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Distribution Coefficients (distribution + coefficient)
Kinds of Distribution Coefficients Selected AbstractsInfluence of soil pH on the sorption of ionizable chemicals: Modeling advancesENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 3 2009Antonio Franco Abstract The soil,water distribution coefficient of ionizable chemicals (Kd) depends on the soil acidity, mainly because the pH governs speciation. Using pH-specific Kd values normalized to organic carbon (KOC) from the literature, a method was developed to estimate the KOC of monovalent organic acids and bases. The regression considers pH-dependent speciation and species-specific partition coefficients, calculated from the dissociation constant (pKa) and the octanol,water partition coefficient of the neutral molecule (log Pn). Probably because of the lower pH near the organic colloid,water interface, the optimal pH to model dissociation was lower than the bulk soil pH. The knowledge of the soil pH allows calculation of the fractions of neutral and ionic molecules in the system, thus improving the existing regression for acids. The same approach was not successful with bases, for which the impact of pH on the total sorption is contrasting. In fact, the shortcomings of the model assumptions affect the predictive power for acids and for bases differently. We evaluated accuracy and limitations of the regressions for their use in the environmental fate assessment of ionizable chemicals. [source] Development of negligible depletion hollow fiber,protected liquid-phase microextraction for sensing freely dissolved triazinesENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 2 2009Xialin Hu Abstract A new sampling method, termed negligible depletion hollow fiber,protected liquid-phase microextraction, was developed for sensing the freely dissolved concentration (Cfree) and evaluating the availability of atrazine (ATR), desethyl atrazine (DEA), and simazine (SIM) in water. The sampling device was prepared by impregnating 1-octanol to both the pores and the lumen of a piece of polypropylene microporous hollow fiber membrane. After equilibrium and negligible depletion extraction, the 1-octanol in the lumen of the hollow fiber (10 ,l) was collected for determination of triazines. Determination of Cfree and the distribution coefficient to 1-octanol (DOW) can be performed with this technique. A wide linear working range (1,200 ,g/L) and low detection limits (0.1,1 ,g/L) were obtained for triazines. Measured log DOW values of DEA (1.44 ± 0.04), SIM (2.06 ± 0.06), and ATR (2.33 ± 0.05) agreed well with those reported in the literature. The measured DOW values were independent of the chemical concentration and sample pH (pH 3,10) and negligibly affected by the sample salinity (0,500 mM), suggesting that environmentally relevant pH and salinity have no significant effects on the availability of triazines. Although a slight (,31%) increase of Cfree was observed, one-way analysis of variance indicated the Cfree of triazines were not significantly affected by the presence of Aldrich humic acid, Acros humic acid, and bovine albumin V (dissolved organic carbon [DOC], 0,100 mg/L). From 3 to 36% of the spiked triazines, however, were found to associate with the dissolved organic matter (DOM) in surface-water samples (DOC, 32.0,61.9 mg/L), suggesting the origin of the DOM is a key parameter in determining its association with and, thus, the availability of triazines. [source] Estimation of the soil,water partition coefficient normalized to organic carbon for ionizable organic chemicals,ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 10 2008Antonio Franco Abstract The sorption of organic electrolytes to soil was investigated. A dataset consisting of 164 electrolytes, composed of 93 acids, 65 bases, and six amphoters, was collected from literature and databases. The partition coefficient log KOW of the neutral molecule and the dissociation constant pKa were calculated by the software ACD/Labs®. The Henderson-Hasselbalch equation was applied to calculate dissociation. Regressions were developed to predict separately for the neutral and the ionic molecule species the distribution coefficient (Kd) normalized to organic carbon (KOC) from log KOW and pKa. The log KOC of strong acids (pKa < 4) was not correlated to these parameters. The regressions derived for weak acids and bases (undissociated at environmental pH) were similar. The highest sorption was found for strong bases (pKa > 7.5), probably due to electrical interactions. Nonetheless, their log KOC was highly correlated to log KOW. For bases, a nonlinear regression was developed, too. The new regression equations are applicable in the whole pKa range of acids, bases, and amphoters and are useful in particular for relatively strong bases and amphoters, for which no predictive methods specifically have been developed so far. [source] Factors influencing the sorption of oxytetracycline to soilsENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 4 2005Aaryn D. Jones Abstract Veterinary antibiotics such as oxytetracycline (OTC) increasingly are found in the environment and often come into direct contact with soils via the release of animal wastes. Oxytetracycline is known to sorb strongly to soils by interaction with soil organic matter, clay minerals, and metal oxides. However, current knowledge of the influence of soil properties on OTC sorption is limited, as is our ability to predict OTC sorption to soils. This work was aimed at identifying properties that most influence the extent of OTC sorption in a suite of soils from the eastern United States representing a wide range in soil properties. Thirty soils were well characterized, an OTC soil-water distribution coefficient (Kd) was determined for each soil, and statistical analyses were employed to determine appropriate soil descriptors of OTC sorption. Soil texture, cation exchange