Sorption of four s-triazine herbicides on natural zeolite and clay mineral materials with microporosity

Abstract

Sorption is an important process that controls the fate and transport of herbicides in the soil and aquifer environment. However, sorption of these pollutants on natural microporous mineral materials has received little attention. This study investigated the sorption of four model s-triazine herbicides, namely, atrazine, metribuzin, ametryn, and prometryn, on two common natural zeolites (i.e., clinoptilite and Fe-mordenite) and two common clays (i.e., kaolin and attapulgite) with microporosity in batch experiments. The results showed that the sorption of s-triazines on the natural zeolites increased in the order of metribuzin < atrazine < ametryn < prometryn, which corresponded approximately to the order of increasing sorbate hydrophobicity and indicated the significant contribution of the hydrophobic effect. In contrast, the sorption of s-triazines on the natural clays increased in the order of metribuzin < atrazine < prometryn < ametryn, which corresponded to the order of increasing sorbate basicity and indicated the significant contribution of cation exchange and electrostatic interactions. The sorption of these s-triazines on the mineral materials was strongly pH-dependent, resulting from the pH dependence of the mineral surface charge and protonation/deprotonation of the s-triazines. Common alkali and alkaline earth cations at low concentrations had no significant impact on the sorption of s-triazines on the natural microporous mineral materials. Meanwhile, the presence of Fe3+ (at 0.05 mmol/L) significantly enhanced the sorption of atrazine, ametryn, and prometryn by increasing the protonation of s-triazine molecules in the interfacial region between the mineral surface and bulk solution (due to hydrolysis of Fe3+). Co-sorption of s-triazines on the natural microporous mineral materials exhibited significant competitive effects when these herbicides are sorbed through similar mechanisms. The findings of this study provide insights on the interactions of s-triazine herbicides with natural microporous mineral materials, which help better understand and predict their occurrence and behavior in the subsurface.