BioRisk | fs | 47-| 55 (2022) Apeer-reviewed open-access journal

doi: 10.3897/biorisk.17.77435 REVIEW ARTICLE & RR te) Re IS k

https://biorisk.pensoft.net

Natural zeolites as detoxifiers and modifiers of the
biological effects of lead and cadmium
in small rodents: A review

Michaela Beltcheva!, Peter Ostoich!, Iliana Aleksieva!, Roumiana Metcheva!

| Institute of Biodiversity and Ecosystem Research — Bulgarian Academy of Sciences, 1 Tsar Osvoboditel blvd.,
Sofia 1000, Bulgaria

Corresponding author: Michaela Beltcheva (mnedialkova@gmail.com)

Academic editor: Stephka Chankova | Received 1 November 2021 | Accepted 13 December2021 | Published 21 April2022

Citation: Beltcheva M, Ostoich PB, Aleksieva I, Metcheva R (2022) Natural zeolites as detoxifiers and modifiers of the
biological effects of lead and cadmium in small rodents: A review. In: Chankova S, Peneva V, Metcheva R, Beltcheva
M, Vassilev K, Radeva G, Danova K (Eds) Current trends of ecology. BioRisk 17: 147-155. https://doi.org/10.3897/
biorisk.17.77435

Abstract

The present investigation analyzes the literature about the toxicity of Cd and Pb in small rodents’ organisms
and the role of natural zeolites as modifiers of the biological effects. An array of ecotoxicological, morpho-
physiological, hematological, genetic and biochemical methods as most representative are under discussion
as a basic point for further exploration of biological effects in laboratory mice. The review of existing results
demonstrated that there is abundant data on the sorption of lead and cadmium by modified natural zeolites
in water and soils. Nevertheless, there is insufficient data on the ion exchange capacity and biological ef-
fects of this sorbent in living organisms, especially regarding Cd detoxification. On the basis of the current
review, it is possible to conclude that future investigations in this field will elucidate the potential of the

use of zeolites as successful detoxifiers against heavy metals and other toxic elements in living organisms.

Keywords

Cadmium, lead, natural zeolites, small mammals

Introduction

Pollution of the environment by different xenobiotics is a problem that still raises ma-
jor concerns about the resilience of ecosystems. Despite serious measures having been
taken, there is still the problem of removing toxicants already accumulated in the soil

Copyright Michaela Beltcheva et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC
BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

148 Michaela Beltcheva et al. / BioRisk 17: 147-155 (2022)

and water. Their slow elimination and high rate of accumulation at higher levels of the
food chains is expected to create problems in the next decades. Discovering suitable
sorbents with detoxifying properties that reduce the content of heavy metals in nature
will help to solve a number of problems related to the environment and the risk to
public health. In recent years, natural zeolites are increasingly used in this field. There is
a growing discussion of a direct positive effect of these exceptional crystalline minerals
on improving the state of ecosystems and the quality of life.

Sorption properties of natural and modified zeolites and their ap-
plication

Natural zeolites form one of the most unique groups among minerals, some of which,
for example clinoptilolite, mordenite, and chabazite, have good potential due to their
high sorption capacity and the presence of deposits with huge reserves in many coun-
tries. Bulgaria is one of the first countries in the world for the presence of quality
clinoptilolites (Aleksiev and Djourova 1976; Aleksiev et al. 1997). They have a specific
microporous crystal structure, permeated by channels and voids, which are formed by
different numbers of SiO,“ and AlO,?- tetrahedral rings. Skeletal charge-compen-
sating cations (Na*, K*, Ca**, Mg” and Ba”) and coordinating water molecules are
located in the channels and gaps. ‘The structural configuration of the mineral predeter-
mines its unique properties related to ion exchange and selective sorption (Mumpton
1999). It is possible to modify zeolite surfaces by adding specific surfactants, and thus
to improve the ion exchange capacity to remove cations, anions, and different organic
compounds (Apreutesei et al. 2008). ‘This procedure is very important for enlarging the
sorption capacity of these minerals.

