Changes in pesticide adsorption with time at high soil to solution ratios

Conversion of fungicide cyprodinil to salts with organic acids: preparation, characterization, advantages

BACKGROUND

As an effective strategy to improve the basic properties of drugs, salt formation was less used in the field of pesticides than the medicine field. It is worth trying to improve the inherent shortcomings of cyprodinil (high K_ow values; polymorphism) in this way to enhance its practicality.

RESULTS

Eight cyprodinil salts (CYP-Salts) were prepared. The properties of CYP-Salts, including solubility in various solvents, polymorphic behavior, soil absorption, photolysis in aquatic water, in vitro fungicidal activity and curative activity, were assessed. It was observed that compared with those of cyprodinil, CYP-Salts had lower soil adsorption, while also having lower log K_ow values and could be more easily photodegraded in water. That is, CYP-Salts have lower impacts on water bodies and aquatic organisms than cyprodinil. Three CYP-Salts showed higher in vitro antifungal activities and curative activity. CYP-Salts have enhanced practicality, as they could avoid possible agglomeration caused by recrystallization.

CONCLUSION

Salt forming enhanced the properties of Cyprodinil in many aspects. CYP-Salts may potentially become a better substitute for cyprodinil. This study offers a more economical and effective strategy to prepare better alternatives to existing fungicides. © 2022 Society of Chemical Industry.

Pesticide sorption and mitigation efficiency of a detention pond in a Champagne vineyard catchment

Detention ponds (DPs) are used to reduce the pesticide inputs from runoff to surface water. This study aimed to assess the role of the sorption process in the mitigation of a DP made up of four successive units and built at the outlet of a vineyard catchment in Champagne (France) to treat runoff waters. Sorption kinetics and isotherms were studied for four pesticides with contrasting properties, cyazofamid (CYA), fludioxonil (FLX), fluopicolide (FLP) and oryzalin (ORY), in the presence of copper in sediments and four emergent macrophyte roots and rhizomes sampled in the DP units 2 (photodegradation) and 3 (phytoremediation). The adsorption equilibrium time (from 24 to 96 h) was less than the hydraulic residence times in the two units (6 and 18 days on average) between November 2016 and November 2017. Sorption equilibrium could then be reached in situ in 85 % of cases. The K_d coefficients of the four pesticides were overall greater in plant roots (14–6742 L kg⁻¹) than in sediments (6–163 L kg⁻¹) because of their affinity for organic matter and the molecular and porous structure of the plant matrices. Typha latifolia and Iris pseudacorus exhibited greater K_d coefficients than Mentha aquatica and Phragmites australis, probably due to their greater specific surface area. The pesticide adsorption capacity in sediments and in T. latifolia and I. pseudacorus roots (ORY ≥ FLX > CYA > FLP) was linked to their K_ow. The estimated total annual amounts of the four pesticides adsorbed in situ were determined to be 1236 mg for unit 2 and 1570 mg for unit 3. The four plants improved the removal efficiency of the unit 3 by 33%. Thus, the establishment of suitable and effective plants should be promoted to optimize sorption processes and DP efficiency in reducing water pollution.

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Pesticide sorption on various substrates of a vineyard detention pond was assessed.

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The measured equilibrium time was less than the in situ hydraulic residence time.

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Plant roots and rhizomes showed greater pesticide adsorption capacity than sediments.

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Sorption capacity was higher on cattail and iris than on mint and reed.

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Affinity of selected pesticides for roots and sediments was related to their Kow.

NCs-Delivered Pesticides: A Promising Candidate in Smart Agriculture

Pesticides have been used extensively in the field of plant protection to maximize crop yields. However, the long-term, unmanaged application of pesticides has posed severe challenges such as pesticide resistance, environmental contamination, risk in human health, soil degradation, and other important global issues. Recently, the combination of nanotechnology with plant protection strategies has offered new perspectives to mitigate these global issues, which has promoted a rapid development of NCs-based pesticides. Unlike certain conventional pesticides that have been applied inefficiently and lacked targeted control, pesticides delivered by nanocarriers (NCs) have optimized formulations, controlled release rate, and minimized or site-specific application. They are receiving increasing attention and are considered as an important part in sustainable and smart agriculture. This review discussed the limitation of traditional pesticides or conventional application mode, focused on the sustainable features of NCs-based pesticides such as improved formulation, enhanced stability under harsh condition, and controlled release/degradation. The perspectives of NCs-based pesticides and their risk assessment were also suggested in this view for a better use of NCs-based pesticides to facilitate sustainable, smart agriculture in the future.

