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BIOACCUMULATION PROCESSES

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					Bioconcentration and Bioaccumulation of
          Organic Substances




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BIOACCUMULATION PROCESSES
The importance of studying the uptake of organic compounds by biota is (1) to determine toxicity
to wildlife, (2) to determine the human health effects of consuming contaminated aquatic life,
and (3) to determine the distribution and transport in the biotic phase, esp. for microbiota such as
algae. Living resources may be a risk in polluted environments. Partitioning plays an important
role in the transfer of organic compounds from water to biota, and biota can perform catalyzed
reactions to degrade contaminants. The following notes will describe the uptake of organic
compounds from various sources, including water and food.



I. Terms

A. Uptake - interfacial transport process from source matrix to biota;
transport across membrane

B. Bioconcentration - uptake from water

C. Bioconcentration Factor - Cbiota/Cwater at equilibrium or steady state
from water uptake

D. Bioaccumulation - uptake from all sources

E. Bioaccumulation Factor - Cbiota/Ci at equilibrium or steady state; i
= any biotic phase

F. Ingestion/Egestion - input of food and output via gut

G. Depuration - diffusive chemical release from gills and kidneys

H. Metabolism - biotransformation of chemical

I. Growth Dilution - reduction of Cbiota from growth

J. Bioaccumulation Cycle - sum of processes involved in uptake,
metabolism, and release of chemical

K. Biogeochemical Processing - fate and cycling of chemical through
biota in ecosystem


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L. Fugacity - chemical potential of solute in environmental phase;
escaping tendency (pascals, atmospheres, etc.)

M. Diffusive Processes - diffusion limited process

N. Bulk Flow Processes - flow limited process


II. Thermodynamics Of Bioconcentration Models

A. Microorganisms -- (e.g., bacteria, algae)

1. Microbs are often treated as small particles, where biouptake modeled
as sorption process. In this case sorption models similar to sediments are
employed.

a. Ci,s = KdCi,w                  (linear)

b. Ci,s = KdCi,wn                 (Freundlich)

c. Ci,s = QKCi,w/(1 + KCi,w)      (Langmuir)

d. (BET)

B. Larger Organisms -- (e.g., fish, mollusks, crustacea)

1. Thermodynamic process of uptake from water. Organisms are
modeled as an organic "phase" into which organic compounds partition
from water. The organism is treated as a membrane enclosing a milieu
of biomacromolecules, of which lipids play the most important role in
determining the degree of exposure and uptake. The following
equations outline the thermodynamics governing the uptake of organic
chemicals by biota.

a). For the organism: (yi = volume fraction of ith phase in biota)

1. fugacity is a equal for all biotic phases and water at thermodynamic
equilibrium
                                    3
f1 = f 2 = f 3 = f 4 = f n

2. fugacity is related to concentration as

fi = CiviifR                                                   (1)

3. the total biota concentration is expressed as

CB = Ciyi = (fb/fR)[yi/(vii)]                                (2)

b). For water:

1. fugacity is related to concentration as

fw = CwvwwfR                                                   (3)

2. at thermodynamic equilibrium fb = fw, and

CwvwwfR = CbfR{[yi/(vii)]}-1                                 (4)

3. KB = thermodynamic bioconcentration factor (ratio of biota
concentration to water concentration @ equilib.)

CB/Cw = KB = vww [yi/(vii)]                                  (5)

4. KB = bioconcentration factor; in many KB estimations

                   CB = mg/kg and Cw = mg/L

                      thus, KB = L/kg




                                        4
        y1                                                 y4
        C11                                               C44
                            y2             y3
                            C22           C33

Biochemical compartments: e.g., y1 protein; y2 carbohydrates; y3 lipids;
y4 nucleic acids; at equilibrium all Cii terms are equal, and

yiCi = CB

Compartment with smallest  has largest concentration, thus, y3C3  CB

c). Hydrophobic organic compounds (HOCs)

1. lipid phase has very small i term and behaves as a good solvent;
therefore Ci is very large at equal f, such that

CLyL/(Ciyi)  1                                                        (6)

biota concentrations are often normalized to fraction of total lipids

2. lipid normalization does not account for all variability observed in
KB; it is common to relate KB to Kow (octanol/water) by

KB/Kow = yLovo/(LvL)                                                  (7)

KB = yLKow                                                              (8)


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Mackay (1981) has shown that bioconcentration factors can be estimated
using the relation:

KB = 0.048 Kow                                                     (9)

where the constant 0.048 represented the approximate volume fraction of
lipids in the organism under study.

