Koc Values Research Articles

Contribution of Soil Components on the Sorption of Chlorpyrifos

It is generally assumed that the sorption of a nonionic pesticide on soil depends mainly on the content of soil organic matter (SOM); however, there are other factors that can contribute to this process. The possible causes of variation in the carbon-normalized partition coefficient (K OC) for chlorpyrifos (CPF) for a diverse set of ten soils have been investigated. On the one hand, the analysis of the chemical composition of the SOM was analyzed, and on the other hand, the likely interactions between the organic matter and the mineral phase were assessed. Sorption experiments of CPF were performed on whole soil, on soils treated with 2% hydrofluoric acid (HF), and onto calcined soil at 550 °C. Organic matter chemistry of soil was determined by 13C CP/MAS NMR spectroscopy; K OC values were positively correlated with aryl C relative proportion and negatively correlated with alkyl C and O-aryl C proportions and prediction equation of K OC was found (R 2 = 0.82, p < 0.001). To evaluate possible organo-mineral interactions, a mathematical model was proposed which calculates the concentration of CPF at equilibrium (C cal) considering adsorption coefficients for the organic (K DHF) and inorganic (K D550 °C) soil constituents, separately. The comparison between C cal and the equilibrium concentration obtained from experimental data (C exp) onto whole soil allowed us to confirm that interactions between the OM and clay affect the adsorption of CPF in whole soil. Such findings should be taken into account in the development of predictive models for the evaluation of the fate and transport of this pesticide in soil.

Sorption mechanisms of sulfamethazine to soil humin and its subfractions after sequential treatments

Sorption mechanisms of an antibiotic sulfamethazine (SMT) to humin (HM) isolated from a peat soil and its subfractions after sequential treatments were examined. The treatments of HM included removal of ash, O-alkyl carbon, lipid, and lignin components. The HF/HCl de-ashing treatment removed a large amount of minerals (mainly silicates), releasing a fraction of hydrophobic carbon sorption domains that previously were blocked, increasing the sorption of SMT by 33.3%. The de-O-alkyl carbon treatment through acid hydrolysis greatly reduced polarity of HM samples, thus weakening the interaction between sorbents with water at the interfaces via H-bonding, leaving more effective sorption sites. Sorption of SMT via mechanisms such as van der Waals forces and π-π interactions was enhanced by factors of 2.04–2.50. After removing the lipid/lignin component with the improved Soxhlet extraction/acid hydrolysis, the organic carbon content-normalized sorption enhancement index Eoc was calculated. The results demonstrated that the Eoc-lipid for SMT (16.9%) was higher than Eoc-lignin (10.1%), implying that removal of unit organic carbon mass of lipid led to a higher increase in sorption strength than that of lignin. As each component was progressively removed from HM, the sorption strength and isotherm nonlinearity of the residual HM samples for SMT were gradually enhanced. The Koc values of SMT by HM samples were positively correlated with their aromatic carbon contents, implying that π-π electron donor-acceptor interactions between the benzene ring of sorbate and the aromatic domains in HM played a significant role in their interactions.

Occurrence, distribution, and fate of pesticides in an intensive farming region in the Brazilian semi-arid tropics (Jaguaribe River, Ceará)

This study is the first to assess herbicide levels in sediment of the Jaguaribe River, which is located in one of the largest agricultural areas of the tropical Brazilian semi-arid region (Ceara state), with the goal of evaluating the occurrence of herbicides as well as their geographical distribution pattern and environmental fate in this river. Sediment collection was performed in October 2012, and nine points were selected located along the river to its mouth, and the levels of atrazine (ATR), alachlor (ALA), bromacil (BRO), ethalfluralin (ETHA), fluridone (FLU), metolachlor (METO), norflurazon (NOR), simazine (SIM), tebuthiuron (TEB), and trifluralin (TRI) were determined. Sediment from each site was characterized by granulometric analysis and total organic carbon (TOC), humic acid (HA), and fulvic acid (FA) contents. ALA, BRO, ETHA, FLU, NOR (a prohibited compound), and TEB were detected in all samples analyzed, whereas herbicides ATR, METO, SIM, and TRI were not detected at any of the sampling sites. The total herbicide concentrations (∑herb) (the sum of six herbicides detected) per sampling sites ranged from 174.86 to 348.84 ng g−1 (fluvial zone) and 111.43 to 419.35 ng g−1 (estuary zone). The results showed that the sum of ETHA concentrations at all sampling points (ΣETHA) (1687.47 ng g−1) was the most abundant herbicide, followed by ΣBRO (201.43 ng g−1), ΣALA (41.73 ng g−1), ΣTEB (24.41 ng g−1), ΣFLU (1.33 ng g−1), and ΣNOR (0.43 ng g−1). The physicochemical properties of ATR, METO, and SIM favor their dispersion in the aqueous fraction; thus, the lack of these herbicides in the sediments is due to environmental partitioning. The environmental and geographical distribution pattern indicates that the estuarine zone is a sink that controls the fate of TEB, ALA, and FLU herbicides. Differently from expected results, data based on non-linear Spearman’s coefficient show that herbicides with high log K ow and K oc values were more likely to be found in the sediment inorganic fraction, whereas herbicides with high solubility and low K oc values were dispersed between both the organic and inorganic fractions of sediments.