Comparison of soil sorption parameters of pesticides measured by batch and centrifugation methods using an andosol

We compared the soil sorption coefficient (K _d) measured by batch and centrifugation methods using a Japanese andosol and ten pesticides. Although the K _d values measured by both methods increased with time, those obtained via the batch method tended to be higher during the test period. The difference in K _d values between the two methods affected pesticide concentrations estimated in the soil solution, and the results estimated using K _d values obtained via the batch method underestimated the observed trends.

Relationship between geosorbent properties and field-based partition coefficients for pesticides in surface water and sediments of selected agrarian catchments: Implications for risk assessment

Studies on pesticide behavior, adsorption-likelihood, and bioavailability vis-a-vis geosorbent properties and seasons, are critical for understanding pesticide-fate and risks in pesticide-prone environments. We examined the relationship between geosorbent profiles of sediments (percentage sand, silt, clay, organic carbon content) across seasons and occurrence of pesticide residues in surface water and sediment of agricultural catchments at Owan, Ogbesse and Illushi communities of Edo State, Nigeria. Pesticide concentrations were measured monthly in samples of surface water and sediments across the selected sites for 18-months. Pesticide behavior and sorption-likelihoods were examined using partition coefficients K_d (sediment-water coefficient), K_oc (sediment-water coefficient normalized for organic carbon) and Log K_ow (octane-water coefficient); the relationship between K_d and K_oc was also examined. Results of the principal component analysis (PCA) indicated that pesticide levels in sediment and surface water were positively associated with the rainy season, total organic content (TOC), percentage silt and clay in sediment. Field-derived pesticide partition coefficients (K_d < 100 and log K_oc < 3) indicated that pesticide species were largely mobile and less likely to be retained in sediments by adsorption. As such, pesticides irrespective of solubility would end up in surface water, increasing risks for pelagic biota and humans sourcing river water for domestic use. Values of Log K_ow indicate that organochlorines including DDT, dieldrin, endrin and heptachlor epoxide portend significant bioaccumulation risks to humans and biota across sites. The relationship between K_d and K_oc for each site fitted into a quadratic model; it depicted a biphasic behavior of pesticide adsorption and desorption to sediments revealing that concentration of organic carbon across study sites was a limiting factor determining the extent of pesticide adsorption. This study demonstrates that understanding pesticide mobility using field-based partition coefficients could give a clearer picture of pesticide risks to biota and human populations.

Is pesticide sorption by constructed wetland sediments governed by water level and water dynamics?

Constructed wetlands (CWs) are used to reduce the pesticide inputs from tile drainage or run-off to surface water. Their effectiveness appears variable and remains to be better characterized and understood. The aim of this study was to assess the influences of two hydraulic parameters (i.e., dynamics and water level) on the sorption process occurring in CWs. Then, two solid/liquid ratios were studied (1/1 and 1/5) to mimic the water level variation in the field, and two agitation speeds were used (none and gentle agitation) to simulate different water dynamics (stagnation and flow pass, respectively). Sorption kinetics and isotherms were obtained for four pesticides with contrasting properties. The pesticide adsorption coefficients were classified as follows: boscalid (BSC) > cyproconazole (CYP) > isoproturon (IPU) ∼ dimethachlor (DMT) at any ratio or agitation, in agreement with their water solubilities and K _ow values. The effect of the solid/liquid ratio was evidenced for all conditions. Indeed, the adsorption equilibrium time was reached more quickly for the 1/1 ratio (24-72 h) than for the 1/5 ratio (96-120 h). In addition, the adsorption coefficients (K _f^ads) were larger for the 1/1 ratio (1.8-11.2 L kg^-1) than for the 1/5 ratio (1.0-5.9 L kg^-1). The agitation effect was more evidenced for the 1/5 ratio and for the more hydrophobic molecules, such as BSC and CYP, for which adsorption equilibrium time was never reached with agitation (>120 h), while it was reached at 96 h without agitation. Moreover, the K _f^ads values were larger with agitation than without agitation for BSC and CYP, whereas they were similar for the two agitations for IPU and DMT. Our results demonstrated that the hydrodynamic function of CWs could influence pesticide sorption with variable effects according to the molecular properties and consequently influence the mitigation effect of CWs throughout the year.

