The abilities of the humic acids (HAS) to bind Cr(II1) were evaluated by a model on the basis of binding between Cr3' and CrOHZf species and the HAS. This model equation denotes the relationship between the concentration of unbound ([Cr(III)],,) and bound ([Cr(III)],) species. The aqueous solution containing Cr(III) and HA was equilibrated, and then [Cr(III)], was determined by flame atomic absorption spectrometry after a pre-separation with the cation-exchanger, sulphopropyl Sephadex C-25 (C-25), in which the cationic unbound C&II) species retained on the C-25 could be calculated by subtracting [Cr(III)], in the effluent from the total concentration of C&II) added initially ([Cr(III)],>. An excellent curve-fitting was obtained in the range of pH 3.2-4.3 by the non-linear least square regression analyses of the proposed model equation to the experimental data set ([Cr(III)],,, [Cr(III)],
). The calculated conditional binding constants (K&J+, K' CrOH~+) and the binding capacities (N) were as follows: log K&3+, 3.4-4.9; log K&2+, 4.9-5.3; N, 0.8-2.4 (mmol g-r of C), and these values increased with increasing pH. The conditional binding constants of the Cr(III)-HA complex (pH 3.25, I = 0.01) were compared with those of other metal-HA complexes in the literature (pH 3.00, I = 0.01). The logK&+ value was comparable to those of metal ions as Ca'+, Znzt and the 1ogK' crOH2+ value was comparable to those of PbzC, and Cu2+ and Ni*'. The N values were larger than those in the case of copper(U) ions. The Cr(III) binding abilities of the peat HAS were also evaluated by the present model. It was found that the N values of the HAS correlated to the carboxylic contents. Comparison of FTIR (Fourier transform infrared) spectra between the protonated I-L4 and Cr(III)-HA complex suggested the existence of Cr(III)-carboxylate binding.