent, thin, or hard

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Biological Aspects, continued

• Effect of physiognomy
• vegetation - filters / collects / holds
  contaminants
• limited biomass (Arctic systems, alpine) may
  concentrate
• Ecosystem diversity
• impact on movement and accumulation
• highly diverse system - radionuclides (may) be
  partitioned in manner akin to diversity.
• Low diversity systems - the opposite

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BEHAVIOR OF SPECIFIC CHEMICAL GROUPS OF
RADIONUCLIDES

• See Whicker Shultz, Vol 1, Ch 5, Section III p
  147 on

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http//pearl1.lanl.gov/periodic/ Los Alamos
National Laboratory's CST Division
Presents Periodic Table of the Elements Click an
element for more information
Groups are noted by 3 notation conventions.
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Nonmetals (H, C, P, I)

• Electron acceptors
• Include some elements within Group IIIA through
  Group VIIA
• All isotopes of essential biological nutrients.
• Uptake and retention largely controlled by flux
  of essential nutrients through ecosystem
• Movement through food chains is high, but
  biological retention is low (Tbio days)

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Nonmetals 3H

• Multiple modes of production
• natural
• anthropogenic
• Intermediate T1/2 - 12 y
• When released
• follows H,
• tends to follow hydrologic cycle
• Generally does not sorb to sediments or biotic
  surfaces

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Nonmetals 14C 32P 131I

• 14C
• multiple modes of production (natural,
  anthropogenic)
• long lived 5500y
• found in biota same concentration as air, water
•  32P
• human made
• short T1/2
• stable P is relatively scare in biosphere, but
  mobile
• organisms concentrate P, excrete slowly 

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Nonmetals 131I

• 131I
• fission product
• short T1/2 8 d
• readily enters biological systems
• concentrates in thyroid

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Light Metals

• ALKALI METALS, series of six chemical elements in
  group 1 (or Ia) of the periodic table. They are
  soft compared to other metals, have low melting
  points, and are so reactive that they are never
  found in nature uncombined with other elements.
  They are powerful reducing agents, that is, they
  give up an electron easily, and react violently
  with water to form hydrogen gas and hydroxides,
  or strong bases. The alkali metals are, in order
  of increasing atomic number, lithium, sodium,
  potassium, rubidium, cesium, and francium .
  Francium exists only in a radioactive form.

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Group IA (K, Rb, Cs)

• Includes "alkali metals"
• Behavior of K, Rb, Cs very similar,
• Tend to follow behavior of K
• K
• Essential nutrient
• Shows fairly uniform distribution in tissues
• Exhibits moderate to high mobility in food
  chains
• 40K
• naturally occurring
• significant contributor to background dose
• primordial, very long lived (1.3 x 109 y)

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Group IA (K, Rb, Cs)

• Rb
• 16 possible isotopes,
• 87Rb natural, long lived (4.8 x 1010 y)
• Fission product 86Rb exhibits high CRs in
  freshwater and marine elements

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Group IA (K, Rb, Cs)

•  Cs
• 137Cs fission product
• Half life 30 y
• Biologically mobile
• In certain food chains, Cs will increase with
  trophic level - Cs retained longer in body
• Biomagnification not generally observed with
  other nuclides, but some pesticides (DDT)
• Availability depends on ecosystem characteristics
  - particularly soil properties (cation exchange
  properties)

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Group IIA (Ca, Sr, Ba, Ra)

• ALKALINE EARTH METALS, series of six chemical
  elements in group 2 (or IIa) of the periodic
  table. They are strong reducing agents, that is,
  they give up electrons easily. They are less
  reactive than the alkali metals, but reactive
  enough not to be found free in nature. Although
  rather brittle, the alkaline earth metals are
  malleable and extrudable. They conduct
  electricity well, and when heated, burn readily
  in air. The alkaline earth metals are, in order
  of increasing atomic number, beryllium,
  magnesium, calcium, strontium, barium, and radium
  . Their oxides are called alkaline earths.  

