Surface modification is an effective way to enhance adsorption of pollutants by soil. In this study, we investigated the individual adsorption of cadmium ion (Cd2+) and phenol and also in combination by the clay layer of a loessial soil treated with the amphoteric modifier, duodalkylbetaine (BS-12). Three levels of BS-12 modification were compared in this experiment: (1) unmodified soil (CK), (2) modification with an amount of BS-12 equivalent to 50% of the soil's CEC (50BS) and (3) modification with an amount of BS-12 equivalent to 100% of the soil's CEC (100BS). Cd2+ adsorption was 0.92-1.70 times higher in the amphoteric modified soil compared to unmodified soil. Adsorption isotherms for Cd2+ displayed a L1-type shape. Phenol adsorption was 1.25-4.35 times higher in the amphoteric modified soil compared to the unmodified control. The adsorption isotherms of phenol on amphoteric modified soils were generally linear, but changed to L1-type isotherms for modified soil in the Cd2+ + phenol treatment at 40 degrees C. The results clearly showed that amphoteric modified soil had the ability to simultaneously adsorb Cd2+ and phenol. Cd2+ adsorption by the amphoteric modified soil was related to the initial concentration of Cd2+ in the supernatant. Cd2+ adsorption in the 100BS treatment exceeded adsorption in the 50BS treatment when Cd2+ initial concentrations were higher than approximate 200 mu g mL(-1). Phenol adsorption by modified soils decreased in the order: 100BS > 50BS > CK and was primarily determined by the surface hydrophobicity of the soil. For the unmodified soil, total adsorption in the Cd2+ + phenol treatment was slightly lower compared to treatments that contained only Cd2+ or phenol. This indicated an antagonistic effect between the adsorption of Cd2+ and phenol, which was reduced after amphoteric modification. A comparison of temperature effects on Cd2+ and phenol adsorption indicated that Cd2+ was both physically and chemically adsorbed by the amphoteric modified soil, but phenol was primarily adsorbed physically. (C) 2008 Elsevier B.V. All rights reserved.