Adsorption of sulfamethoxazole and ethofumesate in biochar- and organoclay-amended soil: Changes with adsorbent aging in the laboratory and in the field.

Biochars and organoclays have been proposed as efficient adsorbents to reduce the mobility of agrochemicals in soils. However, following their application to soils, these adsorbents undergo changes in their physicochemical properties over time due to their interaction with soil components. In this study, the adsorption capacity of a commercial biochar and a commercial organoclay for the antibiotic sulfamethoxazole (SFMX) and the pesticide ethofumesate (ETFM) was evaluated over aging periods of 3 months in the laboratory and 1 year in the field, subsequent to their application to a Mediterranean soil. The results showed that the adsorption of SFMX and ETFM in the soil amended with the adsorbents was greater than in the unamended soil, but for both chemicals, adsorption decreased with aging of the adsorbents in the soil. Characterization of the adsorbents before and after aging revealed physical blocking of adsorption sites by soil components. The loss of adsorption capacity of the adsorbents upon aging led to higher leaching of SFMX and ETFM in the soil containing field-aged adsorbents, although leaching remained lower than in unamended soil. Our findings reveal that, under the Mediterranean environment studied, the efficacy of the studied materials as adsorbents is maintained to a considerable extent for at least one year after their field application, which would have positive implications in their use for attenuating the dispersion of agricultural contaminants in the environment.

An enantioselective study of the behavior of the herbicide ethofumesate in agricultural soils: Impact of the addition of organoclays and biochar.

Chiral pesticides that are still commercialized and incorporated into the environment as racemic mixtures of enantiomers require evaluation of the enantioselectivity of their biological activity and environmental fate processes for a better prediction of their field efficacy and environmental risks. In this work, we successfully separated the enantiomers of the chiral herbicide ethofumesate (ETFM), determined their absolute configuration, and characterized their herbicidal activity as well as their adsorption, degradation, enantiomerization, and leaching in Mediterranean agricultural soils. While the herbicidal activity of R-ethofumesate to the sensitive species Portulaca grandiflora was greater than that of S-ethofumesate, the adsorption, degradation, and leaching of the herbicide showed negligible enantioselectivity and enantiomer interconversion did not occur in soils. The adsorption of both enantiomers showed a positive correlation with the soil organic carbon content (r = 0.856, P = 0.015), and their degradation in soils occurred slowly (DT50 > 60 days) and at similar rates independent of their application as individual enantiomers or as a racemic mixture of enantiomers. The addition of three highly adsorptive materials to a scarcely adsorptive soil increased the adsorption of the enantiomers of ETFM and delayed their degradation without affecting the non-enantioselective character of the processes. As a result of their high adsorption capacity, the materials were highly effective in reducing the leaching of both enantiomers of ETFM through soil columns. The results of this work indicate that the application of single-enantiomer ETFM formulations, based on a higher herbicidal activity or a lower toxicity to non-target organisms of the formulated enantiomer, would reduce considerable exposure risks associated with incorporating into the environment the less favorable enantiomer, as this would show long persistence and high leaching potential in soils similar to its optical isomer.

Soil Effects on the Bioactivity of Hydroxycoumarins as Plant Allelochemicals.

Soil plays a primary role in the activity of plant allelochemicals in natural and agricultural systems. In this work, we compared the phytotoxicity of three natural hydroxycoumarins (umbelliferone, esculetin, and scopoletin) to different model plant species (Lactuca sativa, Eruca sativa, and Hordeum vulgare) in Petri dishes, and then selected the most phytotoxic compound (umbelliferone) to assess how its adsorption and dissipation in two distinct soils affected the expression of its phytotoxic activity. The root growth inhibitory effect of umbelliferone was significantly greater than that of esculetin and scopoletin, and the dicot species (L. sativa and E. sativa) were more sensitive to the hydroxycoumarins than the monocot species (H. vulgare). For all three plant species tested, the phytotoxicity of umbelliferone decreased in the following order: soilless (Petri dishes) > soil 1 > soil 2. In soil 2 (alkaline), umbelliferone displayed negligible adsorption (Kf < 0.01) and rapid biodegradation (t1/2 = 0.2-0.8 days), and its phytotoxicity was barely expressed. In soil 1 (acid), umbelliferone displayed enhanced adsorption (Kf = 2.94), slower biodegradation (t1/2 = 1.5-2.1 days), and its phytotoxicity was better expressed than in soil 2. When the microbial activity of soil 2 was suppressed by autoclaving, the phytotoxicity of umbelliferone, in the presence of soil, became similar to that observed under soilless conditions. The results illustrate how soil processes can reduce the allelopathic activity of hydroxycoumarins in natural and agricultural ecosystems, and suggest scenarios where the bioactivity of hydroxycoumarins may be better expressed.