Environmental fate of nanopesticides: durability, sorption and photodegradation of nanoformulated clothianidin

A lot of research efforts are currently dedicated to the development of nano-enabled agrochemicals. Knowledge about their environmental behaviour is however scarce, which impedes the assessment of the new risk and benefits relative to currently used agrochemicals. With the aim to advance our understanding of the fate of nanopesticides in the environment and support the development of robust exposure assessment procedures, the main objectives of the study were to (i) investigate the extent to which three nanoformulations can affect the photodegradation and sorption of the insecticide clothianidin, and (ii) evaluate various approaches to estimate durability, a key parameter for the exposure assessment of nanopesticides. The nanoformulations increased the photodegradation half-life in water by a maximum of 21% relative to the conventional formulation. Sorption to soil was investigated by two methods and over time, and results show that sorption was increased by up to 51% and 10%, relative to unformulated clothianidin and the commercial formulation, respectively. Our results generally indicate that nanoformulations may have a greater impact on the fate of pesticide active ingredients than commercial formulations. It is important to note however that differences in fate parameters were generally very moderate, including in realistic worst-case conditions (high pesticide concentration and ionic strength). Our results collectively suggest that clothianidin was rapidly released from the nanocarrier systems and that the durability of the three nanoformulations would be short in water as well as in soil environments (including under realistic soil to solution ratio). The durability of nanoformulations after their application in the environment is an essential parameter that needs to be characterised for the development as well as for the evaluation of nano-enabled agrochemicals. This study illustrates how performances of nano-enabled products can be critically benchmarked against existing products to support an objective assessment of new environmental risks and benefits. In this context, the fate of the nanocarrier system is of great interest and should be the topic of further research.

Influence of commercial formulation on the sorption and leaching behaviour of propyzamide in soil

Experiments compared sorption and leaching behaviour for the herbicide propyzamide when applied to two soils either as technical material or in the commercial formulation Kerb® Flo. Sorption was investigated in batch systems as well as using a centrifugation technique to investigate changes in pesticide concentration in soil pore water over incubation periods of up to 28days. Studies with small soil columns compared leaching of technical and formulated pesticide for irrigation events (6 pore volumes) 1, 7, 14, 21 and 28days after treatment. There were no differences in sorption of technical and formulated propyzamide when measured by batch systems. Sorption of technical material was significantly greater than that of formulated pesticide in sandy loam (p<0.05), but not in sandy silt loam when measured by centrifugation of soil incubated at field capacity. Partition coefficients measured by batch and centrifugation methods were similar after 1day and those measured by centrifugation increased by factors of 5.3 to 7.5 over the next 4weeks. The mass of propyzamide leached from soil columns ranged between 1.1±0.33% and 14.4±3.2% of the applied amount. For all time intervals and in both soils, the mass of propyzamide leached was significantly greater (two-sided t-tests, p<0.001) for the formulated product than for the technical material. Leached losses decreased consistently with time in the sandy loam soil (losses after 28days were 14-17% of those after 1day), but with less consistency in the sandy silt loam. There was a highly significant effect of formulation on the leaching of propyzamide through soil (two-way ANOVA, p<0.001) as well as highly significant effects of time and soil type (p<0.001). Results are consistent with modelling studies where leaching from commercial products in the field could only be simulated by reducing sorption coefficients relative to those measured with technical material in the laboratory.

Adsorption and degradation of phenoxyalkanoic acid herbicides in soils: A review

The primary aim of the present review on phenoxyalkanoic acid herbicides-2-(2,4-dichlorophenoxy) acetic acid (2,4-D), 2-(4-chloro-2-methylphenoxy) acetic acid (MCPA), (2R)-2-(2,4-dichlorophenoxy) propanoic acid (dichlorprop-P), (2R)-2-(4-chloro-2-methylphenoxy) propanoic acid (mecoprop-P), 4-(2,4-dichlorophenoxy) butanoic acid (2,4-DB), and 4-(4-chloro-2-methylphenoxy) butanoic acid (MCPB)-was to compare the extent of their adsorption in soils and degradation rates to assess their potential for groundwater contamination. The authors found that adsorption decreased in the sequence of 2,4-DB > 2,4-D > MCPA > dichlorprop-P > mecoprop-P. Herbicides are predominantly adsorbed as anions-on organic matter and through a water-bridging mechanism with adsorbed Fe cations-and their neutral forms are adsorbed mainly on organic matter. Adsorption of anions of 2,4-D, MCPA, dichlorprop-P, and mecoprop-P is inversely correlated with their lipophilicity values, and modeling of adsorption of the compounds based on this relationship is possible. The predominant dissipation mechanism of herbicides in soils is bacterial degradation. The contribution of other mechanisms, such as degradation by fungi, photodegradation, or volatilization from soils, is much smaller. The rate of bacterial degradation decreased in the following order: 2,4-D > MCPA > mecoprop-P > dichlorprop-P. It was found that 2,4-D and MCPA have the lowest potential for leaching into groundwater and that mecoprop-P and dichlorprop-P have slightly higher potential. Because of limited data on adsorption and degradation of 2,4-DB and MCPB, estimation of their leaching potential was not possible.

