Mineralization in water processing is not desirable as it leads to a significant decrease of membrane efficiency by fouling processes and variation in surface properties. An increase of our understanding of early surface mineralization may lead to a better water quality control and improved surface protection.

 Mineralization occurs under many conditions but of particular interest are condition in stages of the water treatment process where super-saturation occurs. Super-saturation may only arise locally at the surface of membrane systems. Therefore, a key question is whether nucleation and aggregation occurs in solution or at a surface. Recently, a novel theory (Gebauer,Volkel,Colfen) was proposed. In this theory nucleation followed the formation of nano-scale clusters stabe even in undersaturated solution. This opposes earlier notions that nucleation occurs only in supersaturated ions solution by stochastic solute clustering. One of the aims of this work is to investigate if prenucleation plays a role in mineralization at membrane surfaces and how early mineralization events may lead to scaling  over time. Double pulse (DP) experiments were performed, which enable to assess the  mineralization rate and seek evidence for the nature of prenuclation prior to a mineralization process. The DP experiment requires a model mineralization solution with a well-defined  metastable region which depends  on parameters, such as: super- saturation ratio, pH , ionic strength, temperature, etc. Moreover, an improved understanding of processes that occur in model mineralization solutions may lead to a better control of mineralization at surfaces.

 In order to control experimental conditions as well as to monitor time evolution of crystal formation we have set out to develop a micro-fluidic system. The monitoring of prenucleation and mineralization events will be done by micro-Raman spectroscopy and imaging. The micro-fluidic system will for this reason be optically coupled in an efficient way to the Raman system. The ultimate aim will be to detect nano-sized crystals, which will give rise to only feeble Raman spectra.  

 A pneumatic micro-fluidic system has been developed to connect to the Raman micro-spectrometer. The micro-fluidic system will be prepared using standard PDMS technology in combination with masks made by photo-lithography. In order to create small chambers on a chip where prenucleation and mineralization will take place the PDMS walls will be pneumatically closed. The advantage of such a pneumatic micro fluidic device is that it enables a control over mass transport such that experiments can be carried out in a confined volume and under well-defined conditions in solution.  Additionally,  Raman-based micro-spectroscopic methods like spontaneous Raman scattering and coherent Raman imaging can be applied on samples under ambient conditions in the micro-fluidic device in real time. Another aim of this work is to show that micro-fluidic devices are promising tools for studies concerning mineralization processes and will provide rich information on underlying principles. The Raman spectra of minerals are directly informing on the chemical composition and can be directly interpreted in terms of the detailed symmetry of the crystals in case of different polymorphs. Furthermore, a great advantage  is the sensitivity which is sufficient to detect nano-sized mineral particles.

 Mineralization has been assumed to be highly dependent on the nature of the surface. A number of polymer materials was selected for  mineralization experiments. The selected organic polymers are common in membrane technology and water filtration systems, namely  polysulfone and poly(p-phenylene-oxide). Polystyrene has been added as a common material in technology. In order to observe the smallest possible  nano-crystals at a polymer surface the Raman scattering of the crystals must be of the some order of magnitude as the Raman scattering from the organic layer. It was therefore decided to prepare very thin polymer films of the order of nanometers. These polymer films should preferably be homogeneous in thickness and contiguous. Quantitative Raman micro-spectroscopy and imaging was used to investigate thickness and contiguity in spin-coated systems. Extremely thin polymer films of less than 50 nm thickness can be easily observed with Raman micro-spectroscopy.

Results will be presented of hybrid micro-fluidic system-Raman micro-spectroscopy and imaging for the study of mineralization processes.

This research was supported by grant issued by FOM and Wetsus

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