Study of the degradation of the herbicides 2,4-D and MCPA at different depths in contaminated agricultural soil

Natural Attenuation of 2,4-Dichlorophenol in Fe-Rich Soil during Redox Oscillations: Anoxic-Oxic Coupling Mechanism

Natural attenuation of organic contaminants can occur under anoxic or oxic conditions. However, the effect of the coupling anoxic-oxic process, which often happens in subsurface soil, on contaminant transformation remains poorly understood. Here, we investigated 2,4-dichlorophenol (2,4-DCP) transformation in Fe-rich soil under anoxic-oxic alternation. The anoxic and oxic periods in the alternating system showed faster 2,4-DCP transformation than the corresponding control single anoxic and oxic systems; therefore, a higher transformation rate (63.4%) was obtained in the alternating system relative to control systems (27.9-42.4%). Compared to stable pH in the alternating system, the control systems presented clear OH- accumulation, caused by more Fe(II) regeneration in the control anoxic system and longer oxygenation in the control oxic system. Since 2,4-DCP was transformed by ion exchangeable Fe(II) in soil via direct reduction in the anoxic process and induced ·OH oxidation in the oxic process, OH- accumulation was unbeneficial because it competed for proton with direct reduction and inhibited •OH generation via complexing with Fe(II). However, the alternating system exhibited OH--buffering capacity via anoxic-oxic coupling processes because the subsequent oxic periods intercepted Fe(II) regeneration in anoxic periods, while shorter exposure to O2 in oxic periods avoided excessive OH- generation. These findings highlight the significant role of anoxic-oxic alternation in contaminant attenuation persistently.

Fermented biochar has a markedly different effect on fate of pesticides in soil than compost, straw, and a mixed biochar-product

Current knowledge about how biochars affect the fate of pesticides in soil is based on studies that used pure biochars. After finding that an additional biological post-pyrolysis treatment, such as co-composting or lactic fermentation, is required for biochars for superior performance in temperate arable soils, a knowledge gap formed of how such further processed biochar products would affect the fate of pesticides in soil. This study compared the effects of a novel fermented biochar alone or mixed with biogas residues on the fate of two pesticides, 4-chloro-2-methylphenoxyacetic acid (MCPA) and metalaxyl-M, in a temperate arable soil to the traditional organic amendments wheat straw and compost. The fate of 14C-labeled MCPA was markedly affected in different ways. Fermented biochar effectively reduced the water-extractability and mineralization due to adsorption that was comparable to adsorption strengths reported for pure biochars. However, this effect was weak for the biochar mixed with biogas residues. Straw reduced water-extractable amounts due to increased biodegradation and formation of likely biogenic non-extractable residues of MCPA. In contrast, compost decelerated mineralization and increased the water solubility of the MCPA residues due to released dissolved organic matter. The amendments' effects were minor regarding 14C-metalaxyl-M, except for the fermented biochar which again reduced water-extractability and delayed degradation due to adsorption. Thus, the effects of the organic amendments differed for the two pesticide compounds with only the fermented biochar's effect being similar for both. However, this effect was no longer present in the mixed product containing 20% biochar. Our findings clearly show that biologically treated biochar-containing products can affect the fate of pesticides in soil very differently, also when compared to traditional organic amendments. Such impacts and their desirable and undesirable ecotoxicological implications need to be considered before the large-scale application of biochars to temperate arable soils.

Mechanistic Study of the Effects of Agricultural Amendments on Photochemical Processes in Paddy Water during Rice Growth

The photochemical properties of paddy water might be affected by the commonly used amendments in rice fields owing to the associated changes in water chemistry; however, this important aspect has rarely been explored. We examined the effects of agricultural amendments on the photochemistry of paddy water during rice growth. The amendments significantly influenced the photogenerated reactive intermediates (RIs) in paddy water, such as triplet dissolved organic matter (³DOM*), singlet oxygen, and hydroxyl radicals. Compared with control experiments without amendments, the application of straw and lime increased the RI concentrations by up to 16.8 and 11.1 times, respectively, while biochar addition had limited effects on RI generation from paddy water in in situ experiments under sunlight. Fluorescence emission-excitation matrix spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry, and structural equation modeling revealed that upon the addition of straw and lime amendments, humified DOM substances contained lignins, proteins, and fulvic acids, which could produce more RIs under irradiation. Moreover, the amendments significantly accelerated the degradation rate of 2,4-dichlorophenol but led to the ³DOM*-mediated formation of more toxic and stable dimeric products. This study provides new insights into the effects of amendments on the photochemistry of paddy water and the pathways of abiotic degradation of organic contaminants in paddy fields.

Degradation of Atrazine, Simazine and Ametryn in an arable soil using thermal-activated persulfate oxidation process: Optimization, kinetics, and degradation pathway

This study examined the feasibility of applying thermal-activated persulfate (PS) oxidation for remediation of soil co-contaminated with s-triazine herbicides including Atrazine (ATZ), Simazine (SIM) and Ametryn (AME). Homogeneous activation using heating method (50 °C) was selected. Results showed that thermal-activated PS oxidation process may successfully degrade ATZ in soil and degradation efficiency was increased along the arising activation temperature. Higher PS dosages and depressed initial pH were beneficial for degradation while increasing initial ATZ concentration may hamper the degradation. The oxidation process may lead to changes of surface functional groups on soil. The presence of Cl^-, HCO₃^- and H₂PO₄^- at both of low and high concentrations may inhibit the degradation of ATZ. Soil depths may apparently influence the ATZ degradation which followed 0-10 < 10-30 < 30-60 cm mainly depending on the soil organic matter (SOM) contents. Thermal-activated PS may effectively degrade ATZ, SIM and AME under co-contaminated condition and the more favorable of ethyl group towards SO₄^- than isopropyl and methylation groups was detected. Both of SO₄^- and HO were identified to be responsible for degradation. Finally, degradation intermediates of ATZ, SIM and AME were identified by LC-Q-TOF-MS and detailed transformation pathways for three pesticides were proposed, respectively.

Gene-Centric Model Approaches for Accurate Prediction of Pesticide Biodegradation in Soils

Pesticides are widely used in agriculture despite their negative impact on ecosystems and human health. Biogeochemical modeling facilitates the mechanistic understanding of microbial controls on pesticide turnover in soils. We propose to inform models of coupled microbial dynamics and pesticide turnover with measurements of the abundance and expression of functional genes. To assess the advantages of informing models with genetic data, we developed a novel "gene-centric" model and compared model variants of differing structural complexity against a standard biomass-based model. The models were calibrated and validated using data from two batch experiments in which the degradation of the pesticides dichlorophenoxyacetic acid (2,4-D) and 2-methyl-4-chlorophenoxyacetic acid (MCPA) were observed in soil. When calibrating against data on pesticide mineralization, the gene-centric and biomass-based models performed equally well. However, accounting for pesticide-triggered gene regulation allows improved performance in capturing microbial dynamics and in predicting pesticide mineralization. This novel modeling approach also reveals a hysteretic relationship between pesticide degradation rates and gene expression, implying that the biodegradation performance in soils cannot be directly assessed by measuring the expression of functional genes. Our gene-centric model provides an effective approach for exploiting molecular biology data to simulate pesticide degradation in soils.

