Three-Parameter Modeling of the Soil Sorption of Acetanilide and Triazine Herbicide Derivatives

Herbicides have widely variable toxicity and many of them are persistent soil contaminants. Acetanilide and triazine family of herbicides have widespread use, but increasing interest for the development of new herbicides has been rising to increase their effectiveness and to diminish environmental hazard. The environmental risk of new herbicides can be accessed by estimating their soil sorption (logK_oc), which is usually correlated to the octanol/water partition coefficient (logK_ow). However, earlier findings have shown that this correlation is not valid for some acetanilide and triazine herbicides. Thus, easily accessible quantitative structure–property relationship models are required to predict logK_oc of analogues of the these compounds. Octanol/water partition coefficient, molecular weight and volume were calculated and then regressed against logK_oc for two series of acetanilide and triazine herbicides using multiple linear regression, resulting in predictive and validated models.     

Sorption of acetochlor, S-metolachlor, and atrazine in surface and subsurface soil horizons of Argentina

Understanding herbicide sorption within soil profiles is the first step to predicting their behavior and leaching potential. Laboratory studies were conducted to determine the influence of surface and subsurface soil properties on acetochlor, atrazine, and S-metolachlor sorption. Soil samples were taken from horizons A, B, and C of two loamy soils of the humid pampas of Argentina under no-till management; horizon A was divided into two layers, A(0) (0-5 cm) and A(1) (5 cm to the full thickness of an A horizon). Sorption isotherms were determined from each sampled horizon using the batch equilibrium method and seven concentrations (0, 0.1, 0.5, 2.0, 5.0, 10.0, and 20.0 mg?L(-1)). Sorption affinity of herbicides was approximated by the Freundlich equation. The sorption strength K(f) (mg(1 - 1/n) kg(-1) L(1/n) ) over the soils and horizons studied followed the order S-metolachlor (16.51-29.19)?>?atrazine (4.85-12.34) ≥ acetochlor (5.17-11.97), which was closely related to the hydrophobicity of herbicides expressed as octanol-water partition coefficient (K(OW) ). The K(f) values of the three herbicides were positively correlated with soil organic carbon, with a significance of p < 0.01. Values of K(f) for the three herbicides decreased with depth in the two soils, indicating greater sorption onto surficial soil horizons and possibly a delayed transport toward subsurface soils and subsequent pollution of groundwater.     

Sorption of Nine Pesticides to Three Aquatic Macrophytes

The sorption of nine pesticides to the aquatic macrophytes Chara globularis, Elodea nuttallii, and Lemna gibba was studied. A batch equilibrium method was used to study the sorption at five concentration levels to fresh shoots of the macrophytes. The results for the herbicides atrazine and linuron were described by nonlinear Freundlich equations, with Freundlich exponents ranging from 0.53 to 0.60. The results for the other compounds showed almost linear sorption isotherms, with Freundlich exponents ranging from 0.9 to 1.1. The highest sorption was measured for chlorpyrifos, with sorption coefficients ranging from 1,660 to 2,150 L/kg. Sorption coefficients for C. globularis tended to be lower than those for the other two macrophytes. Correlation (R² = 0.80) was found for the relation between the sorption coefficient (K_d) of six pesticides and their solubility in water (S). The equation log K_d= 3.20 − 0.65 log  S can be used for a first estimate of the sorption coefficient of a pesticide to aquatic macrophytes. Received: 9 December 1998/Accepted: 1 April 1999     