The establishment of significant reserves of clinoptilolite in Bulgaria (Eastern Rho-
dopes) and their growing importance as an effective sorbent of heavy metals, deter-
mines the interest in their use as suitable detoxifiers of living organisms. Zeolitized
tuffs are found in large areas in the Eastern Rhodopes and contain the largest deposits
of zeolites in Bulgaria, formed during the Oligocene (Aleksiev and Djourova 1976; Ju-
rova and Alexiev 1984; Jurova and Alexiev 1989; Djourova and Aleksiev 1990, 1995;
Aleksiev et al. 1997; Raynov et al. 1997; Yanev and Ivanova 2010) (Fig. 1).

The study on zeolites’ properties began in the 18" century, but they remained a
curiosity for scientists and collectors until the middle of the past century, when their
unique physicochemical features attracted the attention of many researchers. The ab-
sorption capacity of natural zeolites was first developed by Ames (1967) and Mercer
et al. (1970), who demonstrated the effectiveness of clinoptilolite for extracting from
municipal and agricultural waste streams and when materials with enhanced sorption
capacity were necessary for nuclear waste management. In the past 60 years there has
been an extraordinary development in zeolite science. Many investigations have been
published on these promising silicate minerals, but none has been devoted specifi-
cally to natural zeolites, even though this theme may be of interest not only to earth

Natural zeolites: a review 149

Figure |. The largest deposits of zeolites in Bulgaria (Eastern Rhodopes).

scientists, but also to chemists and biologists, as the information obtained from natural
samples complete and integrate the characterization of many zeolites.

Other areas of applications of natural zeolites are the purification of waters and the
industrial and urban wastewaters (Boulinguez 2005). Natural zeolites were mainly in-
vestigated and applied as sorbents for ammonium ions from urban pollution, as well as
for heavy metals (Pitcher et al. 2004) and dyes from industrial wastewaters. The removal
of nutrient species (e.g. NH,*, H,PO,?/HPO,), quantitative precipitation and recovery
in slow-release fertilizer was also reported in the literature (Filippidis et al. 2008).

Nowadays natural zeolites have wide applications to limit the consequences of
pollution and for purifying Cs- and Sr-isotopes from the nuclear industry and even-
tual accidents (Borai et al. 2009). Very effective zeolite pills have been prepared for
humans to counteract Chernoby] fallout in Bulgaria (Filizova 1993), as well as for
the workers in the nuclear power plant. The mechanism of action of zeolite is that
the ion exchanges of '°’Cs and ?°Sr are carried out in the gastrointestinal tract and
is eliminated by the excrements, thus minimizing assimilation into the blood sys-
tem. The removal of heavy metals (e.g. Fe, Pb, Cd, Zn) from acidic mine drainage
is another field of potential environmental applications of natural zeolitic materials
(Xu et al. 2010).

Moreover, natural zeolites are used in agriculture to increase soil fertility through
their useful adsorption and sorption, and cation-substituting catalytic properties. The
most effective of the natural zeolites, is clinoptilolite. A lot of tests have been performed
on rocks containing clinoptilolite on different types of soils, in different climatic con-
ditions and geographical regions, in clear form or modified with mineral and organic
fertilizers. The action of rocks containing clinoptilolite is diverse and affects both the
characteristics of the soil and the growth, development and productivity of plants, as
well as the quality of production. The use of clinoptilolite as a principal ingredient of

150 Michaela Beltcheva et al. / BioRisk 17: 147-155 (2022)

artificial soil was developed in Bulgaria in the late 1970s. A nutrient-treated substrate
used for growing crops and the rooting of saplings in greenhouses produced greater de-
velopment of root systems and larger yields of vegetables and fruits (Petrov et al. 1982).

