Adsorption and Desorption of Herbicides by Soil

Abstract

The adsorption of herbicides from aqueous solution by muck (organic soil), bentonite, an anion exchanger, and a cation exchanger was studied. The nature of the adsorbent, pH, and temperature all influenced adsorption, and different herbicides responded differently to changes in these factors. Lowering of the pH resulted in increased adsorption by bentonite of all of the herbicides studied, except 6,7-dihydrodipyrido (1,2-a: 2', l'-c) py- razidiinium salt (diquat) which was completely adsorbed at both a high and a low pH. The influence of pH was greatest for 4,6- dinitro-o-sec-butylphenol (DNBP) and 2-chloro-4-ethylamino-6-iso- propylamino-s-triazine (atrazine). The DNBP was adsorbed by an anion exchanger, but not by a cation exchanger, while isopropyl N-(3-chlorophenyl)carbamate (CIPC), 3-(p-chlorophenyl) - 1,1 - di - methylurea (monuron), and atrazine were adsorbed by both. Diquat was completely adsorbed by the cation exchanger and only slightly by the anion exchanger. All were adsorbed by muck but to vary- ing extents as follows: diquat > CIPC > DNBP > monuron > atrazine. No relationship was found between water solubility and adsorption. Adsorption by bentonite (pH 8.5) was much greater at OC than at 50C, but adsorption by muck was the same at both temperatures. Desorption studies were conducted using C14-labeled atrazine, monuron, 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine), 3-ami- no-2,5-dichlorobenzoic acid (amiben), and 2,4-dichlorophenoxyacetic acid (2,4-D). Repeated extractions with distilled water desorbed these herbicides from bentonite and muck and their desorption was more readily accomplished from bentonite than from muck.