Dicamba adsorption-desorption on organoclays

The adsorption-desorption of the herbicide dicamba (pK(a) = 1.9, water solubility, 6.5 g kg (-1)) by organoclays (OCls) was studied at different concentrations and pH levels. Two smectites (SAz and SWy), varying in surface properties, were reacted with amounts of different alkylammonium cations [octadecyl(C18)-, hexadecyl-trimethyl(HDT)- and dioctadecyl-dimethyl(DOD)ammonium] equal to 50% or 100% of the clays' CEC. Adsorption isotherms of dicamba on diverse OCl were some of L-type and others of S-type, but both resulted in sigmoid form when extended to higher concentration. Adsorption was greater for OCls with high-layer charge, basal spacing, alkylammonium size and organocation saturation close to CEC. Dicamba adsorption by OCls seems to involve hydrophobic and polar interactions for which the availability of interlayer room between organocations is very important. Adsorption data at different pH levels and two different concentrations (0.05 and 1 mM) indicated that molecular dicamba is the main adsorbing species,, especially at high concentration. Desorption isotherms were reversible, except in OCls with primary alkylammonium (C18) and largest quaternary (DOD), for which there were moderate hysteresis as a result of stronger polar contribution in the primary alkylammonium and the difficulty for diffusion in the case of the quaternary, bulky OCl. The treatment of an artificially dicamba-contaminated soil with highly adsorptive OCls rendered a dramatic decrease in the CaCl2-released- or mobile dicamba, suggesting these OCls as potential immobilising agents. The amount of herbicide immobilised by the OCl was partially extractable with methanol/CaCl2, solution, suggesting its biovailability and hence, its possible combination with bioremediation technique. (C) 2001 Elsevier Science B.V. All rights reserved.