o/L is not Const. for all lipid, but o/L  1

3. Triglycerides and wax esters are most nonpolar lipids; expect most
storage in these lipids

   4. Expressions for bioaccumulation

KB = bioconcentration factor = CB/Cw (exposure from water only)
KL = lipid-normalized bioconcentration factor = KB/fL
NB = bioaccumulation factor = CB/Cw (exposure from all sources)
NL = lipid-normalized bioaccumulation factor = NB/fL
NBS = biota-sediment bioaccumulation factor = CB,L/Cs,oc
MB = biomagnification factor = CB/Cf

III. Kinetic Bioconcentration Models

   A. Kinetic models are employed to describe bioaccumulation as a
      complement to thermodynamic models or as an alternative to
      thermodynamic models when they are not applicable.

   Two Compartment Bioconcentration Model (water and orgam)

dCB/dt = k1Cw - k2CB                                               (10)

k1 = uptake clearance constant (L/kg-hr)
k2 = release rate constant (1/hr)
Cw = chemical concentration in water (mg/L)
CB = chemical concentration in biota (mg/kg)


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Integrated model is expressed as (when Cw = Const.)

CB = k1Cw/k2[1-exp(-k2t)]                                            (11)

When Cw is not constant, expression is ( = first order decay constant
for chemical in water)

CB = k1/k2[Cwexp(-t)] X [exp(-t) - exp(-k2t)]                      (12)


B). Depuration of HOC's (depuration is diffusive flux from biota to
water; depuration occurs through gills and kidneys)

When exposed organism placed in contaminant-free water

dCB/dt = -k2CB                          (release term)

ln CB/CBo = -k2t                        (first-order release)

C). Uptake of organic substances may occur from sediments; the kinetic
profile of uptake from sediments follows the same rate equation as eq.
10 above, but uptake rates are much slower

dCB/dt = k1Cs - k2CB                                            (10-b)

Cs = chemical concentration in sediment (mg/L)

Bioaccumulation factors (NL) where biota concentrations are normalized
to lipids and sediment concentrations normalized to percent organic
matter are near unity

NL = CL/Coc  1 for hydrophobic organic substances.

D). Refining bioconcentration model to account for metabolism and
growth – loss terms



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1). Metabolism of HOC; metabolism acts to lower tissue concentrations
or fugacity of HOC in organism through first order process; adds to
bioconcentration model as loss term as

Rate = -k3CB

2). Growth acts to lower concentrations or fugacity of HOC through
growth dilution or volume dilution; adds to bioconcentration model as
loss term; organismal growth rate can be estimated by

G = w-

G = net growth rate (1/hr)
 = 0.002 at 10 oC (growth coefficient)
w = wet weight in g
 = 0.2 - 0.3 (growth exponent)
E). Refined bioconcentration model to account for metabolism and
growth dilution

dCB/dt = k1Cw - k2CB - k3CB - GCB                                    (13)

At steady-state the KB is

KB = k1/(k2 + k3 + G)                                                (14)

F). Bioavailability of dissolved HOC's

1). Uptake of HOC's influenced by particles and colloids in water;
natural water contains carbon pools of varying sizes.
Uptake rates decrease due to binding of HOC's to particles and/or
colloids; effectively lowers dissolved HOC [] and/or fugacity.

Cw = CT(1 + Kd[TSM] + KDOC[DOC])-1

Cw = CT(1 + 0.41Kow[TSM] + 0.41Kow[DOC])-1


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Cw = concentration in dissolved phase (mg/L)
CT = total concentration in water (mg/L)
Kd = particle/water distribution constant (L/kg)
TSM = total suspended matter concentration (kg/L)
Kow = n-octanol/water partition coefficient
KDOC = colloid/water distribution constant (L/kg)
[DOC] = dissolved organic carbon concentration (kg/L)

IV. Bioaccumulation From Water And Food

A. Transport of HOC's through ecosystem includes several uptake and
release terms for all inputs and outputs from all biotic components;
bioconcentration model can be further refined to include input from food
sources, and as such, can be used to model food chain accumulation and
biogeochemical cycling.