New Evidence for High Sorption Capacity of Hydrochar for Hydrophobic Organic Pollutants.

This study investigated the sorption potential of hydrochars, produced from hydrothermally carbonizing livestock wastes, toward organic pollutants (OPs) with a wide range of hydrophobicity, and compared their sorption capacity with that of pyrochars obtained from conventional dry pyrolysis from the same feedstock. Results of SEM, Raman, and 13C NMR demonstrated that organic carbon (OC) of hydrochars mainly consisted of amorphous alkyl and aryl C. Hydrochars exhibited consistently higher log Koc of both nonpolar and polar OPs than pyrochars. This, combined with the significantly less energy required for the hydrothermal process, suggests that hydrothermal conversion of surplus livestock waste into value-added sorbents could be an alternative manure management strategy. Moreover, the hydrochars log Koc values were practically unchanged after the removal of amorphous aromatics, implying that amorphous aromatic C played a comparable role in the high sorption capacity of hydrochars compared to amorphous alkyl C. It was thus concluded that the dominant amorphous C associated with both alkyl and aryl moieties within hydrochars explained their high sorption capacity for OPs. This research not only indicates that animal-manure-derived hydrochars are promising sorbents for environmental applications but casts new light on mechanisms underlying the high sorption capacity of hydrochars for both nonpolar and polar OPs.

Bioavailability of polycyclic aromatic hydrocarbons (PAHs) and Environmental Risk (ER) Assessment: The case of the Ogbese river, Nigeria

Pollution from PAHs in porewater and surficial sediments of the rapidly contaminated Ogbese River, Ondo State, Nigeria was investigated. Samples were extracted using Hexane/methylene chloride mixture and the complex PAH mixtures were analyzed by gas chromatography-mass spectrometry. There was greater influence of sediment fine particles (R2=0.978) in partitioning of PAHs over other factors. Total concentration of PAHs in the sediments and porewater ranged 14.45 μg g−1 to 26.78 μg g−1 (n=8, dry weight) and 6.25 μg L−1 to 13.93 μg L−1 (n=8) respectively. The measured partition coefficients (i.e., Koc values) of the individual PAHs between sediment organic matter (SOM) and porewater across all locations ranged from 5.027 (±0.157 SD) to 5.074 (±0.169 SD). The correlation in log Koc (observed vs predicted) for high molecular weight/low molecular weight-PAHs suggests variability in specific interactions and unequal partition efficiencies with SOM under natural condition. Based on equilibrium partitioning and narcosis model, three PAHs {benzo(b)fluoranthene, benzo(a)pyrene and dibenz(a,h)anthracene }has toxic unit (TU) values of concern. These results represent a baseline report for future monitoring programs of PAHs.

