Fixed-Bed Modification of Zeolitic Tuffs and Their Application for Cr(VI) Removal

Natural clinoptilolite tuff (CL) and chabazite-clinoptilolite tuff (CH) were modified in fixed-bed column by immobilization of hexadecyltrimethylammonium bromide (HDTMA-Br), then investigated as a sorbent for inorganic anions of Cr(VI). The proposed modification technique combined with surfactant solution batching allows minimizing the surfactant loses through foaming and crystallization and creation of stable organic coverage. The HDTMA loading depended on the mineral composition of the zeolitic tuff, the topology of its external surface, and process conditions. The maximum surface coverage was obtained by gradually dosing surfactant solution in the smallest volume of batches and corresponded up to 100% and 182% of external cation exchange capacity (ECEC) for mono and double layer coverage, respectively. In case of mono layer coverage, modification proceeds until the exhaustion of surfactant in supply solution, while in the double layer one, until equilibrium of HDTMA concentration in both zeolitic and liquid phases was established. The efficiency of Cr(VI) uptake by prepared surface modified zeolites (SMZs) increased with increasing of HDTMA loading. In the case of mono layer SMZs, the capacities of CH-HDTMA and CL-HDTMA were 10.3 and 5.4 mg/g, respectively, while in the case of double layer SMZs, the amount of Cr uptake on CH-HDTMA and CL-HDTMA were 16.8 and 15 mg/g, respectively. Ion exchange is the predominant mechanism of Cr(VI) sorption but it takes place only if modification resulted in at least partial double layer coverage. The XPS analysis reveals Cr(VI) reduction to a less-toxic Cr(III) by the electron donating N-containing groups and by reaction with Fe+2 ions on the zeolite external surface.

Adsorption and Release Characteristics of Purified and Non-Purified Clinoptilolite Tuffs towards Health-Relevant Heavy Metals

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 13574 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.

Concomitant oral intake of purified clinoptilolite tuff (G-PUR) reduces enteral lead uptake in healthy humans

Lead exposure can cause substantial organ damage. Enteral lead absorption may be reduced by concomitant intake of clinoptilolite tuff, a zeolite from natural sources. This study aimed to assess the effect of purified clinoptilolite tuff (G-PUR) on enteral lead uptake in adults using stable lead isotope 204Pb as a tracer. In this randomized, placebo-controlled, double-blind, parallel-group study, 42 healthy participants were randomized to receive oral G-PUR 2.0 g, 2 * 2.0 g, or placebo, together with 2.5 µg of 204Pb in water. The enrichment of 204Pb caused by the tracer in blood and urine was measured by mass spectrometry. G-PUR was well tolerated. The mean maximum 204Pb enrichment of 0.505% of total blood lead was significantly higher (p < 0.0001) in the placebo group compared to G-PUR 2.0 g (0.073%) or G-PUR 2 * 2.0 g (0.057%) group. Normalized 204Pb AUC0-192 was 86.5, 11.9, and 8.5% * h without and with G-PUR 2.0 g, and G-PUR 2 * 2.0 g, respectively (p < 0.0001 vs. placebo). This smaller 204Pb exposure was paralleled by a reduced urinary excretion in subjects receiving G-PUR. Concomitant oral intake of purified clinoptilolite tuff reduced enteral uptake of 204Pb in healthy humans by approximately 90%. The reduced bioavailability is demonstrable by a decrease of 204Pb tracer enrichment in blood and urine.Trial registration: clinicaltrials.gov identifier: NCT04138693, registered 24/10/2019.