Time-lapse 3D imaging by positron emission tomography of Cu mobilized in a soil column by the herbicide MCPA

Phenoxyalkanoic acids like the 4-chloro-2-methylphenoxyacetic acid (MCPA) are the second highest used xenobiotic herbicides worldwide after glyphosate because of their apparently favorable environmental properties. Experimental batch equilibration data suggested a reduced Cu adsorption efficiency with the soil mineral goethite below pH 6 in presence of MCPA. This has been verified by advanced surface complexation adsorption modelling involving dissolved Cu-MCPA complexation constants. Positron emission tomography is a non-invasive molecular imaging method for time-resolved three-dimensional information commonly applied on non-retarded tracers in soil core scale experiments. Mineral surface reactive tracers like Cu-64 are too immobile for the relatively short observation times available with this advanced imaging technique. However, Cu-64 radiolabeled Cu-MCPA complex migration could be observed in as long as 10-cm artificial soil test columns where break-through occurred within a few days. For the first time, time-lapse movies of Cu migration in the opaque soil columns were recorded using this novel reactive transport process tomography approach.

Distribution and incorporation mode of the herbicide MCPA in soil derived organo-clay complexes

The incorporation of xenobiotics into soil, especially via covalent bonds or sequestration has a major influence on the environmental behavior including toxicity, mobility, and bioavailability. The incorporation mode of 4-chloro-2-methylphenoxyacetic acid (MCPA) into organo-clay complexes has been investigated under a low (8.5 mg MCPA/kg soil) and high (1000 mg MCPA/kg soil) applied concentration, during an incubation period of up to 120 days. Emphasis was laid on the elucidation of distinct covalent linkages between non-extractable MCPA residues and humic sub-fractions (humic acids, fulvic acids, and humin). The cleavage of compounds by a sequential chemical degradation procedure (OH^-, BBr₃, RuO₄, TMAH thermochemolysis) revealed for both concentration levels ester/amide bonds as the predominate incorporation modes followed by ether linkages. A possible influence of the soil microbial activity on the mode of incorporation could be observed in case of the high level samples. Structure elucidation identified MCPA as the only nonextractable substance, whereas the metabolite 4-chloro-2-methylphenol was additionally found as bioavailable and bioaccessible compound.

Rapid Biodegradation of the Herbicide 2,4-Dichlorophenoxyacetic Acid by Cupriavidus gilardii T-1

Phytotoxicity and environmental pollution of residual herbicides have caused much public concern during the past several decades. An indigenous bacterial strain capable of degrading 2,4-dichlorophenoxyacetic acid (2,4-D), designated T-1, was isolated from soybean field soil and identified as Cupriavidus gilardii. Strain T-1 degraded 2,4-D 3.39 times more rapidly than the model strain Cupriavidus necator JMP134. T-1 could also efficiently degrade 2-methyl-4-chlorophenoxyacetic acid (MCPA), MCPA isooctyl ester, and 2-(2,4-dichlorophenoxy)propionic acid (2,4-DP). Suitable conditions for 2,4-D degradation were pH 7.0-9.0, 37-42 °C, and 4.0 mL of inoculums. Degradation of 2,4-D was concentration-dependent. 2,4-D was degraded to 2,4-dichlorophenol (2,4-DCP) by cleavage of the ether bond and then to 3,5-dichlorocatechol (3,5-DCC) via hydroxylation, followed by ortho-cleavage to cis-2-dichlorodiene lactone (CDL). The metabolites 2,4-DCP or 3,5-DCC at 10 mg L^-1 were completely degraded within 16 h. Fast degradation of 2,4-D and its analogues highlights the potential for use of C. gilardii T-1 in bioremediation of phenoxyalkanoic acid herbicides.

2, 4-Dichloro-6-nitrophenol, a photonitration product of 2, 4-dichlorophenol, caused anti-androgenic potency in Chinese rare minnows (Gobiocypris rarus)

2,4-Dichloro-6-nitrophenol (DCNP) is an environmental transformation product of 2,4-dichlorophenol that has been identified as widespread in effluent wastewater, but little is known about its toxicity because this compound is not regulated. Therefore, to investigate the endocrine disruption potency of DCNP in Chinese rare minnows (Gobiocypris rarus), adult and juvenile fish were exposed to various concentrations of DCNP (2, 20, and 200 μg/L) for 28 d. After 28 d exposure, the plasma vitellogenin (VTG) levels were reduced in females while increased in males and juvenile fish considerably, as compared with the control. These results suggested that DCNP affects the HPG-axis in a sex-dependent way. Testosterone (T) levels in the plasma were significantly lower in adult and juvenile fish and were accompanied by an increased estradiol (E2)/T ratio. Histopathological observation revealed hypertrophy of the hepatocytes and nuclear pyknosis in the liver, the inhibition of spermatogenesis in the testes, and the degeneration of oocytes in the ovaries after DCNP exposure. The expression pattern of selected genes indicated that the nuclear receptor, steroidogenesis and gonadotropin regulation pathways were perturbed after DCNP exposure. Above all, our results demonstrated that DCNP clearly had anti-androgenic activity in both adult and juvenile fish and can therefore be considered as an endocrine-disrupting chemical.

Evidence for the importance of litter as a co-substrate for MCPA dissipation in an agricultural soil

Environmental controls of 2-methyl-4-chlorophenoxyacetic acid (MCPA) degradation are poorly understood. We investigated whether microbial MCPA degraders are stimulated by (maize) litter and whether this process depends on concentrations of MCPA and litter. In a microcosm experiment, different amounts of litter (0, 10 and 20 g kg(-1)) were added to soils exposed to three levels of the herbicide (0, 5 and 30 mg kg(-1)). The treated soils were incubated at 20 °C for 6 weeks, and samples were taken after 1, 3 and 6 weeks of incubation. In soils with 5 mg kg(-1) MCPA, about 50 % of the MCPA was dissipated within 1 week of the incubation. Almost complete dissipation of the herbicide had occurred by the end of the incubation with no differences between the three litter amendments. At the higher concentration (30 mg kg(-1)), MCPA endured longer in the soil, with only 31 % of the initial amount being removed at the end of the experiment in the absence of litter. Litter addition greatly increased the dissipation rate with 70 and 80 % of the herbicide being dissipated in the 10 and 20 g kg(-1) litter treatments, respectively. Signs of toxic effects of MCPA on soil bacteria were observed from related phospholipid fatty acid (PLFA) analyses, while fungi showed higher tolerance to the increased MCPA levels. The abundance of bacterial tfdA genes in soil increased with the co-occurrence of litter and high MCPA concentration, indicating the importance of substrate availability in fostering MCPA-degrading bacteria and thereby improving the potential for removal of MCPA in the environment.