Adsorption of organophosphorus pesticides in tropical soils: The case of karst landscape of northwestern Yucatan

This article discusses the adsorption of four organophosphorus pesticides—diazinon, dimethoate, methyl parathion, and sulfotep—in soil samples from four sites—Komchén, Xcanatún, Chablekal and Mocochá— in the northwest of Yucatan, Mexico. These pesticides have been detected in groundwater at concentrations greater than 5 (μg/L) during recent monitoring campaigns in the study area. In this region, groundwater contamination is exacerbated by its karst aquifer, which is susceptible to contamination and is considered very vulnerable. The experimental work was carried out using the batch equilibrium technique. Pesticide analyses by solid-phase extraction and gas chromatography were performed. The equilibrium adsorption data were analyzed by Henry, Langmuir and Freundlich models. The results indicate that the Freundlich model provides the best correlation of the experimental data. Freundlich adsorption coefficients Kf were in the range of 1.62–2.35 for sulfotep, 2.43 to 3.25 for dimethoate, from 5.54 to 9.27 for methyl parathion, and 3.22 to 5.17 for diazinon. Freundlich adsorption coefficients were normalized to the content of organic carbon in the soil to estimate the sorption coefficient of organic carbon (KOC). KOC values were in the range of 9.45–71.80, indicated that four pesticides have low adsorption on the four studied soils, which represents a high risk of contamination to the aquifer.

Occurrence and distribution of synthetic musks, triclosan and methyl triclosan in a tropical urban catchment: Influence of land-use proximity, rainfall and physicochemical properties

This study involved a comprehensive thirteen month survey of synthetic musks, triclosan (TCS) and methyl triclosan (MTCS) in surface water, as well as suspended particular matter (SPM) and bottom sediments in a tropical urban catchment in Singapore. The polycyclic musk, Galaxolide (HHCB), exhibited the highest concentration among musk compounds, ranging from 5.16 to 42.9ng/L in surface water, 11.0 to 108ng/g dry wt. in sediments and 44.1 to 81.3ng/g dry wt. in SPM. Concentrations of musk ketone, the dominant nitroaromatic musk, ranged from 0.08 to 6.45ng/L in water, 0.082 to 0.72ng/g dry wt. in sediments and 1.75 to 5.50ng/g dry wt. in SPM. Concentrations of MTCS ranged from 0.0056 to 5.6ng/L in water, 0.01 to 0.17ng/g dry wt. in bottom sediments and 0.75 to 2.81ng/g dry wt. in SPM. These concentrations are below predicted no-effect concentrations (PNEC). Principal components analysis (PCA) results showed that synthetic musk concentrations were positively correlated, indicating common source emissions. Rainfall amount and land-use index were found to be key determinants of hydrophobic organic contaminant concentrations in this catchment. Concentrations of TCS and its methylated degradation product, MTCS, were also positively correlated. However, the relative composition of MTCS to total triclosans was relatively low in water (2.8±2.5%) and bottom sediments (0.3±0.1%), suggesting only minor transformation of TCS to MTCS. The organic carbon-water distribution ratio, log Koc (observed), ranged between 3.8 and 5.4 for musks, TCS and MTCS, indicating relatively strong partitioning from dissolved to solid phases. These field-derived log Koc (observed) values are comparable to estimated values based on physicochemical properties. The results provide insight into the occurrence, transport pathways and exposure risks of synthetic musks, triclosan and methyl triclosan in this tropical catchment.

The partitioning behavior of persistent toxicant organic contaminants in eutrophic sediments: Coefficients and effects of fluorescent organic matter and particle size

In the shallow lakes, the partitioning of organic contaminants into the water phase from the solid phase might pose a potential hazard to both benthic and planktonic organisms, which would further damage aquatic ecosystems. This study determined the concentrations of polycyclic aromatic hydrocarbons (PAHs), organochlorine pesticides (OCPs), and phthalate esters (PAEs) in both the sediment and the pore water from Lake Chaohu and calculated the sediment – pore water partition coefficient (KD) and the organic carbon normalized sediment – pore water partition coefficient (KOC), and explored the effects of particle size, organic matter content, and parallel factor fluorescent organic matter (PARAFAC-FOM) on KD. The results showed that log KD values of PAHs (2.61–3.94) and OCPs (1.75–3.05) were significantly lower than that of PAEs (4.13–5.05) (p < 0.05). The chemicals were ranked by log KOC as follows: PAEs (6.05–6.94) > PAHs (4.61–5.86) > OCPs (3.62–4.97). A modified MCI model can predict KOC values in a range of log 1.5 at a higher frequency, especially for PAEs. The significantly positive correlation between KOC and the octanol – water partition coefficient (KOW) were observed for PAHs and OCPs. However, significant correlation was found for PAEs only when excluding PAEs with lower KOW. Sediments with smaller particle sizes (clay and silt) and their organic matter would affect distributions of PAHs and OCPs between the sediment and the pore water. Protein-like fluorescent organic matter (C2) was associated with the KD of PAEs. Furthermore, the partitioning of PARAFAC-FOM between the sediment and the pore water could potentially affect the distribution of organic pollutants. The partitioning mechanism of PAEs between the sediment and the pore water might be different from that of PAHs and OCPs, as indicated by their associations with influencing factors and KOW.