capacity, and iron oxide content seemed to most influence the extent of OTC sorption in soils with organic carbon (OC) content between 0 and 4%. Thus, the knowledge of these three soil properties would be key to anticipating the extent of OTC sorption and gaining insight into OTC fate within a given soil system. Notably, OC content appeared to influence OTC sorption only in a soil with 9% OC. [source] A thermodynamics-based estimation model for adsorption of organic compounds by carbonaceous materials in environmental sorbentsENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 6 2003Paul C. M. van Noort Abstract A model was developed to estimate Langmuir affinities for adsorption of low-polarity organic compounds from either water or air by carbonaceous sorbents. Sorption enthalpies and entropies provided the basic information for the description of sorption affinities in terms of the entropy of melting and either solubility in water or vapor pressure. For m -xylene, polycyclic aromatic hydrocarbons (PAHs) and chlorobenzenes on 10 different sorbents, 80% of the measured sorption affinities fall within a factor of four of the model estimates. Equations for the limiting distribution coefficients in terms of either octanol,air (KOA) or octanol,water partition (Kow) coefficients were derived from regressions of calculated affinities combined with an estimated relation between experimental Langmuir sorption capacities and Kow. Estimated soot,water distribution coefficients were within a factor of three of measured data for polychlorobiphenyls (PCBs) and lower molecular weight PAHs on automotive soot samples and captured the dependence of PCB distribution coefficients on the extent of ortho substitution. For higher molecular weight PAHs, sorption was underestimated. For soot in sediment,water distribution coefficients of PAHs and PCBs, estimated values captured both the trend of measured data with Kow and the dependence on sorbate planarity. Tentative application to aerosol,air distribution explained the observed independence of distribution coefficient,KOA relations for PCBs on the extent of ortho substitution and suggested nonequilibrium conditions for PAHs in comparison with recent measurements. [source] Partitioning of metals (Cd, Co, Cu, Ni, Pb, Zn) in soils: concepts, methodologies, prediction and applications , a reviewEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 4 2009F. Degryse Summary Prediction of the fate of metals in soil requires knowledge of their solid,liquid partitioning. This paper reviews analytical methods and models for measuring or predicting the solid,liquid partitioning of metals in aerobic soils, and collates experimental data. The partitioning is often expressed with an empirical distribution coefficient or Kd, which gives the ratio of the concentration in the solid phase to that in the solution phase. The Kd value of a metal reflects the net effect of various reactions in the solid and liquid phases and varies by orders of magnitude among soils. The Kd value can be derived from the solid,liquid distribution of added metal or that of the soil-borne metal. Only part of the solid-phase metal is rapidly exchangeable with the solution phase. Various methods have been developed to quantify this ,labile' phase, and Kd values based on this phase often correlate better with soil properties than Kd values based on total concentration, and are more appropriate to express metal ion buffering in solute transport models. The in situ soil solution is the preferred solution phase for Kd determinations. Alternatively, water or dilute-salt extracts can be used, but these may underestimate in situ concentrations of dissolved metals because of dilution of metal-complexing ligands such as dissolved organic matter. Multi-surface models and empirical models have been proposed to predict metal partitioning from soil properties. Though soil pH is the most important soil property determining the retention of the free metal ion, Kd values based on total dissolved metal in solution may show little pH dependence for metal ions that have strong affinity for dissolved organic matter. The Kd coefficient is used as an equilibrium constant in risk assessment models. However, slow dissociation of metal complexes in solution and slow exchange of metals between labile and non-labile pools in the solid phase may invalidate this equilibrium assumption. [source] Soil solution concentration of Cd and Zn canbe predicted with a CaCl2 soil extractEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 1 2003F. Degryse Summary Risk assessment of heavy metals in soil requires an estimate of the concentrations in the soil solution. In spite of the numerous studies on the distribution of Cd and Zn in soil, few measurements of the distribution coefficient in situ, Kd, have been reported. We determined the Kd of soils contaminated with Cd and Zn by measuring metal concentrations in the soil and in the soil solution and attempted to predict them from other soil variables by regression. Soil pH explained most of the variation in logKd (R2 = 0.55 for Cd and 0.70 for Zn). Introducing organic carbon content or cation exchange capacity (CEC) as second explanatory variable improved the prediction (R2 = 0.67 for Cd and 0.72 for Zn), but these regression models, however, left more than a factor of 10 of uncertainty in the predicted Kd. This large degree of uncertainty may partly be due to the variable degree of metal fixation in contaminated soils. The labile metal content was measured by isotopic dilution (E value). The E value ranged from 18 to 92% of the total metal content for Cd and from 5 to 68% for Zn. The prediction of Kd improved when metals in solution were assumed to be in equilibrium with the labile metal pool instead of the total metal pool. It seems necessary therefore to discriminate between ,labile' and ,fixed' pools to predict Kd for Cd and Zn in field contaminated soils accurately. Dilute salt