Alternatives for the determination of the soil adsorption coefficient, KOC, of non-ionicorganic compounds - a review

The determination of the soil adsorption behaviour of an environmental chemical is very important to the evaluation of potential dangers for man and nature. One of the major problems for European regulators working in the field of environmental protection and risk assessment is the lack of complete and comparable soil sorption data, e.g. K_oc values. The large number of existing chemicals makes it necessary to develop and apply fast methods as an alternative to classical batch or column studies. Based on this context numerous approaches to estimate K_oc have been published during the past two decades.

In this paper more than 200 existing relationships for K_oc estimations have been reviewed and divided into four categories. Regression equations for the most important classes of non-ionic organic environmental chemicals able to estimate soil adsorption coefficients are summarised, and the advantages and drawbacks of the different approaches are discussed briefly.

Due to the fact that a broad application of the proposed approaches is limited by the quality and comparability of the underlying experimental K_oc values reference soils are postulated to create a common basis for the comparison of soil adsorption coefficients.     

Reliable QSAR for estimating Koc for persistent organic pollutants: correlation with molecular connectivity indices

Several recent studies have shown that n-octanol/water partition coefficients may not be a good predictor for estimating soil sorption coefficients of persistent organic pollutants (POPs), defined here as chemicals with log Kow greater than 5. Thus, an alternative QSAR model was developed that seems to provide reliable estimates for the soil sorption coefficients of persistent organic pollutants. This model is based on a set of calculated molecular connectivity indices and evaluated soil sorption data for 18 POPs. The chemical's size and shape, quantified by 1chi, 3chiC and 4chiC(v) indices, have a dominant effect on the soil sorption process of POPs. The developed QSAR model was rationalized in terms of potential hydrophobic interactions between persistent organic pollutants and soil organic matrix. Its high predictive power has been verified by an extensive internal and external validation procedure.     

Sorption of fipronil and its metabolites on soils from South Australia

This paper reports on the sorption of fipronil [(+/-)-5-amino-1-(2,6-dichloro-alpha,alpha,alpha-trifluoro-p-tolyl)-4-trifluoromethyl-sulfinylpyrazole-3-carbonitrile] and its two main metabolites, desulfynil and sulfide derivatives on a range of soils from South Australia. The Freundlich sorption coefficient (Kf) values for fipronil on the soils ranged from 1.94 to 4.84 using a 5% acetonitrile/water mixture as the soil solution. Its two metabolites had a higher sorption affinity for soils, with Kf values ranging from 11.09 to 23.49 for the sulfide derivative and from 4.70 to 11.77 for the desulfynil derivative. Their sorption coefficients were found to be better related to the soil organic carbon than clay content. The presence of cosolvents in soil solutions had a significant influence on the sorption of fipronil. The Freundlich sorption coefficients showed a log linear relationship with the fractions of both acetonitrile and methanol in solutions. The sorption coefficient of fipronil on Turretfield soil in the aqueous solution was estimated to be from 13.80 to 19.19. Methanol had less effect on the sorption of fipronil than acetonitrile. The Kd values for fipronil on the eight soils using a 5% methanol/water mixture were from 5.34 to 13.85, which reflect more closely the sorption in the aqueous solution. The average Koc value for fipronil on the eight South Australian soils was calculated to be 825+/-214.     

Determination of the distribution coefficient (log Kd) of oxytetracycline,tylosin A,olaquindox and metronidazole in manure

Olaquindox (log Kow = -2.3) and metronidazole (log Kow = -0.1) both have low tendencies to sorp to particles in manure. This corresponds with the negative log Kow values of these antibiotics. Tylosin (log Kow = 1.63) and oxytetracycline (log Kow = -1.12) sorp relatively strongly to the manure particles and have log Kd values between 1.5 and 2.0. The tendency to bind to manure was ranked after increasing binding as follows: metronidazole < olaquindox < tylosin A and oxytetracycline. This order of ranking is consistent with results of sorption in soil. Our experiments illustrate that for some antibacterial agents estimation of the partitioning coefficients, Kd, cannot be made from Kow and f(oc) alone. Sorption of oxytetracycline to manure is much higher than expected from the negative log Kow value of the compound. It is believed that sorption of oxytetracycline to manure is influenced by ionic binding to divalent metal ions as such Mg2+ and Ca2+ as well as other charged compounds in the matrix. Binding of oxytetracycline to soil is stronger than the binding to manure. This is most likely due to the strong mineral related metal complexes formed between soil, metal ion and oxytetracycline. These complexes are not known to exist in manure. The relatively strong sorption of tylosin A to manure corresponds with data found for soil sorption of tylosin. Tylosin has a log Kow value of 2.5, thus it is not surprising that this drug binds strongly to manure.     

