Study on the Sorption of Eu(III) onto Natural Red Earth and its Solid Components by Linear and Non-Linear Methods

The sorption of Eu(III) on natural red earth (NRE), iron oxides removed red earth (IORRE), and organic matter removed red earth (OMRRE) were studied under ambient conditions. The kinetic and thermodynamic studies of Eu(III) sorption were discussed by linear and non-linear approaches. The results showed that sorption of Eu(III) on the three red earth was strongly dependent on ionic strength below pH similar to 7.0 and independent of ionic strength above pH similar to 7.0. Ion exchange, outer-sphere surface complexes, and inner-sphere surface complexes were controlling Eu(III) sorption in the whole observed pH range. Furthermore, the presence of iron oxides can inhibit Eu(III) sorption to some extent; however, organic matters enhance Eu(III) sorption in a certain range of pH and temperature. Sorption of Eu(III) on red earth samples can be represented by a pseudo-second-order rate equation, and the non-linear pseudo-second-order kinetic expression could fit the kinetics better compared with the linear forms. Freundlich non-linear isotherm model is the best-fit model to Eu(III) sorption on the samples. Meanwhile, non-linear method is found to be a better way to describe the sorption kinetics and thermodynamics than the linear method.