The binding of a weakly acidic herbicide (N‐2, 6‐difluorophenyl‐5‐methyl‐1,2,4‐triazolo(1,5a)pyrimidine‐2‐sulfonamide) to soil is presented as an ionization of the weak acid in solution, and the partitioning of both the anionic and neutral forms of the compound into the soil organic matter. This theoretical description is combined with experimentally determined bulk adsorption constants to give the organic C adsorption constants (Koc) for neutral (650 ± 40 L kg–1) and anionic (12 ± 1 L kg–1) forms of the molecule. Additional experiments with four soils adjusted to different pH levels gave an average Koc of 750 L kg±1 for the neutral form. Describing the soil sorption as a combination of anionic and neutral forms provided an adequate description of the measured sorption.

Summary: Résumé: Zusammenfassung The degradation of DE‐498 (proposed common name flumetsulam) was studied in 21 US soils as a first step in developing a management plan for this new herbicide. Degradation half‐lives were shorter in soils that adsorbed the compound less. Adsorption was lower in soils with higher pH and lower organic carbon content. Degradation half‐lives were thus influenced by both pH and organic carbon: they were in the range 2–4 weeks in higher pH soils (pH 5 7.0) unless the organic carbon content was above 2.5%, at which half‐lives were in the range 1–3 months. In medium pH soils (6.4 pH 6.9) half‐lives were 1–2 months, while in lower pH soils (5.9 pH 6.3) they were 1–4 months. The laboratory data were supported by the response of sunflowers (Helianthus annuus L.) planted 1 year after application of flumetsulam to different soils. A quantitative model relating half‐life to sorption Kd (r2= 0.85) was coupled with an additional equation relating sorption Kd to the proportion of neutral and anionic forms of the compound at different pH values. The study allows estimates of this herbicide's degradation in soil to be made if its pH and organic carbon content are known. Degradation d'un herbicide sulfonamide en fonction de la capacité d'absorption du sol La dégradation du flumetsulam a étéétudiée dans les sols aux Etats Unis comme premier stade dans le développement d'un plan d'utilisation de ce nouvel herbicide. Les demi‐vies de dégradation étaient plus courtes dans les sols qui absorbaient moins le produit. L'absorption était plus faible dans les sols à pH élevé et à faible teneur en carbone organique. Les demi‐vies étaient ainsi sous l'influence du pH et du carbone organique: elles étaient de 2 à 4 semaines pour les pH de sols élevés (pH 7) sauf si le carbone organique était au dessus de 2,5%, dans ce cas, les demi‐vies etaient de 1 à 3 mois. Dans les sols à pH moyen (6,4 pH 6,9) les demi‐vies étaient de 1 à 2 mois, tandis que dans les sols à pH faible (5,9 pH 6,3) elles étaient de 1 à 4 mois. Les résultats de laboratoire ont été confirmés par le comportement de tournesols plantés un an aprfès l'application de flumetsulam dans différents sols. Un modèle reliant la demi‐vie au coefficient d'absorption Kd (r2= 0.85) a été couplé avec une équation additionelle reliant le coefficient d'absorption Kd à la proportion de particules neutres et anioniques de la molécule aux différents pH. L'étude permet de faire des estimations de la dégradation de cet herbicide dans le sol dès que le pH et la teneur en carbone organique sont connus. Abbau eines Sulfonamid‐Herbizids als Funktion der Bodensorption Der Abbau von Flumetsulam (DE‐498) wurde in 21 Böden untersucht, um eine Grundlage zur Produktentwicklung dieses neuen Herbizids zu gewinnen. Die 50%ige Verlustrate (DT50) war in schwächer sorptiven Böden kürzer. Bei hohem pH‐Wert und geringem Gehalt an organischer Substanz war die Adsorption schwächer. Die DT50 wurde durch den pH‐Wert und den Gehalt organischer Substanz wie folgt beeinflußt: Sie be...

To better understand the environmental fate of organopolysiloxanes (silicones), this study investigated the degradation of a 14 C-labeled poly(dimethylsiloxane) (PDMS) on 12 Ca-saturated clay minerals. The rates and products of PDMS degradation were determined at 22 °C and 32% relative humidity, via sequential extraction and highperformance size exclusion chromatography (HPSEC). The results showed that all of the clays tested were catalysts for PDMS degradation. However, clay minerals varied substantially in their catalytic activity: kaolinite, beidellite, and nontronite were the most active; goethite and allophane were least active. In addition, PDMS degradation products bound more strongly to goethite and smectites. These results demonstrated that soil factors such as clay content and clay type are very important in determining the degradation rates of PDMS in soil.

Chlorpyrifos [O,O‐diethyl O‐(3,5,6‐trichloro‐2‐pyridyl) phosphorothioate] is an organophosphorus insecticide applied to soil to control pests both in agricultural and in urban developments. Typical agricultural soil applications (0.56 to 5.6 kg ha−1) result in initial soil surface residues of 0.3 to 32 μg g−1. In contrast, termiticidal soil barrier treatments, a common urban use pattern, often result in initial soil residues of 1000 μg g−1 or greater. The purpose of the present investigation was to understand better the degradation of chlorpyrifos in soil at termiticidal application rates and factors affecting its behaviour. Therefore, studies with [14C]chlorpyrifos were conducted under a variety of conditions in the laboratory. Initially, the degradation of chlorpyrifos at 1000 μg g−1 initial concentration was examined in five different soils from termite‐infested regions (Arizona, Florida, Hawaii, Texas) under standard conditions (25°C, field moisture capacity, darkness). Degradation half‐lives in these soils ranged from 175 to 1576 days. The major metabolite formed in chlorpyrifos‐treated soils was 3,5,6‐trichloro‐2‐pyrid‐inol, which represented up to 61% of applied radiocarbon after 13 months of incubation. Minor quantities of [14C]carbon dioxide (< 5%) and soil‐bound residues (⩽ 12%) were also present at that time. Subsequently, a factorial experiment examining chlorpyrifos degradation as affected by initial concentration (10, 100, 1000 μg g−1), soil moisture (field moisture capacity, 1.5 MPa, air dry), and temperature 15, 25, 35°C) was conducted in the two soils which had displayed the most (Texas) and least (Florida) rapid rates of degradation. Chlorpyrifos degradation was significantly retarded at the 1000 μg g−1 rate as compared to the 10 μg g−1 rate. Temperature also had a dramatic effect on degradation rate, which approximately doubled with each 10°C increase in temperature. Results suggest that the extended (3–24 + years) termiticidal efficacy of chlorpyrifos observed in the field may be due both to the high initial concentrations employed (termite LC 50 = 0.2– 2 μg g−1) and the extended persistence which results from employment of these rates. The study also highlights the importance of investigating the behaviour of a pesticide under the diversity of agricultural and urban use scenarios in which it is employed.

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