Enantiomer-Selective Characterization of the Adsorption, Dissipation, and Phytotoxicity of the Plant Monoterpene Pulegone in Soils.

Plant monoterpenes have received attention for their ecological functions and as potential surrogates for synthetic herbicides, but very little is known about the processes that govern their behavior in the soil environment, and even less about the possible enantioselectivity in the functions and environmental behavior of chiral monoterpenes. We characterized the adsorption and dissipation of the two enantiomers of the chiral monoterpene pulegone in different soils, and their phytotoxicity to different plant species through Petri dish and soil bioassays. R- and S-pulegone displayed a low-to-moderate non-enantioselective adsorption on the soils that involved weak interaction mechanisms. Soil incubation experiments indicated that, once in the soil, R- and S-pulegone are expected to suffer rapid volatilization and scarcely enantioselective, biodegradation losses. In Petri dishes, the phytotoxicity of pulegone and its enantioselectivity to Lactuca sativa, Hordeum vulgare, and Eruca sativa was species-dependent. Lactuca sativa was the most sensitive species and showed higher susceptibility to S- than to R-pulegone. Biodegradation and volatilization losses greatly reduced the phytotoxic activity of S-pulegone applied to soil, but the addition of a highly-adsorptive organoclay stabilized the monoterpene and increased its phytotoxic effect. Stabilization by adsorption may represent an important mechanism by which the bioactivity of plant monoterpenes in soils can be increased.

Soil modification with organic amendments and organo-clays: Effects on sorption, degradation, and bioactivity of the allelochemical scopoletin.

We assessed the effect of three organic amendments and two organo-clays on sorption, persistence, and phytotoxicity of scopoletin, an allelochemical compound with potential as bioherbicide, in a Mediterranean alkaline soil. The aim was to elucidate whether the phytotoxicity of scopoletin could be expressed better in amended than unamended soil. The three organic amendments were fresh solid olive-mill waste (OMW), composted solid olive-mill waste (OMWc), and biochar (BC) prepared from OMWc. The two organo-clays were a commercial organo-montmorillonite (Cloi10) and lab-synthesized oleate-modified hydrotalcite (HT-OLE). The amendments enhanced sorption of scopoletin by the soil consistently with their individual affinities for the allelochemical: Cloi10 ≫ OMW > BC > OMWc > HT-OLE. The soil persistence of scopoletin increased significantly because of the addition of Cloi10, OMW, and BC. This increase was attributed to a combination of sorption, which protected the allelochemical from rapid biodegradation, and microbial activity changes. Although the inhibitory effect produced by the amendments themselves obscured the phytotoxicity of scopoletin to Lactuca sativa L. in soil treated with OMW and Cloi10, applying scopoletin to BC-amended soil led to a marked reduction in root length and aerial biomass of the emerged seedlings even though BC alone did not negatively affect these parameters. This inhibitory effect of scopoletin in BC-amended soil was in contrast to the negligible effect exerted by the allelochemical when applied to unamended soil. The results show that soils treated with suitable amendments, such as BC, might provide a scenario in which the herbicidal properties of 7-hydroxycoumarins could be better expressed.

Enfermedad de Castleman versus tuberculosis generalizada: un caso de fiebre de origen desconocido

Se presenta el caso de una mujer de 25 años con fiebre de origen desconocido, cuyo estudio de biopsia de ganglio cervical se reportó como enfermedad de Castleman, pero que al seguimiento resulto siendo tuberculosis multisistémica. Recibió tratamiento antituberculoso con buena evolución clínica y de laboratorio en el seguimiento. Se enfatiza que, ante enfermedades de baja prevalencia, como la enfermedad de Castleman, se debe descartar tuberculosis en países como el nuestro.

