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Diffusion and advection with sorption of anions, cations and non-polar molecules in organo-clays at varying thermo-chemical conditions - validation by analytical methods and molecular simulation

Antragstellerin Dr. Birgit Schampera
Fachliche Zuordnung Bodenwissenschaften
Förderung Förderung von 2012 bis 2016
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 210539988
 
Erstellungsjahr 2016

Zusammenfassung der Projektergebnisse

The project focused on combination of experimental and theoretical investigations on the molecular scale structure of organo-clays, the related physicochemical surface properties and consequences for diffusion transport. Organo-montmorillonites were prepared from bentonite by systematic exchange of inorganic by the monovalent organic cations hexadecylpyridinium (HDPy) and hexadecyltrimethylammonium (HDTMA). X-ray photoelectron spectroscopy (XPS) and classical molecular dynamics (MD) simulations were combined to define the properties of the solid-liquid interface of organo-montmorillonites. XPS survey spectra verified the varying elemental composition of the external surface with increasing amount of adsorbed organic surfactant. For the hypothesis that interface properties of organo-clays are related to type of organic cation, and their density and arrangement at the clay surfaces, complementary transmission electron microscopy (TEM) provided detailed nanoscale insights on HDTMA/HDPy-montmorillonite (M) surface structure. TEM revealed a relation between the thickness of the organic coating and the amount of loading and gave insights in real structure phenomena: Organic coating thickness varied for clay platelets from the same sample indicating heterogeneity of surface composition. Changes in elemental composition determined by XPS correlated with the organic coating thickness identified by TEM and surface charge measurements. The high-resolution C 1s spectra depict sensitively the surface arrangement and showed changes in C-N binding energies, attributed to varying local environment of organic head groups. MD showed a successive transformation of the organic cation coating of the surface from thin disordered monolayers at low up to disordered bilayer or quasi paraffin-type arrangements at high surfactant coverages. For organo-clays with low organic cation loading no significant difference was observed but at high loading, surface packing density was higher for HDTMA than for HDPy. Experimental and MD study of the anion diffusion with nitrate revealed that the diffusivity in the interfacial area is strongly affected by the packing density of the organic coating. It verified that the retardation increases with increasing amount of organic cations adsorbed. NO3- diffusion velocity in HDTMA- and HDPy-M is in a similar range. MD studies showed comparable trends the diffusion coefficient on the content of organic cations on the surface. Calculations revealed that the positively charged head groups of the organic cations play a key role in decreasing diffusivity by electrostatic interactions. NO3-diffusion coefficient at different concentrations showed influences of the surface charge, due to anion exclusion at the surface, with an increase of diffusivity by ionic strength. In contrast, especially for the positive charged of HDPy- 180-M, the diffusion velocity decreased with ionic strength. Diffusion coefficients of the anions selenite and rhenium were in the same range as nitrate diffusion. In contrast, the diffusion velocity of cations and nonpolar compounds was higher in almost all laboratory experiments. At low applied amounts of organic cations diffusion of nonpolar compounds decreased reflecting the interactions with the hydrophobic C-chains. Diffusion experiments on preheated (up to 130°C and 298d) organo-clays showed an increase of velocity of one magnitude. Temperature controlled diffusion experiments performed up to 80°C in a new bigger sized diffusion cell showed that diffusivity increase depended mainly on temperature increase but on material properties, too. Temperature increase in a first step directly reduces diffusion velocity, and is subsequently followed by an increase of diffusion velocity. At approximate 50°C some samples show a release of H2O which remains over longer times (in the experiment case up to 24 h) reducing diffusion velocity. Diffusion and advection experiments with the new cell are promising especially regarding new detailed insides into temperature affected processes. The combination of experimental and modeling approach was shown as very effective tool for detailed molecular-scale structure and arrangement characterization of organic cations in the montmorillonite interlayer and on the external surface. Changes of surface properties were directly related to the varying amount of organic cations at the external surface. The diffusion experiments with selected ions and nonpolar molecules showed the variable retention capability of the organo-clays with respect to chemical species and applied amount and type of organic cations. MD provided detailed description of the interaction mechanism of the diffusing species at the organo-clay-water interface. The predicted diffusion coefficients from simulations correlated well with the experimental data. Organo-clays are very promising materials with tailor-made properties that can be used in various technological applications such as for contaminate protection of soils or in water purification systems.

Projektbezogene Publikationen (Auswahl)

 
 

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