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.

Adsorption of the herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA) by goethite

Interaction between the goethite surface and 4-chloro-2-methylphenoxyacetic acid (MCPA) herbicide was studied using density functional theory (DFT) calculations combined with molecular dynamics (MD). The important step made here lies in the use of a periodic DFT method enabling the study of a mineral surface of different protonation states, in strong contrast with previous molecular modeling studies limited to single protonation state corresponding to the point of zero charge. Different surface OH groups and MCPA proton states were used to mimic the strong effects of pH on the outer- and inner-sphere surface complexes that are theoretically possible, together with their binding energies, and their bond lengths. Modeling both a solvated and a protonated (110) goethite surface provided a major breakthrough in the acidic adsorption regime. An outer-sphere complex and a monodentate inner-sphere complex with the neutral MCPA molecule were found to be the most energetically stable adsorbate forms. MD modeling predicted that the latter forms via the sharing of the carbonyl oxygen between the MCPA carboxylate group and a singly coordinated surface hydroxyl group, releasing an H2O molecule. All the other complexes, including the bidentate inner-sphere complex, had higher relative energies and were therefore less likely. The two most likely DFT-optimized structures were used to constrain a surface complexation model applying the charge distribution multisite complexation (CD-MUSIC) approach. The adsorption constants for the complexes were successfully fitted to experimental batch equilibrium data.

Modeling of pH-dependent adsorption and leaching of MCPA in profiles of Polish mineral soils

This study aimed to determine the leaching potential of MCPA and the contribution of major soil components to its retention in 27 profiles of common Polish arable soils. Results of experiments and modeling of the pH-dependent adsorption indicated that the adsorption of the neutral and anionic forms of MCPA on soil organic matter are the predominant adsorption mechanisms, even in soil horizons with very low organic matter contents. The third most important mechanism was the adsorption of MCPA anions, most likely through the water bridging mechanism on sorption sites created by exchangeable Fe cations adsorbed in soil. It was found that in acidic soils with very low organic matter contents, the neutral form of MCPA was also adsorbed on the quartz and silica sorption sites of silt and sand. The value of r(2) was 78% when it was assumed that MCPA was adsorbed only on organic matter and 93% when all indicated mechanisms of adsorption on mineral soil components were included in the model fitted to data from all soil profiles. Simulations using FOCUS PELMO indicated that in two profiles, the 80th percentiles of the average yearly concentrations of MCPA in the leachate exceeded the European Union (EU) contamination limit of 0.1 μgL(-1). It was shown that when an assumption was made in the model of pH-dependent adsorption that MCPA was adsorbed only on organic matter, the leaching potential of MCPA was overestimated in the examined soils. Much better predictions were obtained when adsorption on mineral soil components was also included in the model.

Evaluation of polyethersulfone performance for the microextraction of polar chlorinated herbicides from environmental water samples

In this work, the suitability of bulk polyethersulfone (PES) for sorptive microextraction of eight polar, chlorinated phenoxy acids and dicamba from environmental water samples is assessed and the analytical features of the optimized method are compared to those reported for other microextraction techniques. Under optimized conditions, extractions were performed with samples (18 mL) adjusted at pH 2 and containing a 30% (w/v) of sodium chloride, using a tubular PES sorbent (1 cm length × 0.7 mm o.d., sorbent volume 8 µL). Equilibrium conditions were achieved after 3h of direct sampling, with absolute extraction efficiencies ranging from 39 to 66%, depending on the compound. Analytes were recovered soaking the polymer with 0.1 mL of ethyl acetate, derivatized and determined by gas chromatography-mass spectrometry (GC-MS). Achieved quantification limits (LOQs) varied between 0.005 and 0.073 ng mL(-1). After normalization with the internal surrogate (IS), the efficiency of the extraction was only moderately affected by the particular characteristics of different water samples (surface and sewage water); thus, pseudo-external calibration, using spiked ultrapure water solutions, can be used as quantification technique. The reduced cost of the PES polymer allowed considering it as a disposable sorbent, avoiding variations in the performance of the extraction due to cross-contamination problems and/or surface modification with usage.

Isolation and characterization of a novel 2-methyl-4-chlorophenoxyacetic acid-degrading Enterobacter sp. strain SE08

A bacterial strain (SE08) capable of utilizing 2-methyl-4-chlorophenoxy acetic acid (MCPA) as the sole carbon and energy source for growth was isolated by continuous enrichment culturing in minimal salt medium (MSM) from a long term MCPA exposed soil. This bacterial strain was identified as Enterobacter sp. based on morphological, physiological and biochemical tests, as well as 16S rRNA sequence analysis. Its ability to degrade MCPA was determined using high performance liquid chromatography. The strain SE08 can tolerate unusually high MCPA concentrations (125-2000mg/L). The influences of culturing factors (initial concentration, pH, and temperature) on the bacterial growth and substrate degradation were studied. The results showed that the optimal MCPA degradation occurred at an MCPA concentration of 500mg/L, 30°C and pH 6.0. Under these conditions, 68.5 percent of MCPA in MSM was degraded by SE08, and the OD600nm reached 0.64 after culturing for 72h. The degradation of MCPA could be enhanced by addition of both carbon and nitrogen sources. At an initial MCPA concentration of 500mg/L, when 5g/L glucose and 2.5g/L yeast extract were added into the MSM media, the MCPA degradation was significantly increased to 83.8 percent, and OD600nm was increased to 1.09 after incubation at 30°C and pH 6.0 for 72h. This is the first study showing that an Enterobacter sp. strain is capable of degrading MCPA, which might provide a new approach for the remediation of MCPA contaminated soil and contribute to the limited knowledge about the function of Enterobacter species.

Behavior of MCPA in four intensive cropping soils amended with fresh, composted, and aged olive mill waste

An evaluation was made of the impact of olive mill waste and its organic matter transformation on the sorption, desorption, leaching, and degradation of the herbicide MCPA when the waste was applied to four Mediterranean soils. The soils were amended in the laboratory with fresh, composted, and field-aged olive mill waste (OW, COW, and AOW treatments, respectively). It was found that the greater the amount of OW applied to the soils, but especially the greater its organic matter maturity, the greater the adsorption of MCPA. Compared with unamended soils, at the 5% rate of application the adsorption capacity increased by between 9.8% and 40%, 148% and 224%, and by 258% for the OW, COW, and AOW amended soils, respectively. The hysteresis coefficients were significantly lower in the OW-amended soils than in AOW or COW-amended soils, indicating that the adsorbed MCPA could be easily desorbed in OW-amended soils if the amendment is not aged or composted. While the OW addition greatly extended the persistence of MCPA, the application of COW enhanced MCPA degradation in all the soils, as corresponded to the increased soil microbial activity indicated by the higher levels of soil dehydrogenase activity. Fresh OW amendment significantly increased the amount of MCPA leached (from 13.7% in the most alkaline soil to 36.7% in the most acidic, at the 5% rate of application), favored by the higher levels of water soluble organic carbon content. However, leaching losses of the herbicide were reduced by up to 39.9% and 55.3% in the COW- and AOW-amended soils at the 5% loading rate, respectively. The use of OW with a high degree of organic matter maturity may be regarded as a potentially useful management practice to reduce MCPA leaching in soils with low organic matter content. The application of fresh OW, however, could well increase the risk of groundwater contamination by this herbicide, especially in acidic soils.

