Chlorothalonil binding to aquatic humic substances assessed from gas purge studies

Abstract

Fate of the fungicide chlorothalonil (TCIN) binding to dissolved organic acid fractions was quantified using gas‐purge desorption studies. Binding studies were conducted to measure the dissolved organic carbon partition constant (K_DOC) with aquatic fulvic and humic acid fractions purified from cranberry bog water. Desorption studies at DOC concentrations up to 50 mg L^‐1 resulted in mean log K_DOC values of 4.63 (s.d.=0.5, n=8) and 4.81 (s.d.=0.7, n=7) for fulvic and humic acids, respectively. These values deviated from reported K_OC (organic carbon) values by 0.5 to 1.5 orders of magnitude. The relationship between K_OC and K_DOC did not conform to accepted ratios of 10: 1 to 3: 1, although these studies were conducted with the strong hydrophobic fraction of DOC. Binding was rapid suggesting hydrophobic partitioning or weak Van Der Waals forces as binding mechanisms. The strong binding potential for TCIN to aquatic humic substances corresponds to increased solubility in the aqueous system. Sorption to DOC suggests a possible transport mechanism which may result in elevated concentrations of TCIN in cranberry bog systems.