To adsorb and recover uranium(VI) from seawater, an improved macroporous fibrous polymeric adsorbent containing amidoxime chelating functional group (FPAO) was synthesized by irradiation-induced graft polymerization utilizing methacrylic acid (MAA) as a hydrophilic monomer. The adsorption mechanism of U(VI) on FPAO adsorbent was investigated in a wide pH range by the method of the thermodynamics. It was found that the pH value at different conditions had significant effect on the adsorption of U(VI) on FPAO adsorbent. The adsorption ability of FPAO for U(VI) increased from pH 2.0 to pH 6.0 and decreased from pH 6.0 to pH 9.0. U(VI) was effectively adsorbed by FPAO adsorbent at around pH 6.0. The predominant composition of resulting the mononuclear complex was determined as UO2.2R-C(NO)NH2 by investigating the effect of the concentrations of U(VI), FPAO, H+ and Cl- on the adsorption reaction. The adsorption equilibrium equation of U(VI) on FPAO adsorbent was expressed as UO2(OH)((aq))(+) + 2R(aq) C(NOH)NH2(fp) <----> UO2.2R-C(NO)NH2(fp) + H-(aq)(+) + H2O. The overall adsorption equilibrium constant was calculated to be K-ad = 7.08 x 10(7) M-1 at ionic strength of 0.1 M at 25 degreesC. A monolayer molecule adsorption mechanism between U(VI) and FPAO was also determined by examining the relationships between the adsorbed amount of U(VI) and contact time. (C) 2003 Elsevier B.V. All rights reserved.