Sorption and dissipation of the allelochemicals umbelliferone and salicylic acid in a Mediterranean soil environment: Effect of olive-mill waste addition.

Allelochemicals are receiving much attention as natural alternatives to synthetic pesticides. Very little is known, however, about the processes to which allelochemicals are subjected once they reach the soil environment, despite the fact that it is widely recognized that such processes can dramatically influence their bioactivity and applicability as eco-friendly pesticides. The objectives of this study were to characterize the sorption and dissipation of two phenolic allelochemicals, umbelliferone (UM) and salicylic acid (SA), after their simultaneous application to a Mediterranean agricultural soil and to assess to what extent sorption and dissipation were affected by amending the soil with an agro-industrial organic waste (olive-mill waste, OMW), as a common agronomic practice in Mediterranean agricultural systems. In experiments conducted under standard laboratory conditions, UM (pKa = 7.5) showed greater sorption than SA (pKa = 2.8) and both allelochemicals displayed very short half-lives in the tested soil (DT50 < 1 day). Furthermore, the addition of OMW increased the sorption of UM and the half-lives of both SA and UM in the soil. A field experiment conducted on unamended and OMW-amended soil plots confirmed the ability of OMW to increase the persistence of SA and UM under a real Mediterranean soil environment and showed that, for all treatments, the allelochemicals displayed higher half-lives in the field than under standard laboratory conditions. This was attributed to reduced biodegradation of UM and SA under progressive soil drying, which was thus identified as a factor that can prolong the persistence of allelochemicals in semi-arid soil environments. We highlight the need to test the environmental fate of allelochemicals under specific agro-climatic scenarios and illustrate how management practices can help increase their soil persistence so that their bioactivity can be better expressed.

Sorption, persistence, and leaching of the allelochemical umbelliferone in soils treated with nanoengineered sorbents.

Coumarins represent an important family of allelochemicals with fungicidal, bactericidal, insecticidal, nematicidal, and herbicidal properties. Like for other allelochemicals, the short persistence of coumarins in soils can reduce their biological activity and hamper their application as environmentally friendly agrochemicals. We evaluated the sorption of the coumarin umbelliferone by eight soils and six sorbent materials, and then selected two nanoengineered sorbents, hexadecyltrimethylammonium-modified Arizona montmorillonite (SA-HDTMA) and olive-mill waste biochar (BC), to assess the effect of their addition to two distinct soils on umbelliferone sorption, persistence, and leaching. Umbelliferone was sorbed to a greater extent by the acid soils (A1-A2, Kd > 4.0 L kg-1) than by the alkaline soils (B1-B6, Kd < 0.5 L kg-1). The addition of BC and SA-HDTMA at a rate of 4% to alkaline soil (B2) increased the umbelliferone sorption Kd value from 0.3 to 1.6-2.0 L kg-1, whereas their addition to acid soil (A1) increased the Kd value from 4.6 to 12.2-19.0 L kg-1. Incubation experiments showed that BC had more impact than SA-HDTMA on the persistence of umbelliferone in the soils, increasing its half-life from 0.3-2.5 to 1.2-14.4 days, depending on the soil. Furthermore, the addition of BC to the top 0-5 cm of soil columns reduced leaching of umbelliferone and led to accumulation of umbelliferone residues in the top 0-5 cm soil layer. The addition of nanoengineered materials, such as organoclays and biochars, could thus be a suitable strategy to increase the persistence and reduce the mobility of coumarins in the rhizosphere with the aim of prolonging their biological activity.

Nanoengineered Sorbents To Increase the Persistence of the Allelochemical Carvone in the Rhizosphere.

