Mineralogische Charakterisierung natürlicher und für die Aufnahme von Ammonium optimierter Zeolithe für die Abwasserreinigung

Zeolites are aluminosilicates with a tetrahedral (Si,Al)O4 framework structure, with larger (nm) channel-like cavities and high ion exchange capacities. In the ReNOx 2.0 project, this property is being exploited to simultaneously recover nitrogen and phosphorus from wastewaters. Zeolites from ZEOCEM a.s. (Slovakia) were used for this purpose. The Nižný Hrabovec deposit in the eastern Slovak Basin yields zeolites that formed from the Hrabovec tuffs through diagenetic processes.
The zeolites were treated with acids (HCl 20% and 5%), salts (NaCl 20% and CaCl2 5.26%), and bases (NaOH 13.6% and Na2CO3 10%) to increase their cation exchange capacity. Various methods were used to analyse the modification effects: electron microprobe micro analysis (EPMA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). Raman and FTIR analysis proved to be unsuccessful for qualitative detection of nitrogen and phosphorus uptake in zeolite.
Due to their high crystal water content, zeolites are difficult to analyse with EPMA; HENCE; a customised protocol (15 kV, 2 A, 10 µm spot) was developed. For the Raman analysis, measurement repetitions (3), a duration of 60 s, and wave number ranges of 50–1500 cm⁻¹ (mineral identification) and 50–4000 cm⁻¹ (nitrogen/phosphorus detection) were specified.
The samples studied consist of a fine-grained zeolite matrix with larger mineral phases. EPMA and XRD identified major clinoptilolite, plagioclase, potassium feldspar with an admixtures of minor quartz and biotite, as well as some accessory heavy minerals like zircon, titanium/iron oxides, apatite and monazite. The untreated reference sample has a Si/Al ratio of 4.78 (crystals) and 4.96 (matrix). The modification primarily affected the reactive matrix, while the zeolite crystals remained largely unchanged.
Acid treatments led to a depletion of Ca, Na, and K, while the combined NaCl/HCl treatment resulted in Na+ loading, evidenced by a slight shoulder in the FTIR spectrum. With increasing HCl concentration, variable dealuminisation of the matrix was observed, reaching a maximum Si/Al value of 10.04 at 20% HCl, which is reflected in a shift of the main FTIR peak to higher wavenumbers. XRD analyses showed an increase in amorphous phases, and biotite was found to be non-acid resistant at 20% HCl, but was acid-resistant at 5% HCl. FTIR indicated removal of Al³⁺ from the pores, TGA a reduction in bound crystal water, while DSC showed no significant differences compared to the reference sample.
Base treatments (NaOH and Na2CO3) caused desilification (removal of amorphous SiO₂) and Na+ uptake, with NaOH showing a significantly stronger effect. There was a slight depletion of Ca and K, while Al and Mg remained unchanged. FTIR measurements showed a shift of the main peak to lower wavenumbers. The XRD analysis revealed no increase in amorphous phases, but a slight peak shift. Both base treatments resulted in increased total weight loss (more crystal-bound water) and the DSC showed an earlier thermal decomposition.
The salt treatments (CaCl2 and NaCl) had the least impact. The EPMA analysis showed only slight Si depletion and dealuminisation with little Na+ uptake, while XRD, FTIR, and TGA/DSC detected no significant changes compared to the reference sample.
Raman measurements were limited due to strong fluorescence of the samples. Nitrogen and phosphorus were not detected neither with Raman nor FTIR.
This work highlights the importance of methodological diversity in applied mineralogical investigations. The combination of the different methods enables recommendations for ReNOx 2.0 and subsequent projects. Since a high Na+ loading is crucial for Nitrogen and Phosphor exchange, NaOH followed by Na2CO3 treatments proved to be optimal. Another aspect is of course the verification/quantification of the effects of the modifications in an industrial process. In this respect, EPMA turned out to be the most accurate (but also most expensive) method, although the FTIR measurements also showed differences. FTIR could therefore be a time- and cost-effective alternative to EPMA in an industrial process.