Adsorption of pesticides onto quartz, calcite, kaolinite, and alpha-alumina

The fate of pesticides in aquifers is influenced by the small but not insignificant adsorption of pesticides to mineral surfaces. Batch experiments with five pesticides and four minerals were conducted to quantify the contributions to adsorption from different mineral surfaces and compare adsorption characteristics of selected pesticides. Investigated mineral phases included quartz, calcite, kaolinite, and alpha-alumina. Selected pesticides comprised atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine), isoproturon [3-(4-isopropylphenyl)-1,1-dimethylurea)], mecoprop [(RS)-2-(4-chloro-2-methylphenoxy)propionic acid], 2,4-D (2,4-dichlorophenoxyacetic acid), and bentazone [3-isopropyl-1H-2,1,3-benzothiadiazin-4-(3H)-one 2,2-dioxide]. Specific surface area and mineral surface charge proved to be important for the adsorption of these pesticides. Detectable adsorption of the anionic pesticides (mecoprop, 2,4-D, and bentazone) was only measured when positive sites were present on the mineral surface. However, when CaCl2 was added as an electrolyte, a detectable adsorption of mecoprop and 2,4-D was also measured on kaolinite (which exhibits a negative surface charge), probably due to formation of Ca-pesticide--surface complexes. Adsorption of the uncharged pesticides (atrazine and isoproturon) was detected only on kaolinite. The lack of adsorption on alpha-alumina indicates that the uncharged pesticides have a greater affinity for the silanol surface sites (=SiOH) than for the aluminol surface sites (=AlOH) in kaolinite. No measurable effect of ionic strength was found for the uncharged pesticides. The results indicate that quartz and calcite play a smaller role than clay minerals.     

Adsorption of arsenate and arsenite on ferrihydrite in the presence and absence of dissolved organic carbon

The adsorption of As(V) and As(III) on synthetic two-line ferrihydrite in the presence and absence of a peat humic acid (HAp), Suwannee River fulvic acid (FA), or citric acid (CA) was investigated. Previous work with goethite has demonstrated the ability of dissolved organic carbon (DOC) to decrease As(V) and As(III) adsorption. The results obtained demonstrate that As(V) adsorption on ferrihydrite was decreased only in the presence of CA. Arsenate decreased the adsorption of all organic acids except HAp. Both FA and CA reduced As(III) adsorption on ferrihydrite, while HAp had no effect. Fulvic and citric acid adsorption on ferrihydrite was decreased in the presence of As(III); however, FA and CA adsorption increased at lower pH, which was consistent with decreased As(III) adsorption. Peat humic acid did not decrease As(III) adsorption, and we believe that the adsorption process of HAp and As(III) and As(V) on ferrihydrite are independent of each other. Previously, we observed that As(V) adsorption on goethite decreased in the presence of HAp > FA > CA, while As(III) adsorption on goethite was decreased similarly to that on ferrihydrite in the presence of CA > FA approximately HAp, yet As(III) adsorption on ferrihydrite was greater than on goethite. The observed differences between this study and the earlier study on goethite are believed to be an intricate function of ferrihydrite's surface characteristics, which affect the mechanisms of adsorption and hence the affinity of organic acids such as HAp, FA, and CA for the ferrihydrite surface. As such, the adsorption of DOCs to ferrihydrite are assumed to be less favorable and to occur with a fewer number of ligands, resulting in lower surface coverage of weaker bond strength.     