Study of sorption kinetics of some ionic liquids on different soil types

In the present contribution sorption kinetics experiments under static conditions were utilized in three selected ionic liquids cations (1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium chlorides) study with five type of soil, differing in total organic carbon (TOC) content. The experimental results indicate the sorption capacity growth with increase in TOC content and hydrophobicity of ionic liquid cation. The obtained kinetic sorption parameters as well as distribution coefficients (K_d) were used to estimate the sorption properties of the soil types towards the ionic liquids in question. The Gibbs free energy values indicate that ionic liquid cations sorption on soils could be generally considered as a physical adsorption with exothermic effect. But the values of −dG for studied cations sorption on soil with very high of TOC content in soil (45%) may testify to nature of chemical adsorption. Sorption of the analyzed compounds occurs probably by means of hydrogen bonds, electrostatic and π  π interaction with the organic matter and the clay minerals of the soils.     

Sorption behavior of a new acidic herbicide in soils

The sorptive behavior of the experimental herbicide quinmerac (7-chloro-3-methyl-quinoline-8-carboxylic acid) was investigated in soils of different organic carbon content. Distribution coefficients are low (K_d = 0.03 - 12 mL g⁻¹) and are mainly determined by the soil organic carbon content. The adsorption mechanism operating in neutral to slightly acid agricultural soils is supposed to be cation bridging with the anionic form of quinmerac. Under acid conditions (pH 5.2) the predominating sorption mechanism is hydrophobic interaction. Therefore soil pH and cationic composition are also major factors determining the sorptive capacity of soils for quinmerac.

Adsorption kinetics, equilibrium adsorption and desorption isotherms were determined in batch experiments. Sorption kinetics were investigated at various times from 15 min to 96 hours. A two-step sorption behavior with time was found for the anionic form indicating two types of sorption mechanisms or sorption sites. Equilibrium for the first type was reached at a time-scale of minutes and for the second type after 24 hours. Adsorption isotherms were determined for two soil/solution ratios 1/5 and 1/3. Alteration of the adsorbent concentration exerted a strong influence on the adsorption isotherms. An increase of sorption was found with increasing adsorbent concentration. Under natural soil conditions sorption is therefore expected to be higher compared to the batch experiments. Desorption isotherms were obtained using the consecutive desorption method. Desorption hysteresis was not observed which indicates weak interactions. Implications of the results for the movement of quinmerac under field conditions and for models describing transport are discussed.     

Investigations on the sorption of phenols to dissolved organic matter by a QSAR study

The sorption of various phenols to Aldrich-HA and BSA was investigated by solid phase microextraction (SPME). The Aldrich-HA sorption with log K(DOC)-values between 2 and 3 was determined, whereas the sorption to BSA with log K(DOC)-values between 2 and 6 was much stronger. To enable an estimation of sorption constants a QSAR model was investigated. The linear free energy relationship (LFER) model showed a good correlation between the sorption constants and the log K(OW)-values with correlation coefficients of R = 0.910 and R = 0.878 for Aldrich-HA and BSA, respectively.     