extracts (e.g. 0.01 m CaCl2) can mimic soil solution and are unlikely to extract metals from the fixed pool. Concentrations of Cd and Zn in the soil solution were predicted from the concentrations of Cd and Zn in a 0.01 m CaCl2 extract. These predictions were better correlated with the observations for field contaminated soils than the predictions based on the regression equations relating logKd to soil properties (pH, CEC and organic C). [source] Irreversible Phosphorus Sorption in Septic System Plumes?GROUND WATER, Issue 1 2008W. D. Robertson The mobility of phosphorus (P) in septic system plumes remains a topic of debate because of the considerable reactivity of this constituent. In this study, a septic system plume in Ontario was monitored over a 16-year period with detail that clearly shows the advancing frontal portion of the P plume. This monitoring record provides insight into the extent of secondary P attenuation in the ground water zone beyond that available from previous studies. A P plume 16 m in length developed over the monitoring period with PO4 -P concentrations (3 to 6 mg/L) that approached the concentrations present under the tile bed. Simulations using an analytical model showed that when first-order solute decay was considered to account for the possibility of secondary P attenuation in the ground water zone, field values could only be matched when decay was absent or occurred at an exceedingly slow rate (half-life greater than 30 years). Thus, hypothesized secondary P attenuation mechanisms such as slow recystallization of sorbed P into insoluble metal phosphate minerals, diffusion into microsites, or kinetically slow direct precipitation of P minerals such as hydroxyapatite were inactive in the ground water zone at this site or occurred at rates that were too slow to be observed in the context of the current 16-year study. Desorption tests on sediment samples from below the tile bed indicated a PO4 distribution coefficient (Kd) of 4.8, which implies a P retardation factor of 25, similar to the field apparent value of 37 determined from model calibrations. This example of inactive secondary P attenuation in the ground water zone shows that phosphorus in some ground water plumes can remain mobile and conservative for decades. This has important implications for septic systems located in lakeshore environments when long-term usage scenarios are considered. [source] Effect of Heterogeneity on Radionuclide Retardation in the Alluvial Aquifer Near Yucca Mountain, NevadaGROUND WATER, Issue 3 2001S. Painter The U.S. Department of Energy is currently studying Yucca Mountain, Nevada, as a potential site for a geological high-level waste repository. In the current conceptual models of radionuclide transport at Yucca Mountain, part of the transport path to pumping locations would be through an alluvial aquifer. Interactions with minerals in the alluvium are expected to retard the downstream migration of radionuclides, thereby delaying arrival times and reducing ground water concentrations. We evaluate the effectiveness of the alluvial aquifer as a transport barrier using the stochastic Lagrangian framework. A transport model is developed to account for physical and chemical heterogeneities and rate-limited mass transfer between mobile and immobile zones. The latter process is caused by small-scale heterogeneity and is thought to control the macroscopic-scale retardation in some field experiments. A geostatistical model for the spatially varying sorption parameters is developed from a site-specific database created from hydrochemical measurements and a calibrated modeling approach (Turner and Pabalan 1999). Transport of neptunium is considered as an example. The results are sensitive to the rate of transfer between mobile and immobile zones, and to spatial variability in the hydraulic conductivity. Chemical heterogeneity has only a small effect, as does correlation between hydraulic conductivity and the neptunium distribution coefficient. These results illustrate how general sensitivities can be explored with modest effort within the Lagrangian framework. Such studies complement and guide the application of more detailed numerical simulations. [source] EFFECT OF INSOLUBLE PARTICLES UPON SOLID INCLUSION LEVELS IN ICE FORMED ON A SUBCOOLED STAINLESS STEEL SURFACEJOURNAL OF FOOD PROCESS ENGINEERING, Issue 2 2000PING CHEN ABSTRACT This study employed an insoluble solid particle, i.e. a potato starch used as an example, to investigate the impact of the concentration of such particles suspended in water (with solid loading of 5, 10, 20 and 30 wt%) upon the solid inclusion levels in ice layers formed on a sub-cooled smooth stainless steel plate surface. The effects of ice growth rate, bulk concentration and suspension velocity on insoluble solid inclusion, i.e. potato starch mass fraction, in ice layer were studied. The experiments, where potato starch is added into aqueous sucrose solutions or the reverse where sucrose is added into starch suspension, were also carried out to investigated effect of the starch particles on sucrose inclusion in ice and effect of solute (sucrose) on starch particle inclusion in ice. It has been found that solid inclusion in ice increases with increasing bulk concentration and average ice growth rate, at constant solution and coolant velocities, and increasing suspension velocity can help pure ice formation. The average distribution coefficient of sucrose in ice layer formed from sucrose solution does not appear to be affected by the addition of potato starch. However, the average distribution coefficient of potato starch in ice layer formed from suspension is influenced by sucrose concentration quite significantly. [source] Prediction of the Solubility, Activity Coefficient and Liquid/Liquid Partition Coefficient of Organic CompoundsMOLECULAR INFORMATICS, Issue 9 2004H. Hilal Abstract Solvation models, based on fundamental chemical structure theory, were