Author ORCID Identifier

https://orcid.org/0000-0003-0716-5070

Year of Publication

2020

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department/School/Program

Chemical and Materials Engineering

First Advisor

Dr. Isabel Escobar

Abstract

Phosphorene is a two-dimensional material exfoliated from bulk phosphorus. Specifically, relevant to the field of membrane science, the band gap of phosphorene provides it with potential photocatalytic properties, which could be explored in making reactive membranes able to control the accumulation of compounds on the surface during filtration, or fouling. Another reason phosphorene is a promising candidate as a membrane material additive is due to its catalytic properties which can potentially destroy foulants on the membrane surface.

The first goal of this study was to develop an innovative and robust membrane able to control and reverse fouling with minimal changes in membrane performance. To this end, for proof of concept, membranes were embedded with phosphorene. Membrane modification was verified by the presence of phosphorus on membranes, along with changes in surface charge, average pore size, and hydrophobicity. After modification, phosphorene-modified membranes were used to filter methylene blue (MB) under intermittent ultraviolet light irradiation. Phosphorene-modified and unmodified membranes displayed similar rejection of MB; however, after reverse-flow filtration was performed to mimic pure water cleaning, the average recovered flux of phosphorene-modified membranes was four times higher than that of unmodified membranes. Furthermore, coverage of MB on phosphorene membranes after reverse-flow filtration was four times lower than that of unmodified membranes, which supported the hypothesis that phosphorene membranes operated under intermittent ultraviolet irradiation became self-cleaning.

Once it was determined that a successful synthesis of a phosphorene-modified membrane was possible, the next goal was to characterize structural and morphological changes arising from the addition of phosphorene to polymeric membranes. Here, phosphorene was physically incorporated into a blend of polysulfone (PSf) and sulfonated poly ether ether ketone (SPEEK) dope solution. Protein and dye rejection studies were carried out to determine the permeability and selectivity of the membranes. Since the loss of material additive during filtration processes is a challenge, the stability of phosphorene nanoparticles in different environments was also examined. Furthermore, given that phosphorene is a new material, toxicity studies with a model nematode, Caenorhabditis elegans, were carried out to provide insight into the biocompatibility and safety of phosphorene. Results showed that membranes modified with phosphorene displayed a higher protein rejection but lower flux values. Phosphorene also led to a 70% reduction in dye fouling after filtration. Additionally, data showed that phosphorene loss was negligible within the membrane matrix irrespective of the pH environment. Phosphorene caused toxicity to nematodes in a free form, while no toxicity was observed for membrane permeates.

After gaining an understanding of the membrane characteristics, phosphorene’s ability to degrade contaminants was investigated. Nanomaterials with tunable properties show promise because of their size-dependent electronic structure and controllable physical properties. The purpose of this portion of the research was to develop and validate environmentally safe nanomaterial-based approaches for the treatment of drinking water including degradation of per- and polyfluorinated chemicals (PFAS). PFAS are surfactant chemicals with broad uses that are now recognized as being a significant risk to human health. They are commonly used in household and industrial products. They are extremely persistent in the environment because they possess both hydrophobic fluorine-saturated carbon chains and hydrophilic functional groups, along with being oleophobic. Traditional drinking water treatment technologies are usually ineffective for the removal of PFAS from contaminated waters because they are normally present in exiguous concentrations and have unique properties that make them persistent. Therefore, there is a critical need for safe and efficient remediation methods for PFAS, particularly in drinking water. The proposed novel approach has also a potential application for decreasing PFAS background levels in analytical systems. In this study, a 99% rejection of perfluorooctanoic acid (PFOA) was attained alongside a 99% removal from the PFOA that accumulated on the surface of the membrane. This was achieved using nanocomposite membranes made of sulfonated poly ether ether ketone (SPEEK) with two-dimensional phosphorene with pore sizes smaller than the size of PFOA. To then remove the PFOA that accumulated on the surface to foul the membranes, these were exposed to ultraviolet (UV) photolysis and liquid aerobic oxidation.

The last portion of this study investigated the biocidal properties of SPEEK and phosphorene membranes under an alternating electrical potential. SPEEK and phosphorene-based membranes were synthesized and analyzed using cross-flow filtration to determine their biocidal properties. Serratia marcescens was the model bacteria and filtration was performed under alternating positive and negative voltage bias conditions. The biofouled membranes were examined for bacteria growth after three days. In the case of the SPEEK membrane, without voltage, the biofilm covered approximately 60% of the membrane surface, and under voltage, that decreased to 44%. On the other hand, the presence of an alternating voltage did not impact the microbial surface coverage on the phosphorene membranes. It is proposed that because phosphorene membranes were more hydrophobic and less charged as compared to SPEEK membranes, microbial growth adhered more strongly to the phosphorene membranes. Therefore, the alternating voltage was not effective in desorbing the strongly adsorbed biofilm layer from the phosphorene membranes. On the other hand, the employment of an alternating current on the more hydrophilic and more negatively charged SPEEK membranes was more effective at desorbing some of the attached biofilms from the membrane surface.

For the first time, nanocomposite membranes were fabricated using phosphorene. This opens the field to a new class of potentially reactive membranes, or at the least, easier to clean membranes. Due to phosphorene’s properties, these membranes have the potential to be used for multiple purposes, such as compound destruction and self-cleaning membranes, etc. Membrane separations of the future will not favor static membranes, i.e., membranes that only serve the function of rejecting compounds, since accumulated and potentially hazardous compounds on the surface will be released on backwash/cleaning water to make that hazardous and make the membranes hazardous at the time of disposal. Hence, dynamic self-cleaning membranes that can simultaneously remove compounds and destroy them provide the field with an alternative.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2020.471

Funding Information

1)Harmful Algal Bloom Research Initiative grant from the Ohio Department of Higher Education and partially supported by the National Science Foundation under Cooperative Agreement No.1355438 in 2015

2)This research was funded by the National Science Foundation (NSF) under Cooperative Agreement No.1355438 and by the NSF Kentucky EPSCoR Program in 2015

3)The National Institute of Food and Agriculture, U.S Department of Agriculture, under NC-1194 in 2018

4)Summer Undergraduate Research in Environmental Sciences (SURES) funded by the National Institute of Environmental Health Sciences (NIEHS) R25ES027684 in 2018

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