Sorption and predicted mobility of herbicides in Baltic soils

This study was undertaken to determine sorption coefficients of eight herbicides (alachlor, amitrole, atrazine, simazine, dicamba, imazamox, imazethapyr, and pendimethalin) to seven agricultural soils from sites throughout Lithuania. The measured sorption coefficients were used to predict the susceptibility of these herbicides to leach to groundwater. Soil-water partitioning coefficients were measured in batch equilibrium studies using radiolabeled herbicides. In most soils, sorption followed the general trend pendimethalin > alachlor > atrazine approximately amitrole approximately simazine > imazethapyr > imazamox > dicamba, consistent with the trends in hydrophobicity (log K(ow)) except in the case of amitrole. For several herbicides, sorption coefficients and calculated retardation factors were lowest (predicted to be most susceptible to leaching) in a soil of intermediate organic carbon content and sand content. Calculated herbicide retardation factors were high for soils with high organic carbon contents. Estimated leaching times under saturated conditions, assuming no herbicide degradation and no preferential water flow, were more strongly affected by soil textural effects on predicted water flow than by herbicide sorption effects. All herbicides were predicted to be slowest to leach in soils with high clay and low sand contents, and fastest to leach in soils with high sand content and low organic matter content. Herbicide management is important to the continued increase in agricultural production and profitability in the Baltic region, and these results will be useful in identifying critical areas requiring improved management practices to reduce water contamination by pesticides.     

HPLC-screening method for the determination of the adsorption-coefficient on soil-comparison of different stationary phases

The application of a HPLC method for the determination of soil sorption constants was tested and validated. Different stationary and mobile phases including buffer solutions were compared using a large set of reference substances from different chemical classes. Cyanopropyl columns turned out to be the most suitable. The results show that the investigated HPLC method provides a quick and reliable method to determine the sorption constants.     

Sorption and predicted mobility of herbicides in Baltic soils

This study was undertaken to determine sorption coefficients of eight herbicides (alachlor, amitrole, atrazine, simazine, dicamba, imazamox, imazethapyr, and pendimethalin) to seven agricultural soils from sites throughout Lithuania. The measured sorption coefficients were used to predict the susceptibility of these herbicides to leach to groundwater. Soil-water partitioning coefficients were measured in batch equilibrium studies using radiolabeled herbicides. In most soils, sorption followed the general trend pendimethalin > alachlor > atrazine～ amitrole～ simazine > imazethapyr > imazamox > dicamba, consistent with the trends in hydrophobicity (log K_ow) except in the case of amitrole. For several herbicides, sorption coefficients and calculated retardation factors were lowest (predicted to be most susceptible to leaching) in a soil of intermediate organic carbon content and sand content. Calculated herbicide retardation factors were high for soils with high organic carbon contents. Estimated leaching times under saturated conditions, assuming no herbicide degradation and no preferential water flow, were more strongly affected by soil textural effects on predicted water flow than by herbicide sorption effects. All herbicides were predicted to be slowest to leach in soils with high clay and low sand contents, and fastest to leach in soils with high sand content and low organic matter content. Herbicide management is important to the continued increase in agricultural production and profitability in the Baltic region, and these results will be useful in identifying critical areas requiring improved management practices to reduce water contamination by pesticides.     

Sorptive behavior of sorgoleone in ultisol in two solvent systems and determination of its lipophilicity

Sorgoleone (SGL) exuded by sorghum roots inhibits the development of some weeds. Due to its high hydrophobicity, it is expected that SGL presents low soil mobility and limited allelopathic activity in the field. This work aims to evaluate the sorptivity of sorgoleone in octanol-water and in soil under two solvent systems. The two solvent systems were methanol:water (60:40) (MeOH:H2O) and pure methanol (MeOH). These two solvent systems promote different conditions for SGL solubility. Treatments were arranged in a 2 x 6 factorial (solvent systems x equilibrium concentrations in the solution (EC)). For each solvent, the sorption was achieved by shaking 500 mg of soil with 10 ml of 0, 5, 10, 15, 25, 40, and 60 mg L-1 of SGL solution, during 24 h. After centrifugation, the supernatant was filtered and the SGL concentration was determined by high performance liquid chromatography (HPLC). Data of sorbed amount of SGL were submitted to variance analysis, using a hierarchic factorial model. The data of sorbed amount (x/m) and equilibrium concentration (C) were fitted to the linear (x/m = a + KdC) and to the Freundlich (x/m = KfC1/n) models. The isotherm obtained for the MeOH:H2O system presented linear shape, whereas for the MeOH system a two subsequent linear isotherm was fitted. Sorgoleone is a highly hydrophobic compound, presenting a log Kow of 6.1. The sorption of sorgoleone to the soil was very high. The organic environment stimulated the sorgoleone sorption to the soil.     