developed in the SPARC mechanistic tool box to predict a large array of physical properties of organic compounds in water and in non-aqueous solvents strictly from molecular structure. The SPARC self-interaction solvation models that describe the intermolecular interaction between like molecules (solute-solute or solvent-solvent) were extended to quantify solute-solvent interaction energy in order to estimate the activity coefficient in almost any solvent. Solvation models that include dispersion, induction, dipole-dipole and hydrogen bonding interactions are used to describe the intermolecular interaction upon placing an organic solute molecule in any single or mixed solvent system. In addition to estimation of the activity coefficient for 2674 organic compounds, these solvation models were validated on solubility and liquid/liquid distribution coefficient in more than 163 solvents including water. The RMS deviations of the calculated versus observed activity coefficients, solubilities and liquid/liquid distribution coefficients were 0.272,log mole fraction, 0.487,log mole fraction and 0.44,log units, respectively. [source] Structure-Activity Relationships for the Toxicity of Substituted Poly-hydroxylated Benzenes to Tetrahymena pyriformis: Influence of Free Radical FormationMOLECULAR INFORMATICS, Issue 6 2003Tatiana Abstract The aim of this study was to develop quantitative structure-activity relationships for the toxicity to Tetrahymena pyriformis of 30 substituted poly-hydroxylated benzenes. Physico-chemical descriptors for the expression of free radical formation, associated with the OH moiety on the aromatic ring, were calculated. These included one-electron equilibrium constants that did and did not account for the oxidation of an OH-group, homolytic bond dissociation energy (BDE), electronegativity (EN) and absolute hardness (AH), in addition to the distribution coefficient (log D) as a measure of hydrophobicity. The reactivity descriptors were calculated using the semi-empirical AM1 Hamiltonian in the MOPAC molecular orbital software. Statistically significant two-parameter QSARs for toxicity were obtained by combination of log D with either BDE or AH. The QSARs suggested that toxicity is associated with hydrophobicity and the probability of radical formation. [source] Electrical and structural properties of p -type nanocrystalline silicon grown by LEPECVD for photovoltaic applicationsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3-4 2010Gabriel Micard Abstract p-doped hydrogenated nanocrystalline silicon (p-nc-Si:H) is one of the most critical layers in thin film silicon solar cells. LEPECVD is a new technique for the growth of nc-Si at high growth rate without compromising the layer quality, using a dense but low energy plasma. Thin p-nc-Si:H layers are grown on glass and ZnO:Al coated glass and their structural and electrical properties are investigated as a function of the silane dilution (d) and of the doping ratio (DR). The influence of the substrate on the structural properties is investigated and discussed. The incubation layer is clearly observed on both substrate types and its thickness is estimated. LEPECVD distinguishes itself from other high growth rate methods by a very low impurity distribution coefficient to obtain a comparable conductivity and boron density. The conduction path is shown to be dependent on the density of boron in the layer while a significant decrease of conductivity at high DR is not observed in the studied range (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Estimation of plasmalemma conductivity to ascorbic acid in intact leaves exposed to ozonePHYSIOLOGIA PLANTARUM, Issue 4 2000Irina Bichele To establish the capacity of the leaf mesophyll plasmalemma of Phaseolus vulgaris L. to supply ascorbate (ASC) into the cell wall by simple diffusion, a method for calculating plasmalemma diffusional conductivity to ascorbic acid (AA) in intact leaves was evaluated. The core of the approach is that in the presence of a sink for ascorbate in the cell wall, cell wall total ascorbic acid concentration [TAA]cw (=[ASC]cw+[AA]cw) reaches zero at some positive whole-leaf total ascorbic acid concentration [TAA]l. It is shown that [TAA]l at [TAA]cw=0 is proportional to the sink for ASC in the cell wall and the reciprocal of plasmalemma conductivity. The predicted proportional relationship between [TAA]cw and [TAA]l was confirmed by decreasing TAA levels in leaves through predarkening. Furthermore, increasing the sink intensity for ASC in the cell wall by the acute exposure of leaves to 450 nmol ozone mol,1 during re-illumination, [TAA]cw reached zero at 2.7-fold higher [TAA]l than without ozone, and the slope of the relationship increased twofold. Plasmalemma diffusional conductivities to AA of 2.9×10,6 and 1.8×10,6 m s,1, needed to maintain [TAA]cw at the observed level, were calculated from the increase in [TAA]l at [TAA]cw=0 and from the two different estimates of the sink for ASC. A value of 1.3×10,6 m s,1 was calculated on the basis of the oil-water distribution coefficient for TAA. It is concluded that the demand for ASC in the mesophyll cell wall of the investigated leaves could be met by simple diffusion of AA through the plasmalemma. From the measured increase in the slope of the relationship [TAA]cw versus [TAA]l, an increase in the cell wall pH of 0.3 units was estimated under the influence of ozone. [source] Permeability and toxicological profile estimation of organochlorine compounds by biopartitioning micellar chromatographyBIOMEDICAL CHROMATOGRAPHY, Issue 4 2009L. Escuder-Gilabert Abstract This paper points out the usefulness of biopartitioning micellar chromatography (BMC) as a high-throughput primary screening tool providing key information about the oral absorption, skin permeability (Kp), brain,blood distribution coefficient (BB) and ecotoxicological parameters such as median lethal concentration (LC50) and bioconcentration