Detoxification properties of natural zeolites and biological re-
sponse of the organism

The use of natural clinoptilolites for animal nutrition began in Japan in the middle of
the past century. Tests were initially made in Japan offering zeolite as a food additive
to swine and poultry, and later to other animals. The experiments were successful.
Researchers obtained good or excellent results with significant increases in productiv-
ity (Torii 1974, 1978; Mumpton 1978). Using clinoptilolite as a food additive could
increase not only the animal production. Pigs fed with clinoptilolite gain significant
weight and are more resistant to disease than pigs fed on a normal diet. Crucially they
show regular digestion, increase in appetite, and the meat content increases due to the
absence of needless fat (Veldman and Vanderaar 1997).

More of the biological applications of zeolites include ammonium removal from
wastewaters and animal manure (Bernal and Lopez-Real 1993) air filtration, deodor-
ants (Miner 1980) and soil fertilization (Mumpton 1999).

Clinoptilolite has incredible healing properties and has the power as mentioned
above to remove from the body all kinds of toxins and poisonous substances, including
radioactive particles, locking them in itself, so they can no longer be released.

Zeolite addition to cattle’s fodder improves the caloric intake, digestion, appetite
and the animal’s body mass. Using clinoptilolite as a food additive to the diet of pigs
and poultry also improves their body weight and leads to gains in the feed conversion
ratios. Clinoptilolite binds some mycotoxins, absorbing toxins dangerous to animals’
health. It also is useful for controlling toxins in animal food. This way the mineral de-
creases the mortality from digestive stress and alleviates the antibiotic needs. It is also
able to absorb toxins produced by molds and microscopic parasites and thus improve
the absorption of food by animals (Cooney et al. 1999).

One of the important questions is whether zeolites are able to adsorb vitamins and
microelements which can potentially disturb physiological balance in organisms. By
the way, Tomasevic-Canovic et al. (2000) found that amino acids and fat-soluble vita-
mins such as A, D and E do not get absorbed by clinoptilolite. Nevertheless, natural
zeolites have some low affinity for binding vitamin B6 in vitro. This process closely
depends on the crystalline and mineralogical structure of the zeolites. Investigations
have demonstrated that, compared to all minerals with similar crystalline structure and
functions, clinoptololite associates least with them.

Martin-Kleiner et al. (2001) report the effects of the natural clinoptilolite on he-
matopoiesis, serum electrolytes and other essential biochemical indicators of kidney
and liver function in mice. The authors found a small increase in potassium levels
in mice reared on a zeolite-rich diet but without any other changes in serum chem-
istry. Erythrocyte, hemoglobin and thrombocyte levels in peripheral blood were not

Natural zeolites: a review [51

affected. There are studies that demonstrated that a natural clinoptilolite maintained
the good condition of the mouse organism and prevented negative effects from the
iron overload. When zeolites are given with iron in the form of intraperitoneal injec-
tions, mice showed near normal histology in comparison to mice injected with iron
only. Such a pilot study examined potential possibilities to current treatments, and the
results show positives in using zeolites in iron excess conditions (Fan et al. 2018).

Zeolite use in ecotoxicology and medicine is relatively recent, but its application
in these areas has reached significant importance in the last few decades. Antioxidant
and immunostimulatory properties of natural zeolites have been reported in different
tumor types, and prove that zeolite acts as a serious immune system modulator (Pave-
lic et al. 2001, 2002). Montinaro at al. (2013) investigates the antioxidant properties
of natural zeolites against oxidative stress, which is one of the main causes leading to
the aging and degeneration process. The authors suggest zeolites as a novel potential
adjuvant in counteracting oxidative stress and plaque accumulation in the context of
neurodegenerative diseases.