Atrazine and metribuzin sorption in soils of the Argentinean humid pampas

Laboratory studies were conducted to determine the influence of surface and subsurface properties of three representative soils of the humid pampas of Argentina on atrazine and metribuzin sorption. Atrazine and metribuzin sorption isotherms were constructed for each soil at four depths. Sorption affinity of herbicides was approximated by the Freundlich constant (K(f)), distribution coefficient (Kd), and the normalized Kd based on organic carbon content (K(oc)). Multiple regression of the sorption constants against selected soil properties indicated that organic carbon content (OC) and silt were related positively and negatively, respectively, to atrazine K(f) coefficient (r2 = 0.93), while Kd coefficient of atrazine was related positively to organic carbon content and negatively to both silt and cation exchange capacity (CEC) (r2 = 0.96). For metribuzin, only organic matter content was related positively to Kr coefficient (r2 = 0.51). Lower K(f) values for atrazine were obtained for all soils with increasing depth, indicating lesser sorption at greater depths. Metribuzin sorption was quite similar across all depths. Sorption constant K(f) of atrazine ranged from 2.06 to 7.82, while metribuzin K(f) values ranged from 1.8 to 3.52 and were lower than atrazine for all soils and depths, indicating a greater leaching potential across the soil profile.     

Development of the fragment constant method for estimating the partition coefficients of nonionic organic mixtures

The fragment constant model for estimating partition coefficient of nonionic organic mixtures is developed based on group contribution theory. For 60 mixtures randomly composed of eight substituted benzenes, the C₁₈-Empore™ disks/water partition coefficients (K _MD) are determined. Establishment of a model between these K _MD and the mole fraction of the fragments (Cl, Br, I, NO₂, CH₃) acquires the fragments constants for K _MD. The significant linear regression equation, between these fragment constants and that of Hansch, provides an approach for calculating K _MD of untested nonionic organic mixtures by using the fragment constants of Hansch. Received: 11 November 2000/Accepted: 14 April 2001     

Sorption of Chlorophenolates in Soils and Aquifer and Marine Sediments

This article describes the sorption behavior of 3 hydrophobic ionizable chlorophenols—2,4,6-trichlorophenol, 2,3,4,6-tetrachlorophenol, and pentachlorophenol—in different types of natural sorbents. A series of experiments was carried out with 11 topsoil samples, 9 aquifer sediments, and 12 marine sediments differing in pH, organic-matter content, and mineral composition and presumably also in type of organic matter due to their differing origins. Ionized forms of chlorophenols dominated in almost all sorption experiments. Freundlich isotherm coefficients K_f and 1/n, as well as organic-matter sorption coefficient (log K_om) and free-energy change (G^o), were calculated for all 3 compounds in all sorbents. The sorption intensity of predominantly ionized chlorophenols increased linearly with the increase of sorbent organic-matter content and decreased with the increasing sorbent pH. Different sorption behavior of all 3 compounds in marine sediments with respect to topsoils and aquifer sediments was indicated by significant differences in K_f and 1/n coefficients as well as in log K_om and G^o values. The highest K_f and log K_om values were obtained for sorption of chlorophenolic compounds in topsoils and the lowest in marine sediments, although both groups of sorbents had similar organic-matter content. The 1/n coefficient, reflecting the isotherm nonlinearity, was considerably lower than unity for all compounds in almost all sorbents. The most significant deviation of sorption isotherms from linearity was observed in marine sediments. Only marine sediments showed a linear increase in sorption intensity of all 3 compounds with the increase in sorbent-specific surface area. These results pointed to a different mechanism of sorption in marine and terrestrial sorbents and confirmed that the capacity of sorption was related to amount as well as type and origin of organic matter.     

Role of exposure mode in the bioavailability of triphenyl phosphate to aquatic organisms