Evaluation of simplifying assumptions on pesticide degradation in soil

There is evidence that degradation of pesticides in simple laboratory systems may differ from that in the field, but it is not clear which of the simplifications inherent in laboratory studies present serious shortcomings. Laboratory experiments evaluated several simplifying assumptions for a clay loam soil and contrasting pesticides. Degradation of cyanazine [2-(4-chloro-6-ethylamino-1,3,5-triazin-2-ylamino)-2-methylpropiononitrile] and bentazone [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] at fluctuating temperature and moisture was predicted reasonably well based on parameters derived from degradation under constant conditions. There was a tendency for slower degradation of cyanazine and bentazone in soil aggregates of 3 to 5 mm in diameter (DT50 at 15 degrees C and 40% maximum water holding capacity of 25.1 and 58.2 d, where DT50 is the time for 50% decline of the initial pesticide concentration) than in soil sieved to <3 mm (DT50 of 19.1 and 37.6 d), but the differences were not significant for most datasets. Degradation of cyanazine, isoproturon [3-(4-isopropylphenyl)-1,1-dimethylurea], and chlorotoluron [3-(3-chloro-p-tolyl)-1,1-dimethylurea] was measured in soil amended with different amounts of lignin. The effect of lignin on degradation was small despite considerable differences in sorption. The DT50 values of cyanazine, isoproturon, and chlorotoluron were 16.2, 18.6, and 33.0 d, respectively, in soil without lignin and 19.0, 23.4, and 34.6 d, respectively, in soil amended with 2% lignin. Degradation of bentazone and cyanazine in repacked soil columns was similar under static and flow conditions with 50.1 and 47.2% of applied bentazone and 74.7 and 73.6% of applied cyanazine, respectively, degraded within 20 d of application. Thus, the assumptions underpinning laboratory to field extrapolation tested here were considered to hold for our experimental system. Additional work is required before general conclusions can be drawn.     

Adsorption of 2,4-dichlorophenoxyacetic acid by an Andosol

To identify the important soil components involved in 2,4-dichlorophenoxyacetic acid (2,4-D) adsorption on Andosols, 2,4-D adsorption on a surface horizon of an Andosol was compared with that on hydrogen peroxide (H2O2)-treated (soil organic matter [SOM] was removed), acid-oxalate (OX)-treated (active metal hydroxides and SOM were removed), and dithionite-citrate-bicarbonate (DCB)-treated (free and active metal [hydr]oxides and SOM were removed) soil samples at equilibrium pHs ranging from 4 to 8. Although the untreated soil contained a large amount of organic C (71.9 g kg-1), removal of SOM had little effect on 2,4-D adsorption. Active surface hydroxyls, which were attached to the active and free metal (hydr)oxides and metal SOM complexes, were identified as the most important soil functional group for 2,4-D adsorption. The dominant mechanism of the 2,4-D adsorption was a ligand exchange reaction in which the carboxylic group of 2,4-D displaced the active surface hydroxyl associated with metals and formed a strong coordination bond between the 2,4-D molecule and soil solid phase. The ligand exchange reaction reasonably accounted for the selective adsorption of 2,4-D over Cl-, competitive adsorption of phosphate over 2,4-D, reduction in plant-growth-inhibitory activity of soil-adsorbed 2,4-D, and the high 2,4-D adsorption ability of Andosols. Although a humic acid purified from the soil did not adsorb 2,4-D, the presence of the humic acid increased 2,4-D adsorption on Al and Fe, probably by inhibiting the hydrolysis and polymerization of Al and Fe resulting in the preservation of available adsorption sites on these metals. The adsorption behavior of 2,4-D on soils could be a good index for predicting the adsorption behavior of other organic acids in soils.     