Zeolite application as a food supplement in animal feeding is based on effects as
a drug or detoxifier. These effects have been demonstrated in preliminary longtime
investigations and critical assessments of its toxicological effects. Many studies dem-
onstrate that zeolite, especially clinoptilolite addition in the feeding process, even for
several months, showed no evidence of negative reactions or any pathological changes
of animals’ biochemical and hematological profiles. Topashka-Antcheva et al. (2012)
tested the modified natural clinoptilolite KLS-10-MA to establish the LD,, of the used
sorbent. The results show that all test group animals survived successfully up to the end
of the experiment, with significant increases in body weight compared to the control
group, and demonstrated a good activity and physiological condition. No symptoms
of increased toxicity were recorded during the experiment. Any pharmacological ef-
fects were not established. Also, unusual behavior was not observed. The results on the
toxicity of natural modified and nonmodified zeolites, and especially clinoptilolites,
on the basis of their ion exchange capacity, have allowed research groups to investigate
the detoxification possibilities of the mineral on animals and humans. Studies have
been conducted in contaminated areas to eliminate some of the accumulated Pb in
the body of hyperactive children. Clinoptilolite nanoparticles in the form of injection
were applied. The idea was to minimize the effect of the lead on their central nervous
system. It is important to note that these treatments have not been clinically approved
yet (Delavarian et al. 2013). It was established that consumption of alcohol in parallel
with clinoptilolite decreased the ethanol level in the blood in drinkers (Federico et al.
2015). Microencapsulated urease-zeolite sorbent was also investigated. The aim was to
remove the urea from the organism during uremia (Cattaneo and Chang 1991). The
established kinetic curves clearly showed the selective adsorption of zeolite for lead
ions, as well as a significant removal of cadmium not only from wastewaters and soil
but from the animal organism (Malion et al. 1992; Beltcheva et al. 2012).

On the basis of everything mentioned above, Beltcheva et al. (2012) and Topash-
ka-Antcheva et al. (2012) provided a series of long term ecotoxicological experiments
to test the levels of clinoptololite sorption capacity against Pb cations in the animal

152 Michaela Beltcheva et al. / BioRisk 17: 147-155 (2022)

organism. Laboratory white mice were used in 90-day variant experiments. Animals
were divided into four experimental settings: with and without zeolite supplement as
food additive, and with and without lead supplementation. The results showed that the
sorbent reduced Pb concentrations in the body and different tissues and organs as fol-
lows — 84% in carcass, 89% in liver, 91% in kidneys, 77% in bones and the adsorbed
quantity of Pb was excreted via the feces. A mathematical model for the bioaccumula-
tion of lead in bones has been developed and the trends in animals fed with and with-
out zeolite in the diet are presented. The absorption coefficient of Pb from the gastro-
intestinal mucosa was derived and a difference of about 4.5 times was found. In mice
with zeolite, it is 703.53% and in those without additives — n015%. The biological
response of the organism was analyzed at the organ, tissue, cellular and subcellular lev-
els. Changes in chromosome structure, mitotic index, erythrocyte shape and positive
changes in animal body mass were recorded. The main results show that in animals fed
with Pb and clinoptilolite as food additive, 2.3 times lower frequency of chromosomal
aberrations, 2.5 times higher mitotic index and 1.5 times higher percentage of normal
erythrocytes were observed, as well as 1.3 times increase in body weight compared to
those exposed to Pb intoxication, but without the addition of a mineral sorbent.

It is well known that in regions with industries that seriously contaminate the en-
vironment with lead, high levels of cadmium in the environment are also reported. A
future investigation aims to demonstrate that zeolite modifications in the direction of
inset additional active sites of the inner mineral surface and adequate activation of the
clinoptilolite sorbent will increase its sorption capacity compared to the natural mate-
rial. This would increase the detoxification potential and reduce Cd and combinated
Cd+Pb-induced oxidative stress and overall toxicity at the cellular and organism level.