A laboratory study was conducted to investigate the role of the route of triphenyl phosphate (TPP) entry on its aquatic bioavailability and acute biological effects. Three TPP treatments were used for exposures of fish and invertebrates. These consisted of TPP dosed directly into water with and without clean sediment and TPP spiked onto sediment prior to aqueous exposures. Results of static acute toxicity tests (no sediment) were 0.78 mg/L (96-h LC50) for bluegill, 0.36 mg/L (48-h EC50) for midge, and 0.25 mg/L (96-h EC50) for scud. At 24 h, the sediment (1.1% organic carbon)/water partition coefficient (Kp) for TPP was 112. Use of this partition coefficient model to predict the sediment-mediated reduction of TPP concentration in water during toxicity tests resulted in a value that was only 10% less than the nominal value. However, the required nominal concentration of TPP to cause acute toxicity responses in test organisms was significantly higher than the predicted value by the model for both clay and soil-derived sediment. Direct spiking of TPP to soil minimized TPP bioavailability. Data from parallel experiments designed to track TPP residues in water through time suggest that sorption kinetics control residue bioavailability in the initial 24 h of exposure and may account for observed differences in LC50 and EC50 values from the sediment treatments.     

Bioconcentration of persistent organic pollutants in four species of marine phytoplankton

The uptake of polychlorinated biphenyls (PCBs) was studied in four species of marine algae. A novel experimental system to establish and maintain constant dissolved concentrations of PCBs was employed. Headspace sampling was used to verify that the freely dissolved concentrations remained constant with time. The headspace analysis also allowed sorption to dissolved organic carbon (DOC) to be quantified for all but the most lipophilic PCB congeners. Equilibration with the dissolved phase was rapid for three of the four algae species (<1 d for the majority of congeners). Organic carbon-normalized algae/water partition coefficients (KAlgW) were similar for three of the four species, but were lower by a factor of 10 to 20 for Phaeodactylum tricornutum. The KAlgW values of the first three species were similar to the octanol/water partition coefficient (Kow) for those PCB congeners for which DOC sorption could be quantified. These KAlgW values also agreed well with organic carbon-normalized bioconcentration factors for PCBs in suspended particulate matter (BCF(SPM)) sampled in Baltic Sea surface water during the summer.     

Measuring the bioavailability of two hydrophobic organic compounds in the presence of dissolved organic matter

Bioavailability of benzo[a]pyrene (BaP) and 3,3',4,4'-tetrachlorobiphenyl (TCB) was studied in natural lake water containing dissolved organic matter (DOM). Lake water was diluted to give a dissolved organic carbon (DOC) range of 1 to 20 mg/L. Partition coefficients for the model compounds were assessed at different DOM concentrations and over time with three different methods, namely equilibrium dialysis and reverse-phase and liquid-liquid extraction. In addition, biological partition coefficients were estimated from the difference in the bioconcentration of the model compounds in Daphnia magna in the presence and absence of DOM. Results showed that bioavailability of the model compounds was reduced by the presence of DOM. The equilibrium dialysis method gave the best estimates for bioavailability of the model compounds when compared with biologically determined values. Both the reverse-phase and the liquid-liquid extraction overestimated the bioavailable fraction. The more pronounced overestimation of bioavailable fraction of TCB suggested that the sorption of TCB was not only lower but the interaction was also weaker than that of BaP. Increasing DOM concentration produced lower partition coefficients and the effect seemed to be more pronounced when measured by the reverse-phase and the extraction methods.     

Factors determining the bioaccumulation potential of pesticides in the individual compartments of aquatic food chains

