To clarify the adsorption characteristics of surfactants, it is important to understand the surfactant behavior in the soil and water environments.However,there are few adsorption studies for highly humic soil. In this study, the adsorption characteristics of dodecylbenzene sulfonates in a highly humic soil were investigated. A non‒allophanic Andisol was used since this soil contains a large amount of humic substances and is only negatively charged. Thus, electrically, only repulsive force is generated between the soil and the surfactant. The adsorption amount was measured using the batch method at an electrolyte concentration of 100 mmol L－1 NaCl in order to shield the electric fi eld near the soil particle surface and emphasize hydrophobic reactions. The adsorption isotherm was examined using the Langmuir‒Freundlich‒Hill equation. The adsorbed amount increased sharply with increasing concentrations in the low‒concentration range. The sharp increase indicated cooperative adsorption caused by hydrophobic interaction among the carbon chains of adsorbing surfactants. The adsorption amount became higher at a lower pH because electrostatic repulsion between the soil and the surfactant decreased. The adsorption of dodecylbenzene sulfonate with a linear carbon chain was larger than that with a branched chain due to the diff erence of the carbon chain structure