Evaluating the Salting-Out Effect on the Organic Carbon/Water Partition Ratios (KOC and KDOC) of Linear and Cyclic Volatile Methylsiloxanes: Measurements and Polyparameter Linear Free Energy Relationships

Dissolved inorganic salts influence the partitioning of organic chemicals between water and sorbents. We present new measurements of the salting-out constants (Ks) for partition ratios between water and organic carbon (KOC) and between water and dissolved organic carbon (KDOC) of three cyclic volatile methylsiloxanes (cVMS), two linear volatile methylsiloxanes (lVMS), three polychlorinated biphenyls (PCBs), and α-hexachlorocyclohexane (α-HCH). Ks, KOC, and KDOC were derived from volatilization rates of the chemicals from mixtures of water and organic carbon with varying concentrations of sodium chloride in a purge-and-trap system. KOC and KDOC values at different salinities were determined by fitting their values to reproduce observed volatilization rates using a fugacity-based multimedia model and assuming first-order kinetics for volatilization. The Ks values of cVMS and lVMS ranged from 0.16–0.76. The log KOC of cVMS and lVMS in fresh water interpolated from our measurements ranged from 5.20 to 7.36 an...

Experimental evidence of incomplete fluorescence quenching of pyrene bound to humic substances: implications for Koc measurements.

Fluorescence quenching (FQ) is extensively used for quantitative assessment of partition coefficients (KOC) of polycyclic aromatic hydrocarbons (PAHs) to natural organic materials - humic substances (HS). The presence of bound PAHs with incompletely quenched fluorescence would lead to underestimation of the KOC values measured by this technique. The goal of this work was to prove the validity of this assumption using an original experimental setup, which implied FQ measurements upon excitation into two distinct vibronically coupled electronic states. Pyrene was used as a fluorescent probe, and aquatic fulvic acid (SRFA) and leonardite humic acid (CHP) were used as the humic materials with low and high binding affinity for pyrene, respectively. Excitation of pyrene into the forbidden (S0-S1) and allowed (S0-S2) electronic states yielded two pairs of nonidentical FQ curves. This was indicative of incomplete quenching of the bound pyrene, and the divergence of the two FQ curves was much more pronounced for CHP as compared to SRFA. The two component model of fluorescence response formation was proposed to estimate the KOC values from the data obtained. The resulting pyrene KOC value for CHP (220 ± 20) g L(-1) was a factor 3 higher compared to the KOC value determined with the use of the Stern-Volmer formalism (68 ± 2) g L(-1). At the same time for aquatic FA the difference in FQ curves was almost negligible, which enables the use of the Stern-Volmer formalism for weakly interacting HS and PAHs.

Chromatographic methods in determination of the soil–water partition coefficient

ABSTRACTThe soil–water partition coefficient normalized to the soil organic carbon content (KOC) is one of essential properties governing the fate of organic chemicals in the soil–water compartment. It is a parameter of utmost importance when it comes to the removal of toxic organics in the waste water management facilities or placing a new chemical on the market. Since direct determination of KOC based on tests involving soil-sorption measurements are expensive, time-consuming, tedious, and subject to different experimental difficulties and artifacts, different alternative methods have been developed for rapid indirect experimental determination and estimation of KOC values. In the scope of the present work, we provide critical and historical overview of development and application of the main chromatographic methods, both high-performance liquid chromatography and thin-layer chromatographic techniques for indirect experimental determination of the soil–water partition coefficient as well as mobility assessment of environmentally important compounds in the soil. Chromatographic methods proved to be promising in this field, which is clearly demonstrated by their implementation in official guidelines for testing the chemicals.

