Factors Controlling Sorption of Prosulfuron by Variable‐Charge Soils and Model Sorbents

Abstract

Prosulfuron [1-(4-methoxy-6-methyltriazin-2-yl)-3-[2-(3,3,3-trifluoropropyl) phenylsulfonyl]-urea), a relatively new sulfonylurea herbicide, is a weak acid (pK(a) 3.76), and therefore, will undergo pH-dependent speciation and sorption. Understanding prosulfuron sorption in soils is important for predicting its environmental fate. Soil and solution factors controlling sorption were investigated by measuring prosulfuron sorption on five model sorbents (amorphous silica, alpha-alumina, CaSWy1 montmorillonite, commercial humic acid, and anion exchange resin) and 10 variable-charge soils from CaCl(2) and Ca(H(2)PO(4))(2) solutions as a function of pH and ionic strength. Anion exchange of prosulfuron accounted for up to 82% of overall sorption in the pH range from 3 to 7. The relative importance of anion exchange to prosulfuron sorption was positively correlated to the ratio of anion and cation exchange capacities. Comparison between organic carbon (OC)-normalized sorption (K(oc)) versus pH for humic acid and variable-charge soils show similar trends with sorption decreasing with increasing pH. However, K(oc) values estimated from variable-charge soils in the lower pH range where anion exchange has the greatest contribution to sorption was almost one log unit greater than that estimated from humic acid clearly exemplifying the impact of anion exchange. Similarity in K(oc)-pH curves for humic acid and variable-charge soils may result from the fact that (i) cation exchange capacity increases with increasing OC content, thus effective anion exchange capacity is reduced; and (ii) the relative contribution of hydrophobic and hydrophilic sorption mechanisms was fairly constant. Given that both hydrophilic and hydrophobic sorption of prosulfuron decrease with increasing pH, addition of fertilizer and lime amendments may enhance the potential for off-site leaching of recently applied acidic pesticides.

Imazaquin sorption in surface and subsurface soil samples

“…Therefore, cationic connections can be expected between the anionic herbicide and the positive sites of the oxides, as described by Rocha et al (2002). Such influence of iron and aluminum oxides has also been shown for other ionic compounds, such as 2,4,5-T and imazaquin (Regitano et al, 1997(Regitano et al, , 2001), pentachlorophenol (Hyun et al, 2003) and prosulfuron (Hyun & Lee, 2004). Besides, this high retention of imazaquin can also be explained by the narrow soil: solution relationship adopted in this study (Koskinen & Cheng, 1983).…”

Hydrophilic and Hydrophobic Sorption of Organic Acids by Variable-Charge Soils: Effect of Chemical Acidity and Acidic Functional Group

“…The K oc pH values for all the organic acid-soil combinations from this study were calculated using eq 6 ( Table 3). Likewise, K oc pH values were calculated for PCP and prosulfuron from sorption previously collected on several variable-charge soils as a function of pH from 3 to 7 (9,10). A linear log-log regression (solid line) between K ow pH and K oc pH is shown in Figure 5 along with associated standard deviation (dotted lines).…”

Influence of soil pH on the sorption of ionizable chemicals: Modeling advances

“…Strongly pH‐dependent distributions were found for many chemicals, including basic aromatic amines [3], basic N ‐het‐erocyclic compounds [4], basic pesticides [5], acidic pesticides [5–8], basic pharmaceuticals [9,10], and acidic pharmaceuticals [11]. These and other studies have reported decreasing sorption with increasing pH for acids.…”