Binding and neutralization of C. difficile toxins A and B by purified clinoptilolite-tuff

Clostridioides difficile (C. difficile) infection is a major public health problem worldwide. The current treatment of C. difficile-associated diarrhea relies on the use of antibacterial agents. However, recurrences are frequent. The main virulence factors of C. difficile are two secreted cytotoxic proteins toxin A and toxin B. Alternative research exploring toxin binding by resins found a reduced rate of recurrence by administration of tolevamer. Hence, binding of exotoxins may be useful in preventing a relapse provided that the adsorbent is innocuous. Here, we examined the toxin binding capacity of G-PUR®, a purified version of natural clinoptilolite-tuff. Our observations showed that the purified clinoptilolite-tuff adsorbed clinically relevant amounts of C. difficile toxins A and B in vitro and neutralized their action in a Caco-2 intestinal model. This conclusion is based on four independent sets of findings: G-PUR® abrogated toxin-induced (i) RAC1 glucosylation, (ii) redistribution of occludin, (iii) rarefaction of the brush border as visualized by scanning electron microscopy and (iv) breakdown of the epithelial barrier recorded by transepithelial electrical resistance monitoring. Finally, we confirmed that the epithelial monolayer tolerated G-PUR® over a wide range of particle densities. Our findings justify the further exploration of purified clinoptilolite-tuff as a safe agent in the treatment and/or prevention of C. difficile-associated diarrhea.

REMOVAL OF CESIUM IONS BY NATURAL CLINOPTILOLITE AND COMPOSITE ADSORBENT FROM HIGH-SALT SOLU-TIONS

The development of effective adsorbents for the selective removal of radionuclides from contaminated waters is a topical issue, the solution of which is necessary to reduce the amount of hazardous liquid radioactive waste and to improve safety of the waste management. Natural zeolites have been used for a long time for deactivation of contaminated water, however their wider application for selective removal of radionuclides is limited due to reducing of selectivity caused by increasing solution mineralization, as well as the reversible nature of the radionuclides’ adsorption. Synthesis of composite sorbents based on natural zeolites with incorporated sorption-active inorganic phase is considered as a promising approach to fabricate inexpensive adsorbents with high selectivity to certain radionuclides. The paper presents the results on synthesis of a composite sorbent for the selective removal of cesium ions based on clinoptilolite tuff of the Sokyrnytsia deposit (Ukraine) with the incorporated potassium-copper ferrocyanide phase. Samples of natural and composite clinoptilolite tuff were tested for selective removal of cesium ions from single- and multicomponent model solutions with high content of competing sodium and potassium ions. Mineralization of the solution has been shown to have a significant influence on the adsorption parameters of natural clinoptilolite, while composite adsorbent samples demonstrated high adsorption parameters in sorption of cesium ions from model multi-component solutions with high salt content. The presence of the ferrocyanide phase has been proved to result in an increase in the selectivity of the composite adsorbent in comparison with the natural clinoptilolite samples, as well as in strengthening fixation of adsorbed cesium ions.

Strain localization associated with brittle faulting in a natural clinoptilolite-tuff (open-pit mine Nižný Hrabovec, Slovak Republic)

&lt;p&gt;Clinoptilolite, a micro-porous natural zeolite comprising tetrahedra of silica and alumina that commonly occurs in volcanic tuffs through devitrification of natural glasses, has numerous uses in the manufacturing, agriculture and building industries; it also has applications in veterinary and human medicines. Field observations and microstructural investigations in the natural clinoptilolite-tuff from Ni&amp;#382;n&amp;#253; Hrabovec (Slovak Republic) &amp;#8211; one of the world&amp;#8217;s economically most important high-quality clinoptilolite deposits &amp;#8211; show evidence of strain localization. Brittle faults formed along pre-existing joints with plumose structures that had acted as a pathway for local infiltration of iron-, manganese- and potassium-rich fluids. Fault displacement formed structures that are indicative of both velocity hardening, with dissolution precipitation creep (SC/SCC&amp;#8217; foliation), and velocity weakening, with several phases of ultra-cataclasites forming along principal slip surfaces. Rock-fluid interaction is characterized by a high-mobility of K, with K-feldspar decorating SC/SCC&amp;#8217; foliations, infiltrating fractures in fault damage zones and precipitating as idiomorphic crystals in open cavities and along fault surfaces. Microstructures such as polished slickensides, injection of fluidized cataclasites, clast cortex grains in cataclasites and truncated grains along principal slip surfaces suggest that seismic slip probably occurred along some of the faults.&lt;/p&gt;

Petrogenesis of a Large-Scale Miocene Zeolite Tuff in the Eastern Slovak Republic: The Nižný Hrabovec Open-Pit Clinoptilolite Mine

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.