Analysing the fate of nanopesticides in soil and the applicability of regulatory protocols using a polymer-based nanoformulation of atrazine

For the first time, regulatory protocols defined in the OECD guidelines were applied to determine the fate properties of a nanopesticide in two agricultural soils with contrasting characteristics. The nanoformulation studied had no effect on the degradation kinetics of atrazine indicating that (1) the release of atrazine from the polymer nanocarriers occurred rapidly relative to the degradation kinetics (half-lives 36-53 days) and/or that (2) atrazine associated with the nanocarriers was subject to biotic or abiotic degradation. Sorption coefficients, derived from a batch and a centrifugation technique at a realistic soil-to-solution ratio, were higher for the nanoformulated atrazine than for the pure active ingredient. Results indicate that the nanoformulation had an effect on the fate of atrazine. However, since the protocols applied were designed to assess solutes, conclusions about the transport of atrazine loaded onto the nanocarriers should be made extremely cautiously. The centrifugation method applied over time (here over 7 days) appears to be a useful tool to indirectly assess the durability of nanopesticides under realistic soil-to-solution ratios and estimate the period of time during which an influence on the fate of the active ingredient may be expected. More detailed investigations into the bioavailability and durability of nanopesticides are necessary and will require the development of novel methods suitable to address both the "nano" and "organic" characteristics of polymer-based nanopesticides.

Soil adsorption studies of a rice herbicide, cyhalofop-butyl, in two texturally different soils of India

The ability of herbicides to be adsorbed by the soil and sediment and their tendency to be desorbed are some of the most important factors affecting soil and water contamination. Therefore, a sorption study was conducted to evaluate the adsorption of cyhalofop-butyl, butyl (2R)-2-[4-(4-cyano-2-fluorophenoxy) phenoxy] propanoate, in the sandy clay loam and clayey soils using a batch equilibrium method. The adsorption of cyhalofop-butyl was found positively related with the clay and organic carbon content. Freundlich constants (Kf) of cyhalofop-butyl in the clayey and sandy clay loam were found to be 13.39 and 2.21, respectively. Sorption coefficients (Koc) and distribution coefficients (Kd) were found to be 265.38 and 2,092.79, and 1.38 and 11.48, for sandy clay loam and clayey soils, respectively. The adsorption isotherm suggested a relatively higher affinity of cyhalofop-butyl to the adsorption sites at low equilibrium concentrations. The low value of the soil organic carbon partition coefficient (Koc) of cyhalofop-butyl in the sandy loam soil suggested its weaker adsorption in soil and thus increased its risk of mobility into water sources; hence, it should be used judiciously to prevent groundwater contamination.

Partition, sorption and structure activity relation study of dialkoxybenzenes that modulate insect behavior

Some dialkoxybenzenes are promising new insect control agents. These compounds mimic naturally occurring odorants that modulate insect behavior. Before applying these compounds, however, their persistence and biodegradability at the application site and in the environment should be understood. The fate of organic compounds in the environment is a complex phenomenon which is influenced by many processes such as sorption to soil components, sedimentation, volatilization, and uptake by plants, as well as biotic and abiotic chemical degradation. In this study, the octanol-water partition coefficient, volatility and sorption on soil components (sand, clay and organic matter) of selected dialkoxybenzenes as well as structure activity relationships with regard to partition, volatility and sorption were investigated. Additionally, calculations of partition, molar volume and molecular surface areas were done, to understand structure-activity relationships of the physical properties.

Estimating the toxicity of the weak base carbendazim to the earthworm (Eisenia fetida) using in situ pore water concentrations in different soils

Both sorption by soil and uptake by organisms of ionizable organic pollutants depend on their speciation (i.e., neutral and ionized forms); thus, the bioavailability of ionizable organic pollutants is more complicated than that of neutral organic pollutants in soil. The toxicity of the weak base carbendazim to earthworms (Eisenia fetida) was estimated using Soxhlet extracted concentrations (C(SE)), an excess of water extracted concentrations (C(EEW)), ex situ pore water concentrations (C(EPW)) and in situ pore water concentrations (C(IPW)) in different soils. The results indicated that the median lethal concentrations (LC50) calculated from C(SE) ranged from 2.32 to 34.0 mg kg(-1) in the five tested soils and the coefficient of variation (CV) of LC50s was 69.8%. When the LC50 was calculated from the C(EEW), C(EPW) and C(IPW), the variability of the LC50 gradually became smaller in these soils, with the CVs of LC50s being 58.1%, 50.6% and 38.6% (for C(EEW), C(EPW) and C(IPW), respectively). However, the LC50 based on C(IPW) in strongly acidic soil (where carbendazim partially exists as ionized form) was significantly lower than in other soils, and the values of the LC50 calculated from the in situ pore water concentrations were approximately equal. The results indicated that the in situ pore water concentration could be used to estimate the toxicity of carbendazim in different soils especially in those soils where carbendazim exists in the neutral form.