Future prospects

The results of such types of investigations will have a significant theoretical contribution
towards expanding our understanding of the ability of clinoptilolites to reduce the harm-
ful biological effects of certain heavy metals, which are environmental pollutants. In the
long run, such products may be used as low-cost detoxifiers in areas with a high level of an-
thropogenic pressure and contribute to solving important problems related to the environ-
ment, quality of life and health risk not only in nature but also in the human population.

Acknowledgements

This work was supported by the National Science Fund of the Republic of Bulgaria
under project KI1-06-H44/3 “Crystal-chemical and structural characteristics of modi-
fed natural clinoptilolite and correlation between its sorption properties, ion exchange
capacity for heavy metals and biological response in vivo and in vitro”.

Natural zeolites: a review 153

References

Aleksiev B, Djourova E (1976) Mordenite zeolitites from the Northeastern Rhodopes. Dokladi
na Bulgarskata Akademia na Naukite 29: 865-867.

Aleksiev B, Djourova E, Milakovska-Vergilova Z (1997) Geology of the Oligocene zeolitic
rocks in NE Rhodopes, Bulgaria: a review and new data. In: Kirov G, Filizova L, Petrov O
(Eds) Natural Zeolites, Sofia95, Pensoft, Sofia-Moscow, 249-262.

Ames Jr LL (1967) Proceedings of the 13" Pacific Northwest Industrial Waste Conference.
Washington State Univ., Pullman WA, 135-152.

Apreutesei RE, Catrinescu C, Teodosiu C (2008) Surfactant-modified natural zeolites for en-
vironmental applications in water purification. Journal of Environmental Economics and
Management 7: 149-161. https://doi.org/10.30638/eemj.2008.025

Beltcheva M, Metcheva R, Popov N, Teodorova SE, Heredia-Rojas J, Rodriguez-de la Fuente
AO, Rodriguez-Flores LE, Topashka-Ancheva M (2012) Modified natural clinoptilolite
detoxifies small mammal’s organism loaded with lead I. Lead disposition and kinetic model
for lead bioaccumulation. Biological Trace Element Research 147(1—3): 180-188. https://
doi.org/10.1007/s12011-011-9278-4

Bernal MP, Lopez-Real JM (1993) Natural zeolites and sepiolite as ammonium and ammonia
adsorbent materials. Bioresearch Technology 43(1): 27-33. https://doi.org/10.1016/0960-
8524(93)90078-P

Borai EH, Harjula R, Malinen L, Paajanen A (2009) Efficient removal of cesium from low-level
radioactive liquid waste using natural and impregnated zeolite minerals. Journal of Hazard-
ous Materials 172(1): 416-422. https://doi.org/10.1016/j.jhazmat.2009.07.033

Boulinguez B (2005) Ammonium removal in drinking water using natural zeolite. Ecole Na-
tional de la Chimie de Rennes. Proj. No. 11.1577.700/30.6419.080, 14 pp.

Cattaneo MV, Chang TM (1991) The potential of a microencapsulated urease-zeolite oral sorb-
ent for the removal of urea in uremia. ASAIO Transactions (37): 80—87.

Cooney EL, Booker NA, Shallicross DC, Stevens GW (1999) Ammonia Removal from Waste-
waters Using Natural Australian Zeolite. II. Pilot-Scale Study Using Continuous Packed
Column Process. Separation Science and Technology 34(14): 2741-2760. https://doi.
org/10.1081/SS-100100802

Delavarian M, Hassanvand A, Gharibzadeh S (2013) Increasing performance in children with
ADHD by trapping lead with a nano-zeolite. The Journal of Neuropsychiatry and Clinical
Neurosciences 25(1): e23. https://doi.org/10.1176/appi.neuropsych.12010014

Djourova E, Aleksiev B (1990) Zeolitic rocks related to the 2"! acid Paleogene Volcanism to
the east of the town of Kardjali. Geologica Rhodopica. 2" Hellenic-Bulgarian Symposium.
Aristotle University, Thessaloniki, 279-489.