Experimental variability in characterization of cyfluthrin sorption to soil

To assess the risk of a pesticide to leach to groundwater or to run off to surface water after application, it is necessary to characterize the sorption of the pesticide to soil. For pyrethroids, their hydrophobicity, strong sorption to various materials, and low solubility make it difficult to accurately characterize sorption processes. The objective of this research was to evaluate the variability in cyfluthrin ((RS)-alpha -cyano-4-fluoro-3-phenoxybenzyl (1RS,3RS;1RS,3SR)-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate) sorption to soil as affected by experiment conditions. To minimize cyfluthrin sorption on the walls of glass, silanized-glass, stainless steel, and PTFE centrifuge tubes, cyfluthrin solution was added to aqueous soil slurries or directly to soil, after which it was equilibrated with aqueous solution. Depending on the soil, variation in sorption coefficients, Koc, obtained using different experimental methodologies with one soil can be comparable to the variation in Koc values obtained for soils with different physical and chemical properties using one method. Koc values for cyfluthrin ranged from 56,000 to 300,000 L kg-1. Sorption methodology needs to be evaluated before sorption coefficients are used in predictive transport models.     

Estimation of soil adsorption coefficients of organic compounds by HPLC screening using the second generation of the European reference soil set

The European reference soil set was introduced as common basis for a better comparability of soil sorption data measured within the framework of chemical testing of environmental chemicals. The success of the EUROSOILS, as the set is commonly called, convinced the European Commission's Joint Research Centre to evaluate the possibility of producing a remake of these unique and new type of reference materials maintaining the principal sorption-controlling properties of the soils. In this paper the recently proposed second generation of the EUROSOILS is used to evaluate a HPLC-screening technique for the estimation of soil adsorption coefficients of organic chemicals. It could be shown that the derived correlations between HPLC capacity factors of the test substances and the respective soil adsorption coefficients resulting from batch experiments with the second version of the EUROSOILS agreed with those derived for the first generation of reference soils at a different occasion.     

Evaluation of reverse phase liquid chromatography/mass spectrometry for estimation of n-octanol/water partition coefficients for organic chemicals

A reverse-phase high pressure liquid chromatography/mass spectrometry (HPLC/MS method was developed for estimating n-octanol/water partition coefficients (K_ow) of anthropogenic molecules in complex chemical mixtures (e.g., complex effluents and solid waste leachates). The average error for an estimated log K_ow was ca. 0.5 and this error was similar for both aliphatic and aromatic compounds. The minimum level of detection using the total ion current profile generally decreased with increasing molecular weight between 100 and 600 daltons. Results obtained demonstrate that the HPLC/MS method is a viable technique for estimating log K_ow's of anthropogenic chemicals in complex environmental samples.     

Quantifying the sorption of organic chemicals on sediments

The use of a reference compound to quantify the sorption of nonpolar organic chemicals is proposed. This is because organic carbon normalized sorption coefficients (K_OC) do appear to be dependent on the type of sediment, and are thus not generally applicable to characterize the sorption properties of chemicals. Therefore, in this paper the hypothesis that nonpolar chemicals sorb in a constant ratio, independent of the sediment, has been investigated. Evidence for this hypothesis is shown with data from the literature. This enables one to compare sorption properties of nonpolar compounds on different sediments, if the differences between the sediments are normalized with a reference chemical rather than with the organic carbon content. Sediments with an organic carbon content of less than 0.1% seem to be unsuitable, because the compounds do not sorb mainly on the organic carbon, but also on other parts of the sediment. Sorption coefficients of compounds with aqueous solubilities in the μg per liter range or octan-1-ol water partition coefficients of more than 10⁵ are strongly influenced by the experimental techniques used. For these compounds the sorption coefficients measured by different techniques are less comparable. To enable comparison of sorption coefficients of hydrophobic chemicals, the use of a chlorobenzene as a reference compound in sorption experiments is suggested.     