Assessment of phenylurea herbicides sorption on various Mediterranean soils affected by irrigation with wastewater

The retention values of two herbicides, chlorotoluron and isoproturon, in five Mediterranean soils were assessed by two different approaches, a dynamic method, using a batch technique (BT) and a static method, using a soil saturated paste (SP). The SP method led in all cases to lower herbicide sorption when compared with BT, although pesticide distribution constants from both methods were linearly related for the set of used soils (R(2)⩾0.99) showing that both methods similarly reflected the behaviour of the different soils. Low-quality water, evaluated by employing recycled urban wastewater, did not modify herbicide sorption when compared with high quality water, in any soil and with any method.

Sorption and dissipation of aged metolachlor residues in eroded and rehabilitated soils

BACKGROUND

Sorption and dissipation of aged metolachlor were characterized in rehabilitated and eroded prairie soils using sequential batch slurry (conventional) and accelerated solvent extraction (ASE).

RESULTS

In spite of an almost twofold difference in soil organic carbon (OC) content, S-metolachlor sorption coefficients (K(d)) and dissipation rates (DT(50)) were the same in soils from different landscape positions within an eroded landform. Soil was moved within the landform to increase productivity. In areas receiving topsoil addition, S-metolachlor K(d) was higher and DT(50) was longer than in eroded areas. The efficiency of extraction was higher for ASE than for conventional extractions. No consistent aging effect on K(d) was observed. Mineralization in 8 weeks accounted for < 10% of the applied metolachlor.

CONCLUSION

The results of this laboratory study support a field dissipation study. Both showed that S-metolachlor has the same retention and dissipation rate throughout an eroded landform, which was not expected owing to the large variability in soil properties, including OC concentrations. Altering soil properties by adding topsoil increased metolachlor sorption and persistence. The method of extraction (conventional versus ASE) affected calculated sorption coefficients and dissipation rates. In all cases, groundwater ubiquity scores (GUSs) categorized metolachlor as having intermediate mobility.

Sorption behavior of EE2 on soils subjected to different long-term organic amendments

The transport and fate of hydrophobic organic contaminants in the environment involve complex phenomena that are influenced by many processes that include sorption by soil components. Sorption behavior of EE2 molecules onto different soil samples was studied and results correlated with the content and type of organic matter present. The highest K value, among all soils presented in this study, was obtained for soil fertilized with compost (1.22) which presented the highest organic carbon content. Also the sorption behavior depends greatly on the soil specific organic matter characteristics. A strong positive correlation was observed between aromatic and carboxylic units and K(OC) values. The results also suggested an association of the EE2 aromatic nuclei face to face with the surface and/or another EE2 molecule and also sorbent-sorbate interactions due to hydrogen or covalent bonding, likely to occur due to the presence of phenolic function at C-3 and hydroxyl function at C-17 of the EE2 molecules that can react with carboxylic functional groups of soil organic matter. The stronger EE2 sorbs to soil organic matter lower is the leaching into drinking water resources and runoff to rivers and surface water, minimizing its residual toxicity.

Aging of methabenzthiazuron, imidacloprid, and N,N-dimethylsulfamide in silty soils and effects on sorption and dissipation

Differences in soil properties can influence the fate of plant protection agents in the environment. The present study aims to investigate the sorption behavior and related aging processes of imidacloprid (IMI; insecticide), methabenzthiazuron (MBT; herbicide), and N,N-dimethylsulfamide (DMSA; degradate of the fungicide tolylfluanid) in six soils of silty texture but otherwise varying properties. The sorption behavior of these ¹⁴C-labeled compounds exhibiting different physicochemical properties was characterized by applying a three-step sequential extraction procedure. After 119 d, MBT revealed strongest sorption (K'(tot) 47.4-200.4 L/kg), followed by IMI (K'(tot) 11.7-30.6 L/kg), and DMSA with K'(tot) close to zero. Aged sorption factors (AFs) were calculated to characterize aging processes over time exhibiting a 2.6-3.5-fold (IMI), a 1.8-4.5-fold (MBT), and no (DMSA) increase of sorbed amounts within 84 d. Sorption and aging varied widely in the group of silty soils, which differed with respect to organic matter content, C/N-ratio, and microbial soil parameters. The time-dependent increase of adsorption of MBT and IMI was more pronounced in those soils that had a lower organic carbon and low microbial biomass content. Concomitantly, MBT and IMI degradation decelerated, presumably because of aged sorption at inner binding sites leading to a lower accessibility. In contrast, in the soils with a higher organic carbon content a strong initial (but later still reversible) sorption of MBT and IMI, occurring presumably at outer surface sites, reduced the extent of time-dependent diffusion toward inner binding sites.