Djourova E, Aleksiev B (1995) Zeolitic rocks in the Northeastern Rhodopes. Sofia Zeolite
Meeting’95 International Symposium, 20-30.

Fan X, McLaughlin C, Ravasini J, Anthony M (2018) Zeolite protects mice from iron-in-
duced damage in a mouse model trial, FEBS Open Bio. https://doi.org/10.1002/2211-
5463.12477

154 Michaela Beltcheva et al. / BioRisk 17: 147-155 (2022)

Federico A, Dallio M, Gravina AG, Iannotta C, Romano M, Rossetti G, Somalvico F, Tuccillo
C, Loguercio C (2015) A pilot study on the ability of clinoptilolite to absorb ethanol in
vivo in healthy drinkers: Effect of gender. Journal of Physiology and Pharmacology 2(66):
441-447,

Filippidis A, Apostolidis N, Filippidis S, Paragios I (2008) Purification of industrial and urban
wastewaters, production of odorless and cohesive zeolite-sewage sludge, using Hellenic
natural zeolite. Proceedings of the 2°¢ International Conference on Small and Decentral-
ized Water and Wastewater Treatment Plants. Skiathos, 403-408.

Filizova L (1993) Program and Abstracts: Zeolite ’93: 4° International Conference on the Oc-
currence, Properties, and Utilization of Natural Zeolites, Boise, Idaho. abstr., 88-90.
Gottardi G, Galli E (1985) Natural Zeolites. Minerals and Rocks (18): 412-418. https://doi.

org/10.1007/978-3-642-465 18-5

Heard M, Chamberlain A, Sherlock JC (1983) Uptake of lead by humans and effect of
minerals and food. The Science of the Total Environment 30(1): 245-253. https://doi.
org/10.1016/0048-9697(83)90016-5

Hunt V, Radford E, Segall A (1963) Comparison of concentrations of alpha-emitting elements in
teeth and bones. International Journal of Radiation Biology and Related Studies in Physics,
Chemistry, and Medicine 7(7): 277-287. https://doi.org/10.1080/09553006314551191

Malion E, Malamis M, Sakellarides PO (1992) Lead and Cadmium Removal by Ion Exchange.
Water Science and Technology 25(1): 133-138. https://doi.org/10.2166/wst.1992.0020

Martin-Kleiner I, Flegar-MeStri¢ Z, Zadro R, Breljak D, Stanovi¢ Janda S, Stojkovi¢ R, Borani¢
M (2001) The effect of the zeolite clinoptilolite on serum chemistry and hematopoiesis in
mice. Food and Chemical Toxicology 39(7): 717-727. https://doi.org/10.1016/S0278-
6915(01)00004-7

Mercer BW, Ames Jr LL, Touhill CJ, Van Slyke WJ, Dean RB (1970) Ammonia Removal from
Secondary Effluents by Selective Ion Exchange. Journal — Water Pollution Control Federa-
tion 42: 95-107.

Miner JR (1980) Odor control — The challenge to livestock producers. Agricultural Engineer-
ing (61): 22-24.

Montinaro M, Uberti D, Maccarinelli G, Bonini SA, Ferrari-Toninelli G, Memo M (2013)
Dietary zeolite supplementation reduces oxidative damage and plaque generation in the
brain of an Alzheimer’s disease mouse model. Life Sciences 92(17—19): 903-910. https://
doi.org/10.1016/j.1fs.2013.03.008

Mumpton FA (1978) Natural zeolites: A new industrial mineral commodity. In: Sand LB,
Mumpton FA (Eds) Natural Zeolites: Occurrence, Properties, Use. Pergamon Press, Elms-
ford, 327 pp.

