We are facing the world’s biggest unprecedented environmental problem - contamination with plastic debris. Since first plastic products were made in 1950’s, the world’s production of various types of plastic has been constantly increasing and in 2015 it reached 320 million tons (Cai et al. 2018). Together with mass production and usage, also the burden on the environment is increasing, where plastic debris accumulates in terrestrial and aquatic environmental compartments. Once discarded into the environment, plastic pieces are subjected to different mechanical, chemical and biological processes (Andrady 2011), which lead to their break down and fragmentation into smaller pieces (Cai et al. 2018).

The term “microplastics” first appeared in the literature in 2004 (Thompson et al. 2004). In 2008 the size of the particles was defined and it was proposed that the term “microplastics” encompasses small plastic pieces in the size range below 5 mm in diameter (Fendall and Sewell 2009; Betts 2008; Moore 2008). Nowadays, the term microplastics includes broken-down plastic waste, synthetic fibres, tire particles, beads used in personal hygiene products and many other forms. Two types of microplastics are generally distinguished (Sundt et al. 2014), primary and secondary microplastics. Primary microplastics are plastic particles (fibres, microbeads, fragments), which initially enter the environment, while secondary microplastics are derived from these larger plastic debris due to mechanical, chemical or biological degradation. Although the first reports of plastic litter were published in the 1970’s (Carpenter and Smith 1972; Colton and Knapp, 1974), no real concern was expressed by the scientific community at that time (Andrady 2011). In the following decades, the scientific reports accumulated and showed, that the presence of microplastics in fresh and sea water presents a great risk for aquatic life and potentially humans. Wastewater treatment plants (WWTPs) are believed to be a significant point sources in the case of surface water’s pollution with microplastics (Murphy et al. 2016). On the other hand, best global estimates suggest that 80 percent of plastic litter in oceans come from land-based sources and the rest from marine sources (W. C. Li, Tse, and Fok 2016).

Cavitation, an advanced oxidation process, is one of the promising treatment techniques that could be used for this purpose. It is a physical phenomenon, which describes a phase change from liquid to gas and back to homogeneous liquid at approximately constant temperature. It is generated when small vapor bubbles form within the liquid, due to local pressure drop. When the formed bubbles collapse, extreme energies can be released which lead to formation of predominately OH radicals. Cavitation most often occurs on turbine machines, valves and some parts of fuel injection system by internal combustion engines and is in most cases an undesirable phenomenon. Despite this, cavitation is nowadays successfully used for surface cleaning, in pharmaceutical industry, in food industry, for medical purposes and nanoparticles preparation. The extreme conditions generated by cavitation bubble collapses provide unique chemical environment that can be exploited in numerous different ways from guiding chemical reactions (K. S. Suslick 1989), to organic micropollutants removal in WWTPs (M. Zupanc et al. 2014) and microorganism’s (Šarc et al. 2018) and virus destruction (Kosel et al. 2017).

One of the more important sources of aquatic contamination with microplastic debris are WWTP effluents. A critical subgroup of primary microplastics are fibres from textile materials, which originate from domestic washing and are flushed through the WWTPs into the rivers and oceans (Napper and Thompson 2016; Cesa et al. 2017; De Falco et al. 2018). It is reported, that a single washing load of 5 kg of polyester fabrics, can release up to several million microfibers, depending on the washing conditions and type of detergent used (De Falco et al. 2018).  Domestic washing machines do not only contribute to pollution with microfibres from synthetic textiles (De Falco et al. 2018), but also from the usage of softeners. These most commonly contain micro spherical capsules that hold aromatic oils for the long-term fragrances of the clothes. Since the use of domestic washing machines and synthetic textiles is globally increasing, the magnification of the pollution effect from the clothes’ washing processes is a very likely scenario. Most of these polluted waters are released into the sewage channels and through the WWTP effluents into the aquatic environment, mostly rivers and the oceans. Once in the environment, UV light from the sun can initiate the photo-oxidation process, but the degradation of microplastics gets severely retarded when it is not directly exposed to UV (i.e. under water or when adsorbed onto particles).

In the scope of the proposed interdisciplinary project the team of mechanical and chemical engineers will use an advanced technique – cavitation, to perform (one of) the first experiments with microplastics. Nowadays, state of the art (SOTA) is detection of microplastics in the environment but with the proposed interdisciplinary project we will move SOTA to the level, where the potential mechanisms for microplastics degradation will be investigated. Since cavitation can only appear within the liquid, it seems an ideal technology to “attack” the microplastics litter before it enters the aquatic environment (i.e. present in WWTP influents). In this way, by achieving degradation of microplastics at least to a certain degree, its complete degradation in the natural environment would be facilitated and could happen in a shorter time.

