Effect of pH on Sorption and Desorption of Imazaquin and Imazethapyr on Clays and Humic Acid

Abstract

AbstractThe interaction of imazaquin (2‐[4,5‐dihydro‐4‐methyl‐4‐(1‐methylethyl)‐5‐oxo‐1H‐imidazol‐2‐yl]‐3‐quinolinecarboxylic acd) and imazethapyr ((±)‐2‐4,5‐dihydro‐4‐methyl‐4‐(1‐methylethyl)‐5‐oxo‐1H‐imidazol‐2‐yl]‐5‐ethyl‐3‐pyridinecarboxylic acid) with clay suspensions and humic acid was studied using a batch equilibrium method and by fluorescence emission spectroscopy. The objectives of this study were to: (i) determine the effect of different sorbents and pH on the sorption of imazaquin and imazethapyr, and the hysteresis in adsorption‐desorption isotherms, and ii) evaluate the effect of pH on the mechanisms of imidazolinone herbicide sorption by specimen hectorite. Sorption of both herbicides increased as pH decreased, but the increased sorption was most evident as soil pH decreased from 5 to 3. Illite and kaolinite, which exhibit more pH‐dependent charge characteristics and less total surface area than hectorite, can impart important adsorptive character to soil, especially with regard to imazethapyr. Humic acid had much greater affinity for both herbicides than clay did, indicating that organic matter content could be a major factor controlling the sorption of these herbicides in soils. The rate of desorption varied among absorbents and between herbicides; imazethapyr desorbed more readily than imazaquin from most adsorbents. The results of fluorescence emission spectroscopy indicated that imazaquin and imazethapyr molecules were protonated and sorbed on the clay surface under acidic conditions. The extent of protonation was greater in the clay suspensions than in clay‐free solutions.