Interactions between microplastics and the liquid/gaseous interface for environmental remediation

Abstract

In 2021, 2.5 million metric tons of plastic packaging waste was generated in the UK. The plasticene age has allowed for the development of numerous highly versatile materials useful throughout all aspects of society, but the consequence of our success has resulted in extensive plastic pollution. One end life cycle-point of plastic waste is into water, with almost 10 million tons entering oceans every year. These, once durable materials can now break down into smaller fragments, eventually resulting in an abundance of microplastics and even much smaller nanoplastics, with an estimated 5 trillion plastic particles within the world’s surface waters. Due to their prevalence in all aspects of our environment and recently found presence within human circulatory systems, these plastic pieces have become a focus of worldwide attention that require urgent remediation. As such their interactions with liquids, both bulk and droplets, and subsequent modes of removal are paramount to understanding how to bring this problem under control. The work within this thesis explored how microplastics interacted with water in two distinct forms, droplets and in bulk, and assessed how particle properties relate to various parameters. This work also considered how both scenarios could be effectively used to remove microplastics from water, be it in the form of liquid marbles for droplets or a bubble and oil system for removal from bulk water. Liquid marbles allow for various liquids to be encapsulated by hydrophobic particles, thus ensuring isolation from the external environment. They occur when a droplet, often water, is rolled through a powder bed causing the particles to decorate the exterior of the droplet. This unique structure allows for combinations of solid-liquid-gas reactions to occur, providing that stability is maintained throughout. This stability is regularly described in terms of lifetime and is one of their main drawbacks, often impacted by particle characteristics. The impact the coating material has on lifetime is mainly linked to thickness of coating layers and contact angle. Additionally other factors, such as internal and external environments play a major role in overall lifetime, making it possible to engineer a robust long-lived liquid marble. It is therefore useful to know factors which have a positive impact on lifetime and by using a reactor engineering approach it is possible to model and predict the overall lifetime of various coatings on liquid droplets. As such, this phenomenon has been applied to emerging applications, such as unconventional computing, cell mimicry and soft lithography, as well as within green and environmental applications, such as energy conversion, heavy metal recovery, CO2 sequestering, oil removal and microplastic capture. II Microplastics in the environment are more commonly found dispersed within bulk liquids rather than decorating a droplet, with various concentrations found within salt, fresh and potable water. Methods commonly employed to remove microplastics from water, such as membrane filtration, can achieve high rates of removal, however, polymer membranes are susceptible to mechanical and chemical failure due to backwashing and ageing. Another simple yet effective removal method of microplastics from water using oil was investigated and showed early promise, however, this technique is currently incorporated into systems that are unsuitable for scale up into a continuous removal method. Bubbles travel up through the water column attached to polymer particles, transporting them to the surface where they migrated into and subsequently trapped within an ‘environmentally friendly’ oil layer due to their hydrophobic and oleophilic nature, without a concern of secondary pollution. This waste to capture waste method was demonstrated to achieve high removal efficiencies of > 99.4 % of 6 of the most prevalent microplastic types (low-density polyethylene, polypropylene, polystyrene, nylon, poly(ethylene terephthalate) and polyvinyl chloride) and sizes. This bubble-oil method was adapted into a continuous system, where a reactor prototype was built and flow rates, liquid volume, liquid type, particle type and concentration variables were examined to optimise the removal process for vertical and horizontal systems.