This study investigates the changes in sorption/desorption, dissipation, and leaching of the two enantiomeric forms of the allelochemical carvone, R-carvone and S-carvone, after amending an agricultural soil sample with two nanoengineered sorbents: biochar (BC) and organoclay (OCl). The sorption of carvone enantiomers was nonenantioselective and similarly improved by the addition of OCl and BC to the soil. However, OCl-amended soil showed reversible sorption, whereas BC-amended soil displayed sorption-desorption hysteresis. Dissipation of carvone enantiomers was enantioselective. Both amendments increased the half-life of the enantiomers in the soil. This effect was more pronounced for BC-amended soil and for S-carvone. Leaching of R- and S-carvone through soil columns was scarce in unamended soil (<7%), due to their rapid degradation during leaching, and null for OCl- and BC-amended soil, for which much of the applied R- and S-carvone remained in the top 0-5 cm of the amended soil layer. Addition of biochars and organoclays could help increase the persistence of carvone enantiomers in the rhizosphere, which may favor their use as residual pest-management substances.

A clay-based formulation of the herbicide imazaquin containing exclusively the biologically active enantiomer.

BACKGROUND: Imazaquin is a chiral herbicide which displays high mobility in soils. Like other imidazolinones, imazaquin is available for use only as racemic mixture of its enantiomers. In this work, several clay materials were assayed as adsorbents of imazaquin, and then the most suitable material was selected to prepare a clay-based slow release imazaquin nanoformulation containing exclusively the biologically active R-enantiomer. Next, laboratory experiments were conducted to illustrate the benefits of using the clay-based R-imazaquin formulation over the free (non-supported) racemic herbicide or the free pure R-imazaquin enantiomer regarding its leaching behavior and bioefficacy. RESULTS: The clay material selected as a carrier for R-imazaquin, hexadecyltrimethylammonium-saturated montmorillonite (SA-HDTMA), combined a high affinity for the herbicide and a high stability of the clay-herbicide adsorption complex. In a simulated scenario of high water input shortly after herbicide application, the clay-based R-imazaquin formulation displayed reduced leaching and increased bioefficacy compared to free racemic imazaquin and free R-imazaquin. CONCLUSION: The new clay-R-imazaquin formulation prepared, besides avoiding the environmental impact caused by the application of the less active S-enantiomer, reduced the herbicide leaching losses and prolonged the herbicidal activity, by increasing the residence time of the herbicide in the topsoil. © 2018 Society of Chemical Industry.

Assessing the Effect of Organoclays and Biochar on the Fate of Abscisic Acid in Soil.

The potential use of allelopathic and signaling compounds as environmentally friendly agrochemicals is a subject of increasing interest, but the fate of these compounds once they reach the soil environment is poorly understood. This work studied how the sorption, persistence, and leaching of the two enantiomers of the phytohormone abscisic acid (ABA) in agricultural soil was affected by the amendments of two organoclays (SA-HDTMA and Cloi10) and a biochar derived from apple wood (BC). In conventional 24-h batch sorption experiments, higher affinity toward ABA enantiomers was displayed by SA-HDTMA followed by Cloi10 and then BC. Desorption could be ascertained only in BC, where ABA enantiomers presented difficulties to be desorbed. Dissipation of ABA in the soil was enantioselective with S-ABA being degraded more quickly than R-ABA, and followed the order unamended > Cloi10-amended > BC-amended > SA-HDTMA-amended soil for both enantiomers. Sorption determined during the incubation experiment indicated some loss of sorption capacity with time in organoclay-amended soil and increasing sorption in BC-amended soil, suggesting surface sorption mechanisms for organoclays and slow (potentially pore filling) kinetics in BC-amended soil. The leaching of ABA enantiomers was delayed after amendment of soil to an extent that depended on the amendment sorption capacity, and it was almost completely suppressed by addition of BC due to its irreversible sorption. Organoclays and BC affected differently the final behavior and enantioselectivity of ABA in soil as a consequence of dissimilar sorption capacities and alterations in sorption with time, which will affect the plant and microbial availability of endogenous and exogenous ABA in the rhizosphere.

Evaluation of an organo-layered double hydroxide and two organic residues as amendments to immobilize metalaxyl enantiomers in soils: A comparative study.