Representative members of aquatic food chains, i.e., algae (Scenedesmus acutus), daphnids (Daphnia magna), and catfish (Ictalurus melas), have been used to evaluate the bioaccumulation potential of 10 pesticides. The objectives of the studies were to determine the kinetics of bioacumulation and depuration of pesticides and to estimate the contribution of food chain transfer of pesticide residues to the total magnitude of bioaccumulation. Furthermore, the influence of physicochemical properties and the stability of pesticides in the aquatic environment on their final concentration in aquatic organisms and on their possible availability in the aquatic environment was investigated. In standardized experiments it was shown that bioaccumulation of nonionic organic chemicals from water can essentially be described as sorption or, as a simplification, as a distribution phenomenon which was correlated with the n-octanol/water partition coefficient of the compounds. Increasing the biomass at a given pesticide concentration caused a decrease in the bioaccumulation factor. Depuration of pesticides from aquatic organisms followed second-order reaction kinetics. The half-lives as well as the reciprocal of the rate constants of depuration were found to be highly correlated with the lipophilicity of the pesticides. Transfer of pesticide residues via food chains was found to be of lesser importance compared with direct uptake from water. It did not result in a buildup of residues so that higher residues would have been obtained in higher trophic levels of the aquatic food chains (biomagnification). Even for the highly lipophilic p, p′-DDT only 23 and 44% of the residue of the respective lower food chain organisms (algae, daphnids) were retained by the respective higher food chain organisms (daphnids, catfish). Also the transfer of residues can be estimated by means of the partition coefficient of the pesticide. The possible appearance of a soil-applied pesticide in water systems depends on its soil mobility which was found to be inversely correlated with the partition coefficient and also with the sorption constants of aquatic organisms. Consequently, a high bioaccumulation potential of a lipophilic compound is significantly reduced due to its limited transfer to the aquatic environment.     

Influence of soil pH on the sorption of ionizable chemicals: modeling advances

The soil-water distribution coefficient of ionizable chemicals (K(d)) depends on the soil acidity, mainly because the pH governs speciation. Using pH-specific K(d) values normalized to organic carbon (K(OC)) from the literature, a method was developed to estimate the K(OC) of monovalent organic acids and bases. The regression considers pH-dependent speciation and species-specific partition coefficients, calculated from the dissociation constant (pK(a)) and the octanol-water partition coefficient of the neutral molecule (log P(n)). 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.     

Vertical distribution profiles of linear alkylbenzene sulfonates and their long-chain intermediate degradation products in coastal marine sediments

The variation with depth of the concentration of linear alkylbenzene sulfonates (LASs) and of the long-chain sulfophenyl carboxylic acids (SPCs) resulting from LAS biodegradation was determined in coastal sediments. We analyzed samples of sediment cores taken from three locations in a littoral zone subjected to the discharge of untreated urban effluents in the Bay of Cádiz in the southwestern part of the Iberian Peninsula. The vertical profile of LAS concentrations showed a sharp reduction with depth, whereas the concentration of long-chain SPCs (6-13 carbon atoms) was greatest at 10 to 14 cm depth. At this depth, the conditions in the interstitial water are strictly anoxic (Eh = -380 mV). The partition coefficients between the solid phase of the sediment versus the interstitial water are very different for LAS and for its degradation intermediates. For LAS, the organic carbon-based partition coefficient values were between 2.4 x 10(3) and 6.6 x 10(5)L/kg for the C10 and C13 homologues, respectively; these values are similar to those obtained from laboratory tests for the sorption of LAS onto marine sediments. For the long-chain SPCs, the partition coefficients are several orders of magnitude less as a consequence of their lower hydrophobicity.     

Adsorption, desorption, and degradation of three pesticides in different soils

Adsorption, desorption, and degradation of two herbicides (metolachlor and pendimethalin) and a nematicide (cadusafos) were evaluated under laboratory conditions with six soils (ferralsol, regosol, andosol, fluvisol, and two vertisols), selected in a tropical zone (Martinique, French West Indies) and in the Mediterranean area (Languedoc, South of France). Adsorption parameters were calculated using the Freundlich equation, and desorption parameters were evaluated using a 2-compartment model corresponding to two different energy levels. Degradation rate was calculated with a first order equation. Combining values of Koc and half-life in soil, an estimation of the mobility of these pesticides in the soil was made using GUS (ground water ubiquity score). Consideration of pesticide adsorption and resistance to desorption allowed classification of these three compounds according to their mobility: pendimethalin < cadusafos < metolachlor. This classification correlates with some physico-chemical properties of the molecules (water solubility and octanol/water partition coefficient). The equations describing these relations were evaluated. Furthermore, the six soils were classified according to their tendency to adsorb the three pesticides: vertisols > ferralsol > regosol > andosol > fluvisol. Clay or sand content (and cation exchange capacity) of the soils clearly affected adsorption-desorption parameters. The degradation study in the six soils did not show significant differences, but revealed the influence of temperature. GUS values indicated that pendimethalin could be considered as a non-leacher compound, and that cadusafos and metolachlor exhibit a moderate tendency to leaching.     

