Ionic exchange mechanisms between erionite-Na and simulated lung fluids

An investigation of the ionic exchange mechanisms occurring between a sample of fibrous zeolite, erionite, and simulated lung fluids has been carried out the aim being to provide a sound background for establishing the mechanism/s involved in the development of toxic reactions into the human body. In fact, erionite samples from different localities have been recently thoroughly investigated because of their linking with malignant mesothelioma (MM) [1]. The sample selected for this investigation is an erionite-Na from Rome, Oregon, USA. To simulate different interstitial conditions occurring into the lung, Artificial Lysosomal Fluid (ALF) and Gamble's solution were used. ALF is analogous to the fluid with which inhaled particles would come into contact after phagocytosis by alveolar and interstitial macrophages. Gamble's solution simulates the interstitial fluid deep within the lung. ALF solution has an acidic pH of 4.5, lower than Gamble's solution (7.4), and a much higher organic content [2]. The sample, having a chemical formula of (Na4.19K1.91Mg0.63)[Al7.36Si28.64O72.00] • 29.49H2O, has been characterized from both a structural and chemical point of view by using X-ray powder diffraction (XRPD) and Scanning Electron Microscopy (SEM-EDS). Subsequently, part of the sample (ca. 10 mg) was maintained in contact, for 48 hours at room temperature, with the lung solutions (ca. 2ml). Moreover, an additional experiment was performed, keeping the erionite fiber in contact with ALF solution for 4 months. The sample were recovered by filtering and subsequently dried at room temperature. The structural and chemical modifications of the samples were evaluated by XRPD data than for SEM microanalysis adopting the same operating conditions used for characterizing the starting material. From a chemical point of view, the Mg content was found to marginally decrease after being in contact with ALF. Besides, after the reaction with the Gamble’s solution, Ca was detected in the corresponding EDS spectra, whose occurrence was counterbalanced by a proportional reduction of the Na content. Moreover, a further effect of the sample reaction with ALF was a partial dissolution of erionite, confirmed by ICP, certainly due to the acidic pH of the solution. A series of structural refinements by Rietveld method, indicated modifications of both the a and c cell parameters of erionite after reaction, consistently leading to a contraction of the cell volume. Calculated site scattering (s.s) of cation sites were found in good agreement with that obtained by chemical analyses. In general, the reaction with lung fluids causes a depletion of the Ca1 site (preferentially occupied by Mg), and an increase of the s.s at the Ca2 site. This behavior was attributed to the partial lost of Mg, confirmed by chemical analyses, and a simultaneous migration of Na and Ca ions towards Ca2 site. The relevant variations of the individual water molecules sites occupancy did not cause remarkable modification on the overall water content, as indicated by the final total water s.s. ranging from ca. 265 e- to ca. 285 e-.