Mechanically stable and highly permeable porous alumina foam with immobilized copper hexacyanoferrate (Cu-HCF@AF) for the removal of radioactive cesium (137Cs) from seawater

Abstract

Copper hexacyanoferrate (Cu-HCF) is a highly effective adsorbent known for its selective and efficient removal of 137Cs+ ions from seawater. To advance its practical application for seawater remediation, it is essential to granulate Cu-HCF. However, the use of polymer binders presents significant stability challenges in seawater because of their propensity to swell under high osmotic pressure and tendency to decompose when exposed to the radioactivity of 137Cs. This study explores a highly durable and efficient form of Cu-HCF immobilized on alumina foam (Cu-HCF@AF) for the remediation of seawater contaminated with 137Cs+. The Cu-HCF@AF was prepared by first coating copper(II) oxide (CuO) onto alumina foam (AF), then converting it to Cu-HCF through a 1,3,5-benzenetricarboxylic acid (H3BTC)-catalyzed reaction with K3Fe(CN)6 solution. Analysis by scanning electron microscopy showed that spherical CuO particles formed on the AF surface and that 100 nm cubic Cu-HCF crystals developed on these spheres. The synthesis conditions for the Cu-HCF@AF, including the pore size of the AF, the Cu precursor concentration, the number of precursor coatings, and the Cu/reactant ratio, were systematically optimized to achieve optimal Cs+ adsorption performance. Thermogravimetric analysis indicated that the amount of Cu-HCF immobilized on the AF was 3.24 % of the total sample mass. The maximum Cs+ adsorption capacity was determined to be 30.36 mg/g, and the Langmuir isotherm model was found to well fit the adsorption data, suggesting a homogeneous Cs+ adsorption process. The Cu-HCF@AF exhibited outstanding adsorption performance, exceeding 35 mg/g, at pH levels from 7 to 12, making it suitable for use not only in seawater but also in highly alkaline waste streams. In seawater, monovalent ions such as Na+ and K+, which have an ionic radius similar to that of hydrated Cs+, interfered more significantly with Cs+ adsorption compared with divalent ions such as Mg2+ and Ca2+. Finally, we demonstrated the removal of radioactive 137Cs+ in actual seawater, confirming the feasibility of using Cu-HCF@AF in practical applications.