Paraquat

CAS RN: 4685-14-7

Soil Adsorption / Mobility

Koc values for paraquat in various soil have been reported to be 15,473-1,000,000(1-6). According to a classification scheme(7), these Koc values suggest that paraquat is expected to be immobile in soil. Both soil thin layer chromatographic and soil leaching studies also show that paraquat will be almost immobile in soil and no significant leaching of paraquat from soil will occur(8-9). However, some leaching of paraquat may occur under conditions that are favorable for soil colloid formation (eg, with water of low electrolytic conductivity)(10). In such cases, passage of soil colloids containing adsorbed paraquat through soil pores may account for the transport of paraquat(10). Paraquat is strongly adsorbed from aquatic phase to suspended solids and sediments in water(11). Fifteen computer models (based on LEACHM and various experimental parameters) were applied to the Morgan Creek watershed (Chesapeake Bay area) and predicted that 0.00% of applied paraquat would be leached to groundwater(5).
Double positively charged paraquat cation may react with clay minerals in soil by forming complexes with the negatively charged sites on the clay minerals(1-3). Paraquat may also be adsorbed in soil by forming complexes with humic and fulvic materials present in soil(3-5). Therefore, adsorption of paraquat to soil will generally increase with the increase in clay and organic content of soil(SRC).
Paraquat was found to have high Freundlich adsorption distribution coefficients (28.7 and 1419) using Malaysian muck and sandy loam soils, respectively, that correspond to Koc values of 2207 and 4652(1); desorption studies observed negligible paraquat desorption from the muck soil and very low desorption from the sandy loam (0-0.42%)(1); soil column leaching studies found that paraquat leaching was not evident in either the sandy loam or muck soil(1). Using a dialysis technique to measure sorption and desorption to humic acid fractions extracted from soil, paraquat was rapidly adsorbed (nearly complete in 2-4 hours) with no to very low desorption over a 24-hr period(2). In sorption-desorption studies using two vineyard soils from Spain, paraquat exhibited very strong adsorption (Kd partition coefficient of 106-1280) with very small desorption(3). Paraquat has a very low soil leaching potential (5 on a scale of 100) and has a low groundwater contamination potential rating with respect to North Carolina soils and groundwaters(4). A soil column leaching study using a surface layer fine sand soil (1.3% organic carbon) and a subsurface layer fine sand (0.1% organic carbon) found a surface layer Koc of 24,441 with very slow desorption in both soils(5).
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