Abstract
Zeolites and zeolite rocks are commonly used in different industrial applications. Natural zeolites present an attractive material for environmental applications because of their high abundance, availability and low costs. Depending on geological settings and conditions during mineral formation, natural zeolite deposits usually represent a heterogeneous mixture of zeolite minerals together with varying amounts of gangue minerals (e. g. quartz, feldspars and phyllosilicates). Hence, profound mineralogical knowledge and a detailed characterization of natural zeolites are essential for tapping their full potential in any practical application. However, this is rarely done as detailed mineralogical characterizations are elaborate and often neglected in favour of bulk chemical analyses (e. g. XRF).In this paper we describe typical technical applications for natural zeolites as well as the requirements demanded for their use. An analytical protocol has been developed for the detailed characterization of natural zeolites for ammonium exchange consisting of a combination of mineralogical and chemical methods and is exemplarily demonstrated. The methodology comprises mineralogical investigations with X‑ray diffraction (XRD) and electron microprobe analyses (EPMA) to provide qualitative data on mineral compositions and in-situ analysis of mineral chemistry (Si/Al-ratio, cation contents). The examinations are accompanied by bulk chemical analysis (XRF) as well as thermoanalytical investigations (TG/DSC) to distinguish between certain zeolite minerals. Finally, ion exchange experiments for ammonium have been carried out to determine the cation exchange capacity (CEC) of zeolite samples for a defined range of concentrations.The aim of the study is to develop an analytical routine in order to enable the detailed characterization of natural zeolite samples with standardized means of mineralogical/chemical analysis prior to their application in technical processes. This is important for quality assurance and for identifying the best suited zeolites for specific technical processes, especially when the proposed applications are not standardized.
Highlights
Zeolites are hydrated alumosilicates with exchangeable cations and open channel systems in their lattice
The more silica is substituted by aluminium, it is expressed by the Si/Al ratio, the higher the ability to incorporate cations which enhance the properties of zeolite minerals as ion exchangers
The X-ray diffraction pattern shows HEU-type zeolite as the main phase (Fig. 1)
Summary
Zeolites are hydrated alumosilicates with exchangeable cations and open channel systems in their lattice. They consist of infinitely extending three dimensional networks of SiO44- and AlO43- tetrahedra linked by shared oxygen atoms [1,2,3,4]. The frameworks form voids or cages and channels in which exchangeable cations can enter. Water can reversibly enter these structures forming partial hydration spheres around the cations where the negative dipole of the water molecules point towards the positively charged monovalent and divalent cations [5]. Clinoptilolite belongs to the HEU structure type and forms a continuous solid solution series with heulandite [9]. According to IMA (International Mineralogical Association) nomenclature [10] heulandite has Si/Al 4
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