Characterization and Tuning DOLLOPs in Potable Water
Talie Zarei is a PhD student in the Department of Optical Sciences. (Co)Promotors are prof.dr.ir. H.L. Offerhaus, dr.Dipl.-Ing. E.C. Fuchs from the Faculty of Science & Technology, prof. J. Woisetschlager, TU Kratz and prof. D. Gebauer University of Hannover.
Calcium carbonate (CaCO₃) nucleation and scaling pose significant challenges in various industrial and environmental contexts, particularly in potable water systems where mineral scaling can impair infrastructure and water quality. The traditional understanding of CaCO₃ nucleation has evolved from classical theories to encompass non-classical pathways involving prenucleation clusters and dynamically ordered liquid-like oxyanion polymers (DOLLOPs). This research aimed to characterize and tune DOLLOPs in potable water to mitigate scaling, but an unexpected discovery redirected the research focus: the presence of intrinsic nanobubbles in alkaline solutions.
The research unveiled that intrinsic CO₂ nanobubbles naturally form in alkaline aqueous solutions without external generation methods. Using advanced analytical techniques such as Field-Flow Fractionation coupled with Multi-Angle Light Scattering (FFF-MALS) and Zeta Nanoparticle Tracking Analysis (Z-NTA), these nanobubbles were characterized in terms of size (approximately 100 nm in diameter), number density, and zeta potential (negative surface charge). The intrinsic nanobubbles were found to be stable entities inherent to alkaline environments.
Subsequent investigations explored the influence of magnetic fields on nanobubble formation. Rotating magnetic fields significantly enhanced the formation of charged nanobubbles, increasing their number density and negative surface charge while affecting their size distribution. This magnetic enhancement indicates that magnetic fields can be utilized to control nanobubble populations in aqueous systems.
The role of intrinsic nanobubbles in the early stages of CaCO₃ formation was further examined. It was discovered that charged nanobubbles inhibit CaCO₃ nucleation by stabilizing the dense liquid calcium carbonate phase, preventing its aggregation and coalescence into solid amorphous calcium carbonate (ACC), and thus delaying the formation of solid ACC. Magnetically generated nanobubbles, with higher negative surface charge and number density, had an amplified effect on delaying nucleation. This suggests that nanobubbles interact with the dense liquid calcium carbonate phase, influencing the non-classical nucleation pathway of CaCO₃ by stabilizing this intermediate state, delaying the formation of solid ACC.
Additionally, the use of Asymmetric Flow Field-Flow Fractionation (AF4) coupled with Multi-Angle Light Scattering and Inductively Coupled Plasma Mass Spectrometry (AF4-MALS-ICP-MS) enabled the detection and characterization of nanoparticles, including potential (calcium magnesium carbonate-) DOLLOPs, in drinking water samples. Metals such as magnesium were found to be associated with very small particles (1.5 to 10 nm), indicative of prenucleation clusters or DOLLOPs.
The findings of this project highlight the pivotal role of intrinsic nanobubbles in the nucleation and scaling processes of calcium carbonate in potable water systems. By recognizing nanobubbles as inherent components rather than external additives, the research provides a new perspective on mineral scaling phenomena. The ability to manipulate nanobubble populations and properties through magnetic fields offers a novel approach to controlling CaCO₃ nucleation and scaling. This work contributes to a broader understanding of non-classical nucleation pathways and presents potential applications in water treatment technologies aimed at mitigating mineral scaling.
