Modified (Bio)polyelectrolytes for Removing Natural Organic Matter (NOM) From Water – A Fundamental Investigation for the Case of Humic Acid as Model NOM

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In this project we want to investigate from a fundamental colloid science perspective the important topic of NOM (Natural Organic Matter) removal from drinking water during the water treatment process, which is essential for human health. For surface water typically cationic polyelectrolytes (cPEs) are employed to bind and precipitate the negatively charged NOM. Despite the importance of this topic only few studies have addressed it from a fundamental point of view. This is the approach taken here, where we will employ purified humic acid (HA; main component of NOM) as model system to study its complexation with differently modified cationic (quaternised) chitosan (q-Chit). Proof-of-concept tests carried out with Australian collaborators already showed very promising results of simple q-Chit in NOM removal. Its main advantage are biocompatibility and the variability of the molecular build-up due to the ease of chemical modification. q-Chit will be tailor-made by appropriate synthesis within this project, systematically varying parameters like charge density, Mw, and hydrophobicity. In our study we want to determine the phase behaviour as a function of mixing ratio, with an emphasis on quantifying the remaining amounts of HA in the two-phase equilibrium. This will be complemented by comprehensive studies of the thermodynamics of interaction (ITC) and of the mesoscopic structure of the formed complexes by means of light, x-ray and neutron scattering. Important is also the temporal evolution of the systems, to be studied by kinetic investigations of the structural changes taking place during complexation. This comprehensive thermodynamic, structural and kinetic characterisation shall allow to deduce systematic correlations between the molecular architecture of the cPEs and the strength of their binding to HA. Based on this we will identify which molecular motifs are essential for optimising this interaction, presumably a proper balance between cationicity and hydrophobicity, and thereby control HA removal from water. These motifs will be fed back to the synthesis in order to optimise the cPEs further. While the focus is on q-Chit, we will later in the project also address the effect of quaternised hyperbranched polyethylene imine (PEI), a rather compact, globular cPE with high charge density. Its effect on phase behaviour and structures in mixtures will be studied but with an emphasis on mixtures with (linear) q-Chit as we expect pronounced synergism from having these very different cPEs interact with the diverse anionic HA molecules. In summary, we expect to gain a thorough fundamental understanding of the conditions prevailing in mixtures of cPEe and HA, based on their comprehensive physico-chemical characterisation. This shall deliver a sound scientific basis for future developments in removing NOM from drinking water, one of the big current technological challenges of mankind.