Sorption behaviour of acetochlor,atrazine, carbendazim,diazinon, imidacloprid and isoproturon on Hungarian agricultural soil

Laboratory studies were conducted to determine the sorption behaviour of six commonly used pesticides (acetochlor, atrazine, carbendazim, diazinon, imidacloprid and isoproturon) on Hungarian brown forest soil with clay alluviation (Luvisol) using the batch equilibrium technique. The sorption isotherms could be described by the Freundlich equation in non-linear form (n < 1) for all compounds, however in case of diazinon using the extended Freundlich equation proved to be a better approach. The adsorption constant related soil organic carbon content (Koc) calculated from Freundlich equation were 314 for acetochlor, 133 for atrazine, 2805 for carbendazim, 1589 for diazinon, 210 for imidacloprid and 174 for isoproturon. The octanol-water partition coefficients (Pow), which can be a useful parameter to predict of adsorption behaviour of a chemical on soil, and dissociation coefficients of these pesticides were calculated based on the chemical structure of them using a computerized expert system. The octanol-water partition coefficients were determined experimentally from high performance liquid chromatographic parameters as well. Good agreement was observed between experimental and the computer expert system estimated data. Computer estimated log Pow values ranged 0.5 and 3.86 for the examined pesticides, with imidacloprid and diazinon being the least and most hydrophobic respectively. Experimentally determined logPow ranged between 0.92 and 3.81 with the same tendency. It can be concluded that the Freundlich adsorption constants (Kf) are slightly related to the octanol-water partition coefficients of investigated chemicals, nevertheless no close correlation could be established because of the influence of further characteristics of solutes and soil.     

Soot-water distribution coefficients for polychlorinated dibenzo-p-dioxins,polychlorinated dibenzofurans and polybrominated diphenylethers determined with the soot cosolvency-column method

For many types of hydrophobic compounds, sorption non-linearity and solid-water distributions in the field well above expectations from organic matter partitioning models have lead to the proposition that strong adsorption to soot surfaces may not be limited to polycyclic aromatic hydrocarbons but may extend as a significant process for many aromatic compound classes. Here, the soot-water distribution coefficients (Ksc) were determined with the soot cosolvency-column method for homolog series of five polychlorinated dibenzo-p-dioxins (PCDDs), five polychlorinated dibenzofurans (PCDFs) and for two polybrominated diphenylethers (PBDEs). All compounds exhibited significantly stronger association with soot carbon than expected from estimates of their bulk organic-carbon normalized partition coefficients (Koc). The Ksc/Koc ratios (at aqueous concentrations of around 0.1-1 microg/l) were for PCDDs (up to tetrachlorination) 19-130 (median 25), for PCDFs (also up to tetrachlorination) 150-490 (median 300), and for both the tetra- and pentabrominated PBDEs a factor of 60. The particularly strong soot sorption for the PCDFs is of similar enhancement factors as previously elucidated for polycyclic aromatic hydrocarbons. Compound-class specific correlations between log Ksc and octanol-water partition coefficients (log Kow) were significant for both PCDDs and PCDFs (and with R2 > 98%). These may prove useful for anticipating variable fractions of dissolved exposures between different environmental regimes and putative remediation objects.     

Partition coefficients of organochlorine pesticides on soil and on the dissolved organic matter in water

The partition coefficients of organochlorine pesticides (OCPs) between the organic matter of Taichung soil and water (K_oc) were evaluated with batch-type experiments. The partition coefficients of OCPs between Aldrich humic acid and water (K_doc) were estimated with solubility enhancement method as well. In this study, the K_ocs of aldrin, heptachlor, and p,p′-DDT are greater than their K_docs, and the relationship of dieldrin and heptachlor epoxide are opposite. The variations of partition coefficients are discussed. For predicting K_doc, a log-log regression relationship of K_doc and K_ow is determined.     

Structural and chromatographic predictors of n-octanol/water partition coefficients

The estimation of octanol/water coefficients by means of chromatographic and structural parameters may be a useful method for the prediction of ecotoxicological behaviour of chemical compounds. Experimental aspects and validity of some parameters are discussed with respect to heterocyclic aromatic hydrocarbons, including benzofurans. It is concluded that reverse phase chromatographic systems and molecular connectivities are promising predictors of log K_ow.