factors of 15 organochloride compounds. The retention data of compounds in BMC conditions were interpolated in previously developed quantitative,retention activity relationships by our research group. Results show that the compounds studied readily cross the intestinal barrier (oral absorption >ercnt;) and the blood,brain barrier (log BB >p;0.4). In addition, the organochlorines DDE, chlorobenzene, 1,3-dichlorobenzene and 1,2-dichlorobenzene are the compounds which can more quickly cross the skin barrier (log Kp >nus;0.74 cm/h). From a ecotoxicological point of view, it can be concluded that the most retained compounds, DDE, DDD, hexachlorobenzene and dicofol, are the most toxic and bioacumulative. Copyright © 2008 John Wiley & Sons, Ltd. [source] Drug,liposome distribution phenomena studied by capillary electrophoresis-frontal analysisELECTROPHORESIS, Issue 16 2008Jesper Østergaard Professor Abstract The potential of using CE frontal analysis (CE-FA) for the study of low-molecular-weight drug,liposome interactions was assessed. The interaction of bupivacaine, brompheniramine, chlorpromazine, imipramine, and ropivacaine with net negatively charged 80/20,mol% 1-oleoyl-2-palmitoyl- sn -glycero-3-phosphocholine/egg yolk phosphatidic acid liposome suspensions in HEPES buffer at pH,7.4 was investigated. The fraction of free drug as a function of lipid concentration was measured and apparent liposome , buffer distribution coefficients were determined for the basic drug substances. The distribution coefficients increased in the order ropivacaine, bupivacaine, brompheniramine, imipramine, and chlorpromazine. The developed CE method was relatively fast allowing estimates of drug,liposome affinity to be obtained within 15,min. CE-FA may have the potential to become a valuable tool for the characterization of drug,liposome interactions in relation to estimation of drug lipophilicity and for the evaluation of drug distribution in liposomal drug delivery systems. [source] Rapidly profiling blood,brain barrier penetration with liposome EKCELECTROPHORESIS, Issue 14 2007Yongjun Wang Abstract This report intended to study the potential of liposome EKC (LEKC) as a convenient and high-throughput screening tool to assess drug penetration across the blood,brain barrier (BBB). The retention factors (k) of 24 structurally diverse compounds were determined with LEKC and vesicle EKC (VEKC), respectively. Principal component analysis of the steady-state concentrations ratio of compounds in the brain and in the blood expressed as log,BB, log,kLEKC, log,kVEKC, and other lipophilic descriptors including octanol/water partition coefficient (Clog,P), octanol/water distribution coefficients (log,D7.4), and polar surface area (PSA), showed the maximum similarity of partitioning processes in LEKC to drug penetration across the BBB. Furthermore, the log,BB were correlated with the above five lipophilic descriptors, and the results showed that log,kLEKC gave the better correlation coefficient (r2,=,0.811, p <0.0001) than those of log,D7.4, Clog,P, PSA, and log,kVEKC (r2,=,0.730, 0.672, 0.627, and 0.620, p <0.0001). This is the first report of the use of LEKC as a promising rapid tool to profile drug penetration across the BBB. [source] Sorption of male hormones by soils and sedimentsENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 2 2007Il Kim Abstract ,This paper reports the sorption of two male hormones, testosterone and androstenedione, by four soil and sediment samples at both equilibrium and rate-limiting conditions. Unlike prior studies, androstenedione was studied independently of testosterone. Apparent sorption equilibrium is achieved in one to two weeks when the initial aqueous hormone concentrations (C0) at 10,000 ,g/L (,30% of their solubility limits [Sw]) and two to three weeks when the C0 is 300 ,g/L (less than 1% of Sw). The Freundlich model parameter n ranged from 0.698 to 0.899 for all soil,solute systems indicating nonlinear sorption isotherms. Isotherm nonlinearity leads to an inverse correlation between single-point organic carbon,normalized sorption distribution coefficients (KOC) and equilibrium androgen concentration (Ce). When Ce/Sw = 0.012, the log KOC values for testosterone and androstenedione on the various sorbents ranged from 6.18 to 6.75 and 6.83 to 6.04, respectively, compared to 6.30 to 6.80 and 6.16 to 6.92 when Ce/Sw = 0.004. This study suggests that male hormones may exhibit slow rates of sorption over 14 d or longer and that soils and sediments may have greater sorption distribution coefficients when concentrations fall into the ng/L range. [source] A thermodynamics-based estimation model for adsorption of organic compounds by carbonaceous materials in environmental sorbentsENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 6 2003Paul C. M. van Noort Abstract A model was developed to estimate Langmuir affinities for adsorption of low-polarity organic compounds from either water or air by carbonaceous sorbents. Sorption enthalpies and entropies provided the basic information for the description of sorption affinities in terms of the entropy of melting and either solubility in water or vapor pressure. For m -xylene, polycyclic aromatic hydrocarbons (PAHs) and chlorobenzenes on 10 different sorbents, 80% of the measured sorption affinities fall within a factor of four of the model estimates. Equations for the limiting distribution coefficients in terms of either octanol,air (KOA) or octanol,water partition (Kow) coefficients were derived from regressions of calculated affinities combined with an estimated relation between experimental Langmuir sorption capacities and Kow. Estimated soot,water distribution coefficients were within a factor of three of measured data for polychlorobiphenyls (PCBs) and lower molecular weight PAHs on automotive soot samples and captured the dependence of PCB distribution coefficients on the extent of ortho substitution. For higher molecular weight