The role of soil humic and fulvic acid in the sorption of endosulfan (alpha and beta)

Abstract

The sorption behaviour of alpha-and beta-endosulfan in soil organic matter was investigated using standard soil humic acid (HA) and soil fulvic acid (FA) with a modified solubility enhancement method and a dialysis bag technique. For HA, all the experiments were conducted at an ionic strength of 0.001 mol/L, in both the presence and absence of calcium and at an ionic strength of 0.01 mol/L. For FA, the experiments were conducted at two ionic strengths: 0.001 mol/L (with calcium) and 0.01 mol/L. This study is the first to describe the striking differences in the sorption behaviours of the two stereoisomers of endosulfan in HA and in FA. The sorption coefficients of alpha-endosulfan in HA and FA were significantly higher than those of beta-endosulfan. Beta-endosulfan has comparable sorption coefficients (1.5-5.4 L/g) in HA and in FA. Ionic strength and the presence of calcium have no significant effect on the sorption of beta-endosulfan in HA. However, calcium can significantly (p = 0.01) enhance the solubility of alpha-endosulfan in HA. Changes in ionic strength by one order of magnitude also affect the solubility of alpha-endosulfan in HA. The sorption coefficients of alpha-endosulfan in HA (10-36 L/g) were greater than those in FA (9-14 L/g). The chirality of the alpha-isomer was hypothesised to be the primary reason behind its higher sorption in soil organic matter relative to the beta-isomer. In the presence of dissolved HA and FA found in natural soil environments, solubility of endosulfan can be increased by five times than the aqueous solubility of endosulfan without HA and FA.

Environmental and Agricultural Relevance of Humic Fractions Extracted by Alkali from Soils and Natural Waters

“…Extraction procedures for young SOM fractions are often based on disrupting soil agents that stabilize SOM against short‐term cycling, including binding to clay mineral surfaces (Christensen, 1992), aggregation (Tisdall and Oades, 1982; Six et al, 2004; Sutton and Sposito, 2005), and polyvalent cations (Baldock and Skjemstad, 2000). One of the most common soil cations, Ca 2+ , can stabilize organic ligands against microbial decomposition in controlled conditions, and its soil abundance has been positively linked to SOM levels in the field (Baldock and Skjemstad, 2000).…”