Determination of phenoxy herbicides in water samples using phase transfer microextraction with simultaneous derivatization followed by GC-MS analysis

A sensitive and accurate method for the determination of two model phenoxy herbicides, 4-chloro-2-methylphenoxy acetic acid and 4-chloro-2-methylphenoxy propanoic acid, in water is explained. This method utilizes a simple phase transfer catalyst-assisted microextraction with simultaneous derivatization. Factors affecting the performance of this method including pH of the aqueous matrix, temperature, extraction duration, type and amount of derivatization reagents, and type and amount of the phase transfer catalyst are examined. Derivatization and the use of phase transfer catalyst have proven to be especially vital for the resolution of the analytes and their sensitive determination, with an enrichment factor of 288-fold for catalyzed over noncatalyzed procedure. Good linearity ranging from 0.1 to 80 μg L(-1) with correlation of determination (r(2) ) between 0.9890 and 0.9945 were obtained. Previous reported detection limits are compared with our new current method. The low LOD for the two analytes (0.80 ng L(-1) for 4-chloro-2-methylphenoxy propanoic acid and 3.04 ng L(-1) for 4-chloro-2-methylphenoxy acetic acid) allow for the determination of low concentrations of these analytes in real samples. The absence of matrix effect was confirmed through relative recovery calculations. Application of the method to seawater and tap water samples was tested, but only 4-chloro-2-methylphenoxy propanoic acid at concentrations between 0.27 ± 0.01 and 0.84 ± 0.06 μg L(-1) was detected in seawater samples.

The OH-induced degradation mechanism of 4-chloro-2-methylphenoxyacetic acid (MCPA) with two forms in the water: a DFT comparison

The initial degradation mechanisms of ⁱOH and 4-chloro-2-methylphenoxyacetic acid (MCPA) including molecular form and anionic form are studied at the MPWB1K/6-311+G(3df, 2p)//MPWB1K/6-31+G(d, p) level. Possible reaction pathways of H-atom abstraction and ⁱOH addition are considered in detail. By result comparison analysis, it is found that the reaction mechanisms for ⁱOH and two forms of MCPA are different, and most reactions for anionic MCPA are easier than those for molecular MCPA. For H-atom abstraction reactions, the calculated energies show that ⁱOH abstracting H-atom from -CH(3) group of molecular MCPA is the most kinetically favorable process; the potential energy surface for anionic MCPA indicates that H-atom in -CH(2) group is slightly easier to be abstracted than that in -CH(3) group. For ⁱOH addition reactions, the addition of ⁱOH to the C1 site is the initial step for molecular MCPA and the predominant product is 4-chloro-2-methylphenol (denoted P3), while the C4 site is the most reactive site for anionic MCPA and the primary product results from the hydroxylation of the aromatic ring, which is in good agreement with the experimental observation. In additional, results from PCM calculations show that most reactions in water phase are more kinetically favorable than those in gas phase, though the mechanisms discussed above will not be changed.

Modeling of phenoxy acid herbicide mineralization and growth of microbial degraders in 15 soils monitored by quantitative real-time PCR of the functional tfdA gene

Mineralization potentials, rates, and kinetics of the three phenoxy acid (PA) herbicides, 2,4-dichlorophenoxyacetic acid (2,4-D), 4-chloro-2-methylphenoxyacetic acid (MCPA), and 2-(4-chloro-2-methylphenoxy)propanoic acid (MCPP), were investigated and compared in 15 soils collected from five continents. The mineralization patterns were fitted by zero/linear or exponential growth forms of the three-half-order models and by logarithmic (log), first-order, or zero-order kinetic models. Prior and subsequent to the mineralization event, tfdA genes were quantified using real-time PCR to estimate the genetic potential for degrading PA in the soils. In 25 of the 45 mineralization scenarios, ∼60% mineralization was observed within 118 days. Elevated concentrations of tfdA in the range 1 × 10(5) to 5 × 10(7) gene copies g(-1) of soil were observed in soils where mineralization could be described by using growth-linked kinetic models. A clear trend was observed that the mineralization rates of the three PAs occurred in the order 2,4-D > MCPA > MCPP, and a correlation was observed between rapid mineralization and soils exposed to PA previously. Finally, for 2,4-D mineralization, all seven mineralization patterns which were best fitted by the exponential model yielded a higher tfdA gene potential after mineralization had occurred than the three mineralization patterns best fitted by the Lin model.

Metabolic fate of 2,4-dichlorophenol and related plant residues in rats

This study compared the metabolic fate of [(14)C]-DCP, [(14)C]-residues from radish plants, and purified [(14)C]-DCP-(acetyl)glucose following oral administration in rats. A rapid excretion of radioactivity in urine occurred for [(14)C]-DCP, [(14)C]-DCP-(acetyl)glucose, and soluble residues, 69, 85, and 69% within 48 h, respectively. Radio-HPLC profiles of 0-24 h urine from rats fed [(14)C]-DCP and [(14)C]-DCP-(acetyl)glucose were close and qualitatively similar to those obtained from plant residues. No trace of native plant residues was detected under the study conditions. The structures of the two major peaks were identified by MS as the glucuronide and the sulfate conjugates of DCP. The characterization of a dehydrated glucuronide conjugate by MS and NMR of DCP was unusual. In contrast to soluble residues, bound residues were mainly excreted in feces, 90% within 48 h, whereas total residues were eliminated in both urine and feces. For total residues, the radioactivity in feces was higher than expected from the percentage of soluble and bound residues in radish plants. This result highlighted that less absorption took place when residues were present in the plant matrix as compared to plant-free residues and DCP.

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.

Peroxidase-catalyzed stabilization of 2,4-dichlorophenol in alkali-extracted soils

Horseradish peroxidase- (HRP) mediated stabilization of phenolic contaminants is a topic of interest due to its potential for remediation of contaminated soils. This study evaluated the sorption of 2,4-dichlorophenol (DCP) and its HRP-mediated stabilization in two alkali-extracted soils. Alkali extraction reduced the soil organic matter (SOM) contents of the geomaterials and enriched the residual SOM with humin C. Sorption of DCP on these sorbents was complete within 1 d. However, most of the sorbed DCP was removed from the geomaterials by water and methanol, suggesting weak solute-sorbent interactions. The addition of HRP resulted in the generation of DCP polymerization products (DPP), which partitioned between the aqueous and solid phases. The DPP phase distribution was rapid and complete within 24 h. Between 70 and 90% of the added DCP was converted to DPP and up to 43% of the initial aqueous phase contaminant was transformed into a residue that was resistant to extraction with methanol. Bound residues of DPP increased with initial aqueous phase solute concentration and remained fairly constant after 7 d of contact. Contaminant stabilization was noted to be high in the humin-mineral geomaterial. Results illustrate that HRP may be effective in stabilizing phenolic contaminants in subsoils that are likely to contain SOM enriched in humin C.

