Over 170,000 metric tonnes of high-grade clinoptilolite tuff were extracted from the open-pit mine in Nižný Hrabovec (eastern Slovak Republic) in 2018, making it one of the world’s major natural clinoptilolite producers. The mine is hosted in a Miocene volcanogenic-sedimentary deposit in the East Slovak basin, with estimated 150 million tonnes of clinoptilolite tuff—the economically most important reserve in the European Union. The 100-m-thick tuff horizon is under- and overlain by SW-dipping shallow-marine Badenian sediments (16.30–15.03 Ma ≅ Langhian stage). The tuff is rhyolitic (74.3–77.6 wt % SiO2, 4.09–5.49 wt % Na2O + K2O) with a generally high-K calc-alkaline affinity (3.07–4.28 wt % K2O), comparable to volcanic rocks in the central segment of the Carpathian-Pannonian region of similar age. Although some parts of the tuff underwent slight geochemical changes during formation, this did not significantly change the whole-rock composition of the strikingly homogeneous raw material present in the mine. Rare earth elements (ΣREE = 117–141 ppm) show a uniform pattern, with enriched light REEs and negative Eu anomalies of 0.42 to 0.6 (when normalized to chondrite), comparable to the upper continental crust composition. Mean 87Sr/86Sr = 0.70880 and 143Nd/144Nd = 0.512463 indicate an enriched magmatic source, with a dominant crustal contribution. X-ray diffraction data and electron probe microanalyses show that clinoptilolite-Ca is the only zeolite phase present in the deposit. High-resolution electron probe microanalytical imaging and measurement techniques reveal that clinoptilolite is present as (1) coarse patches that either form pseudomorphs of volcanic glass shards or grew in voids and (2) ultrafine material making up the matrix of the tuff. Both textural types have Si/Al >4, and their evolution is connected to dissolution-(transport)-precipitation reactions from acidic volcanic ash under alkaline fluid influence and slightly elevated pressure-temperature (44°–84°C) conditions. Authigenic cristobalite, detected as ultrafine-grained matrix crystallites, formed during the zeolitization process from excess Si. The large-scale, remarkably homogeneous and monomineralic natural zeolite deposit formed from a well-equilibrated magmatic source, with no syn- to postsedimentary reworking and with essentially isochemical conditions during the zeolitization process.

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The occurrence of health-relevant contaminants in water has become a severe global problem. For treating heavy-metal-polluted water, the use of zeolite materials has been extended over the last decades, due to their excellent features of high ion exchange capacity and absorbency. The aim of this study was to assess the effect of heavy metal uptake of one purified (PCT) and two non-purified clinoptilolite tuffs (NPCT1 and NPCT2) in aqueous solutions on monovalent ions Ni+, Cd+, Cs+, Ba+, Tl+, and Pb+. Experiments were furthermore carried out in artificial gastric and intestinal fluids to mimic human digestion and compare removal efficiencies of the adsorbent materials as well as release characteristics in synthetic gastric (SGF) and intestinal fluids (SIF). Batch experiments show low sorption capacities for Ni+ and Cd+ for all studied materials; highest affinities were found for Ba+ (99–100%), Pb+ (98–100%), Cs+ (97–98%), and Tl+ (96%), depending on the experimental setup for the PCT. For the adsorption experiments with SGF, highest adsorption was observed for the PCT for Pb+, with an uptake of 99% of the lead content. During artificial digestion, it was proven that the PCT did not release Ba+ cations into solution, whereas 13,574 ng·g−1 and 4839 ng·g−1 of Ba+ were measured in the solutions with NPCT1 and NPCT2, respectively. It was demonstrated that the purified clinoptilolite tuff is most effective in remediating heavy-metal-polluted water, particularly during artificial digestion (99% of Pb+, 95% of Tl+, 93% of Ba+). In addition, it was shown that the released amount of bound heavy metal ions (e.g., barium) from the non-purified clinoptilolite tuffs into the intestinal fluids was significantly higher compared to the purified product.