PAHs, sorption was underestimated. For soot in sediment,water distribution coefficients of PAHs and PCBs, estimated values captured both the trend of measured data with Kow and the dependence on sorbate planarity. Tentative application to aerosol,air distribution explained the observed independence of distribution coefficient,KOA relations for PCBs on the extent of ortho substitution and suggested nonequilibrium conditions for PAHs in comparison with recent measurements. [source] The potential for estradiol and ethinylestradiol to sorb to suspended and bed sediments in some English riversENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 12 2002Karlijn I. E. Holthaus Abstract The endocrine-disrupting impact of steroid estrogens on fish will be strongly influenced by their distribution between sediment and water. Laboratory studies were performed to investigate the potential for sorption of 17,-estradiol (E2) and 17,-ethinylestradiol (EE2) to bed and suspended sediments taken from five British rivers. Sediment material was collected from the Rivers Aire and Calder (located in urban and industrialized areas in Yorkshire, UK), the River Thames (at a relatively rural site in Oxfordshire, UK), and from the estuaries of the Rivers Tees and Tyne. Using anaerobic conditions to inhibit biodegradation, it was found that 80 to 90% of binding to bed sediments was complete within 1 d, but that an equilibrium had not been reached after 2 d. Bed sediments gave distribution coefficients (Kd) ranging from 4 to 74 L/kg for E2 and from 8 to 121 L/kg for EE2 for samples taken over a range of seasons and locations. Sorption to suspended sediment gave Kd values ranging from 21 to 122 L/kg for E2 and 19 to 260 L/kg for EE2. However, these Kd values suggest less than 1% removal of the steroid estrogens from the aqueous phase given the ambient suspended sediment concentration. In the bed sediments, higher Kd values were associated with smaller particle size and higher organic carbon content. In most cases, the Kd values obtained for EE2 were higher than those for E2 by a factor of up to three. [source] Partitioning of copper at concentrations below the marine water quality criteria,ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 5 2001Anthony J. Paulson Abstract Partitioning of Cu between the aqueous and particulate phases and among their components was examined in six ambient Puget Sound, Washington State, USA, samples (6,10 nM Cu). Most of the particulate Cu (4,12% of the total Cu) was associated with particulate organic matter, and resulted in distribution coefficients (Kd) ranging between 104,55 and 105,1. For the dissolved phase, the portion of Cu extracted by C18 -packed cartridges averaged 44% (+ 11%). Radioactive 64Cu was added to these samples to total stable Cu concentrations (17,33 nM). After 24 h of equilibration, the portion of 64Cu associated with the particulate matter in five of the six samples (Kd between 1047 and 1053) was an average of 70% higher than that of natural Cu in the ambient samples. In contrast, only 19 ± 7% of the 64Cu was extracted by C18 -packed cartridges. The partitioning of natural Cu and 64Cu onto particles was not significantly different when the equilibria were based on dissolved Cu passing through the C18 cartridges. Further research is warranted on utilizing the hydrophilic component of the dissolved phase as a parameter on which water quality criteria are based. [source] Geochemical Factors Controlling Radium Activity in a Sandstone AquiferGROUND WATER, Issue 4 2006Tim Grundl Geochemical processes behind the occurrence of radium activities in excess of the U.S. EPA's drinking water limit of 5 pCi/L combined radium were investigated in a regional sandstone aquifer located in southeastern Wisconsin. Geochemical speciation modeling (PHREEQC 2.7) combined with a detailed understanding of the regional flow system provided by recent flow modeling efforts was used to determine that radium coprecipitation into barite controls radium activity in the unconfined portion of the aquifer. As the aquifer transitions from unconfined to confined conditions, radium levels rise and the water becomes more sulfate rich yet the aquifer remains at saturation with barite throughout. Calculations based on published distribution coefficients and the observed Ra:Ba atomic ratios indicate that barite contains ,12 ,g/kg coprecipitated radium. Confined portions of the aquifer have high concentrations of sulfate, and barium concentrations become too low to be an effective control on radium activity. Additional, as yet undefined, controls on radium are operative in the downgradient, confined portion of the aquifer. [source] Contaminant Transport in Fractured Chalk: Laboratory and Field ExperimentsGROUND WATER, Issue 6 2003K. Witthüser Laboratory experiments were performed on chalk samples from Denmark and Israel to determine diffusion and distribution coefficients. Batch tests were used to define sorption isotherms for naphthalene and o-xylene. Linear sorption isotherms were observed and described with Henry-isotherms. Because of the high purity and low contents of clay minerals and organic carbon, Danish and white Israeli chalk generally have low retardation capacities. Con-trarily, gray Israeli chalk, with organic carbon fractions as high as 1.092%, remarkably retards organic contaminants. The Koc concept is not applicable to predicting distribution coefficients based on the organic carbon content in the chalk samples. Effective diffusivities of o -xylene, naphthalene, and several artificial tracers were determined using through-diffusion experiments. Based on measured diffusion coefficients and available literature values, a chalk specific exponent of 2.36 for Archie's law was derived, allowing a satisfactory estimate of relative diffusivities in chalk. A field-scale tracer test with uranine and lithium was performed in the Negev desert (Israel) to examine the transfer-ability of diffusivities