Soil-dissipation kinetics of twelve herbicides used on a rain-fed barley crop in Spain

This study evaluated the dissipation kinetics under actual field conditions of twelve herbicides in a typical xerofluvent soil in Castilla y León (north central Spain) sustaining barley. The type of soil selected was that typically used in the Castilla y León region to cultivate barley under a rain-fed alternating crop-fallow rotation regimen. Treatments were conducted in spring as two replicates and the soil was sampled every day during the first week, once a week for the following few weeks and thereafter once every month. Soil samples were extracted with a suitable mixture of acetone, water and acetic acid (30:7.5:0.3) before their analysis by GC-MS (gas chromatography-mass spectrometry). Dissipation of the herbicides was well described by a biphasic kinetics pattern. The dissipation times DT50 and DT90 were in general lower than those reported in the literature, owing to a high initial dissipation rate because of volatilization and photolysis processes caused by high environmental temperatures. Herbicide degradation was also enhanced by their lack of sorption by this low colloid-content soil. However, the most persistent herbicides, triallate, flamprop, pendimethalin, terbutryn, and isoproturon, remained for 286 to 372 days in the soil, because low water and organic carbon content impaired microbial growth. In contrast, the phenoxy acid herbicides dissipated rapidly, with no detectable residues detected on harvesting the crop.

Spatial variation in 2-methyl-4-chlorophenoxyacetic acid mineralization and sorption in a sandy soil at field level

The phenoxyacetic acid herbicide MCPA (2-methyl-4-chlorophenoxyacetic acid) is frequently detected in groundwater beneath Danish agricultural fields. We investigated spatial variation in microbial MCPA mineralization potential in a flat agricultural field of fine sandy soil (USDA classification: Humic Dystrudept) located on the Yoldia plains of Northern Jutland, Denmark. Samples for determination of MCPA mineralization and sorption were collected from the Ap and Bs horizons at 51 sampling sites located in a 200 x 220 m grid. Spatial variation in sorption was low in both horizons (distribution coefficient, 0.36-4.16 L kg(-1)). Sorption correlated strongly with soil organic carbon content in both horizons (CV, 93 and 83%, respectively) and negatively with soil pH. [Ring-(14)C]-MCPA mineralized readily in the Ap horizon, with 49 to 62% of the (14)C-MCPA being converted to (14)CO(2) during the 67-d incubation period. With the subsoil, mineralization of (14)C-MCPA varied considerably between samples (0.5-72.8%). At neither depth was there correlation between (14)C-MCPA mineralization and sorption, soil pH, organic carbon content, clay content, number of colony-forming units (CFU), pseudomonad CFU, or any of the four microbial activity parameters measured. The presence of microbial genes encoding for the TfdA enzyme was quantified using real-time polymerase chain reaction. No correlation was found between MCPA mineralization potential and the natural background number of tfdA genes present in the soil samples. The degradation kinetics suggests that the high (14)C-MCPA mineralization rate detected in soil samples was linked to growth of the MCPA-degrading soil microbial community.

Impact of basic soil parameters on pesticide disappearance investigated by multivariate partial least square regression and statistics

Dissipation time is a key parameter when studying and modeling the environmental fate of pesticides. This study was conducted to characterize the variability of pesticide disappearance in soil and to identify possible controlling parameters related to intrinsic soil properties and microbiology. Multivariate data analysis was used to study spatial variability in three horizons from 24 sandy soil profiles. The time for 50% disappearance (DT(50)) was characterized for two herbicides, metribuzin (MBZ) and MCPA, and methyltriazine amine (MTA; transformation product of metsulfuron-methyl, tribenuron-methyl, thifensulfuron-methyl, and chlorsulfuron). Normal and log-normal distributions were compared for DT(50) and soil properties and descriptive statistics were calculated. Conformity with log-transformed distributions was observed and assuming normality of the DT(50) data would cause 5 to 35% overestimation. Mean DT(50) were: MCPA 9.5, MBZ 168, and MTA 127. Significant effect of soil depth on DT(50) was shown for MCPA and MBZ, with low values in deeper horizons. Simple linear correlation for combinations of MCPA, MTA, pH, and total organic carbon (TOC) was observed. Using partial least squares regression (PLS) 71 to 85% of the total DT(50) variance was explained. A specific predictor variable could not be identified as the controlling components differed within horizons and compounds. For MCPA the overall important predictor variables were microbiology and TOC, whereas for MTA and MBZ it was inorganic variables (Al, Fe, cation exchange capacity, base saturation percent, and pH) and microbiology. The study indicates that PLS generated input data can improve pesticide fate modeling and reduce the uncertainty in dissipation estimation.

Occurrence and fate of pesticides in four contrasting agricultural settings in the United States

Occurrence and fate of 45 pesticides and 40 pesticide degradates were investigated in four contrasting agricultural settings--in Maryland, Nebraska, California, and Washington. Primary crops included corn at all sites, soybeans in Maryland, orchards in California and Washington, and vineyards in Washington. Pesticides and pesticide degradates detected in water samples from all four areas were predominantly from two classes of herbicides--triazines and chloroacetanilides; insecticides and fungicides were not present in the shallow ground water. In most samples, pesticide degradates greatly exceeded the concentrations of parent pesticide. In samples from Nebraska, the parent pesticide atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] was about the same concentration as the degradate, but in samples from Maryland and California atrazine concentrations were substantially smaller than its degradate. Simazine [6-chloro-N,N'-diethyl-1,3,5-triazine-2,4-diamine], the second most detected triazine, was detected in ground water from Maryland, California, and Washington. Metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] rarely was detected without its degradates, and when they were detected in the same sample metolachlor always had smaller concentrations. The Root-Zone Water-Quality Model was used to examine the occurrence and fate of metolachlor at the Maryland site. Simulations accurately predicted which metolachlor degradate would be predominant in the unsaturated zone. In analyses of relations among redox indicators and pesticide variance, apparent age, concentrations of dissolved oxygen, and excess nitrogen gas (from denitrification) were important indicators of the presence and concentration of pesticides in these ground water systems.