Extending proteochemometric modeling for unraveling the sorption behavior of compound–soil interaction

Contamination of ground water by industrial chemicals presents a major environmental and health problem. Soil sorption plays an important role in the transport and movement of such pollutant chemicals. In this study, proteochemometric (PCM) modeling was used to unravel the origins of interactions of 17 phthalic acid esters (PAEs) against 3 soil types by predicting the organic carbon content normalized sorption coefficient (log Koc) values as a function of fingerprint descriptors of 17 PAEs and physical and textural properties of 3 soils. The results showed that PCM models provided excellent predictivity (R2=0.94, Q2=0.89,QExt2=0.85). In further validation of the model, our proposed PCM model was assessed by leave-one-compound-out (QLOCO2=0.86) and leave-one-soil-out (QLOSO2=0.86) cross-validations. The transparency of the PCM model allowed interpretation of the underlying importance of descriptors, which potentially contributes to a better understanding on the outcome of PAEs in the environment. A thorough analysis of descriptor importance revealed the contribution of secondary carbon atoms on the hydrophobicity and flexibility of PAEs as significant properties in influencing the soil sorption capacity.

Development and application of screening tools for biodegradation in water–sediment systems and soil

Two new screening-test systems for biodegradation in water–sediment systems (WSST; Water–Sediment Screening Tool) and soil (SST; Soil Screening Tool) were developed in analogy with the water-only test system OECD 301C (MITI-test).The test systems could be applied successfully to determine reproducible experimental mineralization rates and kinetics on the screening-test level for fifteen organic chemicals in water (MITI), water-sediment (WSST) and soil (SST). Substance-specific differences were observed for mineralization compared among the three test systems. Based on mineralization rate and mineralization half-life, the fifteen compounds could be grouped into four biodegradation categories: substances with high mineralization and a half-life <28days in (1) all three test systems, (2) only in the MITI test and in the WSST, (3) only in the SST, and (4) none of the test systems. The observed differences between the MITI results and the WSST and SST biodegradation rates of the compounds do not reflect their (reversible) sorption into organic matter in terms of experimental Koc values and log D values for the relevant pH range. Regarding mineralization kinetics we recommend to determine the lag-phase, mineralization half-life and mineralization rate using a 5-parameter logistic regression for degradation curves with and without lag-phase. Experimental data obtained with the WSST and the SST could be verified by showing good agreement with biodegradation data from databases and literature for the majority of compounds tested. Thus, these new screening-tools for water–sediment and soil are considered suitable to determine sound and reliable quantitative mineralization data including mineralization kinetics in addition to the water-only ready biodegradability tests according to OECD 301.

Shifts in production of perfluoroalkyl acids affect emissions and concentrations in the environment of the Xiaoqing River Basin, China

Perfluoroalkyl acids (PFAAs) have been widely used in surfactant applications, especially as processing acids for fluoropolymer production. This study provides an analysis of sources of certain PFAAs emitted from the intensive fluoropolymer facilities in the Xiaoqing River Basin of China. Concentrations of perfluorooctanoic acid (PFOA) as great as 0.97mg/L in surface water and 10.5μg/g dry weight in surface sediment have been detected near the effluent of one facility (F1) that produces polytetrafluoroethylene (PTFE) and other fluoropolymers with massive capacity. With the great emission of PFAAs to water in natural conditions, the log KOC values decreased for short-chain PFCAs. Mass loads of PFAAs indicated that emissions of PFAAs from other facilities or sources were much less than those from F1, which emitted 174kg/d of PFAAs including 159kg/d of PFOA to the rivers. Even though production and emissions of PFOA have been strictly controlled in other countries since 2006, production of PFOA as well as several other fluoropolymers that use PFOA as processing aids has been increasing at F1 in recent years. We recommended that production shift should be taken into consideration in PFOA elimination actions.

