Origin

In 1756, the mineralogist Baron Crönsted discovered the Stilbite. Under fast heating conditions this mineral seemed to be boiling due to its water loss. Crönsted named it “zeolite”, from greek word “zeo”, meaning “to boil” and “lithos” meaning “stone”. The zeolite family grew ever since that first discovery and is among the most numerous minerals on Earth. Some two hundred zeolites types are currently known including forty which are natural ones.

Zeolites are hydrated aluminosilicates. They are cation exchangers, and microporous minerals.

After several hundreds of thousand years, natural zeolites formed from volcanic ashes deposited in seas or lakes. With the passing of time and due to an alcalin environment, the ashes have been alterated and cristalised to form zeolites. Natural zeolites are exploited in open-air mines. The first synthesis of zeolite took place in 1862 but it is only in 1956 when the first synthetic zeolite not existing in the nature was able to be made.

Definition and structures

Natural or synthetic zeolites are defined by specific chemical compositions and structural arrangements. The table below gives some zeolites common nomenclature.

Nomenclature of main zeolites

Name	Origin	Abbreviation
Zeolite “A”	Synthetic	LTA
Faujasite	Natural or synthetic	FAU
Chabazite	Natural	CHA
Mordenite	Natural or synthetic	MOR
Ferrierite	Natural or synthetic	FER
Phillipsite	Natural	PHI
Clinoptilolite (heulandite )	Natural	HEU
ZSM-5	Synthetic	MFI

Zeolites are hydrated aluminosilicates. Their structure consists in a three dimensional framework of AlO4 and SiO4 tetrahedra coordinated by oxygen atoms. These connections must respect the Loëwenstein rule namely that one oxygen can not be linked to two aluminum atoms.

The general zeolites formula is written as follows : M2/nO Al2O3 xSiO2 yH2O . M is the cation of valence n, which ensures the electroneutrality of the structure. These are generally mono cations (M+) and divalent (M2+). In solution, they are potentially exchangeable by other monovalent cations, divalent or trivalent.

Zeolite properties

1. Gas adsorption

Zeolites can adsorb organic and mineral molecules in gas phase without any modification of their structure. This adsorption is due to their high specific surface (20 to 800 m2/g), some hydrophobic-hydrophilic surface effects and their structure.

2. Molecular sieves

Zeolites pores with constant diameter only let the smallest molecules pass their inner apertures. Hence they enable a selective separation of gas or liquid mixtures: they are molecular sieves.

3. Water adsorption / desorption

Some zeolites have a high affinity for water. This is shown by an adsorption capacity which may reach 30% by weight without any volume modification. Regeneration takes place by eliminating water thanks to pressure and/or temperature effects. In other processing conditions, adsorbed water naturally returns when the environment is extremely dry. This reversibility of water adsorption according to the hydric balance turns zeolites into some perfect humidity stabilizers.

4. Organic liquids and minerals adsorption

As for gases and water, zeolites can adsorb organic or mineral molecules in the water solution or not. This adsorption is specific to each zeolite. This property enables the application of zeolites in the treatment of pesticide, organic chlorine or hydrocarbons-loaded effluents.

5. Cation exchange capacity

The cation in charge of the electronic neutrality of the zeolitic structure can be exchanged. This is a selective cation exchange according to the zeolite affinity for the replacing cation. The total cation exchange capacity (CEC) and the selectivity are specific to each type of zeolite. This property makes zeolites especially useful and efficient as no other for cation elimination or to achieve control of their concentration in drinking and waste waters in many other fields.

6. Catalysis

In their inner structure zeolites can exhibit sites able to catalyze chemical reactions. This property is widely used in petrochemistry and it enables many reduction, oxidation or acid-base reactions. Insofar as the reactions occur within the zeolite matrix, only molecules which require a space less than that available in the cavities may be formed.

7. Energy storage and recovery

In the case of zeolites, the adsorption of water is accompanied by a heat release. The adsorption/desorption cycle can be renewed indefinitely and the heat transferred through compressors or heat transfer fluids. This property allows produce hot or cold air on the principle of the heat pump.

Zeolites stability

The use of zeolites to collect, retain or confine, implies that they keep their properties over comparable time periods of the pollutants life duration. This stability must be mechanical, thermal, chemical, but also by high radiation resistance (removal of radionuclides).

This stability depends mainly on the material history, i.e. the synthesis and shaping techniques for synthetic zeolites, geological genesis and extraction methods for natural zeolites. Finally, it should be noted that zeolites thermal, mechanical and chemical resistances are generally higher than for materials with comparable physicochemical properties like resins and activated carbon.

Exchange capacity

Among the natural minerals, zeolites are the materials that have the highest cation exchange capacities.

In the environmental protection and ecological balance, zeolites are applied in the following fields:

• The removal of the ammonium ion, a nitrate precursor, for the treatment of industrial effluents, sewage and drinking water.
• Heavy metal cations removal, for the treatment industrial waste waters,
• Radioactive cations removal, especially cesium , strontium and cobalt for the treatment of various nuclear industry effluents.
• Calcium removal by “A” type zeolites, in the phosphate-free detergents.

From the studies of zeolites used as ion exchangers (“A”, “X”, chabazite, clinoptilolite, etc), it is possible to outline the exchange properties of these materials:

• Selectivity is favorable to monovalent cations and in many case with cesium in leading position.
• Retention is greater for higher valence cations, whence the role of Ca²⁺ in the genesis of zeolites.

Origin and shaping

When we speak about natural zeolites, it is very important to know what it is. Unlike synthetic zeolites which are industrially manufactured and very pure, natural zeolites are extracted from quarries. They are not absolutely pure, so it is more accurate to speak of zeolite rock, zeolite tuff or even “zéolitite”. In the case of natural zeolites, mineralogists have adopted the method of giving the name of the zeolite which it mainly consists provided that it contains at least 50% of this zeolite. So be called clinoptilolite, mordenite or chabazite any rock composed at least half of these zeolites.

Other minerals that make up the rock that could be described as “impurities” are of paramount importance. Depending on their nature and concentration they condition the rock and its scope of application. For example, two rocks from two mines having the same content the same zeolite can be inapplicable to the same areas.