Adsorption and desorption variability of four herbicides used in paddy rice production

This investigation was performed to determine the effect of physicochemical soil properties on penoxsulam, molinate, bentazon, and MCPA adsorption-desorption processes. Four soils from Melozal (35 degrees 43' S; 71 degrees 41' W), Parral (36 degrees 08' S; 71 degrees 52' W), San Carlos (36 degrees 24' S; 71 degrees 57' W), and Panimavida (35 degrees 44' S; 71 degrees 24' W) were utilized. Herbicide adsorption reached equilibrium after 4 h in all soils. The Freundlich L-type isotherm described the adsorption process, which showed a high affinity between herbicides and sorption sites mainly because of hydrophobic and H-bonds interaction. Penoxsulam showed the highest adsorption coefficients (4.23 +/- 0.72 to 10.69 +/- 1.58 mL g-1) and were related to soil pH. Molinate showed Kd values between 1.72 +/- 0.01 and 2.3 +/- 0.01 mL g-1and were related to soil pH and organic matter, specifically to the amount of humic substances. Bentazon had a high relationship with pH and humic substances and its Kd values were the lowest, ranging from 0.11 +/- 0.01 to 0.42 +/- 0.01 mL g-1. MCPA Kd ranged from 0.14 +/- 0.02 to 2.72 +/- 0.01 mL g-1, however its adsorption was related to humic acids and clay content. According to these results, the soil factors that could explain the sorption process of the studied herbicides under paddy rice soil conditions, were principally humic substances and soil pH. Considering the sorption variability observed in this study and the potential risk for groundwater contamination, it is necessary to develop weed rice management strategies that limit use of herbicides that exhibit low soil adsorption in areas with predisposing conditions to soil leaching.