Selectivity of Ar/O2 plasma-treated carbon nanotube membranes for Sr(II) and Cs(I) in water and wastewater: Fit-for-purpose water treatment

Abstract

The need for high-performance and novel technologies for selective treatment of radionuclides in water has recently stimulated broad research. In this study, we examined the ability of carbon nanotube (CNT) membranes in selective removal of strontium (Sr2+) and cesium (Cs+) from water and wastewater. In single-metal systems that contained only Sr2+, Cs+ or their competitive metal ions, the distribution coefficients (Kd) of functionalized CNT membranes for divalent cations, i.e., Sr2+, Ca2+, and Mg2+, were measured as 4.41 mmol g−1, 2.14 mmol g−1, and 1.30 mmol g−1, respectively, and those for monovalent cations, i.e., Cs+, K+ and Na+, were 0.81 mmol g−1, 0.79 mmol g−1, and 0.81 mmol g−1, respectively. When Cs+ or Sr2+ co-existed with a competing cation, both pristine and functionalized CNT membranes showed excellent selectivity to Sr2+ over Mg2+ and Ca2+ while their selectivity to Cs+ over K+ and Na+ diminished. Moreover, the CNT membranes remained their selectivity for Sr2+ in a wastewater effluent despite the complex water composition. Overall, the selectivity sequence of the CNT membranes for divalent cations support the occurrence of inner-sphere complexation on CNT surfaces, while that for the monovalent cations was probably determined by electrostatic interactions. This study demonstrates that Ar/O2 plasma treatment is a greener technology for the production of CNT functionalized membranes for selective removal of contaminants from water to achieve “fit-for-purpose” treatment.