Alachlor and bentazone losses from subsurface drainage of two soils

Atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) is frequently detected at high concentrations in ground water. Bentazone [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] plus alachlor (2-chloro-2',6'-diethyl-N-methoxymethylacetanilide) is a potential herbicide combination used as a substitute for atrazine. Thus, the objective of this study was to assess the environmental risk of this blend. Drainage water contamination by bentazone and alachlor was assessed in silty clay (Vertic Eutrochrept) and silt loam (Aquic Hapludalf) soils under the same management and climatic conditions. Drainage volumes and concentrations of alachlor and bentazone were monitored after application. Herbicides first arrived at the drains after less than 1 cm of net drainage. This is consistent with preferential flow and suggests that about 3% of the pore volume was active in rapid transport. During the monitoring periods, bentazone losses were higher (0.11-2.40% of the applied amount) than alachlor losses (0.00-0.28%) in the drains of the silty clay and silt loam. The rank order of herbicide mass losses corresponded with the rank order of herbicide adsorption coefficients. More herbicide residues were detected in drainage from the silty clay, probably due to preferential flow and more intensive drainage in this soil than the silt loam. Surprisingly, herbicide losses were higher in the drains of both soils in the drier of the two study years. This could be explained by the time intervals between the treatments and first drainage events, which were longer in the wetter year. Results suggest that the drainage phases occurred by preferential flow in the spring-summer period, with correspondingly fast leaching of herbicides, and by matrix flow during the fall-winter period, with slower herbicide migration.     

On the interaction mechanisms of atrazine and hydroxyatrazine with humic substances

Atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) is retained against leaching losses in soils principally by sorption to organic matter, but the mechanism of sorption has been a matter of controversy. Conflicting evidence exists for proton transfer, electron transfer, and hydrophobic interactions between atrazine and soil humus, but no data are conclusive. In this paper we add to the database by investigating the role of (i) hydroxyatrazine (6-hydroxy-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) and (ii) hydrophobicity in the sorption of atrazine by Brazilian soil humic substances. We demonstrate, apparently for the first time, that hydroxyatrazine readily forms electron-transfer complexes with humic substances. These complexes probably are the cause of the well-known strong adsorption by humic acids and they may be the undetected cause of apparent electron-transfer complexes between soil organic matter and atrazine, whose transformation to the hydroxy form is facile. We also present evidence that supports the important contribution of hydrophobic interactions to the pH-dependent sorption of atrazine by humic substances.     

Potential mineralization of four herbicides in a ground water--fed wetland area

Herbicides may leach from agricultural fields into ground water feeding adjacent wetlands. However, only little is known of the fate of herbicides in wetland areas. The purpose of the study was to examine the potential of a riparian fen to mineralize herbides that could leach from an adjacent catchment area. Slurries were prepared from sediment and ground water collected from different parts of a wetland representing different redox conditions. The slurries were amended with O2, NO3-, SO4(2-), and CO2, or CO2 alone as electron acceptors to simulate the in situ conditions and their ability to mineralize the herbides mecoprop, metsulfuron-methyl, isoproturon and atrazine. In addition, the abundance of bacteria able to utilize O2, NO3-, SO4(2-) + CO2, and CO2 as electron acceptors was investigated along with the O2-reducing and methanogenic potential of the sediment. The recalcitrance to bacterial degradation depended on both the type of herbicide and the redox conditions pertaining. Mecoprop was the most readily degraded herbicide, with 36% of [ring-U-14C]mecoprop being mineralized to 14CO2 under aerobic conditions after 473 d. In comparison, approximately 29% of [phenyl-U-14C]metsulfuron-methyl and 16% of [ring-U-14C]isoproturon mineralized in aerobic slurries during the same period. Surprisingly, 8 to 13% of mecoprop also mineralized under anaerobic conditions. Neither metsulfuron-methyl nor isoproturon were mineralized under anaerobic conditions and atrazine was not mineralized under any of the redox conditions examined. The present study is the first to report mineralization of meco-prop in ground water in a wetland area, and the first to report mineralization of a phenoxyalcanoic acid herbicide under both aerobic and anaerobic conditions.     

