Modification of pore surface physicochemistry in unsaturated media by formation of biogeochemical interfaces - Contact angle analysis and relevance for sorption and transport of solutes and colloids

Principle investigators: Prof. Dr. J. Bachmann, Dr. J. Rühlmann

Co-workers: Dr. M.-O. Göbel, G. Mühl, Dr. S.K. Woche

Objectives
The reactivity of soil solid surfaces depends on their surface chemistry and spatial accessibility. Beside geometrical factors such as surface roughness and pore topology, the accessibility of solid surfaces is determined by the interfacial (wetting) properties of the solid and liquid phase as they control the distribution of fluid phases (liquid/gas) in the matrix. The project intends to investigate the impact of solid interfacial properties (solid surface free energy, particle surface charge) on the formation of biogeochemical interfaces and their relevance for transport and sorption of organic chemicals and colloids. Particularly the comparison between saturated and unsaturated porous media will provide information about the importance of the gas-water interface for the transport and sorption behaviour of differently wettable and differently charged colloidal substances and organic chemicals.

Hypotheses
• The interfacial properties of soil particles control the formation of biogeochemical interfaces (conditioning films).
• The accessibility of surfaces and the distribution of the fluid phases (liquid/gas) in the soil matrix depends on the physicochemical structure and composition of conditioning films.
• The sorption and transport properties for colloids and organic chemicals of the soil matrix is affected by the distribution and continuity of the liquid/gas phases.

Scientific approach
The first experimental step is the creation of model soil systems with defined wetting properties (quartz sand, glass beads) and the formation of conditioning films by adsorption of dissolved organic matter (DOM) on the mineral surfaces. The specific properties of the adsorbed DOM components are controlled by the modification of the initial particle wetting properties. The experiments will be carried out in flow chambers which provide the control of moisture conditions in the matrix by the variation of matric potential. This will allow the adjustment of defined degrees of saturation as well as the application of wetting and drying cycles.
The physicochemical properties of mineral surfaces and conditioning films will be characterised by the determination of contact angles, solid surface free energy and particle surface charge. The wetting behaviour on the microscale will be analysed by dynamic wetting experiments carried out with an environmental scanning electron microscope (ESEM). To evaluate the significance of solid surface wetting properties for film formation processes, it is planned to investigate the sorption behaviour of microspheres on the solid surfaces by confocal laser scanning microscopy (LSM) and comparing specific locations for sorption to derive general conclusions for the colloidal sorption processes. To identify specific functional groups of the adsorbed organic matter and to analyse the homogeneity of the conditioning films , Fourier-transform infrared spectroscopy (FT-IR) and LSM will be applied.
The sorption and transport behaviour of the model systems will be analysed by breakthrough curves obtained from flow chamber and column experiments with bromide (as a conservative tracer), colloids (microspheres), and organic chemicals different in wetting properties and water solubility (in accordance with other research groups). Finally, the results obtained from the model soil systems will be verified by experiments using natural soil material of different wettability with comparable particle size distribution.

Cooperation within the priority programme:
PD Dr. T. Baumann, Prof. Dr. M. Kaupenjohann, Prof. Dr. I. Kögel-Knabner, Dr. A. Miltner, Prof. Dr. G. Schaumann, Prof. Dr. T. Streck, Prof. Dr. K.U. Totsche, PD Dr. H.-J. Vogel

Publications:
Mühl, G.J.H., Rühlmann, J., Göbel, M.-O., Bachmann. J. 2012. Application of confocal laser scanning microscopy (CLSM) to visualize the effect of porous media wettability on unsaturated pore water configuration. J. Soil. Sediment. 12, 75-85.

Bachm1

ESEM images showing water condensed on hydrophobic glass beads (approx. 105 µm diameter, contact angle = 158°) (left), and wettable glass beads (contact angle = 48°) (right). On the hydrophobic glass beads the water condensed as small drops. The discontinuous distribution of water on the surfaces causes a reduced water connectivity between individual glass beads. Contrastingly, the wettable glass beads are wetted uniformly and thin water films are formed which resulted in enhanced water connectivity. The images illustrate the importance of solid surface wetting properties for the distribution and arrangement of the fluid phases (liquid/gas) in the soil matrix.

Bachm2

Position of water films in partly saturated glass beads; Images taken with confocal laser scanning microscope (LSM 510 META) with x×y×z = 460×460×179 µm; middle: plan view, top and right: cross section. This could be used to investigate water film thickness or contact angle.



Poster Bachmann