Author ORCID Identifier

https://orcid.org/0000-0003-4635-7139

Date Available

7-12-2024

Year of Publication

2023

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Engineering

Department/School/Program

Chemical and Materials Engineering

Advisor

Dr. Dibakar Bhattacharyya

Abstract

In a world where clean water predictability is challenged by both global warming and the contamination of our natural resources, it is our responsibility to advance water separation technologies into more efficient processes and to lessen their environmental footprint. Pore and surface functionalization of membranes can enhance the separation of ions from water and even help overcome intrinsic challenges that some current separation technologies possess. If we begin with a complete fundamental understanding of the nanoscale interactions that occur in the process of separating ions from water, then we can engineer new functionalized composite membranes to provide alternative processes keeping life cycle aspects, lowering energy consumption, and keeping minimum supply needs in consideration.

This research evaluated the synthesis and performance of three new membrane platforms: a positively charged nanofiltration (NF) membrane, a dual-functional NF and adsorptive membrane, and a set of adsorptive/ion exchange membranes. The application of these technologies include: the recovery of valuable lanthanide ions (rare earth elements) from water, and the removal of organic and inorganic ions and pollutants, such as per- and polyfluoroalkyl substances (PFAS) and arsenic, among other ions being studied. A comprehensive study that involved the synthesis of the membrane technologies, thorough materials characterization, analysis of the separation performance, and modeling of the solute transport through these membranes, was performed for each one of these three materials/applications.

First, positively charged nanofiltration membranes were created by adding commercially available polyallylamine hydrochloride (PAH, primary amine) to the conventional interfacial polymerization technique. The NF membranes with added PAH had high and stable lanthanides retention rates, with values around 99.3% in mixtures with high ionic strength (100 mM, equivalent to ~6000 ppm), 99.3% rejection at 85% water recovery (and high Na+/La3+ selectivity, with 0% Na+ rejection starting at 65% recovery), and both constant lanthanum rejection and permeate flux even at pH 2.7. The Donnan steric pore model with dielectric exclusion elucidated the transport mechanism of lanthanides and sodium, proving the potential of highly selective separation with acceptable water permeability using positively charged NF membranes.

Second, the rejections of ionogenic compounds by nanofiltration (NF) membranes at pH values near the membrane isoelectric point were compared to the size-transfer-dependent and mass-transfer-dependent modeled rejection rates of these compounds in an ionized state. This provided new insight on the transport mechanism of PFAS through the NF layer. The synthesis of an additional support barrier with thermo-responsive adsorption/desorption properties (quantified by water permeance variation) was proposed. This allowed for enhanced separations of PFAS and preventing other compounds, such as NOM, to enter the backing adsorption domain. The synthesis was made possible by successfully adding an NF layer on top of a poly-N-isopropylacrylamide (PNIPAm) pore-functionalized microfiltration support structure. The support layer adsorbed organics (178 mg perfluorooctanoic acid (PFOA) adsorbed/m2), and the simultaneous exclusion (50-70%) from the NF layer allows separations at a high water flux of 100 L/m2-h at 7 bar.

Finally, three types of adsorptive polymers comprised of strong/weak acid, strong base, and iron-chitosan complex groups were synthesized in the pores and on the surface of microfiltration (MF) membranes. These were tested for removal of organic and inorganic cations and anions from water, including arsenic, per- and polyfluoroalkyl substances (PFAS), and calcium (hardness). An increase in the relative adsorption capacity by up to eight times when compared to beads and crushed-down resins, as well as up to two orders of magnitude faster adsorption kinetics were found. This research includes an analysis of the simple synthesis approach, consideration for how to apply the results of this research on a larger scale, and comparison against commercial adsorptive/ion exchange technologies, ultimately quantifying the gap in adsorption capacities between adsorptive membranes and adsorptive resins.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2023.340

Funding Information

The National Institute of Environmental Health Sciences of the National Institutes of Health under Award Number P42ES007380 (2020-2023)

Honeywell UOP (2018-2021)

NSF KY EPSCoR grant (Grant no: 1355438) (2018-2019)

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