Equilibrium partitioning Approach

Pavlou, S.P (1987) The use of equilibrium partition approach in determining safe levels of contaminants in marine sediments, p. 388 -12. In Fate and Effects of Sediments-Bound Chemicals in Aquatic Systems. Dickson, K.L., Maki, A.W., Brungs, W.A., Editors. Proceedings of the Sixth Pellston Workshop, Florissant, Colorado, August 12-17,1984. SETAC Special Publ. Series, Ward, C.H., Walton, B.T., Eds., Pergamon Press, N.Y. 

Webster, J. Ridgway, I. (1994) The application of the equilibrium partitioning approach for establishing quality criteria in two UK sea disposal and outfall sites. Marine Pollution Bulletin 28, 653-61. 

Swartz, R.C., D.W. Schults, T.H. Dewitt, G.R. Ditsworth, and J.O. Lamberson. 1990. Toxicity of fluoranthene in sediments to marine amphipods a test of the equilibrium partitioning approach to sediment quality criteria. Environ. Toxicol. Chem. 9 1071-1080. 

Efforts have been undertaken mainly be the United States Environmental Protection Agency to develop standard procedures and criteria for the assessment of environmental impact of sediment-associated pollutants. Initial discussions (Anon., 1984, 1985) suggested five methodological approaches which merit closer consideration (i) "background approach", (ii) "water quality/pore water approach", (iii) sediment/water equilibrium partitioning approach , (iv) sediment/organism equilibrium" approach, and (v) "bioassay" approach. Of these possibilities, applications of "bioassays" and "background approach" have been outlined in sections 6.1 and 6.2, respectively. 

Calculated using the LOEC for Daphnia magna using equilibrium partitioning approach [42]... 

THERMODYNAMICS OF PERSISTENT ORGANIC CHEMICALS THE EQUILIBRIUM PARTITIONING APPROACH... 

Of course, the equilibrium partition approach has some limitations (a) it considers only a phase equilibrium situation and does not take into account the rate of transport between air and soil, (b) the fine distribution of compounds within surface soils may influence the calculation of fugacities and (c) it provides only a snap-shot for a given set of enviromnental conditions. 

Since the passive root uptake of organic chemicals is driven by the amount of water transpired, the simple lipid-water equilibrium partitioning approach that is applied so successfiilly to animals and fish is less applicable to plants. Transpiration rates vary diumally and seasonally as plants respond to changes in climatic conditions (e.g., humidity, temperature, sunlight, and wind speed). The simplest and obvious response to these complexities has been to measure chemical concentrations in specific plant fluids or tissues at specific times or over specific intervals and relate them to external exposure concentrations in hydroponic solution, soil or soil pore water. The BCFs and RCFs described earlier are examples of these ratios and have been correlated to properties of the chemical such as Kow- An example of one of earliest and the most widely used plant BCE—chemical properties relationship was derived by Travis and Arms (1988) ... 

Johansson and coworkers [182-184] have analyzed polyacrylamide gel structure via several different approaches. They developed an analytical model of the gel structure using a single cylindrical unit cell coupled with a distribution of unit cells. They considered the distribution of unit cells to be of several types, including (1) Ogston distribution, (2) Gaussian distribution of chains, and (3) a fractal network of pores [182-184]. They [183] used the equilibrium partition coefficient... 

Our final task in this chapter is to demonstrate how partition constants/coefficients can be used to calculate the equilibrium distribution of a compound i in a given multiphase system. As already pointed out earlier, for simplicity, we consider only neutral species. As we will see in Chapter 8, the equilibrium partitioning of ionogenic compounds (i.e., compounds that are or may also be present as charged species, as, for example, acids or bases) is somewhat more complicated to describe. However, the general approach discussed here is the same. 

Explain in words the basic idea behind simple one-parameter LFERs for evaluation and/or prediction of equilibrium partition constants. What are the most common approaches What are the dangers when using such LFERs ... 

Rogers, H.R., 2002. Assessment of PAH contamination in estuarine sediments using the equilibrium partitioning-toxic unit approach. Sci. Total. Environ. 290, 139-155. 

Metal concentrations and metal activities in the pore water are dependent upon both the metal concentration in the solid phase and the composition of both the solid and the liquid phase. In matrix extrapolation, and with emphasis on the pore water exposure route, it is therefore of great practical importance to have a quantitative understanding of the distribution of heavy metals over the solid phase and the pore water. A relatively simple approach for calculating the distribution of heavy metals in soils is the equilibrium-partitioning (EP) concept (Shea 1988 van der Kooij et al. 1991). The EP concept assumes that chemical concentrations among environmental compartments are at equilibrium and that the partitioning of metals among environmental compartments can be predicted based on partition coefficients. The partition coefficient, Kp, used to calculate the distribution of heavy metals over solid phase and pore water is defined as... 