The proposed project aims to address the break-up, chemical and mechanical, of one of the world’s currently most critical pollutant for the first time ever on the laboratory scale. We will adapt an existing advanced oxidation process - cavitation in a way that will degrade and oxidise the microplastic litter to a degree that could be more easily “devoured” in WWTP and/or nature. The main objective of the proposed project is the investigation of the effects, cavitation will have on microfibres released from synthetic textiles and micro spherical capsules from textile softeners, which both are critical pollutants that come from the domestic washing machines.

The proposed interdisciplinary project will consist of different work packages (WPs), each of them including several tasks. The WPs will involve participation of both project partners, Faculty of Mechanical Engineering (FME) and National Institute of Chemistry (NIC), where tight cooperation will result in high success.  

WP1: Preliminary tests of microplastics cavitation treatment with currently developed laboratory test rigs (FME, NIC; Months 1-6)

WP2: Design and development of a novel hydrodynamic cavitation set-up for microplastic treatment (FME; Months 6-24)

WP3: Preparation and characterization of microplastics (NIC, Months 1-34 )

WP4: Testing the efficiency of a novel hydrodynamic cavitation device alone for microplastics degradation (FME, NIC; Months 18-24)

WP5: Coupling acoustic/hydrodynamic cavitation with other oxidation processes (UV, addition of H₂O₂ and TiO₂ film deposited on the stator) (FME, NIC; Months 24-34)

WP6: Evaluation of the optimized treatment coupling for scale up (FME, NIC; Months 30-36)

Reported scientific contributions in the scope of J7-1814 project:

• PETKOVŠEK, Martin, ZUPANC, Mojca, ŽAGAR, Ema, KRŽAN, Andrej. Degradation of water soluble PVOH with acoustic cavitation : pointers for microplastics. V: ESS-JSS-AOSS 1st Joint Sonochemistry Conference (online), November 8-10, 2021 : abstract book. [S. l.]: Japan Society of Sonochemistry, 2021. Datoteka posters (f. 26), ilustr. [COBISS.SI-ID 84911875]
• KOLBL REPINC, Sabina, BIZJAN, Benjamin, BUDHIRAJA, Vaibhav, DULAR, Matevž, GOSTIŠA, Jurij, BRAJER HUMAR, Barbara, KAURIN, Anela, KRŽAN, Andrej, LEVSTEK, Meta, MORALES ARTEAGA, Juan Francisco, PETKOVŠEK, Martin, RAK, Gašper, STRES, Blaž, ŠIROK, Brane, ŽAGAR, Ema, ZUPANC, Mojca. Integral analysis of hydrodynamic cavitation effects on waste activated sludge characteristics, potentially toxic metals, microorganisms and identification of microplastics. Science of the total environment. Feb. 2022, vol. 806, pt. 4, str. 1-14, ilustr. ISSN 0048-9697. https://www.sciencedirect.com/science/article/pii/S0048969721064925, DOI: 10.1016/j.scitotenv.2021.151414. [COBISS.SI-ID 83741955]
• PODBEVŠEK, Darjan, PETKOVŠEK, Martin, OHL, Claus-Dieter, DULAR, Matevž. Kelvin-Helmholtz instability governs the cavitation cloud shedding in Venturi microchannel. International journal of multiphase flow. Sep. 2021, vol. 142, str. 1-7, ilustr. ISSN 0301-9322. https://www.sciencedirect.com/science/article/pii/S0301932221001488?via%3Dihub, DOI: 10.1016/j.ijmultiphaseflow.2021.103700. [COBISS.SI-ID 66420227]
• ZUPANC, Mojca, PETKOVŠEK, Martin, ZEVNIK, Jure, KOZMUS, Gregor, ŠMID, Alenka, DULAR, Matevž. Anomalies detected during hydrodynamic cavitation when using salicylic acid dosimetry to measure radical production. Chemical engineering journal. 2020, vol. 396, str. 1-11, ilustr. ISSN 1385-8947. https://www.sciencedirect.com/science/article/pii/S1385894720313814#!, DOI: 10.1016/j.cej.2020.125389. [COBISS.SI-ID 15039491]
• PETKOVŠEK, Martin, HOČEVAR, Marko, DULAR, Matevž. Visualization and measurements of shock waves in cavitating flow. Experimental thermal and fluid science. [Print ed.]. Nov. 2020, vol. 119, str. 1-10, ilustr. ISSN 0894-1777. https://www.sciencedirect.com/science/article/pii/S0894177720307196?via%3Dihub, DOI: 10.1016/j.expthermflusci.2020.110215. [COBISS.SI-ID 23172099]
• PETKOVŠEK, Martin, HOČEVAR, Matej, GREGORČIČ, Peter. Surface functionalization by nanosecond-laser texturing for controlling hydrodynamic cavitation dynamics. Ultrasonics Sonochemistry. 2020, vol. 67, str. 1-10, ilustr. ISSN 1350-4177. https://www.sciencedirect.com/science/article/pii/S1350417719313173?via%3Dihub#!, DOI: 10.1016/j.ultsonch.2020.105126. [COBISS.SI-ID 17154075]