Many pollutants released into the environment as a result of human activities are chiral. Pollution control strategies generally consider chiral compounds as if they were achiral and rarely consider enantiomers separately. We compared the performance of three different materials, an organically-modified anionic clay (HT-ELA) and two organic agro-food residues (ALP and ALPc), as amendments to immobilize the chiral fungicide metalaxyl in two soils with different textures, addressing the effects of the amendments on the sorption, persistence, and leaching of each of the two enantiomers of metalaxyl (R-metalaxyl and S-metalaxyl) separately. The effects of the amendments were both soil- and amendment-dependent, as well as enantiomer-selective. The organo-clay (HT-ELA) was much more efficient in increasing the sorption capacity of the soils for the two enantiomers of metalaxyl than the agro-food residues (ALP and ALPc), even when applied at a reduced application rate. The enhanced sorption in HT-ELA-amended soils reduced the bioavailability of metalaxyl enantiomers and their leaching in the soils, mitigating the particularly high leaching potential of the more persistent S enantiomer. The immobilizing capacity of the agro-food residues was more variable, mainly because their addition did not greatly ameliorate the sorption capacity of the soils and had variable effects on the enantiomers degradation rates. HT-ELA showed potential to reduce the bioavailability and mobility of metalaxyl enantiomers in soil and to mitigate the contamination problems particularly associated with the higher leaching potential of the more persistent enantiomer.

Evidence for the effect of sorption enantioselectivity on the availability of chiral pesticide enantiomers in soil.

Although enantioselective sorption to soil particles has been proposed as a mechanism that can potentially influence the availability of individual chiral pesticide enantiomers in the environment, environmental fate studies generally overlook this possibility and assume that only biotic processes can be enantioselective, whereas abiotic processes, such as sorption, are non-enantioselective. In this work, we present direct evidence for the effect of the enantioselective sorption of a chiral pesticide in a natural soil on the availability of the single pesticide enantiomers for transport. Batch sorption experiments, with direct determination of the sorbed amounts, combined with column leaching tests confirmed previous observations that from non-racemic aqueous solutions the sorption of the chiral fungicide metalaxyl on the soil appeared to be enantioselective, and further demonstrated that the enantiomer that was sorbed to a greater extent (R-metalaxyl, Kd = 1.73 L/kg) exhibited retarded leaching compared to its optical isomer (S-metalaxyl, Kd = 1.15 L/kg). Interconversion and degradation of the pesticide enantiomers, which are potential experimental artifacts that can lead to erroneous estimates of sorption and its enantioselectivity, were discarded as possible causes of the observed enantioselective behavior. The results presented here may have very important implications for a correct assessment of the environmental fate of chiral pesticides that are incorporated into the environment as non-racemic mixtures, and also of aged chiral pesticide residues that have been transformed from racemic to non-racemic by biologically-mediated processes.

Imazamox-clay complexes with chitosan- and iron(III)-modified smectites and their use in nanoformulations.

BACKGROUND: Imazamox is an ionisable herbicide, weakly retained and with high soil vertical mobility, that is used for the control of the root-parasitic plants Orobanche spp. A natural smectite (SW) modified with the biopolymer chitosan (Ch) or with Fe(3+) cation was assayed as adsorbent or carrier for imazamox controlled-release formulations (CRFs). RESULTS: The greatest adsorption (74%) was observed for SWFe at high initial concentration (500 µM) and low pH (4.3). The interaction mechanism of imazamox on SWFe implies interlayer polar adsorption, followed by protonation of the imidazolinone ring, whereas ionic, polar and hydrophobic interactions seemed to occur in imazamox adsorption on SWCh. The herbicide release into water was inversely related to the strength of the imazamox-clay interactions and ranged in the first 10 min for imazamox-SWFe and imazamox-SWCh complexes from 27 to 75%, whereas commercial imazamox released 86%. The imazamox-SWCh weak complex (SWCh6 WC) showed similar herbicidal activity to the commercial formulation but produced a reduction of 15% in the total soil leaching losses and a reduction of 40% in the peak maximum concentration in soil column leachates. CONCLUSION: The imazamox-clay weak complex (WC) of SWFe and SWCh and the strong complex (SC) with SWCh showed appropriate behaviour as nanopesticides or smart delivery systems to be incorporated in CRFs. © 2015 Society of Chemical Industry.

