Depuration rate constant

Bioaccumulatlon of some pesticides (fenitrothion, aminocarb, permethrin) with real or potential application in forestry in Canada has been examined in laboratory experiments using larval rainbow trout and common duckweed. Bioaccumulation of an aromatic hydrocarbon, fluorene, has also been examined since some commercial formulations employ hydrocarbon solvents. Laboratory exposures of fish or plants were carried out by placing the organisms in dilute aqueous solutions of C labelled pesticide or hydrocarbon, and by measuring transfer of radioactivity from water to fish or plants. After transfer of fish or plants to untreated water, loss of radioactivity was measured similarly. These measures allowed calculation of uptake and depuration rate constants which were used to predict residue accumulations under various exposure conditions. Predicted residue accumulations agreed substantially with other predictive equations in the literature and with reported field observations. 

The calculated biconcentration factors were taken as the ratio (K1/K2) of uptake rate constants (Kl) to depuration rate constants (K2). The measured bioconcentration factors were taken as the ratio of measured radioactivity in fish to that in exposure water after 24 hours exposure. 

Compound Uptake rate Constant (hr"1) Depuration rate Constant (hr 1) Calculated BCF (K1/K2) Measured BCF... 

We can apply kinetic principles to the phenomenon of bioconcentration. It is possible to describe the uptake (rate constant k ), clearance (also called depuration, rate constant fci), and metabolism (rate constant k, ) of non-polar pollutants into organisms such as fish by means of kinetic equations. Analysis of the dependence of the concentration of the toxic substance with time is called toxicokinetics... 

After determining a concentration of test compound which elicits no visually detectable response or effect in the aquatic species over a period of 48 hours (Step 1), fresh animals are placed in the chamber, exposed to known concentrations of test chemical (usually 14C-labelled), and the uptake rate and major metabolites determined (Step 2). Depuration rate from the dosed animals also can be estimated at this point by transfer to untreated water. Fresh animals also can be exposed to a constant flow of test solution until an absorption-excretion equilibrium (steady state) has been established, dosed briefly with labelled compound, and release (turnover) rate determined (Step 3). 

The intent of this study was to derive rate constants describing uptake and depuration of some forest pesticides using fish (rainbow trout, Salmo gairdneri) and an aquatic macrophyte (duckweed, Lemna minor) in laboratory tests. Since some formulations of forest pesticides also contain solvents of petroleum distillates, experiments were also carried out with a hydrocarbon, fluorene, which is a component of fuel oil (16). 

In spite of the limitations on values produced by simple laboratory experiments reported here (use of radioactivity to indicate a compound, single exposure concentrations, steady state assumptions, lack of allowance for growth dilution, first order depuration kinetics), the rate constants derived have given a surprisingly good estimate both of some published field biconcentration measurements and of other predictions based on larger amounts of data. 

A negative linear correlation between k2 (depuration constant) and log Kow (or BCF) has been shown in fish by several authors (e.g. Spade and Hamelink, 1982 Gobas et al., 1989 Petersen and Kristensen, 1998), whereas k (uptake rate constant) is more or less independent of the lipophilicity of the substance (Connell, 1990). The resultant BCF will thus generally increase with increasing lipophilicity of the substances, i.e. log BCF and log Kow correlate for substances which do not undergo extensive metabolism. 

Box 3. Application of kinetic constants for BCF calculations (after Bunce, 1994) Bioconcentration factor, BCF, can be related to the rate constants of uptake and depuration, including metabolism and excretion of a solute... 

Metabolism clearly reduces the BCFs of compounds by enhancing the depuration rates with respect to the uptake rate. This can be shown by extending the fish-water two compartment model to include metabolic transformation, represented by the transformation parameter Dp and the first order metabolic rate constants koi... 

In a laboratory study, a certain fish species was observed to metabolize and/or excrete 2,4, 5-trichlorinated biphenyl (a PCB congener) with a first-order rate constant of 0.021/day. Estimate how long it will take for a contaminated fish, on being placed in clean water, to undergo depuration (cleansing of pollutants) if the initial biphenyl concentration in the fish exceeds regulatory standards by a factor of three. 

The Richards model reduces the unexplained statistical variation in the accumulation of PCBs by phytoplankton, but it does not provide any information about the mechanisms responsible for the observed pattern. Numerous causes are possible for deviation from the classical pattern of accumulation. However, violations of assumptions associated with the classical model (i.e., constant uptake rate, instantaneous mixing within a single compartment, and a time-independent probability of depuration) are most likely the cause. With phytoplankton, several physiological mechanisms can potentially contribute to a sigmoidal accumulation curve. 

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