determined on small rock samples in the laboratory. Due to low recovery rates of the tracer, a modified single fissure dispersion model was used for inverse modeling of the breakthrough curves. Resulting diffusivities deviate insignificantly from the laboratory values, which are considered to be representative for the investigated part of the aquifer and applicable in transport models. [source] Gravel-Corrected Kd ValuesGROUND WATER, Issue 6 2000Daniel I. Kaplan Standard measurements of solute sorption to sediments are typically made on the <2 mm sediment fraction. This fraction is used by researchers to standardize the method and to ease experimental protocol so that large labware is not required to accommodate the gravel fraction (>2 mm particles). Since sorption is a phenomenon directly related to surface area, sorption measurements based on the <2 mm fraction would be expected to overestimate actual whole-sediment values for sediments containing gravel. This inaccuracy is a problem for ground water contaminant transport modelers who use laboratory-derived sorption values, typically expressed as a distribution coefficients (Kd), to calculate the retardation factor (Rf), a parameter that accounts for solute-sediment chemical interactions. The objectives of this laboratory study were to quantify the effect of gravel on strontium Kd and Rf values and to develop an empirical method to calculate gravel-corrected Kdgc values for the study site (Hanford Site in Richland, Washington). Three gravel corrections, Kd values, were evaluated: a correction based on the assumption that the gravel simply diluted the Kd<2mm and had no sorption capacity (Kdgc,g=0), a correction based on the assumption that the Kd of the intact sediment (Kdtot was a composite of the Kd<2mm and the Kd>2mm (Kdgc,g = x), and a correction based on surface area (Kdgc,surf). On average, Kd<2mm tended to overestimate Kdtot by 28% to 47%; Kdgc,g = x overestimated Kdtot by only 3% to 5%; and Kdgc,g = 0 and Kdgc,surf underestimated Kdtot by 10% to 39%. Kdgc,g = x provided the best estimate of actual values (Kdtot); however, Kdgc,g = 0 was appreciably easier to acquire. Although other contaminants will likely have different gravel-correction values, these results have important implications regarding the traditional approach to modeling contaminant transport which uses Kd<2mm values. Such calculations may overestimate the tendency of gravel-containing sediments to retard contaminant migration. [source] Copper and cadmium extraction from highly concentrated phosphoric acid solutions using calcium alginate gels enclosing bis(2,4,4-trimethylpentyl)thiophosphinic acidJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 8 2006Argune Ocio Abstract The availability of alginate gels enclosing Cyanex 302 [bis(2,4,4-trimethylpentyl)thiophosphinic acid] for the uptake of cadmium and copper from highly concentrated solutions of industrial phosphoric acid wet process phosphoric acid (WPA)] was studied. For this purpose, beads of alginate gels enclosing microdrops of kerosene solutions of the industrial extractant Cyanex 302 at different concentrations were prepared. The experimental procedure gives rise to a composite bead in which alginate is the continuous phase and the organic extractant forms the discrete homogeneously distributed phase within the bead. The equilibrium in this three-phase system (phosphoric acid,extractant solution,alginate gel) was modelled in terms of the corresponding distribution factors, the main chemical reactions and their equilibrium constants. Retention isotherms of both metal ions were obtained experimentally at four concentrations (1.0, 2.5, 5.0 and 7.5 mol L,1) of pure phosphoric acid. High metal removal efficiency, due to liquid,liquid extraction processes, was observed even in the most acidic conditions. High values of the extraction constants were estimated, with the distribution coefficients between aqueous and alginate phase being near unity. Finally, the results obtained with industrial WPA are in close agreement with those predicted by the physicochemical model developed in synthetic media. Copyright © 2006 Society of Chemical Industry [source] Adsorption and desorption behaviour of taurine on macroporous adsorption resinsJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 7 2001Tang Zhigang Abstract Adsorption of taurine on S-8, NKA2 and Amberlite XAD-1 resins was measured with respect to time and the results indicated that equilibrium was reached in 50,min. The adsorption isotherms of taurine on D4006, AB-8, S-8, NKA2 were recorded and compared with those using Amberlite XAD-1, XAD-3, XAD-6 and XAD-7 at 28,°C. For an aqueous concentration range of 0,100,mg,g,1, each isotherm could be represented as a straight line. S-8 and XAD-1 resins had the highest solid/liquid distribution coefficients of 0.92 and 0.9. Since the locally produced S-8 resin is less expensive than XAD-1 resin, it was selected for further studies with adsorption isotherms being measured over the aqueous concentration range of 0,160,mg,g,1. These experimental results could be fitted by the Langmuir equation. The effects of pH, salting-out and temperature on the adsorption were studied with the results showing that the influence of temperature was the most important. A temperature-swing adsorption process was then tested to separate taurine from aqueous solutions and gave a overall yield >90% when taurine was adsorbed at 28,°C and eluted by deionized water at 70,°C. © 2001 Society of Chemical Industry [source] Application of ALOGPS to predict 1-octanol/water distribution coefficients, logP, and logD, of AstraZeneca in-house databaseJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 12 2004Igor V. Tetko Abstract The ALOGPS 2.1 was developed to predict 1-octanol/water partition coefficients, logP, and aqueous solubility of neutral compounds. An exclusive feature of this program is its ability to incorporate new user-provided