Environmental fate of the fungicide metalaxyl in soil amended with composted olive-mill waste and its biochar: An enantioselective study.

A large number of pesticides are chiral and reach the environment as mixtures of optical isomers or enantiomers. Agricultural practices can affect differently the environmental fate of the individual enantiomers. We investigated how amending an agricultural soil with composted olive-mill waste (OMWc) or its biochar (BC) at 2% (w:w) affected the sorption, degradation, and leaching of each of the two enantiomers of the chiral fungicide metalaxyl. Sorption of metalaxyl enantiomers was higher on BC (Kd ≈ 145 L kg(-1)) than on OMWc (Kd ≈ 22 L kg(-1)) and was not enantioselective in either case, and followed the order BC-amended>OMWc-amended>unamended soil. Both enantiomers showed greater resistance to desorption from BC-amended soil compared to unamended and OMWc-amended soil. Dissipation studies revealed that the degradation of metalaxyl was more enantioselective (R>S) in unamended and OMWc-amended soil than in BC-amended soil. The leaching of both S- and R-metalaxyl from soil columns was almost completely suppressed after amending the soil with BC and metalaxyl residues remaining in the soil columns were more racemic than those in soil column leachates. Our findings show that addition of BC affected the final enantioselective behavior of metalaxyl in soil indirectly by reducing its bioavailability through sorption, and to a greater extent than OMWc. BC showed high sorption capacity to remove metalaxyl enantiomers from water, immobilize metalaxyl enantiomers in soil, and mitigate the groundwater contamination problems particularly associated with the high leaching potential of the more persistent enantiomer.

Enantioselective sorption of the chiral fungicide metalaxyl on soil from non-racemic aqueous solutions: Environmental implications.

Mechanisms governing the enantioselectivity of the processes that determine the behavior of chiral pollutants in the environment need to be better understood. Understanding these mechanisms should help improve predictions of the hazards and risks chiral compounds can pose to people and the environment. We report the results of batch sorption experiments indicating that the sorption of the chiral fungicide metalaxyl on soil from non-racemic initial solutions was enantioselective. While from a racemic initial solution the two enantiomers of metalaxyl were sorbed on the soil to the same extent, increasing the fraction of R-enantiomer in the initial solution led to enhanced sorption of this enantiomer and to reduced sorption of the S-enantiomer. Considering the shape of the sorption isotherms (S-type) and the sorption behavior of model sorbents, we attributed this effect to molecular interactions between metalaxyl enantiomer species at the sorbed state, where R-R metalaxyl interactions appeared to be more favorable than R-S metalaxyl interactions. We discuss important environmental implications of the proposed mechanism, such as those related to the fact that the biological degradation of metalaxyl is known to be an enantioselective process that can yield non-racemic residues in soils shortly after application of the fungicide as a racemic mixture.

Effect of formulation and repeated applications on the enantioselectivity of metalaxyl dissipation and leaching in soil.

BACKGROUND: Soil incubation and column leaching experiments were conducted to address the question of whether the type of formulation (unsupported versus clay supported) and repeated applications of the chiral fungicide (RS)-metalaxyl affected the enantioselectivity of its dissipation and leaching in a slightly alkaline, loamy sand agricultural soil. RESULTS: Regardless of the type of formulation and the number of fungicide applications, the R-enantiomer of metalaxyl was degraded faster than the S-enantiomer, but the individual degradation rates of R- and S-metalaxyl were highly affected by the different application regimes assayed (t1/2 = 2-104 days). Repeated applications accelerated the degradation of the biologically active R-metalaxyl enantiomer, whereas they led to slower degradation of the non-active S-metalaxyl enantiomer. The type of formulation had less influence on the dissipation rates of the enantiomers. For all formulations tested, soil column leachates became increasingly enriched in S-enantiomer as the number of fungicide applications was increased, and application of metalaxyl to soil columns as clay-based formulations reduced the leaching of both enantiomers. CONCLUSION: Pesticide application conditions can greatly influence the enantioselective dissipation of chiral pesticides in soil, and hence are expected to exert a great impact on both the biological efficacy and the environmental chiral signatures of pesticides applied as mixtures of enantiomers or racemates to agricultural soils.

