Fluridone Retention and Release in Soils

Abstract

AbstractAqueous adsorption/desorption studies showed that fluridone [1‐methyl‐3‐phenyl‐5(3‐trifluoromethyl) phenyl]‐4‐(1H)‐pyridinone) adsorbed to both organic matter and montmorillonite clay surfaces in 18 soils. Adsorption ranged from a high of 0.81 mmol/kg on Earle clay (Vertic Haplaquets) [3.7% organic matter (OM), 59% clay] to a low of 0.02 mmol/kg on an Amarillo loamy sand (Aridic Paleustalfs) (0.5% OM, 8% clay). Fluridone adsorption was highly correlated with montmorillonite clay or organic matter contents of the soils, but the highest correlation coefficients (r = 0.92) resulted when adsorption was correlated with both clay and organic matter contents. Adsorption isotherms for fluridone on three soils (Earle and Tunica [Vertic Haplaquets], and Falaya [Aeric Fluvaquents] were of the S‐type on soils containing low amounts of organic matter and high amounts of montmorillonite clay, and of the L‐type on soils high in organic matter and with low mixed clay mineralogy (Cape Fear [Typic Umbraquults] and Drummer [Typic Haplaquolls]). Isotherms for soils low in organic matter and clay content (Fincastle [Aeric Ochraqualfs] and Norfolk [Typic Paleudults]) were linear (C‐type) or of the L‐type. Fluridone adsorption increased by 38 to 42% when solution pH was decreased from 6.4 to 3.5 and the herbicide was not readily desorbed with water at the low pH level (11–27% released), suggesting that the weakly basic herbicide protonated and was bound by cation exchange forces. Large amounts (58%) were desorbed with water where adsorption occurred at higher pH levels (6.4), suggesting that only physical binding was involved. Incubation of fluridone adsorbed on soil pellets under wet, 50°C conditions reduced fluridone that could be displaced with water by 10 to 20%, as compared with incubations at 25°C, or incubations under dry conditions (25 or 50°C), suggesting that climatic factors may affect adsorption/desorption, mobility in soil, bioactivity, and persistence of the herbicide. X‐ray diffraction studies revealed that the herbicide was adsorbed in the interlayer surfaces of montmorillonite clay and Δd spacing ranged from 0.2 to 0.4 nm.