Alumina-Boosted Fly Ash Geopolymers Capture Radioactive Metals More Efficiently

Scientists at India’s Bhabha Atomic Research Centre have discovered a counterintuitive recipe for trapping radioactive metals: make the material’s surface smaller, not larger. By modifying fly ash–based geopolymers with gamma-phase aluminum oxide (γ-Al₂O₃), the research team produced a sorbent that pulls trivalent actinides out of water with more than 97 percent efficiency across a wide pH range, despite actually having less physical surface area than the unmodified original. The finding, published in Environmental Science and Pollution Research, upends the conventional wisdom that adsorption performance is primarily a function of how much surface a material exposes.

Geopolymers are aluminosilicate materials formed when industrial precursors such as coal fly ash are activated with alkaline solutions. Rather than burning energy in high-temperature kilns the way ordinary Portland cement does, geopolymers harden through a chemical dissolution-and-reassembly process, weaving amorphous networks of aluminate and silicate units. Because fly ash is an abundant byproduct of coal combustion, geopolymers built from it are cheap, and they have long been investigated both for waste encapsulation and as low-cost adsorbents for removing heavy metals and radionuclides from contaminated water.

The challenge addressed in the new study concerns trivalent actinides, a family of radioactive elements that includes americium-241, a component of spent nuclear fuel and legacy weapons-site waste. In aqueous environments, trivalent actinides behave chemically much like their lanthanide cousins, which is why the researchers used europium(III) as a nonradioactive chemical surrogate alongside trace amounts of americium-241 as a radiotracer. This pairing allowed the team to measure uptake quantitatively and to probe the chemistry of the bound metal ions with spectroscopic precision.

The researchers prepared two materials: a pristine fly ash geopolymer, designated FA-GP, and a modified version, FA-Al-GP, in which gamma-alumina was integrated into the geopolymer framework. Structural characterization confirmed that the γ-Al₂O₃ had genuinely become part of the matrix rather than merely sitting in it as an inert filler. Nitrogen adsorption measurements using the Brunauer–Emmett–Teller method revealed something unexpected: the modification cut the specific surface area from 41 square meters per gram in the pristine material down to just 16 square meters per gram. By the usual logic of adsorption science, that alone should have crippled its performance.

Solid-state nuclear magnetic resonance told a different story. Using ²⁷Al magic-angle spinning NMR, the team found that the modified geopolymer contained a higher proportion of octahedrally coordinated aluminum sites. These six-fold coordinated aluminum centers, along with associated aluminol groups, act as Lewis-basic anchors, chemically receptive to hard trivalent metal cations. In effect, the alumina modification traded sheer surface for a denser matrix whose surfaces are far richer in the specific chemical functionality that binds actinides and lanthanides.

Batch sorption experiments bore this out decisively. The modified geopolymer removed more than 97 percent of the target metal ions across a broad pH window from 4.0 to 8.0, a range relevant to many real waste streams and groundwater conditions. Its maximum adsorption capacity reached approximately 66 milligrams per gram at 328 kelvin, compared with roughly 39 milligrams per gram for the pristine fly ash geopolymer. In other words, the material with 60 percent less surface area captured about 70 percent more metal per unit mass.

Kinetic and thermodynamic analysis showed that uptake in both materials followed pseudo-second-order kinetics and fit Langmuir isotherms, the classic signatures of monolayer chemisorption. The process was found to be spontaneous and endothermic, meaning the metal ions form genuine chemical bonds with the surface rather than simply sticking through weak physical forces. That distinction matters enormously for nuclear waste management: chemically bound radionuclides resist being leached back out by changes in water chemistry, whereas physisorbed contaminants can be released when conditions shift.

To see exactly where and how the metals were binding, the team turned to time-resolved fluorescence spectroscopy and X-ray photoelectron spectroscopy. The fluorescence measurements, which exploit the sharp, environment-sensitive emission of europium(III), provided molecular-level evidence about the coordination environment of the sorbed ions, while XPS tracked shifts in binding energies that revealed the formation of strong inner-sphere complexes. Both techniques pointed to the same conclusion: the reactive aluminol and silanol groups on the geopolymer surface are the active sites that grip the trivalent metals directly, displacing their hydration shells.

The broader implication of the study is a design principle for next-generation sorbents. Rather than chasing ever-larger surface areas, materials engineers can boost performance by concentrating highly accessible, chemically reactive sites on the surfaces they already have. Because surface chemistry, not bulk area, dictates capacity, targeted modification strategies like alumina integration could improve a wide range of geopolymer and mineral sorbents. The approach also strengthens the case for geopolymers as dual-purpose nuclear materials, capable of both immobilizing waste in solid form and scrubbing actinides from aqueous streams, all while converting an industrial waste product, coal fly ash, into a functional environmental technology.

For the nuclear industry, where cleanup costs at legacy sites run into the billions and the safe management of actinides remains one of the field’s hardest problems, a sorbent made from power-plant ash that outperforms its unmodified counterpart is more than a laboratory curiosity. The work, led by Atanu Das and Aishwarya Soumitra Kar with colleagues at the Bhabha Atomic Research Centre, Homi Bhabha National Institute, and the Tata Institute of Fundamental Research, suggests that smarter surface engineering of abundant, inexpensive materials may offer a practical path toward more efficient actinide sequestration in contaminated waters around the world.

Subject of Research: Enhanced sorption of trivalent actinides by γ-Al₂O₃-modified fly ash-based geopolymers

Article Title: Structural evolution and enhanced sorption performance of γ-Al₂O₃-modified fly ash-based geopolymers for trivalent actinide sequestration

Article References: Das, A., Kar, A. S., Arunachalam, V., Tyagi, D., & Parayil, R. T. (2026). Structural evolution and enhanced sorption performance of γ-Al₂O₃-modified fly ash-based geopolymers for trivalent actinide sequestration. Environmental Science and Pollution Research, 33(28), 14505-14526. https://doi.org/10.1007/s11356-026-38180-2

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38180-2

Keywords: geopolymers, fly ash, gamma-alumina, actinides, americium-241, europium, sorption, chemisorption, nuclear waste, radionuclide sequestration, XPS, fluorescence spectroscopy