Technological characterization of natural zeolites for the removal of inorganic pollutants

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Giovanni Varriale

Technological characterization of natural zeolites for the removal of inorganic pollutants

by Giovanni Varriale*

Natural zeolites represent some of the most important inorganic ionic exchangers. Nowadays, considering their high availability and low cost, zeolites are used in a wide range of technological processes which comprise the treatment and the purification of waters from pollutants (inorganic, organic, potential radionuclides) as well as agricultural and zootechnical use.

A zeolite can be defined as crystalline, hydrated aluminosilicate of alkali and alkaline earth cations having an open, three-dimensional structure. It is further able to lose and gain water reversibly and to exchange extraframework cations, both without change of crystal structure (Mumpton 1998). Their crystalline framework is based on structural cavities connected by entry channels. On removal of water by heating at 350-400°, small molecules can pass through entry channels but larger molecules are excluded, thus they can behave as “molecular sieves”. Furthermore zeolites are able to adsorb polar molecules with high selectivity.

The weakly bonded extraframework cations can be removed or exchanged through ionic exchange reactions by washing the zeolite with solutions containing interfering cations. The framework of a crystalline zeolite will be able to dictate its selectivity (Eisenman,1966 – Sherry, 1967) toward competiting ions. From a technological point of view, the amount of exchangeable ions from the considered zeolite well known as the cation exchange capacity (CEC – meq/g), represents one of the main properties and it depends both on zeolite chemical and structural features.

Starting from a Phillipsite enriched sample, whose origin was related to the alteration of volcanic glass coming from the Neapolitan Yellow Tuff eruption (c.a.14k years ago), the work has been focused on a technological characterization based on chemical, mineralogical and physical analysis. The main object was represented by the understanding of phillipsite selectivity toward zinc (Zn2+) as a potential pollutant through: 1) a comparison between theoretical vs measured CEC (the former through EMPA-EDS analysis); 2) the realization of Zn2+ isotherms considering the Ciavatta isotherm model (1980), as a result it was possible to understand the amount of zinc entered the zeolite after the ionic exchange reactions (using a Perkin Elmer Optima 2100DV ICP-OES).

The natural zeolite sample named P4ter, was collected from the north western part of the Campi Flegrei volcanic field (Licola, Pozzuoli – Italy).

P4ter sample was prepared and brought in Na+ form (P4ter Na+ ) in order to guarantee the best efficiency from the 2Na+ Zn2+ ion exchange reaction.

A first part of the technological characterization highlighted a high potassium content as extraframework cation for the P4ter standard sample, in accord with literature (de’Gennaro e Langella, 1996; Gatta, Cappelletti, Langella, 2010), while the mineralogical analysis, as well as showing, as expected, a high content in Phillipsite, shown a discrete content in Chabazite for the P4ter standard sample (not Na+ exchanged). Due to the SEM – EDS analysis, both visual informations about the unusual shape and morphology of the phillipsite crystals, maybe because of the enrichment processes, as well as specific chemical informations which confirmed the potassic character of the P4ter standard sample and the entering of Na into the zeolite for P4ter (Na) sample were obtained. Thermal analysis (TG, DTG, DSC) for the understanding of their physical properties in terms of loss in mass were also conducted, but they didn’t represent the main part of the job.

The comparison between theoretical vs measured CEC, showed that the cationic population present into the zeolite (represented by the theoretical CEC) didn’t match the amount of the actually exchanged cations (measured CEC). The thermodynamic parameters obtained, Ka (thermodynamic equilibrium constant) and ΔG 0 (standard free energy), would prove a non selective Zn2+ exchange: only a fraction of the cationic sites seemed to prefer Zn2+ with respect to Na+ . This condition perfectly reflects Eisenman & Sherry theory (1966 – 1967) : a wide number of zeolites appear to be non selective toward divalent cation with high standard free energy just like Zn2+ .

Moreover while the Zn2+ isotherm highlighted a medium – low selectivity toward the zinc as a potential pollutant, the Na+ isotherm showed how part of the cationic sites preferred the sodium with respect to the zinc. It is a noteworthy group of zinc selective cationic sites.

One of the more likely reasons for this scenario resides in the fact that, when hydrated, Zn2+ ionic radius grows about six times with respect to no hydration condition, differently from Na2+ which grow about two times (Railsback, 2006). Hence, hydrated Zn2+ dimension wouldn’t allow it to enter the zeolite.

Deeper Transmission Electron Microscopy (TEM) studies would be able to evaluate the effective entering of the zinc into the phillipsite framework.

Generally, once ion exchange processes are accomplished the Zn2+ exchanged zeolites could be treated with ceramization processes by which they would be inertizated.

Furthermore, since Zn2+ exchange reactions appear to be completely reversible, column type processes based on the recovery of zinc from the solutions, could be arranged. In these cases zinc, opportunely concentrated, could be reused several times and earmarked to a wide range of technological processes: among these, zinc coating could be noteworthy.

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*Giovanni Varriale, Born on 19/06/1989 in Naples, Italy

Education:

  • 2012 – Bachelor degree in Geologic Sciences, Full marks with honor; Polytechnic School of Basic Sciences, Department of Earth, Environment and Resources Sciences (DiSTAR) – Università Degli Studi di Napoli – Federico II.
  • 2015 – I Level Master Degree in Geology and Applied Geology – Georesources curriculum, Full marks with honor; Polytechnic School of Basic Sciences, Department of Earth, Environment and Resources Sciences (DiSTAR) – Università Degli Studi di Napoli – Federico II.

Professional experience: 

  • From 2017 – Marine Surveyor Geologist; INSTALL SRL. Agnano – Pozzuoli (Italy)
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