1. Introduction
High-level liquid waste (HLLW) from spent nuclear fuel contains various fission products, including alkali metals, alkaline earth metals, and rare earth elements. Conventional solvent extraction methods are expensive and generate secondary wastes. Supported liquid membrane (SLM) systems are promising alternatives due to their low solvent consumption, operational simplicity, and high selectivity.
Nanohydroxyapatite (nHAp) has excellent ion-exchange properties for divalent and trivalent metal ions, while activated carbon (AC) provides high surface area and mechanical strength. Their combination as a composite support material for SLMs could enhance transport efficiency and membrane durability.
2. Materials and Methods
2.1 Synthesis of nHAp-Activated Carbon Composite
- Activated carbon powder was impregnated with nHAp nanoparticles (via precipitation from calcium nitrate and ammonium phosphate precursors).
- The mixture was dried at 80 °C, calcined at 400 °C for 2 h, and ground to <100 µm particle size.
2.2 Supported Liquid Membrane Preparation
- PVDF membrane (0.45 µm pore) was impregnated with 0.2 M organophosphorus extractant (e.g. tri-n-butyl phosphate, TBP) dissolved in kerosene.
- The nHAp-AC composite was coated as a thin layer to improve mechanical stability and active surface area.
2.3 Simulated Waste Feed
- A simulated feed solution containing Cs⁺ (10 ppm), Sr²⁺ (10 ppm), and Nd³⁺ (20 ppm) in 0.1 M nitric acid was used to represent fission products.
3. Results and Discussion
3.1 Physicochemical Characterization of nHAp-AC
Table 1. Physicochemical properties of nHAp-AC composite
| Property | Activated Carbon | nHAp | nHAp-AC Composite |
|---|---|---|---|
| BET Surface Area (m²/g) | 900 | 65 | 560 |
| Average pore diameter (nm) | 2.1 | 45 | 3.8 |
| Zeta potential (mV) | -20 | -4 | -14 |
| Water contact angle (°) | 58 | 73 | 64 |
3.2 Membrane Transport Performance
Table 2. Transport efficiency of fission products using nHAp-AC supported liquid membrane
| Ion | Initial Conc. (ppm) | Final Conc. (ppm) | Removal Efficiency (%) | Permeability (cm/min ×10⁻⁵) |
|---|---|---|---|---|
| Cs⁺ | 10 | 1.5 | 85 | 2.8 |
| Sr²⁺ | 10 | 0.9 | 91 | 3.2 |
| Nd³⁺ | 20 | 3.0 | 85 | 2.4 |
3.3 Effect of pH on Ion Transport
Table 3. Effect of feed pH on Sr²⁺ transport efficiency
| Feed pH | 2.0 | 3.0 | 4.0 | 5.0 | 6.0 |
|---|---|---|---|---|---|
| Removal Efficiency (%) | 42 | 65 | 81 | 91 | 76 |
Optimal Sr²⁺ transport occurred at pH 5.0 due to reduced competition from H⁺ ions.
3.4 Membrane Stability and Reusability
Table 4. Stability of nHAp-AC SLM over multiple cycles
| Cycle Number | Removal Efficiency of Sr²⁺ (%) |
|---|---|
| 1 | 91 |
| 2 | 89 |
| 3 | 87 |
| 4 | 83 |
| 5 | 80 |
The membrane retained >80% efficiency even after five cycles, indicating good stability.
4. Conclusion
This study demonstrated that nHAp-AC composite significantly enhances the performance of supported liquid membranes for removing fission products from simulated nuclear waste. The membrane showed high ion removal efficiency, chemical stability, and good reusability. The synergistic effect of nHAp for ion exchange and AC for structural support makes this material a promising candidate for nuclear waste treatment technologies.
5. References
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