Sorption of 2,4-D and other phenoxy herbicides to soil, organic matter, and minerals

We review 2,4-dichlorophenoxyacetic acid (2,4-D) and other phenoxy herbicide sorption experiments. A database with 469 soil-water distribution coefficients K (d) (in liters per kilogram) was compiled: 271 coefficients are for the phenoxy herbicide 2,4-D, 9 for 4-(2,4-dichlorophenoxy)butyric acid, 18 for 2-(2,4-dichlorophenoxy)propanoic acid, 109 for 2-methyl-4-chlorophenoxyacetic acid, 5 for 4-(4-chloro-2-methylphenoxy)butanoic acid, and 57 for 2-(4-chloro-2-methylphenoxy)propanoic acid. The following parameters characterizing the soils, solutions, or experimental procedures used in the studies were also compiled if available: solution CaCl2 concentration, pH, pre-equilibration time, temperature, soil organic carbon content (f (oc)), percent sand, silt and clay, oxalate extractable aluminum, oxalate extractable iron (Oxalate Fe), dithionite-citrate-bicarbonate extractable aluminum, dithionite-citrate-bicarbonate extractable iron (DCB Fe), point of zero negative charge, anion exchange capacity, cation exchange capacity, soil type, soil horizon or depth of sampling, and geographic location. K (d) data were also compiled characterizing phenoxy herbicide sorption to the following well-defined sorbent materials: quartz, calcite, alpha-alumina, kaolinite, ferrihydrite, goethite, lepidocrocite, soil humic acid, Fluka humic acid, and Pahokee peat. The data review suggests that sorption of 2,4-D can be rationalized based on the soil parameters pH, f (oc), Oxalate Fe, and DCB Fe in combination with sorption coefficients measured independently for humic acids and ferrihydrite, and goethite. Soil organic matter and iron oxides appear to be the most relevant sorbents for phenoxy herbicides. Unfortunately, few authors report Oxalate Fe and DCB Fe data.