Predicted bioconcentration factors and soil sorption coefficients of pesticides and other chemicals

Equations were used to calculate soil sorption coefficients (K_oc) and bioconcentration factors (BCF) for 358 compounds, mostly pesticides, from known water solubility values. Bioconcentration factor (BCF) values were also calculated from K_oc values. Comparisons were made between calculated and actual values where possible. The highest experimental BCF values were associated with persistent chemicals having water solubilities below 0.1 ppm. With some exceptions, such as certain ionic compounds, the highest K_oc values were also associated with low water solubility. Calculated values for BCF or K_oc in excess of 1000 should be experimentally confirmed in order to be certain of their environmental significance. Of the values calculated from water solubility, 10% of the BCF values were over 1000 and 30% of the K_oc values were over 1000. BCF and K_oc values are simple to calculate from water solubility and are useful for estimation of partitioning in soil, and in animal tissues for contributing to early assessment of potential hazard. Low water solubility is not a good indicator for differentiating between insecticides, fungicides, and herbicides.     

Estimation of soil sorption coefficients of veterinary pharmaceuticals from soil properties

Environmental exposure assessment of veterinary pharmaceuticals requires estimating the sorption to soil. Soil sorption coefficients of three common, ionizable, antimicrobial agents (oxytetracycline [OTC], tylosin [TYL], and sulfachloropyridazine [SCP]) were studied in relation to the soil properties of 11 different soils. The soil sorption coefficient at natural pH varied from 950 to 7,200, 10 to 370, and 0.4 to 35 L/kg for OTC, TYL, and SCP, respectively. The variation increased by almost two orders of magnitude for OTC and TYL when pH was artificially adjusted. Separate soil properties (pH, organic carbon content, clay content, cation-exchange capacity, aluminum oxyhydroxide content, and iron oxyhydroxide content) were not able to explain more than half the variation observed in soil sorption coefficients. This reflects the complexity of the sorbent-sorbate interactions. Partial-least-squares (PLS) models, integrating all the soil properties listed above, were able to explain as much as 78% of the variation in sorption coefficients. The PLS model was able to predict the sorption coefficient with an accuracy of a factor of six. Considering the pH-dependent speciation, species-specific PLS models were developed. These models were able to predict species-specific sorption coefficients with an accuracy of a factor of three to four. However, the species-specific sorption models did not improve the estimation of sorption coefficients of species mixtures, because these models were developed with a reduced data set at standardized aqueous concentrations. In conclusion, pragmatic approaches like PLS modeling might be suitable to estimate soil sorption for risk assessment purposes.     

Accumulation of lipophilic chemicals in plant cuticles: Prediction from octanol/water partition coefficients

Partition coefficients of lipophilic organic compounds have been determined for the system plant cuticle/water. The cuticles were isolated enzymatically from leaves of rubber plant (Ficus elastica) and bitter orange (Citrus aurantium) and from fruits of tomato (Lycopersicon esculentum) and green pepper (Capsicum annuum). The sorption in cuticles of 4-nitrophenol, (2,4-dichlo-rophenoxy) acetic acid (2,4-D), 6-chloro-N-ethyl-N-(1-methylethyl)-1,3,5-triazine-2,4-diamine (atrazine), (2,4,5-trichlorophenoxy)acetic acid (2,4,5-T), pentachlorophenol (PCP), hexachlorobenzene (HCB), perylene, and 1,2-benzenedicarboxylic acid bis(2-ethylhexyl)ester (DEHP, bis(2-ethyl-hexyl)phthalate) can be predicted with fair accuracy from octanol/water partition coefficients. Logarithms of cuticle/water and octanol/water partition coefficients between 10² and 10⁸ are correlated by linear regression equations with coefficients of correlation from 0.980 to 0.988. The ecotoxicological significance of sorption in cuticles is discussed in a simplified distribution model.     