Mumpton FA (1999) La roca magica: Uses of natural zeolites in agriculture and industry. Pro-
ceedings of the National Academy of Sciences of the United States of America 96(7):
3463-3470. https://doi.org/10.1073/pnas.96.7.3463

Pavelic K, Hadzija M, Bedrica L, Pavelic J, Dikic I, Katic M (2001) Natural zeolite clinoptilo-
lite: New adjuvant in anticancer therapy. Journal of Molecular Medicine 78(12): 708-720.
https://doi.org/10.1007/s00 1090000176

Natural zeolites: a review 155

Pavelic K, Katic M, Sverko V, Marotti T, Bosnjak B, Balog T, Stojkovic R, Radacic M, Colic
M, Poljak-Blazi M (2002) Immunostimulatory effect of natural clinoptilolite as a possible
mechanism of its antimetastatic ability. Journal of Cancer Research and Clinical Oncology
128(1): 37-44. https://doi.org/10.1007/s00432-001-0301-6

Petrov GS, Petkov IA, Etropolski HI, Dimitrov DN, Popov NN, Uzunov AI (1982) U.S. Pat-
ent 4,337,078.

Pitcher SK, Slade RCT, Ward NI (2004) Heavy metal removal from motorway stormwater
using zeolites. The Science of the Total Environment 334-335: 161-166. https://doi.
org/10.1016/j.scitotenv.2004.04.035

Raynov N, Popov N, Yanev Y, Petrova P, Popova P, Hristova V, Atanasova R, Zancharska R
(1997) Geological, mineralogical and technological characteristics of zeolitized (clinop-
tilolitized) tuffs deposits in the eastern Rhodopes Bulgaria. Pensoft, Sofia. 249-262.

Tomasevic-Canovic M, Dakovic A, Markovic V, Radosavljevic-Mihajlovic A, Vukicevic J
(2000) Adsorption effects of mineral adsorbents part iii: Adsorption behaviour in the pres-
ence of vitamin b6 and microelements. Acta Veterinaria 50: 23-29.

Topashka-Ancheva M, Beltcheva M, Metcheva R, Heredia-Rojas JA, Rodriguez-De la Fuente
AO, Gerasimova T, Rodriguez-Flores LE, Teodorova SE (2012) Modified natural clinop-
tilolite detoxifies small mammal’s organism loaded with lead II: Genetic, cell, and physi-
ological effects. Biological Trace Element Research 147(1-3): 206-216. https://doi.
org/10.1007/s12011-011-9289-1

Torii K (1974) Utilization of sedimentary zeolites in Japan. In: Mumpton FA (Ed.) Abstracts
and Proceedings of a Seminar on the Occurrence, Origin, and Utilization of Sedimentary
Zeolites in the Circum-Pacific Region. U.S. Japan Cooperative Science Program, Menlo
Park California, USA, 49.

Torii K (1978) Utilization of natural zeolites in Japan. In: Sand LB, Mumpton FA (Eds) Natu-
ral Zeolites: Occurrence, Properties, Use. Pergamon Press, Elmsford, N.Y., USA, 441-450.

Veldman A, Vanderaar PJ (1997) Effects of dietary inclusion of a natural clinoptilolite
(Mannelite(TM)) on piglet performance. Agribiological research 50: 289-294.

Xu W, Li LY, Grace JR (2010) Zinc removal from acid rock drainage by clinoptilolite in a
slurry bubble column. Applied Clay Science 50(1): 158-163. https://doi.org/10.1016/j.
clay.2010.07.005

Yanev Y, Ivanova R (2010) Zeolite 2010. M. Drinov Academic Publishing House, Sofia Bul-
garia, 34 pp.

Yanev Y, Cochemé JJ, Ivanova R, Grauby O, Burlet E,Pravchanska R (2006) Zeolites and
zeolitization of acid pyroclastic rocks from paroxysmal Paleogene volcanism, Eastern Rho-
dopes, Bulgaria. Neues Jahrbuch ftir Mineralogie — Abhandlungen: Journal of Mineralogy
and Geochemistry 182(3): 265-283. https://doi.org/10.1127/0077-7757/2006/0050