Spiking solvent, humidity and their impact on 2,4-D and 2,4-DCP extractability from high humic matter content soils

The 2,4-dichlorophenoxyacetic acid (2,4-D) is a hormone-like herbicide widely used in agriculture. Although its half life in soil is approximately two weeks, the thousands of tons introduced in the environment every year represent a risk for human health and the environment. Considering the toxic properties of this compound and its degradation products, it is important to assess and monitor the 2,4-D residues in agricultural soils. Furthermore, experiments of phyto/bioremediation are carried out to find economic and environmental friendly tools to restore the polluted soils. Accordingly, it is essential to accurately measure the amount of 2,4-D and its metabolites in soils. There is evidence that 2,4-D extraction from soil samples seriously depends on the physical and chemical properties of the soil, especially in those soils with high content of humic acids. The aim of this work was to assess the variables that influence the recovery and subsequent analysis of 2,4-D and its main metabolite (2,4-dichlorophenol) from those soils samples. The results showed that the recovery efficiency depends on the solvent and method used for the extraction, the amount and kind of solvent used for dissolving the herbicide and the soil water content at the moment of spiking. An optimized protocol for the extraction and quantification of 2,4-D and its main metabolite from soil samples is presented.

Runoff of pesticides from rice fields in the Ile de Camargue (Rhône river delta, France): field study and modeling

A field study on the runoff of pesticides was conducted during the cultivation period in 2004 on a hydraulically isolated rice farm of 120 ha surface with one central water outlet. Four pesticides were studied: Alphamethrin, MCPA, Oxadiazon, and Pretilachlor. Alphamethrin concentrations in runoff never exceeded 0.001 microg L(-1). The three other pesticides were found in concentrations between 5.2 and 28.2 microg L(-1) in the runoff water shortly after the application and decreased thereafter. The data for MCPA compared reasonably well with predictions by an analytical runoff model, accounting for volatilization, degradation, leaching to groundwater, and sorption to soil. The runoff model estimated that runoff accounted for as much as 18-42% of mass loss for MCPA. Less runoff is observed and predicted for Oxadiazon and Pretilachlor. It was concluded that runoff from rice paddies carries important loads of dissolved pesticides to the wetlands in the Ile de Camargue, and that the model can be used to predict this runoff.

Dissipation of 2,4-D in soils of the Humid Pampa region, Argentina: a microcosm study

Phenoxy herbicides like 2,4-dichlorophenoxyacetic acid (2,4-D) are widely used in agricultural practices. Although its half life in soil is 7-14d, the herbicide itself and its first metabolite 2,4-dichlorophenol (2,4-DCP) could remain in the soil for longer periods, as a consequence of its intensive use. Microcosms assays were conducted to study the influence of indigenous microflora and plants (alfalfa) on the dissipation of 2,4-D from soils of the Humid Pampa region, Argentina, with previous history of phenoxy herbicides application. Results showed that 2,4-D was rapidly degraded, and the permanence of 2,4-DCP in soil depended on the presence of plants and soil microorganisms. Regarding soil microbial community, the presence of 2,4-D degrading bacteria was detected even in basal conditions in this soil, possibly due to the adaptation of the microflora to the herbicide. There was an increment of two orders of magnitude in herbicide degraders after 15d from 2,4-D addition, both in planted and unplanted microcosms. Total heterotrophic bacteria numbers were about 1x10(8) CFUg(-1) dry soil and no significant differences were found between different treatments. Overall, the information provided by this work indicates that the soil under study has an important intrinsic degradation capacity, given by a microbial community adapted to the presence of phenoxy herbicides.

Occurrence of 2,4-dichlorophenol and of 2,4-dichloro-6-nitrophenol in the Rhone River Delta (Southern France)

The compounds 2,4-dichlorophenol (2,4-DCP) and 2,4-dichloro-6-nitrophenol (6-nitro-2,4-DCP) have been detected at microg L(-1) levels (10(-9)-10(-8) M) during the summer season 2005 in the water of the Rhône river delta. Compound 2,4-DCP would mainly derive from the transformation of the herbicide dichlorprop, heavily used in flooded rice farming (1400 kg in the delta region in 2005), in addition to being an impurity of the commercial herbicide. Field data show a fast concentration decrease of 2,4-DCP in the period June 21st to July 5th, accompanied by a corresponding increase of 6-nitro-2,4-DCP. This could imply a possible nitration process of 2,4-DCP into 6-nitro-2,4-DCP, with quite elevated yield (33%). Nitration of 2,4-DCP can be induced by photoproduced *NO2, the reaction kinetics (calculated in the presence of Fe(III) + nitrite under irradiation as model system) being d[6-nitro-2,4-DCP]/dt = 650 [2,4-DCP] [*NO2]. Interestingly, the yield of the process (38%) is similar to that suggested by field data. An indirect assessment of [*NO2] in surface water in different sites of the Rhône delta indicated that 2,4-DCP could be transformed into 6-nitro-2,4-DCP in a couple of weeks or less in the shallow water (10 cm depth) of the rice fields, a time scale that is compatible with field data. Photonitration of 2,4-DCP is thus a possible process to account for the occurrence of 6-nitro-2,4-DCP in the Rhône delta.

Incorporating variations in pesticide catabolic activity into a GIS-based groundwater risk assessment

The catabolic activity of incumbent microorganisms in soil samples of eleven dissimilar soil series was investigated, with respect to the herbicide isoproturon. Soils were collected from a 30x37 km area of river catchment to the north-west of London, England. Catabolic activity in each soil type during a 500 h assay was determined by 14C-radiorespirometry. Results showed four soils that exhibited high levels of catabolic activity (33-44% mineralisation) while the remaining seven soils showed lower levels of catabolic activity (12-16% mineralisation). There was evidence to suggest that soils exhibiting high catabolic activity had low (<22%) clay content and tended towards lower organic carbon content (<2.7%), but that these higher levels of catabolic activity were also related to pre-exposure to isoproturon. The 14C-radiorespirometric results were used to produce a GIS layer representing levels of catabolic activity for the dissimilar soils across the study area. This layer was combined with other GIS layers relating to pesticide attenuation, including soil organic carbon content, depth to groundwater and hydrogeology, to produce a map showing risk of groundwater contamination by isoproturon. The output from this approach was compared with output from an attenuation-only approach and differences appraised. Inclusion of the catabolism layer resulted in a lowering of risk in the model in 15% of the study area. Although there appears to be limited benefit in including pesticide catabolic activity in this regional-scale groundwater risk model, this type of addition could be useful in a site-specific risk assessment.

Sorption, desorption and mineralisation of the herbicides glyphosate and MCPA in samples from two Danish soil and subsurface profiles

The vertical distribution of the sorption, desorption and mineralisation of glyphosate and MCPA was examined in samples from two contrasting soil and subsurface profiles, obtained from a sandy agricultural site and a non-agricultural clay rich site. The highest mineralisation of [14C-methylen]glyphosate, with 9.3-14.7% degraded to 14CO2 within 3 months was found in the deepest sample from the clay site. In the deeper parts of the sandy profile high sorption and low desorption of glyphosate coincided with no or minor mineralisation indicating a limited glyphosate bioavailability. MCPA was readily mineralised except in the deepest samples from both sites. The highest MCPA mineralisation was detected just below the surface layers with 72% or 44% degraded to 14CO2 at the sandy or the clay sites, respectively. MCPA sorped to a minor extent in all samples and no indications of sorption-controlled mineralisation was revealed. None of the herbicides were mineralised under anoxic conditions.

