Retention and release of diquat and paraquat herbicides in soils

The sorption and desorption behaviour of diquat and paraquat, in the presence of different salt concentrations, has been studied in some Western Australian soils varying in physical and chemical properties. Sorption isotherms were found to be of the 'L' type according to the Giles classification with the data fitting well to the Langmuir equation. The sorption capacities of soils for diquat ranged from very high in clay soil (146 400 µmol kg-1) to very low in sand (1765 µmol kg-1) and followed the order of clay content in the soils. The clay soil also showed the highest value for the Langmuir coefficient representing bonding energy. The sorption capacities for the herbicides were less than the CEC of the soils. Both the type and content of clay minerals present in soil were important for sorption and subsequent desorption of the dipyridylium herbicides. An increase in the salt concentration of the soil solution (from 0.005 to 0.05 M CaCl2) resulted in decreases in sorption capacities for the herbicides ranging from some 17% to 40% in the different soils. Both Ca2+ and Na+ cations competed for the sorption sites with the herbicides, but Na+ was not as effective as Ca2+. Desorption of the herbicides was also significantly affected by the salt concentrations of the extractant. After five successive extractions with 0.005 m CaCl2, the total amounts of equilibrium sorption desorbed from the Bassendean surface soil were 13% and 7% for diquat and paraquat, respectively. The percentages of diquat removed by extractions decreased with increasing clay contents in the soils studied. Desorption of diquat was higher than that of paraquat at all salt concentrations. Hysteresis was evident between sorption and desorption isotherms.. As sorption increased, the desorption became easier, indicating that the herbicides were less strongly held. Sorption which occurred in the presence of higher ionic strength solutions of inorganic cations was relatively less susceptible to desorption.