Polycyclic Aromatic Hydrocarbons in Freshwater Isopods and Field-Partitioning Between Abiotic Phases

An assessment was made of the in situ bioaccumulation of 13 polycyclic aromatic hydrocarbons (PAHs) in freshwater isopods in relation to their partitioning between sediments, particulate matter (>0.7 μm), and dissolved phases in eight different water systems of The Netherlands. Large differences in total (Γ PAHs) concentrations and in relative abundance of individual PAHs were observed between organisms and abiotic compartments and among sampling stations. Principal component analysis revealed distinct differences between PAH profiles in sediments and water. High molecular weight PAHs dominated in the sediments, fluoranthene and pyrene in the isopods, and naphthalene in water. Apparent lipid-based bioconcentration factors (BCFs) increased with increasing hydrophobicity (n-octanol/water partition coefficient; K_ow). The total range of the BCFs varied only one order of magnitude, ranging from 10^5.1 (naphthalene) to 10^6.1 (benzo[a]pyrene). For PAHs with log K_ow > 6.1 lower BCFs than expected were observed, which was attributed to reduced bioavailability, to the operational definition of the dissolved phase, and to growth dilution preventing equilibrium to be reached within the lifetime of the isopods. Abiotic partitioning coefficients, such as K_oc (organic carbon normalized sediment–water partition coefficient) and K_pm (particulate matter–water distribution coefficient) increased with hydrophobicity for PAHs having a log K_ow < 6.1. Sediment-water partition coefficients (K_d) increased with the organic carbon content of the sediments for most PAHs. It is concluded that isopods have a marked ability to accumulate PAHs and that their tissue residues tend to reflect spatial and temporal variations in the bioavailability of PAHs in littoral freshwater environments. Received: 9 April 1997/Accepted: 3 January 1998     

Effects of Pig Slurry on the Sorption of Sulfonamide Antibiotics in Soil

Sorption of p-aminobenzoic acid (pABA) and five sulfonamide antibiotics to loess Chernozem topsoil amended with varied additions of pig slurry was investigated in batch trials. In unfertilized soil, partition coefficients (K_d) of sulfonamides ranged from 0.3 to 2.0. Strong sorption nonlinearity (1/n = 0.5 to 0.8) was best fitted by the Freundlich isotherm (R² = 0.7 to 1.0) and was indicative for specific sorption mechanisms. Adsorption to pig slurry was much stronger, and nondesorbable portions were increased compared with soil. However, in a mixture of soil and slurry (50:1 w/w), sorption of the antibiotics was significantly decreased at a lower concentration range of pABA and the sulfonamides. This was attributed to competitive adsorption of dissolved organic matter (DOM) constituents from manure. An increase in pig slurry amendment resulted in increased total organic matter, DOM concentration, and ionic strength, but pH decreased. As a result, the nonadsorbed portions of pABA, sulfanilamide, and sulfadiazine (logD_ow < –0.4) ranged from 47% to 82% of the applied concentration in the differently manured substrates. Dissolved fractions of the antibiotics reached a maximum at a soil–slurry ratio of 9:1 and decreased with further addition of manure. This decrease was related to the formation of less-effective DOM associates in solution. The adsorbed and desorbed portions of the less-polar substances—sulfadimidine, sulfadimethoxine, and sulfapyridine (logD_ow > 0.1)—remained nearly constant in the presence of increased manure input. The pH changes caused by manure amendment strongly affected ionisation status of the latter compounds, thus resulting in increased adsorption, which compensated the mobilizing effect of DOM. It is suggested that the effect of manure be considered in test methods to determine the soil retention of pharmaceutical substances.