Degradation of 4-chloro-2-methylphenoxyacetic acid in top- and subsoil is quantitatively linked to the class III tfdA gene

The tfdA gene is known to be involved in the first step of the degradation of the phenoxy acid herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA) in several soil bacteria, but bacteria containing other tfdA-like genes have been isolated as well. A quantitative real-time PCR method was used to monitor the increase in the concentration of tfdA genes during degradation of MCPA in sandy topsoil and subsoil over a period of 115 days. Quantitative PCR revealed growth in the tfdA-containing bacterial community, from 500 genes g(-1) soil to approximately 3 x 10(4) genes g(-1) soil and to 7 x 10(5) genes g(-1) soil for topsoil initially added to 2.3 mg MCPA kg(-1) (dry weight) soil and 20 mg MCPA kg(-1) (dry weight) soil, respectively. We analyzed the diversity of the tfdA gene during the degradation experiment. Analyses of melting curves of real-time PCR amplification products showed that a shift in the dominant tfdA population structure occurred during the degradation period. Further denaturing gradient gel electrophoresis and sequence analysis revealed that the tfdA genes responsible for the degradation of MCPA belonged to the class III tfdA genes, while the tfdA genes present in the soil before the occurrence of degradation belonged to the class I tfdA genes. The implications of these results is that the initial assessment of functional genes in soils does not necessarily reflect the organisms or genes that would carry out the degradation of the compounds in question.

The evaluation of different sorbents for the preconcentration of phenoxyacetic acid herbicides and their metabolites from soils

A procedure using alkaline extraction, solid-phase extraction (SPE) and HPLC is developed to analyze the polar herbicides 2,4-dichlorophenoxyacetic acid (2,4-D) and 4-chloro-2-methylphenoxyacetic acid (MCPA) together with their main metabolites in soils. An ion-pairing HPLC method is used for the determination as it permits the baseline separation of these highly polar herbicides and their main metabolites. The use of a highly cross-linked polystyrene-divinylbenzene sorbent (PS-DVB) gives the best results for the analysis of these compounds. This sorbent allows the direct preconcentration of the analytes at the high pH values obtained after quantitative alkaline extraction of the herbicides from soil samples. Different parameters are evaluated for the SPE preconcentration step. The high polarity of the main analytes of interest (2,4-D and MCPA) makes it necessary to work at low flow rates (< or =0.5 mL min(-1)) in order for these compounds to be retained by the PS-DVB sorbent. A two stage desorption from the SPE sorbent is required to obtain the analytes in solvents that are appropriate for HPLC determination. A first desorption with a 50:50 methanol:water mixture elutes the most polar analytes (2,4-D, MCPA and 2CP). The second elution step with methanol permits the analysis of the other phenol derivatives. The humic and fulvic substances present in the soil are not efficiently retained by PS-DVB sorbents at alkaline pH's and so do not interfere in the analysis. This method has been successfully applied in the analysis of soil samples from a golf course treated with a commercial product containing esters of 2,4-D and MCPA as the active components.

Time-dependent sorption-desorption behavior of 2,4-dichlorophenol and its polymerization products in surface soils

Contact time-dependent sorption-desorption of 2,4-dichlorophenol (DCP) and DCP polymerization products (DPP) was investigated in the context of agricultural and woodland soils. DPP was generated in soil slurry reactors by the addition of H2O2 to solutions containing horseradish peroxidase (HRP) and DCP. Size-exclusion chromatography confirmed the formation of oligomeric products including dimers, trimers, and tetramers. DCP removal from HRP-amended, soil-free solutions occurred as a result of DPP formation and sorption of DCP to the oligomeric precipitate. In reactors containing soil particles, additional removal occurred due to sorption of residual DCP and DPP to the soil. Sorption of DCP and DPP to the surface soils was rapid and appeared to be complete within 1 day. DPP sorbed to a greater extent than DCP, especially at higher solute concentrations. Water-extraction data indicated that while sorbed DCP was readily released into solution, sorbed DPP was more resistant to desorption. Both DCP and DPP were more readily extracted from the woodland soil than the agricultural soil. Solute extractability decreased with contact time, indicating that sorbent-solute interactions proceeded long after apparent sorption equilibrium at the particle or aggregate scale. Results from this study show that "slow" sorption processes occurring at the molecular scale continue long after apparent sorption equilibrium at the particle scale. This "aging effect" was operational over longer periods for DPP than the parent DCP, thereby ensuring long-term reductions in contaminant mobility in soils treated with peroxidase enzyme.

Influence of plants on the chemical extractability and biodegradability of 2,4-dichlorophenol in soil

This study investigated the fate and behaviour of [UL-(14)C] 2,4-dichlorophenol (DCP) in planted (Lolium perenne L.) and unplanted soils over 57 days. Extractability of [UL-(14)C] 2,4-DCP associated activity was measured using calcium chloride (CaCl(2)), acetonitrile-water and dichloromethane (DCM) extractions. Biodegradability of [UL-(14)C] 2,4-DCP associated activity was assessed through measurement of (14)CO(2) production by a degrader inoculum (Burkholderia sp.). Although extractability and mineralisation of [UL-(14)C] 2,4-DCP associated activity decreased significantly in both planted and unplanted soils, plants appeared to enhance the sequestration process. After 57 days, in unplanted soil, 27% of the remaining [UL-(14)C] 2,4-DCP associated activity was mineralised by Burkholderia sp., and 13%, 48%, and 38% of (14)C-activity were extracted by CaCl(2), acetonitrile-water and DCM, respectively. However, after 57 days, in planted soils, only 10% of the [UL-(14)C] 2,4-DCP associated activity was available for mineralisation, whilst extractability was reduced to 2% by CaCl(2), 17% by acetonitrile-water and 11% by DCM. This may be due to the effect of plants on soil moisture conditions, which leads to modification of the soil structure and trapping of the compound. However, the influence of plants on soil biological and chemical properties may also play a role in the ageing process.

Chlorinated pesticides (2,4-D and DDT) biodegradation at high concentrations using immobilized Pseudomonas fluorescens

Degradation of two chlorinated pesticides (2,4-D and DDT) using a 54-mL glass column packed with tezontle (a low-cost basaltic scoria) was tested. Bacteria were cultured in YPG (yeast, peptone, and glucose) liquid medium at 32 degrees C. The rich medium was pumped during 24 h through the column to inoculate it. Later, the wasted medium was discharged and the pesticide added. Optical densities, TOC, and pesticide concentration were determined. Pesticide removals for 2,4-D (with initial concentration between 100 and 500 mg/L) were about 99%. DDT removal (at initial concentration of up to 150 mg/L) was as high as 55-99%. TOC removals for 2,4-D was in the 36-87% interval, whereas for DDT they were as high as 36-78%.