Effect of olive-mill waste addition to agricultural soil on the enantioselective behavior of the chiral fungicide metalaxyl.

Certain soil management practices can affect the enantioselective behavior of chiral pesticide enantiomers in agricultural soils. In this work, laboratory experiments were conducted to study the effects of olive-mill waste (OMW) addition to a Mediterranean agricultural soil on the enantioselectivity of sorption, degradation, and leaching processes of the chiral fungicide metalaxyl. Sorption-desorption isotherms indicated that the sorption of metalaxyl enantiomers by unamended and OMW-amended soil (2% w/w) was non-enantioselective and that OMW addition had little effect on the extent of sorption of metalaxyl enantiomers by the soil. Soil incubation experiments revealed that the degradation of metalaxyl in unamended soil was highly enantioselective, with R-metalaxyl being degraded faster (t1/2 = 12 days) than S-metalaxyl (t1/2 = 39 days). OMW addition to the soil increased the half-life of the biologically-active R-metalaxyl enantiomer from 12 to 28 days, and decreased the half-life of the non-active S-metalaxyl enantiomer from 39 to 33 days. Consequently, the enantioselectivity of metalaxyl degradation in the soil was greatly reduced upon OMW addition. Column leaching data were consistent with batch sorption and incubation results, showing similar retardation of S- and R-metalaxyl in unamended and OMW-amended soil and enantioselective leaching of the fungicide only in unamended soil. The results have important implications regarding the biological efficacy and environmental impact of the fungicide when applied as a mixture of enantiomers or racemate to OMW-treated soils.

Reanalysis of experiments to quantify irreversibility of pesticide sorption-desorption in soil.

Previously published research used an isotope-exchange technique to measure irreversibility of pesticide sorption-desorption in soil. Results indicated significant irreversibility (6-51%) in sorption in five pesticide-soil systems measured over 72 h. Here, we propose a three-site model to reanalyze the experimental data. The model adds a slow but reversible binding on nonequilibrium sorption sites in addition to instantaneously reversible sites and irreversible sites. The model was able to match experimental data very closely, but only if irreversible sorption was assumed to be absent. Observed asymmetry in the binding of (12)C- and (14)C-pesticide was explained on the basis of nonattainment of sorption equilibrium over the study period. Results suggest that irreversible sorption may be less significant than previously considered with important implications for understanding the fate of pesticides applied to soil.

Comparative sorption and leaching study of the herbicides fluometuron and 4-chloro-2-methylphenoxyacetic acid (MCPA) in a soil amended with biochars and other sorbents.

Biochar, the solid residual remaining after the thermochemical transformation of biomass for carbon sequestration, has been proposed to be used as a soil amendment, because of its agronomic benefits. The effect of amending soil with six biochars made from different feedstocks on the sorption and leaching of fluometuron and 4-chloro-2-methylphenoxyacetic acid (MCPA) was compared to the effect of other sorbents: an activated carbon, a Ca-rich Arizona montmorillonite modified with hexadecyltrimethylammonium organic cation (SA-HDTMA), and an agricultural organic residue from olive oil production (OOW). Soil was amended at 2% (w/w), and studies were performed following a batch equilibration procedure. Sorption of both herbicides increased in all amended soils, but decreased in soil amended with a biochar produced from macadamia nut shells made with fast pyrolysis. Lower leaching of the herbicides was observed in the soils amended with the biochars with higher surface areas BC5 and BC6 and the organoclay (OCl). Despite the increase in herbicide sorption in soils amended with two hardwood biochars (BC1 and BC3) and OOW, leaching of fluometuron and MCPA was enhanced with the addition of these amendments as compared to the unamended soil. The increased leaching is due to some amendments' soluble organic compounds, which compete or associate with herbicide molecules, enhancing their soil mobility. Thus, the results indicate that not all biochar amendments will increase sorption and decrease leaching of fluometuron and MCPA. Furthermore, the amount and composition of the organic carbon (OC) content of the amendment, especially the soluble part (DOC), can play an important role in the sorption and leaching of these herbicides.