Study On Se(â…£)Adsorption Onto Clay Minerals

As an important physical and chemical phenomena in soild-liquid interface, sorption has been widely applied in environmental science research areas. Investigating the sorption behavior of interesting elements in soild-liquid interface can be helpful to clarify the retention, distribution and migration behavior of pollutants (e.g. radionuclides) in the environment, as well as to give a theoretical guidance for the development of new adsorbents and their engineering applications. In this work, the sorption behavior of Se(IV) onto anatase, bentonite, kaolinite and illite/smectite mixture were investigated. Sorption kinetics and the influencing factors such as pH, concentration of Se(IV), temperature, ionic strength, concentration of adsorbents, coexisting ions like Eu(â…¢) and Ca(II) and the state of adsorbent (dispersed and compacted states) on the sorption of Se(IV) onto different adsorbents were studied in details. Corresponding equilibrium constants of the surface reactions were obtained according to the modelling results using surface complexation model. The possible sorption mechanism was also deduced based on the findings above.(1) Surface acid-base properties of anatase, bentonite and kaolinite were characterized by potentiometric titration technologies. The site densities, acid-base equrlibrium constants and other parameters of the adsorbents were obtained using surface complexation models through computer software calculation.(2) The sorption kinetics of Se(IV) onto anatase, bentonite. kaolinite and illite/smectite mixture were well studied. It was found that the sorption rate of Se(IV) onto these adsorbents is relatively fast and24h is enough to reach the equrlibrium state. The sorption results were well fitted by pseudo-second-order model. The conditional rate constants decreased with the increase of amount of Se(IV) sorbed and varied with an exponential law.(3) pH is a major factor on the sorption of Se(IV) onto these adsorbents, the lower the pH, the higher Se(IV) sorbed. The effects of ionic strengths on the sorption of Se(IV) onto anatase and bentonite are not obvious, which indicate that inner-sphere surface complexes might be formed between Se(IV) species and active sites of the adsorbent surface. The sorption abilities of these four adsorbents to Se(IV) in the same conditions are:bentonite> illite/smectite mixture> kaolinite> anatase, this is mainly based on the spatial structure and surface area of the adsorbents.(4) The sorption of Se(IV) can be enhanced by the presence of Eu(â…¢), which is mainly based on the formation of ternary surface complexes as well as Eu2(SeO3)3precipitate in relatively high concentration of Se(â…£) and Eu(â…¢). The effect of Ca(â…¡) on the sorption of Se(â…£) onto illite/smectite mixture is not obvious in our experiment.(5) A study on the effect of very high concentration of adsorbents (compacted state) was carried out using a capillary method. The results show that the compated effect is not obvious. There are two possible explanations:(i) there is no relationship between the sorption behavior of Se(â…£) and the existing state of adsorbents. The sorption abilities of these adsorbents to Se(IV) might not be changed either in dispersed or compacted state if the contacted time is long enough.(ii) the adsorbents is not compated enough.(6) A surface complexation model (constant capacitance model) was successfully applied to model the experimental data, and two inner-sphere surface complexes:SSeO3-and (=S)2SeO3were considered when modelling.