Sequestration of selenate and selenite onto titanate nanotube: A combined classical batch and advanced EXAFS approach

Abstract

The sequestration of selenate (Se(VI)) and selenite (Se(IV)) onto titanate nanotubes (TNTs) via an adsorption was studied using a combination of macroscopic interaction data, zeta potential determinations and X-ray absorption fine structure (XAFS) analyses. The macroscopic interaction results showed generally increased adsorption of Se(VI)/Se(IV) onto TNTs with pH decreasing, and indicated an ionic strength-dependent adsorption of Se(VI) onto TNTs, whereas an ionic strength-(in)dependent adsorption of Se(IV) onto TNTs. Besides, we observed that both Se(VI) and Se(IV) decreased the zeta potential of TNTs upon adsorption. Nevertheless, only Se(IV) adsorption decreased the pHPZC of TNTs. The X-ray absorption fine structure (XAFS) results pointed to substantial differences of the adsorption mode for Se(VI)/Se(IV) onto TNTs, with Se(VI) adsorbing as an outer-sphere surface complex, and Se(IV) coordinating as an inner-sphere surface complex. Combined, the results presented here showed that pH and ionic strength are major factors in determining the adsorption extent and model of Se(VI)/Se(IV) onto TNTs. In a word, TNTs exhibited excellent adsorption capacity to both Se(VI) and Se(IV) regardless their different adsorption mode. Hence, as the synthesis equipment is cheap and simple, TNTs can be regarded as a low-cost sequestrator for the efficient trapping of selenium in water.