Differentiating nonpoint sources of deisopropylatrazine in surface water using discrimination diagrams

Pesticide degradates account for a significant portion of the pesticide load in surface water. Because pesticides with similar structures may degrade to the same degradate, it is important to distinguish between different sources of parent compounds that have different regulatory and environmental implications. A discrimination diagram, which is a sample plot of chemical data that differentiates between different parent compounds, was used for the first time to distinguish whether sources other than atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) contributed the chlorinated degradate, deisopropylatrazine (DIA; 6-chloro-N-ethyl-1,3,5-triazine-2,4-diamine) to the Iroquois and Delaware Rivers. The concentration ratio of deisopropylatrazine to deethylatrazine [6-chloro-N-(1-methylethyl)1,3,5-triazine-2,4-diamine], called the D2R, was used to discriminate atrazine as a source of DIA from other parent sources, such as cyanazine (2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methylpropionitrile) and simazine (6-chloro-N,N'-diethyl-1,3,5-triazine-2,4diamine). The ratio of atrazine to cyanazine (ACR) used in conjunction with the D2R showed that after atrazine, cyanazine was the main contributor of DIA in surface water. The D2R also showed that cyanazine, and to a much lesser extent simazine, contributed a considerable amount (approximately 40%) of the DIA that was transported during the flood of the Mississippi River in 1993. The D2R may continue to be a useful discriminator in determining changes in the nonpoint sources of DIA in surface water as cyanazine is currently being removed from the market.     

Iron and aluminium based adsorption strategies for removing arsenic from water

Arsenic is a commonly occurring toxic metal in natural systems and is the root cause of many diseases and disorders. Occurrence of arsenic contaminated water is reported from several countries all over the world. A great deal of research over recent decades has been motivated by the requirement to lower the concentration of arsenic in drinking water and the need to develop low cost techniques which can be widely applied for arsenic removal from contaminated water. This review briefly presents iron and aluminium based adsorbents for arsenic removal. Studies carried out on oxidation of arsenic(III) to arsenic(V) employing various oxidising agents to facilitate arsenic removal are briefly mentioned. Effects of competing ions, As:Fe ratios, arsenic(V) vs. arsenic(III) removal using ferrihydrite as the adsorbent have been discussed. Recent efforts made for investigating arsenic adsorption on iron hydroxides/oxyhydroxides/oxides such as granular ferric hydroxide, goethite, akaganeite, magnetite and haematite have been reviewed. The adsorption behaviours of activated alumina, gibbsite, bauxite, activated bauxite, layered double hydroxides are discussed. Point-of-use adsorptive remediation methods indicate that Sono Arsenic filter and Kanchan™ Arsenic filter are in operation at various locations of Bangladesh and Nepal. The relative merits and demerits of such filters have been discussed. Evaluation of kits used for at-site arsenic estimation by various researchers also forms a part of this review.     

A hydrological modeling framework for defining achievable performance standards for pesticides

This paper proposes a hydrological modeling framework to define achievable performance standards (APSs) for pesticides that could be attained after implementation of recommended management actions, agricultural practices, and available technologies (i.e., beneficial management practices [BMPs]). An integrated hydrological modeling system, Gestion Intégrée des Bassins versants à l'aide d'un Système Informatisé, was used to quantify APSs for six Canadian watersheds for eight pesticides: atrazine, carbofuran, dicamba, glyphosate, MCPB, MCPA, metolachlor, and 2,4-D. Outputs from simulation runs to predict pesticide concentration under current conditions and in response to implementation of two types of beneficial management practices (reduced pesticide application rate and 1- to 10-m-wide edge-of-field and/or riparian buffer strips, implemented singly or in combination) showed that APS values for scenarios with BMPs were less than those for current conditions. Moreover, APS values at the outlet of watersheds were usually less than ecological thresholds of good condition, when available. Upstream river reaches were at greater risk of having concentrations above a given ecological thresholds because of limited stream flows and overland loads of pesticides. Our integrated approach of "hydrological modeling-APS estimation-ecotoxicological significance" provides the most effective interpretation possible, for management and education purposes, of the potential biological impact of predicted pesticide concentrations in rivers.     