data by means of self-learning properties of Associative Neural Networks. Using this feature, it calculated a similar performance, RMSE,=,0.7 and mean average error 0.5, for 2569 neutral logP, and 8122 pH-dependent logD7.4, distribution coefficients from the AstraZeneca "in-house" database. The high performance of the program for the logD7.4 prediction looks surprising, because this property also depends on ionization constants pKa. Therefore, logD7.4 is considered to be more difficult to predict than its neutral analog. We explain and illustrate this result and, moreover, discuss a possible application of the approach to calculate other pharmacokinetic and biological activities of chemicals important for drug development. © 2004 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 93:3103,3110, 2004 [source] Design, Synthesis and Evaluation of N -Basic Substituted 3-Hydroxypyridin-4-ones: Orally Active Iron Chelators with Lysosomotrophic PotentialJOURNAL OF PHARMACY AND PHARMACOLOGY: AN INTERNATI ONAL JOURNAL OF PHARMACEUTICAL SCIENCE, Issue 3 2000ZU D. LIU To investigate the possibility of targeting chelators into the lysosomal iron pool, nine bidentate 3-hydroxypyridin-4-ones with basic chains have been synthesized. As the turnover of ferritin iron is centred in the lysosome, such strategy is predicted to increase chelator efficacy of bidentate ligands. The pKa values of the ligands together with their distribution coefficients between 1-octanol and 4-morpholinepropane sulphonic acid (MOPS) buffer pH 7.4 have been determined. The in-vivo iron mobilization efficacy of these basic 3-hydroxypyridin-4-ones has been investigated in a 59Fe-ferritin-loaded rat model. No obvious correlation was observed between efficacy and the pKa value of the side chain, although those with pKa > 7.0 tended to be more efficient than those with pKa < 7.0. The imidazole-containing molecules are much less effective than the tertiary amine derivatives. A dose-response study suggested that basic pyridinones are relatively more effective at lower doses when compared with N -alkyl hydroxypyridinones. Optimal effects were observed with the piperidine derivatives 4h and 4i. The derivative 4i at a dose of 150 ,mol kg,1 was more effective than 450 ,mol kg,1 deferiprone, the widely adopted clinical dose. [source] Prediction of the Solubility, Activity Coefficient and Liquid/Liquid Partition Coefficient of Organic CompoundsMOLECULAR INFORMATICS, Issue 9 2004H. Hilal Abstract Solvation models, based on fundamental chemical structure theory, were developed in the SPARC mechanistic tool box to predict a large array of physical properties of organic compounds in water and in non-aqueous solvents strictly from molecular structure. The SPARC self-interaction solvation models that describe the intermolecular interaction between like molecules (solute-solute or solvent-solvent) were extended to quantify solute-solvent interaction energy in order to estimate the activity coefficient in almost any solvent. Solvation models that include dispersion, induction, dipole-dipole and hydrogen bonding interactions are used to describe the intermolecular interaction upon placing an organic solute molecule in any single or mixed solvent system. In addition to estimation of the activity coefficient for 2674 organic compounds, these solvation models were validated on solubility and liquid/liquid distribution coefficient in more than 163 solvents including water. The RMS deviations of the calculated versus observed activity coefficients, solubilities and liquid/liquid distribution coefficients were 0.272,log mole fraction, 0.487,log mole fraction and 0.44,log units, respectively. [source] Transport and distribution of lindane and simazine in a riverine environment: measurements in bed sediments and modellingPEST MANAGEMENT SCIENCE (FORMERLY: PESTICIDE SCIENCE), Issue 5 2004Ian J Allan Abstract Aquatic sediments often remove hydrophobic contaminants from fresh waters. The subsequent distribution and concentration of contaminants in bed sediments determines their effect on benthic organisms and the risk of re-entry into the water and/or leaching to groundwater. This study examines the transport of simazine and lindane in aquatic bed sediments with the aim of understanding the processes that determine their depth distribution. Experiments in flume channels (water flow of 10 cm s,1) determined the persistence of the compounds in the absence of sediment with (a) de-ionised water and (b) a solution that had been in contact with river sediment. In further experiments with river bed sediments in light and dark conditions, measurements were made of the concentration of the compounds in the overlying water and the development of bacterial/algal biofilms and bioturbation activity. At the end of the experiments, concentrations in sediments and associated pore waters were determined in sections of the sediment at 1 mm resolution down to 5 mm and then at 10 mm resolution to 50 mm depth and these distributions analysed using a sorption,diffusion,degradation model. The fine resolution in the depth profile permitted the detection of a maximum in the concentration of the compounds in the pore water near the surface, whereas concentrations in the sediment increased to a maximum at the surface itself. Experimental distribution coefficients determined from the pore water and sediment concentrations indicated a gradient with depth that was partly explained by an increase in organic matter content and specific surface area of the solids near the interface. The modelling showed that degradation of lindane within the sediment was necessary to explain the concentration profiles, with the optimum agreement between the measured and theoretical profiles obtained with differential degradation in the oxic and anoxic zones. The compounds penetrated to a depth of 40,50 mm over a period of 42 days. Copyright © 2004 Society of Chemical Industry [source] | ||||||||||||||||