clay and metalaxyl

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

Topics: Alanine; Aluminum Silicates; Clay; Environmental Restoration and Remediation; Fungicides, Industrial; Humans; Hydroxides; Soil; Soil Pollutants; Stereoisomerism

Metalaxyl adsorption and desorption behavior in acid soils were evaluated via batch and stirred-flow chamber experiments. On the basis of batch experiments (adsorption curves of the Giles C-type), metalaxyl has a low affinity for acid soils. Also, as derived from batch and stirred-flow chamber tests, its adsorption in acid soils is dictated mainly by their organic matter and clay contents. The high correlation between these two variables makes it rather complicated to resolve their effects. Metalaxyl adsorption occurs largely (80-99%) via fast adsorption reactions. On the other hand, the pesticide is desorbed in variable proportions (30-100%). The desorption parameters obtained by fitting the results to a pseudo-first-order reaction were correlated with no edaphic variable; however, the q(0)/q(max) ratio, which is a measure of reversibility in the adsorption-desorption process, exhibited significant negative correlation with the organic matter and clay contents.

Topics: Adsorption; Alanine; Aluminum Silicates; Clay; Fungicides, Industrial; Hydrogen-Ion Concentration; Kinetics; Soil

Sorption of three pesticides (chlorpyrifos, metalaxyl and penconazole) has been measured on a commercial clay montmorillonite and on the same mineral modified with either of two cationic-surfactant micelles. Both micelle-clay complexes, commercial names Cloisite 20A and Cloisite 30B, showed a good capacity to sorb all three pesticides from water, whereas their sorption on the natural montmorillonite was not described by an isotherm. Modelling sorption on both micelle-clay complexes showed that the Freundlich sorption constant (K(F)) was higher for chlorpyrifos on Cloisite 20A (K(F) = 7.76) than on Cloisite 30B (K(F) = 5.91), whereas the sorption of metalaxyl was stronger on Cloisite 30B (K(F) = 1.07) than on Cloisite 20A (K(F) = 0.57). Moreover the micelle-clay complex Cloisite 20A also showed a good affinity for penconazole, the maximum quantity adsorbed (q(m)) of 6.33 mg g(-1) being 45% more than that on Cloisite 30B. Single-batch adsorption of each pesticide onto both micelle-clay complexes was studied using the Freundlich isotherm for chlorpyrifos and metalaxyl and the Langmuir isotherm for penconazole. The Cloisite 20A micelle-clay complex was predicted to require 23% less adsorbent to treat certain volumes of wastewater containing 30 mg L(-1) chlorpyrifos, 43% more to treat metalaxyl similarly and 57% less to treat penconazole compared with Cloisite 30B.

Topics: Adsorption; Alanine; Aluminum Silicates; Bentonite; Cations; Chlorpyrifos; Clay; Kinetics; Micelles; Pesticides; Solutions; Surface Properties; Surface-Active Agents; Thermodynamics; Triazoles; Waste Disposal, Fluid; Water

Sorption-desorption of two different hydrophobic fungicides, penconazole and metalaxyl, by a series of clay minerals layered and non-layered (montmorillonite, illite, kaolinite, muscovite, sepiolite and palygorskite) modified with the two-chain cationic surfactant, dihexadecyldimetylammonium (DHDDMA) was studied by first time in this work. DHDDMA-clays showed a good capacity to sorb both fungicides from water. Freundlich sorption constants (K(f)) increased 22-268-fold for penconazole and 4-112-fold for metalaxyl in relation to natural clays. High sorption irreversibility was observed for penconazole for all modified clays, while for metalaxyl this occurred only for some of the modified clays. Additionally, a comparative study of the sorption of the fungicides by clay minerals modified with organic cations of different structure was carried out by a statistical approach. Non-layered and layered clay minerals modified with the two-chain alkylammonium organic cation DHDDMA, and with single-chain organic cations octadecyltrimethylammonium (ODTMA) and hexadecylpyridinium (HDPY) were used as sorbents. The study indicated a significant linear regression between the distribution coefficients K(d) of fungicides and the organic carbon (OC) content of the organo clays (r(2)>or=0.80, p<0.001). According to this sorption of fungicides by partition between the aqueous solution and the organic medium created by the alkyl chains of the exchanged ammonium cations regardless of the type and structure of these cations is suggested. However, linear relationships between K(d) values of each pesticide by the different organo clays and their relative OC contents revealed higher increase in sorption for HDPY-clays (penconazole) and for ODTMA-clays (metalaxyl) pointing out different effectiveness of the OC provided by each organic cation. These effects were also supported by the values of K(d) relative to OC, K(d)(oc). The results evidenced the different relative weight of alkyl cations to make organo clays effective barriers to prevent the mobility of pesticides from a point source of pollution.

Topics: Alanine; Aluminum Silicates; Carbon; Clay; Fungicides, Industrial; Kinetics; Quaternary Ammonium Compounds; Surface-Active Agents; Thermodynamics; Triazoles

Adsorption-desorption studies of metalaxyl in fifteen agricultural soil samples from Southern Portugal and Spain were performed following a batch equilibration method. Very high sorption was observed on a clay soil of high content in altered illite, but, when this soil was excluded from regression analysis, organic matter (OM) was the most important single soil property affecting sorption at low concentrations of metalaxyl. At higher concentrations, no correlation was observed with any soil property. The relevance of OM on sorption was also corroborated by the increase in sorption coefficients when soil OM was increased artificially by the addition of an organic amendment. Sorption studies with the colloidal fraction of selected soils also revealed the importance of mineral surfaces in metalaxyl sorption. Sorption of metalaxyl in most of the soil samples was hysteretic. Selected soil samples were incubated in the dark, sampled periodically and extracted for their fungicide content. Metalaxyl half-lives increased with sorption and OM content of the soil, and were specially high in the amended soil. Leaching studies in hand-packed columns under saturated/unsaturated flow conditions showed an inverse relation between leaching and sorption. Recoveries from the soil columns were close to 80% of the metalaxyl applied, except for the soil which OM was artificially increased, in which recovery was lower and this has been attributed to the much higher irreversibility of metalaxyl sorption in the amended soil.

Topics: Adsorption; Alanine; Aluminum Silicates; Clay; Environmental Monitoring; Minerals; Molecular Structure; Portugal; Soil; Soil Pollutants; Spain; Time Factors