Quantifying constraints imposed by calcium and iron on bacterial reduction of uranium(VI)

Uranium is a redox active contaminant of concern to both human health and ecological preservation. In anaerobic soils and sediments, the more mobile, oxidized form of uranium (UO(2)(2+) and associated species) may be reduced by dissimilatory metal-reducing bacteria. Despite rapid reduction in controlled, experimental systems, various factors within soils or sediments may limit biological reduction of U(VI), inclusive of competing electron acceptors and alterations in uranyl speciation. Here we elucidate the impact of U(VI) speciation on the extent and rate of reduction, and we examine the impact of Fe(III) (hydr)oxides (ferrihydrite, goethite, and hematite) varying in free energies of formation. Observed pseudo first-order rate coefficients for U(VI) reduction vary from 12 +/- 0.60 x 10(-3) h(-1) (0 mM Ca in the presence of goethite) to 2.0 +/- 0.10 x 10(-3) h(-1) (0.8 mM Ca in the presence of hematite). Nevertheless, dissolved Ca (at concentrations from 0.2 to 0.8 mM) decreases the extent of U(VI) reduction by approximately 25% after 528 h relative to rates without Ca present. Imparting an important criterion on uranium reduction, goethite and hematite decrease the dissolved concentration of calcium through adsorption and thus tend to diminish the effect of calcium on uranium reduction. Ferrihydrite, in contrast, acts as a competitive electron acceptor and thus, like Ca, decreases uranium reduction. However, while ferrihydrite decreases U(VI) in solutions without Ca, with increasing Ca concentrations U(VI) reduction is enhanced in the presence of ferrihydrite (relative to its absence)-U(VI) reduction, in fact, becomes almost independent of Ca concentration. The quantitative framework described herein helps to predict the fate and transport of uranium within anaerobic environments.     

Behavior of 14C-atrazine in Argentinean topsoils under different cropping managements

Atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) behavior was studied in four surface soils during incubations in laboratory conditions. Soils were chosen in relation to their cropping management (tillage and no tillage) and crop rotation system (continuous soybean [Glycine mar (L.) Merr.] and maize (Zea mays L.)-soybean rotation). A natural soil under brushwood was sampled as a reference. Atrazine use in field conditions was associated with maize cropping, thus only one soil received atrazine every other year. Atrazine behavior was characterized through the balance of 14C-U-ring atrazine radioactivity among the mineralized fraction, the extractable fraction, and the nonextractable bound residues. Soil organic matter capacity to form bound residues was characterized using soil size fractionation. Accelerated atrazine mineralization was only observed in the soil receiving atrazine in field conditions. Atrazine application every other year was enough to develop a microflora adapted to triazine ring mineralization. Bound residue formation was rapid and increased with soil organic matter content. The coarsest soil size fractions (2000-200 and 200-50 microm) containing the nonhumified organic matter presented the highest capacity to form bound residues. No effect of tillage system was observed, probably because of the uniform sampling depth at 20 cm, hiding the stratification pattern of soil organic matter in non-tilled soils.     

In vitro pesticide degradation in turfgrass soil incubated under open and sealed conditions

Degradation of selected pesticides was conducted in a turfgrass soil from a golf course under open (i.e., allowing gas exchange with atmosphere) and sealed systems. The time required for 50% of the initial dose of fenitrothion (O,O-dimethyl O-4-nitro-m-tolyl phosphorothioate), diazinon (O,O-dimethyl O-2-isopropyl-6-methylpyrimidin4-yl phosphorothioate), iprodione [3-(3,5-dichlorophenyl)-N-isopropyl-2,4-dioxo-imidazolidine-1-carboxamide], mecoprop [(RS)-2-(4-chloro-otolyloxy)propionic acid], and asulam (4-aminophenylsulfonyl-carbamate) to dissipate (half-life, t 1/2) was less than 2 wk under both conditions. The t 1/2 values of dithiopyr (S,S'-dimethyl 2-difluoromethyl-4-isobutyl-6-trifluoro-methylpyridine-3,5-dicarbothioate) were 324 and 185 d under the open and sealed conditions, respectively. The t 1/2 values of isoprothiolane (di-isopropyl 1,3-dithiolan-2-ylidene-malonate), flutolanil (alpha,alpha,alpha-trifluoro-3'-isopropoxy-o-toluanilide), and benefin (N-butyl-N-ethyl-alpha,alpha,alpha-trifluoro-2,6-dinitro-p-toluidine) under the open conditions were 154, 336, and 47 d, respectively. The t 1/2 values of these pesticides increased slightly under the sealed conditions. The t 1/2 values of terbutol (2,6-di-tert-butyl-4-methylphenyl N-methycarbamate) and one of the major degradation products, N-demethyl-terbutol (2,6-di-tert-butyl-4-methylphenyl carbamate), were 182 and 291 d under the open conditions and increased by six- and threefold under the sealed conditions, respectively. The degradation system under the sealed conditions could characterize the persistence of terbutol and N-demethyl-terbutol, which were the most persistent in the field.     

