Huge amounts of herbicides are being used every year to control weeds all around the world. These chemicals not only have negative impacts on the environment but also harm human health. Contaminated surface and ground waters can arise from run-off and leaching downward of applied herbicides through the soil profile. 2,4-Dichlorophenoxyacetic acid (2,4-D) is a chlorinated phenoxy compound and the most commonly used herbicide in the world to control a wide range of broadleaf weeds. This herbicide is one of the most important herbicides that are used excessively in Iran, as well. 2,4-D is a water soluble herbicide and, due to its low pKa value at 2.76, exists predominantly in anionic form. Such anionic organic compounds are weekly retained by most soil and subsurface materials; therefore, they are very mobile in the environment. Ecologically acceptable organoclay-based slow-release herbicide formulations have recently been developed and have demonstrated improved herbicidal activity, inhibition of leaching and migration, and reduced photochemical degradation and volatilization. Little attempts have been made in Iran, however, to develop 2,4-D slow-release formulations. Therefore, the aim of the present study was to evaluat the sorption capacity of diverse organoclays for 2,4-D herbicide, synthesis of slow-release 2,4-D formulations and the efficiency of these formulations to release the herbicide and control herbs compared with commercial compound. Bentonite and zeolite minerals modified with cetyltrimethylammonium (CTMA) were used to sorb 2,4-D herbicide. Result showed that natural bentonite and zeolite were poor sorbents for 2,4-D, but CTMA-modified clays were very good adsorbents for 2,4-D. Sorption of 2,4-D increased as the surfactant loading on the clays increased. The highest amount of 2,4-D sorption was related to modified bentonte with CTMA surfactant concentration equivalent to 200% of its CEC . Desorption of 2,4-D by Na-bentonite was highly reversibile. But 2,4-D desorption by the modified clays was much lower and decreased as the surfactant loading on the clays increased. The adsorption-desorption isotherm of 2,4-D on organoclays showed that significant hysteresis occurred between adsorption-desorption reactions. Commercial formulation released almost 100% of 2,4-D in the early hours, but the slow release formulations gradually released 2,4-D during 168 hours. Greenhouse bioassay with clover and lolium plants showed that all slow release formulations were nearly effective as the commercial formulation in the control of clover.. Leaching of 2,4-D from slow release formulation was less than that from the commercial formulation. Finally, application of slow-release herbicide formulations is recommended to reduce leaching and increase the efficiency and performance of the herbicide.
