Fluridone Retention and Release in Soils

Aqueous 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.