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Alkaline Earth Metals, continued

• 2 oxidation state
• Chemically reactive, seldom occur in free state
• Commonly form salts such as carbonates, sulfate
  and chlorides.
• Ca is most abundant, and is essential.
• Sr, Ba, and Ra behavior inferred from Ca. Tend
  to accumulate in bone structure

59
Alkaline Earth Metals, continued

• Ca
• 45Ca man-made
• Not sufficient quantities to be of concern
•  Sr
• 89,90Sr Fission products
• Form soluble compounds - mobile in ecosystems
• 90Sr has progeny, 90Y, also radioactive
• OBSERVED RATIO Sr/Ca ratio in an organism to
  Sr/Ca ratio in diet or soil
• OR soil to plant 1. Plant to animal lt1

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Alkaline Earth Metals, continued

• 140Ba
• like Sr, fission product,
• short T1/2 12.8 d
• Ra
• important naturally occurring.
• 226Ra and 228Ra most significant
• found in geological deposits
• uptake in plants lt Sr or Ca
• Concentrated in plants 10 - 100 x less than soil

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Noble Gases (Ar, Kr, Xe, Rn)

• Elements of Group VIIIA are called "Noble" or
  inert gases, because of electronic structure.
• Exist as gas, do not combine with other elements.
  No nutrient analogs - expose biota through
  submersion and external irradiation

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Noble Gases, continued

•  41Ar
• Activation product, short lived
•  85Kr
• Fission product,
• High yield
• Intermediate half-life (10 y)

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Noble Gases, continued

•  Xe
• Multiple isotopes,
• Fission products
• Rn
• 222Rn and 220Rn -natural
• ubiquitous
• multiple progeny
• alpha-emitters
• do not generally concentrate, unless
• formed insitu from parent decay (Ra)

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Heavy Metals

• (Cr, Mn, Fe, Co, Zn, Zr Tc, Ru, Pb, Po)
• Exhibit complex and varied chemistry - exhibited
  in food chain transport, critical organs,
  assimilation and retention.
• Nutrient analogs are micronutrients for specific
  biochemical functions.
• Those with nutrient analogs have moderate to high
  food chain mobility, but so do others with no
  analogs.
• Common feature - most end up in soil "sink" after
  introduction to environment.

65
Heavy Metals, continued

• 51Cr,
• 54Mn,
• 55,59 Fe,
• 60Co,
• 65Zn,
• 95Zr,
• 99Tc,
• 103, 106 Ru,
• 210,212...Pb,
• 210Po

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Rare Earths

• (Y, La, Ce, Pr, Pm)
• All fission products, no nutrient analogs.
• Tend towards poor assimilation and low food chain
  transport.
• In ecosystems, tend to be associated with soils
  and sediments.
• In vertebrates, tend to bony tissues.
• Biological half-times are long.
• Stable isotopes occur in trace quantities in
  nature - but no essential biological role

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Rare Earths, continued

• Tend toward 2, 3 valences, but in aquatic
  systems form hydroxides and insoluble
  particulates
•  Tend to exhibit low biotic uptake, minimum food
  chain transport, but may readily sorb to
  particles and surfaces
•  Principle isotopes of interest include 90,91Y,
  140La, 141,144Ce, 144Pr, 147Pm

68
Actinides

• Include atomic 89 and higher.
• Ac, Th, Pa, U as well as transuranium elements.
• All can exist as 3, but 4 is is more stable
  for Th, Pu, Np, and 6 for U.
• Chemically similar to rare Earths - makes
  separation difficult.
• Many are alpha emitters, varied t1/2 , some have
  chemical toxicity higher than radiotoxicity (U)
• Tend to form insoluble compounds in environment -
  not biologically mobile
• Passive interaction with environment (go along
  for the ride) exposure largely through surface
  contact/ingestion or inhalation

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Summary

• Multitude of factors impact nuclide movement
• May be opposing
• Or competing
• Need to understand system in order to model or
  explain behavior