Combined application of QEM-SEM and hard X-ray microscopy to determine mineralogical associations and chemical speciation of trace metals

We describe the application of quantitative evaluation of mineralogy by scanning electron microscopy in combination with techniques commonly available at hard X-ray microprobes to define the mineralogical environment of a bauxite residue core segment with the more specific aim of determining the speciation of trace metals (e.g., Ti, V, Cr, and Mn) within the mineral matrix. Successful trace metal speciation in heterogeneous matrices, such as those encountered in soils or mineral residues, relies on a combination of techniques including spectroscopy, microscopy, diffraction, and wet chemical and physical experiments. Of substantial interest is the ability to define the mineralogy of a sample to infer redox behavior, pH buffering, and mineral-water interfaces that are likely to interact with trace metals through adsorption, coprecipitation, dissolution, or electron transfer reactions. Quantitative evaluation of mineralogy by scanning electron microscopy coupled with micro-focused X-ray diffraction, micro-X-ray fluorescence, and micro-X-ray absorption near edge structure (mXANES) spectroscopy provided detailed insights into the composition of mineral assemblages and their effect on trace metal speciation during this investigation. In the sample investigated, titanium occurs as poorly ordered ilmenite, as rutile, and is substituted in iron oxides. Manganese's spatial correlation to Ti is closely linked to ilmenite, where it appears to substitute for Fe and Ti in the ilmenite structure based on its mXANES signature. Vanadium is associated with ilmenite and goethite but always assumes the +4 oxidation state, whereas chromium is predominantly in the +3 oxidation state and solely associated with iron oxides (goethite and hematite) and appears to substitute for Fe in the goethite structure.     

Anionic polyacrylamide effects on soil sorption and desorption of metolachlor,atrazine, 2,4-D,and picloram

Polyacrylamide (PAM) treatment of irrigation water is a growing conservation technology in irrigated agriculture in recent years. There is a concern regarding the environmental impact of PAM after its application. The effects of anionic PAM on the sorption characteristics of four widely used herbicides (metolachlor, atrazine, 2,4-D, and picloram) on two natural soils were assessed in batch equilibrium experiments. Results showed that PAM treatment kinetically reduced the sorption rate of all herbicides, possibly due to the slower diffusion of herbicide molecules into interior sorption sites of soil particles that were covered and/or cemented together by PAM. The equilibrium sorption and desorption amounts of nonionic herbicides (metolachlor and atrazine) were essentially unaffected by anionic PAM, even under a high PAM application rate, while the sorption amounts of anionic herbicides (2,4-D and picloram) were slightly decreased and their desorption amounts increased little. The impact mechanisms of PAM were related to the molecular characteristics of PAM and herbicides. The negative effects of PAM on the sorption of anionic herbicides are possibly caused by the enhancement of electrostatic repulsion by presorbed anionic PAM and competition for sorption sites. However, steric hindrance of the large PAM molecule weakens its influence on herbicide sorption on interior sorption sites of soil particles, which probably leads to the small interference on herbicide sorption, even under high application rates.     

