Sorption of the herbicide aminocyclopyrachlor by cation-modified clay minerals

Aminocyclopyrachlor (6-amino-5-chloro-2-cyclopropyl-4-pyrimidinecarboxylic acid), a new herbicide, was approved by US-EPA (United States Environmental Protection Agency) in September 2010 for the control of broadleaf weeds, grasses, vines and woody species in non-crops, turf, sod farms and residential areas. At typical soil pH levels, aminocyclopyrachlor is anionic. Anionic pesticides are generally weakly retained by most soil and sediment components and are subject to potential transport offsite. One strategy to minimize the risk of offsite transport associated with weakly sorbed, highly mobile pesticides is the application of these chemicals supported on a sorbent, such as modified clay minerals which contain exchangeable inorganic and organic cations. The aim was to determine the ability of unmodified and modified clay minerals to sorb aminocyclopyrachlor. Two montmorillonites (Ca-Arizona montorillonite (SAz-1) and Na-Wyoming montmorillonite (SWy-2)) were used as unmodified clay minerals and were also modified with an inorganic cation (Fe3+), soil constituents (ferrihydrite, humic acid) and organic cations (hexadecyltrimethylammonium (HDTMA), hexadimethrine, spermine). Greater sorption of aminocyclopyrachlor was observed in the sorbentsolution systems with lower pH values, presumably because of protonation of the herbicide anion at low pH and subsequent sorption of molecular species and cationic species. FeSWy-2 (SWy-2 montmorillonite saturated with Fe3+; system pH = 4.6) was the best sorbent for aminocyclopyrachlor (86% of applied). The best organo-clay (SA-HDTMA) (system pH = 6.1) sorbed 53% of the applied herbicide. Binary and ternary model sorbents (system pH > 6.7) did not sorb aminocyclopyrachlor. At the lowest pH value studied (2.8), the organo-clay SW-Sper was the best sorbent system for aminocyclopyrachlor. Our recommendation for the best adsorbent is the clay mineral modified with Fe3+, although at pH values <3, the use of the organo-clay SW-Sperm could be also convenient.