The binding of Eu(III) to a biosorbent derived from cultured cells of the plant Datura innoxia, have been investigated through elucidation of apparent affinity constants associated with different chemical environments present on the cell wall. Adsorption isotherms for separate types of binding sites were generated using metal ion luminescence measurements. Application of regularized regression analysis to these isotherm data for four chemically distinguishable sites revealed the presence of sites exhibiting both low (mean log K-app = -0.3 to 0.6) and higher (mean log K-app = 3.2-3.5) apparent affinities for pH conditions of 2.0, 4.0, and 5.0. Low affinity sites were observed for all pH conditions and attributed to non-specific binding of the metal ions to the negatively charged biomaterial. The pH-dependent higher affinity sites are ascribed to specific sites involving either an ion-exchange mechanism or formation of weak surface-metal ion complexes. These results differed significantly from a similar analysis of total metal binding isotherms that indicated mean log K-app values of -0.5 to 0.25 (low affinity) and 5.6-6.0 (high affinity). (C) 2009 Elsevier B.V. All rights reserved.