Determining the influence of rainfall patterns and carbendazim on the surface activity of the earthworm Lumbricus terrestris

Carbendazim is highly toxic to earthworms and is used as a standard control substance when running field-based trials of pesticides, but results using carbendazim are highly variable. In the present study, impacts of timing of rainfall events following carbendazim application on earthworms were investigated. Lumbricus terrestris were maintained in soil columns to which carbendazim and then deionized water (a rainfall substitute) were applied. Carbendazim was applied at 4 kg/ha, the rate recommended in pesticide field trials. Three rainfall regimes were investigated: initial and delayed heavy rainfall 24 h and 6 d after carbendazim application, and frequent rainfall every 48 h. Earthworm mortality and movement of carbendazim through the soil was assessed 14 d after carbendazim application. No detectable movement of carbendazim occurred through the soil in any of the treatments or controls. Mortality in the initial heavy and frequent rainfall was significantly higher (approximately 55%) than in the delayed rainfall treatment (approximately 25%). This was due to reduced bioavailability of carbendazim in the latter treatment due to a prolonged period of sorption of carbendazim to soil particles before rainfall events. The impact of carbendazim application on earthworm surface activity was assessed using video cameras. Carbendazim applications significantly reduced surface activity due to avoidance behavior of the earthworms. Surface activity reductions were least in the delayed rainfall treatment due to the reduced bioavailability of the carbendazim. The nature of rainfall events' impacts on the response of earthworms to carbendazim applications, and details of rainfall events preceding and following applications during field trials should be made at a higher level of resolution than is currently practiced according to standard International Organization for Standardization protocols.

Sorption-desorption behavior of atrazine on soils subjected to different organic long-term amendments

Sorption of atrazine on soils subjected to three different organic amendments was measured using a batch equilibrium technique. A higher K(F) value (2.20 kg(-1)(mg L(-1))(-N)) was obtained for soil fertilized with compost, which had a higher organic matter (OM) content. A correlation between the K(FOC) values and the percentage of aromatic carbon in OM was observed. The highest K(FOC) value was obtained for the soil with the highest aromatic content. Higher aromatic content results in higher hydrophobicity of OM, and hydrophobic interactions play a key role in binding of atrazine. On the other hand, the soil amended with farmyard manure had a higher content of carboxylic units, which could be responsible for hydrogen bonding between atrazine and OM. Dominance of hydrogen bonds compared to hydrophobic interactions can be responsible for the lower desorption capacity observed with the farmyard manure soil. The stronger hydrogen bonding can reduce the leaching of atrazine into drinking water resources and runoff to rivers and other surface waters.

