Adsorption behavior of radioiodides in hexadecylpyridinium-humate complexes

The adsorption and desorption of the radionuclides iodide (I-125(-)) and cesium (Cs-134(+)) to and from humates with different hexadecylpyridinium (HDPy+) loadings were studied with the batch technique using bidistilled water and synthetic ground water (SGW) as equilibrium solutions. Furthermore, original humates, HDPyCl molecules, and the organo-humate samples were characterized by X-ray diffraction (XRD) and thermogravimetric (TG) measurements, differential thermo-analysis (DTA), and FT-IR spectroscopy. In HDPy+-modified humates, iodide ions exhibited increased adsorption (characterized by the distribution coefficient, K-d), while the adsorption of the cationic cesium was negligible. Higher distribution coefficients were obtained for the desorption than for the adsorption steps indicating that part of the iodide ions were bound irreversibly (hysteresis). In a log-log plot, adsorption and desorption isotherms for iodide exhibited non-linearity over wide concentration ranges of the carrier ions investigated, suggesting that besides ion exchange other types of adsorption mechanism also exist. The diffraction measurements revealed a distinct crystalline character of the HDPy-humate samples in contrast to the amorphous state of the original humic substances. These implications were confirmed by the thermal data and from the results of the FT-IR spectroscopy. (C) 2001 Elsevier Science B.V. All rights reserved.