분자량 크기별 토양 휴믹산(HA)의 구조적 특성 및 페난트렌 흡착 반응특성 비교

A sample of soil humic acid (HA) was divided by ultrafiltration (UF) into five fractions of different molecular size (UF1: > 300, UF2: 100~300, UF3: 30~100, UF4: 10~30, UF5: 1~10 kilodaltons). Apparent average molecular weight (Mw) of the HA fractions were measured using high performance size exclusion chromatography (HPSEC), and the chemical and structural properties of the five HA fractions were characterized by elemental compositions (H/C, O/C and w ((2O + 3NH)/ C)) and ultraviolet-visible absorption ratios (SUVA, A4/6). The organic carbon normalized-sorption coefficients (Koc) for the binding of phenanthrene to the HA fractions were determined by fluorescence quenching and relationship between the sorption coefficients and structural characteristics of the HA fractions were investigated. The elemental analysis and UV-vis spectral data indicated that the HA fractions with higher molecular weights have grater aliphatic character and lower contents of oxygen, while the HA fractions with lower molecular size have greater aromatic character and molecular polarity that correspond to greater SUVA and internal oxidation values (w). The log Koc values (L/kg C) were gradual increased from 4.45 for UF5 to 4.87 for UF1. The correlation study between the structural descriptors of the HA fractions and log Koc values of phenanthrene show that the magnitude of Koc values positively correlated with and H/C, while negatively correlated with the independent descriptors of the O/C, w, SUVA and A4/6.

Do constructed wetlands in grass strips reduce water contamination from drained fields?

This study evaluates the efficiency of two small constructed wetlands installed in the regulatory grass strips between a drained plot and a river. The observed nitrate removal efficiencies were independent of the season or type of constructed wetland and ranged from 5.4 to 10.9% of the inlet amounts. The pesticide mass budgets ranged from −618.5 to 100%, depending on the molecule. The negative efficiencies were attributed to runoff and remobilization. In contrast, the highest efficiencies were associated with pesticides with high Koc and low DT50 (half-life) values, suggesting sorption and degradation. However, the effectiveness of these wetlands is limited for pesticides with low Koc or high DT50 values; thus, the use of these molecules must be reduced. Increasing the number of these small, inexpensive and low-maintenance wetlands in the agricultural landscape would reduce the level of water pollution whilst preserving the extent of cultivated land, but their long-term effectiveness should be evaluated.

Extended sorption partitioning models for pesticide leaching risk assessments: Can we improve upon the koc concept?

Models used to assess leaching of pesticides to groundwater still rely on the sorption koc value, even though its limitations have been known for several decades, especially for soils of low organic carbon content (i.e. subsoils). This is mainly because the general applicability of any improved model approach that is also simple enough to use for regulatory purposes has not been demonstrated. The objective of this study was to test and compare alternative models of sorption that could be useful in pesticide risk assessment and management. To this end, a database containing the results of batch sorption experiments for pesticides was compiled from published studies in the literature, which placed at least as much emphasis on measurements in subsoil horizons as in topsoil. The database includes 785 data entries from 34 different published studies and for 21 different active substances. Overall, the apparent koc value, koc(app), roughly doubled as the soil organic carbon content decreased by a factor of ten. Nevertheless, in nearly half of the individual datasets, a constant koc value proved to be an adequate model. Further analysis showed that significant increases in koc(app) in subsoil were found primarily for the more weakly adsorbing compounds (koc values<ca. 100–200Lkg−1) and that sorption to clay in loamy and clayey-textured subsoil horizons was the main cause. Tests with the MACRO model demonstrated that sorption to clay minerals may significantly affect the outcome of regulatory exposure and risk assessments for leaching to groundwater. The koc concept currently used in leaching models should therefore be replaced by an alternative approach that gives a more realistic representation of pesticide sorption in subsoil. The two alternative models tested in this study appear to have widespread applicability and are also simple enough to parameterize for this purpose.

The kinetics of solid-phase microextraction measured for freshly added and aged hydrophobic compounds in two different soils

The proper choice of exposure times is critical if the freely dissolved concentration of chemicals in soil porewater is to be measured via the equilibrium solid-phase microextraction (SPME) as the times to equilibrium may vary depending on compound and soil properties. To reveal the effects of compound hydrophobicity, ageing and soil organic matter content on times to equilibrium, the SPME uptake was measured for five freshly added and aged hydrophobic organic compounds (phenanthrene, pyrene, lindane, p,p′-DDT and polychlorinated biphenyl (PCB) 153) in two contrasted soils (arable and forest soil). The tested compound-soil systems behaved kinetically different. Longer equilibrium times were observed with increasing hydrophobicity of compounds for aged compared to freshly added chemicals and for the forest soil in comparison to the arable soil. The calculated soil–porewater partition coefficients (i.e. sorption coefficients, Kd) of chemicals differed between soil types mainly due to various organic carbon (OC) contents as evidenced by the comparable Koc values (i.e. Kd values normalised to soil OC content). Similar Koc values were also found with the various extent of ageing, indicating that both the freshly added and aged compounds linearly partitioned between the soil organic matter and porewater. Our results suggest that, for a respective compound, variations in equilibrium times may be expected depending upon the residence time and the organic matter content in soil where the longest equilibrium times seems to appear for a combination of aged compounds and high organic soils. With regard to this outcome, the effect of the level of sample depletion due to the SPME extraction (LDSPME) on equilibrium times was assessed. At LDsSPME of up to 10%, equilibrium times increases linearly with LDsSPME for p,p′-DDT and PCB 153. For phenanthrene (LDSPME<10%), and for lindane and pyrene (1.2% < LDSPME > 40%), no clear relationships were observed.