Bioavailability of xenobiotics in the soil environment

It is often presumed that all chemicals in soil are available to microorganisms, plant roots, and soil fauna via dermal exposure. Subsequent bioaccumulation through the food chain may then result in exposure to higher organisms. Using the presumption of total availability, national governments reduce environmental threshold levels of regulated chemicals by increasing guideline safety margins. However, evidence shows that chemical residues in the soil environment are not always bioavailable. Hence, actual chemical exposure levels of biota are much less than concentrations present in soil would suggest. Because "bioavailability" conveys meaning that combines implications of chemical sol persistency, efficacy, and toxicity, insights on the magnitude of a chemicals soil bioavailability is valuable. however, soil bioavailability of chemicals is a complex topic, and is affected by chemical properties, soil properties, species exposed, climate, and interaction processes. In this review, the state-of-art scientific basis for bioavailability is addressed. Key points covered include: definition, factors affecting bioavailability, equations governing key transport and distributive kinetics, and primary methods for estimating bioavailability. Primary transport mechanisms in living organisms, critical to an understanding of bioavailability, also presage the review. Transport of lipophilic chemicals occurs mainly by passive diffusion for all microorganisms, plants, and soil fauna. Therefore, the distribution of a chemical between organisms and soil (bioavailable proportion) follows partition equilibrium theory. However, a chemical's bioavailability does not always follow partition equilibrium theory because of other interactions with soil, such as soil sorption, hysteretic desorption, effects of surfactants in pore water, formation of "bound residue", etc. Bioassays for estimating chemical bioavailability have been introduced with several targeted endpoints: microbial degradation, uptake by higher plants and soil fauna, and toxicity to organisms. However, there bioassays are often time consuming and laborious. Thus, mild extraction methods have been employed to estimate bioavailability of chemicals. Mild methods include sequential extraction using alcohols, hexane/water, supercritical fluids (carbon dioxide), aqueous hydroxypropyl-beta-cyclodextrin extraction, polymeric TENAX beads extraction, and poly(dimethylsiloxane)-coated solid-phase microextraction. It should be noted that mild extraction methods may predict bioavailability at the moment when measurements are carried out, but not the changes in bioavailability that may occur over time. Simulation models are needed to estimate better bioavailability as a function of exposure time. In the past, models have progressed significantly by addressing each group of organisms separately: microbial degradation, plant uptake via evapotranspiration processes, and uptake of soil fauna in their habitat. This approach has been used primarily because of wide differences in the physiology and behaviors of such disparate organisms. However, improvement of models is badly needed, Particularly to describe uptake processes by plant and animals that impinge on bioavailability. Although models are required to describe all important factors that may affect chemical bioavailability to individual organisms over time (e.g., sorption/desorption to soil/sediment, volatilization, dissolution, aging, "bound residue" formation, biodegradation, etc.), these models should be simplified, when possible, to limit the number of parameters to the practical minimum. Although significant scientific progress has been made in understanding the complexities in specific methodologies dedicated to determining bioavailability, no method has yet emerged to characterized bioavailability across a wide range of chemicals, organisms, and soils/sediments. The primary aim in studying bioavailability is to define options for addressing bioremediation or environmental toxicity (risk assessment), and that is unlikely to change. Because of its importance in estimating research is needed to more comprehensively address the key environmental issue of "bioavailability of chemicals in soil/sediment."

The kinetics of sorption by retarded diffusion into soil aggregate pores

This study investigates time-dependent sorption of pesticides in soil aggregates. We tested if the sorption kinetics of pesticides in soil aggregates can be described by modeling diffusion into aggregates for a range of soils and pesticides. Our hypothesis is that the rate of sorption is negatively related to sorption strength due to retardated diffusion. Natural aggregates of 3-5 mm diameter were separated from three soils: a clay, a silty clay loam, and a clay loam. The aggregates were stabilized with alginate gel, and adsorption of azoxystrobin, chlorotoluron, and atrazine was measured in batch experiments with eight equilibration times up to 28 days. Equilibrium sorption appeared to be reached within the 28-day period for each pesticide. An intra-aggregate diffusion model was employed to describe the increase of sorption with time. The model describes diffusion of the dissolved pesticides through the pore space inside the aggregates and sorption on internal surfaces. Sorption could be described by pore diffusion into the aggregates with diffusion coefficients between 0.5 x 10(-10) and 1.5 x 10(-10) m(2) s(-1). The model fits support the theory that pore diffusion is the rate-limiting process for sorption of pesticides in aggregates, although the diffusion coefficients were a factor 3-10 smaller than the theoretical diffusion coefficient for diffusion in water. Comparing the results from the different pesticide-soil combinations showed that the extent of nonequilibrium increased with increasing sorption strength. This confirmed that sorption takes longer to reach equilibrium for pesticides and soils with stronger sorption. The differences between the different pesticides and soils were fully accounted for in the model by stronger retardation of the more strongly sorbed pesticides. The results imply that diffusion into aggregates may be the major time-limiting process for sorption of pesticides in structured soils. Commonly performed sorption experiments with sieved soil fail to account for this process.

Batch experiments versus soil pore water extraction--what makes the difference in isoproturon (bio-)availability?

