Lung Toxicity Analysis of Nano-Sized Kaolin and Bentonite: Missing Indications for a Common Grouping.

Martin Wiemann,Antje Vennemann,Wendel Wohlleben

doi:10.3390/nano10020204

Abstract

Kaolin and bentonite (nanoclay NM-600) are nanostructured aluminosilicates that share a similar chemical composition, platelet-like morphology, and high binding capacity for biomolecules. To investigate if these material-based criteria allow for a common grouping, we prepared particle suspensions of kaolin and bentonite with a similar hydrodynamic diameter and administered them to NR8383 alveolar macrophages in vitro and also to a rat lung using quartz DQ12 as a reference material. Bentonite was far more bioactive in vitro, indicated by a lower threshold for the release of enzymes, tumor necrosis factor α, and H2O2. In addition, in the lung, the early effects of bentonite exceeded those of kaolin and even those of quartz, due to strongly increased numbers of inflammatory cells, and elevated concentrations of total protein and fibronectin within the bronchoalveolar lavage fluid. The pro-inflammatory effects of bentonite decreased over time, although assemblies of particle-laden alveolar macrophages (CD68 positive), numerous type-2 epithelial cells (immunopositive for pro-surfactant protein C), and hypertrophic lung epithelia persisted until day 21. At this point in time, kaolin-treated lungs were completely recovered, whereas quartz DQ12 had induced a progressive inflammation. We conclude that bentonite is far more bioactive than equally sized kaolin. This argues against a common grouping of aluminosilicates, previously suggested for different kaolin qualities.

Highlights

IntroductionKaolin (Al2Si2O5(OH)4) and bentonite (Al2H2Na2O13Si4) are platelet-like aluminosilicates (or mixed Si-Al-oxides), which are a challenging group of particles for risk assessment
Kaolin (Al2Si2O5(OH)4) and bentonite (Al2H2Na2O13Si4) are platelet-like aluminosilicates, which are a challenging group of particles for risk assessment
In time kaolin was localized in alveolar macrophages (AM) (Figure 5), and no structural changes in the lung parenchyma (Figure S1) and in the numbers of CD68-positive AM or pro-surfactant protein C (pSP-C) positive cells were noted (Figure 6)

Summary

Introduction

Kaolin (Al2Si2O5(OH)4) and bentonite (Al2H2Na2O13Si4) are platelet-like aluminosilicates (or mixed Si-Al-oxides), which are a challenging group of particles for risk assessment. Their chemical sum formula places them in the substance class of silicates, but their physical structure and biopersistence differ from pure silicates. Solubility studies on mineral fibers have shown that at acidic pH, mixed Si-Al-oxides have a significantly higher biodissolution rate and reduced biopersistence rate than pure Si-oxides [1]. The toxicity assessment of aluminosilicates should, be based on a common evaluation of material properties, solubility data, and biological effects. We investigate whether or not the high degree of structural and chemical similarity justifies an allocation of kaolin and bentonite to the same group of nanomaterials (NMs) according to current concepts comprising in vitro and in vivo testing [4]

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Conclusion