Pesticide sorption and leaching potential on three Hawaiian soils

On the Hawaiian Islands, groundwater is the principal source of potable water and contamination of this key resource by pesticides is of great concern. To evaluate the leaching potential of four weak acid herbicides [aminocyclopyrachlor, picloram, metsulfuron-methyl, biologically active diketonitrile degradate of isoxaflutole (DKN)] and two neutral non-ionizable herbicides [oxyfluorfen, alachlor], their sorption coefficients were determined on three prevalent soils from the island of Oahu. Metsulfuron-methyl, aminocylcopyrachlor, picloram, and DKN were relatively low sorbing herbicides (Koc = 3–53 mL g−1), alachlor was intermediate (Koc = 120–150 mL g−1), and oxyfluorfen sorbed very strongly to the three soils (Koc > 12,000 mL g−1). Following determination of Koc values, the groundwater ubiquity score (GUS) indices for these compounds were calculated to predicted their behavior with the Comprehensive Leaching Risk Assessment System (CLEARS; Tier-1 methodology for Hawaii). Metsulfuron-methyl, aminocyclopyrachlor, picloram, and DKN would be categorized as likely leachers in all three Hawaiian soils, indicating a high risk of groundwater contamination across the island of Oahu. In contrast, oxyfluorfen, regardless of the degradation rate, would possess a low and acceptable leaching risk due to its high sorption on all three soils. The leaching potential of alachlor was more difficult to classify, with a GUS value between 1.8 and 2.8. In addition, four different biochar amendments to these soils did not significantly alter their sorption capacities for aminocyclopyrachlor, indicating a relatively low impact of black carbon additions from geologic volcanic inputs of black carbon. Due to the fact that pesticide environmental risks are chiefly dependent on local soil characteristics, this work has demonstrated that once soil specific sorption parameters are known one can assess the potential pesticide leaching risks.

Sorption–desorption of imidacloprid onto a lacustrine Egyptian soil and its clay and humic acid fractions

Sorption–desorption of the insecticide imidacloprid 1-[(6-chloro-3-pyridinyl)-methyl]-N-nitro-2-imidazolidinimine onto a lacustrine sandy clay loam Egyptian soil and its clay and humic acid (HA) fractions was investigated in 24-h batch equilibrium experiments. Imidacloprid (IMDA) sorption–desorption isotherms onto the three sorbents were found to belong to a non-linear L-type and were best described by the Freundlich model. The value of the IMDA adsorption distribution coefficient, Kdads, varied according to its initial concentration and was ranged 40–84 for HA, 14–58 for clay and 1.85–4.15 for bulk soil. Freundlich sorption coefficient, Kfads, values were 63.0, 39.7 and 4.0 for HA, clay and bulk soil, respectively. The normalized soil Koc value for imidacloprid sorption was ∼800 indicating its slight mobility in soils. Nonlinear sorption isotherms were indicated by 1/nads values <1 for all sorbents. Values of the hysteresis index (H) were <1, indicating the irreversibility of imidacloprid sorption process with all tested sorbents. Gibbs free energy (ΔG) values indicated a spontaneous and physicosorption process for IMDA and a more favorable sorption to HA than clay and soil. In conclusion, although the humic acid fraction showed the highest capacity and affinity for imidacloprid sorption, the clay fraction contributed to approximately 95% of soil-sorbed insecticide. Clay and humic acid fractions were found to be the major two factors controlling IMDA sorption in soils. The slight mobility of IMDA in soils and the hysteresis phenomenon associated with the irreversibility of its sorption onto, mainly, clay and organic matter of soils make its leachability unlikely to occur.