Two approaches to determine pesticide (bio-)availability in soils (i) batch experiments with "extraction with an excess of water" (EEW) and (ii) the recently introduced "soil pore water (PW) extraction" of pesticide incubated soil samples have been compared with regard to the sorption behavior of the model compound isoproturon in soils. A significant correlation between TOC and adsorbed pesticide amount was found when using the EEW approach. In contrast, there was no correlation between TOC and adsorbed isoproturon when using the in situ PW extraction method. Furthermore, sorption was higher at all concentrations in the EEW method when comparing the distribution coefficients (K(d)) for both methods. Over all, sorption in incubated soil samples at an identical water tension (-15 kPa) and soil density (1.3 g cm(-3)) appears to be controlled by a complex combination of sorption driving soil parameters. Isoproturon bioavailability was found to be governed in different soils by binding strength and availability of sorption sites as well as water content, whereas the dominance of either one of these factors seems to depend on the individual composition and characteristics of the respective soil sample. Using multiple linear regression analysis we obtained furthermore indications that the soil pore structure is affected by the EEW method due to disaggregation, resulting in a higher availability of pesticide sorption sites than in undisturbed soil samples. Therefore, it can be concluded that isoproturon sorption is overestimated when using the EEW method, which should be taken into account when using data from this approach or similar batch techniques for risk assessment analysis.

In situ mass distribution quotient (iMDQ) - a new factor to compare bioavailability of chemicals in soils?

Aim of this work was the development of a new non-biological factor to determine microbial in situ bioavailability of chemicals in soils. Pesticide residues were extracted from ten highly different agricultural soils that had been incubated with the (14)C-herbicide isoproturon (IPU) under comparable soil conditions (water tension - 15 kPa; soil density 1.3 g cm(-3)). Two different pesticide extraction approaches were compared: (i) (14)C-pesticide residues were measured in the pore water (PW) which was extracted from soil by centrifugation; (ii) (14)C-pesticide residues were extracted from soil samples with an excess of water (EEW). We introduce the pesticide's in situ mass distribution quotient (iMDQ) as a measure for pesticide bioavailability, which is calculated as a quotient of adsorbed and dissolved chemical amounts for both approaches (iMDQ(PW), iMDQ(EEW)). Pesticide mineralization in soils served as a reference for real microbial availability. A highly significant correlation between iMDQ(PW) and mineralization showed that PW extraction is adequate to assess IPU bioavailability. In contrast, no correlation exists between IPU mineralization and its extractability from soil with EEW. Therefore, it can be concluded that soil equilibration at comparable conditions and subsequent PW extraction is vital for a isoproturon bioavailability ranking of soils.

Kinetic distribution of 14C-metsulfuron-methyl residues in paddy soils under different moisture conditions

Rice paddy soils undergo several cycles of drying and wetting during a growing season. A laboratory study was conducted to determine the effect of soil moisture conditions on the distribution and kinetics of extractable and bound residues of 14C-metsulfuron-methyl in six Chinese paddy soils during 84 d of incubation at 15 degrees C with moisture contents varying from 20 to 50% of the field water-holding capacity. The amount of extractable residues consistently increased and bound residues decreased with increasing soil moisture content. At the end of the incubation experiments, extractable residues and bound residues accounted for 34.5 to 84.4% and 11.6 to 53.3% of applied radioactivity in soils, respectively. Soil pH and soil microbial biomass carbon were the most predominant factors affecting the formation and relative distribution of herbicide residues between extractable and bound residue forms. In high-pH soils, bound residues decreased and extractable residues increased, suggesting an increased leaching risk for metsulfuron-methyl in alkaline soils. High precipitation rates, along with the common practice of liming in southeastern China, may lead to enhanced herbicide leaching as well as phytotoxicity to rotation plants and should be considered in overall pest management practices.

Sorption kinetics of 2,4-D and carbaryl in selected agricultural soils of northern Iraq: application of a dual-rate model

Agriculture in northern Iraq (Kurdistan) relies on the widespread use of pesticides to promote crop performance. Over-application of many pesticides is commonplace, however, and may compromise soil and water quality, and ultimately human health, within the region. The aim of this study was to investigate the sorption-desorption kinetics and equilibrium partitioning of two selected pesticides in agricultural soils from northern Iraq. This was achieved by fitting a dual-rate sorption-desorption model to time-dependent data obtained from batch experiments. 2,4-D and carbaryl were selected for scrutiny since both are in common use in the region. Six agricultural soils, sampled around the city of Erbil, were investigated. These were low in organic carbon (OC) compared with many agricultural soils from more temperate regions. However, there was still a clear trend of increasing sorption of both 2,4-D and carbaryl with increasing % OC. In the case of both compounds, fast and slow adsorption rate coefficients and 48 h experimental K(d) values were positively correlated with % OC. It was assumed that K(OC) would provide a simple and reliable predictor of K(d). However, while this assumption holds true for short-term (48 h) experimental data, longer-term sorption in some soils (as indicated by theoretical K(d) values estimated from kinetic parameters in our study) appears to be under-predicted by K(OC) alone. The data presented here provide a useful starting point for further site-specific investigations of pesticide impacts in the Kurdistan region of Iraq.