1. Uptake from food sources is normally described as

Rate = EoFCF                                                      (15)

Eo = extraction efficiency of chemical from food
F = specific food consumption rate (kg/kg-hr)
CF = concentration of chemical in food

2. Bioaccumulation model can be described as

dCB/dt = k1Cw + EoFCF - k2CB - k3CB - GCB                         (16)

At steady-state the bioaccumulation factor (NB) is

N = k1 + EoF/(k2 + k3 + G)                                        (17)

3. Food chain accumulation at steady-state; consider three-level food
chain where organism 1 is at the base of the food chain (e.g.,
phytoplankton) and is eaten by organism 2 which in turn is eaten by
organism 3


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Orgam 1: KB(1) = k1/(k2 + k3 + G) (water exposure only)             (18)

Orgam 2: NB(2) = KB(2) + f21KB(1)
    (19)

Orgam 3: NB(3) = KB(3) + f32NB(2)

                   = KB(3) + f32KB(2) + f32f21KB(1)                 (20)

Orgam 4: NB(4) = KB(4) + f43KB(3) + f43f32KB(2) + f43f32f21KB(1)
                                                            (21)
fi,i-1 = (Eo(i,i-1) i,i-1)/(k2,i + k3,i + Gi)

4. Food chain "biomagnification;" if ecosystem is at steady-state then
fugacities in all compartments should be equal such that

                   fw = f(1) = f(2) = f(3)

However, it has been observed that fugacities of top predators in aquatic
ecosystems >> fugacity of water; how and why????

Food chain model provides insights to dilemma; NB(i) is a function of
organismal food consumption rate, Eo's (gill and food assimilation),
respiration, and growth rates.

Important variables across food chain are

k1, K', and Eo(food)

(K' = k2 + k3 + G)

In larger, older organisms food consumption and assimilation
efficiencies have not changed substantially through the life history;
however, depuration rates become much smaller as the volume of the
organism increases. This kinetic effect is to trap contaminants in the
organism, increasing their concentrations; also, if enhancement is
occurring gradually up the food chain because f(food) increases, then
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exposure is very high from food sources; top aquatic predators show
largest fugacity ratios relative to water for recalcitrant HOC's.

An alternative view is from a thermodynamic perspective. At
thermodynamic equilibrium, net uptake from food should not occur
against fugacity gradient. However, as organism consumes prey with
contaminant present digestion alters fugacities between the gut and
blood. As food is digested, lipids are saponified and absorbed into the
bloodstream. The loss of lipids from the food source decreases the
fugacity capacity of the food. When the fugacity of the food decreases,
the fugacity of the food increases and is temporarily larger than the
fugacity of the blood. Transport occurs into the bloodstream against an
overall fugacity gradient between water and organism.

In Fig. 2, it is apparent that biomagnification becomes important for
hydrophobic compounds, esp. Compounds with octanol/water partition
coefficients greater than log 5.

In addition, notice that observed fugacity ratios <1 in most instances
except for chlordanes. If water concentration rate of decrease is larger
than depuration, the fugacity ratio >1. It can be seen that generally,
fugacity ratios are greater the higher the trophic level. Another theory
explaining fugacity ratios >1 is provided by Gobas et al. 1993. The
question is how does a predator, higher in the food chain, accumulate
chemical when fugacities are equal between water and predator; there is
an apparent lack of chemical gradient. This model is shown below (Fig.
3) and states that as the predator consumes prey, fats are digested in the
gut which contain the contaminant in the prey species. As fats are
digested and absorbed in the gut, the fugacity of the gut contents
increases thereby creating a fugacity gradient. The gradient exists
between the gut and the bood, not between the predator and water

V. Important Implications For Bioaccumulation

A. Chemical transport -- biota play important role in the fate of aquatic
contaminants (primarily microbiota).


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B. Ecosystem Hazard Assessment -- hazard of chemical in
environment is dependent on both exposure and toxicity.
C. Human Health -- effects of consumption of contaminated food
supply.

D. Environmental Monitoring -- enhanced detection of contaminants;
biota good integrators of HOC contamination; assessment of 1
bioavailable substances; regulating inputs.

VI. Effect Of Humic Substances On Bioconcentration

It is generally recognized that humic substances effectively reduce the
magnitude of bioconcentration from water. The mechanism has been
discussed previously, in that humics increase the apparent water
solubility of the compound in water, thereby lowering the
bioconcentration factor.

VII. Linear Free Energy Relations

Correlations with Kow:

a. KB = 0.048 Kow                       Mackay (1982) ES&T 16,274

b. log KB,L = 0.893 log Kow + 0.607          Chiou (1985) ES&T 19,57

c. log KB = 0.76 log Kow - 0.23              Veith et al. (1980) J. Fish.
                                             Res. Board Can.
Correlations with Ktw:

a. log KB,L = 0.957 log Ktw + 0.245          Chiou (1985) ES&T 19,57

KB = wet weight bioconcentration factor
KBL = lipid-based bioconcentration factor




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