Sorption-controlled degradation kinetics of MCPA in soil

The relationship between sorption strength and degradation kinetics has been studied for the pesticide MCPA in a sandy top- and subsoil. After adding two types of sorbents (crushed peat and activated carbon) in various amounts to the sandy soils, sorption, desorption, and mineralization of 14C-MCPA were measured. The obtained Freundlich constants (KF) varied between 0.7 and 27.2 mg(1-nF) x L(nF)/kg, and the first-order mineralization rate constants varied between 0.001 and 0.128 d(-1). The results showed an inverse relationship between sorption strength and mineralization. A higher KF value corresponded to a smaller mineralization rate and less mineralization. A correlation coefficient of r2 = 0.934 between the log-transformed Freundlich desorption coefficient (K(F,des)) and the log-transformed mineralization rate constant (k) was obtained. After 7, 14, 22, and 35 days of incubation, soil samples were consecutively extracted by water, methanol, and 5 M NaOH to separate the remaining 14C into 3 different pools. The extractions showed that the mineralization only proceeded from the water extractable pool of MCPA. Thin-layer chromatography revealed a formation of small amounts of metabolites; <7% of initially added 14C was present as other compounds than 14C-MCPA in the water and methanol extractable pools. The study showed mineralization to be strongly correlated with sorption, represented by the desorption coefficient, and hence stresses the significance of bonding strength for estimating pesticide degradation in soil.

Can degradation products be used as documentation for natural attenuation of phenoxy acids in groundwater?

In situ indicators of degradation are important tools in the demonstration of natural attenuation. A literature survey on the production history of phenoxy acids and degradation pathways has shown that metabolites of phenoxy acid herbicides also are impurities in the herbicide products, making the bare presence of these compounds useless as in situ indicators. These impurities can make up more than 30% of the herbicides. Degradation of phenoxy acids was demonstrated in microcosm experiments using groundwater and sediment contaminated with MCPP, dichlorprop, and related compounds such as other phenoxypropionic acids and chlorophenols. Field observations at two phenoxy acid-contaminated sites showed the occurrence of several impurities including metabolites in the groundwater. Neither the microcosm experiments nor the field observations verified that metabolites were actually produced or accumulated in situ. However, it was demonstrated that the impurity/parent herbicide ratios can be useful in situ indicators of degradation.

Toxicity of chlorinated phenoxyacetic acid herbicides in the experimental eukaryotic model Saccharomyces cerevisiae: role of pH and of growth phase and size of the yeast cell population

The inhibitory effect of the herbicides 2-methyl-4-chlorophenoxyacetic acid (MCPA) and 2,4-dichlorophenoxyacetic acid (2,4-D) in Saccharomyces cerevisiae growth is strongly dependent on medium pH (range 2.5-6.5). Consistent with the concept that the toxic form is the liposoluble undissociated form, at values close to their pK(a) (3.07 and 2.73, respectively) the toxicity is high, decreasing with the increase of external pH. In addition, the toxicity of identical concentrations of the undissociated acid form is pH independent, as observed with 2,4-dichlorophenol (2,4-DCP), an intermediate of 2,4-D degradation. Consequently, at pH values above 3.5 (approximately one unit higher than 2,4-D pK(a)), 2,4-DCP becomes more toxic than the original herbicide. A dose-dependent inhibition of growth kinetics and increased duration of growth latency is observed following sudden exposure of an unadapted yeast cell population to the presence of the herbicides. This contrasts with the effect of 2,4-DCP, which essentially affects growth kinetics. Experimental evidences suggest that the acid herbicides toxicity is not exclusively dependent on the liposolubility of the toxic form, as may essentially be the case of 2,4-DCP. An unadapted yeast cell population at the early stationary-phase of growth under nutrient limitation is significantly more resistant to short-term herbicide induced death than an exponential-phase population. Consequently, the duration of growth latency is reduced, as observed with the increase of the size of the herbicide stressed population. However, these physiological parameters have no significant effect either on growth kinetics, following growth resumption under herbicide stress, or on the growth curve of yeast cells previously adapted to the herbicides, indicating that their role is exerted at the level of cell adaptation.

Solid-phase extraction method for the determination of free and conjugated phenol compounds in human urine

A rapid flow system for automatic sample conditioning for the determination of phenol compounds in human urine has been developed and optimised. Free phenols are detected directly in urine samples while total phenols require acid hydrolysis to convert their conjugate fraction into free phenols, all compounds then being cleaned up and preconcentrated by solid-phase extraction. Separation and determination are done by gas chromatography, using mass spectrometry operating in the selective ion monitoring mode for quantitation. The linear range was 1-160 ng/ml of urine for most of the phenols. Limits of detection for phenol compounds (phenol, alkylphenols and chlorophenols) in the nanogram-per-millilitre range (0.3-0.6 ng/ml) are thus achieved by using 1 ml of urine; also, the repeatability, as RSD, is less than 6.5%. Based on the results for urine samples from unexposed individuals, 2-methylphenol, 2-chlorophenol and 2,4-dichlorophenol are largely detected in hydrolysed urine samples, whereas phenol and 4-methylphenol are detected in hydrolysed and unhydrolysed urine. Other chlorophenols such as trichlorophenols and pentachlorophenol are not detected. The results obtained in the analysis of urine from an individual before and after dietary intake reveal that the levels of phenol compounds in urine look related to food intake.

Analysis of phenoxyalkanoic acid herbicides and their phenolic conversion products in soil by microwave assisted solvent extraction and subsequent analysis of extracts by on-line solid-phase extraction-liquid chromatography

A multiresidue method for the determination of phenoxyalkanoic acid herbicides and their phenolic conversion products in soil was developed. The method was based on microwave-assisted solvent extraction (MASE) of soil samples by an aqueous methanolic mixture and subsequent analysis of extracts by automated solid-phase extraction followed by on-line high-performance liquid chromatography and diode array detection. MASE parameters (extraction temperature and time, composition of the extraction mixture and extraction volume) were optimized with respect to analyte recoveries. The method was validated with two types of soils containing 1.5 and 3.5% organic matter, respectively, both types containing fresh and aged residues of sought analytes. Under the selected analytical conditions when soils with fresh residues were analyzed all target analytes were recovered above 80% from the soil containing 1.5% organic matter, while limits of identification at the level of 20-40 ng/g were achieved. From the soil containing 3.5% organic matter the least polar phenolic analytes exhibited slightly reduced recoveries, while identification limits of 30-50 ng/g were achieved. Samples with aged residues exhibited reduced recoveries for some analytes, the reduction amounting up to 6-12% within 1 month of aging period depending on soil organic matter.