Anaerobic degradation of atrazine and metolachlor and metabolite formation in wetland soil and water microcosms

The half-lives, degradation rates, and metabolite formation patterns of atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] were determined in an anaerobic wetland soil incubated at 24 degrees C for 112 d. At 0, 7, 14, 28, 42, 56, and 112 d, the soil and water were analyzed for atrazine and metolachlor, and their major metabolites. The soil oxidation-reduction potential reached -200 mV after 14 d. Degradation reaction rates were first-order for atrazine in anaerobic soil and for metolachlor in the aqueous phase. Zero-order reaction rates were best fit for atrazine in the aqueous phase and metolachlor in anaerobic soil. In anaerobic soil, the half-life was 38 d for atrazine and 62 d for metolachlor. In the aqueous phase above the soil, the half-life was 86 d for atrazine and 40 d for metolachlor. Metabolites detected in the anaerobic soil were hydroxyatrazine and deethylatrazine for atrazine, and relatively small amounts of ethanesulfonic acid and oxanilic acid for metolachlor. Metabolites detected in the aqueous phase above the soil were hydroxyatrazine, deethylatrazine, and deisopropylatrazine for atrazine, and ethanesulfonic acid and oxanilic acid for metolachlor. Concentrations of metabolites in the aqueous phase generally peaked within the first 25 d and then declined. Results indicate that atrazine and metolachlor can degrade under strongly reducing conditions found in wetland soils. Metolachlor metabolites, ethanesulfonic acid, and oxanilic acid are not significantly formed under anaerobic conditions.     

Infiltration and adsorption of dissolved atrazine and atrazine metabolites in buffalograss filter strips

Vegetated filter strips (VFS) potentially reduce the off-site movement of herbicides from adjacent agricultural fields by increasing herbicide mass infiltrated (Minf) and mass adsorbed (Mas) compared with bare field soil. However, there are conflicting reports in the literature concerning the contribution of Mas to the VFS herbicide trapping efficiency (TE). Moreover, no study has evaluated TE among atrazine (6-chloro-N-ethyl-N'-isopropyl-[1,3,5]triazine-2,4-diamine) and atrazine metabolites. This study was conducted to compare TE, Minf, and Mas among atrazine, diaminoatrazine (DA, 6-chloro[1,3,5]triazine-2,4-diamine), deisopropylatrazine (DIA, 6-chloro-N-ethyl-[1,3,5]triazine-2,4-diamine), desethylatrazine (DEA, 6-chloro-N-isopropyl-[1,3,5]triazine-2,4-diamine), and hydroxyatrazine (HA, 6-hydroxy-N-ethyl-N'-isopropyl-[1,3,5]triazine-2,4-diamine) in a buffalograss VFS. Runoff was applied as a point source upslope of a 1- x 3-m microwatershed plot at a rate of 750 L h(-1). The point source was fortified at 0.1 microg mL(-1) atrazine, DA, DIA, DEA, and HA. After crossing the length of the plot, water samples were collected at 5-min intervals. Water samples were extracted by solid phase extraction and analyzed by high performance liquid chromatography (HPLC) photodiode array detection. During the 60-min simulation, TE was significantly greater for atrazine (22.2%) compared with atrazine metabolites (19.0%). Approximately 67 and 33% of the TE was attributed to Minf and Mas, respectively. These results demonstrate that herbicide adsorption to the VFS grass, grass thatch, and/or soil surface is an important retention mechanism, especially under saturated conditions. Values for Mas were significantly higher for atrazine compared with atrazine's metabolites. The Mas data indicate that atrazine was preferentially retained by the VFS grass, grass thatch, and/or soil surface compared with atrazine's metabolites.     

