Interactions of two sulfonylurea herbicides with organoclays

The sorption of two sulfonylurea herbicides (SU), metsulfuron methyl and nicosulfuron, on pure clays and organoclays was investigated. Three clays (Arizona smectite, SAz-1, Wyoming smectite, SWy-2, and hectorite, SHCa-1), were treated with amounts of octadecylammonium (ODA) or dioctadecyldimethylammonium (DODMA) cations equal to similar to50 and 100% of the clays' cation exchange capacity (CEC). Sorption isotherms were fitted to the Freundlich equation. While no measurable sorption was found on the pure clays (K-f = 0), organoclays prepared using both primary and quaternary amines were effective as SU sorbents. The metsulturon methyl K-f values ranged between 196 and 1498 mumol(1-1/n) kg(-1) L-1/n, and K-f values for nicosulfuron, which were lower than those of metsulfuron methyl, ranged from 35 to 198 mumol(1-1/n) kg(-1) L-1/n. As shown by sorption coefficients, K-d and K-OC, SWy-2 treated with DODMA at similar to100% of the CEC was the most effective sorbent for metsulfuron, K-d = 684 L kg(-1) and K-OC = 2138 L kg(-1). For nicosulfuron the most effective sorbent was SAz-1 with ODA at similar to50% of the CEC (Kd = 147 L kg(-1) and K-OC = 1233 L kg(-1)). In contrast to other weak-acid herbicides, such as phenoxy and picolinic acids, no clear relationships were found between sorption and layer charge, organic carbon content, and basal spacing of the organoclays for both sulfonylurea herbicides. Sorption of both herbicides on organoclays was assumed to involve hydrophobic and polar interactions for which the availability of interlayer room between organocations was a very important factor.