The TU approach, combined with equilibrium partitioning (EqP) and (Q)SAR modeling, was also used by Swartz and DiToro (1997) to develop the XPAII model to predict the toxicity of sediment-associated PAH compounds. A (Q)SAR and EqP method is also presented by Swartz and DiToro (1997) for modeling narcotic chemicals in sediments. In this approach, the sediment quality guideline for a mixture of narcotic chemicals that exhibit additive toxicity could be expressed as the sum of the fraction of the OC-normalized sediment concentrations divided by the SQG for each... 

Equilibrium partitioning and mass transfer relationships that control the fate of HOPs in CRM and in different phases in the environment were presented in this chapter. Partitioning relationships were derived from thermodynamic principles for air, liquid, and solid phases, and they were used to determine the driving force for mass transfer. Diffusion coefficients were examined and those in water were much greater than those in air. Mass transfer relationships were developed for both transport within phases, and transport between phases. Several analytical solutions for mass transfer were examined and applied to relevant problems using calculated diffusion coefficients or mass transfer rate constants obtained from the literature. The equations and approaches used in this chapter can be used to evaluate partitioning and transport of HOP in CRM and the environment. 

In contrast to the equilibrium-controlled approach which ends with a true equUibrium, in the protease-catalyzed kinetically controlled synthesisf l the product appearing with the highest rate and disappearing with the lowest velocity would accumulate. This approach requires the use of acyl donor esters as carboxy components (Ac-X) and is limited to proteases which rapidly form an acyl-enzyme intermediate (Ac-E). Serine and cysteine proteases are known to catalyze acyl transfer from specific substrates to various nucleophihc amino components via an acyl-enzyme intermediate. In reactions of this type, the protease reacts rapidly with an amino acid or peptide ester, Ac-X, to form a covalent acyl-enzyme intermediate, Ac-E, that reacts, in competition with water, with the amino acid or peptide-derived nucleophile HN to form a new peptide bond (Scheme 3). The partitioning of the acyl-enzyme intermediate between water and the added nucleophile is the rate-limiting step. Under kinetic control, and if k4[HN] k3[H20], the peptide product Ac-N should accumulate. However, the soluble peptide product will be degraded if the reaction is not terminated after the acyl donor ester is consumed. 

Despite these considerations, the first approach in method development for ESl-MS is the formation of preformed ions in solution, i.e., protonation of basic analytes or deprotonation of acidic analytes. Thus, for basic analytes, mixtures of ammonium salts and volatile acids like formic and acetic acid are applied. Alternatively, formic or acetic acid may be added to the mobile phase, just to set a low pH for the generation of preformed ions in solution. The latter approach is successful if sufficient hydrophobic interaction between preformed aiialyte ions and the reversed-phase material remains. The concentration of buffer is kept as low as possible, i.e., at or below 10 nunol/1 in ESl-MS. The buffer concentration is obviously determined by the buffer capacity needed to achieve stable pH conditions upon repetitive injection of the samples. Constantopoulos et al. [99] derived an equilibrium partitioning model to predict the effect of the salt concentration on the analyte response in ESI. If the salt concentration is below 10 moFl, the analyte response is proportional to its concentration. The response is found to decrease with increasing salt concentration. 

The IPAH model incorporated a number of factors that can modify the toxicity of the sediment-borne PAHs. Equilibrium partitioning was used to estimate the concentration of each PAH in the pore water of the sediment. The assumption was that the pore water material is the fraction that is bioavail-able. QSAR was also used to estimate the interstitial water concentration based on the octanol-water partition coefficient of several PAHs. Amphipods were used as the test organism to represent environmental toxicity. A toxic unit (TU) approach was used and the toxicity is assumed to be additive. The assumption of additivity is justified since each of the PAHs has a similar mode of action. Finally, a concentration-response model was formulated using existing toxicity data to estimate the probability of toxicity. 

Sediment/water equilibrium partitioning. This approach is related to a relative broad toxicological basis of water quality data. The distribution coefficient Kp, which is determined from laboratory experiments, is defined as the quotient of equilibrium concentration of a certain compound in sediment (Csx, e.g. in mg/kg) and in the aqueous phase (Cwx e.g. in mg/1). In practice, three categories of compounds can be distinguished ... 

Pankow (1987) consolidated, and subsequently refined early approaches (Pankow and Bidleman 1991, 1992) to describe the equilibrium partitioning between gas and particles based on adsorption theory (Junge 1977 Yamasaki et al. 1982). Much attention has also been paid to the effect of relative humidity (RH) (Thibodeaux et al. 1991 Pankow et al. 1993 Storey et al. 1995) and nonexchangeable matter (Pankow 1988). Pankow (1988) defined the gas-particle partitioning coefficient, Kp (m pg ), which can be related to measured fractions/ concentrations of gas (Ca,g— retained by adsorbent) and particulate phase concentrations (Ca,p— retained by the filter) and the total suspended particle concentration (TSP), in pg m ... 

Additional experimental evidence continues to support the idea that local phase equilibrium is approached closely in a sandstone core under chemical flood at flow rates comparable to flow rates in the field. Since phases formed inside the core are not generally of equal mobility, components of the chemical slug may separate during a chemical flood if they partition differently among the phases. 

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