Use of a low-cost biosorbent to remove pesticides from wastewater

A lignocellulosic substrate (LS) obtained from our local agroindustry was used as a low-cost and effective adsorbent for the removal of pesticides from wastewaters. The studied pesticides were terbumeton (N-(1,1-dimethyl)-Nethyl-6-methoxy-1,3,5-triazine-2,4-diamine), desethyl terbumeton (N-(1,1-dimethylethyl)-6-methoxy-1,3,5-triazine-2,4-diamine), dimetomorph (4-[3-(4-chlorophenyl)-3-(3,4-dimethoxyphenyl)acryloyl]morpholine), and isoproturon (3-(4-isopropylphenyl)-1,1-dimethylurea). Batch and column experiments were conducted as a function of pH and pesticide concentration under laboratory and industrial conditions. The concentration range studied for the pesticides varied from 2 x 10(-7) to 3 x 10(-4) mol L(-1). The influence of organic and inorganic pollutants was assessed by studying the retention of pesticide in the presence of copper(II) and a surfactant. These experiments indicated that LS is an efficient adsorbent toward the investigated pesticides and has little influence of the other pollutants. The kinetic adsorptions are fast, and the amounts of adsorbed pesticide varied from 1 to 8 g kg(-1) of LS. These retention capacities show that LS can provide a simple, effective, and cheap method for removing pesticides from contaminated waters. Thus, this biomaterial may be useful for cleaning up polluted waters.     

Laboratory degradation studies of bentazone, dichlorprop, MCPA, and propiconazole in Norwegian soils

Laboratory degradation studies were performed in Norwegian soils using two commercial formulations (Tilt and Triagran-P) containing either propiconazole alone or a combination of bentazone, dichlorprop, and MCPA. These soils included a fine sandy loam from Hole and a loam from Kroer, both of which are representative of Norwegian agricultural soils. The third soil was a highly decomposed organic material from the Froland forest. A fourth soil from the Skuterud watershed was used only for propiconazole degradation. After 84 d, less than 0.1% of the initial MCPA concentration remained in all three selected soils. For dichlorprop, the same results were found for the fine sandy loam and the organic-rich soil, but in the loam, 26% of the initial concentration remained. After 84 d, less than 0.1% of the initial concentration of bentazone remained in the organic-rich soil, but in the loam and the fine sandy loam 52 and 69% remained, respectively. Propiconazole was shown to be different from the other pesticides by its persistence. Amounts of initial concentration remaining varied from 40, 70, and 82% in the reference soils after 84 d for the organic-rich soil, fine sandy loam, and loam, respectively. The organic-rich soil showed the highest capacity to decompose all four pesticides. The results from the agricultural soils and the Skuterud watershed showed that the persistence of propiconazole was high. Pesticide degradation was approximated to first-order kinetics. Slow rates of degradation, where more than 50% of the pesticide remained in the soil after the 84-d duration of the experiment, did not fit well with first-order kinetics.     

Fate of atrazine in sandy soil cropped with sorghum

A field study was conducted to determine the fate of atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) within the root zone (0 to 90 cm) of a sandy soil cropped with sorghum [Sorghum bicolor (L.) Moench] in Gainesville, Florida. Atrazine was uniformly applied at a rate of 1.12 kg ai. ha(-1) to a sorghum crop under moderate irrigation, optimum irrigation, and no irrigation (rainfed), 2 d after crop emergence. Bromide as a tracer for water movement was applied to the soil as NaBr at a rate of 45 kg Br ha(-1), 3 d before atrazine application. Soil water content, atrazine, and Br concentrations were determined as a function of time using soil samples taken from the root zone. Atrazine sorption coefficients and degradation rates were determined by depth for the entire root zone in the laboratory. Atrazine was strongly adsorbed within the upper 30 cm of soil and most of the atrazine recovered from the soil during the growing season was in that depth. The estimated half-life for atrazine was 32 d in topsoil to 83 d in subsoil. Atrazine concentration within the root zone decreased from 0.44 kg ai. ha(-1) 2 days after application (DAA) to 0.1 kg a.i. ha(-1) 26 DAA. Negligible amounts of atrazine (approximately 5 microg kg(-1)) were detected below the 60-cm soil depth by 64 DAA. Most of the decrease in atrazine concentration in the root zone over time was attributed to degradation. In contrast, all applied bromide had leached past the 60-cm soil depth during the same time interval.